PPACK-thrombin is a noncompetitive inhibitor of α-thrombin binding to human platelets

Recent studies from our laboratory indicate that purified kininogens are noncompetitive inhibitors of human α-thrombin but not PPACK-thrombin, binding to human washed platelets. In order to understand the mechanism by which the kininogens inhibit α-thrombin binding, investigations were initiated to determine if α-thrombin and PPACK-thrombin bound to the same site on human platelets. Initial investigations reveal that α-thrombin is a more potent inhibitor of 125I-PPACK-thrombin binding than PPACK-thrombin. Further studies show that PPACK-thrombin is a noncompetitive inhibitor of 125I-α-thrombin binding to platelets. These studies suggest that human α-thrombin binds on the platelet surface to a different site or binds differently to the same site from PPACK-thrombin. These data indicate that the ability of the kininogens to block α-thrombin binding to platelets but not PPACK-thrombin binding results from these thrombins having either two different binding sites or one binding site on the platelet surface which they interact with differently.