Characterization of plasminogen binding to human capillary and arterial endothelial cells

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Abstract
Phenotypic diversity of endothelial cells that line the various vascular spaces has been well established. However, it is not known if biochemical differences also exist, particularly in the numbers of receptors for plasma proteins. Equilibrium binding techniques were used to assess potential differences in the binding of 125I-labelled plasminogen to cultured human umbilical arterial endothelial cells and capillary endothelium, as compared with umbilical venous cells. The kinetic behaviour of plasminogen binding to all three types of cells was similar, with optimal binding occurring between 20 and 30 min of incubation. Binding of plasminogen to arterial, capillary, and venous cells was concentration dependent and reversible upon addition to excess unlabelled plasminogen. Scatchard analyses showed that artery, capillary, and venous endothelial cells all possess low affinity sites for plasminogen with Kd values of 0.30 ± 0.07, 0.40 ± 0.06, and 0.40 ± 0.08 μM, respectively. Vein cells also possess an additional higher affinity binding site with a Kd of 0.07 ± 0.01 μM, exhibiting a 6-fold greater affinity for plasminogen than the lower affinity sites on capillary and arterial endothelial cells. Assuming a stoichiometry of 1:1 for binding, the data indicate that arterial and capillary endothelial cells contain approximately 4.2 (± 0.9) × 106 and 4.1 (± 0.6) × 106 plasminogen receptors per cell. Venous cells contain both low and high density binding sites with 6.2 (± 0.8) × 106 and 12.4 (± 2.4) × 106 sites per endothelial cell. The presence of a higher affinity site on vein cells, but not on artery or capillary cells, may signal functional differences relating to fibrinolytic activity on the surface of these cells. Ligand blotting experiments, in which labelled plasminogen was adsorbed to polypeptides recovered from endothelial cell lysates, identified polypeptides of 46, 45, and 37 kDa, which may constitute the plasminogen-binding sites–receptors on endothelial cells.Key words: plasminogen, endothelial cells, receptors, fibrinolysis.

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