INHIBITION OF HUMAN BETA-FACTOR-XIIA BY SQUASH FAMILY SERINE PROTEINASE-INHIBITORS

Many inhibitors of trypsin and human β-factor XIIa have been isolated from squash and related seeds and sequenced (Wieczorek et al., Biochem. Biophys. Res. Comm.(1985) 126, 646-652). The association equilibrium constants (Ka) of several of these inhibitors have now been determined with human β-factor XIIa using a modification of the method of Green and Work (Park et al., Fed. Proc. Fed. Am. Soc. Exp. Biol.(1984) 43, 1962). The Ka's range from 7.8 × 104 M-1 to 3.3 × 108 M-1. Two isoinhibitors from Cucurbita maxima seeds, CMTI-I and CMTI-III, differ in only a single glutamate to lysine change in the P'4 position. This results in a factor of 62 increase in the Ka of the lysine inhibitor, CMTI-III (Ka = 3.3 × 108 M-1). To our knowledge, this is the largest effect ever seen for a residue substitution at the P'4 position of a serine proteinase inhibitor. The result is even more surprising because β-Factor XIIa's natural substrate, Factor XI, contains Gly in the P'4 position. © 1990.