The binding of high molecular weight kininogen to cultured human endothelial cells

Binding of 125I-labeled high molecular weight kininogen (high Mr kininogen) to cultured human endothelial cells derived from the umbilical cord was demonstrated. The binding was time-dependent, specific, and saturable and required the presence of zinc ions. Maximal binding of 125I-labeled high Mr kininogen was observed at 50 microM Zn2+. Calcium ions inhibited the Zn2+-dependent binding of 125I-labeled high Mr kininogen to endothelial cells. In the presence of 3 mM CaCl2 the total binding of 125I-labeled high Mr kininogen was significantly decreased, and a concentration of 100 microM Zn2+ was then required for the binding of 125I-labeled high Mr kininogen to the cells. Higher calcium concentrations did not further decrease the binding of 125I-labeled high Mr kininogen. Analysis of the binding data by the LIGAND computer program indicated 3.2 X 10(6) binding sites per cell for high Mr kininogen with an apparent Kd of 35 nM at 50 microM ZnCl2 and 1 mM CaCl2. Binding of high Mr kininogen also occurred at physiological plasma Zn2+ concentrations. Saturation of the high Mr kininogen binding sites at 25 microM ZnCl2 occurred at 80 nM of high Mr kininogen with 2.8 X 10(6) molecules of high Mr kininogen bound per cell. At 10 microM ZnCl2, the high Mr kininogen binding sites appeared to be saturated at 130 nM with 1.6 X 10(6) molecules bound per cell. In addition it was demonstrated that endothelial cells internalize 125I-labeled high Mr kininogen, since 125I-labeled high Mr kininogen was detected in solubilized endothelial cells after the cell-bound 125I-labeled high Mr kininogen had been removed with dextran sulfate.