The effect of glycoprotein-processing inhibitors on the secretion of glycoproteins by Madin-Darby canine kidney cells

The effects of various glycoprotein-processing inhibitors on the biosynthesis and secretion of N-linked glycoproteins was examined in cultured Madin-Darby canine kidney (MDCK) cells. Since incorporation of [2-3H]mannose into lipid-linked saccharides and into glycoproteins was much greater in phosphate-buffered saline (PBS) than in serum-supplemented basal medium (BME), most experiments were done in PBS. Castanospermine, an inhibitor of glucosidase I, caused the formation of glycoproteins having mostly Glc3Man7-9(GlcNAc)2 structures; deoxymannojirimycin, an inhibitor of mannosidase I, gave mostly glycoproteins with Man9(GlcNAc)2 structures; swainsonine, an inhibitor of mannosidase II, caused the accumulation of hybrid types of oligosaccharides. Castanospermine and swainsonine, either in PBS or in BME medium, had no effect on the incorporation of [2-3H]mannose or [5,6-3H]leucine into the secreted glycoproteins and, in fact, there was some increase in mannose incorporation in their presence. These inhibitors also did not affect mannose incorporation into cellular glycoproteins nor did they affect the biosynthesis as measured by mannose incorporation into lipid-linked saccharides. On the other hand in PBS medium, deoxymannojirimycin, at 25 micrograms/mL, caused a 75% inhibition in mannose incorporation into secreted glycoproteins, but had no effect on the incorporation of [3H]leucine into the secreted glycoproteins. Since deoxymannojirimycin also strongly inhibited mannose incorporation into lipid-linked oligosaccharides in PBS, the decreased amount of radioactivity in the secreted and cellular glycoproteins may reflect the formation of glycoproteins with fewer than normal numbers of oligosaccharide chains, owing to the low levels of oligosaccharide donor. However, in BME medium, there was only slight inhibition of mannose incorporation into lipid-linked saccharides and into cellular and secreted glycoproteins.     

High molecular weight forms of adrenocorticotropic hormone are glycoproteins.

Mouse pituitary tumor cells (AtT-20/D-16v) were incubated in medium containing [3H] glucosamine or [3H] mannose. By analyzing immunoprecipitates of cell extracts and culture medium it was shown that [3H] glucosamine and [3H] mannose were incorporated into all three high molecular weight forms of ACTH; label was not incorporated into Mr=4,500 ACTH (which is thought to be similar to the 39 amino acid polypeptide form of ACTH, alpha(1-39)). Based on sodium dodecyl sulfate-polyacrylamide gel electrophoresis the apparent molecular weights of these glycoprotein forms of ACTH were 31,000, 23,000, and 13,000. Gel filtration in 6 M guanidine HCl indicated that the molecular weights of these forms of ACTH were substantially lower; sodium dodecyl sulfate-polyacrylamide gel electrophoresis has often been found to overestimate the molecular weight of glycoproteins. A significant fraction of the high molecular weight ACTH in tumor cell extracts binds to columns of concanavalin A-agarose and can be eluted with 0.2 M alpha-methyl-D-mannopyranoside; porcine alpha(1-39) does not bind to concanavalin A-agarose. High molecular weight glycoprotein ACTH can be detected in extracts of mouse and bovine pituitary by using concavalin A affinity chromatography.     

Biosynthesis of prolyl hydroxylase: evidence for two separate dolichol-media pathways of glycosylation

Prolyl hydroxylase is a glycoprotein containing two nonidentical subunits, alpha and beta. The alpha subunit of prolyl hydroxylase isolated from 13-day-old chick embryos contains a single high mannose oligosaccharide having seven mannosyl residues. Two forms of alpha subunit have been shown to exist in enzyme purified from tendon cells of 17-day-old chick embryos, one of which (alpha) appears to be identical in molecular weight and carbohydrate content with the single alpha of enzyme from 13-day-old chick embryos, as well as another form (alpha') that contains two oligosaccharides, each containing eight mannosyl units [see Kedersha, N. L., Tkacz, J. S., & Berg, R. A. (1985) Biochemistry (preceding paper in this issue)]. Biosynthetic labeling studies were performed with chick tendon cells using [2-3H]mannose, [6-3H]glucosamine, [14C(U)]mannose, and [14C(U)]glucose. Analysis of the labeled products using polyacrylamide gel electrophoresis in sodium dodecyl sulfate showed that only the oligosaccharides on alpha' incorporated measurable mannose or glucosamine isotopes; however, both alpha subunits incorporated 14C amino acid mix and [14C(U)]glucose [metabolically converted to [14C(U)]mannose] under similar conditions. Pulse-chase labeling studies using 14C amino acid mix demonstrated that both glycosylated polypeptide chains alpha and alpha' were synthesized simultaneously and that no precursor product relationship between alpha and alpha' was apparent. In the presence of tunicamycin, neither alpha nor alpha' was detected; a single polypeptide of greater mobility appeared instead. Incubation of the cells with inhibitory concentrations of glucosamine partially depressed the glycosylation of alpha' but allowed the glycosylation of alpha.(ABSTRACT TRUNCATED AT 250 WORDS)     

