APC-mediated inactivation of FVa-W1920R mutant. FV variants (8 nM) were incubated with thrombin (100 nM) for 5 minutes, and the reaction was terminated by the addition of hirudin (25 U/mL). FVa variant samples (2 nM) were reacted with APC (25 pM) and PL (20 μM) in the presence (A) or absence (B) of PS (30 nM). After dilution, FVa activity was measured in an aPTT-based 1-stage clotting assay. The symbols used are as follows: open circles, WT; closed circles, W1920R; open squares, R506Q. Initial activities of FVa variants were regarded as 100%. The plotted data were fitted in an equation of single exponential decay. All experiments were performed at least 3 separate times, and the average values are shown.