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PubMed1995

Functional expression and subcellular localization of a high-Km hexose transporter from Leishmania donovani.

We have used expression in Xenopus oocytes to characterize a new hexose transporter from the parasitic protozoan Leishmania donovani. This transporter utilizes the hexoses glucose, fructose, and mannose as substrates. A substrate saturation curve for 2-deoxy-D-glucose reveals a very high Km, estimated to be approximately 150 mM. Immunolocalization of the protein with an antibody directed against the COOH terminus indicates that the transporter is present primarily in the parasite plasma membrane but is not detectable in the flagellar membrane. Since this protein is expressed in the insect stage promastigotes but not in the intracellular amastigotes, it may be specialized to function following an insect sugar meal when the concentrations of sugars surrounding the parasite are high.

Animals↗

PubMed1984

Human erythrocyte hexose transporter activity is governed by bilayer lipid composition in reconstituted vesicles.

Purified membrane protein band 4.5 (a sugar transport protein) from human erythrocytes was reconstituted by reverse-phase evaporation into a variety of bilayers formed from the synthetic lecithins. The number of proteins reconstituted was estimated by determination of D-glucose-sensitive cytochalasin B binding sites. D-Glucose transport activity in reconstituted vesicles was assessed by monitoring cytochalasin B sensitive D-glucose fluxes using microturbidimetric analysis. The major points are as follows: The sugar transport activity of the reconstituted system is directly proportional to the number of cytochalasin B binding sites reconstituted. The ratio of cytochalasin B binding sites per band 4.5 protein is 0.8. These data suggest that the functional protein unit is a monomer. Inhibitor studies of reconstituted hexose transfer support the notion that the kinetics of reconstituted transport are intrinsically symmetric. The turnover number for transport is not consistent with transport proceeding via water-filled pores. The absolute activity of the reconstituted system (Vmax per reconstituted cytochalasin B binding protein) is governed by the bulk lipid composition of the synthetic membrane. At temperatures where bilayers formed from dimyristoyl- or dipalmitoyllecithin (DML and DPL, respectively) are "crystalline", hexose transport activity is not observed. Over the same temperature range, however, crystalline bilayers formed from the longer chain lecithins, distearoyl-, diarachidonoyl-, and dielaidoyllecithin (DSL, DAL, and DEL, respectively), support significant protein-mediated transport activity. In a given synthetic membrane, the bilayer transition from the liquid-crystalline to the fluid state results in increased protein-mediated sugar transport activity. In the one synthetic membrane (DEL) in which the activation energy (Ea) for transport could be measured both above and below the bilayer phase transition, Ea was unaffected by the phase change. Ea and the Arrhenius constant (A) for transport are dependent on lecithin acyl chain length and saturation. For both parameters, the order of increase is DML less than DPL = dipalmitoleoyllecithin (DPOL) less than DSL less than DAL less than DEL = dioleoyllecithin (DOL). This means that at 59-60 degrees C, the order of catalytic activity follows the lipid sequence DML less than DPOL less than DAL less than DOL much less than DPL = DEL less than DSL. Cholesterol (48 mol%) restores protein-mediated transport activity to crystalline DPL bilayers and reduces the activity supported by fluid DPL bilayers. This effect is not simply related to the effects of cholesterol on the bilayer partial specific volume. T

Binding Sites↗

PubMed1977

Purification and initial rate kinetics of acyl-phosphate-hexose phosphotransferase from Aerobacter aerogenes.

The enzyme acyl-phosphate-hexose phosphotransferase from Aerobacter aerogenes was purified to electrophoretic homogeneity. The molecular weight of the enzyme as determined on Sephadex gels is 150 000. The enzyme possesses potent phosphotransferase and phosphohydrolase activities. Initial rate kinetics were used to investigate the mechanism of acyl-phosphate-hexose phosphotransferase. These studies, which involved a number of different phosphoryl donors and substrate analogues, suggest that the kinetic mechanism is of the rapid equilibrium random Bi Bi type. A number of other enzymes that exhibit both transferase and hydrolase activities involve obligatory covalent enzyme-substrate intermediates in their mechanisms of action.

Enterobacter↗

PubMed2007

Degradation of the hexose transporter Hxt5p in Saccharomyces cerevisiae.

