Thrombin induced platelet reactions: Effect of substrates and inhibitors on binding of thrombin and serotonin release

Abstract

Binding of 125I-thrombin to platelets and subsequent serotonin release were confirmed. The binding of unaltered thrombin to platelets was also measured by a new technique using a chromogenic substrate (S-2238). As compared to 125I-thrombin, this method gave similar constants for binding to the high affinity binding site, but lower for binding to the low affinity binding site. Furthermore, the results suggest that the platelets have two classes of independent binding sites. Substrates and inhibitors of thrombin inhibited thrombin induced serotonin release, suggesting that the release reaction depends on the proteolytic activity of thrombin. The serotonin release was more inhibited than the binding of thrombin, suggesting that the platelet binding site and the active site of thrombin are located in different parts of the thrombin molecule.