Patients with Congenital Factor V Deficiency have Decreased Factor Xa Binding Sites on their Platelets

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Abstract
Human platelets have binding sites for plasma coagulation Factor Xa that are available only after the platelet release reaction. Platelets from 15 normal donors bound 216±52 (SD) molecules of Factor Xa per platelet. The association of Factor Xa with its platelet surface receptor results in a 300,000-fold increase in the catalytic activity of Factor Xa in forming thrombin from prothrombin. The turnover number for platelet-bound Factor Xa was 1,850±460 mol thrombin/ml per min per mol Factor Xa in experiments with platelets from 15 normal donors. Platelets from five patients with varying degrees of Factor V deficiency were investigated to determine whether or not coagulation Factor V participates in either aspect of the Factor Xa-platelet interaction. The binding of Factor Xa to platelets and the accompanying increase in rate of thrombin formation were either reduced in parallel or absent in each case with values ranging from 0 to 45% of control values. The apparent affinity of Factor Xa from Factor V-deficient patients was normal when platelet binding was detected. The supernate from thrombin-treated control platelets, which contains Factor V activity, corrected the Factor Xa binding deficiency of the platelets from three patients tested. Immunoreactive Factor V determined with an homologous antibody corresponded to the functional Factor V activity of platelets from one patient with Factor V deficiency, suggesting that the patient's platelets have a decreased amount of normal Factor V. The ability of platelets from the patients to bind Factor Xa and increase the rate of thrombin formation correlated with the severity of each patient's bleeding disorder better than the plasma level of Factor V. The results indicate that Factor V is required for the Factor Xa-platelet interaction and that thrombin formation at the platelet surface is important in normal hemostasis.