THE IMPORTANCE OF GLYCOSYLATION EVENTS IN THE RAT LIVER TO THE BIOSYNTHESIS, DISPOSITION AND FUNCTION OF SECRTETORY AND MEMBRANE GLYCOPROTEINS

Abstract

The role of dolichylsaccharide synthesis in mediating and regulating the production of functional secretory and plasma membrane glycoproteins by the liver of normal, partially hepatectomized or laparotomized rats was evaluated in vivo and in vitro. Dolichylsaccharide-mediated glycoprotein biosynthesis, when examined in vitro using liver slices incubated in the presence of 6-('3)H glucosamine, was found to be stiimulated two-fold in slices from laparotomized rats compared to either partially hepatectomized or control animals. The specific synthesis and appearance in the serum of ceruloplasmin (CPN), a prototypic secretory glycoprotein synthesized by the liver, was assessed in vivo by immunoprecipitation of ('3)H -leucine and ('14)C -glucosamine labeled protein and by its copper binding activity. Synthesis of functional ceruloplasmin was stimulated three-fold following laparotomy, but, in contrast, it was inhibited 45% after partial hepatectomy in comparison to steady-state controls. Thus, laparotomy caused generalized stimulation of glycoprotein biosynthesis and secretion in the rat liver, and was a useful model to study aspects of the regulation of these phenomena.;The dolichylsaccharides synthesis inhibitor, tunicamycin (TM), was used to limit the availability of dolichylsaccharides and, as a consequence, evaluate the importance of glycosylation to the production and proper disposition of functional CPN and hepatic binding protein (HBP), an asialoglycoprotein receptor on the hepatocyte internal and surface membranes. Following administration of a single dose of TM (50 ug/150-200g rat), dolichylsaccharide synthesis was not detectable after 3.5 hr and was restored to only 12% of control values after 6 days. The appearance of CPN-bound ('64)CU in the serum was sharply depressed (60-80%) within 3.5 hr and this inhibition was maintained for at least 3 days. Immunoprecipitation of CPN from the serum of TM-treated laparotomized rats injected with 4,5-('3)H leucine and U-('14)C glucosamine, revealed that ('3)H CPN peptide synthesis was depressed only 10-20%, whereas the glycosylation of the protein was totally inhibited 48 hr after antibiotic administration. Thus, while glycosylation was not required for CPN peptide synthesis nor secretion, it appeared to be critical for proper Cu binding. As a consequence, CPN oxidase activity, a functional measure of total CPN concentration in serum that is dependent on CPN-bound Cu, was also suppressed by about 40% within 48 hr after TM administration. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMI.