Prothrombin inhibits APC inactivation of factor Va more effectively than it inhibits APC/PTGla. Factor Va (0.2 nmol/L) was incubated for 30 minutes with APC (A and B) or APC/PTGla (C) at the concentrations indicated. Either PS/PC vesicles or PE/PS/PC vesicles were present at a final concentration of 20 μg/mL. Residual factor Va activity was assessed as described in Materials and Methods. (A) Compared with the absence of prothrombin (○) on PS/PC vesicles, physiological concentrations of prothrombin (1.4 μmol/L) shifted the APC concentration dependence of factor Va inactivation about 10-fold (◊). Compared with the absence of prothrombin (•) on PE/PS/PC vesicles, physiological prothrombin concentrations shifted this concentration dependence about 30-fold (⧫). (B) The above experimental conditions were used except that protein S (72 nmol/L) was present. The symbols are the same as in (A). (C) With APC/PTGla, addition of physiological concentrations of prothrombin had little effect on the enzyme concentration dependence of factor Va inactivation. The experiments were performed in the absence of protein S, as it does not influence chimera activity. The symbols are as described in (A).