Deficiency of factor X(a)-factor V(a) binding sites on the platelets of a patient with a bleeding disorder

Factor V (V(a)) is essential for binding of factor X(a) to the surface of platelets. After thrombin treatment, normal platelets release at least five times more factor V(a) activity than is required for maximal factor X(a) binding. The concentration of factor V activity obtained after thrombin stimulation of 10⁷ normal platelets in sufficient to allow half-maximal factor X(a) binding to 10⁸ platelets (10% normal, 90% factor-V deficient). Therefore, factor V(a) activity is not limiting in platelet-surface factor X(a) 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 V(a) factor X(a) binding. The patient's platelet factor V(a) activity released after thrombin treatment is normal, but factor X(a) 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.