Two distinct forms of Factor VIII coagulant protein in human plasma. Cleavage by thrombin, and differences in coagulant activity and association with von Willebrand factor.

We have characterized Factor VIII coagulant protein, present in normal human plasma, that reacts with a specific human 125I-labeled anti-human VIII:C antigen Fab antibody fragment. Two major Factor VIII coagulant antigen populations were present. The first, approximately 85% of the total antigen, was bound to von Willebrand factor and when tested in a standard one-stage assay had Factor VIII coagulant activity. The second antigenic population, eluting near fibrinogen when plasma was gel filtered, was not bound to von Willebrand protein, did not have Factor VIII coagulant activity unless activated, but did block anti-VIII:C Fab neutralization of clotting activity. The two antigenic populations were separable by cryoprecipitation and agarose gel electrophoresis. Although the two antigenic populations differed in their Factor VIII coagulant activity and in their binding to von Willebrand factor, the principal member of both populations is of mol wt 2.4 X 10(5). Both antigens, when proteolyzed by thrombin, were quickly converted to a 1 X 10(5)-mol wt form in association with the appearance of VIII:C activity. The 1 X 10(5)-mol wt antigen was further slowly degraded to an 8 X 10(4)-mol wt form while Factor VIII coagulant activity declined. These results demonstrate the presence of an inactive Factor VIII coagulant protein in plasma, not associated with von Willebrand factor, that can react with thrombin to yield Factor VIII coagulant activity.