Structural and functional characterization of platelet receptor-mediated factor VIII binding.

Optimal rates of factor X (FX) activation require occupancy of receptors for factor IXa (FIXa), factor VIII (FVIII), and FX on the activated platelet surface. The presence of FVIII and FX increases 5-fold the affinity of FIXa for the surface of activated platelets, and the presence of FVIII or FVIIIa generates a high affinity, low capacity specific FX-binding site on activated platelets. We have now examined the effects of FX and active site-inhibited FIXa (EGR-FIXa) on the binding of both FVIII and FVIIIa to activated platelets and show the following: (a) von Willebrand factor inhibits FVIII binding (K(i) = 0.54 nM) but not FVIIIa binding; (b) thrombin and the thrombin receptor activation peptide (SFLLRN amide) are the most potent agonists required for FVIII-binding site expression, whereas ADP is inert; (c) FVa does not compete with FVIIIa or FVIII for functional platelet-binding sites; and (d) Annexin V is a potent inhibitor of FVIIIa binding (IC(50) = 10 nM) to activated platelets. The A2 domain of FVIII significantly increases the affinity and stoichiometry of FVIIIa binding to platelets and contributes to the stability of the FX-activating complex. Both FVIII and FVIIIa binding were specific, saturable, and reversible. FVIII binds to specific, high affinity receptors on activated platelets (n = 484 +/- 59; K(d) = 3.7 +/- 0.31 nM) and FVIIIa interacts with an additional 300-500 sites per platelet with enhanced affinity (K(d) = 1.5 +/- 0.11 nM). FVIIIa binding to activated platelets in the presence of FIXa and FX is closely coupled with rates of F-X activation. The presence of EGR-FIXa and FX increases both the number and the affinity of binding sites on activated platelets for both FVIII and FVIIIa, emphasizing the validity of a three-receptor model in the assembly of the F-X-activating complex on the platelet surface.