Studies on Soybean Trypsin Inhibitors

Soybean inhibitor C-II, which inhibits trypsin, alpha-chymotrypsin, and elastase, was reduced and S-carboxymethylated, and digested with trypsin. The amino acid sequences of the resulting tryptic peptides were determined by conventional methods, establishing the complete 76-amino acid sequence of the inhibitor. Inhibitor C-II was found to be homologous with soybean (Glycine max) Bowman-Birk inhibitor and more closely related to an inhibitor from garden beans (Phaseolus vulgaris). The homology with these inhibitors and the limited proteolysis of C-II indicated the reactive sites of C-II for elastase and trypsin to be alanine-22 and arginine-49, respectively. Arginine-49 was also identified as a reactive site for alpha-chymotrypsin. It was found that only a few replacements of one or two amino acid residues around the reactive sites resulted in considerable alteration of the inhibitory specificity.