Biosynthetic studies on mannolipids and mannoproteins of normal and vitamin A-depleted hamster livers.

The incorporation of [1-14C]mannose into hamster liver glycolipids and glycoproteins was studied in normal and vitamin A-depleted hamsters. Severly (25% weight loss) and mildly (no weight loss) deficient animals were compared to vitamin A-fed controls. The incorporation of [14C]mannose into glycolipids and glycoproteins decreased in mild and severe vitamin A deficiency by 63-90% compared to vitamin A-fed animals. These results were essentially the same whether expressed per g of wet liver, per DNA or per protein. The size of the pools of mannose, glucose and galactose and their specific radioactivity in liver were determined by gas-liquid chromatography of the boronates of the hexitols (Eisenberg, Jr, F. (1972) Methods Enzymol. XXVIIIB, 168-178) in normal and vitamin A-deficient conditions. It was found that the amount of free hexoses per g of liver was very similar in normal and vitamin A-deficient conditions. The specific radioactivities for mannose and glucose were greater in vitamin A deficiency, thus excluding the possibility that the observed severe decrease in glycopeptide and glycolipid synthesis is a reflection of a similar decrease in the specific radioactivity of the precursor pools. Quantitation of mannose in glycoprotein showed a 79% decrease in vitamin A deficiency. Specific radioactivity of mannose in glycoproteins, 20 min after injection of the label, was 187 dpm/mug of mannose in the normal and 48 kpm/mug of mannose in the vitamin A-deficient livers. It is concluded that vitamin A is necessary for the biosynthesis of liver mannose-containing glycoproteins and glycolipids.     

Effects of Inhibitors of Oligosaccharide Processing on P0Protein Synthesis and Incorporation into PNS Myelin

Four inhibitors of oligosaccharide processing were used to investigate their effects on the transport of PNS myelin glycoproteins through the secretory pathway, as well as to gain further insight into the structure of the oligosaccharide chains of the P0 and 19-kDa glycoproteins. Several different inhibitors of oligosaccharide processing were incubated with chopped peripheral nerves from young rats (21-24 days of age) and the uptake of 14C-amino acid and [3H]fucose or [3H]mannose was measured in P0 and the 19-kDa glycoprotein after separation of homogenate and myelin proteins on polyacrylamide gels. [3H]Mannose was not found as suitable as [3H]fucose as an oligosaccharide precursor because glucose used as an energy source profoundly inhibited the uptake of [3H]mannose. The substitution of pyruvate as an energy source, however, resulted in incomplete glycosylation, poor amino acid uptake, and truncated oligosaccharide chains. Endoglycosidase H cleaved approximately 50% of the P0 labeled with [3H]fucose and 14C-amino acid. The lower molecular weight protein resulting from endoglycosidase H cleavage contained approximately one-half the [3H]fucose label on the protein, whereas one-half remained on the oligosaccharide chain of the undegraded P0, indicating that at least one-half the P0 has a hybrid structure. Deoxynojirimycin, deoxymannojirimycin, and castanospermine inhibited incorporation of [3H]fucose into the oligosaccharide chains of P0 and the 19-kDa glycoprotein as predicted from their action in blocking various stages of trimming of high mannose structures before the addition of fucose. P0 synthesized in the presence of these inhibitors was cleaved to a greater extent by endoglycosidase H than the normal protein, indicating increased vulnerability to this enzyme with arrest of normal processing. Similar results were obtained for the 19-kDa glycoprotein. Both the incompletely processed P0 and the 19-kDa glycoprotein formed in the presence of these inhibitors appeared to be transported normally into myelin.     

Metabolism of 2-deoxy-2-fluoro-D-[3H]glucose and 2-deoxy-2-fluoro-D-[3H]mannose in yeast and chick-embryo cells.

2-Deoxy-2-fluoro-D-[3H]glucose and 2-deoxy-2-fluoro-D-[3H]mannose have been prepared by tritiation of the corresponding unlabeled 2-fluoro sugars. The tritiated 2-fluoro sugars are phosphorylated and activated by UTP and by GTP to yield UDP-2-deoxy-2-fluoro-D-[3H]glucose, UDP-2-deoxy-2-fluoro-D-[3H]mannose, GDP-2-deoxy-2-fluoro-D-[3H]glucose and GDP-2-deoxy-2-fluoro-D-[3H]mannose in both cell types. The nucleotide derivatives could also be labeled in the nucleotide moiety by feeding the cells with [14C]uridine or [14C]guanosine in the presence of unlabeled 2-fluoro sugar. No evidence was obtained for metabolic steps in which the six-carbon chain of 2-fluoro sugars was not preserved. No epimerisation of the label to 2-deoxy-2-fluoro-D-[3H]galactose could be observed by radioactive gas-liquid chromatography of the enzymatic cleavage products of the different 2-fluoro sugar metabolites isolated from either cell type. Yeast and chick embryo cells both incorporate 2-deoxy-2-fluoro-D-[3H]glucose and 2-deoxy-2-fluoro-D-[3H]mannose specifically into glycoproteins, although this incorporation is very low when compared to the incorporation of 2-deoxy-D-[3H]glucose.     

The effect of glycoprotein-processing inhibitors on fucosylation of glycoproteins

The influenza viral hemagglutinin contains L-fucose linked alpha 1,6 to some of the innermost GlcNAc residues of the complex oligosaccharides. In order to determine what structural features of the oligosaccharide were required for fucosylation or where in the processing pathway fucosylation occurred, influenza virus-infected MDCK cells were incubated in the presence of various inhibitors of glycoprotein processing to stop trimming at different points. After several hours of incubation with the inhibitors, [5,6-3H]fucose and [1-14C]mannose were added to label the glycoproteins, and cells were incubated in inhibitor and isotope for about 40 h to produce mature virus. Glycopeptides were prepared from the viral and the cellular glycoproteins, and these glycopeptides were isolated by gel filtration on Bio-Gel P-4. The glycopeptides were then digested with endo-beta-N-acetylglucosaminidase H and rechromatographed on the Bio-Gel column. In the presence of castanospermine or 2,5-dihydroxymethyl-3,4-dihydroxypyrrolidine, both inhibitors of glucosidase I, most of the radioactive mannose was found in Glc3Man7-9GlcNAc structures, and these did not contain radioactive fucose. In the presence of deoxymannojirimycin, an inhibitor of mannosidase I, most of the [14C]mannose was in a Man9GlcNAc structure which was also not fucosylated. However, in the presence of swainsonine, an inhibitor of mannosidase II, the [14C]mannose was mostly in hybrid types of oligosaccharides, and these structures also contained radioactive fucose. Treatment of the hybrid structures with endoglucosaminidase H released the [3H]fucose as a small peptide (Fuc-GlcNAc-peptide), whereas the [14C]mannose remained with the oligosaccharide. The data support the conclusion that the addition of fucose linked alpha 1,6 to the asparagine-linked GlcNAc is dependent upon the presence of a beta 1,2-GlcNAc residue on the alpha 1,3-mannose branch of the core structure.     

Detection of a glycosylated, incompletely folded form of chorionic gonadotropin beta subunit that is a precursor of hormone assembly in trophoblastic cells

The alpha and beta subunits of human chorionic gonadotropin are secreted both as a combined, noncovalently linked dimer form as well as uncombined, free forms by human trophoblastic cells. We have utilized the cultured choriocarcinoma cell line JAR to determine what regulates the combination of the two subunits. The human chorionic gonadotropin subunits produced by JAR cells were biosynthetically labeled with [35S] cysteine or [3H]mannose by a pulse-chase protocol, purified by immunoprecipitation with specific antisera that recognize free or combined subunits, and separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing or reducing conditions. Radioactively labeled bands were eluted from the gels and analyzed for total counts/minute incorporated, the ratio of free thiols to intramolecular cystine disulfides, and oligosaccharide composition. In some experiments, labeled gel bands were eluted with trypsin under nonreducing conditions, and the trypsin-released peptides were analyzed by high performance liquid chromatography. Using these procedures, the following results were obtained. The earliest, biosynthetically labeled form of the beta subunit detected in JAR cells contains high mannose N-linked oligosaccharides and has one-half of its incorporated cysteines present as free thiols. This form, termed pre-beta 1, has not yet combined with the alpha subunit even though the biosynthetically labeled alpha subunit is present in the cells at the same time. The pre-beta 1 form has a t1/2 of about 4 min and has a precursor-product relationship with a more completely disulfide-bonded form, termed pre-beta 2, which does combine with the alpha subunit to form a dimer. A subset of beta molecules produced in JAR cells does not attain the same disulfide bonding pattern as the pre-beta 2 form, does not combine with the alpha subunit, and is secreted as a free beta subunit into the culture medium. On the other hand, the earliest detectable form of the alpha subunit in JAR cells has all its thiols present as cystine disulfides, at a time when dimerization with the beta subunit has not yet taken place. These results strongly suggest that intramolecular disulfide bond formation in the beta subunit is the crucial and rate-limiting event in alpha beta dimer formation. The subset of beta molecules that remain free do not appear to form the appropriate intramolecular disulfides and thus do not achieve the correct conformation to combine with the alpha subunit.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effect of media composition on the induction of chorionic gonadotropin by sodium butyrate in HeLa cells

Summary Production of the glycoprotein hormone α-subunit by HeLa cells and its induction by sodium butyrate are dependent on the choice of culture medium. Under identical growth conditions it was found that subunit synthesis in the presence of butyrate was highest in RPMI 1640, lowest in Medium 199 (M199), and intermediate in minimum essential medium (MEM) and Waymouth's MB 752/1. Cell growth was similar in all media examined and was retarded in the presence of butyrate. Alkaline phosphatase activity was also lower in M199 than in RPMI 1640, although, in general, the magnitude of this difference was less than that for the hormone subunit. Incorporation of [1-¹⁴C]butyrate by HeLa cells was simimar in both M199 and RPMI 1640, indicating that uptake and metabolism of the fatty acid were not significantly different under these conditions. In the presence of 3 mM butyrate, mixtures of RPMI 1640 and M199 gave intermediate levels of α-subunit and alkaline phosphatase compared to each medium alone. Intracellular levels of α-subunit as well as that of the culture medium were reduced in M199 compared to RPMI 1640 indicating that synthesis rather than secretion was altered. This work was supported by Grant CA 21534 from the National Institutes of Health, Bethesda, MD.     

Glycosylation in vitro of Semliki-Forest-virus and influenza-virus glycoproteins and its suppression by nucleotide-2-deoxy-hexose

Cell-free enzyme preparations from cultured fibroblasts infected with Semliki forest virus or fowl plague virus (an influenza A virus) incorporate [14C]-mannose into dolichol-phosphate-mannose, lipid-linked oligosaccharides and into endogenous virus-specific glycoproteins. When GDP-2-deoxy-D-[14C]glucose serves as substrate 2-deoxy-D-[14C]glucose is transferred to dolichol phosphate yielding dolichol-monophosphate-2-deoxy-D-[14C]glucose. UDP-2-deoxy-D-[14C]glucose gives rise also to a lipid which, however, is not a polyprenol derivative. The transfer of [14C]mannose to lipid-extractable fractions and glycoproteins in vitro is blocked by GDP-2-deoxy-D-glucose. It can be restored by exogenous dolichol monophosphate only with regard to the formation of dolichol-monophosphate-[14C]mannose-labelled oligosaccharides into glycoproteins. UDP-2-deoxy-D-glucose has no inhibitory effect on transfer reactions of [14C]mannose from GDP-[14C]mannose into various lipid fractions or into glycoprotein. It is concluded therefore, that the inhibition of glycosylation brought about by 2-deoxyglucose in vivo is caused by an interference of its GDP derivative with the formation of a correct lipid-oligosaccharide.     

Differential processing and regulation of thyroid-stimulating hormone subunit carbohydrate chains in thyrotropic tumors and in normal and hypothyroid pituitaries

Thyroid-stimulating hormone (TSH) alpha- and beta-subunit glycosylation was investigated in mouse thyrotropic tumor and in normal and hypothyroid pituitary cells for various periods of time in the presence of [3H]mannose or [3H]galactose. After sequential precipitation with anti-alpha and anti-beta sera, subunits were treated with Pronase followed by endo-beta-N-acetylglucosaminidase H (Endo H) and analyzed by paper chromatography. In primary cultures of thyrotropic tumor cells incubated for 60 min with [3H]mannose, primarily Man9GlcNAc and Man8GlcNAc were found on TSH + alpha subunits, whereas Glc1Man9GlcNAc and Man9GlcNAc were prominent on free beta subunits. After preincubation of cells for 16 h in the presence or absence of glucose followed by a 60-min pulse of [3H]mannose, there was an 8-fold increase in labeled TSH + alpha but only a minimal change in free beta or total proteins. In the absence of glucose, there was a selective accumulation of Man8GlcNAc on TSH + alpha but not on free beta or total proteins; however, there was no detectable accumulation of Endo H resistant forms during glucose starvation on TSH subunits or total proteins. Normal mouse and rat pituitary minces incubated for 60 min with either [3H]mannose or [3H]galactose showed no glucose-containing species on TSH subunits, but equal amounts of Man9GlcNAc and Man8GlcNAc on TSH + alpha, and mostly Man9GlcNAc on free beta subunits. In contrast, hypothyroid mouse and rat pituitaries exhibited an increase in Glc1Man9NAc and Glc1Man8GlcNAc on free beta but not on TSH + alpha or total proteins.(ABSTRACT TRUNCATED AT 250 WORDS)     

Relationships among dolichyl phosphate, glycoprotein synthesis, and cell culture growth

Following treatment of Chinese hamster ovary cells with inhibitors of mevalonate biosynthesis in the presence of exogenous cholesterol, the cellular concentration of phosphorylated dolichol and the incorporation of [3H]mannose into dolichol-linked saccharides and N-linked glycoproteins declined coincident with a decline in DNA synthesis. Addition of mevalonate to the culture medium increased rates of mannose incorporation into lipid-linked saccharides and restored mannose incorporation into N-linked glycoproteins to control levels within 4 h. After an additional 4 h, synchronized DNA synthesis began. Inhibition of the synthesis of lipid-linked oligosaccharides and N-linked glycoproteins by tunicamycin prevented the induction of DNA synthesis by mevalonate, indicating that glycoprotein synthesis was required for cell division. The results suggest that the rate of cell culture growth may be influenced by the level of dolichyl phosphate acting to limit the synthesis of N-linked glycoproteins.     

The hormonal control of protein N-glycosylation in the developing rabbit mammary gland and its effect upon transferrin synthesis and secretion

Pregnant rabbit mammary gland explants cultured with insulin, prolactin and cortisol, synthesise and secrete transferrin radiolabelled with [3H]leucine or [3H]mannose. Omission of prolactin from the culture medium inhibited the incorporation of [3H]leucine into casein but not transferrin. Total transferrin secreted under these conditions was approx. 75% of the control (+prolactin) value measured by rocket immunoelectrophoresis. Little incorporation of [3H]mannose into transferrin was seen in the absence of prolactin suggesting a lack of glycosylation of the protein. Dual label experiments with [3H]mannose and [14C]leucine confirmed this. The decreased incorporation of [3H]mannose into dolichol linked intermediates suggests a general effect on protein N-glycosylation in the absence of prolactin. Thus, while the synthesis of the polypeptide backbone of transferrin does not require prolactin its glycosylation does.     

Direct utilization of mannose for mammalian glycoprotein biosynthesis

Direct utilization of mannose for glycoprotein biosynthesis has not been studied because cellular mannose is assumed to be derived entirely from glucose. However, animal sera contain sufficient mannose to force uptake through glucose-tolerant, mannose-specific transporters. Under physiological conditions this transport system provides 75% of the mannose for protein glycosylation in human hepatoma cells despite a 50- to 100-fold higher concentration of glucose. This suggests that direct use of mannose is more important than conversion from glucose. Consistent with this finding the liver is low in phosphomannose isomerase activity (fructose-6-P<->mannose-6-P), the key enzyme for supplying glucose-derived mannose to the N-glycosylation pathway. [2- 3H] Mannose is rapidly absorbed from the intestine of anesthetized rats and cleared from the blood with a t1/2of 30 min. After a 30 min lag, label is incorporated into plasma glycoproteins, and into glycoproteins of all organs during the first hour. Most (87%) of the initial incorporation occurs in the liver, but this decreases as radiolabeled plasma glycoproteins increase. Radiolabel in glycoproteins also increases 2- to 6-fold in other organs between 1-8 h, especially in lung, skeletal muscle, and heart. These organs may take up hepatic- derived radiolabeled plasma glycoproteins. Significantly, the brain, which is not exposed to plasma glycoproteins, shows essentially no increase in radiolabel. These results suggest that mammals use mannose transporters to deliver mannose from blood to the liver and other organs for glycoprotein biosynthesis. Additionally, contrary to expectations, most of the mannose for glycoprotein biosynthesis in cultured hepatoma cells is derived from mannose, not glucose. Extracellular mannose may also make a significant contribution to glycoprotein biosynthesis in the intact organism.      

