Preparation of an Active Fragment of Potato Proteinase Inhibitor Ha and Its Properties

Potato proteinase inhibitor Ua complexed with trypsin [EC 3.4.4.4] was incubated at pH 8 with a small excess of trypsin, yielding a low molecular weight active fragment. The active fragment, purified by successive gel filtrations and by treatment with trichloroacetic acid, consists of a single peptide chain with a molecular weight of 4,800; it has serine as the amino-terminal residue and arginine as the carboxyl-terminal residue. It exhibits very strong inhibitory activity against trypsin; 1 //g inhibits 5.1 fig of the enzyme. The active fragment also shows weak inhibitory activity against chymotrypsin [EC 3.4. 4. 5], which is considerably reduced as compared with that of the native inhibitor. Moreover, the inhibitory activity of the active fragment against a bacterial proteinase, Nagarse, had become negligible. In 1968, Dlouha et al. (1) prepared a high molecular weight fragment of a complex of trypsin with its basic bovine pancreatic in-hibitor (BPTI) by digesting the complex with trypsin [EC 3.4.4.4] and chymotrypsin [E