Human platelets have binding sites for plasma
coagulation Factor X(a) that are available only after the platelet release reaction. Platelets from 15 normal donors bound 216+/-52 (SD) molecules of
Factor X(a) per platelet. The association of
Factor X(a) with its platelet surface receptor results in a 300,000-fold increase in the catalytic activity of
Factor X(a) in forming
thrombin from
prothrombin. The turnover number for platelet-bound
Factor X(a) was 1,850+/-460 mol
thrombin/ml per min per mol
Factor X(a) 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 X(a)-platelet interaction. The binding of
Factor X(a) 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 X(a) 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 X(a) 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 X(a) 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 X(a)-platelet interaction and that
thrombin formation at the platelet surface is important in normal hemostasis.