May play a role in spermatogenesis. (6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + NAD(+) = 5,10-methenyltetrahydrofolate + NADH 5,10-methenyltetrahydrofolate + H2O = (6S)-10-formyltetrahydrofolate + H(+) Homodimer. Expressed in developing tissues and at high levels in adult tissues. Isoform A shows male-specific expression. This NAD-dependent bifunctional enzyme has very different kinetic properties than the larger NADP-dependent trifunctional enzyme and is unique in that it requires formation of an enzyme-magnesium complex to allow binding of NAD. Belongs to the tetrahydrofolate dehydrogenase/cyclohydrolase family.