Association of alpha 2-macroglobulin-thrombin and alpha 2-macroglobulin-plasmin complexes with isolated hepatocytes

125I-labelled alpha 2-macroglobulin complexed with thrombin or plasmin bound to hepatocytes in a concentration- and time-dependent manner. The apparent Kd values calculated from displacement experiments were 7.9 X 10(-8) M for alpha 2-macroglobulin-thrombin and 8.5 X 10(-8) M for alpha 2-macroglobulin-plasmin. Association of these complexes was only partially reversible; after a 180 min incubation period, 50-60% of the bound radioactivity was internalized by the cells. alpha 2-Macroglobulin itself bound also to hepatocytes, but the affinity of the alpha 2-macroglobulin complexes was higher than that of the inhibitor alone, and alpha 2-macroglobulin was not internalized, either. 125I-labelled thrombin or plasmin bound to hepatocytes as well. These bindings were also concentration-dependent and could be decreased with an excess of unlabelled ligands. Binding rates and amounts of the bound proteinases were higher than those of their alpha 2-macroglobulin complexes. The alpha 2-macroglobulin-thrombin complex competed with the alpha 2-macroglobulin-plasmin complex in binding to hepatocytes, whereas there was no competition between these complexes and the antithrombin III-thrombin complex. These results suggest that the binding sites of hepatocytes for alpha 2-macroglobulin-proteinase and antithrombin III-proteinase complexes are different.