We have developed two assay systems (amidolytic assay and platelet aggregation assay) for platelet factor X activator activity (Pl. FXAA) with purified coagulation factors and gel filtered platelets (GFP), and investigated the properties of Pl. FXAA. The present study shows the correlation of human factor VIII to Pl. FXAA. In platelet aggregation assay, GFP aggregation behavior was observed after addition of factor X, factor II and CaCl2. In amidolytic assay, the rate of factor Xa generation from factor X by GFP was determined with chromogenic substrate S-2222.
GFP from hemophiliacs were tested to Pl. FXAA assay. GFP from hemophilia B patients showed the similar aggregation behavior and amidolytic activity as control. On the other hand, GFP from hemophilia A patients showed low FXAA compared to control GFP, and that was significant in platelet aggregation assay. Furthermore, GFP prepared from factor VIII inhibitor patient or pretreated with factor VIII inhibitor plasma in vitro showed low FXAA, and the reduced extent was very marked in platelet aggregation assay and insignificant in amidolytic assay. While, both inhibitor-GFP and control-GFP showed the similar GFP aggregation after addition of factor Xa, factor II and CaCl2 in platelet aggregation assay. Furthermore, purified human factor VIII added in vitro accelerated dose-dependently Pl. FXAA in both assays.
These results suggest that factor VIII enhances factor Xa generation by GFP, and reduced Pl. FXAA in hemophilia A patients is due to the reduced membranebound factor VIII.
