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
X. Li et Da. Gabriel, 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, Biochemistry, 36(35), 1997, pp. 10760-10767
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.