Enhancement of the TFPIα-mediated inhibition of thrombin generation by free protein S compared with that of protein S bound to C4BP or the isolated C4BP β-chain. Thrombin generation was measured in protein S-depleted plasma supplemented with 50 µM phospholipids and 1 pM TF in the presence and absence of 1 nM TFPIα, and in the presence and absence of protein S (50 or 100 nM) which was either free (A-B) or prebound to saturating (100 or 200 nM) concentrations of the entire C4BP molecule (A) or the isolated C4BP β-chain (B). Representative experiments are shown (n = 4-5). (C-D) Normalized lag time (C) or peak thrombin (D) were plotted against protein S concentration. The lag time and peak height in the presence of TFPIα alone represent 100%. Results are expressed as mean ± SD (n = 4-5). PS, protein S.