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PubMed1977

Biosynthesis of the dimethylbenzene moiety of riboflavin and dimethylbenzimidazole: evidence for the involvement of C-1 of a pentose as a precursor.

The relative incorporations of specially labeled pyruvate, lactate, erythritol, D-erythrose, D-ribose, and D-glucose precursors into the dimethylbenzene carbon atoms of the 5,6-dimethylbenzimidazole unit of vitamin B12 by Propionibacterium shermanii have been determined. The incorporation data provide information regarding the putative four-carbon biosynthetic unit which is involved in the formation of 6,7-dimethyl-8-ribityllumazine and which is the source of the eight dimethylbenzene carbon atoms of both 5,6-dimethylbenzimidazole and riboflavin. The relative incorporations of the labeled lactate and pyruvate precursors are not consistent with either acetoin or 2,3-butanedione functioning as the four-carbon biosynthetic unit. The relative incorporations of the labeled hexose, pentose, and tetrose precursors indicate that the observed incorporation of C-1 of the pentose into the dimethylbenzene carbon atoms does not involve metabolism to a tetrose intermediate, but occurs more directly. It is concluded that the C-1 position of a pentose precursor is involved in the formation of the putative four-carbon biosynthetic unit.

Benzimidazoles↗
PubMed1974

Simulation of the pentose cycle in lactating rat mammary gland.

A computer model representing the pentose cycle, the tricarboxylic acid cycle and glycolysis in slices of lactating rat mammary glands has been constructed. This model is based primarily on the studies, with radioactive chemicals, of Abraham & Chaikoff (1959) [although some of the discrepant data of Katz & Wals (1972) could be accommodated by changing one enzyme activity]. Data obtained by using [1-(14)C]-, [6-(14)C]- and [3,4-(14)C]-glucose were simulated, as well as data obtained by using unlabelled glucose (for which some new experimental data are presented). Much past work on the pentose cycle has been mainly concerned with the division of glucose flow between the pentose cycle and glycolysis, and has relied on the assumption that the system is in steady state (both labelled and unlabelled). This assumption may not apply to lactating rat mammary glands, since the model shows that the percentage flow through the shunt progressively decreased for the first 2h of a 3h experiment, and we were unable to construct a completely steady-state model. The model allows examination of many quantitative features of the system, especially the amount of material passing through key enzymes, some of which appear to be regulated by NADP(+) concentrations as proposed by McLean (1960). Supplementary information for this paper has been deposited as Supplementary Publication SUP 50023 at the British Museum (Lending Division) (formerly the National Lending Library for Science and Technology), Boston Spa, Yorks. LS23 7BQ, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1973) 131, 5.

Animals↗
PubMed1978

New reaction sequences for the non-oxidative pentose phosphate pathway.

1. Reactions leading to the formation of 14C-labelled volatile compounds and compounds volatile under acid conditions were investigated in a system actively synthesizing hexose 6-phosphates from [U-14C]ribose 5-phosphate by reactions catalysed by enzymes prepared from acetone-dried powder of rat liver; no reactions involving 14C-labelled volatile compounds were detected. Similarly the fixation of 14C-labelled volatile compounds into hexose 6-phosphate could not be detected. 2. A complete carbon balance was made for the reactants, intermediates and products of the reactions involved in the conversion of ribose 5-phosphate into hexose 6-phosphate by enzymes of rat liver. Five additional intermediates of pentose 5-phosphate metabolism in liver were detected, namely D-manno-heptulose 7-phosphate, D-altro-heptulose 1,7-bisphosphate, D-glycero-D-ido-octulose 1,8-bisphosphate, D-glycero-D-altro-octulose 1,8-bisphosphate and D-arabinose 5-phosphate. 3. D-Arabinose 5-phosphate was found to be utilized by a rat liver enzyme preparation to produce both hexose 6-phosphate and triose phosphate. 4. D-Arabinose 5-phosphate was reversibly converted into other pentose 5-phosphates. Paper chromatographic and enzymic evidence indicated that the conversion involved an enzyme tentatively named arabinose phosphate 2-epimerase, which catalyses the following reaction: D-arabinose 5-P in equilibrium D-ribose-5-P. 5. A variety of rat tissues also utilized D-arabinose 5-phosphate to produce both hexose 6-phosphate and triose phosphate and at a rate comparable with that obtained with D-ribose 5-phosphate. 6. A new reaction sequence for the non-oxidative pentose phosphate pathway in liver is proposed.

