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PubMed2000

Xylulose fermentation by mutant and wild-type strains of Zygosaccharomyces and Saccharomyces cerevisiae.

Anaerobic xylulose fermentation was compared in strains of Zygosaccharomyces and Saccharomyces cerevisiae, mutants and wild-type strains to identify host-strain background and genetic modifications beneficial to xylose fermentation. Overexpression of the gene (XKS1) for the pentose phosphate pathway (PPP) enzyme xylulokinase (XK) increased the ethanol yield by almost 85% and resulted in ethanol yields [0.61 C-mmol (C-mmol consumed xylulose)(-1)] that were close to the theoretical yield [0.67 C-mmol (C-mmol consumed xylulose)(-1)]. Likewise, deletion of gluconate 6-phosphate dehydrogenase (gnd1delta) in the PPP and deletion of trehalose 6-phosphate synthase (tps1delta) together with trehalose 6-phosphate phosphatase (tps2delta) increased the ethanol yield by 30% and 20%, respectively. Strains deleted in the promoter of the phosphoglucose isomerase gene (PGI1) - resulting in reduced enzyme activities - increased the ethanol yield by 15%. Deletion of ribulose 5-phosphate (rpe1delta) in the PPP abolished ethanol formation completely. Among non-transformed and parental strains S. cerevisiae ENY. WA-1A exhibited the highest ethanol yield, 0.47 C-mmol (C-mmol consumed xylulose)(-1). Other non-transformed strains produced mainly arabinitol or xylitol from xylulose under anaerobic conditions. Contrary to previous reports S. cerevisiae T23D and CBS 8066 were not isogenic with respect to pentose metabolism. Whereas, CBS 8066 has been reported to have a high ethanol yield on xylulose, 0.46 C-mmol (C-mmol consumed xylulose)(-1) (Yu et al. 1995), T23D only formed ethanol with a yield of 0.24 C-mmol (C-mmol consumed xylulose)(-1). Strains producing arabinitol did not produce xylitol and vice versa. However, overexpression of XKS1 shifted polyol formation from xylitol to arabinitol.

Anaerobiosis↗

PubMed2001

Changes of in vivo fluxes through central metabolic pathways during the production of nystatin by Streptomyces noursei in batch culture.

The central carbon metabolism of the nystatin-producing strain Streptomyces noursei ATCC 11455 was evaluated by 13C-labelling experiments. A batch fermentation was examined during the idiophase by GC-MS measurements of the labelling patterns of amino acids in the biomass. The labelling patterns of the amino acids and calculated fluxes of the central metabolism showed that changes in the primary and secondary metabolisms occurred simultaneously. Changes in the profiles for the integrated fluxes showed a decreased flux through the pentose phosphate pathway and an increased flux in the tricarboxylic acid cycle relative to the glucose uptake rate when the culture entered a phase with reduced specific growth rate and enhanced nystatin yield. The flux through the pentose phosphate pathway seemed to be adjusted according to the NADPH requirement during the different phases of the batch fermentation.

Amino Acids↗

PubMed1999

Activity of key enzymes in glucose catabolism during the growth and metacyclogenesis of Leishmania infantum.

This paper follows the development in the activity of the key enzymes of glycolysis and dehydrogenases of the pentose phosphate shunt throughout the in vitro growth and metacyclogenesis of two human strains of Leishmania infantum - one visceral (VL) and the other cutaneous (CL) - together with changes in the glucose, ammonium, and proton concentrations in the culture medium. In the first stage, ammonium was generated and no glucose was consumed. Later on, all the glucose was consumed and, finally, ammonium was generated again. The ammonium concentration increased 16- and 21-fold in cultures of VL and CL strains, respectively. The activities of the glycosomal enzymes hexokinase and phosphofructokinase differed in each strain, always being higher in CL than in VL and increasing throughout the culture period in CL while decreasing in VL. This probably indicates a different capability to adapt to the culture medium conditions. The activities of the pentose phosphate shunt enzymes examined indicate that 6-phosphogluconate dehydrogenase is possibly a rate-limiting enzyme for this pathway. Pyruvate kinase is a cytosolic control enzyme of glycolysis in trypanosomatids, and its activity decreased throughout the growth and differentiation of both strains of L. infantum, as occurs in other trypanosomatids. It was also observed that glucose catabolism was more active in the cutaneous strain than in the visceral one.

