Purines and Pyrimidines in Mitochondrial DNA Content, etc.

In the first sentences of my previous posting, I meant to say that researchers thought extra uridine would allow the wild-type mtDNA copies to be replicated. There are many articles showing that uridine does increase mtDNA content and restore it in cells treated with inhibitors of DNA polymerase-gamma, and uridine definitely has therapeutic effects in a lot of these conditions, either experimental or disease-associated, of respiratory chain dysfunction. But I'm not sure about the mechanistic explanation for the effect (the idea that it's just providing more pyrimidines for DNA replication or even, additionally, for transcription of mtDNA).

Here's that article I was thinking about, and it's really interesting. The authors show that plasma total homocysteine (tHcy) correlates inversely with the mtDNA content of peripheral blood "leukocytes" (lymphocytes, white blood cells of unspecified phenotypes):

http://www.ncbi.nlm.nih.gov/pubmed/11583719

They also found that serum folate correlated positively with mtDNA content, given that serum folate correlated inversely with tHcy. I'm not sure why they didn't just correlate serum folate and mtDNA, but the inverse relationship between tHcy and serum folate is well-known.

That article is significant because it shows there could be an effect of small increases in serum folate, or small reductions of tHcy without elevation of serum folate (as has been shown in some articles), on the mtDNA content in some cells. Actually, the more I think about this article, the more I'm not sure of the meaning of an increase in mtDNA content. It could mean that there are more mitochondria or that there are just more copies of mtDNA per mitochondrion. An increase in mtDNA content wouldn't necessarily preclude heteroplasmy (as in "acquired," age-associated, low-level heteroplasmy). An increase in the density of mitochondria can definitely be a bad thing, but the fact that the mtDNA content correlated inversely with insulin resistance suggests, indirectly, that the increase in mtDNA content, associated with higher folate levels, is, in fact, evidence of a "beneficial" effect.

The authors talk about oxidative damage to mtDNA, but I doubt that's the mechanism by which an increase in serum folate would increase the mtDNA content. Those are very tiny increments of serum folate and tHcy, and I think the more likely mechanism is a decrease in the dUMP/dTMP ratio (produced by folate repletion). I suppose that tiny amount of extra folate could increase de novo purine formation in lymphocytes (the activity of the de novo pathway is high in lymphocytes).

But that's what I was getting at with the discussion of uridine. Extra uridine could increase the dUMP/dTMP ratio, and this article shows that phenomenon in response to exogenous deoxyuridine (increased uracil misincorporation into DNA):

http://www.ncbi.nlm.nih.gov/pubmed/15183762?dopt=Abstract

One could make the argument that the conversion of uridine into deoxyuridine would be regulated by ribonucleotide reductase, but I think that exogenous uridine could elevate the dUMP/dTMP ratio to some extent. That's why I was suggesting that maximizing thymidylate synthase activity, with folate supplementation, might help limit that elevation and enhance the effectiveness of uridine in the treatment of respiratory chain dysfunction due to heteroplasmy.

Part of the reason I mentioned purine depletion, as a mechanism that could show up in response to folate depletion and limit uridine's effectiveness in this context, is that 1) folate depletion tends to reduce purine salvage and deplete purines nonselectively, potentially reducing the availability of purines for mtDNA replication; and 2) folate repletion increases the S-adenosylmethionine (SAM-e)/S-adenosylhomocysteine (SAH) ratio and increases SAM-e levels somewhat. There's research showing that exogenous SAM-e, given by intraperitoneal injection, increases the mtDNA content in cells (presumably myocytes) in the skeletal muscle of rats:

http://jn.nutrition.org/cgi/content/full/137/2/339
(pubmed: http://www.ncbi.nlm.nih.gov/pubmed/17237308?dopt=Abstract)

There are serious bioavailability issues with oral SAM-e, but I think part or most of the way SAM-e, even by i.p. or i.m., works is by elevating the pool of total purines. Here's an article showing that exogenous SAM-e increases adenine nucleotide levels:

http://www.ncbi.nlm.nih.gov/pubmed/11996106

Here's an even more persuasive article that shows that some of the anti-inflammatory effects of exogenous SAM-e are mediated by the adenosine that is derived from SAM-e. This article shows that exogenous adenosine can substitute for SAM-e (produce some of the same anti-inflammatory effects) or restore the anti-inflammatory effects of SAM-e in the presence of an inhibitor of S-adenosylhomocysteine. The article shows that activation of A2 adenosine receptors, by SAM-e-derived adenosine, essentially, mediates many of the effects of SAM-e. This shows that it's the adenosine, not a significant increase in the activities of methyltransferase enzymes per se, that is mediating the effects of SAM-e:

http://www.ncbi.nlm.nih.gov/pubmed/15843034

This article shows the same thing (A2 adenosine receptor activation as a factor mediating the effects of exogenous SAM-e):

http://www.ncbi.nlm.nih.gov/pubmed/15566950

Those articles, particularly the second and third one, are really important. There are others like it, but it suggests that, for example, some of the antidepressant effects and neuroprotective effects of SAM-e are the result of elevations in intracellular adenosine, hypoxanthine, and, following the conversion of hypoxanthine into inosine monophosphate, guanosine nucleotides derived from inosine monophosphate dehydrogenase. This would imply that exogenous adenosine and guanosine monophosphate could substitute for SAM-e (and overcome many of the bioavailability issues that constrain the usefulness of oral SAM-e) in those situations (along with folate repletion, to help prevent the exogenous purines from elevating S-adenosylhomocysteine levels). This article and other articles have shown that excessively high adenosine levels can cause liver toxicity by inhibiting or reversing the activity of S-adenosylhomocysteine hydrolase (leading to glutathione depletion):

http://www.ncbi.nlm.nih.gov/pubmed/8937453

They used concentrations as high as 1 mM, though, and that's very high. I think it's worthwhile to be aware of that effect, though, of excessive adenosine accumulation. S-adenosylhomocysteine hydrolase inhibition can be really damaging to the liver. This article suggests that deoxyguanosine triphosphate, like deoxyadenosine triphosphate, could accumulate in response to a decrease in purine nucleoside phosphorylase activity, as could occur in response to a generalized accumulation of purines, and inhibit ribonucleotide reductase activity:

http://www.ncbi.nlm.nih.gov/pubmed/10859343

The accumulation of purines in the liver could potentially be a limiting factor, as far as dosages would go, for the oral administration of exogenous ATP/AMP/GMP for neuroprotection or whatever other purpose.