Intracellular dissociation of receptor-bound asialoglycoproteins in cultured hepatocytes. A pH-mediated nonlysosomal event.

The binding, internalization, and degradation of 125I-asialo-orosomucoid were studied in primary monolayer cultures of rat hepatocytes. Ligand entered the cell bound to the asialoglycoprotein receptor and subsequently dissociated from the receptor intracellularly. Rate coefficients for each of the transitions that constitute the endocytic pathway were computed. Subcellular fractionation on Percoll gradients revealed that prior to localization in lysosomes, 125I-asialo-orosomucoid resided in a fraction of slightly lower buoyant density than plasma membranes. Neither ammonium chloride (20 mM) nor leupeptin (0.1 mg/ml) affected ligand binding or internalization of prebound ligand. However, both reagents inhibited degradation of ligand by greater than 95%. Of the two, only ammonium chloride inhibited receptor-ligand dissociation. Ammonium chloride treatment resulted in the accumulation of ligand in the prelysosomal fraction. In contrast, exposure of cells to leupeptin led to accumulation of ligand within lysosomes. The results are interpreted in terms of pH-mediated dissociation of ligand-receptor complex within a nonlysosomal endocytic vesicle.