Animals↗

PubMed2006

Glucose metabolism in rat retinal pigment epithelium.

The retinal pigment epithelium (RPE) is the major transport pathway for exchange of metabolites and ions between choroidal blood supply and the neural retina. To gain insight into the mechanisms controlling glucose metabolism in RPE and its possible relationship to retinopathy, we studied the influence of different glucose concentrations on glycogen and lactate levels and CO(2) production in RPE from normal and streptozotocin-treated diabetic rats. Incubation of normal RPE in the absence of glucose caused a decrease in lactate production and glycogen content. In normal RPE, increasing glucose concentrations from 5.6 mM to 30 mM caused a four-fold increase in glucose accumulation and CO(2) yield, as well as reduction in lactate and glycogen production. In RPE from diabetic rats glucose accumulation did not increase in the presence of high glucose substrate, but it showed a four- and a seven-fold increase in CO(2) production through the mitochondrial and pentose phosphate pathways, respectively. We found high glycogen levels in RPE which can be used as an energy reserve for RPE itself and/or neural retina. Findings further show that the RPE possesses a high oxidative capacity. The large increase in glucose shunting to the pentose phosphate pathway in diabetic retina exposed to high glucose suggests a need for reducing capacity, consistent with increased oxidative stress.

Animals↗

PubMed1983

Transfer of reducing equivalents into mitochondria by the interconversions of proline and delta 1-pyrroline-5-carboxylate.

Direct evidence is presented for a proline cycle using a cell-free experimental system which sequentially transfers 3H from [1-3H]glucose to NADP+ to delta 1-pyrroline-5-carboxylate and yields [3H]proline. The formation of [3H]proline depends on the presence of NADP, delta 1-pyrroline-5-carboxylate, and the enzymes glucose-6-phosphate dehydrogenase and delta 1-pyrroline-5-carboxylate reductase. The production of [3H]proline from unlabeled proline in the presence of mitochondria provides direct evidence for one complete turn of a proline cycle which transfers reducing equivalents produced by glucose oxidation in the pentose pathway into mitochondria. In this cycle, proline is oxidized to delta 1-pyrroline-5-carboxylate by mitochondrial proline oxidase. delta 1-pyrroline-5-carboxylate is released from mitochondria and is recycled back to proline by delta 1-pyrroline-5-carboxylate reductase with concomitant oxidation of NADPH. At the maximal rate observed, 60% of delta 1-pyrroline-5-carboxylate produced is recycled back to proline. This cycle provides a mechanism for transferring reducing equivalents from NADPH into mitochondria and is linked to glucose oxidation in the pentose pathway by NADPH turnover.

Animals↗

PubMed1987

Effect of orally administered secondary autoxidation products of linoleic acid on carbohydrate metabolism in rat liver.

The effects of orally administered secondary autoxidation products of linoleic acid in rat liver were investigated. Their administration led to two toxic effects on hepatic carbohydrate metabolism, as compared to the administration of saline or linoleic acid used as controls. One effect was depletion of glucose 6-phosphate and fructose 6-phosphate caused by the reduction of glycolysis and glycogenolysis, accompanied by decreases in glycogen synthesis and pentose phosphate cyclic activity. The reduction in these metabolic systems seems unlikely to occur because phosphofructokinase was regulated by ATP or citrate enzymatically, because their accumulation in the liver was not detected in the secondary products. Another toxic effect was the depletion of oxaloacetate and isocitrate caused by the reduction in enzyme activity of the mitochondrial citrate cycle. On the basis of these results, the hepatotoxic effects of secondary products are discussed as follows: the incorporated secondary products impaired the activities of hexokinase and phosphoglucomutase in the liver. The reduction in these enzyme activities resulted in the depletion of glucose 6-phosphate and fructose 6-phosphate, which led ultimately to decreases in the activities of phosphofructokinase, the pentose phosphate cycle, and glycogen synthesis. Moreover, the secondary products disturbed the mitochondrial membrane, resulting in a decrease in the activity of the citrate cycle, which was accompanied by depletion of its metabolites.