Animals↗
PubMed1994

Coordination of Chloroplastic Metabolism in N-Limited Chlamydomonas reinhardtii by Redox Modulation (II. Redox Modulation Activates the Oxidative Pentose Phosphate Pathway during Photosynthetic Nitrate Assimilation).

The onset of photosynthetic NO3- assimilation in N-limited Chlamydomonas reinhardtii increased the initial extractable activity of the glucose-6-phosphate dehydrogenase (G6PDH), the key regulatory step of the oxidative pentose phosphate pathway. The total activated enzyme activity did not change upon NO3- resupply. The higher activity, therefore, represents activation of existing enzyme. No activation occurred during NH4+ assimilation. Incubation of extracts with DTT reversed the NO3- stimulation of G6PDH activity, indicating that the activation involved redox modulation of G6PDH. Phosphoribulosekinase, an enzyme activated by thioredoxin reduction, was inhibited at the onset of NO3- assimilation. A 2-fold stimulation of O2 evolution and a 70% decrease in the rate of photosynthetic CO2 assimilation accompanied the enzyme activity changes. There was an immediate drop in the NADPH and an increase in NADP upon addition of NO3-, whereas NH4+ caused only minor fluctuations in these pools. The response of C. reinhardtii to NO3- indicates that the oxidative pentose phosphate pathway was activated to oxidize carbon upon the onset of NO3- assimilation, whereas reduction of carbon via the reductive pentose phosphate pathway was inhibited. This demonstrates a possible role for the Fd-thioredoxin system in coordinating enzyme activity in response to the metabolic demands for reducing power and carbon during NO3- assimilation.

Journal Article↗
PubMed1972

Influence of Light Intensity on Reductive Pentose Phosphate Cycle Activity during Photoheterotrophic Growth of Rhodospirillum rubrum.

Light intensity during growth affects the proportion of carbon dioxide fixed by the reductive pentose phosphate cycle relative to that incorporated via C(4) acids in acetate phototrophs of Rhodospirillum rubrum. With cells grown at high light intensity (9000 lux) the specific activities of ribulose-1, 5-diphosphate and propionyl CoA carboxylases were increased compared with cells grown at low light intensity (1500 lux), although pyruvate carboxylase activity was unaltered.Kinetic experiments with cells assimilating acetate at high light intensity showed that when the cells had been grown at high light intensity there was a rapid incorporation of (14)CO(2) into phosphate esters compared with cells grown at low light intensity and fixing (14)CO(2) while assimilating acetate at low light intensity. The percentage of the total radioactivity present in phosphate esters plotted against time gave a negative slope for high light conditions compared with a positive slope for low light conditions. High light-grown cells assimilating acetate at high light intensity showed the greatest combined rate of (14)CO(2) fixation via the reductive pentose phosphate cycle and C(4) acids, and this corresponded to the shortest mean generation time. When cells were grown at high light intensity and allowed to assimilate (14)CO(2) at high light intensity but in the stationary phase, the pattern of (14)CO(2) fixation resembled that for low light-grown cells assimilating acetate and fixing (14)CO(2) at low light intensity, showing that both acetate assimilation and high light intensity were necessary for the rapid incorporation of (14)CO(2) via the reductive pentose phosphate cycle.

Journal Article↗
PubMed1982

The functional significance of the pentose phosphate pathway in synaptosomes: protection against peroxidative damage by catecholamines and oxidants.

