Stoichiometry of the porcine factor VIII-von Willebrand factor association

Factor VIII and von Willebrand factor (vWF) are glycoproteins that form a tightly bound complex in plasma. The interaction of porcine factor VIII with porcine vWF was studied by analytical velocity sedimentation. A single approximately 240-kDa species of factor VIII was isolated for use in the analysis. In contrast, when analyzed by agarose/sodium dodecyl sulfate-polyacrylamide gel electrophoresis, vWF consisted of a population of greater than 10 multimers derived from a 270-kDa monomer. A single boundary (So20,w = 7.2 S) was observed during velocity sedimentation of factor VIII at 260,000 x g. A single boundary also was observed for vWF (weight-average So20,w = 21 S) at 42,000 x g. Under condition of excess factor VIII, the weight-average So20,w of the factor VIII-vWF complex was 40 S at 42,000 x g. At 260,000 x g, the factor VIII-vWF complex had sedimented completely, leaving only free factor VIII. The height of the plateau region of the factor VIII sedimentation velocity curve at 260,000 x g was studied as a function of several starting concentrations of vWF. The experiments were done under conditions in which the effect of radial dilution was negligible so that the plateau height was a measure of the concentration of free factor VIII. The plateau height decreased linearly as the concentration of vWF was increased, indicating that the association was essentially irreversible under the conditions used. A stoichiometry of 1.2 vWF monomers/factor VIII molecule was calculated from the slope of the line. Assuming one factor VIII-binding site/vWF monomer, these results indicate that all factor VIII-binding sites are accessible in the vWF multimer.