1. The pH-dependence of the binding to pepsin of four dipeptide competitive inhibitors is reported. Values of K(i) obtained from equilibrium-dialysis experiments agree closely with those from kinetic measurements. 2. The binding of uncharged N-acyl-dipeptide amides to pepsin is essentially independent of pH from 0.2 to 5.8. Values of K(i) for the corresponding N-acyl-dipeptide acids rise rapidly above pH3.5, and depend on the ionization of a group of apparent pK(a) 3.6. 3. The data indicate that pepsin does not undergo any gross conformation change (at least none that affects binding) over the whole pH range of its catalytic activity. The pH-dependence of the dipeptide acid inhibitors indicates that the acid anions do not bind to pepsin, presumably because of electrostatic repulsion between the inhibitor anion and a negative centre at or near the active site of the enzyme. 4. The binding of all four stereoisomers of N-acetylphenylalanylphenylalanine, of the depside analogues of the l-l- and d-l-compounds and of N-acetylglycyl-l-phenylalanine and N-acetyl-l-phenylalanylglycine was studied at pH2.2. 5. These results throw further light on the binding specificity of pepsin and on the charge nature of the active site of this enzyme.

1. The inhibition of pepsin-catalysed hydrolysis of N-acetyl-l-phenylalanyl-l-phenylalanylglycine by products and product analogues was studied. 2. The non-competitive nature of the inhibition by the product N-acetyl-l-phenylalanine confirms an ordered release of products, and points to a common mechanism (involving an amino-enzyme) for pepsin-catalysed transpeptidation and hydrolysis reactions. 3. N-Acetyl-l-phenylalanine ethyl ester is also a non-competitive inhibitor, but here the inhibition is of the ;dead-end' type. No ethanol is detectable in reaction mixtures, indicating that this ester cannot act as an amino group acceptor in a transpeptidation process. 4. The same is true for N-methanesulphonyl-l-phenylalanine methyl and methyl thiol esters. No methanethiol is liberated when the methyl thiol ester is present as an inhibitor of the hydrolytic reaction, and the hope that such a thiol ester would effectively trap the amino-enzyme was not fulfilled.

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