No abstract

Serine proteinase inhibitors of the squash family were isolated from bitter gourd (Momordica charantia LINN.) seeds by the conventional purification method. Heat treatment of the extract of the seeds allowed removal of large amounts of protein without loss of trypsin and elastase inhibitory activities. From the supernatants thus obtained, the inhibitors were isolated to homogeneity by ion-exchange chromatography, gel filtration, and reversed phase chromatography. One trypsin inhibitor (Momordica charantia trypsin inhibitor-III; MCTI-III) and three elastase inhibitors (Momordica charantia elastase inhibitor-II, -III, and -IV; MCEI-II, -III, and -IV) were newly isolated in addition to trypsin inhibitors MCTI-I and -II and elastase inhibitor MCEI-I previously reported [Hara, S. et. al. (1989). J. Biochem. 105, 88-92]. The primary structures of the four new inhibitors were determined as follows. [sequence: see text] The dissociation constants, Ki, of MCTI-III complex with bovine beta-trypsin, and of MCEI-II, -III, -IV with porcine elastase were determined to be 1.9 x 10(-7) M, 9.4 x 10(-9) M, 4.0 x 10(-9) M, and 4.7 x 10(-9) M, respectively. Although MCTI-III differed from MCTI-I in only two amino acids, having Gly(3) and Gln(13) in place of Arg(3) and Arg(13), the Ki value of MCTI-III was 20-fold larger than that of MCTI-I. Addition of an amino terminal Glu residue, a dipeptide (Glu-Glu-), and a tripeptide (Glu-Glu-Glu-) to MCEI-I strengthened its elastase inhibitory activity by 200-fold.

Squash family inhibitors are the smallest protein serine protease inhibitors, being composed of approximately 30 amino acid residues. We isolated 8 squash family inhibitors from the seeds of bitter gourd, squash, gourd and luffa and examined their effect on serine proteases of the blood coagulation system. Five of them prolonged the activated partial thromboplastin time of human plasma to various extents, but three did not. Only Momordica charantia (bitter gourd) trypsin inhibitor-II prolonged the prothrombin time of human plasma. All inhibitors inhibited the amidolytic activities of factor XIIa, plasma kallikrein, factor Xa, but did not inhibit significantly those of factor XIa, factor IXa, factor VIIa, and thrombin. Ki values for factor XIIa, plasma kallikrein, and factor Xa were in the order of 10(-6)-10(-9), 10(-4)-10(-5), and 10(-4)-10(-6)M, respectively. The prolongation of the activated partial thromboplastin time by inhibitors appeared to correspond to their inhibitory potencies for factor XIIa. Momordica charantia trypsin inhibitor-II, which has the strongest inhibitory potency toward the amidolytic activity of factor Xa, with a Ki value 10-100 times smaller than those of other inhibitors, inhibited the activation of factor X by factor VIIa-tissue factor complex or factor IXa, while others did not.