Thrombin-stimulated human platelets express molecular forms of α-granule factor V (FV), which differ from FVa

Binding of 125 I-Fab fragments of chain-specific antibodies indicate that both heavy and light chains of α-granule factor Va (FVa) were externalized on the platelet membrane after stimulation with thrombin. Using a Mab against the activation peptide of factor V (FV), the epitope appears on the stimulated platelet surface. Half as much light chain and heavy chain (FVa) was expressed campared to the activation peptide, suggesting that expression of α-granule FV occurs after thrombin stimulation. Using an ELISA, we find that 32% of α-granule FV was released and 68% is retained in the platelet pellet