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The activation of prothrombin by platelet-bound factor Xa.

After the platelet release reaction, factor Xa binds to a receptor on the platelet surface with high affinity, Ka 5.2 × 1o9 M−1. Binding of the factor Xa substrate, prothrombin to the platelet surface has not yet been possible to demonstrate. The interaction between prothrombin and platelets was therefore studied by determination of the structural requirements on the prothrombin molecule for activation by platelet-bound factor Xa. A comparison was made between the rates of activation of normal prothrombin, prethrombin 1, prethrombin 2, and acarboxyprothrombin (prothrombin lacking γ-carboxyglutamic acid residues) in the presence of factor Xa and isolated platelets. Both the vitamin-K-dependent fragment 1 of prothrombin and fragment 2 which interacts with factor V were found to be required for rapid prothrombin activation. Acarboxyprothrombin and prethrombin 1 were slowly activated to thrombin by factor Xa on the platelet surface after the platelet release reaction. In the same system prethrombin 2 was activated more slowly to thrombin. In the presence of fragment 2, which binds noncovalently to prethrombin 2, the activation rate was enhanced and equal to that of prethrombin 1 supporting the conclusion that factor V is part of the platelet receptor. The cleavage products of prothrombin by factor Xa in the presence of platelets were found to be the same as those previously described when the activation was catalyzed by the prothrombinase complex. The coefficient for proteolytic efficiency, Kcat/Km, was found to be approximately 50 times higher in the presence of prothrombin than in the presence of acarboxyprothrombin. We conclude that prothrombin interacts with a specific platelet structure consisting of phospholipid and an activated form of factor V exposed on the platelet surface as a result of the release reaction.