In the body 5-methyl-THF (active folate) can regenerate BH4 from oxidised BH2, so good folate levels are important for maintaining BH4 levels. But i learnt recently you can also boost BH4 levels with plain old synthetic folic acid via a different mechanism.
As well as the main de novo (from new) synthesis pathway for BH4, there is a salvage pathway that uses the DHFR enzyme to convert BH2 to BH4 (1,2,6). The DHFR enzyme is at the start of folate metabolism, and normally converts inactive folate forms to active THF forms. Folic acid supplements enter folate metabolism before DHFR and have been shown to boost intracellur BH4 by stimulation of DHFR (2,3,4,5,6). I think the active forms of supplemental folate (folinic acid and methyl-THF) wont stimulate this enzyme much, so i guess combined 5-methyl-THF and folic acid supplementation may be best to support BH4 levels.
There is one hitch though, unfortunately there is 5-fold variability in DHFR activity in humans(8), so some people may not be able to make as good use of the BH4 salvage pathway as others.
Basic folate metabolism
folate → DHF (dihydrofolate) → THF (tetrahydrofolate) ↔ methylene-THF → methyl-THF
BH4 metabolism
References
1. http://www.food-chemistry.com/Research/neu%20biosynthesis%20of%20folates%20and%20BH4.htm
2. laboratory evaluations, richard s lord. (p.35)
3. http://cat.inist.fr/?aModele=afficheN&cpsidt=17594313
4. http://www.ncbi.nlm.nih.gov/pubmed/2862841
5. http://cat.inist.fr/?aModele=afficheN&cpsidt=17676232
6. http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=393638
7. http://www.bh4.org/pdf/channon.pdf
8. http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=2730961&blobtype=pdf
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