Rambler on Magnesium and Sad Ironies

In this article [Vink et al., 1988: (http://www.jbc.org/cgi/reprint/263/2/757.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/3335524)] found that the depletion of intracellular free magnesium (Mg2+) correlated positively with the magnitude of the damage that was produced by experimental brain injuries in rats, and the administration of intravenous Mg2+, 5 minutes before the injuries, prevented much of the damage. The authors cited some interesting data on the pH dependence of the calculations, based on 31P-MRS data, of the intracellular free Mg2+ values, and the authors used data on the dissociation constant of MgATP(2-) at pH 7.2 (50 uM). It's interesting that the mean pre-injury, intracellular free Mg2+ concentration was 1.01 mM (1,010 uM), and the mean concentration was 0.26 mM (260 uM) by 3 hours post-injury. Vink et al. (1988) also cited research (reference 13, cited on p. 761) that had shown the rate of DNA synthesis in cultured fibroblasts to decrease logarithmically at intracellular free Mg2+ concentrations below 0.24 mM (240 uM). In that article, the rate of protein synthesis was down to almost nothing at those low concentrations, also. Resnick et al. (1997) [Resnick et al., 1997: (http://hyper.ahajournals.org/cgi/content/full/30/3/654)(http://www.ncbi.nlm.nih.gov/pubmed/9322999?dopt=Abstract)] found that the intracellular free Mg2+ levels were inversely correlated with the ages of people, and that means the levels decrease as people get older. It's interesting that the mean concentration of intracellular free Mg2+ in people who were hypertensive was 0.284 mM (284 uM), and the mean concentration in normotensive controls was only 0.383 mM (383 uM). One could argue that the rate of DNA synthesis in mitotic cells (fibroblasts) is going to be much higher at specific points in the cell cycle, but then why are the intracellular free Mg2+ levels in the brains of normal rats 3-4 times the levels in the brains of humans? I'll bet one reason is that laboratory animals generally receive higher intakes of magnesium, in addition to phosphate, etc. One could make the argument that the higher zinc or copper intakes of animals eating "rat chow," or whatever, would cause some neurotoxicity and balance out the benefits that have sometimes been associated with higher Mg2+ intakes. But the discrepancies between rat and human diets tend to not be as significant for some of those metals, like copper and zinc. That doesn't sound like a very good situation, with intracellular free Mg2+ concentrations being that low. This basic search on magnesium in relation to neuroprotection or neurotoxicity yielded 45,000+ results (http://scholar.google.com/scholar?hl=en&q=magnesium+neuroprotective+OR+neurotoxic+OR+ischemia+OR+neurodegenerative). In that search, there's one of the articles (Harkema et al., 1992) in which researchers have discussed the use of parenteral MgATP(2-) to protect against different kinds of trauma. If only someone could market a simple acylated prodrug of ATP (or the adenosine prodrug along with dibasic orthophosphate in a 1:3 ratio or something) that would release adenosine slowly enough not to produce hypotension but quickly enough to outperform the effects of oral ATP. They could have done it back in the 1960's, when researchers were obtaining the first use patents for acylated nucleosides. Think of the effects that type of simple approach (or a prodrug of inosine, etc.) could have had in clinical neuroscience, even in the years since 1992. It's moving up on 20 years since 1992. It's sadly ironic, in my mind, that it's ATP and nucleotides that have all of these effects, that have been researched heavily, and that aren't being utilized and probably won't be for a long time, in spite of the thousands of articles on all of these things. But the irony is that researchers have been dumping nucleotide triphosphates into PCR machines all over the place and have been using them in experiments like candy or "hamburger helper." In any case, no one would think magnesium could be a standalone treatment for strokes or anything, and there are all of those details, as discussed in past postings, related to the fact that magnesium supplementation tends to decrease serum phosphate levels, sometimes very significantly. But those issues are not all that difficult to address, as long as one is aware of the potentially-large magnitude of the interaction, etc.

