Summary
Binding of 125I-Fab fragments of chain-specific antibodies indicate that both heavy and light chains
of a-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 compared to the activation peptide, suggesting that expression
of α-granule FV occurs after thrombin stimulation. Using an ELISA, we find that 32%
of a-granule FV was released and 68% is retained in the platelet pellet. Immunoblots
of platelets indicate that FV exists in 200 kDa und 150 kDa forms, representing incomplete
cleavage, while the releasate demonstrates a more complete cleavage by proteases.
We conclude that expression of α-granule FV is quantitatively greater than that released
and exists in molecular forms which cannot be completely explained by the binding
of FVa.
