Characterization of the dihydropterin reductase activity of pig liver methylenetetrahydrofolate reductase.

Pig liver methylenetetrahydrofolate reductase catalyzes the reduction of quinonoid dihydropterins in vitro. Either NADPH or methyltetrahydrofolate can serve as the electron donor. Methylenetetrahydrofolate reductase can also suppor phenylalanine hydroxylation in vitro by regeneration of the tetrahydropterin cofactor. These results lend support to the proposal that reduction of methylenetetrahydrofolate proceeds by tautomerization of the 5-iminium cation to form quinonoid 5-methyldihydrofolate, which is then reduced to methyltetrahydrofolate (Matthews, R. G., and Haywood, B. J. (1979) Biochemistry 18, 4845-4851). Under Vmax conditions, the turnover numbers for the NADPH-linked reductions of the quinonoid forms of 6,7-dimethyldihydropterin, dihydrobiopterin, and dihydrofolate are all about the same as that for the reduction of methylenetetrahydrofolate. The Km values for racemic mixtures of the same quinonoid acceptors are 40, 30, and 20 microM, respectively, while the Km for (6R,S)methylenetetrahydrofolate is 20 microM at pH 7.2 in phosphate buffer. The reduction of quinonoid dihydropterins is inhibited by adenosylmethionine and dihydropteroylhexaglutamate, which are known to modulate methylenetretrahydrofolate reductase activity.