The influence of prothrombin on the inactivation of factor Va and factor Va Leiden. Factor Va or factor Va Leiden (0.2 nmol/L) in TBS, 5 mmol/L CaCl₂, 5 mg/mL BSA, pH 7.4 were incubated for 30 minutes at room temperature with APC at different concentrations in the presence of the prothrombin concentrations indicated. The reactions contained 40 μg/mL PE/PS/PC with or without 72 nmol/L protein S. The APC concentration required for 50% factor Va inactivation (y-axis) was calculated from the APC concentration dependence curve of factor Va inactivation under each condition. The reaction mixtures were: •, factor Va, APC, and protein S; ○, factor Va, APC, and no protein S; ▾, factor Va Leiden, APC, and protein S; ▿, factor Va Leiden, APC, and no protein S.