Interaction of Thrombin with Human Platelets

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Abstract
Summary The first step in thrombin-induced aggregation of blood platelets is binding of thrombin to specific receptors on the platelet membrane. Elucidation of the nature of this receptor in human platelets was attempted using radioactively labelled thrombin. In disc gel electrophoresis an extract of thrombin-treated platelets showed one peak of radioactivity near the origin. Gel filtration of the platelet extract through Sephadex G-200 showed mainly one protein peak at the void volume which contained radioactivity. After column fractionation the final isolate reacted with antiserum to thrombosthenin but did not react with antiserum to serum, fibrinogen or soluble platelet proteins. Gel electrophoresis of the reduced isolate in the presence of sodium dodecyl sulphate showed a pattern similar to thrombosthenin. To explore the possibility that thrombosthenin might be the receptor of thrombin, attempts were made to complex the receptor sites with thrombosthenin antibody or its univalent fragment. It was observed that complexing these receptors potentiates, rather than inhibits, platelet aggregation by thrombin or by adenosine diphosphate. Univalent fragment of antibody to albumin failed to cause this potentiation. Thus, blocking of the thrombosthenin sites is necessary for sensitization of platelets.