Summary
We have investigated the binding of blood coagulation factor XIIIa to thrombin-stimulated
platelets using cytofluorometric analysis. Washed thrombin-stimulated platelets bound
exoge-nously added factor XIIIa in a calcium-dependent reaction. The expression of
endogenous platelet factor XIII was also detected on the surface of thrombin-stimulated
platelets. When fluorescence analysis was performed based on particle size, factor
XIIIa bound to the surface of greater than 95% of particles which contained more than
one platelet, but only 50% of single platelets. The binding of factor XIIIa to thrombin-stimulated
platelets was inhibited by plasmin. Plasmin also inhibited thrombin-dependent expression
of the factor XIIIa binding site on platelets. Experiments in which thrombin-stimulated
platelets were incubated with factor XIIIa in the presence of 125I-dimethyl-casein or 3H-putrescine demonstrated that platelets bear both glutamyl and lysyl substrates for
factor XIIIa. Thrombin increased the expression of factor XIIIa substrates by platelets.
Plasmin inhibited both the expression of factor XIIIa substrates and degraded them.
The binding of factor XIIIa to thrombin-stimulated platelets and the availability
of factor XIIIa substrates on the platelet surface could provide a mechanism by which
factor XIIIa stabilizes the hemostatic plug by promoting crosslinking reactions between
platelet membrane proteins and adhesive glycoproteins. In contrast, plasmin inhibition
of factor XIIIa binding and crosslinking could disrupt hemostasis.
Key words
Factor XIII - Platelets - Plasmin