INHIBITION OF SERINE PROTEASES OF THE BLOOD-COAGULATION SYSTEM BY SQUASH FAMILY PROTEASE INHIBITORS

Squash family inhibitors are the smallest protein serine protease inhi bitors, being composed of approximately 30 amino acid residues. We iso lated 8 squash family inhibitors from the seeds of bitter gourd, squas h, gourd and luffa and examined their effect on serine proteases of th e blood coagulation system. Five of them prolonged the activated parti al thromboplastin time of human plasma to various extents, but three d id not. Only Momordica charantia (bitter gourd) trypsin inhibitor-II p rolonged the prothrombin time of human plasma. All inhibitors inhibite d 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. K-i values for factor XIIa, plasma kallikr ein, 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 p artial thromboplastin time by inhibitors appeared to correspond to the ir inhibitory potencies for factor XIIa. Momordica charantia trypsin i nhibitor-II, which has the strongest inhibitory potency toward the ami dolytic activity of factor Xa, with a K-i value 10-100 times smaller t han those of other inhibitors, inhibited the activation of factor X by factor VIIa-tissue factor complex or factor IXa, while others did not .