Galactose‐Specific Endocytosis in Rat Liver

Galactosylated bovine serum albumin (galBSA) and its peroxidase‐conjugate (galBSA‐HRP) are rapidly cleared after intravenous injection and accumulate in rat liver. Competition experiments indicate that these galBSA derivatives are taken up by the galactose‐specific receptor. By cytochemistry on liver slices, galBSA‐HRP is found almost exclusively in hepatocytes, in similar structures as those reported for asialoorosomucoid. After differential centrifugation of liver homogenates, galactosylated ligands are concentrated in the light mitochondrial (L) and the microsomal (P) fractions. Together, these two fractions contain roughly 80% of the galBSA or of its peroxidase‐conjugate present in the homogenate 10 min after injection to the animals. By isopycnic centrifugation of L or combined LP fractions in sucrose gradients, ligand‐containing structures equilibrate at low densities (1.11–1.13 g/ml). Up to 52‐fold enrichment over the homogenate can be achieved for these structures, with a 14% yield. By isopycnic centrifugation, the distribution of ligand‐containing structures is clearly distinct from the bulk of plasma membrane (5′‐nucleotidase), endoplasmic reticulum (glucose‐6‐phosphatase) and lysosomes (cathepsin B), but considerable overlapping with galactosyltransferase, a marker for the Golgi complex, is observed. GalBSA‐HRP‐containing structures have been identified by ultrastructural cytochemistry. In low density fractions, labelled structures are vesicles or tubules heterogenous in size and shape and surrounded by a smooth membrane. Other components of such preparations are mostly Golgi and endoplasmic reticulum elements. Quantitative assessment of the purity in our best preparations of ligand‐containing structures leads to the inference that they are about 50% pure. It may nevertheless be concluded that they constitute a biochemically and morphologically distinct intracellular compartment. This compartment could be specialized in ligand‐receptor and ligand‐ligand sorting.