Formiminotransferase–cyclodeaminase from porcine liver. A sulfhydryl essential for the deaminase activity of the bifunctional enzyme

Abstract

Reaction of the bifunctional enzyme formiminoglutamate:tetrahydrofolate formiminotransferase (EC 2.1.2.5) - formiminotetrahydrofolate cyclodeaminase (EC 4.3.1.4) with the sulfhydryl reagent 5,5'-dithiobis (2-nitrobenzoic acid) selectively inactivates the cyclodeaminase. Loss of activity correlates with the modification of two sulfhydryl groups per subunit. The inhibitor folic acid reduces the rates of inactivation and sulfhydryl modification, and protection experiments demonstrate that only one of the two sulfhydryls modified is important for enzyme activity. The results indicate the presence of a cyclodeaminase site on each polypeptide, assuming one sulfhydryl per site, in agreement with a quaternary structure containing identical polypeptides. Modification does not cause dissociation of the enzyme and is reversible with dithiothreitol.