BACKGROUND INFORMATION: Hxt5p is a member of a multigene family of hexose transporter proteins which translocate glucose across the plasma membrane of the yeast Saccharomyces cerevisiae. In contrast with other major hexose transporters of this family, Hxt5p expression is regulated by the growth rate of the cells and not by the external glucose concentration. Furthermore, Hxt5p is the only glucose transporter expressed during stationary phase. These observations suggest a different role for Hxt5p in S. cerevisiae. Therefore we studied the metabolism and localization of Hxt5p in more detail. RESULTS AND CONCLUSIONS: Inhibition of HXT5 expression in stationary-phase cells by the addition of glucose, which increases the growth rate, led to a decrease in the amount of Hxt5 protein within a few hours. Addition of glucose to stationary-phase cells resulted in a transient phosphorylation of Hxt5p on serine residues, but no ubiquitination was detected. The decrease in Hxt5p levels is caused by internalization of the protein, as observed by immunofluorescence microscopy. In stationary-phase cells, Hxt5p was localized predominantly at the cell periphery and upon addition of glucose to the cells the protein translocated to the cell interior. Electron microscopy demonstrated that the internalized Hxt5p-HA (haemagglutinin) protein was localized to small vesicles, multivesicular bodies and the vacuole. These results suggest that internalization and degradation of Hxt5p in the vacuole occur in an ubiquitination-independent manner via the endocytic pathway.

Monosaccharide Transport Proteins↗

PubMed1974

Hexose phosphate synthase from Methylcoccus capsulatus makes D-arabino-3-hexulose phosphate.

The product of the reaction catalysed by hexose phosphate synthase prepared from Methylococcus capsulatus was dephosphorylated and the sugar moiety purified. The sugar and derivatives were compared by various chromatographic and other methods with authentic samples of allulose (psicose), d-erythro-l-glycero-3-hexulose and d-erythro-d-glycero-3-hexulose. The sugar is not allulose, as was previously thought on the basis of less extensive evidence (Kemp & Quayle, 1966), but is in fact d-erythro-l-glycero-3-hexulose (d-arabino-3-hexulose). This identification is consistent with recent studies which have shown that hexose phosphate synthase catalyses the condensation of formaldehyde with d-ribulose 5-phosphate rather than with d-ribose 5-phosphate (Kemp, 1972).

Aldehyde-Lyases↗

PubMed1974

Hexose phosphate synthase in trimethylamine-grown bacterium 2B2, a facultative methylotroph.

Hexose phosphate synthase and hexulose phosphate isomerase activities were found in trimethylamine-grown bacterium 2B2, a facultative methylotroph. The activities were separated by column chromatography of cell extracts on DEAE-cellulose. Hexulose phosphate isomerase activity was measured spectrophotometrically by using the product of the hexose phosphate synthase reaction as substrate.

Bacteria↗

PubMed1982

Lipid peroxidation and haemoglobin degradation in red blood cells exposed to t-butyl hydroperoxide. Effects of the hexose monophosphate shunt as mediated by glutathione and ascorbate.

Lipid peroxidation and haemoglobin degradation were the two extremes of a spectrum of oxidative damage in red cells exposed to t-butyl hydroperoxide. The exact position in this spectrum depended on the availability of glucose and the ligand state of haemoglobin. In red cells containing oxy- or carbonmono-oxy-haemoglobin, hexose monophosphate-shunt activity was mainly responsible for metabolism of t-butyl hydroperoxide; haem groups were the main scavengers in red cells containing methaemoglobin. Glutathione, via glutathione peroxidase, accounted for nearly all of the hydroperoxide metabolizing activity of the hexose monophosphate shunt. Glucose protection against lipid peroxidation was almost entirely mediated by glutathione, whereas glucose protection of haemoglobin was only partly mediated by glutathione. Physiological concentrations of intracellular or extracellular ascorbate had no effect on consumption of t-butyl hydroperoxide or oxidation of haemoglobin. Ascorbate was mainly involved in scavenging chain-propagating species involved in lipid peroxidation. The protective effect of intracellular ascorbate against lipid peroxidation was about 100% glucose-dependent and about 50% glutathione-dependent. Extracellular ascorbate functioned largely without a requirement for glucose metabolism, although some synergistic effects between extracellular ascorbate and glutathione were observed. Lipid peroxidation was not dependent on the rate or completion of t-butyl hydroperoxide consumption but rather on the route of consumption. Lipid peroxidation appears to depend on the balance between the presence of initiators of lipid peroxidation (oxyhaemoglobin and low concentrations of methaemoglobin) and terminators of lipid peroxidation (glutathione, ascorbate, high concentrations of methaemoglobin).

