The role of factor VIII in blood coagulation

Factor VIII, a component of the intrinsic pathway of blood coagulation, has yet to be purified to homogeneity. It appears that, in vivo, the factor VIII coagulant protein is closely associated with one or more other proteins (factor VHI-related antigen and platelet aggregating factor). The material normally isolated from bovine plasma as 'factor VIII' possesses all three activities and is therefore either a mixture or a complex of the various proteins. In the present study, bovine factor VIII:C was purified approximately fivethousand- fold by a combination of ion-exchange chromatography and fractional precipitation. The factor VIII coagulant activity can be separated from the other activities of the 'factor VIII complex' but the procedures involved are not suitable for preparative use as the factor VIII:C which is obtained is unstable. During coagulation, factor VIII:C is required during the activation of factor X. Studies with purified bovine clotting factors indicate that factor IXa is the enzyme responsible for the cleavage of factor X, in a calcium-dependent reaction which is stimulated by phospholipid. Factor VIII:C further accelerates the rate at which factor Xa is generated. Preliminary investigations of the kinetic parameters of the reaction indicate that the stimulation by factor VIII:C occurs through a marked increase in the Vmax of the reaction; factor VIII:C does not affect the Km for factor X. The coagulant activity of factor VIII is enhanced by exposure to thrombin, but the 'activated' factor VIII:C which is produced is not itself capable of activating factor X in the absence of factor IXa. Thus, the 'activation' of factor VIII:C, in contrast to the activation of, for example, factors IX and X, does not appear to result in the formation of an enzyme. That is, factor VIII:C is a non-enzymic, high molecular weight cofactor for factor IXa.