ON THE MECHANISM OF PEPSIN ACTION

The problem of the mechanism of pepsin action is considered in relation to recent data on the kinetics and specificity of the enzyme, as well as the finding, reported here, that pepsin exhibits a deuterium isotope effect in the cleavage of a peptide bond. The kinetic parameters for the hydrolysis of the Phe(NO 2 )-Phe bond of Gly-Gly-Gly-Phe(NO 2 )-Phe-OMe by pepsin have been determined in H 2 O and in D 2 O. The finding of a significant deuterium isotope effect ( k H2O / k D2O = ca . 2) supports the hypothesis that the catalytic mechanism of pepsin involves the participation, in the rate-limiting step, of a proton donor (probably an enzymic carboxyl group) in addition to an enzymic carboxylate group acting as a nucleophile.