Proposed Mechanism for the Peroxynitrite-Mediated Oxidation of Uric Acid to Allantoin

God, this "Marvin" can be a real ditty to use and can still take a little time, even though I guess it is easier than the other one for this type of thing. This shows two one-electron oxidations of uric acid by ferryl heme (just because I couldn't think of another oxidant I could show the reactions of, easily) and then a series of two-electron oxidations of the products, yielding allantoin. These are proposed mechanisms that I've tried to piece together. The last reaction is shown as a two-electron oxidation, but it wouldn't really be possible in that step. Oxygen would only have 6 electrons after that. Peroxynitrite can, actually, act as either a one- or two-electron oxidant, but not in that case. I didn't feel like showing another one-electron oxidation, assuming that peroxynitrite can even bind to that carbon. The intermediates are, however, known to be intermediates in the oxidation of uric acid by peroxynitrite.

Potential Interactions of Urate and Inorganic Phosphate with Xenobiotic Substrates and Physiological Substrates of Organic Anion Transporters

One thing I was going to mention is that an excessive intake of inorganic phosphate, alone or in combination with oral purine nucleotides, could conceivably interact with prescription or nonprescription drugs that are substrates of organic anion transporters (OAT's) or multidrug resistance (MDR) protein transporters. The main types of drug-drug interactions that are given attention in the literature are the interactions that involve the noncompetitive inhibition, induction, or competitive inhibition of cytochrome P450 enzymes. But another type of interaction that would be more difficult to predict or even measure could be the competition of two substrates for export, across the canalicular, or apical, membranes of biliary epithelial cells, into the bile. Urate and phosphate can compete for export into the blood or bile by OAT's on the plasma membranes of different cell types in the liver, and bilirubin (http://scholar.google.com/scholar?q=bilirubin+%22organic+anion%22&hl=en), bile acids (http://scholar.google.com/scholar?hl=en&q=%22bile+acids%22+%22organic+anion%22), and other compounds are also substrates of various OAT's. A lot of different drugs are also substrates of OAT's (http://scholar.google.com/scholar?q=drugs+transport+%22organic+anion%22&hl=en) and might compete with urate or phosphate or xanthine, for example, conceivably, for export into the bile. It's unlikely that these interactions would be significant, in my opinion, except at high or excessive dosages of uricogenic purines or inorganic phosphate or in people who have liver or kidney disease. As I've mentioned in past postings, however, some neuraminidase inhibitors and other drugs or metabolites of drugs that are excreted unchanged or otherwise eliminated primarily by renal excretion might interact more significantly with high dosages of oral purines or with excessive amounts of inorganic phosphate. The effect that could conceivably be problematic would be a slowing, in response to an increase in intracellular urate or phosphate, etc., of the rate of biliary or renal excretion of a given drug. That's one reason it's always necessary to discuss these things with one's doctor.

Nonetheless, urate has been used to treat various forms of liver disease in animal models [one example: Garcia-Ruiz et al., 2006: (http://www.ncbi.nlm.nih.gov/pubmed/16941682)], and researchers have shown that urate can protect against mitochondrial dysfunction induced by a wide variety of treatments that produce mitochondrial dysfunction by increasing peroxynitrite formation (http://scholar.google.com/scholar?hl=en&q=mitochondrial+peroxynitrite+urate+OR+uric). A lot of factors and disease states can increase peroxynitrite formation, and the "antioxidant" or "nitrosative-degradation-by-proxy," more accurately, effects of urate, along with its apparent capacity to decrease or directly inhibit PARP-1 activity, make it more useful than many other compounds or antioxidants, in my opinion. As I've discussed in past postings, it may well be advantageous for an antioxidant, such as urate, to not be regenerated. Nonetheless, urate can, for example, regenerate melatonin and guanosine radical species by apparently-nonenzymatic mechanisms (http://scholar.google.com/scholar?hl=en&q=melatonin+regeneration+urate). And, as far as the rest of this posting is concerned, there's evidence that hypophosphatemia and intracellular phosphate depletion in the liver may contribute to liver damage in some cases and disease states (see past postings). One of the most important considerations in the context of phosphate homeostasis is to be aware that, in my opinion, the "phosphate" contained in inositol hexakisphosphate and other phytate compounds, in cereal grains and "plant proteins," etc., is unlikely to provide much, if any, utilizable phosphate in humans [see here: (http://hardcorephysiologyfun.blogspot.com/2009/08/phytates-as-potentially-poor-sources-of.html); (http://hardcorephysiologyfun.blogspot.com/2009/07/phytates-inositol-hexaphosphate-and.html)]. As far as my own calculation of my "dietary phosphate" intake went, I didn't even bother to include a contribution of cereal-grain phosphate. I put a big "NOTH-THING" by the spot on the page for the mg phosphate derived from phytate-containing foods. But I can't make that determination or calculation for anyone except myself. If I had been in the business of obtaining "hocus-pocus-microbial-phytase-derived-phantom-phosphate" phosphate from foods, maybe I'd have listed an actual number. But anyway, as with any compound, bizzarely-high dosages could cause problems. In response to massive dosages of either uricogenic purines or inorganic phosphate, those problems could take the form of interactions with other OAT substrates.

Potential for Competition Among Phosphate, Uric Acid (Urate), and Antivirals Used to Treat Influenza for Transport by Organic Anion Transporters

The authors of this article [Yabuuchi et al., 1998: (http://jpet.aspetjournals.org/cgi/reprint/286/3/1391)(http://www.ncbi.nlm.nih.gov/pubmed/9732402?dopt=Abstract)] describe the capacity of the type I Na(+)/Pi cotransporter (NPT1), a sodium and inorganic phosphate (Pi) transporter, to transport either organic anions, including probenecid, or inorganic phosphate (Pi) out of the liver and into the blood. Yabuuchi et al. (1998) noted that probenecid can compete with Pi for transport by NPT1, and this could conceivably mean that a higher intake of Pi might inhibit the efflux of uric acid (urate, UA), an organic anion whose reabsorption by proximal tubule epithelial cells can be inhibited by probenecid (http://scholar.google.com/scholar?hl=en&q=urate+probenecid), from the liver or otherwise influence the efflux or uptake of urate or xanthine by cells in the liver or kidneys, etc. (http://scholar.google.com/scholar?hl=en&q=%22inorganic+phosphate%22+anion+transporter). It's also conceivable that increases in extracellular or, in a more likely event, intracellular Pi could slow the elimination of antiviral drugs used to treat influenza. For example, Oo et al. (2002) [Oo et al., 2002: (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=127254&blobtype=pdf)(http://www.ncbi.nlm.nih.gov/pubmed/12019123)] noted that the active metabolites of some neuraminidase inhibitors are mostly excreted unchanged, such as through their uptake by the proximal tubule cells, from the peritubular capillaries, and efflux across the luminal (apical) membranes of proximal tubule epithelial cells into the tubular fluid. Karie et al. (2006) [Karie et al., 2006: (http://ndt.oxfordjournals.org/cgi/reprint/21/12/3606.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/16799172)] noted that some neuraminidase inhibitors do not serve as substrates for cytochrome P450 oxidoreductases in the liver and do not inhibit those enzymes either, and that's a major reason that their active metabolites are mostly excreted unchanged by renal tubular excretion. Probenecid competes with some of these These interactions would probably not be very likely and would be most likely to occur, if at all, in people whose kidney function has already been diminished, as a result of age or other factors. This is a hastily-chosen article that describes the capacity of probenecid to inhibit the transport and, hence, renal excretion of some neuraminidase inhibitors or their active metabolites [(http://www.cdc.gov/Mmwr/preview/mmwrhtml/rr4814a1.htm); (http://scholar.google.com/scholar?q=probenecid+neuraminidase+inhibitor&hl=en)], and that basically means that oral purines or phosphate supplementation could conceivably slow the elimination of some neuraminidase inhibitors, and that wouldn't necessarily be desirable. It might sound good, and some people have proposed the use of probenecid to allow for the use of neuraminidase inhibitors at lower dosages (thereby allowing more people to be treated with antivirals, in the event of a "1970's-style shortage" of antivirals). But that could be a dangerous approach, given that the movement and constant efflux of some neuraminidase inhibitors is necessary to prevent the potentially problematic effects of their accumulation intracellularly, in cells in the liver or kidneys.

Thus, if one were taking an antiviral to treat an influenza infection and also taking some oral purine compound or source of inorganic phosphate (Pi), one might need to reduce the dosages of those or, as discussed by Karie et al. (2006), reduce the dosages of the antivirals. It would seem that reducing the dosage of the antiviral would not be the better approach, in theory, but one would obviously want to discuss this with one's doctor. Some of the major old M2 protein inhibitors, used as antivirals in the treatment of influenza, are derivatives of 1-aminoadamantane and are therefore also excreted unchanged. Aminoadamantane derivatives are apparently transported by organic cation transporters and would seem to not compete with UA or phosphate, but probenecid is a weak base and can sometimes inhibit the transport of substrates of organic cation transporters (http://scholar.google.com/scholar?hl=en&q=probenecid+aminoadamantane). There are strange ways in which substrates of organic cation transporters can influence the transport of other substrates (drugs or physiological compounds) of organic anion transporters [Khamdang et al., 2002: (http://jpet.aspetjournals.org/cgi/content/full/303/2/534)(http://www.ncbi.nlm.nih.gov/pubmed/12388633?dopt=Abstract)], maybe because they, like probenecid, are weak bases and could either be protonated or deprotonated or because they contain more than one ionizable group. There can be pH extremes and variations in the tubular fluid, for example, and there could be indirect interactions. An increase in the reabsorption of UA could, for example, be pH dependent and thereby produce an indirect, pH-sensitive reduction in the excretion of a drug that UA, by its binding to an efflux transporter intracellularly, in proximal tubule cells, and relative failure to serve as a substrate for transport by that transporter, competes with for transport, etc.

Another implication is that increases in the intracellular Pi concentration could reduce the loss of purine nucleotides both by inhibiting adenosine deaminase (and by activating adenosine kinase, arguably) and by reducing the efflux of cAMP or cGMP or other purine substrates of some organic anion transporters or multidrug resistance proteins that transport purines out of cells. This might mean that phosphate could, apart from its role in promoting normal purine salvage, serve as a dose-reducing agent for oral purines, such as ATP disodium, even in the absence of an influenza infection, obviously. But that's more theoretical, and these are just my opinions. Obviously, other medications, including but not limited to some antibiotics, are transported by organic anion transporters, too, and that's another reason one should discuss this type of thing with one's doctor.

