Selective binding of heparins to human endothelial cells. Implications for pharmacokinetics

Cultured endothelial cells isolated from human umbilical vein bind heparin and heparin fragments. The binding capacity of endothelial cells for 35S-heparin was for 38% composed of high affinity binding sites (Kd = 11 X 10(-8) M) and for 62% of sites with much lower affinity (Kd = 14 X 10(-7) M). The affinity of unlabeled compounds for heparin binding sites was determined by competition with binding of 125I-heparin. I50 found for unlabeled heparin was 16 X 10(-8) M, which is in agreement with the Kd for binding of heparin to high affinity sites. PK 10169, a low molecular weight fragment of heparin, competed only at relatively high concentrations (I50 = 10(-5) M). Competition experiments with subfractions of PK 10169 showed that I50 was inversely correlated with molecular weight. Gelfiltration of 35S-heparin and 35S-PK 10169 before and after binding to endothelial cells demonstrated a selective binding of high molecular weight molecules from polydisperse heparin and PK 10169 preparations. Bound heparin and PK 10169 molecules were detached from the cell-surface by proteolytic treatment and tested for antifactor-Xa and antifactor-IIa activity. Released heparin is slightly more active in antifactor-Xa and antifactor-IIa activity than its parent preparation. Released PK 10169 was 4 fold more active in antifactor-Xa and 8 fold more active in antifactor-IIa assays than heparin.