The present study was initiated to establish the functional
factor V concentration in platelets of patients with a mild
bleeding disorder ascribed to a
gray platelet syndrome. This inherited platelet disorder has been characterized by a specific deficiency of alpha-granules and subsequent deficiencies in the alpha-granule
proteins. We found that the concentration of plasma
factor V was slightly decreased (70% of normal values). In contrast, platelet
factor Va formation was severely impaired. Besides a much lower
factor V content than in control platelets (10-20% of normal), the dependency of platelet
factor Va formation on
thrombin concentration was altered. Increasing the
thrombin concentration 4-fold compared to the concentration that results in maximal
factor Va generation from normal platelets did not result in a maximal
factor Va formation from gray platelets. When a
suspension of washed gray platelets was incubated with a
prostacyclin analogue prior to the stimulation with
thrombin, a 10-fold lower
factor Va activity was measured. Thus,
thrombin-induced
factor Va formation in a
suspension of gray platelets is the result of a release reaction, followed by the
thrombin-catalyzed activation of released
factor V. Whereas the kinetics of the former reaction are apparently impaired, the kinetics of the latter one were found to be identical to those observed for normal platelet and plasma
factor V activation.