Subcellular localization and secretion of factor V from human platelets.

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Abstract
Factor V, a plasma protein cofactor necessary for optimal conversion of prothrombin to thrombin, is also present in considerable concentration in blood platelets (9.9 U/109 platelets). Subcellular fractionation by 2 methods has localized factor V in the .alpha. granules of unstimulated platelets. ADP and epinephrine cause release of 4.6 and 6.4%, respectively, of the total factor V, a process completely inhibited by cyclooxygenase alkylation by aspirin. In contrast, collagen causes release of 25% of platelet factor V, a process only partially suppressed by aspirin. Secretion of factor V depends on the availability of metabolic energy, because antimycin A, an inhibitor of aerobic metabolism, and 2-deoxyglucose, an inhibitor of anaerobic glycolysis, together almost totally inhibited the secretion of factor V induced by collagen. Factor V is not normally available on unstimulated platelets but can be secreted from .alpha. granules upon stimulation with physiological agents such as ADP, epinephrine and collagen. Because factor V is known to serve as a receptor for factor Xa, the exposure of factor V on platelets consequent to release would accelerate the process of blood coagulation.