Open Discussion on Multiple Sclerosis, UVB, and Energy Metabolism

I was going to mention that one way of looking at the age-dependence of the protective effect of "migration" in multiple sclerosis (migration to an equatorial latitude) is that the axonal transport capacity or generalized "robustness" is greater in people who are relatively "younger" than in people who are older. There's also a latitude gradient in Epstein-Barr Virus-associated malignancies (Burkitt's lymphoma, etc.), but it's not clear if that's explainable in terms of the lesser extents to which people living in Subsaharan Africa have access to health care and nutrition, etc. (http://scholar.google.com/scholar?hl=en&q=Burkitt%27s+lymphoma+latitude). That's not a great search. But the point is that that could provide indirect support to the ideas people have had about late EBV infection being associated with abnormal neural development in the offspring of people who have had late EBV infections (see past postings and the articles cited in them). I just don't think one can ignore that epidemiology (the latitude gradient and research on migration) in relation to multiple sclerosis, and I don't think it's written in stone that there's no capacity for protection after age 15 or something (no capacity for protection to occur in response to migration or to the environmental, protective factors(s) that have been associated with migration to equatorial latitudes). I've seen research suggesting, for example, that migration may be protective through age 27 or something. That would tend to imply that migration *could* be protective at later ages but that people's behaviors, related to time spent outdoors or to nutritional factors that would interact with that or to activity levels and all other things, have become ingrained, perhaps, by the time people are 16 or 17. On average, how many people radically change everything about their habits after age 18? I'm talking about drastic changes in UVB exposure, etc. I mean, I'm sorry to say it (and I'm in no way suggesting that people go out in the sun without talking with their doctor), but 10 minutes of sun exposure at noon, on the hands and face, is not likely to increase serum 25-hydroxyvitamin D levels all that much and is not going to have the kinds of immunomodulatory effects that depend on hundreds of billions of neutrophils infiltrating the UVB-irradiated skin and the immunosuppressive cytokine milieu associated with it. In my past comments, the main thing I wanted to convey is that I don't think people should expect magic from UVB exposure and that there are many other factors that come into play in the etiology of multiple sclerosis. But, for example, Epstein-Barr Virus infects keratinocytes (http://scholar.google.com/scholar?hl=en&q=Epstein-Barr+keratinocytes), and I don't have to say what that means. It means that there's the *potential* for the induction of tolerance to EBV latent and lytic cycle proteins on a kind of mass scale, following UVB exposure. But there's also the potential for that tolerance to turn into seriously aberrant, Th2-driven immunity and to worsen matters. It's difficult to control or predict the responses, and that's especially true in disease states (in which there's potential for disastrous effects). But, then again, UVB is known to suppress both Th2 and Th1 immunity (the Th1/Th2 dichotomy is a bit outdated but still has some usefulness as a crude framework for looking at these things). But my point is that the release of Th1 cytokines almost disappears, in some cases, from the lymph nodes of UVB-irradiated animals, and the idea that everything boils down to vitamin D and hands-and-face, anemic, Victorian-Era, parasol-carrying approaches is a bit absurd to me. That said, I can't make any recommendations on these things, because I can't give medical advice and, to say the least, can't make any guarantees whatsoever about safety. Avis et al. (1995) (http://scholar.google.com/scholar?hl=en&q=sudden+death+sun+exposure+%22multiple+sclerosis%22) discussed case(s) of people with multiple sclerosis dying after sitting in the sun.

I think it's telling that thermoregulatory dysfunction features prominently in multiple sclerosis, but I don't claim to know how it relates to the supposed UVB-mediated trigeminohypothalamic thermoregulation that may occur in humans. Part of the difficulty is that, for example, the neuropathological effects of Epstein-Barr Virus, in some extreme case studies, can be highly diffuse, and that could be explained in any number of ways (in terms of "diffuse" B-cell infiltration or latent infection of astrocytes and microglia and other perivascular, monocyte-macrophage-lineage cells). I tend to think it's a result of astrocytic infection by EBV, but that's just my opinion. The relapsing-remitting quality could be explained in terms of the devastating effects that pro-inflammatory cytokines can have on energy metabolism, and that could explain the apparent absence of overt inflammation in some research in multiple sclerosis (I can't say anything more specific without looking at the specific articles that people have cited, and I don't want to do that now).

I just don't understand why there would be such resistance to the consideration of all the mechanisms at work. I've seen articles make statements that there's no problem with energy metabolism in the brains of people who have multiple sclerosis. That makes no sense, in my opinion, because axonal degeneration implies profound problems with energy metabolism. And if one buys into the idea that a lot of pro-inflammatory cytokines are being released from activated T-cells infiltrating the CNS, then one would expect major problems with energy metabolism from that. Here's a not-very-good search that shows some of the vast amounts of research showing rapidly-induced mitochondrial dysfunction induced by TNF-alpha and other pro-inflammatory cytokines (http://scholar.google.com/scholar?hl=en&q=TNF+mitochondrial+dysfunction+astrocytes). There's one article, there, in which the authors probably discuss the concept that, in my opinion, optic neuropathies are frequently associated with mitochondrial dysfunction and can be caused by that. The energetic demands of neurotransmission in the optic nerve fibers are enormous. I'm not saying that reducing pro-inflammatory cytokine production by T-cells or other cell types doesn't have the potential to improve energy metabolism. I just think that it would help to acknowledge the deficits in energy metabolism that are very likely to exist in a neurodegenerative disease, such as multiple sclerosis, and to try to develop therapeutic strategies for addressing those deficits in more direct ways. These are just my crude, unrefined thoughts on some of these topics, and I'd strongly urge anyone to discuss things with one's doctor before doing anything.

I'm deliberately discussing some of these things from an idealistic, somewhat impractical point of view, because dogged pragmatism and dogma haven't seemed to be all that beneficial. Even something like parenteral guanosine could be viewed as an energy-metabolism-based strategy, because de novo purine biosynthesis is metabolically costly (and the brain has very little capacity for de novo purine biosynthesis). Also, anticonvulsant medications and adenosine receptor activation or modulation are known to be able to increase or preserve the phosphocreatine to creatine ratio and the adenylate charge. Additionally, purine nucleotide availability is likely, in my opinion, to be a limiting or nearly-limiting factor in mtDNA replication and in other aspects of mitochondrial functioning. I'm not saying guanosine would be a cure-all, but I'm just saying that there are many ways to address energy metabolism.

Elevating uric acid (UA) levels intracellularly, in neurons and astrocytes, such as through the administration of exogenous, parenteral purine nucleotides, would, in my opinion, have the potential to be therapeutic as a result of, among other mechanisms, the UA-mediated improvements in mitochondrial functioning (as a result of peroxynitrite scavenging). That article I cited awhile back, on UA in relation to the sympathetic nervous system and goal-oriented behavior, found intramitochondrial UA levels of 60 uM or something. That's very significant, in my opinion. The peroxynitrite scavenging effects of UA may not look all that special or unique in some articles, but one has to consider the fact that the suppression of nitric oxide (NO) output from activated macrophages, by UA, can occur in the face of these massive increases in the output of iNOS-derived NO. I forget what the variable was--NO output or NADPH oxidase activity--that increases the most dramatically. I think the mRNA or protein content of iNOS can increase 20-50 fold or something, in activated macrophages, and I think the NO output can increase by something like 1000-fold or even more. Some articles show these little graphs of the effects of UA (suppression of NO output by cultured monocyte-macrophage-lineage cells, etc.), and the graphs don't capture what's going on. The UA can suppress NO output drastically at physiological concentrations (of UA), and that's no small feat. NO and peroxynitrite produce strongly detrimental effects on mitochondrial function [a lot of these articles show, rather incidentally or in a manner that doesn't showcase the effects of UA, that UA can ameliorate peroxynitrite-induced mitochondrial dysfunction in various cell types: (http://scholar.google.com/scholar?hl=en&q=mitochondrial+peroxynitrite+urate+OR+uric)].

Adenosine vs. Ribose vs. AICAriboside for the Restoration of Adenosine Nucleotides in the Heart Following Ischemia

In this article [Mauser et al., 1985: (http://circres.ahajournals.org/cgi/reprint/56/2/220.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/3918804)], Mauser et al. (1985) show that the intraarterial infusion of adenosine produced a 90-fold increase in the rate of adenosine nucleotide resynthesis (mostly by purine salvage pathways), following cardiac ischemia in dogs, and infusions of equimolar dosages of ribose or AICAriboside produced only between 5-fold and 9-fold increases in the rate of adenosine nucleotide formation, by either the salvage or de novo pathways. Only adenosine significantly restored ATP levels, following ischemia. All of the compounds were infused intraarterially, into the left coronary arteries of the dogs.

This article shows, in my opinion, that adenosine is far superior to ribose as an approach to restoring adenosine nucleotide levels following their depletion, and the research casts doubt on the idea, as suggested by the authors of some articles, that the entries of the carbons of ribose, derived from exogenous purine nucleotides, into the nonoxidative pentose cycle and into glycolytic pathways make a substantial contribution to the purine-mediated protection of cultured cells, such as astrocytes, against death due to glucose deprivation or other conditions. In most of those articles, the only evidence that ribose mediates the protective effects is that purine nucleoside phosphorylase (PNP) inhibitors sometimes block the protective effects of exogenous purines. But, as discussed by other authors, that doesn't mean that the use of ribose as a glycolytic substrate mediates the protective effects of purines or that ribose can substitute for the preformed nucleotides. It may just mean that ribose-1-phosphate has to be derived from nucleoside phosphorolysis to maintain purine salvage and that, paradoxically, more purine nucleobases are lost when ribose is "locked" in nucleosides and nucleotides than are lost when some turnover of nucleoside-derived ribose is allowed to occur, via the PNP-mediated formation of ribose-1-phosphate and purine bases. Also, the inhibition of nucleoside phosphorolysis prevents the formation of uric acid from the exogenous purines, and some authors have suggested that uric acid-induced peroxynitrite scavenging may partially mediate the protective effects of purines on cultured cells. There's a lot of research showing that uric acid can maintain mitochondrial functioning, in cells in the liver or in other cells, by preventing the inactivation of respiratory-chain enzymes by peroxynitrite, etc. Additionally, adenosine is normally kept at a very low concentration intracellularly, and massive amounts of adenosine have to be supplied, normally, to produce adenosine-mediated toxic effects and S-adenosylhomocysteine accumulation, etc., in cells [usually 1 mM (1000 uM) or higher of extracellular adenosine is required, a concentration that is supraphysiological] [Adair, 2005: (http://ajpregu.physiology.org/cgi/content/full/289/2/R283)(http://www.ncbi.nlm.nih.gov/pubmed/16014444)]. If adenosine were to accumulate as a result of PNP inhibition, that accumulation could produce toxic effects. But adenosine is normally metabolized extremely rapidly.

