Comparison of the APC cofactor activity of wild-type rPS and PS/FII chimeras in plasma clotting assays. (A) The APC cofactor activity of each PS/FII chimera was investigated in an FXa 1-stage clotting assay. Increasing concentrations of wild-type rPS (▪), Gla_FII-ΔTSR-PS (□), and Gla_FII-PS (♦) were incubated with APC (7.3 nM) in the presence of PC/PS/PE phospholipid vesicles, RVV-X, and PS-deficient plasma. Clotting was triggered by the addition of Ca2+ ions. For all the PS mutants tested in the study, mean basal clotting time in the presence of 7.3 nM APC was 112 ± 12 seconds. Data represent mean values (±SD) of 2 separate experiments. (B) APC cofactor activity of wild-type rPS (▪), Gla_FII-ΔTSR-PS (□), and Gla_FII-PS (♦) was assessed in an APTT assay, as described in “Materials and methods.” For all PS mutants used in the study, mean basal clotting time in the presence of APC alone was 77 ± 2 seconds. In both plasma clotting assays, the APC cofactor activity of the different PS species was evaluated by determining the following ratio: clotting time with APC + PS/clotting time with APC only.