STRUCTURE AND FUNCTION STUDIES ON THE GOLGI APPARATUS AND GERL OF RAT LIVER

Abstract

A rapid procedure has been developed to isolate from rat liver homogenates a subcellular fraction that is highly enriched in Golgi and GERL, and in which the spatial relations between Golgi elements and GERL are retained. This fraction equilibrates during centrifugation at a density of 1.10-1.14 in sucrose and as a single band in colloidal silica-PVP gradients. It contains 1.2-1.4 mg/gm liver and represents 0.75-1.0% of the protein in the homogenate. Golgi and GERL elements retained their mutally exclusive staining for thiamine pyrophosphatase and acid phosphatase, respectively. Biochemical analysis of this fraction revealed approximately 100-fold enrichment, over the homogenate, for UDP-galactose:N-acetylglucosamine galactosyltransferase. Acid phosphatase activity, with (beta)-glycerolphosphate as substrate, was enriched approximately 6-fold over the homogenate.;The hepatic clearance of asialoglycoproteins from the circulation was investigated using asialoglycoproteins conjugated to cytochemically-demonstrable markers. These conjugates were recognized by their glycoprotein moieties and were used to delineate cytochemically the pathways of receptor-mediated endocytosis in hepatocytes. Subsequent to binding at the sinusoidal surface to a variety of structures, including coated pits, the conjugates were translocated to GERL elements, at the pericanalicular poles of the cell. Lactosaminated ferritin an electron-dense neoglycoprotein was also used to delineate this pathway. It demonstrated the existence of an extensive lysosomal system within the hepatocyte.;The intimate structural association of the Golgi apparatus and GERL, even when subcellularly isolated, suggests a possible functional relationship, the nature of which is unknown. The observations reveal an extensive lysosomal system that has heretofore been appreciated only in macrophages and in livers of beige mice, an animal homologue of the Chediak-Higashi syndrome.