Inhibition of thrombin binding and serotonin secretion from platelets by a 74,000 dalton protein

A 74,000 dalton protein has been isolated from solubilized human platelet membranes by affinity chromatography with a nonagglutinating derivative of wheat germ agglutinin. The protein as well as the lectin derivative blocked platelet aggregation by thrombin while aggregation induced by several other agents was unchanged. This protein inhibited the binding of thrombin to platelets in a competitive manner. The secretion of serotonin from platelets by thrombin was also blocked by the protein. These effects of the protein were observed when platelets were suspended in either plasma or buffer. This protein may have functional significance in the activation of platelets by thrombin.