Ascorbic Acid↗

PubMed1994

Presence and differential expression of SGLT1, GLUT1, GLUT2, GLUT3 and GLUT5 hexose-transporter mRNAs in Caco-2 cell clones in relation to cell growth and glucose consumption.

Seven clones from the Caco-2 cell line, three isolated from passage 29 (PD7, PD10, PF11) and four from passage 198 (TB10, TC7, TF3, TG6), all of them selected on the basis of differences in the levels of expression of sucrase-isomaltase and rates of glucose consumption, were analysed for the expression of hexose-transporter mRNAs (SGLT1, GLUT1-GLUT5) in relation to the phases of cell growth and the associated variations of the rates of glucose consumption. All clones showed a similar pattern of evolution of the rates of glucose consumption, which decreased from the exponential to the late-stationary phase, but differed, in a 1-40-fold range, in the values observed at late postconfluency. According to these values, clones could be divided into high- (PD10, PF11) and low-glucose-consuming cells (PD7, TB10, TC7, TF3 and TG6). GLUT1 and GLUT3 mRNAs were expressed in all clones and showed a similar pattern of evolution: their level decreased, from the exponential to the stationary phase, in close correlation with the decrease in rates of glucose consumption, with only high-glucose-consuming clones maintaining high levels in the stationary phase. In contrast, SGLT1, GLUT2 and GLUT5 mRNAs were only expressed, like sucrase-isomaltase mRNA, in the low-glucose-consuming clones, and their level increased from the exponential to the stationary phase, in parallel with the differentiation of the cells. GLUT4 was undetectable in all the clones. Glucose deprivation generally resulted in a discrete decrease in the levels of all transporter mRNAs in all clones, one exception being GLUT2, which in the high-glucose-consuming clones is only detectable when the cells are grown in low glucose. These clones should be ideal tools with which to study in vitro, at the single-cell level, how these transporters concur to the utilization and transport of hexoses and how their exclusive or co-ordinated expression is regulated.

Carrier Proteins↗

PubMed2003

Structural and serological studies on a new 4-deoxy-d-arabino-hexose-containing O-specific polysaccharide from the lipopolysaccharide of Citrobacter braakii PCM 1531 (serogroup O6).

The O-specific polysaccharide of Citrobacter braakii PCM 1531 (serogroup O6) was isolated by mild acid hydrolysis of the lipopolysaccharide (LPS) and found to contain d-fucose, l-rhamnose, 4-deoxy-d-arabino-hexose and O-acetyl groups in molar ratios 2 : 1 : 1 : 1. On the basis of methylation analysis and 1H and 13C NMR spectroscopy data, the structure of the branched tetrasaccharide repeating unit of the O-specific polysaccharide was established. Using various serological assays, it was demonstrated that the LPS of strain PCM 1531 is not related serologically to other known 4-deoxy-d-arabino-hexose-containing LPS from Citrobacter PCM 1487 (serogroup O5) or C. youngae PCM 1488 (serogroup O36). Two other strains of Citrobacter, PCM 1504 and PCM 1505, which, together with strain PCM 1531, have been classified in serogroup O6, were shown to be serologically distinct from strain PCM 1531 and should be reclassified into another serogroup.

Animals↗

PubMed2005

The metabolisable hexoses D-glucose and D-mannose enhance the expression of IRS-2 but not of IRS-1 in pancreatic beta-cells.

D-glucose regulates maintenance and function of pancreatic beta-cells. Several studies have shown that IRS-2, but not IRS-1, is necessary to maintain and sufficient to expand functional beta-cell mass. We therefore analyzed the expression of IRS-2 and IRS-1 in beta-cells after culture in the presence of various concentrations of D-glucose and other metabolisable or non-metabolisable hexoses. D-glucose increased Irs-2 transcription and IRS-2 accumulation in a dose-dependent manner (1.6 to 25 mmol/l), with a 3-fold increased plateau after 10 h. In contrast, the expression of IRS-1 remained unaffected. D-glucose also induced phosphorylation of IRS-2 while non-metabolisable hexoses did neither affect expression nor phosphorylation. D-glucose-mediated elevation and phosphorylation of IRS-2 were independent of autocrine insulin action although insulin itself could transiently and slightly enhance IRS-2 expression.

