Binding of native and thrombin activated factor VIII to platelets.

The interaction of human Factor VIII with platelets has been studied using discontinuous albumin gradient centrifugation. When purified Factor VIII complex was mixed with released platelets about 10% of the recovered Factor VIII activity became associated with the platelets. In the corresponding experiment with thrombin-activated Factor VIII about half of the Factor VIII activity was bound to the platelets. This binding was reversible; dilution of the platelet suspension containing bound Factor VIII resulted in dissociation of part of the Factor VIII activity. With non-released platelets very little binding of Factor VIII activity occurred. A mechanism for the in vivo formation of the Factor X activator complex is suggested.