Enhancement of TFPI in the inhibition of FXa by WT protein S and protein S/Gas6 chimeras I to III. FXa activity (0.5 nM) was followed in real time through cleavage of S-2765 (200 µM) at 405 mm in the presence of 25 μM phospholipids, the presence or absence of TFPI (2 nM), and in increasing concentrations (0-160 nM) of WT protein S (A), chimera I (B), chimera II (C), or chimera III (D). Results from representative experiments are shown (n = 3). (E) The initial velocity (V₀) was calculated and plotted against protein S concentration. Results are given as mean ± SD and are expressed as percentage of the V₀ in the presence of TFPI alone (n = 3). PS, protein S.