Catecholamines added in vitro in rat brain synaptosomes activate the decarboxylation of glucose radioactively labelled on carbon 1, suggesting an effective activation of the pentose phosphate pathway. Stimulation also occurred with phenazine methosulphate, reduced glutathione and hydrogen peroxide. The activation of the pentose phosphate pathway by 5-hydroxytryptamine, noradrenaline and dopamine is ascribed to the activation of monoamine oxidase, producing both the respective biogenic aldehyde and hydrogen peroxide. Evidence is presented that the further metabolism of the aldehyde by aldehyde reductase and the removal of hydrogen peroxide by glutathione peroxidase both release the limitation of NADP+ availability for the pentose phosphate pathway by leading to the oxidation of NADPH. The relevance of the maintenance of reduced NADP+ on brain is discussed in relation to the metabolism of glutathione and to lipid peroxidation.

Aldehyde Oxidoreductases↗
PubMed1983

Alternative pathways of glucose utilization in brain: changes in the pattern of glucose utilization and of the response of the pentose phosphate pathway to 5-hydroxytryptamine during aging.

The oxidation of differentially labelled glucose, pyruvate and glutamate in brain slices from rats aged 20 days to 26 months has been studied and the partition of the glucose used into the glycolytic-tricarboxylic acid cycle pathway, the pentose phosphate pathway and the glutamate-GABA shunt has been calculated. Over the time range 4 to 26 months, there is an approximately 20% decrease in the production of CO2 via the glycolytic-tricarboxylic acid cycle route, as there is in the rate of glucose phosphorylation. The glutamate-GABA pathway falls by about 50% over this same time span. The broad activity of the pentose phosphate pathway falls rapidly and cannot be detected in the brains of rats aged 18 months or more, whereas the fully stimulated pathway, i.e. in the presence of the artificial electron acceptor phenazine methosulphate, declines only marginally over this period, falling sharply only after 23 months. The pentose phosphate pathway is stimulated by the presence of 5-hydroxytryptamine and this stimulation appears to increase with age.

Aging↗
PubMed1988

Effect of pentoses and pentitols on fermentation of hay by mixed populations of ruminal microorganisms.

Consecutive batch culture, a technique which involves sequential transfer of cultures to fresh medium at regular intervals, was used to establish mixed ruminal-microbial populations in an anaerobic medium containing highly digestible hay. Once volatile fatty acid production was stable, perturbations were imposed in consecutive cultures by the addition of one of each of the following pentoses or analogous pentitols: l-arabinose, d-lyxose, d-ribose, d-xylose, l-arabitol, d-arabitol (lyxitol), ribitol, and xylitol. With the exception of d-lyxose, the addition of pentoses caused marked increases in propionate and valerate production, and except for d-arabitol, pentitol addition caused increases in butyrate and valerate production. On transfer to and continued incubation in the control medium, volatile fatty acid production reverted to preperturbed levels. The presence of pentitols and pentoses significantly reduced the endpoint pH of cultures and the proportion of hay that was fermented. With all added substrates, the response to the perturbation was at its maximum within one incubation (i.e., within 48 h). Similarly, the variables being monitored all returned to control levels within one incubation. On the basis of these results, it is suggested that changes were related to the need to maintain a redox balance within anaerobic cultures rather than any significant changes in the microbial population that was present.

Journal Article↗
PubMed1966

Reductive pentose phosphate cycle in Nitrosocystis oceanus.

Campbell, Ann E. (Woods Hole Oceanographic Institution, Woods Hole, Mass.), Johan A. Hellebust, and Stanley W. Watson. Reductive pentose phosphate cycle in Nitrosocystis oceanus. J. Bacteriol. 91:1178-1185. 1966.-Assays in cell-free extracts of Nitrosocystis oceanus, a marine chemoautotrophic bacterium, have demonstrated the presence of all of the enzymes of the reductive pentose phosphate cycle, with activities high enough to account for the normal growth rate of the cells. Studies on ribulosediphosphate carboxylase activity in these extracts showed that it is inhibited by MgCl(2) (30% at 0.01 m), MnCl(2) (70% at 0.01 m), NaCl and KCl (100% at 0.5 m, 63% at 0.2 m), and by sulfate (35% at 0.01 m); phosphate, glutathione, and ethylenediaminetetraacetic acid had no effect. The bacterial enzyme differs from the spinach enzyme with respect to its affinity for bicarbonate and its pH optimum. Whole cells were incubated with C(14)O(2), and the acid-soluble fraction was analyzed by paper chromatography and autoradiography. Phosphoglyceric acid and the sugar phosphates were the earliest labeled compounds; several amino acids and organic acids were also labeled. It is concluded that N. oceanus incorporates CO(2) primarily via the reductive pentose phosphate cycle.