The rate of de novo purine biosynthesis is extremely slow, and this is one reason, as Mauser et al. (1985) discussed, that AICAr did not produce very significant restorative effects on adenosine nucleotide levels. Ribose did not even appear to contribute much to purine salvage, in my opinion, in comparison to the effects of adenosine. Even if the effects of ribose depended on its metabolism into glycolytic intermediates, the presence of the preformed purine nucleotides, such as can be derived from exogenous adenosine, appear to be crucial and to be a limiting factor in the rate of ATP resynthesis and nucleotide replenishment following ischemia. There are other articles that provide similar data.

In the case of pyrimidines, the research showing that uridine phosphorylase (UP) inhibition can abolish the cytoprotective effects of exogenous uridine, as in astrocytes, tends to not take into account the role that UP is thought to play in the salvage of uracil in rodents and cultured astrocytes, etc. The traditional view is that pyrimidine bases are not salvaged and that pyrimidine salvage occurs only at the level of the whole nucleoside, meaning that the main salvage pathway for uridine, for example, is its phosphorylation to UMP by uridine kinase. Some of these articles discuss the fact that, in the brains of rodents, UP appears to play an "anabolic," rather than catabolic, role and to be required for pyrimidine salvage [Mascia et al., 1999, etc.: (http://scholar.google.com/scholar?q=%22uridine+phosphorylase%22+salvage&hl=en&lr=)]. Also, inhibition UP may indirectly inhibit purine salvage, given that UP inhibition would prevent uridine from serving as a source of ribose-1-phosphate. This could decrease the formation of PRPP from uridine-derived ribose-1-phosphate, thereby increasing the loss of purines and compromising both ATP formation and the ATP-dependent salvage of uridine, etc. The depletion of purines has been shown to lead to secondary depletion of pyrimidines, even in the context of the fructose-induced depletion of ATP from the liver. Fructose has been shown to transiently elevate plasma uridine levels, and that's very much a pathological effect, in my opinion. Some people seem to think that the fructose-induced elevation of plasma uric acid levels is "good" or desirable, given that uric acid scavenges peroxynitrite. But the elevations in uric acid levels, following fructose ingestion or infusion, are mainly the result of pronounced ATP depletion in the liver, in my opinion (and as shown by countless articles). That's not desirable. In any case, the articles in these areas are interesting.

Serum Uric Acid, Energy Metabolism, Sympathetic Activation, and Goal-Oriented Behavior or "Grant-Money-Getting" Behavior

This article [Hunter et al., 1990: (http://www.ncbi.nlm.nih.gov/pubmed/2345757)] is one that I cited in a previous posting (http://hardcorephysiologyfun.blogspot.com/2009/03/interactions-of-caffeine-with-purine.html), but I didn't have time to discuss some interesting research that Hunter et al. (1990) discuss and cite. There's old research showing positive associations of serum uric acid (UA) levels with goal-oriented behavior and, essentially, activity level in general, and there's also some more recent research looking at UA per se as being a supposedly-reliable mediator of hyperactivity or mania or whatever other conditions. What interests me is the reasons why UA might be associated with goal-oriented behavior. I remember that a professor I took a class from once mentioned research showing that higher serum UA levels were associated with more success in getting grant money (in successfully getting grants awarded, etc.). He was referring to old research, from the 1950's or 1960's, but I wonder if it doesn't have to do with brain activity in some generalized sense. There's a vast amount of research showing that electrical stimulation or glutamatergic stimulation or noradrenergic activity increases extracellular-fluid (ECF) adenosine and UA levels in the brain. It's a generalized response that may just have to do with an increase in the metabolic demands of neurons. I tend to think that the cerebral metabolic activity or noradrenergic activity might just be making people slightly more aggressive or driven and might be accompanied by increases in sympathetic outflow from the central nervous system, and that could account for the serum UA elevations. That type of process, however, would not mean that low serum UA levels could not also be associated with excessively-prolonged increases in noradrenergic activity and sympathetic activation. There could be a pathological activation that would eventually compromise beta-adrenergic sensitivity, such as in people with multiple sclerosis (in whom the serum UA levels tend to be very low). Astrocytic beta2-adrenoreceptor density and sensitivity has been reported to be very low in people with MS, and there's research associating prescriptions for asthma (specifically beta2-adrenoreceptor agonists) with lower incidences of MS. Obviously, taking beta-agonists would be potentially dangerous for people with MS, and one would want to discuss that type of thing with one's doctor. The association only was found when researchers looked at medical records across many years, also, although beta2-adrenoreceptor activation does tend to be anti-inflammatory and immunosuppressive. One could make the argument that robust increases and equally-robust decreases in noradrenergic activity in the brain, accompanied by augmentations in sympathetic outflow, would produce elevations in UA that would account for the associations of high serum UA with goal-oriented behavior. Poorly-regulated noradrenergic activity could conceivably lead to gradual, "functional sympathectomy-like" changes (reduced beta-adrenoreceptor sensitivity) that could produce decreases in serum UA, etc. This is very general and imprecise, but it's interesting to think about. Low serum UA is a generalized feature of a variety of intracranial disease states and is thought to be partially a result of poor osmoregulation in the brain, such that the sympathetic innervation of the kidneys changes. The decreases in functional, sympathetic innervation of the kidneys is thought to play more of a role in the etiology of cerebral salt wasting (CSW) than in the etiology of syndrome of inappropriate antidiuretic hormone secretion (SIADH). But there must be some more precise neurobiological changes that could account for the UA depletion that occurs in SIADH and CSW, and I'm not convinced that it only has to do with osmoregulatory failures per se or with changes in renal UA reabsorption. I think it might have to do with derangements in energy metabolism. Here's an interesting article that shows that the intramitochondrial UA levels are higher in rats with diabetes [Kristal et al., 1999: (http://www.ncbi.nlm.nih.gov/pubmed/10496973)]. But as the disease process and poor glycemic control in the rats' diabetes became more pronounced, the UA production normalized or decreased again. That's potentially really important for understanding why UA is low in people with MS, and it also casts serious doubt on the use of all of these association studies showing UA to be some kind of "independent" risk factor for (or variable independently-associated with) cardiovascular disease. One could claim to be able to control for insulin sensitivity in some association study, but that's unlikely to be possible. Energy metabolism, as related to insulin sensitivity, is far too complex to control for in an association study that looks at some blood tests from 20,000 people.

Elevation of Plasma Hypoxanthine During and After Glycogen-Depleting Exercise: Relevance to Energy Metabolism and Nucleotide Turnover in the Brain

This is a really interesting article [Sahlin et al., 1999: (http://www.ncbi.nlm.nih.gov/pubmed/10502075)], and the authors found that plasma hypoxanthine increased by a mean of 7.78-fold during exercise in humans. There was some variation in the magnitudes of the increases in plasma hypoxanthine among individuals. The increases in plasma uric acid (urate) and xanthine, which is derived from the metabolism of hypoxanthine by xanthine oxidoreductase (xanthine oxidase is technically a modified form of xanthine reductase that's been modified by proteolytic cleavage, etc., but people typically refer to the enzyme activity as being "xanthine oxidase" activity), were larger in terms of the amounts of those purines formed, but the percent increases were lower (mean increases of 5 percent for urate and 223%, or a 2.23-fold increase, for xanthine). The effect of that increase in plasma hypoxanthine on the brain should not be underestimated. In a past posting, I discussed some of the research showing neuroprotective effects of remarkably low doses of hypoxanthine [Mink and Johnston, 2007, cited and discussed here: (http://hardcorephysiologyfun.blogspot.com/2009/03/protection-against-postischemic-damage.html)]. There's still the fact that the exercise-induced increases in neuronal activity in the brain would be expected to substantially, albeit transiently, increase the rates of purine export from neurons. I can't immediately find any articles showing elevated cerebrospinal fluid hypoxanthine and xanthine and urate levels, following exercise, but it's very likely that those elevations would occur, in my opinion, especially following high-intensity exercise. So that supposed depleting effect of exercise on neuronal and astrocyte purine nucleotide levels would be expected to lessen the impact of an increase in the plasma hypoxanthine level. I would think that, as a person's muscle mass increased over time, the muscles' capacity to export hypoxanthine would be increased and would produce more significant effects on the brain. But the person would, in my opinion, keep having to push the limits and produce glycogen depletion, as discussed below. Other researchers have discussed the effects of muscle-derived purines on the brain. There's one article from 1978 or 1979 that discusses the potential effects of exercise-induced elevations in extracellular ATP, released from endothelial cells, on the brain, but the authors focused too much on ATP per se. Extracellular ATP is rapidly degraded to ADP and then adenosine and hypoxanthine, etc., and so one would expect to see much more of an effect of exercise on adenosine or hypoxanthine than on ATP. I mean that the purines that might be expected to enter the brain in significant amounts, during exercise, and to also exert meaningful effects on nucleotide pools in neurons or astrocytes would be hypoxanthine or, conceivably, adenosine and not ATP, in my opinion. Xanthine is not salvaged efficiently (but can, in fact, be salvaged in small amounts to xanthosine and then guanosine), but hypoxanthine and adenosine are salvaged relatively efficiently by cells in the brain.

