Collagen-bound von Willebrand factor has reduced affinity for factor VIII

von Willebrand factor (vWf) is a multimeric adhesive glycoprotein that serv es as a carrier for factor VIII in plasma. Although each vWf subunit displa ys a high affinity binding site for factor VIII in vitro, in plasma, only 2 % of the vWf sites for factor VIII are occupied. We investigated whether in teraction of plasma proteins with vWf or adhesion of vWf to collagen may al ter the affinity or availability of factor VIII-binding sites on vWf, When vWf was immobilized on agarose-linked monoclonal antibody, factor VIII boun d to vWf with high affinity, and neither the affinity nor binding site avai lability was influenced by the presence of 50% plasma. Therefore, plasma pr oteins do not alter the affinity or availability of factor Vm-binding sites . In contrast, when vWf was immobilized on agarose-linked collagen, its aff inity for factor VIII was reduced 4-fold, with K-D increasing from 0.9 to 3 .8 nM, However, one factor VIII-binding site remained available on each vWf subunit. A comparable reduction in affinity for factor VIII was observed w hen vWf was a constituent of the subendothelial cell matrix and when it was bound to purified type VI collagen. In parallel with the decreased affinit y for factor VIII, collagen-bound vWf displayed a 6-fold lower affinity for monoclonal antibody W5-6A, with an epitope composed of residues 78-96 with in the factor VIII-binding motif of vWf, We conclude that collagen induces a conformational change within the factor VIII-binding motif of vWf that lo wers the affinity for factor VIII.