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PubMed1977

Location of the gene specifying hexose phosphate transport (uhp) on the chromosome of Escherichia coli.

The uhp gene, which specifies the uptake of hexose phosphates, and several other genes in the vicinity of minute 81 on the E. coli linkage map have been located by phage-mediated transductions. The order found is mtl-gpsA-pyre-gltc-uhp-tna-dnaa. Alleles specifying the Uhp- and Uhp+ characters were separated from that specifying constitutivity of hexose phosphate uptake (Uhpc). Although cotransduction frequencies between gltC and uhp as high as 90%, and between uhp and tna as high as 80%, were observed, these frequencies were unusually strongly dependent on which marker was selected. This may be due to the proximity of the uhp region to the point of origin of chromosome replication.

Chromosome Mapping↗
PubMed1995

In Saccharomyces cerevisiae deletion of phosphoglucose isomerase can be suppressed by increased activities of enzymes of the hexose monophosphate pathway.

Saccharomyces cerevisiae mutants defective in the structural gene PGI1 lack phosphoglucose isomerase and hence cannot grow on glucose. Spontaneous mutants were isolated by selecting for the regained ability to grow on YEPD (yeast extract/peptone/glucose). Three complementation groups called spg29-31 (suppressor of pgi1 delta) were identified. The metabolism of [2-13C]glucose was studied by 13C NMR spectroscopy. This led to the conclusion that in a spg29 mutant suppression of the glycolytic defect was achieved by increased carbon flux through the hexose monophosphate pathway. The specific activities of enzymes of the hexose monophosphate pathway (except glucose-6-phosphate dehydrogenase) and NAD- and NADP-dependent glutamate dehydrogenase were increased in the bypass mutant.

Enzymes↗
PubMed2003

Sugar-regulated expression of a putative hexose transport gene in grape.

Different lengths of the promoter of grape (Vitis vinifera) VvHT1 (Hexose Transporter 1) gene, which encodes a putative hexose transporter expressed during the ripening of grape, have been transcriptionally fused to the beta-glucuronidase reporter gene. In transgenic tobacco (Nicotiana tabacum) transformed with these constructs, VvHT1 promoters were clearly responsible for the sink organ preferential expression. The potential sugar effectors of VvHT1 promoter were studied in tobacco cv Bright-Yellow 2 cells transformed with chimeric constructs. Glucose (56 mM), sucrose (Suc; 58 mM), and the non-transported Suc isomer palatinose doubled the beta-glucuronidase activity conferred by the VvHT1 promoter, whereas fructose did not affect it. These effects were the strongest with the 2.4-kb promoter, which contains all putative sugar-responsive elements (activating and repressing), but they were also significant with the 0.3-kb promoter, which contains only activating sugar boxes. The induction of VvHT1 expression by both Suc and palatinose was confirmed in the homologous grape berry cell culture. The data provide the first example of a putative sugar transporter, which is induced by both glucose and Suc in higher plants. Although induction of VvHT1 expression by Suc does not require transport, the presence of glucosyl moiety is necessary for Suc sensing. These results provide new insights into sugar sensing and signaling in plants.

Carbohydrates↗
PubMed1966

Hexose-, inositol-, and nucleoside phosphate esters in germinating seeds of crested wheatgrass.

Net synthesis of phosphate esters was measured in crested wheatgrass seeds [Agropyron desertorum (Fisch.) Schult.] to learn what phosphorylating reactions occur in early stages of germination. Phosphate esters were separated on ion exchange columns and identified by cochromatography and chemical analyses. Synthesis of adenosine triphosphate and UDP-hexose was detected 6 hours after the beginning of water absorption. This synthesis occurred during the period of rapid increase in water potential. Synthesis of hexose phosphate and uridine triphosphate was detected 12 hours after the beginning of water absorption. The concentrations of these esters increased during 48 hours. Increases in concentration of inositol di-, tri-, and tetraphosphate suggested that hydrolysis of inositol hexaphosphate began after 12 hours.

