Degradation of factor VIII coagulant antigen by proteolytic enzymes.

The factors responsible for the lability of factor VIII coagulant activity (VIII:C) and factor VIII coagulant antigen (VIII:CAg) are poorly understood. In this study the VIII:C and VIII:CAg are studied after incubation with plasmin, trypsin or alpha-chymotrypsin. Both isolated human VIII:CAg and VIII:CAg associated with factor VIII-related antigen (VIII R:Ag) are evaluated. The antigenic sites of the VIII:CAg are somewhat more stable to the action of these enzymes than the functional activity, although both follow a generally parallel degradation. A biphasic decay curve is seen in the initial time points. No stabilization of the functional or antigenic reactivity is observed in the presence of the VIII R:Ag. Lower concentrations of each enzyme cause an initial rise in the factor VIII:C in the presence of VIII R:Ag, but not in the isolated VIII:CAg. Higher concentrations of alpha-chymotrypsin cause activation of VIII:C and a slight decrease in the VIII:CAg values in both preparations. These enzymes may play a modulating role in the coagulation cascade through the activation and degradation of VIII:C and VIII:CAg.