DEFICIENCY OF FACTOR-XA-FACTOR-VA BINDING-SITES ON THE PLATELETS OF A PATIENT WITH A BLEEDING DISORDER

Factor [F] V (V8) is essential for binding of F Xa to the surface of platelets. After thrombin treatment, normal platelets released at least 5 times more F Va activity than required for maximal FXa binding. The concentration of F V activity obtained after thrombin stimulation of 107 normal platelets was sufficient to allow half-maximal F Xa binding to 108 platelets (10% normal, 90% F-V deficient). F Va activity was not limiting in platelet-surface F Xa binding and prothrombin activation in normal platelets; some other components limited the number of binding sites. Studies of a patient with a moderate to severe bleeding abnormality whose platelets were deficient in the platelet-surface component required for the F Va-F Xa binding, were reported. The patient''s platelet F Va activity released after thrombin treatment was normal, but F Xa binding was 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.