This letter [Gajdos et al., 1966: (http://www.ncbi.nlm.nih.gov/pubmed/5968309)] is really interesting and shows that the accumulation of protoporphyrin and coproporphyrin, induced in rats by dietary orotic acid administration, can be reversed by exogenous adenine-based purine nucleotides (adenosine 5'-monophosphate, ADP, or ATP). There are lots of old letters and case reports of the use of intravenous AMP to treat different porphyrias [here's one: Gajdos, 1974: (http://www.ncbi.nlm.nih.gov/pubmed/4129728)], but I know the FDA banned the use of injectable AMP in either the 1970s or 1980s. I think it was the mid-1970s. Intravenous (i.v.) AMP can cause bradycardia and hypotension, among other cardiac problems, and i.v. AMP is no longer used for anything other than specific medical procedures. It's interesting, though, that Kichenin et al. (2000) found that as little as 5 mg/kg/d of intrajejunal ATP (mimicking oral administration) increased ATP levels in the red blood cells of rabbits [Kichenin et al., 2000: (http://jpet.aspetjournals.org/cgi/content/full/294/1/126) (http://www.ncbi.nlm.nih.gov/pubmed/10871303)]. Either 3 or 20 mg/kg bw did not produce hypotension or bradycardia, did induce peripheral vasodilation, and produced bronchodilation, elevation of the arterial PaO2 (the partial pressure of oxygen in the arterial blood), and a decrease in the respiratory rate. There's reason to think some of these effects would occur, to some extent, in humans, and I'm not going to go into all the research now. But to administer adenine-based purine nucleotides orally, it would be necessary to provide the purine in some form other than a "slow-release" tablet. If the purine were released slowly, all of it would be converted into uric acid by the intestinal epithelial cells. Additionally, only some purine salts and only purine nucleotides, as opposed to the dephosphorylated nucleosides, are soluble. I know in the research on oral guanosine, the researchers first used guanosine and found that the low solubility limited the dosage they could give the rats (because they could only prepare water solutions with x molar guanosine, and the rats could only drink so much water every day). They had to start using guanosine monophosphate disodium.
Kichenin et al. (2000) discuss a little bit about the way red blood cells can make and maintain their purine pools via ATP derived from glycolysis, and the orotic acid was shown by Gajdos et al. (1966) to produce porphyria by inducing ATP depletion and adenine nucleotide depletion, more broadly, in red blood cells. I know the transport of iron into mitochondria of erythroblasts is ATP-dependent, and scientists have hypothesized that mitochondrial activity is required for ferrochelatase activity. I suppose the purines could, in some cases, interfere with the cell cycle in reticulocytes or other erythrocyte precursor cells and thereby indirectly suppress heme biosynthesis, as a byproduct of a cytostatic effect. But I don't think that would occur in vivo, given the rapidity with which adenosine is metabolized. In cell culture studies, large concentrations of exogenous hypoxanthine or other purines can rescue folate-depleted erythroblasts. It's conceivable that the purines suppress de novo purine biosynthesis in erythrocyte precursor cells and spare ATP by that mechanism.
It's interesting that folate and cobalamin (vitamin B12) deficiencies can cause orotate accumulation. People with mutations in glutamate formiminotransferase (GFIT) can have orotic aciduria and megaloblastic anemia, and one group of researchers hypothesized that the folate- and cobalamin-depletion-induced decreases in GFIT activity could reduce histidine recycling and thereby reduce histidine availability for hemoglobin biosynthesis. Exogenous uridine could reasonably be expected to suppress orotate accumulation, given that uridine nucleotides inhibit de novo pyrimidine (uridine) formation of uridine at the carbamoyl phosphate synthetase II step, upstream of orotic acid formation. I'm not up for collecting the references from my computer now. Researchers have also sometimes found evidence that histidine loading can be used to either diagnose or treat anemias due to folate deficiency, but that sounds like a bad idea to me. The concern I would have would be with the potential for histamine biosynthesis to increase in mast cells and cause adverse effects. Histamine can be stored in mast cells, and histidine loading could conceivably produce prolonged, mast-cell-mediated injury to the blood-brain-barrier or other tissues, etc. Histamine can produce secretagogue-like effects on mast cells and cause them to release much more destructive mediators (proteases, pro-inflammatory cytokines, etc.). I can at least link to one of the articles showing folate or cobalamin depletion can increase orotate accumulation. It's an old article, but the findings are actually consistent with the effects that result from decreases, in response to folate or cobalamin depletion, in the activity of GFIT (and possibly other folate-derived-cofactor-dependent enzymes): [Van der Weyden et al., 1979: (http://www.ncbi.nlm.nih.gov/pubmed/465362)]. Folate and cobalamin depletion are also well-known to reduce purine levels in proliferating cells, in particular, and one article shows folate reduces purine salvage in the liver.
I'm actually starting to think that some of the decreases in purine salvage that occur in response to folate and cobalamin depletion, especially in tissues such as the liver and brain, could be the result of mtDNA depletion. I've linked to multiple articles on this in past postings, and the decreases in mtDNA replication or transcription occur fairly rapidly in the liver (within four weeks, I think, in rats). The effect would take longer in humans, but I still think it's possible. Some of the effects probably do have to do with the accumulation of AICAR (ZMP) and the nucleotide depletion that ZMP can produce. Small increases in ZMP could produce phosphate sequestration, and AICAriboside (and possibly ZMP, too, by binding to the cAMP binding site) can inhibit S-adenosylhomocysteine hydrolase, etc. There could be a "cascade" of different mechanisms that would lead to feed-forward depletion of purines. But that dissociation between megaloblastic anemia and neurological symptoms, in either cobalamin or folate depletion, is subjectively reminiscent of the situation in mitochondrial disorders. Different types of proliferating cells can sometimes be affected and show heteroplasmy, and the degree of heteroplasmy can increase and decrease in mitotic cells, but the cardiac myocytes, skeletal muscle myocytes, liver, and central nervous system tend to be severely affected in people with mtDNA mutations. Even though that picture doesn't completely fit with the picture that is seen in cobalamin and folate deficiencies, some of those articles on folate-responsive neuropathies show profound muscle weakness and other symptoms reminiscent of mitochondrial disorders.
A more interesting question is this: If folate or cobalamin depletion can produce neurological symptoms and fatty liver (mitochondrial injury) or muscle weakness and can also produce mtDNA damage and depletion (which may or may not be causal in the symptomatology), then why do exogenous folates improve the neurological symptoms? One could argue that the reduced folates replenish the purine and pyrimidine pools and reduce further accumulation of mitochondrial uracil in DNA, but would that really explain a therapeutic effect in an acquired condition that is characterized by large deletions in mtDNA, as shown in animal studies? Given what's known about mtDNA, I don't see why it would be possible to treat such severe damage to mtDNA in tissues such as the brain.
It's possible that there are other mechanisms, such as the effects on gluconeogenic and glycolytic enzymes, as I've mentioned previously. I don't know. Maybe there isn't that much mtDNA damage in a lot of cases, and the reduced folates simply replenish the purine (and thymidine) pools. Purines have strong, trophic effects in the brain.
It's also conceivable that reduced folates increase the proliferation of neuronal progenitor cells and that even the apoptotic progenitor cells donate their purine pools to adjacent cells, thereby buffering the intracellular purine pools in adjacent cells and increasing the trophic effects that occur by extracellular actions of the purines.
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