Comparison of the ability of prothrombin and factor IX to alter the APC concentration dependence of factor Va inactivation in the purified system. The APC concentration dependence of factor Va (0.2 nmol/L) inactivation was determined with varying concentrations of prothrombin or factor IX as indicated. PE/PS/PC vesicles (40 μg/mL) were used. The reactions were performed in TBS, 5 mmol/L CaCl₂, 5 mg/mL BSA, pH 7.4 at room temperature. Residual factor Va activity was determined using the prothrombinase assay described in Materials and Methods (•, with added factor IX; ○, with added prothrombin).