von Willebrand factor as a regulator of intrinsic factor X activation

Factor VIII is an important cofactor in the intrinsic activation of factor X. To function effectively as a cofactor, factor VIII must be activated. In plasma, factor VIII circulates in a complex with von Willebrand factor, and although thrombin can activate complexed factor VIII, the activation by activated factor X is inhibited by von Willebrand factor. In this study, the effect of von Willebrand factor on the generation of factor Xa by the factor IXa-VIII complex was investigated. Purified human factors VIII, IXa, and X were incubated on human umbilical vein endothelial cells or phospholipid vesicles in the presence of calcium ions, and the generation of factor Xa was followed. In the presence of von Willebrand factor, a prolonged lag-phase and a dose-dependent inhibition of factor X activation was observed. These effects were not observed when von Willebrand factor was preincubated with a monoclonal antibody directed against von Willebrand factor that blocks factor VIII binding. When factor VIII was activated with thrombin before the incubation, neither the monoclonal antibody nor von Willebrand factor had an effect on the rate of factor X activation. Preincubation of endothelial cells with the monoclonal antibody resulted in a somewhat higher rate of factor X activation. When endothelial cells from a patient with von Willebrand's disease type I were used, preincubation of the monoclonal antibody had no effect on the rate of factor X activation. We conclude that von Willebrand factor on the surface of endothelial cells can modulate the intrinsic factor X activation. This effect is greatly enhanced, however, by the addition of exogenous von Willebrand factor.(ABSTRACT TRUNCATED AT 250 WORDS)