Binding of Heparin and Low Molecular Weight Heparin Fragments to Human Vascular Endothelial Cells in Culture

The interaction of standard heparin and some low molecular weight heparin fragments (CY 222, mw 1,500-8,000 daltons) with human vascular endothelium in culture was studied using both 125I and 3H labeled ligands. A specific and saturable binding was shown for both labeled standard heparins. Two populations of binding sites for 3H-standard heparin could be distinguished: one of high affinity (KD = 0.12 microM), and another of lower affinity (KD = 1.37 microM). Total binding capacity was in the order of 10(7) molecules per cell. The same high level of affinity was calculated for unlabeled compounds from competition experiments with 125I-standard heparin. No other glycosaminoglycans, except a highly sulfated heparan (fraction IIA) could compete for heparin binding sites. A specific binding was also shown for 125I-CY 222. The affinity of unlabeled CY 222 was approximately ten times lower than that of unfractionated heparin. However, CY 222 could compete for approximatively 30% of standard heparin binding. Binding was not completely reversible. Even in the presence of a large excess of unlabeled compounds, a fraction of 25% of radioactive heparins remained bound to the endothelium. This fraction was three times lower if incubation was carried out at +4 degrees C, suggesting a possible incorporation of heparin into the endothelial cells.