Interaction of coagulation factor V and factor Va with platelets.

Abstract

Bovine platelet and plasma Factor V were compared using antisera raised to purified single chain plasma Factor V and the quantity of Factor V associated with the bovine platelet was established using a radioimmunoassay developed with the same antisera. Bovine platelet Factor V is immunochemically indistinguishable from bovine plasma Factor V using both radioimmunoassay and immunodiffusion as comparative techniques. Radioimmunoassay quantitation of platelet Factor V indicated that 593 + 139 Factor V molecules were present/platelet. Bioassay assessment of the quantity of Factor V present in the bovine platelet gave a value similar to this (666 f 124), further suggesting that the platelet and plasma derived Factor V are identical. The binding of both iz51-Factor V and -Factor Va to platelets was also measured. When incubated with washed bovine platelets, 1251-Factor Va underwent saturable and exchangeable binding. There are high affinity binding sites to which approximately 900 Factor Va molecules are bound/platelet with an apparent dissociation constant of 3 x lo-” M, as well as binding sites of slightly lower affinity (Z& = 3 X lo-’ M) to which as many as 3500 Factor Va molecules are bound/platelet. The binding of Factor V to platelets is also saturable and exchangeable. Approximately 800 Factor V molecules bind to a single class of sites with a dissociation constant of 3 x lo-’ M. Exchange studies indicated that Factor V and Factor Va both bind to the lower affinity sites; however, Factor V does not bind to the high affinity Factor Va binding sites. Thrombin-induced platelet activation was not required for, nor had any effect on, the binding of either Factor V or Factor Va. The number of Factor V molecules associated with the platelet is similar to the number of Factor V molecules bound to the low affinity platelet binding sites, and that value is also similar to the number of high affinity Factor Va binding sites.