Journal Article↗
PubMed1978

Metabolic Studies on Intermediates in the myo-Inositol Oxidation Pathway in Lilium longiflorum Pollen: I. Conversion to Hexoses.

The myo-inositol oxidation pathway was investigated in regard to its role as a source of carbon for products of hexose monophosphate metabolism in germinated pollen of Lilium longiflorum Thunb., cv. Ace. myo-[2-(14)]Inositol and d-[1-(14)C]glucuronate had similar distributions of radioactivity, contributing about three times more label to polysaccharide-bound glucose than myo-[2-(3)H]inositol. In the course of glucogenesis label from the latter appeared as tritiated water in the medium. This exchange could be enhanced by supplying d-[5R,5S-(3)H]xylose instead of myo-[2-(3)H]inositol. When the former was administered, [(3)H]glucose was the only labeled sugar residue found in polysaccharide products. The soluble constituents of d-[5R,5S-(3)H]xylose-labeled pollen contained no traces of labeled xylose despite massive uptake and utilization.l-[1-(14)C]- and l-[5-(14)C]Arabinose produced similar labeling patterns in germinated pollen including incorporation of arabinosyl units into pollen tube polysaccharides and substantial glucogenesis which led to utilization of arabinose for respiration and further incorporation of labeled glucosyl units into pollen tube polysaccharides.d-[5-(3)H]Galacturonate was rapidly taken up by germinated pollen but slowly utilized, without conversion to other sugars, for incorporation into pollen tube polysaccharides. l-[6-(14)C]Gulonate was not taken up by pollen.Results strongly support a scheme of conversion from myo-inositol to hexose monophosphate and subsequent products of glucose metabolism that involves the myo-inositol oxidation pathway.

Journal Article↗
PubMed1985

Hexose kinases from the plant cytosolic fraction of soybean nodules.

The enzymes responsible for the phosphorylation of hexoses in the plant cytosolic fraction of soybean (Glycine max L. Merr cv Williams) nodules have been studied and a hexokinase (ATP:d-hexose 6-phosphotransferase EC 2.7.1.1) and fructokinase (ATP:d-fructose 6-phosphotransferase EC 2.7.1.4) shown to be involved. The plant cytosolic hexokinase had optimum activity from pH 8.2 to 8.9 and the enzyme displayed typical Michaelis-Menten kinetics. Hexokinase had a higher affinity for glucose (K(m) 0.075 millimolar) than fructose (K(m) 2.5 millimolar) and is likely to phosphorylate mainly glucose in vivo. The plant cytosolic fructokinase had a pH optimum of 8.2 and required K(+) ions for maximum activity. The enzyme was specific for fructose (apparent K(m) 0.077 millimolar) but concentrations of fructose greater than 0.4 millimolar were inhibitory. The native molecular weight of fructokinase was 84,000 +/- 5,000. The roles of these enzymes in the metabolism of glucose and fructose in the host cytoplasm of soybean nodules are discussed.

Journal Article↗
PubMed1988

Characterization of the hexose transport system in maize root tips.

Sugar-depleted excised maize (Zea mays L.) root tips were used to study the kinetics and the specificity of hexose uptake. It was found that difficulties induced by bulk diffusion and penetration barriers did not exist with root tips. Several lines of evidence indicate the existence of a complex set of uptake systems for hexoses showing an overall biphasic dependence on external sugar concentrations. The results suggest that the high and the low affinity components might be located on the same carrier. One uptake system was specific for fructose, but the high affinity component was repressed by high concentrations of external glucose. A second system was specific for glucose and its analogs (2-deoxy-d-glucose and 3-O-methyl-d-glucose), and a third one, more complex, had a high affinity for glucose and its analogs but could transport fructose when glucose was not present in the external solution. A simple method is proposed to determine the inhibitor constants in competition experiments.

