Substrate specificity of a sulfhydryl protease (P-Ia) purified from germinating corn was investigated by using synthetic substrates and oxidized insulin B-chain. P-Ia showed a potent activity for p-nitrophenyl esters of various amino acid derivatives, except for those of carbo-benzoxy-L-proline and carbobenzoxy-L-valine. Benzoylarginine-β-naphthylamide, a good substrate for papain and cathepsin B1, was not hydrolyzed by P-Ia. An investigation with acyl dipeptides showed that P-Ia hydrolyzed preferably the peptide bond adjacent to the carboxyl group of the aromatic amino acid. Oxidized insulin B-chain was hydrolyzed at the peptide bonds Gln4-His5, Glu13-Ala14, Ala14-Leu15, Leu15-Tyr16, Tyr16-Leu17 and Tyr26-Thr27. P-Ia, in spite of a sulfhydryl protease, seems to be characterized by its similarity to pepsin rather than papain, as far as the substrate specificity studied in the present work is concerned.
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Abe et al. (1978) studied this question.