Deficiency of factor Xa-factor Va binding sites on the platelets of a patient with a bleeding disorder.

Factor V (Va) is essential for binding of factor Xa to the surface of platelets. After thrombin treatment, normal platelets release at least five times more factor Va activity than is required for maximal factor Xa binding. The concentration of factor V activity obtained after thrombin stimulation of 10(7) normal platelets is sufficient to allow half-maximal factor Xa binding to 10(8) platelets (10% normal, 90% factor-V deficient). Therefore, factor Va activity is not limiting in platelet-surface factor Xa binding and prothrombin activation in normal platelets; some other components limit the number of binding sites. We report studies of a patient (M.S.) with a moderate to severe bleeding abnormality whose platelets are deficient in the platelet-surface component required for the factor Va-factor Xa binding. The patient's platelet factor Va activity released after thrombin treatment is normal, but factor Xa binding is 20%-25% of control values at saturation. Abnormal prothrombin consumption in a patient with normal plasma coagulation factors and platelet function suggests a disorder in platelet-surface thrombin formation.