THE PHYSICAL EXCHANGE OF FACTOR-VIII (FVIII) BETWEEN VON-WILLEBRAND-FACTOR AND ACTIVATED PLATELETS AND THE EFFECT OF THE FVIII B-DOMAIN ON PLATELET BINDING

Normal hemostasis proceeds through the assembly of coagulant complexes on a lipid surface derived from activated platelets. The activation c omplex assembly is governed by multiple factors including the binding constants (K-d) of the coagulant factors for the lipid surface. The fo rmation of the tenase complex requires delivery of factor WI (FVIII) t o the activated lipid surface by von Willebrand actor (VWF). Using ele ctrophoretic quasi-elastic light scattering (ELS), we have examined th e interaction of FVIII in the presence and absence of VWF with both re sting and activated gel-filtered human platelets. Resting platelets do not bind FVIII. Platelets activated by thrombin, epinephrine, or SFLL RN, but not ADP or collagen, bind unactivated FVIII if VWF is not pres ent. In the absence of VWF, unactivated FVIII binds to activated plate lets with a K-d of 10.4 nM. B-domain deleted FVIII binds to activated platelets with a K-d of 5.1 nM, Thrombin-activated FVIII (FVIIIa) bind s to activated platelets with a K-d of 1.7 nM, The activation of FVIII while bound to the platelet surface can be monitored as a function of time, In the presence of VWF, binding of unactivated FVIII to activat ed platelets was inhibited, but not the binding of FVIIIa. Displacemen t of bound unactivated FVIII from the platelet surface occurs when VWF is added to the FVIII-platelet complex, The binding of FVIII to activ ated platelets is affected by the B-domain, the state of FVIII activat ion, and the presence of soluble VWF and proceeds as a multistep proce ss, FVIII binding by activated platelets is not affected by platelet g pIIb/IIIa or by platelet vWF.