APC blocks conversion of factor V to factor Va on the endothelial cell surface.

Conditions are similar to those for Figure 5. Endothelial cells were incubated with prothrombin, factor X, factor V, protein S, antithrombin III, and TFPI. Unactivated factor V at plasma concentrations was added rather than activated factor V. Accumulation and activation of factor V on the endothelial cells was detected by Western blotting. The plot shows the density of the unactivated factor V (●) and the heavy chain band (○). (A) Results in the absence of APC. (B) Results when 13 nM APC was included in the reaction mixture.