Protein S enhancement of APC-mediated cleavage of FVa in Arg306. Protein S (0-120nM) in the presence of 0.5nM APC was incubated with 0.8nM FVa R506Q/R679Q in the presence of phospholipids for 10 minutes. The remaining FVa actvity was measured with a prothrombinase assay. Results are plotted as mean ± SD from 3 independent experiments performed in duplicate (A). A time course experiment was performed to calculate the apparent pseudo–first-order rate constants of WT protein S and protein S D95A. It is observed that approximately 6-fold more APC is needed in the presence of protein S D95A to obtain a similar amount of APC-mediated FVa R506Q/R679Q inactivation as with WT protein S (B).