Effect of surface on the loss of factor Va activity.

Protein S (100 nM) was incubated with 6 nM factor Va and the indicated surface before the addition of 13 nM APC. At timed intervals, samples were removed and assayed for residual factor Va activity. The amount of prothrombinase activity without APC was defined as one for each surface. The surfaces were lipid vesicles (▴), endothelial cells (○), and platelets (●).