Adenine Nucleotides↗

PubMed1976

Conformation in aqueous medium of the neutral, protonated and anionic forms of 9-beta-D-arabinofuranosyladenine.

Proton magnetic resonance spectroscopy was employed to study the solution conformations of the neutral, protonated and dissociated forms of the therapeutically active 9-beta-D-arabinofuranosyladenine (araA). In particular, in strongly basic medium, increasing alkalinity led to pronounced changes in chemical shifts and coupling constants of some pentose protons, due to ionization of the pentose hydroxyls, especially the 2'-OH. The neutral form of araA may be characterized as approx. 25% C(2')endo and approx. 60% gauche-gauche, hence somewhat different from that of the therapeutically active 1-beta-D-arabinofuranosylcytosine (araC). By contrast, the conformations of the anionic forms of both of these are identical, predominantly (greater than 80%) C(2')endo and gauche-gauche. With the aid of the 3'-O-methyl derivatives of araA and araC, where only the 2'-OH ionizes, and the accompanying conformational changes are similar, it follows that the conformation C(2')endo and gauche-gauche for all the foregoing is constrained to this form via a strong intramolecular hydrogen bond, viz. O(5')H...O(2')(-). The influence of the foregoing hydrogen bond on the chemical shifts of the adenine H(8) in the araA anion points to the existence of the latter in the form anti. A similar effect of the doubly ionized phosphate group on H(8) in 5'-araAMP shows the nucleotide to also prefer the form anti, as previously demonstrated for 5'-AMP. The conformations of the sugar rings of the neutral forms of araA and adenosine in aqueous medium differ appreciably, whereas in the solid state they are very similar. PMR spectroscopy is shown to be an effective method for following sugar hydroxyl dissociation. The extent of ionization of a given hydroxyl is provided by the resulting chemical shifts of neighbouring (geminal and vicinal) protons. When ionization is accompanied by a change in conformation, the process may be followed also by changes in proton-proton vicinal coupling constants.

Adenine Nucleotides↗

PubMed1986

The outer membrane of Pseudomonas aeruginosa is a barrier against the penetration of disaccharides.

The outer membrane of Pseudomonas aeruginosa acted as a barrier against the penetration of di- (Mr, 342), tri- (Mr, 504) and tetrasaccharides (Mr, 666), whereas the membrane allowed the penetration of pentose (Mr, 150) and methylhexoses (Mr, 194) into the periplasm. When the intact cells of P. aeruginosa were treated with 600 mosM saccharides of various sizes and observed under an electron microscope, saccharides of Mr larger than 342 caused the extensive shrinking of the outer membrane. Whereas the cells treated with the saccharides of Mr less than 194 or with sucrose in the presence of EDTA showed plasmolysis. Determination of the extent of saccharide penetration into the periplasm of the cells treated with 600 mosM sodium chloride or with 600 mosM saccharides of various sizes showed that only pentose and hexoses, so far examined, were penetrable but di-, tri- and tetrasaccharides were impenetrable.

Disaccharides↗

PubMed1988

Comparative effects of insulin and proinsulin in vitro on pathways of glucose utilization and lipid synthesis in the lactating rat mammary gland.

The effects of insulin and proinsulin have been measured on the rates of glucose oxidation via the pentose phosphate pathway and incorporation into lipid in slices of lactating rat mammary gland. Half-maximal stimulation of glucose oxidation was observed with 1-3 x 10(-8)M insulin while 1 x 10(-7)M proinsulin was required to achieve half-maximal stimulation. A similar, approximately 10-fold, difference in potency was observed in relation to lipid synthesis. The present results appear to indicate that the maximum stimulation of either glucose oxidation via the pentose phosphate pathway or lipid synthesis by proinsulin did not reach the same level as that found for insulin.

Animals↗

PubMed1991

Stimulation by D-glucose of the direct conversion of arginine to citrulline in enterocytes isolated from pig jejunum.

