[pH-dependence of the mechanism of pepsin action]

N-Acetyl-L-phenylalanine inhibition of the peptic hydrolysis of N-acetyl-L-phenylalanine-L-tyrosine over the pH range 2-4.5 was studied. The mixed character of inhibition which was partially competitive and partially non-competitive allowed us to infer that the separate steps of the enzymatic hydrolysis were pH dependent. The orderliness of the dissociation of the triple enzyme-product-product complex was also pH dependent. The group with pKa approximately 3 influenced the mechanisms of pepsin hydrolysis as strongly as in the case of pepsin catalyzed oxygen isotopic exchange in the acyl amino acid carboxyl group.