The effect of von Willebrand factor on activation of factor VIII by factor Xa.

Factor VIII has to be activated before it can serve efficiently as a cofactor in the intrinsic pathway of blood coagulation. This activation occurs through specific proteolytic cleavages in the molecule by either thrombin or factor Xa. In this study, we show that von Willebrand factor inhibits the activation of factor VIII by factor Xa. Incubation of factor VIII (30 U/ml) with 0.1 microgram/ml factor Xa resulted in a 1.6-fold activation followed by a decay of coagulant activity. In the presence of 10 micrograms/ml von Willebrand factor, activation and inactivation of factor VIII was completely inhibited. In contrast, the activation of factor VIII by thrombin was not influenced by von Willebrand factor. At high concentrations of factor Xa (10 micrograms/ml), von-Willebrand-factor-bound factor VIII could be cleaved and activated. The generated proteolytic fragments were identical to the fragments produced in the absence of von Willebrand factor and all fragments were released from von Willebrand factor. The major products were light-chain-derived fragments of molecular mass 66/68 kDa and 60 kDa and heavy-chain-derived fragments of 40 and 42 kDa. Also minor products of 12, 20/21, 23, 27 and 30 kDa were observed, most of which were specific for cleavage of factor VIII by factor Xa.