In enterocytes isolated from pig jejunum, L-arginine is metabolized to L-citrulline either directly or indirectly through the sequence of reactions catalysed by arginase and ornithine transcarbamylase. In the presence of 5 mM D-glucose, the direct conversion of 1mM L-[guanido-14C] arginine to L-citrulline was increased more than 4 times. Isolated enterocytes exhibit a high glycolytic capacity. Furthermore, the decarboxylation of 5mM D-[1-14C] glucose was 3.6 fold higher than the decarboxylation of 5 mM D-[6-14C] glucose which suggests the presence of a pentose phosphate pathway in enterocytes. Since the production of labelled L-citrulline from L-[guanido-14C] arginine in pig enterocyte homogenates was markedly increased in the presence of NADPH, it is proposed that the direct conversion of L-arginine to L-citrulline could be stimulated by the production of NADPH from D-glucose in the pentose phosphate pathway.

Animals↗

PubMed1982

Regulation of NADPH-dependent mixed-function oxidation in perfused livers. Comparative studies with sorbitol and ethanol.

Sorbitol and ethanol were shown to have opposite effects on p-nitroanisole O-demethylation in perfused livers from fasted, phenobarbital-treated rats. Sorbitol (2 mM) stimulated drug metabolism by 50% while ethanol (20 mM) caused 80% inhibition. Both sorbitol and ethanol infusion decreased the NAD+/NADH ratio and increased fluorescence of pyridine nucleotides monitored from the liver surface; however, the NADP+/NADPH ratio was decreased by sorbitol but tended to be increased by ethanol. Stimulation of drug metabolism by sorbitol was abolished by pretreatment of fasted rats with 6-aminonicotinamide, an inhibitor of the pentose phosphate shunt, but was not affected by aminooxyacetate, a transaminase inhibitor. These results indicate that sorbitol stimulated p-nitroanisole metabolism by providing NADPH via the pentose phosphate shunt. Ethanol and sorbitol caused changes in intracellular concentrations of NADPH in livers from fasted rats which correlated directly with changes in hepatic levels of citrate and aspartate. Furthermore, aspartate infusion reduced the inhibition of p-nitroanisole O-demethylation by ethanol. This inhibition was also reversed partially by sorbitol in livers from 6-aminonicotinamide-treated rats. It is concluded that ethanol inhibits mixed-function oxidation primarily by decreasing the concentrations of citric acid cycle intermediates which leads to depletion of cytosolic NADPH.

Animals↗

PubMed1988

The effect of aldose reductase inhibitor Statil (ICI 128436) on the glucose over-utilization in kidney of diabetic rats.

The present study examined the effect of the aldose reductase inhibitor Statil (ICI 128436, ICI, Cheshire, U.K.) on the levels of metabolites and activities of enzymes involved in the glycolysis, polyol pathway and pentose phosphate pathway and on the flux of radioactive glucose through these pathways in kidney of streptozotocin diabetic rats. In kidneys of diabetic rats of 30 days duration the level of sorbitol was increased by +82% and fructose concentration was raised by +42%. After treatment with Statil for 9 days (reversal study) a significant fall in kidney sorbitol concentration and kidney fructose concentration was found. Lactate and UDP-glucose concentrations which were both significantly raised in diabetes by +80% and +23% respectively decreased by 20% after Statil treatment, together with a decline in UDP-glucose dehydrogenase activity. Aldose reductase and sorbitol dehydrogenase activities were also significantly lowered by Statil. In the reversal study there was no significant effect of Statil on the flux of glucose via alternative routes in the kidney cortex. In kidneys of diabetic rats of 9 days duration, the level of sorbitol increased by +61% and the concentration of fructose was raised by +30%. The treatment with Statil (25 mg/kg) from the day of induction of diabetes (prevention study) prevented the accumulation of sorbitol, fructose and UDP-glucose. The increase in the incorporation of radioactive glucose through the pentose phosphate pathway seen in diabetes was less marked in the renal cortex of diabetic rats treated with Statil ab initio.

Animals↗

PubMed1995

Structure-activity relationship for the binding of nucleoside ligands to adenosine kinase from Toxoplasma gondii.

