The role of platelet factor V in prothrombin conversion

The contribution of platelet factor V to prothrombin conversion was studied in a purified two-stage system designed to measure the ability of factor V to accelerate prothrombin conversion. When unstimulated gel-filtered platelets (GFP) were substituted for both factor V and phospholipid, thrombin evolution was linear following a long lag time. Gel filtration resulted in considerable phospholipid availability with minimal factor V release. Incubating platelets with collagen in increasing concentrations resulted in marked shortening of the lag time, an increase in the initial rate of thrombin formation, and release of platelet factor V. The inhibition of thrombin formation by preincubation of the platelets with metabolic inhibitors is consistent with previous observations that factor V is released from alpha-granules by collagen in a process requiring metabolic energy. Released platelet factor V added to metabolically inhibited platelets reproduces the acceleration of prothrombin conversion demonstrated in GFP incubated with collagen. Furthermore no acceleration of the clotting time at collagen concentrations used in this study was demonstrated in an assay designed to measure available platelet phospholipid in the presence of excess factor V. The rate of increased thrombin generation produced by collagen stimulation is primarily due to released platelet factor V in the system employed.