Methylenetetrahydrofolate dehydrogenase-methenyltetrahydrofolate cyclohydrolase-formyltetrahydrofolate synthetase from porcine liver. Interaction between the dehydrogenase and cyclohydrolase activities of the multifunctional enzyme.

Methylenetetrahydrofolate dehydrogenase (5,10-methylenetetrahydrofolate: NADP+ oxidoreductase, EC 1.5.1.5) one of the activities of a trifunctional folate-dependent enzyme from porcine liver, uses an ordered kinetic mechannism as determined from initial velocity, product inhibition and dead-end inhibition studies. The final product released from the dehydrogenase is methenyltetrahydrofolate. However, from the time course of appearance of products it is observed that the methenyltetrahydrofolate, rather than equilibrating with the solution, is converted preferentially to formyltetrahydrofolate by the cyclohydrolase, (5,10-methenyletrahydrofolate 5-hydrolase (decyclizing), EC 3.5.4.9) demonstrating a functional interaction between these two enzymic activites.