Endothelial binding sites for heparin. Specificity and role in heparin neutralization

The specificity of endothelial binding sites for heparin was investigated with heparin fractions and fragments differing in their Mr, charge density and affinity for antithrombin III, as well as with heparinoids and other anionic polyelectrolytes (polystyrene sulphonates). The affinity for endothelial cells was estimated by determining I50 values in competition experiments with 125I-heparin. We found that affinity for endothelial cells increases as a function of Mr and charge density (degree of sulphation). Binding sites are not specific receptors for heparin. Other anionic polyelectrolytes, such as pentosan polysulphates and polystyrene sulphonates, competed with heparin for binding to endothelial cells. Fractions of standard heparin with high affinity for antithrombin III also had greater affinity for endothelium. However, these two properties of heparin (affinity for antithrombin III and affinity for endothelial cells) could be dissociated. Oversulphated heparins and oversulphated low-Mr heparin fragments had lower anticoagulant activity and higher affinity for endothelial cells than did their parent compounds. Synthetic pentasaccharides, bearing the minimal sequence for binding to antithrombin III, did not bind to endothelial cells. Binding to endothelial cells involved partial neutralization of heparin. Bound heparin exhibited only 5% and 7% of antifactor IIa and antifactor Xa specific activity, respectively. In the presence of 200 nM-antithrombin III, and in the absence of free heparin, a limited fraction (approx. 30%) of bound heparin was displaced from endothelial cells during a 1 h incubation period. These data suggested that a fraction of surface-bound heparin could represent a pool of anticoagulant.