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A proteinase inhibitor was purified from the seeds of faba bean(Vicia faba L.) to an electrophoretically homogeneous protein by extraction, salting-out, and column chromatographies on CM-Sephadex C-25, Sephadex G-75, and QAE-Sephadex A-25. The inhibitor(FBPI) was a basic protein with a molecular weight of 7000 and an isoelectric point of 8.7. The amino acid composition had a fairly high content of half-cystine and lacked of methionine, isoleucine, and tryptophan. FBPI inhibited bovine trypsin and a-chymotrypsin in a 1:1(M/M) stoichiometry: the KPs were 6.1 × 10^<-9> M and 4.4 × 1^<-8> M, respectively. The inhibitor was stable in acidic and neutral pH, but it lost its activity upon heat treatment at alkaline pH. These properties indicate that FBPI belongs to the Bowman-Birk soybean proteinase inhibitor family.