DIRECT INHIBITION OF PROTEIN CA BY SITE-DIRECTED THROMBIN INHIBITORS - IMPLICATIONS IN ANTICOAGULANT AND THROMBOLYTIC THERAPY

Several synthetic and recombinant antithrombin agents are currently developed for anticoagulation during thrombolytic therapy. Many of these agents are designed to inhibit thrombin at the active site. This active site is similar in enzymes belonging to the family of serine proteases. Since many of the thrombolytic enzymes are serine proteases (the class of enzymes in which thrombin and most of the coagulation enzymes are included), the newly developed antithrombin agents may also inhibit their pharmacologic actions and compromise thrombolytic efficacy. For this reason, it is important to compare the relative antithrombin and antifibrinolytic actions of these agents. A systematic investigation of the antiprotease profile and inhibitory spectrum against fibrinolytic enzymes is also important. Although several thrombin inhibitors have been compared in their ability to inhibit various enzymes involved in the activation of fibrinolysis (1), no reports on the interactions of these inhibitors with activated protein C are available. Since protein Ca has a dual role, both as an anticoagulant in the coagulation cascade (by inactivating factors Va and VIIIa) and as a profibrinolytic enzyme (by inactivating PAI-1 and PAI-III and thus stimulating the activity of t-PA) (2), it is important that its activity is not compromised by thrombin inhibitors, for optimal coagulation and fibrinolytic state. The current studies are therefore designed to investigate the direct inhibitory effects of synthetic and recombinant antithrombin agents on the amidolytic action of protein Ca. Human activated protein C has been made available by plasma fractionation and thus the inhibitory effects of thrombin inhibitors in a defined biochemical system was possible to investigate.