The reaction of sulfhydryl reagents with bovine hepatic monoamine oxidase. Evidence for the presence of two cysteine residues essential for activity.

The bovine liver monoamine oxidase (EC 1.4.3.4) was found to be inactivated by various well-known sulfhydryl reagents like p-mercuribenzoate, methylmercuric iodide and 5,5'-dithiobis-(2-nitro benzoic acid). The present investigation shows that the inactivation of the enzyme results from reactions of these reagents with 2 out of 8 titratable sulfhydryl groups per 10(5) g of the enzyme. The substrate, benzylamine, and competitive inhibitors like benzaldehyde, p-nitrobenzaldehyde, benzyl alcohol protected the enzyme from inactivation by the mercurials or the Ellman reagent. The inactivation experiments with these sulfhydryl reagents, the protection experiments, and the kinetics as well as physicochemical observations suggest that there are only two cysteine residues that are required for activity of the enzyme. It is possible that these two residues may be active-center residues.