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

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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 inactivated the cyclodeaminase. Loss of activity correlated with the modification of 2 sulfhydryl groups per subunit. The inhibitor folic acid reduced the rates of inactivation and sulfhydryl modification, and protection experiments demonstrated that only 1 of the 2 sulfhydryls modified was important for enzyme activity. The results indicated the presence of a cyclodeaminase site on each polypeptide, assuming 1 sulfhydryl per site, in agreement with a quaternary structure containing identical polypeptides. Modification did not cause dissociation of the enzyme and was reversible with dithiothreitol.