Copper and Zinc Ions Differentially Block Asialoglycoprotein Receptor-mediated Endocytosis in Isolated Rat Hepatocytes*

Asialoglycoprotein receptors on hepatocytes lose endocytic and ligand binding activity when hepatocytes are exposed to iron ions. Here, we report the effects of zinc and copper ions on the endocytic and ligand binding activity of asialoglycoprotein receptors on isolated rat hepatocytes. Treatment of cells at 37 °C for 2 h with ZnCl2 (0–220 μm) or CuCl2 (0–225 μm) reversibly blocked sustained endocytosis of 125I-asialoorosomucoid by up to 93% (t 1/2 = 62 min) and 99% (t 1/2 = 54 min), respectively. Cells remained viable during such treatments. Zinc- and copper-treated cells lost ∼50% of their surface asialoglycoprotein receptor ligand binding activity; zinc-treated cells accumulated inactive asialoglycoprotein receptors intracellularly, whereas copper-treated cells accumulated inactive receptors on their surfaces. Cells treated at 4 °C with metal did not lose surface asialoglycoprotein receptor activity. Exposure of cells to copper ions, but not to zinc ions, blocked internalization of prebound 125I-asialoorosomucoid, but degradation of internalized ligand and pinocytosis of the fluid-phase marker Lucifer Yellow were not blocked by metal treatment. Zinc ions reduced diferric transferrin binding and endocytosis on hepatocytes by ∼33%; copper ions had no inhibitory effects. These findings are the first demonstration of a specific inhibition of receptor-mediated endocytosis by non-iron transition metals.