(6S)-5-methyl-5,6,7,8-tetrahydrofolate + NAD(+) <=> (6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + NADH + H(+)
(6s)-5-methyl-5,6,7,8-tetrahydrofolate + nad(+) <=> (6r)-5,10-methylene-5,6,7,8-tetrahydrofolate + nadh + h(+)