Bacteria↗
PubMed1994

CONTRIBUTION OF THE PENTOSE PHOSPHATE SHUNT TO THE FORMATION OF CO2 IN SWIMBLADDER TISSUE OF THE EEL

The contribution of the pentose phosphate shunt to glucose metabolism in the swimbladder tissue of the American eel Anguilla rostrata has been evaluated by comparing the rate of 14CO2 and [14C]lactate production from [1-14C]glucose and [6-14C]glucose. In blood-perfused swimbladder preparations, 0.18±0.07 nmol min-1 of [6-14C]glucose and 3.19±0.57 nmol min-1 of [1-14C]glucose were converted to CO2. The rate of [14C]glucose conversion to [14C]lactate was about the same in preparations perfused with [6-14C]glucose and with [1-14C]glucose. This may indicate that the C5 skeleton formed in the pentose phosphate shunt is not returned to glycolysis and converted to lactate. Although gas deposition was usually not measurable in these blood-perfused swimbladder preparations, 14CO2 was detected in the eel swimbladder gas of preparations perfused with [1-14C]glucose, but not in preparations perfused with [6-14C]glucose. The results confirm the hypothesis that, in the eel swimbladder epithelium, some of the glucose taken up from the blood is metabolized in the pentose phosphate shunt. This results in the formation of CO2, which is released into the swimbladder as well as into the bloodstream.

Journal Article↗
PubMedDate not supplied

[Activity of pentose phosphate pathway enzymes in alkane-oxidizing yeast cells].

The activity of the key enzymes of the pentose phosphate pathway (glucose 6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, transketolase) was determined in cell-free homogenates of Candida lipolytica 695 and Candida tropicalis 303 growing on different carbon sources. The activity of these enzymes remained almost the same in the course of growth of both cultures. The activity of the enzymes differed only slightly in the cells metabolizing hexadecane and glucose. The activity of glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase in the cell-free homogenates of C. tropicalis 303 was twice as high as in the cells of C. lipolytica 695. The activity of transketolase was the same in both cultures. The main role of the pentose phosphate pathway is presumed to consist not in catabolism of the carbon source, but in biosynthesis of pentoses and other important intermediates.

Alkanes↗
PubMed1978

Glucose oxidation in the chick cornea: effect of diamide on the pentose shunt.

Chick embryo corneas (stages 38 and 45) have been used to study variations in pentose shunt activity following the use of a glutathione-specific oxidizing agent, diamide, and a sulfydryl blocking agent, N-ethylmaleimide (NEM). Shunt activity was measured by the ratio of radiolabeled carbon 1 (14C-1) of glucose to radiolabeled carbon 6 (14C-6) of glucose derived as expired 14CO2. Diamide and NEM were both found to increase pentose shunt activity relative to glycolysis, although by different means. Diamide appeared to exert its effect by oxidizing glutathione and creating a demand for higher shunt activity to facilitate glutathione reduction by NADPH. Both C-1 and C-6 oxidation were increased, but C-1 oxidation was increased to a much greater extent. In contrast, NEM decreased both C-1 and C-6 oxidation, with C-6 preferentially affected. Thus NEM appears to preferentially inhibit the enzymatic machinery of the glycolytic-tricarboxylic acid cycle pathway and acts as an effective metabolic stress on the cornea. Our data suggest that the pentose shunt in the cornea may serve as an important alternative pathway under conditions of metabolic stress for glucose utilization and the production of energy (ATP) in the corneal cells.

Animals↗
PubMed1977

Quantitative analysis of the change of metabolite fluxes along the pentose phosphate and glycolytic pathways in Tetrahymena in response to carbohydrates.