Journal Article↗
PubMed1996

Systemic Acquired Resistance Mediated by the Ectopic Expression of Invertase: Possible Hexose Sensing in the Secretory Pathway.

Systemic acquired resistance (SAR) has been reported to be associated with lesion-mimic mutants. Tobacco plants expressing vacuolar and apoplastic yeast-derived invertase (vaclnv and cwlnv, respectively) develop spontaneous necrotic lesions similar to hypersensitive responses caused by avirulent pathogens. Therefore, SAR and metabolic alterations leading to the activation of defense-related responses were studied in these plants. Defense-related gene transcripts, callose content, peroxidase activities, and levels of salicylic acid were found to be elevated. The defense reactions were accompanied by increased resistance toward potato virus Y and were measured as decreased viral spreading and reduced multiplication in systemic leaves of the transgenic plants. Interestingly, the accumulation of pathogenesis-related (PR) protein transcripts (PR-Q) and repression of photosynthetic gene transcripts (chlorophyll a/b binding protein) were inversely correlated and required the same threshold level of hexoses for induction and repression. Expression of a cytosolic yeast-derived invertase in transgenic tobacco plants with equally increased levels of sugars neither displayed SAR responses nor showed decreased levels of photosynthetic genes. It is suggested that hexose sensing in the secretory pathway is essential for mediating the activation of defense-related genes as well as repression of photosynthetic genes in vaclnv and cwlnv plants.

Journal Article↗
PubMed1987

Nucleotide sequence and transcriptional startpoint of the glpT gene of Escherichia coli: extensive sequence homology of the glycerol-3-phosphate transport protein with components of the hexose-6-phosphate transport system.

The nucleotide sequences of the glpT gene of Escherichia coli and its regulatory region have been elucidated and the primary structure of the glycerol-3-phosphate transport protein deduced. Extensive amino acid sequence homology was found with two other cytoplasmic membrane proteins: the functionally related hexose-6-phosphate transport protein, and the UHPC protein involved in regulating hexose-6-phosphate uptake. Although no significant amino acid sequence homology was found with other transport proteins, such as the arabinose, citrate, glucose, melibiose, lactose or xylose transporters, all of these proteins share a common secondary structure arrangement with the GLP T protein as they apparently contain twelve membrane-spanning alpha-helical segments. The promoter for glpT was located by transcript mapping and shown to overlap a site to which catabolite activator protein binds in vitro. These findings indicate how catabolite repression may be mediated but do not explain its physiological significance in glycerol metabolism.

Amino Acid Sequence↗
PubMed1985

Regulation of the human-erythrocyte hexose-monophosphate shunt under conditions of oxidative stress. A study using NMR spectroscopy, a kinetic isotope effect, a reconstituted system and computer simulation.

The regulation of the hexose monophosphate shunt of human erythrocytes under conditions of oxidative stress has been investigated by monitoring the reduction of oxidised glutathione (GSSG) to reduced glutathione (GSH) in erythrocytes containing high levels of GSSG; 1H NMR and a biochemical assay were used to measure the changes. A reconstituted metabolic system prepared with the purified erythrocyte enzymes was used in conjunction with studies of intact cells and haemolysates to determine the dependence of the rate of GSH production on the activities of hexokinase and glucose-6-phosphate dehydrogenase. Both of these enzymes have previously been claimed to be the rate-limiting step of oxidatively stimulated flux through the hexose monophosphate shunt. The absence of a kinetic isotope effect on the rate of GSH production in these systems, when [1-2H]glucose replaced glucose as the source of reducing equivalents, showed that glucose-6-phosphate dehydrogenase activity was not a strong determinant of the flux. The dependence of the rate of GSH production on the concentration of the hexokinase inhibitors glucose 1,6-bisphosphate and glycerate 2,3-bisphosphate showed that, under conditions of oxidative stress, hexokinase was the principal determinant of flux through the shunt. Glucose 1,6-bisphosphate at the concentration present in vivo appears to be more important in limiting hexokinase activity, and thus the rate of glucose utilisation, than was previously assumed. A detailed computer model of the system was developed based on the reported kinetic parameters of the enzymes involved. A sensitivity analysis of this model predicted that the hexokinase reaction would have a sensitivity coefficient of 0.995 with respect to the maximal rate of GSH production.

