Effect of Thrombin Inhibitors on Positive Feedback in the Coagulation Cascade

The coagulation cascade is a series of sequential reactions of limited proteolysis of protein factors resulting in generation of thrombin. Thrombin mediates both positive and negative feedback in regulating this cascade by taking part in activation of several factors. Some thrombin inhibitors, by affecting positive feedback, inhibit generation of thrombin itself. In the current study, we used two thrombin inhibitors: argatroban, a low molecular weight reversible competitive inhibitor that binds to the active site, and bivalirudin, a bivalent oligopeptide that blocks the active site and binding center of protein substrates (exosite I). Appearance rate and total amount of thrombin were measured in a thrombin generation assay (TGA) using a fluorescent substrate. We found that argatroban slows the appearance of thrombin and lowers its amount. Bivalirudin also slows appearance of thrombin, but it does not decrease its amount, perhaps because the region being bound to the active site undergoes hydrolysis so that the inhibitor stops binding to thrombin. Many reactions of the coagulation cascade proceed on the surface of phospholipid micelles (PLMs). In the case of argatroban, PLMs do not affect the results of the TGA, whereas for bivalirudin they lower its inhibitory activity. It seems that PLMs stabilize protein complexes (wherein thrombin exosite I is hindered) mediating positive feedback in the coagulation cascade, e.g. complexes of thrombin with factor V and VIII.