Retinol-binding protein and asialo-orosomucoid are taken up by different pathways in liver cells.

The intracellular transport and degradation of in vivo endocytosed retinol-binding protein was compared with that of asialo-orosomucoid, a marker for receptor-mediated endocytosis through coated pits. The transport pathways were studied in rat liver cells by means of subcellular fractionation in Nycodenz and sucrose density gradients and by immunoelectron microscopy. Retinol-binding protein and asialo-orosomucoid were 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 studies, as well as from immunoelectron microscopy, showed that retinol-binding protein and asialo-orosomucoid were initially localized in different endocytic vesicles. Retinol-binding protein co-localized in density gradients with markers for potocytosis, an alternative endocytic pathway which uses internalization through caveolae instead of clathrin-coated pits. Later, retinol-binding protein and asialo-orosomucoid comigrated in the gradients and they were also observed in the same larger vesicles by immunoelectron microscopy. These data suggest that retinol-binding protein is taken up by liver cells by potocytosis and that a fraction of the retinol-binding protein is later transferred to larger vesicles located deeper in the cytoplasm where degradation takes place.