Sahlin et al. (1999) discuss the fact that researchers have generally found glycogen depletion from the skeletal muscles to be a prerequisite for the most pronounced, exercise-induced increases in plasma purines, including hypoxanthine. True glycogen depletion from a muscle group generally requires exhaustive exercise, and the research has generally shown, in my opinion, that resistance exercise produces more-pronounced degrees of glycogen depletion and purine depletion from the muscles [the general idea is that ATP depletion causes a loss of the capacity to salvage adenosine (and also guanosine), and this causes inosine monophosphate to accumulate and be converted into hypoxanthine and xanthine in the muscle cells] than exercise at low intensity does. Hellsten et al. (1998) [Hellsten et al., 1998: (http://ajpendo.physiology.org/cgi/content/full/274/4/E600)(http://www.ncbi.nlm.nih.gov/pubmed/9575819)] discuss research showing that a high-intensity exercise program causes a 20 percent decrease in the total adenine nucleotide contents of skeletal muscles (when people are not exercising, meaning post-exercise and all the time). That's a remarkable fact and suggests to me that some benefit might be derived from low-dose adenosine or guanosine supplementation, particularly early in an exercise program. But that's my opinion. The effects of hypoxanthine on the brain should not be underestimated, and a single exercise session that produces an 8-fold elevation in plasma hypoxanthine levels, during the hour or few hours following exercise, could have a significant effect on the pools of adenine nucleotides, in particular, in the brain. Mink and Johnston (2007), cited above, discuss the fact that hypoxanthine appeared to be salvaged to a large extent, even during ischemia. The brain has an extremely low capacity to make purines de novo and depends almost entirely on purines exported from either the endothelial cells lining the cerebral blood vessels or from the blood.

Of course, creatine and glutamine have been shown to augment the salvage of purine or pyrimidine nucleotides in various articles [cited and discussed here or in other postings: (http://hardcorephysiologyfun.blogspot.com/2009/02/interactions-of-glutamine-and-arginine.html)], but any supposed improvement in purine or pyrimidine salvage, in the brain, that might occur in response to the administration of those types of supplements might be offset by the increase in, for example, purine nucleotide export that could accompany a creatine- or glutamine-induced increase in exercise intensity. I don't think that's the way it would work, though, as long as the doses of creatine or glutamine are kept low, but that's just my opinion. For example, the combination of glutamine and inosine, which is hypoxanthine riboside (hypoxanthine attached to ribose to make a nucleotide), [Hodges and Snyder, 2004, cited here: (http://hardcorephysiologyfun.blogspot.com/2009/02/contribution-of-glutamine-to-pool-of.html)]. That's just my opinion. In my opinion, low doses of glutamine, creatine and adenosine and guanosine might produce some sort of buffering effect on brain energy metabolism and purine nucleotide pools. Creatine increases or "stimulates" oxidative metabolism, in part by maintaining the intramitochondrial ADP pool, and this would be expected to increase the extent to which glutamine, upon its metabolism into glutamate, can be metabolized into alpha-ketoglutarate and undergo oxidative metabolism in neurons or astrocytes in the brain. People discuss creatine as if it participates only in anaerobic metabolism, but this is just not the case. Creatine can prolong "aerobic" exercise and is known to increase oxidative glucose utilization in cells, etc.

Similarly, cytidine was recently shown to decrease the glutamine+glutamate pool in parts of the brains of humans [Yoon et al., 2009: (http://www.ncbi.nlm.nih.gov/pubmed/19194376)], and that type of effect could be explained in terms of an increase in the flux of substrates through the nonoxidative pentose cycle, via the ribose-1-phosphate derived from cytidine catabolism in the brain, and glycolytic pathways. Uridine is converted into cytidine, and ribose derived from purines might be expected to produce a similar effect, to some extent. Hodges and Snyder (2004), in fact, discuss research showing that either glucose or glutamine can serve as an energy substrate for cultured cells but that inosine or some other "pentose source," meaning ribose derived from inosine, had to be present, in combination with glutamine, to maintain ATP levels in cultured cells that lacked a source of glucose. Exogenous nucleotides have been shown to elevate lactate levels in many articles, and those effects could be partly attributable to the ribose-induced increases in the activities of glycolytic enzymes. The effect of cytidine could also have been the result of some effect on receptors that bind pyrimidine triphosphates, etc., but that type of mechanism, in my opinion, is less plausible. Nonetheless, I don't think ribose, by itself would substitute for uridine, past a certain point.

My overall point is that some of these energy substrates, such as creatine and glutamine and ribose, can increase the flux of intermediates through one pathway and produce beneficial effects that might, in my opinion, be expected to be limited by the "depleting" effect that might result from that shift in the metabolism of one or another intermediates. At high doses, however, these types of supplements start to produce really complicated effects. The glutamine-induced decreases in plasma free fatty acids, for example, could, in my opinion, begin to become counterproductive, with respect to the brain, at higher dosages. Obviously, one would want to discuss any of these supplements with one's doctor before taking any of them or beginning any kind of exercise program.

Research on the Use of Creatine Monohydrate as an Adjunctive to Prescription Antidepressants

These are some more articles discussing the use of creatine in combination with prescription antidepressants or medications to treat Parkinson's disease [Roitman et al., 2007: (http://www.ncbi.nlm.nih.gov/pubmed/17988366); Amital et al., 2006: (http://ajp.psychiatryonline.org/cgi/reprint/163/10/1840-b.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/17012702); Bender et al., 2006: (http://www.ncbi.nlm.nih.gov/pubmed/17030762)]. Amital et al. (2006) researched the effects of creatine at 3 grams per day, for one week, and then 5 grams per day, for four weeks (and then also for eight weeks after the end of the trial), in a person who was being treated with prescription medications for post-traumatic stress disorder and depression with fibromyalgia. The authors found that, during the course of the trial, the person's scores on the Hamilton Depression Rating Scale had decreased, indicating that the creatine had evidently produced an antidepressant effect in the person, and that the person's symptoms of fibromyalgia had improved. Roitman et al. (2007) found that the full antidepressant effect in the patients had required four weeks to emerge, but the greatest reductions in the Hamilton Depression Rating Scale had occurred within the first two weeks in most of the people. Roitman et al. (2007) also found that some people experienced a greater improvement in mood from 3 grams/day of creatine than from 5 grams/day.

Bender et al. (2006) used 4 grams a day of creatine for two years (after an initial phase of 20 grams per day for 6 days and then 2 grams per day for six months), in a randomized, placebo-controlled trial and found that the people who had taken creatine had not had to increase the dosages of their dopaminergic medications for Parkinson's disease to the extent that the people taking the placebo had had to increase their dosages, and that effect was statistically-significant. The other main result of the trial was the finding that creatine had produced a statistically-significant reduction in symptoms of depression, as measured by the "score" on Item 3 of Part I of the Unified Parkinson's Disease Rating Scale (UPDRS). Incidentally, I don't think the "loading dose" concept of creatine makes much sense, especially given the potential for high-dose creatine to produce plasma and extracellular fluid volume expansion in some people [Powers et al., 2003: (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=155510&blobtype=pdf)(http://www.ncbi.nlm.nih.gov/pubmed/12937471)]. To the extent that that effect would occur more with high-dose than with low-dose creatine, one would expect the body to be less able to adapt or compensate to that effect in the context of a "loading dose." But that's just my opinion. Obviously, one would want to discuss this with one's doctor before using creatine or any supplement, particularly given that Roitman et al. (2007) found that, in one of the ten patients, creatine produced transient improvement in the person's mood and then worsened it. Roitman (2007) also found that creatine produced mania or hypomania in the people in the trial who had been diagnosed with bipolar disorder.

In my opinion, creatine would be more effective in this context in combination with adenosine monophosphate/triphosphate or guanosine monophosphate or both, for complex reasons. I think it's a really important point, but it's just my opinion. I could explain my reasoning in more detail, but I'd have to cite a lot more articles and can't do that right now. I discussed some of the rationale for the combination in a past posting (http://hardcorephysiologyfun.blogspot.com/2009/03/creatine-cr-phosphocreatine-pcr-and.html), and I discussed some of the research and details on orally-administered guanosine monophosphate and adenosine monophosphate or adenosine triphosphate in this posting (http://hardcorephysiologyfun.blogspot.com/2009/03/adenosine-and-guanosine-in-animal.html). Other approaches that might potentiate the effects of creatine, in my opinion, would be the use of methylcobalamin at ~5 mg/day or something (with the intent of disinhibiting creatine kinase enzymes and tricarboxylic acid cycle enzymes by reducing methylmalonic acid levels), the use of L-methylfolate or levoleucovorin in combination with methylcobalamin (with the intent of reducing the accumulation of sarcosine, which may be produced from high-dose creatine, given that folate depletion can compromise the metabolism of sarcosine and dimethylglycine and betaine, as I've discussed in the context of research cited in past postings), and the use of adenosine and guanosine to elevate serum uric acid to high-normal levels, under a doctor's supervision (with various "purposes" in mind, including the disinhibition of creatine kinase, which is inhibited by peroxynitrite, through the scavenging of peroxynitrite by uric acid) [discussed and cited here: (http://hardcorephysiologyfun.blogspot.com/2009/03/creatine-cr-phosphocreatine-pcr-and.html)]. Poortmans et al. (2005) [Poortmans et al., 2005: (http://www.ncbi.nlm.nih.gov/pubmed/16260971)] discussed the fact that the elevation of urinary methylamine and formaldehyde, in response to high-dose creatine administration, could have resulted from the metabolism of some of the creatine to sarcosine, evidently by the creatinase enzymatic activity in microorganisms in the G.I. tract, if memory serves (humans don't express a creatinase enzyme, as far as is known). Sarcosine can be metabolized to methylamine by sarcosine reductase or to formaldehyde and glycine, by sarcosine oxidase, and methylamine can be metabolized to formaldehyde and ammonia by semicarbazide-sensitive amine oxidase (SSAO) (Poortmans et al., 2005). Formaldehyde can then be converted into formate, and the activities of the folate cycle enzymes, which are dependent upon adequate pools of reduced folates, such as L-methylfolate or levoleucovorin, and vitamin B12 (i.e. methylcobalamin), are important for the normal metabolism of formate.