Increase in Nucleotide Absorption and Retention During the Fasted State: Bioavailability Issues and Potential Problems With Enteric-Coated Tablets

These articles [Gross et al., 1988: (http://www.ncbi.nlm.nih.gov/pubmed/3390463); Gross and Savaiano, 1991: (http://www.ncbi.nlm.nih.gov/pubmed/2009279)] are really good and show that the retention, by the intestinal tissues themselves, of intrajejunally-administered nucleosides or nucleic acids can be roughly twice as great in the fasted state as in the "fed" state. Those articles are potentially confusing, because, normally, the retention of nucleosides or nucleotides in the intestines (i.e. the salvage of nucleosides and incorporation into nucleotide pools in the epithelial or smooth muscle cells, etc.) would be less-than desirable, from the standpoint of bioavailability. But what those authors are saying is that xanthine oxidase activity is lower in the fasted state. Those articles tell me that administering purines, in particular, during the fasted state, as discussed below and in past postings [see Carver and Walker, 1995, cited and discussed here: (http://hardcorephysiologyfun.blogspot.com/2009/03/adenosine-and-guanosine-in-animal.html); see here, also: (http://hardcorephysiologyfun.blogspot.com/2009/01/details-on-nucleotides-bioavailability.html)], is likely to produce both greater bioavailability, as discussed in those past postings, and greater salvage, by the target cells that the nucleotides or their metabolites enter, of those purines. That's just my opinion. I didn't know the effect had been shown to be that large. Ho et al. (1979) [Ho et al., 1979: (http://jn.nutrition.org/cgi/reprint/109/8/1377.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/458492)] found even larger increases in the absorption of nucleosides or nucleic acids in the fasted state, although those large, relative increases in tissue content, in the fasted state vs. the fed state, appear to be partially or largely due to increases in the absorption of the nucleotides or nucleic acids (purine bases).

It's important to note that the bioavailability is also likely, in my opinion, to be enhanced in the fasted state. The bioavailability is partly a function of how rapidly a physiological compound, such as a nucleotide, enters solution in the intraluminal fluid. I forget where the reference is, but the intraluminal fluid volume in the stomach or along a segment of the small intestine can be remarkably small and can be something like 15-20 mL. The general concept is that many physiological compounds (nucleotides in particular) can be transported into and metabolized by any cell they come in contact with. If a person takes something like guanosine or adenosine, as a free nucleoside, the low solubilities will, in my opinion, significantly limit the bioavailability of those nucleosides by slowing the rate of dissolution in the GI tract. The undissolved nucleosides will slowly enter solution, as the fraction that is dissolved is transported into cells or has diffused away, by passive diffusion. Savaiano et al. (1980) [Savaiano et al., 1980: (http://jn.nutrition.org/cgi/reprint/110/9/1793.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/7411237)] found evidence suggestive of extreme differences in the bioavailabilities of nucleic acids and nucleosides, such that the intravenous (i.v.) administration produced levels of tissue retention that were 3-59 times the levels produced by oral administration. Usually, the ratio of the i.v. to oral bioavailability of a drug, expressed as the ratio of the areas under the serum concentration vs. time curves [AUC(i.v.)/AUC(oral)], is maybe between 2 and 5 or 7 or something like that. Those differences found by Savaiano et al. (1980) are not especially relevant for human dosing, however, because the solubilities of both nucleic acids and nucleosides are drastically lower than the solubilities of the disodium salts of guanosine monophosphate (GMP and adenosine monophosphate (AMP) or triphosphate (ATP), for example [or the disodium salt of inosine monophosphate (IMP)]. Those solubility differences could essentially mean that most of the nucleic acids or nucleosides would be degraded to uric acid, in humans, or to uric acid and then allantoin, in animals, before they could even enter the portal circulation, etc, in my opinion. Other salts of AMP or GMP or ATP also display deficient solubilities, and those solubility data are freely available on countless sites on the internet. Many of the researchers who have used oral guanosine or GMP as anticonvulsants, in animal experiments, have discussed those solubility issues. The authors of many of those older articles were evidently not aware of those issues, however, in my opinion, and they're very important issues.

