Rapid and cooperative binding of factor XII to human umbilical vein endothelial cells

When activated, factor XII (FXII) has been shown to play a role in a series of proteolytic cascades including systems as the fibrinolytic, the coagulation, the kallikrein–kinin and the complement. How FXII is activated in vivo remains poorly understood as the concentration and density of surface bound negative charges known to trigger the activation in vitro is far from sufficient in vivo. Specific binding of FXII to cellular receptors in the blood stream may, however, solve this problem which may be a question of inter molecular vicinity enhanced by binding to any surface. Here we report that the Zn2+-dependent binding of FXII to endothelial cells is rapid, saturable, specific and cooperative. Each endothelial cell from the human umbilical veins was found to bind (417 ± 202) × 103 molecules of FXII with a Kd of (65 ± 23) nM and a Hill coefficient of 2.1. The binding was inhibited by α-FXIIa but not by β-FXIIa. The Kd for binding α-FXIIa was (50 ± 27) nM. The rate of association was found to be 1.9 × 105m−1· min−1. A confirmed inhibition by HK increased the Kd without affecting the maximal number of binding sites and the Hill coefficient. The concentration of HK in serum did not prevent binding of FXII/FXIIa to cells incubated with serum supplemented with Zn2+. The optimal concentration of Zn2+ was 15 µm for binding factor XII/FXIIa whether purified or in serum.