Adenosine and Guanosine in Animal Models of Depression

Several articles show that adenosine [Kaster et al., 2004: (http://www.ncbi.nlm.nih.gov/pubmed/14729225); Kaster et al., 2005a: (http://www.ncbi.nlm.nih.gov/pubmed/16140163); Kaster et al., 2005b: (http://www.ncbi.nlm.nih.gov/pubmed/16202183); Kaster et al., 2007a: (http://www.ncbi.nlm.nih.gov/pubmed/17868670); Kaster et al., 2007b: (http://www.ncbi.nlm.nih.gov/pubmed/17296254)] or guanosine [Eckeli et al., 2000: (http://www.ncbi.nlm.nih.gov/pubmed/10884029)] can produce antidepressant ("antidepressant-like") effects in animal models of depression. It's important to note that guanosine and adenosine have not been proven to be effective or safe in the treatment of depression or any other disease, and one should always talk to his or her doctor before taking any of these supplements. It's also important to take the results from these animal studies with a grain of salt, but the results of a lot of these experiments are actually generally-consistent with the results of other research in humans. My sense is that guanosine would display more of a mild anticonvulsant effect than adenosine would, mainly because of the large amount of research showing anticonvulsant effects of oral or intraperitoneal guanosine in animals [(http://scholar.google.com/scholar?num=100&hl=en&lr=&safe=off&cites=13842524336757775794); (http://hardcorephysiologyfun.blogspot.com/2009/01/anticonvulsant-effects-of-oral.html)]. The apparent effectiveness of SAM-e in the treatment of depression (http://scholar.google.com/scholar?num=100&hl=en&lr=&safe=off&q=antidepressant+%22S-adenosylmethionine%22+OR+%22S-adenosyl-L-methionine%22), in spite of the relatively poor biovailability that I see SAM-e as having, is, in my opinion, an important line of evidence suggesting that adenosine could, in fact, display antidepressant effects and serve some kind(s) of adjunctive role(s) in that regard. SAM-e is rapidly metabolized into adenosine, and, as discussed below, some of the effects of SAM-e on neurons and other cell types can be blocked by adenosine receptor antagonists or mimicked by adenosine itself. Although adenosine is generally viewed as exerting primarily an inhibitory influence on catecholaminergic and glutamatergic transmission, S-adenosylmethionine (SAM-e), which is generally known to increase the pool of adenosine and its nucleotides in the cells it reaches [Smolenski, 2000: (http://www.actabp.pl/pdf/4_2000/1171-1178s.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/11996106)], can nonetheless induce mania or hypomania in some people (an effect that is not consistent with an anticonvulsant effect) (http://scholar.google.com/scholar?q=S-adenosylmethionine+mania+OR+hypomania&hl=en&lr=). Thus, SAM-e can exert excitatory effects under some conditions, such as in the presence of medications that affect adenosine release or adenosine receptor sensitivities in the brain (many drugs used in psychopharmacology). Consistent with this, Saletu et al. (2002) [Saletu et al., 2002: (http://www.ncbi.nlm.nih.gov/pubmed/12486491)] found that the changes in the electroencephalograms of people who had been receiving parenteral SAM-e for 7 days indicated that SAM-e had been producing a mixture of both excitatory and inhibitory effects on overall brain activity. This article is a bit confusing at times but discusses some of the complexities of "adenosinergic" signalling in the context of dopaminergic transmission and psychiatric conditions [Cunha et al., 2008: (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=2423946&blobtype=pdf)(http://www.ncbi.nlm.nih.gov/pubmed/18537674)]. What they're saying is that adenosine itself can exert excitatory effects on striatal dopamine release by activating (apparently-predominantly-presynaptic) "extrastriatal" A2A receptors on glutamatergic neurons, even in the face of the inhibition of dopamine release by the adenosine-induced, tonic activation of A1 receptors on neurons in the striatum (on dopaminergic terminals or interneurons, etc.). Adenosine could, for example, modulate the release of glutamate from glutamatergic, pyramidal neurons that project from the dorsolateral prefrontal cortex and provide excitatory inputs to dopaminergic neurons in the ventral tegmental area (VTA), thereby providing an excitatory influence on dopamine release in the striatum (given that many dopaminergic neurons in the VTA project to the ventral striatum, etc.) [Carr and Sesack, 2000: (http://www.jneurosci.org/cgi/reprint/20/10/3864)(http://www.ncbi.nlm.nih.gov/pubmed/10804226?dopt=Abstract)].

That's a fairly well-established neuronal pathway that's important for cognitive functioning and mood, but the research on the effects of adenosine (and guanosine) in the brain is very vast and extremely complicated. That's one reason I tend to focus more on in vivo studies in animals and to not focus too heavily on articles that discuss, for example, the intracellular signalling cascades that guanosine and adenosine activate, via the binding of GTP to G-proteins or the binding of adenosine to adenosine receptors or intracellular binding sites, etc. But given the importance of dopaminergic transmission to cognitive functioning and mood disorders, it is noteworthy that, as discussed by Carr and Sesack (2000), glutamatergic inputs to striatal dopaminergic neurons are crucially important for the burst firing patterns of those dopaminergic neurons to be maintained normally. Too much or too little dopamine release in the prefrontal cortex, from dopaminergic neurons that project to the prefrontal cortex from different sites in the basal ganglia (including the striatum), is detrimental to working memory and therefore to cognitive functioning, and the firing patterns of glutamatergic neurons in the prefrontal cortex and other "extrastriatal" sites, as would be regulated by, for example, the degree of activation of A2A receptors outside the striatum by adenosine, are a major factor that regulates dopamine release in the prefrontal cortex. In some articles, a discussion of "a receptor in the striatum" can mean that the receptor is on an axon terminal, in the striatum, of a dopaminergic neuron whose cell body is not in the striatum but is in, for example, the midbrain, in the VTA. This type of discussion can just become absurdly complicated and can just get crazy, especially given the concentration-dependence of the effects of adenosine, etc. Cunha et al. (2008) discussed the fact that A2A receptors seem to be activated more by adenosine that is derived from extracellular ATP hydrolysis than adenosine that is derived from other sources (such as by the export from the cytosol, via equilibrative adenosine transporters, etc.). That's just one example of an obscure "mechanistic" explanation for the complexity of the adenosinergic or purinergic modulation of neuronal activity. In any case, those excitatory effects that adenosine, such as can be derived from SAM-e or oral adenosine monophosphate or triphosphate, can sometimes have on some glutamatergic neurons are one mechanism that could account for the acute increases in dopamine and noradrenaline release that have been shown to occur in animals in response to SAM-e administration [cited in Benelli et al., 1999: (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1566059&blobtype=pdf)(http://www.ncbi.nlm.nih.gov/pubmed/10401554)]. In a related vein, Eckeli et al. (2000), cited above, found that the apparent antidepressant effects of guanosine monophosphate in animals was partially dependent on serotonin biosynthesis or release. The antidepressant-like effect of guanosine monophosphate was mimicked by an NMDA receptor antagonist or fluoxetine, a serotonin reuptake inhibitor and prescription antidepressant. In that case, the general idea is essentially that a stressor is producing excessive glutamatergic activation and that the glutamatergic activity is indirectly disturbing the feedback inhibition of the firing rates, which increase in response to acute stress, of noradrenergic neurons in the locus ceruleus. Guanosine or NMDA receptor antagonism may attenuate this excessive glutamatergic activity, as implied by the results of Eckeli et al. (2000). There are more than a dozen articles showing that oral or intraperitoneal guanosine can exert anxiolytic (anti-anxiety) or anticonvulsant or neuroprotective effects that are at least partially a result of the attenuation of glutamate release or augmentation of synaptic glutamate reuptake by guanosine (http://scholar.google.com/scholar?num=100&hl=en&lr=&safe=off&cites=13842524336757775794). This seems confusing, but under conditions of chronic stress or neuronal injury, for example, decreasing glutamatergic transmission (such as by NMDA receptor antagonism) can actually increase dopamine release or, more precisely, increase the responsiveness of (postsynaptic) neurons receiving dopaminergic inputs to, for example, D1 dopamine receptor activation in response to dopamine release [see, for example, Peeters et al., 2002: (http://www.ncbi.nlm.nih.gov/pubmed/12213297); Deep et al., 1999: (http://www.ncbi.nlm.nih.gov/pubmed/10529725); Arai et al., 2003: (http://www.ncbi.nlm.nih.gov/pubmed/12711097); Konradi et al., 1996: (http://www.jneurosci.org/cgi/reprint/16/13/4231)(http://www.ncbi.nlm.nih.gov/pubmed/8753884?dopt=Abstract); Tokuyama et al., 2001: (http://www.ncbi.nlm.nih.gov/pubmed/11408088); Boyce-Rustay et al., 2006: (http://www.ncbi.nlm.nih.gov/pubmed/16482087)]. In the absence of stress (and even in the "presence of stress," up to a point), however, decreasing glutamatergic transmission (let's say to the same "degree" as in the presence of stress and at the same dosages of the same drug, for example) will tend, with some degree of predictability, to decrease dopamine release (many more references on this). In the former case, mild NMDA receptor antagonism, such as by amantadine (which also exerts numerous other effects that confound the discussion, but so be it), produces an excitatory effect on normal, burst-firing-associated dopaminergic transmission, but NMDA receptor antagonism in the latter case--antagonism that is stronger or that occurs in the absence of some factor that is producing an "abberant" or excessive increase in the firing rates of the neurons in question--will tend to inhibit dopaminergic (or noradrenergic) transmission. Here's an article that highlights some of the complexity with which adenosine can modulate the release of dopamine in response to NMDA receptor (a glutamate receptor) activation in the striatum [Quarta et al., 2004: (http://www.ncbi.nlm.nih.gov/pubmed/15525341)].

