Enhancement of TFPI inhibition of FXa by protein S–, C4BP-, and C4BP-bound protein S. 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 protein S (A), 0 to 320 nM C4BP (B), and C4BP-bound protein S in a 2:1 ratio (C). Representative experiments are shown (n = 3). (D) 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.