A metabolic scheme of glycolysis and the pentose phosphate pathway has been constructed, assuming that the reactions occur in a single compartment. From this scheme, equations are written for a system in metabolic and isotopic steady state. These allow computation of the specific activity of every carbon atom of all the intermediates of the glycolytic and pentose phosphate pathways and consequently of the flux of carbon along each step of these pathways. A sufficiently large number of well distributed measurements of incorporation of radioactive label from different positions of several substrates into intermediates or products must be made to determine all the fluxes. This is done by choosing a set of metabolic fluxes, calculating incorporation with the aid of a computer, and then manipulating the flux rates until the computed incorporations match the data. The model is used in this paper to analyze the metabolism of the protozoan Tetrahymena pyriformis. The metabolic scheme of the model is consistent with all available information on the enzyme complement of this ciliate. Cells grown to transition phase in proteose/peptone medium were inoculated into a mixture of glucose (6 mM), fructose (6 mM), ribose (3 mM), and glycerol (3 mM) and incubated for 1 h. In each of these experiments, one of the following labeled substrates was present: [1-, 2-, 6-, or U-14C]glucose; [1- or U-14C]fructose; [1- or U-14C]ribose; [1(3)-or 2-14C]glycerol. The incorporation of label from these substrates into CO2, lipid, glycogen, and RNA was measured. In contrast to earlier studies on the metabolism of 2- and 3-carbon substrates by Tetrahymena, the rate of incorporation of label from some substrates into some products (e.g. from [1-14C]glucose into CO2) changed during the incubation. To treat these time-dependent data within the framework of the steady state model, the 1-h incubation was divided into three 20-min intervals; within each of these, the rates of incorporation were approximately constant, as required for a steady state system. Measurements of the pool sizes of glucose-6-P and fructose-6-P showed that only slow changes in pool sizes occurred after the first 5 min of incubation and indicated that the system was effectively in a metabolic and isotopic steady state throughout most of the incubation. The finding that a low concentration of cycloheximide prevented the acceleration of 14CO2 production from labeled glucose suggests a role for protein synthesis in the slow adaptation to carbohydrate addition and supports the quasi-steady state treatment of this system. The expected incorporation into each product was computed for trial sets of 1, independent flux rates. A set of flux values was found which yielded a good fit to the 29 measurements made for each interval. These flux values therefore constitute a quantitative description of temporal changes in carbon flow along the glycolytic and pentose phosphate pathways during the 1st h of adaptation to the carbohydrate mixture...

Adenosine Triphosphate↗
PubMed1981

[Dependence of effectiveness of the pentose pathway on ATP concentration in erythrocytes].

The dependence of effectivity of the pentose phosphate pathway on ATP concentration in human erythrocytes was established. The effectivity of the pentose phosphate pathway was decreased 2-fold with a fall in ATP concentration by 20% below the physiological values. The rate of glucose uptake remained practically unchanged thereby, i. e. the equilibrium between the pentose phosphate pathway and the Embden-Meyerhof pathway was markedly shifted towards the latter.

Adenosine Triphosphate↗
PubMed1976

Role of pentose-phosphate pathway in haemolytic crisis of chronic copper toxicity of sheep.

Although the rise in blood copper is associated with onset of the acute haemolytic crisis of chronic copper poisoning in sheep, the sudden fall in erythrocyte glutathione is apparently not due to a direct action of the copper. Moreover the reduced glutathione of the red cells is converted to some form that is not capable of regeneration by the pentose-phosphate mechanism. Only negligible inhibition of the pentose-phosphate enzymes occurs. As the haemolysis proceeds, there is a rapid recovery of erythrocyte glutathione levels, and a marked increase in pentose-phosphate enzyme activity, consistent with influx of young red cells. It seems that the release of copper into blood from liver at the haemolytic crisis is associated with an increase of the oxidative state of the blood, possibly by simultaneous release of other components from the liver.

Animals↗
PubMedDate not supplied

[Effect of manganese on transketolase activity and total pentose content in rabbits].