Another major problem with oral purine dosing is the use of enteric coatings, and I've discussed this in detail previously (http://hardcorephysiologyfun.blogspot.com/2009/01/details-on-nucleotides-bioavailability.html). Most enteric coatings would be expected to severely and unnecessarily reduce the bioavailabilities of orally-administered nucleotides or nucleosides, in my opinion. Many enteric coated tablets could potentially not dissolve in the GI tract, in my opinion, because the pH in many people would not be expected to be high enough to allow the coatings to dissolve, as discussed by Fallingborg et al. (1999), cited below. Additionally, the use of tablets could be expected to produce the same, drastic slowing of entry into solution that a low level of solubility would be expected to produce, in my opinion. Persky et al. (2003) [Persky et al., 2003: (http://www.pharmacy.unc.edu/pkpd/AMP%20Articles/Persky%20et%20al%20Clin%20Pharmacok%202003.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/12793840)] discussed the fact that the rate of dissolution of physiological substrates, such as creatine, can be an important pharmacokinetic variable to consider, and these types of pharmacokinetic considerations are potentially more important for maximizing the bioavailabilities of nucleotides or other physiological compounds, in my opinion, than those considerations are for maximizing the bioavailabilities of drugs. Many drugs cannot be extensively or even partially metabolized by every cell in the body. With physiological substrates, time (i.e. pharmacokinetics) is of the essence, so to speak, because there is both the rate of uptake, by endothelial cells or cells in the liver, and the rates of degradation by every cell the substrates are available to. Even in the case of creatine, Deldicque et al. (2008) [Deldicque et al., 2008: (http://www.ncbi.nlm.nih.gov/pubmed/17851680)], discussed here (http://hardcorephysiologyfun.blogspot.com/2009/03/adenosine-and-guanosine-in-animal.html), found that the Cmax of plasma creatine, a reflection of an "improvement" in the kinetics of absorption or entry into the systemic circulation, etc., was higher in response to the administration of creatine monohydrate in a water solution (i.e. pre-dissolved) than in response to its administration in foods, which slow the rate of entry of creatine monohydrate into solution. A lower Cmax could be expected, in my opinion, to decrease the fraction of nucleotides, for example, that would gain entry into the brain and be salvaged, as opposed to being degraded into uric acid, by cells in the brain.

Some of the confusion surrounding these issues may be the result of some lingering misconceptions that many people, even researchers, evidently are holding onto. The fasted state in a human means any time 12 or more hours after the previous meal, although I've seen the 10-hour time point used as a marker for the beginning of the "fasted" state. So this means the only true fasted state is likely to be the time period in the morning, before breakfast. Why is this the case? Fallingborg (1999) [Fallingborg et al., 1999: (http://www.ncbi.nlm.nih.gov/pubmed/10421978)] discussed large numbers of studies on the time course with which food moves out of the stomach and into the duodenum and jejunum and so on, and much of the data that Fallingborg (1999) discussed had been collected from experiments using different types of devices that transmit data on the pH and other variables in the gastrointestinal (GI) tract. Some of those are pill-sized devices that have tiny video cameras in them, but I'm not sure those were in use in 1999. After a person eats the first meal of the day, that food may, depending on the sizes and frequencies of the subsequent meals (meaning any food that is eaten), remain in the stomach for between ~2.6 and 14.5 hours (Fallingborg et al., 1999). The gastric residence time (GRT) of tiny, mechanized capsules, with pH sensors in them (the pH is a measure of acidity, such that pH values below or above 7 are "acidic" or "basic"/"alkaline," respectively) is between 1.1 and 1.9 hours in the fasted state, but the GRT for the same capsule can be *up to 14.5 hours* in a person who takes the capsule at breakfast and eats every couple of hours during the rest of the day. Fallingborg (1999) discusses the fact that a single, small meal, eaten in the morning, has been shown to only increase the GRT of the capsule to ~2.6 hours. Fallingborg (1999) discusses the fact that, in the fasted state, the interdigestive migrating myoelectric complex (IMMC), which is phase III of a series of cyclic, contractile events in the smooth muscle that lines the stomach, allows solid food to exit the stomach about every 2 hours. When a person eats a single meal or, in particular, meals every 2-3 hours, the cyclic or "phasic" aspects of these contractions are abolished or "frozen", and food may not exit the stomach for many hours (up to 14 or 14.5). This is very important for understanding the major problems that exist, in my opinion, with enteric coatings for many preparations of (i.e. products containing) SAM-e or ATP disodium, etc., as discussed previously (http://hardcorephysiologyfun.blogspot.com/2009/01/details-on-nucleotides-bioavailability.html).

This residence time applies to solid substances that cannot enter solution and diffuse into the small intestine. If a person eats a water-soluble nutrient or sugar or amino acid or nucleotide, those substances can exit the stomach in aqueous (water) solution very rapidly. But even the slowing effect that is produced by the food in the stomach can limit the bioavailability of a water-soluble substance, such as creatine, in the "fed" state. So the stomach may not be completely empty until ~4 am or even later, if a person eats his or her last meal at 6 pm or something. The paper by Fallingborg (1999) is superb, and the author cites 183 papers and goes into exhaustive detail on all of these considerations.