One hundred and twenty-eight purine nucleoside analogs were evaluated as ligands of Toxoplasma gondii adenosine kinase (EC 2.7.1.20) by examining their ability to inhibit this enzyme in vitro. Inhibition was quantified by determining apparent Ki (appKi) values for those compounds that inhibited this enzyme by greater than 10% at a concentration of 1 mM. Two compounds, N6-(p-methoxybenzoyl)adenosine and 7-iodo-7-deazaadenosine (iodotubercidin), were found to bind to the enzyme (appKi = 3.9 and 1.6 microM, respectively) better than adenosine. On the basis of these data, a structure-activity relationship for the binding of ligands to T. gondii adenosine kinase was formulated using adenosine as a reference compound. It was concluded that the following structural features of purine nucleoside analogs are required or strongly preferred for binding: (1) "pyridine-type" endocyclic nitrogens at the 1- and 3-positions; (2) an exocyclic hydrogen at the 2-position; (3) 6-position exocyclic substituents in the lactim tautomeric form; (4) a "pyridine-type" endocyclic nitrogen at the 7-position or hydrophobic exocyclic substituents attached to an endocyclic carbon at the 7-position; (5) an endocyclic methine or "pyridine-type" nitrogen at the 8-position; (6) an endocyclic nitrogen at the 9-position; (7) a pentose or "pentose-like" (e.g. hydroxylated cyclopentene) moiety attached to the 9-position nitrogen; (8) hydroxyl groups at the 2'- and 3'-positions in a ribose configuration; (9) a hydroxymethyl or methyl (i.e. 5'-deoxy) group at the 5'-position; (10) a beta-D-nucleoside configuration; and (11) an anti conformation around the N-glycosidic bond. In addition, there appears to be a "pocket" in the catalytic site of T. gondii adenosine kinase, adjacent to the 6-position of adenosine, that can accommodate large (preferably unsaturated or aromatic) substituents (e.g. phenyl). These findings provide the basis for the rational design of additional ligands of T. gondii adenosine kinase.

Adenosine↗

PubMed1991

Deoxygenated and alkylated furanoses: Thorpe-Ingold effects on tautomeric equilibria and rates of anomerization.

2-Deoxy-D-glycero-tetrose, 3-deoxy-DL-glycero-tetrose, 3-deoxy-3,3-di-C-methyl-DL-glycero-tetrose, 3-C-methyl-DL-erythrose, 3-C-methyl-DL-threose, 2-deoxy-5-O-methyl-D-erythro-pentose and 3-deoxy-5-O-methyl-D-erythro-pentose have been prepared, in some cases with 13C-substitution at the anomeric carbon, and characterized by 1H-(300 and 620 MHz) and 13C-n.m.r. (75 MHz) spectroscopy. The proportions of cyclic (alpha and beta furanoses) and acyclic (aldehyde and hydrate) forms were determined in aqueous (2H2O) solution, and ring-opening (kopen) and ring-closing (kclose) rate constants were measured by 1H and 13C saturation-transfer n.m.r. spectroscopy at p2H 5.0 (acetate buffer) and 60 degrees. The degree of furanose ring substitution was found to significantly affect both the thermodynamics and kinetics of furanose anomerization. Increased substitution enhances the proportion of cyclic forms in solution by stimulating furanose kclose. In contrast, furanose kopen was less affected by the degree of substitution; however, kinetic studies of 2-deoxyfuranose anomerization implicate furanose ring conformation as a potential determinant of kopen.

Alkylation↗

PubMed1992

13C and 31P NMR studies of glucose and 2-deoxyglucose metabolism in normal and enzyme-deficient human erythrocytes.

The flux of 13C-labeled glucose through the Embden-Meyerhof and pentose phosphate pathways was studied by 13C NMR in intact erythrocytes isolated from normal subjects or from patients suffering of glucose-6-phosphate dehydrogenase (G6PD, EC 1.1.1.49) deficiency. Similar rates of glucose catabolism and similar fluxes of the 13C-label into 2,3-bisphosphoglycerate and lactate were found, under basal conditions, in normal and in G6PD-deficient erythrocytes incubated in the presence of either [1-13C]- or D[6-13C]glucose. Exposure to oxidative stress by preincubation with tert-butylhydroperoxide induced in normal, but not in G6PD-deficient erythrocytes, a significant enhancement of glucose consumption, as well as a substantial reduction in 13C-label transfer from C1-glucose into lactate. It was also possible, by 31P NMR, to evaluate the conversion of 2-deoxyglucose to its phosphate-containing metabolites. The oxidation and subsequent decarboxylation of 2-deoxyglucose-6-phosphate was assessed in reconstituted systems and could subsequently be evidenced also in ethanolic extracts from normal (but not from G6PD-deficient) erythrocytes which had been exposed to oxidative stress. The results indicate that, in terms of glucose flux through the glycolytic pathway, there is little or no difference between normal and G6PD-deficient erythrocytes, regardless of previous exposure to oxidative stress. Faster consumption of either glucose or 2-deoxyglucose is induced, only in normal cells, by treatment with tert-butylhydroperoxide, essentially as a consequence of the activation of the pentose-phosphate pathway.

2,3-Diphosphoglycerate↗

PubMed1980

Connective tissue metabolism in normal and atrophic skeletal muscle.

The first two enzymes of the pentose-phosphate pathway (G6PD and 6PGD) were studied in rabbit skeletal muscle. Their activities closely paralleled the connective tissue levels in each individual muscle, and they increased in atrophying muscles at the same rate as connective tissue. Skeletal muscle tendons and subcutaneously implanted polyvinyl sponges, both of which consist of relatively pure connective tissue, displayed much higher activity of these enzymes than the muscle homogenates. These results suggest that pentose synthesis in muscle is mostly confined to its connective tissue elements, raising the possibility of a role for connective tissue in the biosynthetic processes of skeletal muscle. The contribution of connective tissue to the results of muscle quantitative studies may be significant, and even overshadow other values in atrophic muscles, where the muscle cell population may be greatly reduced. The interpretation of quantitative biochemical studies on pathological muscle samples should take this factor into consideration.

Animals↗

PubMed1981

Regulation of lipid synthesis in hepatocytes from lean and obese Zucker rats.

Fatty acid synthesis and CO2 production were evaluated in hepatocytes from lean and obese Zucker rats in the presence of 3H2O, and several carbon precursors. The incorporation of 3H2O into fatty acids was greater in obese compared to lean rats in both the isolated hepatocyte and in vivo. The rates of incorporation of 3H2O into fatty acids and cholesterol in hepatocytes of both lean and obese rats were linear for 2 hr, in the absence or presence of 16.7 mM glucose. Rates of fatty acid synthesis were higher in the presence of 16.7 mM glucose compared to the absence of glucose in both lean and obese while rates of cholesterol synthesis were similar. The incorporation of 3H2O into fatty acids, but not into cholesterol, was correlated with increasing glucose concentration and was 2 to three-fold higher in hepatocytes of obese compared to lean rats in the presence of several carbon precursors. Differences in CO2 production between lean and obese rats suggested increased pentose phosphate shunt activity, decreased pyruvate dehydrogenase activity, and lower tricarboxylic acid cycle activity in obese rats. Fatty acid synthesis and CO2 production from 3H2O and [U-14C]glucose in hepatocytes of lean and obese rats was similarly elevated by insulin and depressed by glucagon at several concentrations, suggesting that hepatocytes of obese animals respond to these hormones. These data indicate that rates of hepatic fatty acid synthesis although higher in obese rats respond to modulation in a fashion which is similar to the response in lean rats. The present studies suggest that the oxidation of several carbon precursors in the tricarboxylic acid cycle is diminished in obese compared to lean rats, but pentose phosphate shunt activity is greater in the obese Zucker rats.

Animals↗

PubMed1986

The aqueous humor of rabbit contains high concentrations of pyrroline-5-carboxylate.

Accumulating evidence suggests that the interconversion of proline and pyrroline-5-carboxylate (P5C) transfers oxidizing or reducing potential within or between cells. We report here that rabbit aqueous humor contains P5C at a concentration at least 20 times that found in rabbit plasma or in biologic fluids from other animals, and, in isolated lens, P5C stimulates the activity of the pentose phosphate shunt. However, aqueous humor in other species contains P5C in the same range of concentrations as their respective plasma. The high P5C concentration in rabbit aqueous humor may be due to comparatively low levels of P5C reductase activity in ocular tissues, especially the cornea. The levels in lens epithelium, though lower in the rabbit, nevertheless could mediate the transfer of oxidizing potential from P5C and stimulate the pentose-phosphate shunt. These results suggest that the eye may serve as a convenient model for the study of proline and P5C-related transfer of reducing or oxidizing potential between tissues.

Animals↗