Computers↗
PubMed1990

Effects of ethanol on hexose uptake by cultured rat brain cells.

The effects of ethanol on hexose uptake by glial cells was investigated using primary cultures prepared from term rat fetuses. Specific 3H 2-deoxy-D-glucose (2DG) uptake was significantly reduced by a 4-hr exposure to ethanol at concentrations of 25, 50, and 100 mM, but not 200 or 300 mM. The inhibitory effect of 50 mM ethanol increased with the duration of exposure, with 2DG uptake inhibited by 36% after 18 hr. Astrocytes cultured from the brains of term fetuses of rats fed ethanol during pregnancy showed essentially the same 2DG uptake response to in vitro ethanol treatment. Kinetics of 2DG uptake showed a significant decrease of Vmax in the presence of ethanol. No interaction was found between ethanol and insulin, which stimulated 2DG uptake and protein content of the cultures. The data suggest that ethanol can modulate hexose uptake by astrocytes cultured from fetal rat brain. However, insulin actions on glial cells were not affected by ethanol.

Animals↗
PubMed1986

Activities of some antioxidative and hexose monophosphate shunt enzymes of skeletal muscle in neuromuscular diseases.

The activities of some antioxidative and hexose monophosphate shunt enzymes, as well as of 2 hydrolases were studied in skeletal muscle biopsy specimens taken from 39 patients with neuromuscular diseases and from 15 controls. The activity of Se-dependent glutathione peroxidase was higher in patients with congenital myotonia, whereas in the other diagnostic groups this enzyme activity was the same as in the controls. The Se-independent and total glutathione peroxidase activity of patients in the various diagnostic groups did not differ from the controls. Moreover, no difference were observed in catalase activity between the patient groups and the controls. The activities of the rate limiting enzymes of hexose monophosphate shunt, glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase of muscle biopsy samples of various patient groups did not show any significant difference from controls. The activity of a lysosomal hydrolase, beta-N-acetylglucosaminidase, was increased in patients with polyneuropathy and the activity of a nonlysosomal protease, alkaline protease, was high in patients with Charcot-Marie-Tooth disease. The activities of Se-dependent glutathione peroxidase, 6-phosphogluconate dehydrogenase and of both hydrolases showed a significant correlation to the magnitude of muscle atrophy.

Acetylglucosaminidase↗
PubMed1973

Further effects of chlorpromazine on the hexose permeability of the human erythrocyte.

1. The effect of chlorpromazine on hexose transfer across the human erythrocyte membrane was investigated by optical and isotopic techniques in intact erythrocytes.2. Chlorpromazine produces two phases of inhibition of glucose exit at 17 degrees C in both types of experiment.3. Glucose exchange flux shows a much smaller decrease in flux rate at the same concentrations of chlorpromazine at 17 degrees C.4. Sorbose and glucose entry at 36 degrees C show a similarly complex inhibition by chlorpromazine. The inhibitor constant (K(i)) for these effects with the two sugars is similar.5. An Arrhenius plot shows that the average energy change for the reaction of the carrier with chlorpromazine is -46 kJ mole(-1) and suggests that a drug-induced conformational change in the carrier may occur.6. These findings require models of hexose permeation to contain separate terms in their kinetics for net and exchange fluxes and the predictions of one such model have been compared with the experimental results.

Biological Transport, Active↗
PubMed1971

Dissociation by colchicine of the hexose monophosphate shunt activation from the bactericidal activity of the leukocyte.