A major point that Fallingborg (1999) makes is that statements about taking enteric-coated (acid-resistant) tablets "between meals" or "on an empty stomach, between meals" make no sense, because the stomach does not empty between meals. When a person is told to take some of these enteric-coated SAM-e or ATP disodium tablets (or other enteric-coated tablets) "between meals, on an empty stomach," the tablet may not exit the stomach and have any hope of releasing its contents until up to 14.5 hours after the person has taken it and eaten many subsequent meals. More importantly, Fallingborg (1999) discusses research showing that the mean pH in the duodenum of humans is ~6.22. The duodenal pH has been shown to range from 5.66 to 6.4 in other articles (Fallingborg, 1999). In the jejunum, the upper part of the true small intestine proper, is about 4.92 in the fasted state (a median value) and 6.08 after a meal. The pH is thought to only increase to above 7, to 7.4-7.6 (Fallingborg, 1999), in the distal ileum, which is an almost shocking fact that helps to explain the many problems, such as intestinal strictures and so on, with enteric-coated tablets that have been reported in the literature. The pH in the proximal small intestine, therefore, ranges from ~6.08 to ~6.49 (Fallingborg, 1999), when one looks at the data from multiple articles. But many of the enteric coatings do not dissolve until the pH is some amount greater than 7, and yet the jejunum is the site at which enteric-coated tablets are supposed to dissolve. Enteric coatings are polymeric substances, generally, whose solubility is pH-dependent. That means they can't dissolve in fluids that display pH values below some critical range of numbers, and the lower limit of the range may be 7.5 or 8 or some other value and may depend on the particular formulation used by the manufacturer.

Those data on the pH of the intraluminal fluid mean that the dissolution of enteric coatings could be very problematic, in my opinion. One explanation for the misconceptions about the pH in the intraluminal fluid might be that the pH of bile is ~8.03 (Fallingborg, 1999), and maybe people have thought that the pH of bile will be equivalent to the pH of the intraluminal fluid. It's just not the case. There can be a tendency to rely on 30- and 40-year-old data or research in some of these areas, and that tendency can become problematic, in my opinion. I should mention that, in many disease states, such as in people with liver disease, the jejunal pH can be substantially lower than those median or mean values, measured in apparently healthy people and can decrease progressively throughout the day. The pH-sensitivities of something like an enteric coating should obviously, in my opinion, be engineered so as to allow dissolution at the lower range of intraluminal pH values for anyone. This would not be difficult to do, but it's not something that many manufacturers or other people seem to be aware of the need for (if enteric coatings are still going to be used). Here are some articles reporting gastric or intestinal injuries (i.e. obstruction of the pyloric sphincter or intestinal obstructions/strictures) from poorly-formulated enteric-coated tablets (this poor formulation extends to more or less all enteric-coated tablets, in my opinion, when one looks at the data on the pH-dependences of the polymers used in the coatings) [Harris, 1973: (http://www.ncbi.nlm.nih.gov/pubmed/4764749); Sogge et al., 1977: (http://www.ncbi.nlm.nih.gov/pubmed/22308); Davies, 1999: (http://www.ualberta.ca/~csps/JPPS2(1)/N.Davies/NSAID.htm)(http://www.ncbi.nlm.nih.gov/pubmed/10951657); Sherry, 1979: (http://www.ncbi.nlm.nih.gov/pubmed/287936); (http://scholar.google.com/scholar?num=50&hl=en&lr=&safe=off&q=%22enteric+coated%22+stricture+OR+obstruction)]. Obviously, non-enteric-coated aspirin could cause damage to the stomach or small intestine for other reasons, and one should always talk to one's doctor before making any change in any medication. The benefits of enteric-coated preparations may outweigh any potential problems with the preparations, for many people in many specific disease states. But my point is to show the many problems that exist with the approach, in a functional sense, and with many of the individual preparations, in my opinion.

When researchers refer to a "pyloric obstruction" from an enteric-coated aspirin tablet, the researchers mean that the tablet become "stuck" in the valve-like muscle that opens, periodically, to allow food to pass from the stomach into the duodenum. In some cases, minor or not-so-minor surgical procedures are required to remove these obstructions from the undissolved tablets.

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.

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.