Animals↗

PubMed1986

Hexose transport control in a fibroblast metabolic mutant can be promoted more effectively by D-allose than by glucose.

By studying the energy-requiring control of the hexose transport system (the transport "curb") in a lung fibroblast mutant called the phosphoglucose isomerase mutant (because it is devoid of the enzyme phosphoglucose isomerase) the following features were noted. The aldohexose D-allose, if added over 20 hr to a culture of the mutant, promotes the development of an intense curb of the hexose transport system, greatly surpassing that brought about by incubation with glucose. The allose-mediated curb can be circumvented by various metabolic inhibitors as well as by the presence of other aldohexoses such as mannose.

Animals↗

PubMed1989

Concerted hexose transport curb by tunicamycin is rendered irreversible by glucose or allose in medium containing L-glutamine.

The hexose transport in a hamster fibroblast mutant (DS7), unable to use glucose for generation of energy, is nevertheless subject to a marked down-regulation ("curb") after prolonged incubation of monolayer cultures with glucose; fructose is unable to exert any curb. D-Allose, an all-cis hexose, mediates a vigorous curb of the transport system. Moreover, prolonged coincubation of glucose or allose with tunicamycin (TM) brings about an additional effect that is not an inhibition of the transport system, which we shall call the "concerted" transport curb. This type of concerted transport curb requires L-glutamine in the maintenance medium; moreover, addition of cycloheximide prevents the development of this TM effect. Apparently, cellular protein synthesis or protein turnover or both are required for the development of the TM-concerted transport curb. The concerted transport curb can be reversed in sugar-free or in fructose-containing medium, even upon readdition of TM. In contrast, the sole readdition of glucose or D-allose renders the concerted curb irreversible. This raises the question of whether the cells under the condition of the concerted curb somehow have internalized the TM into the membrane.

Animals↗

PubMed1993

An unusual active hexose transport system in human and mouse preimplantation embryos.

In a metabolic study of human and mouse preimplantation embryos (preembryos), we measured glucose uptake and phosphorylation with nonradioactive 2-deoxyglucose (DG) as tracer. Initial experiments indicated an active hexose transport capacity, a property thought to be restricted in mammals to intestinal villi and kidney tubules [Baly, D. L. & Horuk, R. (1988) Biochim. Biophys. Acta 947, 571-590]. Significant findings are as follows: (i) During a 60-min incubation with a low level of DG, mouse blastocyst DG rose to levels up to 30 times that of the medium. (The intestinal active system does not transport DG [Crane, R. K. (1960) Physiol. Rev. 40, 789-825].) (ii) Active preembryo transport was not blocked (as it would have been in the intestine) by phlorizin [Alvarado, F. & Crane, R. K. (1962) Biochem. Biophys. Acta 56, 170-172 and Sacktor, B. (1989) Kidney Int. 36, 342-350] or by replacement of Na+ with choline+ or K+ [Crane (1960) and Sacktor (1989)]. (iii) Transport of DG was blocked by cytochalasin B (which is not true for the intestinal transporter). We conclude that a distinct active hexose transporter and at least one facilitated transporter are present in preembryos, perhaps appearing in tandem on different membranes during formation of the increasingly complex preembryo structure.

Animals↗

PubMed2005

Hexose-6-phosphate dehydrogenase knock-out mice lack 11 beta-hydroxysteroid dehydrogenase type 1-mediated glucocorticoid generation.

The local generation of active glucocorticoid by NADPH-dependent, 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) oxoreductase activity, has emerged as an important factor in regulating hepatic glucose output and visceral adiposity. We have proposed that this NADPH is generated within the endoplasmic reticulum by the enzyme hexose-6-phosphate dehydrogenase. To address this hypothesis, we generated mice with a targeted inactivation of the H6PD gene. These mice were unable to convert 11-dehydrocorticosterone (11-DHC) to corticosterone but demonstrated increased corticosterone to 11-DHC conversion consistent with lack of 11beta-HSD1 oxoreductase and a concomitant increase in dehydrogenase activity. This increased corticosterone clearance in the knock-out mice resulted in a reduction in circulating corticosterone levels. Our studies define the critical requirement of hexose-6-phosphate dehydrogenase for 11beta-HSD1 oxoreductase activity and add a new dimension to the investigation of 11beta-HSD1 as a therapeutic target in patients with the metabolic syndrome.

11-beta-Hydroxysteroid Dehydrogenase Type 1↗

PubMed2001

In wounded sugar beet (Beta vulgaris L.) tap-root, hexose accumulation correlates with the induction of a vacuolar invertase isoform.

Wounding of sugar beet tap-root causes an induction of invertase activity, which contributes to post-harvest sucrose losses. In this first comprehensive monitoring of wound-induced invertase mRNAs, proteins, enzyme activities, and tissue hexose concentrations, the VI isoform responsible for wound-induced hexose accumulation in mature tap-root could be identified.

Adaptation, Physiological↗

PubMed1977

Glycolytic, hexose monophosphate shunt and bactericidal activities of leukocytes in ascorbic acid deficient guinea pigs.

It is well known that glycolytic and hexose monophosphate shunt activities of leukocytes increase during phagocytosis. The relevance of these metabolic changes to particle uptake and particle destruction is also well established. In the present study, these metabolic activities were studied to assess the phagocytic function of leukocytes isolated from ascorbic acid deficient guinea pigs. Glycolytic activity which provides the necessary energy for particle uptake was found to be decreased in both resting and phagocytizing leukocytes for ascorbic acid deficient guinea pigs. The direct oxidation of glucose through the hexose monophosphate shunt (HMS) was stimulated to a significantly lesser extent during phagocytosis in ascorbic acid deficient leukocytes. There was a progressive decline in phagocytosis induced shunt activity of leukocytes as the deficiency of ascorbic acid progressed. These findings show that particle uptake (as indicated by glycolytic activity) as well as particle destruction (as indicated by HMS activity) by leukocytes are impaired in ascorbic acid deficiency. Bactericidal capacity of leukocytes against Escherichia coli was also found to be low in ascorbic acid deficient guinea pigs as compared to those in the pair-fed control group.

Adrenal Glands↗

PubMed1981

Microsomal reductase for aromatic aldehydes and ketones in guinea pig liver. Purification, characterization, and functional relationship to hexose-6-phosphate dehydrogenase.

An NADPH-specific aromatic aldehyde-ketone reductase located in guinea pig liver microsomes can be effectively solubilized with nonionic detergents, but not with bile salts and hydrolytic enzymes. Destruction of microsomal membranes by nonionic detergents or acetone treatment leads to significant activation of the reductase, indicating that the enzyme is partly latent in intact microsomes. After solubilization with Triton X-100, the reductase has been highly purified. The purified enzyme catalyzes the NADPH-linked reduction of xenobiotic aromatic aldehydes and ketones as well as 3-ketosteroids, notably 5 alpha- and 5 beta-dihydrotestosterones. The reductase activities for xenobiotic carbonyl compounds and for 3-ketosteroids are each inhibited by addition of the other type of substrate and show the same pH optimum, cofactor requirement, and heat stability, indicating the same enzyme is responsible for the reduction of the two types of substrates. Hexose-6-phosphate dehydrogenase, purified from guinea pig liver microsomes, acts as a more effective NADPH generator for the reductase than yeast and guinea pig liver cytosolic glucose-6-phosphate dehydrogenase. Evidence has been obtained that hexose-6-phosphate dehydrogenase undergoes a functional interaction with the reductase, facilitating the provision of NADPH to the reductase activity both in the reconstituted system and in microsomes.

Alcohol Oxidoreductases↗

PubMed1988

A study of the role of the hexose monophosphate pathway with respect to fatty acid biosynthesis in sporulation of Saccharomyces cerevisiae.

13C NMR was used to study the pattern of label incorporation from [2-13C]acetate into trehalose during sporulation in Saccharomyces cerevisiae. A wild-type strain and a strain homozygous for the zwf1 mutation (which affects glucose-6-phosphate dehydrogenase) were used. In the wild-type it was possible to deduce the cycling of glucose 6-phosphate around the hexose monophosphate pathway whilst in the mutant strain this did not occur. The requirement of the hexose monophosphate pathway for providing NADPH for fatty acid biosynthesis was examined using 13C NMR and GC/MS. The wild-type strain produced a typical profile of fatty acids with palmitoleic acid being the most abundant whereas the mutant contained only one-quarter the amount of total fatty acid. As zwf1 homozygous diploids are able to sporulate this indicates that the large amount of fatty acid biosynthesis observed in sporulation of wild-type strains is not essential to the process.

Fatty Acids↗