Specific association of thrombin-antithrombin complexes with a human hepatoma cell line

The clearance of thrombin-antithrombin (TAT) complexes from blood by the liver is through a receptor mediated pathway. We have used the established human hepatoma cell line, Hep G2, to determine if these hepatocytes have the capacity to bind this enzyme-inhibitor complex. The TAT complex was bound to the cells in a time and temperature dependent manner, reaching an apparent steady state at 90 minutes at both 4 and 37 degrees C. Binding at 4 degrees C was 5-7-fold less extensive than at 37 degrees C. The bound TAT was structurally similar to the added ligand. This interaction was specific, as it was inhibited by nonlabeled TAT but not by 50-fold molar excesses of unrelated proteins or by the individual constituents, thrombin or antithrombin. Factor Xa- antithrombin complex inhibited the binding reactions slightly. Specific binding isotherms at 37 degrees C were subjected to Scatchard plots. The apparent dissociation constant was 247 +/- 74 nM, and the number of TAT molecules bound per cell was 5.19 +/- 0.89 X 10(5). Bound TAT complexes did not undergo degradation at 4 or 37 degrees C for up to 2.5 hr, as greater than 85% of the bound ligand was acid precipitable during the time course of binding. Internalization of the TAT complex was compared with transferrin, a molecule known to be internalized by Hep G2 cells, by resistance of the cell-bound ligands to degradation by trypsin or pronase. In contrast to transferrin, most of the TAT complexes remained cell-surface associated for at least 2 hr at both 4 degrees and 37 degrees C, indicating that TAT was not substantially internalized by the Hep G2 cells.