Again, my opinion is that many of the effects of SAM-e on the brain are mediated by adenosine, which SAM-e is metabolized into, and by the nucleotides derived from adenosine and its metabolites (including hypoxanthine and inosine and also some guanosine formed from inosine monophosphate, derived from adenosine nucleotides, etc.). I've discussed some of the evidence for this in past postings, but, for example, Renshaw et al. (2001) suggested that SAM-e may exert antidepressant effects, in part, via its metabolism into adenosine [cited here: (http://hardcorephysiologyfun.blogspot.com/2009/01/details-on-nucleotides-bioavailability.html)]. Additionally, the effects of exogenous adenosine or S-adenosylmethionine or inosine monophosphate, in combination with guanosine and other nucleotides, can produce increases in the phosphocreatine (PCr) to creatine (Cr) ratio (PCr/Cr ratio) that are reminiscent of the effect of exogenous SAM-e on the PCr/NTP ratio [see Silveri et al. (2003) and other articles cited and discussed here: (http://hardcorephysiologyfun.blogspot.com/2009/03/creatine-cr-phosphocreatine-pcr-and.html)]. In reference to the article by Silveri et al. (2003), it is noteworthy that NTP refers to the pool of beta-nucleotide triphosphates, and this "quantity" is thought to primarily reflect the pool of ATP that is measured, in a given part of the brain, by magnetic resonance spectroscopy (MRS). Also, some of the antiinflammatory effects of SAM-e can be antagonized by adenosine receptors and mimicked by the administration of adenosine, as a substitute for SAM-e [Song et al., 2005: (http://www.ncbi.nlm.nih.gov/pubmed/15843034)]. Song et al. (2005) also found that SAM-e elevated the pool of intracellular adenosine in cultured monocytes. Travagli et al. (1994) [Travagli et al., 1994: (http://www.ncbi.nlm.nih.gov/pubmed/7698179)] found that SAM-e reduced the firing rates of neurons in the vagal motor nucleus, and these effects were not only abolished by adenosine receptor antagonists but were shown to be "reversed" in the presence of those antagonists. SAM-e increased the firing rates of those neurons in the presence of the adenosine receptor antagonists, and Travagli et al. (1994) noted that increases in the availability of adenosine, produced from SAM-e via the hydrolysis of S-adenosylhomocysteine (SAH), had probably caused the changes in the neuronal firing rates. The attenuation of the SAM-e-induced increases in the firing rates of the neurons by SAH, in the presence of the adenosine receptor antagonists, might be explained by the inhibition of methyltransferases by SAH. Inhibiting methyltransferase enzymes in the presence of an excess of SAM-e would prevent much of the conversion of the excess SAM-e into SAH and then into adenosine. Alternatively, the excess SAH may itself have been converted into adenosine and thereby produced effects on adenosine receptors or purinergic receptors that were, as a result of the higher concentration of adenosine, in opposition to the effects that had been produced by the exogenous-SAM-e-derived adenosine (i.e. by that lower concentration). In either case, the results could conceivably be explained in terms of the effects of nucleotides or nucleosides derived from adenosine (i.e. inosine, hypoxanthine, xanthine, uric acid, or guanosine, etc.) or the effects of different concentrations of extracellular adenosine, derived from exogenous SAM-e or SAH, on different adenosine receptor subtypes or on other intracellular or extracellular adenosine binding sites on other proteins. There are lots of other articles that show similarly-comparable effects of adenosine and either SAM-e or 5'-methylthioadenosine (MTA), which is converted into adenosine [these are two that I found without any effort, but there are many others: Song et al., 2004: (http://www.ncbi.nlm.nih.gov/pubmed/15566950); Song et al., 2003: (http://www.ncbi.nlm.nih.gov/pubmed/12736147)]. I view the adenosine-receptor dependence of the anti-inflammatory effects of SAM-e, along with the elevation of intracellular adenosine by SAM-e (Song et al., 2005, cited above; Smolenski, 2000, cited above), as evidence that the adenosine nucleotide pool is being elevated and equilibrating with the extracellular pool, thereby causing increases in the activation of extracellular adenosine receptors. There are other interpretations, but that's just my opinion.

As far as the dosage considerations go, a number of articles have shown that adenosine and guanosine can exert effects in animal models at remarkably low dosages. Guanosine has been shown to exert neuroprotective and anticonvulsant effects at 7.5 mg/kg bw per day, given orally or intraperitoneally, in rats. That works out to something like ~111 mg per day, scaled to a dosage for a 70-kg human, and that's an extremely low dosage. I would think there might be some kind of issue with that dosage or the scaling factor (4.71) that I used, but essentially the same dosage (8 mg/kg bw/day, given intraperitoneally), was shown to promote restoration of functioning in animals following experimental spinal cord injuries [Jiang et al., 2008: (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=2072916&blobtype=pdf)(http://www.ncbi.nlm.nih.gov/pubmed/18404454)]. If one were to not scale the dosage and use 8 mg/kg bw per day in a 70-kg human, the dosage would be 560 mg/d. Even under those circumstances, that's a very low dosage, as purines go. That suggests that guanosine is fairly potent, particularly in comparison to inosine. The dosages of inosine used to promote recovery after spinal cord injuries, in animal models, are much higher. For example, Liu et al. (2006) [Liu et al., 2006: (http://www.ncbi.nlm.nih.gov/pubmed/16317421)] used inosine to prevent the ongoing degneration that occurs after a spinal cord injury in adult rats, and the dosage used was 75 mg/kg bw, given intraperitoneally, every eight hours. That's 225 mg/kg bw per day in rats. The fact that Jiang et al. (2008) found therapeutic effects of guanosine at 8 mg/kg bw/day is remarkable and shows, along with the many articles showing anticonvulsant and neuroprotective effects of oral or i.p. guanosine or guanosine monophosphate at 7.5 mg/kg bw/day, that, in my opinion, guanosine would be expected to produce meaningful effects on the brain at doses that would be compatible with human physiology. I say that because the dosage of inosine of 225 mg/kg bw/day (or the 100 mg/kg bw/day used in many other animal experiments showing anti-inflammatory effects of inosine, for example) scales to 3344 mg/day of inosine for a 70-kg human. If one looks at the trials using inosine to elevate uric acid in people with multiple sclerosis, one sees that that dose of inosine could produce hyperuricemia in many humans. Using that dose in the short term, such as after a spinal cord injury, would not be expected to produce hyperuricemia, and researchers would get larger effects on the brain by administering inosine intravenously or intraperitoneally than they would by administering it orally. But guanosine appears to exert many of its effects on the brain at remarkably low dosages.

One can, in my opinion, get a sense of the possible "dosage ranges" for guanosine monophosphate and adenosine monophosphate by looking at the dosages of S-adenosylmethionine and inosine that have been used in trials in humans. Guanosine monophosphate is available, evidently only in combination with other nucleotides, from a limited number of manufacturers [for example: (http://www.google.com/products?q=bluebonnet+nucleotide+complex+&hl=en)], but I'm not sure how much guanosine monophosphate is in that product. Again, I don't have any financial interest in any of these products or in anything I've discussed on this blog. The combinations of nucleotides that are used in research (http://scholar.google.com/scholar?q=dietary+nucleotide&hl=en&lr=) typically contain guanosine monophosphate and the other nucleoside monophosphates (a nucleotide, for the purposes of this discussion, is a nucleoside that's been phosphorylated in the 5'-position) in a either roughly 1:1:1:1 mass ratio or molar ratio. If the nucleotides and nucleosides were in a 1:1:1:1 mass ratio, each of those capsules would contain ~50 mg guanosine monophosphate and 10 mg guanosine and 15 mg guanine (and those same amounts for adenosine monophosphate, adenosine, and adenine). But I don't know if that's correct. One could call the manufacturer, and I'm sure they'd probably tell you what the composition is. But, from the standpoint of uric acid production, the elevation in uric acid is going to be different for every person. That's one reason it's necessary to have one's uric acid monitored by one's doctor, if one's going to take the full range of dosages for oral adenosine or guanosine monophosphates. One way to determine a dosage, with one's doctor, in the context of antidepressant medication, would be to look at the "percent adenosine" in S-adenosylmethionine and the dosage range of inosine used to treat multiple sclerosis. Less uric acid is made from adenosine than from inosine, in general, because adenosine is more efficiently salvaged. The ratio is something like 1.3/2.5, meaning the ratio of the AUC or Cmax (I forget which) for the serum uric acid response to oral adenosine to the uric acid response to oral inosine is about 0.52 or 0.55 or something. SAM-e is ~67.1 percent adenosine (the ratio of the molar mass of adenosine to the molar mass of SAM-e is 267.241/398.44), and the maximum dosage of SAM-e that I've seen used to treat depression is 3600 mg/day [Di Rocco et al., 2000: (http://www.ncbi.nlm.nih.gov/pubmed/11104210)]. If one takes 67.1 percent of that dosage, one gets 2415 mg/day of adenosine (or adenosine + guanosine). That works out to about 16 of those nucleotide capsules per day, assuming about half of the 300 mg is adenosine + guanosine (or purine bases, which are nucleic acids that can would probably be converted into uric acid). I'm not suggesting anyone would want to take any of these sorts of full dosage ranges without talking to one's doctor, because elevations in uric acid are not the only potential side effect. It's conceivable that adenosine or guanosine could produce vasodilation or reduce blood pressure and interact with sedatives or blood pressure medications or any number of medications and produce serious side effects. It's actually the case that adenosine monophosphate and guanosine monophosphate are 70-some percent adenosine and guanosine (the rest is phosphate), and that would affect the "calculation," crude as it is (that would mean the dose would be a little bit higher to be equivalent to the 2415 mg of adenosine that can be derived from 3,600 mg of SAM-e). Adenosine triphosphate disodium is more widely available than guanosine [(http://hardcorephysiologyfun.blogspot.com/2009/02/brief-note-on-purines.html); (http://hardcorephysiologyfun.blogspot.com/2009/01/details-on-nucleotides-bioavailability.html)].

One way to minimize the production of uric acid from adenosine monophosphate and guanosine monophosphate would be to dissolve them in water and take them on an empty stomach (which means before breakfast, not between meals), but I'm not sure if that would meaningfully increase the bioavailabilities of those nucleotides. There's increasingly been a recognition that the bioavailability of physiological substrates can depend on seemingly-trivial factors, such as the rate of dissolution, and this has been discussed in the context of the bioavailability of creatine. When researchers [Deldicque et al., 2008: (http://www.ncbi.nlm.nih.gov/pubmed/17851680)] gave 2 grams of creatine monohydrate dissolved in aqueous solution (water) or in a protein bar or in some kind of complex with "beta-glucan" (I have no idea what the rationale for testing that is), the Cmax for creatine in water was 299 uM (up from a baseline of 10 uM), and that was higher than the Cmax for creatine from the protein bar (237 uM) or the beta-glucan source (174 uM). I've seen multiple articles in which the researchers have dissolved creatine monohydrate in water, ostensibly with the aim of improving the availability of creatine for entry into the brain. I don't know if that type of factor would really matter all that much, and it might be cumbersome to do. But the same concept has been discussed in the context of orally-administered nucleotides. The activity of xanthine oxidase ("xanthine oxidoreductase") is thought to be lower in the fasting state, and this means that less of a dose of adenosine or guanosine may be converted, following the conversion into hypoxanthine or xanthine, into uric acid, in the fasting state, in the short term [see references 75-77, cited on p. 63 of Carver and Walker, 1995: (http://cat.inist.fr/?aModele=afficheN&cpsidt=3477199)]. Note that that paper by Carver and Walker (1995) is not especially up-to-date, as far as information on nucleotide bioavailability is concerned.

