THROMBIN-STIMULATED HUMAN PLATELETS EXPRESS MOLECULAR-FORMS OF ALPHA-GRANULE FACTOR V(FV), WHICH DIFFER FROM FVA

Binding of I-125-Fab fragments of chain-specific antibodies indicate t hat both heavy and light chains of alpha-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 epit ope appears on the stimulated platelet surface. Half as much light cha in and heavy chain (FVa) was expressed compared to the activation pept ide, suggesting that expression of alpha-granule FV occurs after throm bin stimulation. Using an ELISA, we find that 32% of alpha-granule FV was released and 68% is retained in the platelet pellet. Immunoblots o f platelets indicate that FV exists in 200 kDa and 150 kDa forms, repr esenting incomplete cleavage, while the releasate demonstrates a more complete cleavage by proteases. We conclude that expression of alpha-g ranule FV is quantitatively greater than that released and exists in m olecular forms which cannot be completely explained by the binding of FVa.