*HUVECs were preincubated with 20 nmol/L HK for 1 hour at 37°C. Unbound HK was removed, HUVECs were incubated with 20 nmol/L PK for another 1 hour, and the cells were washed. The capacity of the cell-associated PK/HK complex to activate Pro-UK was determined by adding 0.6 mmol/L S2444, 20 nmol/L factor XII (FXII) where indicated, and variable concentrations of pro-urokinase (5 to 1,000 nmol/L). Hydrolysis was measured over 75 minutes at 37°C. Cell-associated urokinase activity was determined using a standard curve made with known amounts of soluble tcuPA. The data presented are the mean ± SD of three experiments.

The reactants added included high molecular weight kininogen (HK), prekallikrein (PK), and factor XII (FXII), as indicated.