title: Xylosylation and glucuronosylation reactions in rat liver Golgi apparatus and endoplasmic reticulum.
abstract: We have studied in rat liver the subcellular sites and topography of xylosylation and galactosylation reactions occurring in the biosynthesis of the D-glucuronic acid-galactose-galactose-D-xylose linkage region of proteoglycans and of glucuronosylation reactions involved in both glycosaminoglycan biosynthesis and bile acid and bilirubin conjugation. The specific translocation rate of UDP-xylose into sealed, "right-side-out" vesicles from the Golgi apparatus was 2-5-fold higher than into sealed right-side-out vesicles from the rough endoplasmic reticulum (RER). Using the above vesicle preparations, we only detected endogenous acceptors for xylosylation in the Golgi apparatus-rich fraction. The specific activity of xylosyltransferase (using silk fibroin as exogenous acceptor) was 50-100-fold higher in Golgi apparatus membranes than in those from the RER. Previous studies had shown that UDP-galactose is translocated solely into vesicles from the Golgi apparatus. In these studies, we found the specific activity of galactosyltransferase I to be 40-140-fold higher in membranes from the Golgi apparatus than in those from the RER. The specific translocation rate of UDP-D-glucuronic acid into vesicles from the Golgi apparatus was 10-fold higher than into those from the RER, whereas the specific activity of glucuronosyltransferase (using chondroitin nonasaccharide as exogenous acceptor) was 12-30-fold higher in Golgi apparatus membranes than in those from the RER. Together, the above results strongly suggest that, in rat liver, the biosynthesis of the above-described proteoglycan linkage region occurs in the Golgi apparatus. The specific activity of glucuronosyltransferase, using bile acids and bilirubin as exogenous acceptor, was 10-25-fold higher in RER membranes than those from the Golgi apparatus. This suggests that transport of UDP-D-glucuronic acid into the RER lumen is not required for such reactions.
mesh_terms: D000818:Animals; D002236:Carbohydrate Conformation; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D005690:Galactose / Q000378:metabolism; D005965:Glucuronates / Q000378:metabolism*; D020723:Glucuronic Acid; D014453:Glucuronosyltransferase / Q000378:metabolism; D006056:Golgi Apparatus / Q000378:metabolism*; D007700:Kinetics; D008099:Liver / Q000648:ultrastructure*; D051381:Rats; D014531:Uridine Diphosphate Galactose / Q000378:metabolism; D014540:Uridine Diphosphate Xylose / Q000378:metabolism; D014994:Xylose / Q000378:metabolism*
chemical_list: D005965:Glucuronates; D014531:Uridine Diphosphate Galactose; D014540:Uridine Diphosphate Xylose; D020723:Glucuronic Acid; D014994:Xylose; D014453:Glucuronosyltransferase; D005690:Galactose