Homologous inhibitors from potato tubers of serine endopeptidases and metallocarboxypeptidases.

Abstract

A potent polypeptide inhibitor of chymotrypsin was purified from Russet Burbank potatoes. The inhibitor has no effect on bovine carboxypeptidases A or B but exhibits homology with a carboxypeptidase inhibitor that is also present in potato tubers. The chymotrypsin inhibitor has a MW of .apprx. 5400 as estimated by gel filtration, amino acid analysis and titration with chymotrypsin. The polypeptide chain consists of 49 amino acid residues, of which 6 are half-cystine, forming 3 disulfide bonds. Its size is similar to that of the carboxypeptidase inhibitor, which contains 39 amino acid residues and also has 3 disulfide bridges. In immunological double diffusion assays, the chymotrypsin inhibitor and the carboxypeptidase inhibitor do not crossreact. Automatic Edman degradation of reduced and alkylated derivatives of the chymotrypsin inhibitor, yielding a partial sequence of 18 amino acid residues at the NH2-terminus, reveals a similarity in sequence to that of the carboxypeptidase inhibitor. Inhibitors directed toward 2 distinct classes of proteases, the serine endopeptidases and the metallocarboxypeptidases, appear to have evolved from a common ancestor.