The effect of increasing protein S concentrations on the APC cleavage at Arg306. Recombinant FV variants (0.8 nM) were activated with thrombin. APC (0.05 nM) was added to the reaction mixture containing protein S (0 to 100 nM) and 10:20:70 wt/wt/wt PS/PE/PC phospholipids (25 μM). The inactivation reactions were stopped after 10 minutes by dilution, and activity was measured as described in the legend to Figure 3. The values were normalized to the value of FVa activity at time zero for each reaction. FVa-Arg506Gln/Arg679Gln, □; FVa-Ile359Thr/Arg506Gln/Arg679Gln, ▴. Each data point represents the mean of 3 independent experiments performed in duplicate. Error bars represent ± SD.