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Thrombin-induced platelet factor Va formation in patients with a gray platelet syndrome
D Baruch1, T Lindhout, E Dupuy
1Department of Biochemistry, University of Limburg, Maastricht, The Netherlands.
Abstract:
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.
Insights
Gray platelet syndrome severely impairs platelet factor Va formation due to reduced factor V content and altered thrombin response. This bleeding disorder affects alpha-granule protein levels.
Area of Science:
- Hematology
- Hemostasis and Thrombosis
- Platelet Biology
Background:
- Gray platelet syndrome is an inherited bleeding disorder characterized by alpha-granule deficiency.
- This deficiency leads to reduced levels of alpha-granule proteins, including factor V.
- Impaired platelet function contributes to bleeding tendencies in affected individuals.
Purpose of the Study:
- To determine the functional factor V concentration in platelets of patients with gray platelet syndrome.
- To investigate the mechanism of impaired factor Va formation in these patients.
- To compare factor V activation in gray platelets with normal platelets.
Main Methods:
- Quantification of plasma factor V concentration.
- Assessment of platelet factor V content and factor Va formation.
- Evaluation of factor Va generation in response to varying thrombin concentrations.
- Analysis of factor Va activity after prostacyclin analogue incubation.
Main Results:
- Plasma factor V was slightly decreased (70% of normal).
- Platelet factor V content was significantly reduced (10-20% of normal).
- Platelet factor Va formation was severely impaired and showed altered dependency on thrombin concentration.
Conclusions:
- Thrombin-induced factor Va formation in gray platelets involves impaired release and normal activation kinetics.
- The study elucidates the specific defect in factor V processing in gray platelet syndrome.
- Findings highlight the critical role of alpha-granules in platelet factor V storage and availability.