Platelet procoagulant activity: physiological significance and mechanisms of exposure.

This review describes an important function of blood platelets in the hemostatic process: the formation of a procoagulant surface. Two essential steps of the coagulation cascade, the formation of factor Xa and the formation of thrombin, require a catalytic surface on which the enzyme complexes can be assembled. This catalytic surface is provided by the phospholipids of the platelet plasma membrane. However, in the quiescent platelet, the negatively charged phospholipids which are essential to the catalytic properties of the surface, are located in the cytoplasmic leaflet of the membrane. Dependent on the activator, the normal asymmetric distribution of phospholipids is lost, resulting in the formation of a procoagulant surface. Although platelets are primarily predestined to exhibit this function, certain pathological conditions can lead to exposure of a procoagulant surface in other cells as well. Current views to explain the mechanisms of exposure of a procoagulant surface include the role of the cytoskeleton, the formation of microvesicles from the plasma membrane as well as the contribution of a membrane protein, which actively transports specific phospholipids from the outer-to inner leaflet of the membrane bilayer.