[8] Chromatography of sugar nucleotides in morpholinium borate. Complex Carbohydrates. Elsevier; 1966. pp. 108-11. [DOI: 10.1016/0076-6879(66)08012-1]

UDPglucose-ceramide glucosyltransferase from porcine submaxillary glands is associated with the Golgi apparatus

Subcellular distribution of pig submaxillary gland UDPglucose-ceramide glucosyltransferase (EC 2.4.1.80), the enzyme which catalyses the first step during the sequential addition of carbohydrate moieties for ganglioside biosynthesis, was studied. The results presented strongly suggest that in pig submaxillary gland, the transfer of glucose on endogenous or exogenous ceramides takes place in the Golgi apparatus: the specific activity of UDPglucose-ceramide glucosyltransferase increased in parallel with the activity of a known marker of the Golgi apparatus, UDPgalactose-ovomucoid galactosyltransferase. The specific activity of the glucosyltransferase was 18-times higher in the purified Golgi membranes than in the postnuclear supernatant and the yield was over 30%. An apparent Km of 22 microM for UDPglucose and 54 microM for ceramides was determined. Maximal glucosylation of endogenous ceramides was achieved at pH 6.5 in the presence of NADH (1 mM) as inhibitor of pyrophosphatases and with Mn2+ (5 mM). It was found that the zwitterionic detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (Chaps) is an efficient activator for the glucosylation of exogenous ceramides.

Evidence for coupling between transport of UDP-glucose and its synthesis by membrane-bound pyrophosphorylase in Golgi apparatus of cat liver

Incubation of sealed vesicles of cat-liver Golgi apparatus with UDP[14C]glucose showed that the vesicles accumulated radioactivity. After Triton X-100 treatment or sonication of washed vesicles, soluble radiolabeled species were released and identified by paper chromatography as UDP[14C]glucose, [14C]glucose 1-phosphate and free glucose. In the incubation medium, UDPglucose was effectively protected by addition of dimercaptopropanol and UTP. Presence of glucose 1-phosphate and glucose within the vesicles most probably arose from luminal pyrophosphatase and phosphatase. A portion of the [14C]glucose moiety became covalently linked to endogenous acceptors. Uptake of UDPglucose was saturable and dependent on time and on the concentration of sugar nucleotide. Together, these results were consistent with a transport system for UDPglucose in Golgi vesicles. Furthermore, penetration rate was considerably higher with UDPglucose synthetized in situ from glucose 1-phosphate by membrane-bound pyrophosphorylase than from added UDPglucose: Vmax values were respectively 10 and 2 pmol/15 min per mg protein. This result allows the conclusion that a coupling between translocase and synthetase is involved in UDPglucose transport through Golgi apparatus membranes. The mechanism of this 'kinetic advantage' is discussed.

Sequential glycosylations of endogenous glycosphingolipids in hamster fibroblasts incubated with nucleotide sugars

Hamster fibroblasts, NIL 2cl cells, which were harvested from culture bottles by the use of chelating agents of divalent cations, incorporated [14C]galactose and N-acetyl[14C]galactosamine (GalnAc) from corresponding nucleotide 14-C-labeled sugars into endogenous glycosphingolipid acceptors. Incorporation of [14C]-GalNAc from UDP-[14C]GalNAc into endogenous acceptors to form tetra- and pentahexosylcermide with terminal GalNAc was enhanced 3.8-gold by preincubation of the cells with UDPglucose and UDPgalactose. In the whole cell reactions, sequential glycosylations of endogenous glycosphingolipids were observed by addition of whole cell reactions, sequential glycosylations of endogenous glycosphingolipids were observed by addition of appropriate nucleotide sugars to the reaction mixture: in a two step incubation experiment, mono-, di= and trihexosylceramides newly glycosylated from UDP-[14C]glucose and UDP-[14C] galactose in the first step of the whole cell reaction were effectively converted to hematoside by addition of CMP-N-[3H]acetylneuraminic acid in the second step. In the sequential glycosylation of endogenous glycolipids observed in the whole cell reaction, quantitative differences were found between cells harvested from confluent cultures and cells harvested from sparse cultures. Furthermore, a result was obtained which indicates the presence of two distinct pools of each glycosphingolipid in NIL cells. Glycolipids in the one pools represent newly glycosylated glycolipids and act as substrates for further glycosylations. On the other hand, precursor-product relationships do not exist among the main pools of each glycolipid.

Inhibition by 2-deoxy-D-glucose of synthesis of glycoprotein enzymes by protoplasts of Saccharomyces: relation to inhibition of sugar uptake and metabolism

The synthesis of the glycoprotein enzymes, invertase and acid phosphatase, by protoplasts of Saccharomyces mutant 1016, is inhibited by 2-deoxy-d-glucose (2-dG) after a 20- to 30-min lag period under conditions (external sugar to 2-dG ratio of 40:1) which cause only a slight decrease in total protein synthesis. Formation of one intracellular enzyme, alpha-glucosidase, is also sensitive, but production of another, alkaline phosphatase, is unaffected. A nonmetabolized glucose analogue, 6-deoxy-d-glucose, had no inhibitory effect. The total uptake of external fructose and maltose was decreased by 2-dG after a lag period of about the same duration as that before the inhibition of synthesis of enzymes or of mannan and glucan; during this time 2-dG was taken up by the protoplasts and accumulated primarily as 2-dG-6-phosphate (2-dG-6-P). Studies in vitro showed that 2-dG-6-P inhibits both yeast phosphoglucose isomerase and phosphomannose isomerase. The intracellular levels of the 6-phosphates of glucose, fructose, and mannose did not increase in the presence of 2-dG. We suggest that the high internal level of 2-dG-6-P blocks synthesis of the cell wall polysaccharides and glycoproteins in two ways. It directly inhibits the conversion of fructose-6-P to glucose-6-P and to mannose-6-P. At the same time, it restricts the transport of fructose and maltose into the cell; however, the continuing limited uptake of the sugars still provides sufficient energy for protein synthesis. The cessation of alpha-glucosidase synthesis is probably a result of depletion of the internal pool of maltose (the inducer). Our findings support the suggestion that restriction of synthesis of the carbohydrate moiety of glycoproteins reduces formation of the active enzyme.