Catalyzes oxidation of cytoplasmic one-carbon units for purine biosynthesis. (6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + NAD(+) = 5,10-methenyltetrahydrofolate + NADH One-carbon metabolism; tetrahydrofolate interconversion. Homodimer. Belongs to the tetrahydrofolate dehydrogenase/cyclohydrolase family.