Specificity of phosphatidylserine-containing membrane binding sites for factor VIII. Studies with model membranes...

Factor VIII functions in an enzyme complex upon the activated platelet membrane where phosphatidylserine exposure correlates with expression of receptors for factor VIII. To evaluate the specificity of phosphatidylserine-containing membrane binding sites for factor VIII, we have developed a novel membrane model in which phospholipid bilayers are supported by glass microspheres (lipospheres). The binding of fluorescein-labeled factor VIII to lipospheres with membranes of 15% phosphatidylserine was equivalent to binding to phospholipid vesicles (KD = 4.8 nM). Purified von Willebrand factor (vWf), a carrier protein for factor VIII, decreased membrane binding of factor VIII with a Ki of 10 micrograms/ml. Likewise, normal plasma decreased bound factor VIII by more than 90% whereas plasma lacking vWf decreased the binding of factor VIII by only 20%. Proteolytic activation of factor VIII by thrombin, which releases factor VIII from vWf, increased liposphere binding in the presence of vWf and in the presence of normal plasma. Although factor V is homologous to factor VIII and binds to lipospheres with the same affinity, purified factor V was not an efficient competitor for the membrane binding sites of factor VIII. These results indicate that phosphatidylserine-containing membrane sites have sufficient specificity to select thrombin-activated factor VIII from the range of phospholipid-binding proteins in plasma.