Methylenetetrahydrofolate dehydrogenase, methenyltetrahydrofolate cyclohydrolase and formyltetrahydrofolate synthetase from porcine liver. Isolation of a dehydrogenase-cyclohydrolase fragment from the multifunctional enzyme.

Tryptic digestion of a multifunctional enzyme from porcine liver containing methylenetetrahydrofolate dehydrogenase (5,10-methylenetetrahydrofolate: NADP+ oxidoreductase, EC 1.5.1.5), methenyltetrahydrofolate cyclohydrolase (5,10-methenyltetrahydrofolate 5-hydrolase, EC 3.5.4.9) and formyltetrahydrofolate synthetase (formate:tetrahydrofolate ligase, EC 6.3.4.3) activities destroys the synthetase. A fragment containing both dehydrogenase and cyclohydrolase activities has been isolated by affinity chromatography on an NADP+-Sepharose affinity column. The purified fragment is homogeneous on dodecyl sulfate-polyacrylamide gel electrophoresis where its molecular weight was determined as 33 000 +/- 1200 compared with 100 000 for the undigested protein. The cyclohydrolase activity retains sensitivity to inhibition by NADP+, MgATP and ATP.