Studies of mannose metabolism and effects of long-term mannose ingestion in the mouse.

Dietary mannose is used to treat glycosylation deficient patients with mutations in phosphomannose isomerase (PMI), but there is little information on mannose metabolism in model systems. We chose the mouse as a vertebrate model. Intravenous injection of [2-3H]mannose shows rapid equilibration with the extravascular pool and clearance t(1/2) of 28 min with 95% of the label catabolized via glycolysis in <2 h. Labeled glycoproteins appear in the plasma after 30 min and increase over 3 h. Various organs incorporate [2-3H]mannose into glycoproteins with similar kinetics, indicating direct transport and utilization. Liver and intestine incorporate most of the label (75%), and the majority of the liver-derived proteins eventually appear in plasma. [2-3H]Mannose-labeled liver and intestine organ cultures secrete the majority of their labeled proteins. We also studied the long-term effects of mannose supplementation in the drinking water. It did not cause bloating, diarrhea, abnormal behavior, weight gain or loss, or increase in hemoglobin glycation. Organ weights, histology, litter size, and growth of pups were normal. Water intake of mice given 20% mannose in their water was reduced to half compared to other groups. Mannose in blood increased up to 9-fold (from 100 to 900 microM) and mannose in milk up to 7-fold (from 75 to 500 microM). [2-3H]Mannose clearance, organ distribution, and uptake kinetics and hexose content of glycoproteins in organs were similar in mannose-supplemented and non-supplemented mice. Mannose supplements had little effect on the specific activity of phosphomannomutase (Man-6-P<-->Man-1-P) in different organs, but specific activity of PMI in brain, intestine, muscle, heart and lung gradually increased <2-fold with increasing mannose intake. Thus, long-term mannose supplementation does not appear to have adverse effects on mannose metabolism and mice safely tolerate increased mannose with no apparent ill effects.     

Metabolism of cartilage proteins in cultured tissue sections.

The asparagine-linked oligosaccharides of the complex acidic-type from [3H]mannose-, [3H]glucosamine- or [3H]galactose-labelled membrane glycoproteins of BHK21 cells and Rous-sarcoma virus were analysed by gel filtration combined with extensive digestion with endo- and exo-glycosidases from bacterial and eukaryotic sources. The neutral products from the digestion with a mixture of exoglycosidases and endo-beta-N-acetylglucosaminidase D from Diplococcus pneumoniae included a series of [3H]mannose- and [3H]glucosamine-labelled neutral oligosaccharides that were all converted by digestion with eukaryotic beta-N-acetylglucosaminidases into free N-acetylglucosamine and a small oligomannosyl core containing two alpha-linked mannose residues and a third mannose residue beta-linked to N-acetylglucosamine. These studies suggested that the complex acidic-type oligosaccharides from cellular and viral membrane glycoproteins contained a common oligomannosyl core region (Man2 alpha leads to Man beta leads to GlcNAc2), with heterogeneity in the number and/or linkage of outer branch N-acetylglucosamine residues resulting in partial resistance to beta-N-acetylglucosaminidase from a bacterial source.     

