The rat hepatic corticosteroid-binding globulin receptor: distinction from the asialoglycoprotein receptor

This investigation was undertaken to ascertain whether rat liver cells contained a receptor for corticosteroid-binding globulin (CBG) that could be differentiated clearly from the asialoglycoprotein receptor. To do this, [125I]CBG, [125I] asialo-CBG, and [125I]asialofetuin were used as probes to differentiate the binding activities of the two receptors. On hepatic membranes, CBG bound to a single set of sites with a Kd of 0.74 microM, asialofetuin bound to a single set of sites with a Kd of 0.018 microM, asialo-CBG bound to two sets of sites with Kd values of 0.004 and 1.4 microM and in the presence of 1 microM asialofetium, asialo-CBG bound to a single set of sites with a Kd of 0.53 microM, not different (P greater than 0.2) from the Kd of CBG. Cross-competition studies using the three 125I-labeled ligands and allowing each to compete with the three radioinert ligands indicated the existence of two separate receptors. Desialylation of hepatic membranes differentially affected the binding of CBG and asialofetuin. Finally, whole cells bound CBG specifically, but internalized it to only a minimal extent (less than 10%). This observation does not support a role for the CBG-receptor system in the entry of steroids into cells.