Immunological evidence for prothrombin in human platelets

An attempt was made to isolate from plasma the platelet surface substrate for thrombin, glycoprotein V (GPV), because a GPV antigen was reported to be present in plasma. Plasma fractionation based on procedures for purification of GPV from platelets revealed a thrombin-sensitive protein with appropriate electrophoretic mobility. The protein was purified; an antiserum against it reacted with detergent-solubilized platelet proteins or secreted proteins in a double diffusion assay, adsorbed a protein from the supernatant solution of activated platelets, and inhibited thrombin-induced platelet activation, but the antiserum did not adsorb labeled GPV. The purified protein was immunochemically related to prothrombin rather than to GPV. Other antibodies against prothrombin were also able to adsorb a protein from platelets. It is concluded that plasma does not contain appreciable amounts of GPV, and platelets contain prothrombin or an immunochemically similar protein.