Lack of effect of butyrate on S phase DNA synthesis despite presence of histone hyperacetylation

The effects of sodium butyrate on [³H]thymidine incorporation and cell growth characteristics in randomly growing and synchronized HeLa S₃ cells have been examined in an attempt to determine what effects, if any, butyrate has on S phase cells. Whereas 5 mM sodium butyrate rapidly inhibits [⁵H]thymidine incorporation in a randomly growing cell populations, it has no effect on incorporation during the S phase in cells synchronized by double thymidine block techniques. This lack of effect does not result from an impaired ability of the S phase cells to take up butyrate, since butyrate administration during this period leads to histone hyperacetylation that is identical with that seen with butyrate treatment of randomly growing cells. Furthermore, the ability to induce such hyperacetylation with butyrate during an apparently normal progression through S phase indicates that histone hyperacetylation probably has no effect on the overall process of DNA replication. Temporal patterns of [³H]thymidine incorporation and cell growth following release from a 24-h exposure to butyrate confirm blockage of cell growth in the G1 phase of the cell cycle. Thus, the inhibition by butyrate of [³H]thymidine incorporation in randomly growing HeLa S₃ cell populations can be accounted for solely on the basis of a G1 phase block, with no inhibitory effects on cells already engaged in DNA synthesis or cells beyond the G1 phase block at the time of butyrate administration.     

Kinetic advantage for transport into hamster intestine of glucose generated from phlorizin by brush border beta-glucosidase

Phlorizin, labeled with tritium only in the glucose moiety, was used as substrate for the beta-glucosidase present in brush border membranes from hamster intestine in order to study, simultaneously, the kinetics of hydrolysis and the fate of the [3H]glucose liberated by the enzyme. The [3H]glucose seems to experience the same hydrolase related transport into the intestinal villi as the hexoses liberated from the common disaccharides byu their respective hydrolases. The released [3H]glucose accumulation rate is only partially inhibited by unlabelled glucose added to the medium as either the free sugar or as the precursors sucrose, lactose or glucose 1-phosphate, and then only when these sugars are present at very high levels. Furthermore, glucose oxidase, added to the medium as a glucose scavenger, has no effect on the uptake rate of the phlorizin hydrolase-liberated sugar. These and other findings are presented as evidence that, under conditions where the Na+-dependent glucose carrier is more than 97% inhibited by phlorizin, the glucose derived from the inhibitor, like the hexoses from disaccharides, has a kinetic advantage for transfer into the intestinal tissue.     

Glycoprotein synthesis in yeast. Identification of Man8GlcNAc2 as an essential intermediate in oligosaccharide processing

Synthesis of the N-linked oligosaccharides of Saccharomyces cerevisiae glycoproteins has been studied in vivo by labeling with [2-3H]mannose and gel filtration analysis of the products released by endoglycosidase H. Both small oligosaccharides, Man8-14GlcNAc, and larger products, Man greater than 20GlcNAc, were labeled. The kinetics of continuous and pulse-chase labeling demonstrated that Glc3Man9GlcNAc2, the initial product transferred to protein, was rapidly (t1/2 congruent to 3 min) trimmed to Man8GlcNAc2 and then more slowly (t1/2 = 10-20 min) elongated to larger oligosaccharides. No oligosaccharides smaller than Man8GlcNAc2 were evident with either labeling procedure. In confirmation of the trimming reaction observed in vivo, 3H-labeled Man9-N-acetylglucosaminitol from bovine thyroglobulin and [14C]Man9GlcNAc2 from yeast oligosaccharide-lipid were converted in vitro by broken yeast cells to 3H-labeled Man8-N-acetylglucosaminitol and [14C]Man8GlcNAc2. Man8GlcNAc and Man9GlcNAc from yeast invertase and from bovine thyroglobulin were purified by gel filtration and examined by high field 1H-NMR analysis. Invertase Man8GlcNAc (B) and Man9GlcNAc (C) were homogeneous compounds, which differed from the Man9GlcNAc (A) of thyroglobulin by the absence of a specific terminal alpha 1,2-linked mannose residue. The Man9GlcNAc of invertase (C) had an additional terminal alpha 1,6-linked mannose and appeared identical in structure with that isolated from yeast containing the mnn1 and mnn2 mutations (Cohen, R. E., Zhang, W.-j., and Ballou, C. E. (1982) J. Biol. Chem. 257, 5730-5737). It is concluded that Man8GlcNAc2, formed by removal of glucose and a single mannose from Glc3Man9GlcNAc2, is the ultimate product of trimming and the minimal precursor for elongation of the oligosaccharides on yeast glycoproteins. The results suggest that removal of a particular terminal alpha 1,2-linked mannose from Man9GlcNAc2 by a highly specific alpha-mannosidase exposes the nascent Man-alpha 1,6-Man backbone for elongation with additional alpha 1,6-linked mannose residues, according to the following scheme: (formula, see text).