Different doses of manganese are shown to exert a positive effect on the transketolase activity in the blood serum and in certain tissues as well as on the amount of total pentoses in the blood serum of rabbits. This confirms intensification of the pentose-phosphate exchange of carbohydrates under the effect of manganese.

Animals↗
PubMed2004

Growth-rate recovery of Escherichia coli cultures carrying a multicopy plasmid, by engineering of the pentose-phosphate pathway.

Expression of plasmid-encoded genes in bacteria is the most common strategy for the production of specific proteins in biotechnological processes. However, the synthesis of plasmid-encoded proteins and plasmid-DNA replication often places a metabolic load (metabolic burden) into the cell's biochemical capacities that usually reduces the growth rate of the producing culture (Glick BR. Biotechnol Adv 1995;13:247-261). This metabolic burden may be related to a limited capacity of the cell to supply the extra demand of building blocks and energy required to replicate plasmid DNA and express foreign multicopy genes. Some of these required blocks are intermediaries of the pentose phosphate (PP) pathway, e.g., ribose-5-phosphate, erythrose-4-phosphate. Due to the important impact of metabolic burden on biotechnological processes, several groups have worked on developing strategies to overcome this problem, like reduction of plasmid copy number (Seo JH, Bailey JE. Biotechnol Bioeng 1985;27:1668-1674; Jones KL, Kim S, Keasling JD. Metab Eng 2000;3:328-338), chromosomal insertion of the gene which product is desired, or changing the plasmid-coded antibiotic resistance gene (Hong Y, Pasternak JJ, Glick BR. Can J Microbiol 1995;41:624-628). However, few efforts have been attempted to overcome the reduction of growth rate due to protein over-expression, by modifying central metabolic pathways (Chou C-H, Bennett GN, San KY. Biotechnol Bioeng 1994;44:952-960). We constructed a high-copy number plasmid carrying the gene for glucose-6-phosphate dehydrogenase, zwf, under the control of an inducible trc promoter (pTRzwf04 plasmid). By transforming a wild-type strain and inducing with IPTG, it was possible to recover growth-rate from 0.46 h(-1) (uninduced) to 0.64 h(-1) (induced). The same transformation in an Escherichia coli zwf(-), allows a growth-rate recovery from 0.43 h(-1) (uninduced) to 0.62 h(-1) (induced). We also studied this effect as part of a laboratory-scale biotechnology process: production of a recombinant insulin peptide by co-transforming E. coli JM101 strain with pTRzwf07, a low-copy-number plasmid that carries the same inducible construction as pTRzwf04, and with the pTEXP-MMRPI vector that carries a TrpLE-proinsulin hybrid gene. In this system, production of TrpLE-proinsulin strongly reduces growth rate; however, overexpression of zwf gene recovers with a growth rate from 0.1 h(-1) in the TrpLE-proinsulin induced strain, to 0.37 h(-1) when both zwf and TrpLE-proinsulin genes were induced. In this paper, we show that the engineering of the pentose phosphate pathway by modulation of the zwf gene expression level partially overcomes the possible bottleneck for the supply of building blocks and reducing power synthesized through the PP pathway, that are required for plasmid replication and plasmid-encoded protein expression.

Cell Proliferation↗
PubMed2006

Pentose phosphate cycle oxidative and nonoxidative balance: A new vulnerable target for overcoming drug resistance in cancer.

The metabolic network of cancer cells confers adaptive mechanisms against many chemotherapeutic agents, but also presents critical constraints that make the cells vulnerable to perturbation of the network due to drug therapy. To identify these fragilities, combination therapies based on targeting the nucleic acid synthesis metabolic network at multiple points were tested. Results showed that cancer cells overcome single hit strategies through different metabolic network adaptations, demonstrating the robustness of cancer cell metabolism. Analysis of these adaptations also identified the maintenance of pentose phosphate cycle oxidative and nonoxidative balance to be critical for cancer cell survival and vulnerable to chemotherapeutic intervention. The vulnerability of cancer cells to the imbalance on pentose phosphate cycle was demonstrated by phenotypic phase plane analysis.

Animals↗