I'll put the discussion of mechanisms in a separate posting. This is too long to put in one posting.

Protection Against Postischemic Damage by Uric Acid and Evidence Arguing Against a Strongly Destructive Role for Xanthine Oxidase Activity

This article [Mink and Johnston, 2007: (http://www.ncbi.nlm.nih.gov/pubmed/17335786)] shows that the intravenous infusion of 600 umol/kg/hr of hypoxanthine (HPX) reduced damage to the brain following experimental hypoxia and cerebral ischemia (8 minutes of each). The authors compared the degree of damage in the group that had received HPX, given during the interval of 30-minutes before and 30 minutes after the induction of ischemia (the 30 minutes "after" had included the 16 minutes during which the animals had been subjected to hypoxia and then ischemia), to the degrees of damage in the groups that had received either an infusion of xanthine (XAN) or an inactive vehicle. The relative degrees of damage were not significantly different in the groups that had received either xanthine or the vehicle, and the authors noted that these data, along with other data, argue against the notion that free-radical production from xanthine oxidase activity contributes strongly to damage following cerebral ischemia. The authors also found that the cerebral HPX levels, measured after the four hours of reperfusion had elapsed, were actually much lower in the group that had received the HPX infusion than in the other groups. Given that these decreases in the post-reperfusion HPX levels had been accompanied by reductions in postischemic brain damage, the authors noted that the HPX had probably been salvaged heavily or converted into xanthine, etc. Some of the XAN in the XAN group may have also been used for salvage (XAN can be salvaged to xanthosine and guanosine).

These results are important and suggest, as noted by the authors, that the protective effects of allopurinol, a xanthine oxidase inhibitor used to treat hyperuricemia, against cerebral ischemia are mediated by mechanisms other than XO inhibition per se. In other words, the reduction in uric acid is probably not protective in and of itself (especially since uric acid has been shown in many studies to protect against experimental, ischemic brain injuries) and, more importantly, the xanthine oxidase activity per se seems unlikely to be a major source of oxidative damage to the brain. The authors provided more XO substrates, which are HPX and XAN, and did not find increases in ischemic damage. The authors noted that the high Km value for XAN binding to XO (88 uM) as a substrate means that XO was not saturated with XAN during the experiments and wasn't in untreated animals either (the brain XAN content in the cerebral cortex of the rabbits was only 18 uM at 30 minutes after the initiation of reperfusion). This means that exogenous XAN is likely to have been utilized as a substrate for XO, as noted by the authors, but also means that the XO activity in the control group might not have necessarily been lower (the XO enzymes in the control rabbits would presumably have had access to plenty of endogenously-produced XAN, etc.). There's an article showing that allopurinol can exert antinociceptive effects that are evidently due to elevations in the cerebrospinal fluid guanosine and adenosine levels, and some of the antinociceptive effects can be blocked by adenosine receptor antagonists [Schmidt et al., 2009: (http://www.ncbi.nlm.nih.gov/pubmed/19133997)]. Those types of elevations in CSF guanosine and adenosine could also explain the neuroprotective effects of allopurinol. Even though the paper by Mick and Johnston (2007) doesn't necessarily preclude a damaging effect of XO-derived reactive oxygen species, the evidence, in my opinion, that XO activity is "all-bad" or that uric acid is a mediator of ischemic damage is not compelling at all, to say the least. Betz et al. (1991) [Betz et al., 1991: (http://www.ncbi.nlm.nih.gov/pubmed/1996699)] also noted that XO activity is unlikely to make a large contribution to oxidative damage following ischemia. To the contrary, uric acid has been shown to protect against postischemic damage, in many articles, in both humans and animals [here are a couple of the articles that show or discuss this: Yu et al., 1998: (http://www.ncbi.nlm.nih.gov/pubmed/9726432); Amaro et al., 2008: (http://www.ncbi.nlm.nih.gov/pubmed/18271711); Chamorro et al., 2004: (http://www.ncbi.nlm.nih.gov/pubmed/14962621); Amaro et al., 2007: (http://www.ncbi.nlm.nih.gov/pubmed/17525395)(http://stroke.ahajournals.org/cgi/reprint/38/7/2173.pdfhttp://stroke.ahajournals.org/cgi/reprint/38/7/2173.pdf); Romanos et al., 2007: (http://www.ncbi.nlm.nih.gov/pubmed/16596120); Teng et al., 2002: (http://www.ncbi.nlm.nih.gov/pubmed/12398932); Keller et al., 1998: (http://www.ncbi.nlm.nih.gov/pubmed/9425011)]. Uric acid and xanthine have both been shown to exert fairly strong feedback inhibition of XO activity [Rubbo et al., 1991: (http://www.ncbi.nlm.nih.gov/pubmed/1653611); Radi et al., 1992: (http://www.ncbi.nlm.nih.gov/pubmed/1322703)], and this feedback inhibition appears to be significant at normal, physiological concentrations in humans [Tan et al., 1993: (http://www.ncbi.nlm.nih.gov/pubmed/8134172)]. But the feedback inhibition has actually been shown to increase superoxide production by XO in vitro (by the isolated enzyme) (Rubbo et al., 1991; Radi et al., 1992). At the same time, Tan et al. (1993) found that 150 or 300 uM uric acid reduced the production of superoxide in human plasma overall by 23.2 and 32.0 percent, respectively. This indicates, in my opinion, that the overall effect of uric acid, despite its potential to increase superoxide production by XO, is to decrease superoxide formation. So elevations in uric acid levels, produced by exogenous purines or exogenous uric acid, during ischemia protect against ischemic damage, may increase free radical production by XO, but appear to decrease superoxide formation in the blood overall. This is consistent with the results of many other articles. The articles I've discussed provide more evidence, in my opinion, that uric acid per se and XO activity per se should not be viewed as being major factors, in the absence of exogenous uric acid or purine precursors of uric acid, contributing to oxidative damage following ischemia.

When one considers the nearly countless articles showing protection by uric acid against experimental autoimmune encephalomyelitis (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=encephalomyelitis+uric+OR+urate) and mitochondrial damage (http://scholar.google.com/scholar?q=mitochondrial+peroxynitrite+uric+OR+urate&hl=en&lr=), etc., these association studies, implying that uric acid is "independently" damaging, become bizarre to see, in my opinion. Hozawa et al. (2006) noted that elevations in serum urate are a risk factor for stroke but that uric acid itself "may" not cause strokes [Hozawa et al., 2006: (http://www.ncbi.nlm.nih.gov/pubmed/16239005)]. It's important to remember that ischemia itself (meaning ongoing, intermittent, low-level cerebral ischemia or ischemia in blood vessels outside the brain) increases purine nucleotide export and XO activity and uric acid production in a very reliable manner. It's worthwhile to remember that statistic-driven association studies are not a substitute for reasoning.

Interactions of Caffeine With Purine Metabolism, Ribose, and Uric Acid

The authors of this article [Herrick et al., 2009: (http://linkinghub.elsevier.com/retrieve/pii/S0306987709000061)] suggest that people could combine D-ribose with caffeine to augment the effect of caffeine and conceivably decrease the adverse effects associated with caffeine intake. The authors are essentially saying that ribose could augment ATP production and either decrease or increase the export of adenosine and its nucleotides from cells, meaning neurons, that have been stimulated by caffeine, etc. There's some validity to this suggestion, but, in my opinion, using purines or uridine as a source of small amounts of ribose would be a safer and more effective approach in the long term. Inosine monophosphate is ~43 percent ribose, and some similar percent of adenosine and guanosine are ribose. I don't feel like looking up the molar masses. In my opinion, high-dose ribose is not really a good idea, but I suppose one approach would be to combine small doses of ribose with purines and uridine or cytidine (uridine has been shown to elevated the cytidine and uridine nucleotide pools to significant extents, and so one doesn't need to take cytidine, really), etc. Barsotti and Ipata (2002) [Barsotti and Ipata, 2002: (http://www.ncbi.nlm.nih.gov/pubmed/11841784)] note that ribose has been shown to more effectively augment ATP repletion, following ischemia, when purines or purine bases are given along with the ribose (references 2 and 4, p. 130). Other articles have also shown that to be the case [Smolenski, 2000: (http://www.actabp.pl/pdf/4_2000/1171-1178s.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/11996106)].

One major reason I say that ribose from purines would, in my opinion, be safer is that ribose has been shown to increase the export of purine nucleotides and nucleosides from skeletal muscle cells and other cell types (including cells in the liver, etc.). The "purine-wasting" effect is not nearly as large as the effect of xylitol (I've cited much of this research in postings in December and early January) or fructose, but there is the potential for that to occur, in my opinion. That essentially means, in my view, that there would be short-term export of purines that would appear to be beneficial and long-term sequalae that would not be desirable. A similar effect occurs with something like zinc, which shows all these apparent "antidepressant" effects in short-term animal studies. Zinc increases the export of adenosine and other purines, partly by serving as a cofactor for a number of nucleotidase enzymes that degrade intracellular and extracellular nucleotides. That can produce short-term benefits, but the articles showing neurotoxicity from excessive zinc supplementation or from derangements in zinc homeostasis, in the absence of supplementation, are almost endless in numbers and are just absolutely appalling to see. I collected about a hundred of them in a list, and I'll try to link to a large number of them one of these days. They're not pleasant to look for or to read. That's an extreme example, in any case. Also, adenosine is exported from neurons in a generalized manner in response to neuronal activity, and that would be part of the rationale for providing actual exogenous purines instead of ribose. This is basically the same thing I've been saying over and over again, and there are almost innumerable articles showing that purine export is a generalized response to the excitation of neurons, either by electrical stimulation or pharmacological manipulations that increase excitatory neurotransmission. This effect, again, to the extent that it would be therapeutic, might not be long-lived in the absence of some attempt to address the resulting deficit in the intracellular purine pools. I came across a reference to an old article suggesting that antidepressants may exert some of their effects by increasing adenosine availability (this was discussed in the context of the capacity of low levels of adenosine to produce activation, rather than inhibition, of adenylate cyclase) [cited as reference 7 on p. 598 in Cooper et al., 1980: (http://www.ncbi.nlm.nih.gov/pubmed/6162091)]. I'm not sure if that cited article is talking about a reduction in the export of adenosine or about the export of adenosine from astrocytes leading to the import of adenosine into neurons. But I have multiple articles showing that either exogenous guanosine, adenosine, or inosine can elevate cAMP in various cell types, and I don't feel like linking to them right now. cAMP signaling is really complex, though, and an increase in the activities of cAMP-dependent protein kinases can be "pathological" or undesirable under some circumstances, and adenosine exerts a very complex set of effects on cAMP signaling.

