Effect of WT protein S and protein S E36A on TFPI cofactor activity through inhibition of FXa. Conversion of 200μM S-2765 by 0.5nM FXa was monitored by the absorbance at 405 nm in the presence (A) or absence (B) of 10μM phospholipids, 1nM TFPI, and 80nM WT protein S. The reaction was started on addition of FXa. WT protein S (C) and protein S E36A (D) were titrated (0-80nM) to the reaction containing 1nM TFPI (n = 3). Representative experiments are shown.