[Photobiochemistry of folates: a photochemical reduction of folic acid].

Exposure of deaerated folic acid solutions containing an electron donor to UV radiation (310-390 nm, I = 0.4 W m(-2)) induced formation of dihydrofolic acid (DHFA), a photoexcitation which gave tetrahydrofolic acid (THFA). Only DHFA was formed in the presence of EDTA (E'o = +0.40 V), while the presence of stronger reductants-NADH (E'o = -0.32 V) and boron hydride (E'o = -0.48 V)-induced photoreduction to THFA. It was demonstrated that UV radiation had no effect on the THFA formylation, giving the coenzyme 5,10-methenyltetrahydrofolic acid and its transformation into another coenzyme, 5-formyltetrahydrofolic acid.