The effect of the in vitro addition of colchicine on various aspects of leukocyte metabolism was investigated. Colchicine in 6 mm concentration reduced the oxygen uptake of phagocytizing leukocytes to below the level of resting cells. A similar concentration of colchicine inhibited the increase in glucose-1-(14)C oxidation by 90% and the quantitative nitroblue tetrazolium reduction by about 50%. Experiments with broken cell preparations indicate that the inhibition of the hexose monophosphate shunt is due to an inhibition of the activity of the shunt enzymes glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase. In spite of these pronounced metabolic changes, the addition of 6 mm colchicine had no observable effect on the bactericidal activity of the leukocyte against three test organisms. These results support a mechanism for phagocytosis in which the hexose monophosphate-shunt activation is a secondary phenomenon which is not required for the actual killing process.

Journal Article↗
PubMed1970

Compartmentation in the induction of the hexose-6-phosphate transport system of Escherichia coli.

The induction of the hexose-6-phosphate transport system was investigated. Glucose-6-phosphate (G6P) at concentrations as low as 10(-4)m was able to induce this system in wild-type cells, as well as in mutants lacking phosphoglucose isomerase or G6P dehydrogenase. Growth in the presence of fructose-6-phosphate (F6P) induced the system only if the cells contained phosphoglucose isomerase. Furthermore, glucose and F6P were found to induce the system only if the extracellular concentration of G6P became appreciable in the medium as a consequence of the leakage of intracellular G6P formed from the glucose or F6P. Intracellular G6P was not an inducer even at high concentrations. The metabolism of glucose inhibited the induction of the hexose-6-phosphate transport system. Hypotheses for this compartmentalization of inducer and membrane-associated induction are presented.

Alkaline Phosphatase↗
PubMed1971

Kinetics of exogenous induction of the hexose-6-phosphate transport system of Escherichia coli.

The kinetics of the exogenous induction of the hexose-phosphate transport system by glucose-6-phosphate (G6P) was investigated. The induction of this system by extracellular but not intracellular G6P was confirmed. The differential rate of synthesis was linear, a function of the extracellular concentration of G6P and independent of the previous induction history of the culture. Neither maintenance nor autocatalysis, phenomena described in the induction of the lac operon, were observed in the exogenous induction of hexose-phosphate transport. Fructose-6-phosphate, a potent competitive inhibitor of G6P influx, had no effect on the induction of the system by G6P, indicating that the transport of inducer was not involved in the induction process.

Biological Transport, Active↗
PubMed1973

Isolation and characterization of mutations affecting the transport of hexose phosphates in Escherichia coli.

Mutants of Escherichia coli defective in the hexose phosphate transport system were isolated. Negative selection by penicillin treatment or positive selection with phosphonomycin was employed. These mutants grew normally on all carbon sources other than hexose phosphates. The map location of the mutations in 18 independently isolated mutant strains was investigated by transduction crosses. All of the mutations were found to lie in the same region of the chromosome, in the region represented by min 72 on the Taylor map. The order of the genes in this region was found to be mtl-cysE-pyrE-uhp-bgl-ilv. Revertants of some of the mutants exhibited altered regulatory control of this transport system.

Alkanesulfonates↗
PubMed1973

Energy coupling of the hexose phosphate transport system in Escherichia coli.

The active transport of hexose phosphates in Escherichia coli was inhibited by many uncouplers or inhibitors of oxidative metabolism. Fluoride and the lipid soluble cation, triphenylmethylphosphonium, had little effect. The uninduced level of transport was sensitive to fluoride, but not to azide. After energy uncoupling of active transport, the cells could equilibrate their intracellular water with the glucose-6-phosphate in the medium and displayed exit counter-flow suggesting the existence of carrier-mediated transport in the energy-uncoupled cells. The uncoupled transport of glucose-6-phosphate was inhibited by fructose-6-phosphate; the uninduced level of glucose-6-phosphate transport was not inhibited by fructose-6-phosphate. After energy uncoupling, the influx had a low affinity suggesting that, unlike the transport of beta-galactosides, the energy coupling for the active transport of hexose phosphate involved a change in the affinity of influx.

Azides↗