Evidence That Reduced Folates Can Serve as Cofactors for Tetrahydrobiopterin (BH4)-Dependent Enzymes

This is a really great article [Smith et al., 1986: (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=2418601&blobtype=pdf)(http://www.ncbi.nlm.nih.gov/pubmed/3540926)], and the authors discuss a lot of in vivo and in vitro research suggesting that reduced folates (i.e. levoleucovorin or L-methylfolate) can substitute (or their "metabolites," meaning tetrahydrofolate or other reduced folates whose concentrations are increased, intracellularly, in response to exogenous L-leucovorin or L-methylfolate) for tetrahydrobiopterin (BH4) as cofactors for tyrosine hydroxylase and other BH4-dependent enzymes. This is one of the articles that provides evidence of this [Lucock et al., 1995: (http://www.ncbi.nlm.nih.gov/pubmed/7566370)], and the authors of both articles discuss the fact that reduced folates are actually less neurotoxic than folic acid in people with dihydropteridine reductase deficiency (a genetic disorder that causes hypofunctionality in that enzyme, which is one of the BH4 biosynthetic enzymes). McCarty (2007) [McCarty, 2007: (http://www.ncbi.nlm.nih.gov/pubmed/17293058)] discussed evidence that methylfolate can serve as a substitute for BH4 ["pinch-hit[ting]" (McCarty, 2007, p. 14)] as a cofactor for endothelial nitric oxide synthase (eNOS).

The articles cited by Smith et al. (1986) also show, uniformly, as far as I can tell, that folic acid has no cofactor activity for tyrosine hydroxylase. That may not be all that relevant to the use of folic acid in brain disorders, because folic acid doesn't enter the brain nearly as readily as reduced folates do (supposedly, *only* reduced folates enter the brain, but this may not be true). But the authors of some of those articles suggest that folic acid could compete with reduced folates for binding to the transporters for reduced folates and thereby crowd out reduced folates from the brain.

The capacity of reduced folates to serve as BH4-mimetics is relevant to research on the uses of reduced folates in people with depression or in whatever other context. Their supposed capacity to serve as BH4 substitutes is different from their supposed capacity to promote BH4 recycling. I've discussed that research in past postings, and there's recent research that casts serious doubt on that "promotion of BH4-recycling," by reduced folates, as a mechanism. But if reduced folates can partially substitute for BH4, this would provide a rationale for the kinds of higher doses of reduced folates that have been used in the treatment of depression [Guaraldi et al., 1993; Di Palma et al., 1994, cited here: (http://hardcorephysiologyfun.blogspot.com/2009/01/thirty-year-old-articles-showing-folate.html)]. Those doses are similar to the maintenance dose of 100 mg/d of oral BH4 that has been used to treat depression [see references here: (http://hardcorephysiologyfun.blogspot.com/2009/03/d-phenylalanine-l-phenylalanine-l.html); Curtius et al., 1982: (http://www.springerlink.com/content/g1447716574g2616/)]. Obviously, anyone would want to discuss this with one's doctor before taking high doses of reduced folates, because a person with vitamin B12 deficiency could be seriously harmed by that approach. But in people who are not deficient in B12, Smith et al. (1986) basically say that reduced folates are less toxic than folic acid. Another important implication of "BH4-mimesis" by reduced folates is that reduced folates could conceivably exert feedback inhibition of GTP cyclohydrolase I, given that both BH4 and BH2 (and, by extension, methylfolate or levoleucovorin, as supposed BH4 analogs with cofactor activity for BH4-dependent enzymes) exert feedback inhibition of GTP cyclohydrolase I activity by binding to GTP cyclohydrolase I feedback regulatory protein (http://hardcorephysiologyfun.blogspot.com/2009/03/d-phenylalanine-l-phenylalanine-l.html). That could be an important mechanism by which BH4 or reduced folates could exert their supposed antidepressant effects, in my opinion, because, in people with BH4-responsive genetic disorders, that feedback inhibition causes reductions in the tissue and plasma concentrations of oxidized biopterins [biopterin, neopterin, different dihydrobiopterins (there's quinonoid dihydrobiopterin and 7,8-dihydrobiopterin, etc.)]. Oxidized biopterins can inhibit folate dependent enzymes and can also inhibit BH4-dependent enzymes, in some cases, by complex mechanisms. They basically disrupt pterin metabolism in a multitude of ways. But it seems as if researchers are reluctant to research those higher dosage ranges (50-90 mg/d), and that reluctance may be unfounded. Obviously, these are just my opinions. The authors that tested 90 mg/d didn't think the dose was toxic and said as much, but, obviously, that's just their opinion, also, and they didn't get any experimental data to support that assertion of safety. That's the type of thing that one would obviously want to not do in the absence of a doctor's supervision. BH4 itself is available by prescription in the U.S. (http://hardcorephysiologyfun.blogspot.com/2009/03/d-phenylalanine-l-phenylalanine-l.html), but it seems as if it might not be covered by insurance in some instances (for some applications). Also, everyone keeps repeating, in the research, that BH4 crosses the blood-brain barrier poorly (in comparison to sepiapterin, etc.), but who knows if that's even true. Researchers could compare BH4 to high (i.e. equimolar) doses of L-leucovorin or L-methylfolate and measure plasma biopterin and neopterin levels (the neopterin to biopterin ratio has tended to be elevated in people with depression, perhaps as a result of inflammation, etc.). Another possibility is that the feedback inhibition of GTP cyclohydrolase I activity by BH4 or L-methylfolate or L-leucovorin would "spare" guanosine triphosphate (GTP) and make more available to eNOS, etc. A lot of researchers have suggested that some of the effects of reduced folates on the brain could be due to increases in GTP or total guanosine nucleotide levels, as a result of L-methylfolate- or L-leucovorin-induced increases in de novo purine formation (i.e. in the brain or cerebral vascular endothelial cells, etc.). But the "feedback sparing" effect could conceivably occur, also. That would be interesting, because the formation of a lot of oxidized biopterins (such as neopterin) could be viewed as constituting a "wasting" and sequestration of guanosine nucleotides, which tend to be in short supply to begin with.