Multiple active forms of thrombin: binding to platelets and effects on platelet function.

Abstract

Partially purified bovine thrombin, which is a mixture of multiple active forms of thrombin, was chromatographed to yield molecular species termed .alpha.-, .beta.- and .gamma.-thrombin, each of which has varying degrees of fibrinogen clotting and esterase activities. A direct correlation was observed between the ability of the different forms of thrombin to clot fibrinogen and to influence platelet function. In general, thrombin with high fibrinogen clotting activity was also a potent inducer of platelet [human] aggregation and the release reaction, while those species with low clotting ability were poor inducers of aggregation and release. Thrombin with both fibrinogen clotting activity and esterase activity nearly completely inactivated by treatment with N-.alpha.-p-tosyl-L-lysine chloromethyl ketone had no aggregation or release inductive ability. Studies performed with partly purified thrombin or individual species of thrombin iodinated with 125I indicated that despite differences in fibrinogen clotting activities, .alpha.-thrombin and its degraded forms were capable of binding to the platelet surface. The possibility exists that thrombin binding to the platelet surface may be a means of strategically anchoring and concentrating large amounts of enzymatically active thrombin within the white thrombus at a site of vascular injury.