RETINOL-BINDING PROTEIN AND ASIALO-OROSOMUCOID ARE TAKEN UP BY DIFFERENT PATHWAYS IN LIVER-CELLS

The intracellular transport and degradation of in vivo endocytosed ret inol-binding protein was compared with that of asialo-orosomucoid, a m arker for receptor-mediated endocytosis through coated pits, The trans port pathways were studied in rat liver cells by means of subcellular fractionation in Nycodenz and sucrose density gradients and by immunoe lectron microscopy, Retinol-binding protein and asialo orosomucoid wer e labeled by covalent attachment of radioiodinated tyramine cellobiose , an adduct which is incapable of crossing cellular membranes and thus provides a marker for the organelles where the protein has been taken up and degraded. The data obtained from subcellular fractionation stu dies, as well as from immunoelectron microscopy, showed that retinol-b inding protein and asialo-orosomucoid were initially localized in diff erent endocytic vesicles, Retinol-binding protein co-localized in dens ity gradients with markers for potocytosis, an alternative endocytic p athway which uses internalization through caveolae instead of clathrin -coated pits. Later, retinol-binding protein and asialo-orosomucoid co migrated in the gradients and they were also observed in the same larg er vesicles by immunoelectron microscopy. These data suggest that reti nol-binding protein is taken up by Liver cells by potocytosis and that a fraction of the retinol-binding protein is later transferred to lar ger vesicles located deeper in the cytoplasm where degradation takes p lace.