PURIFICATION AND CHARACTERIZATION OF A BOWMAN-BIRK TYPE PROTEINASE-INHIBITOR FROM FABA BEANS (VICIA-FABA L)

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 alpha-chymotrypsin in a 1:1 (M/M) stoichiometry: the Ki's were 6.1 x 10(-9) M and 4.4 x 10(-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.