Ribose mainly would contribute to the pool of intermediates in the nonoxidative pentose cycle, and this would mainly assist, to some extent, in the salvage of purines and in the provision of ATP by glycolysis, etc. (the activities of the enzymes of the de novo purine biosynthetic pathway are very low in the brain, and any increase in de novo inosine monophosphate formation from exogenous ribose alone would, in my opinion, be fairly minimal).

This suggestion about ribose is actually sort of minimally interesting in the context of the bizarre formulation of some of these well-known energy drinks advertised heavily on tv. I'm not going to say the brand, but I looked up the ingredients to see what was supposed to be so special about one of them. I see nothing very special about it and don't see the appeal of it. But an ingredient that stands out, in combination with caffeine, as being unusual is glucuronolactone. This is metabolized into glucuronic acid, and some of labeled glucuronolactone is converted into L-xylulose and then ribose [Hiatt, 1958: (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1062823&blobtype=pdf)(http://www.ncbi.nlm.nih.gov/pubmed/13575548)]. So, in my opinion, glucuronolactone is like a third-rate substitute for ribose (the conversion of L-xylulose into xylulose-5-phosphate is ATP-consuming, and this ATP consumption and phosphate sequestration is basically the mechanism whereby xylulose produces more purine depletion than ribose, etc.). These types of differences in the point of entry into the pentose cycle have been shown, fairly clearly, in my opinion, to produce surprisingly significant effects on ATP levels and phosphate sequestration, and it's partly because of the large amounts of the sugar(s) entering cells at one time.

Another advantage of providing the actual purines, as a source of ribose, would be, in my opinion (apart from the peroxynitrite scavenging effect of uric acid), the capacity of uric acid (urate) to influence purine metabolism indirectly (also, oral inosine has been shown to elevate plasma hypoxanthine and xanthine in humans, and the contribution of those elevations in hypoxanthine to the effects associated with inosine should not be underestimated). Hunter et al. (1990) [Hunter et al., 1990: (http://www.ncbi.nlm.nih.gov/pubmed/2345757)] found that elevating uric acid in the blood of rats decreased the density of A1 adenosine receptors in the striatum, a site of action of caffeine. In contrast, caffeine upregulated the A1 adenosine receptor density. This basically shows that uric acid elevations decreased the tolerance to caffeine by downregulating the chronic caffeine-induced upregulation of A1 adenosine receptor density [caffeine is a nonselective adenosine receptor antagonist, but its acute blocking effect on (antagonism of) A1 adenosine receptors figures prominently into its stimulant effects]. Thus, in a person who has never taken caffeine, the antagonism is robust, but the receptor density becomes gradually upregulated in response to the presence of the antagonism by caffeine. Increases in extracellular or intracellular uric acid may oppose that, but Hunter et al. (1990) found that uric acid was not a direct A1 adenosine receptor antagonist at physiologically-relevant concentrations. One mechanism could be feedback inhibition of xanthine oxidase activity [cited as reference 16 in Kroll et al., 1992: (http://www.ncbi.nlm.nih.gov/pubmed/1539702)]. I've never heard anyone talk about that mechanism, and it could be really important for understanding purine metabolism. There's research showing that the Ki for the feedback inhibition of xanthine oxidase by uric acid (urate) may be as low as 200 uM, a physiologically relevant concentration. This could spare ATP, given that xanthine oxidase is an ATP-consuming reaction and consumes reducing equivalents. I saw that the articles showing feedback inhibition of xanthine oxidase by endogenousl purines and purine-based drugs haven't been cited very many times, but, if that effect occurs in humans, it would be a really important mechanism, in my opinion. An excess of intracellular urate can obviously be detrimental, and one would want to discuss any of this with one's doctor and have one's uric acid checked. There's evidence that urate can inhibit glycogen phosphorylase, and caffeine can also inhibit glycogen phosphorylase. That would be counterproductive to any supposed therapeutic effects, and the levels of plasma urate at which those undesirable effects might begin to occur are not well-known. But there's research showing, for example, that hyperuricemia induced by excessive inosine supplementation (in a study in athletes whose urate levels were already high-normal) can worsen exercise performance, and inhibition of glycogen phosphorylase could account for that.

Arginine, Uric Acid, and Peroxynitrite in Neurodegenerative and Psychiatric Conditions

This article [Xia et al., 1996: (http://www.pnas.org/cgi/reprint/93/13/6770.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/8692893)] is one of many showing that depletion of intracellular arginine tends to increase peroxynitrite formation by multiple mechanisms. When nitric oxide synthase (NOS) enzymes are not occupied by arginine or are competitively inhibited by asymmetric N(G),N(G)-dimethylarginine (ADMA) or by other methylarginines, which are inhibitors of NOS enzymes that are produced normally during the breakdown of proteins, the NOS enzymes produce superoxide and can also produce NO and superoxide at the same time. The NOS-derived NO and superoxide tend to react to form peroxynitrite. The depletion of cytosolic arginine by roughly half produced a fivefold increase in the sensitivities of the cells to a cytotoxic stimulus that increased nNOS activity. Similarly, Xia and Zweier (1997) [Xia and Zweier, 1997: (http://www.pnas.org/cgi/content/full/94/13/6954)] found that arginine depletion from activated macrophages produced large increases in peroxynitrite levels, and these increases were almost entirely blocked by either 1 mM extracellular arginine or 1 mM extracellular urate (the form that uric acid is in at physiological pH values).

This is relevant to the effects of arginine and purines in the brain. I think that maintaining an adequate urate level in the cerebrospinal fluid and also intracellularly, in neurons and astrocytes, is likely to be really important for maintaining cellular energy metabolism and also for maintaining the normal nitrergic regulation of noradrenergic and dopaminergic transmission, such as through the effects of nitric oxide on NMDA receptor activation. Roitman et al. (2007) [cited here: (http://hardcorephysiologyfun.blogspot.com/2009/03/arginine-agmatine-and-nitric-oxide-in.html)] noted some of the evidence that impairments in cellular energy metabolism can be found in people with depression and other psychiatric symptoms. Phosphocreatine (PCr) levels have been shown, in research using magnetic resonance spectroscopy techniques, to be drastically depleted in the brains of people with depression, for example (cited in Roitman et al., 2007). Mitochondrial dysfunction and ATP depletion would reasonably be expected to produce PCr depletion, even in the absence of a deficit in the formation of new creatine from arginine, etc. There's actually a large amount of research showing protection by uric acid/urate against mitochondrial damage due to peroxynitrite (inactivation of complex I and mitochondrial dysfunction) (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=mitochondrial+peroxynitrite+uric+OR+urate).

The reason I didn't see those articles in the past is that I think the authors of many articles showing protective effects of urate tend to not mention multiple sclerosis or Parkinson's disease, two neurodegenerative diseases in which inosine, a precursor of urate, is being tested (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=Parkinson%27s+uric+OR+urate); (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=%22multiple+sclerosis%22+uric+OR+urate)]. They would be much better off (and would, in my opinion, get substantially more robust therapeutic effects) using guanosine and adenosine monophosphates or triphosphates as precursors of urate instead of inosine, in my opinion, but that's beside the point. The research tends to be very focused in on one little area, and I've never seen those articles on mitochondrial protection, by urate, cited in the context of Parkinson's disease or MS. But the potential for the protection, by urate (or arginine), against the compromising of cellular energy metabolism, by peroxynitrite, would be very important, in my opinion, in the contexts of those and other disorders. West et al. (2002) [cited here: (http://hardcorephysiologyfun.blogspot.com/2009/03/arginine-agmatine-and-nitric-oxide-in.html)] cited research showing that peroxynitrite tends to decrease dopamine release (as in tonic, excitatory, nitrergically-mediated dopamine release), and that's obviously relevant to cognition and psychiatric conditions. I think the peroxynitrite-reducing potential of arginine would be more likely to be effective in combination with normalization of CSF and intracellular urate levels in the brain. If a person's plasma urate (blood uric acid) is already high, the person wouldn't need to do this. But the notion that any old urate level is as "good" as any other, within the normal range, is not defensible, given the overwhelming evidence, in my opinion, showing major effects across small increments in extracellular and, by extension, intracellular urate concentrations. I don't have time to go into the articles showing high intracellular urate levels, but the main idea is, in my opinion, that the use of urate as a peroxynitrite scavenger makes the use of most other antioxidants look like child's play. The concentrations of urate that scavenge peroxynitrite meaningfully, in vitro, are comparable to achievable and normal in vivo concentrations (this is not the case at all for many antioxidants). The concentrations of urate, both intracellularly and extracellularly, are much, much higher than the concentrations one is going to achieve with most antioxidants, in my opinion. Additionally, urate is less like an antioxidant scavenger of peroxynitrite than it is like a peroxynitrite "sink" and is, for the most part, excreted. It is not regnerated (doesn't need to be regenerated) by oxidoreductase enzymes but can actually be degraded, in a series of intramolecular degradative reactions (to allantoin or other molecules), upon its nitrosylation/nitration/etc. That's a really unique property that sets it apart from most other so-called "antioxidants" that consume reducing equivalents in their regeneration. I do think there's a lot of validity to the peroxynitrite-reducing effects of arginine in the context of brain disorders [Wiesinger, 2001: (http://www.ncbi.nlm.nih.gov/pubmed/11275358)], but I think that approach would work better in combination with normalization of urate levels. Obviously, one should discuss this type of thing with one's doctor.