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PubMedDate not supplied

[Dehydrogenase activity of pentose and glucuronate pathways of glucose utilization in the rat brain during single and repeated cooling].

Repeated supercoolings down to rectal temperatures (19-20 degrees C) results in the different changes in the dehydrogenase activity of pentose and glucuronate pathways in the rat brain: the activity of the pentose cycle oxidative enzymes (glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase) lowers considerably and that of VDP-glucose dehydrogenase rises. The dehydrogenase activity in the pentose cycle is found to be inhibited in rats cooled for the first time, the UDP-glucose dehydrogenase activity being preserved at the control level. In the adapted rats the cooling causes mobilization of the pentose cycle, the UDP-glucose dehydrogenase activity remains unchanged.

Adaptation, Physiological↗
PubMed1985

The significance of sedoheptulose 1,7-bisphosphate in the metabolism and regulation of the pentose pathway in liver.

Rat liver cytosolic enzyme preparation catalyses the formation of sedoheptulose 1,7-P2 (60% of total heptulose-P formed) from hexose 6-P and triose 3-P (reverse mode of pentose pathway operation). Smaller amounts of sedoheptulose 1,7-P2 are also formed from ribose 5-P during the non-oxidative synthesis of hexose 6-P (forward pentose pathway operation). The apparent absence of erythrose 4-P in biological systems may be explained by its contribution to carbons 4,5,6 and 7 of sedoheptulose 1,7-P2 as well as its pronounced ability to exist in dimeric form. Apart from the aldolase catalyzed formation of sedoheptulose 1,7-P2, 6-phosphofructokinase also catalyses its formation from sedoheptulose 7-P and fructose 1,6-bisphosphatase catalyses its dephosphorylation. These three enzymes may contribute to the regulation of carbon flux through the near equilibrium reactions of the non-oxidative pentose phosphate pathway in vivo. The phosphotransferase enzyme of the L-type pentose pathway is also able to catalyse the interconversion of sedoheptulose mono and bisphosphates via D-glycero D-ido octulose-P.

Animals↗
PubMed1984

Evidence that aldolase and D-arabinose 5-phosphate are components of pentose pathway reactions in liver in vitro.

An immunochemical procedure involving the reaction of liver aldolase antibody and rat liver enzyme preparation shows that conversion of ribose 5-P to hexose 6-P by reactions of the non-oxidative pentose pathway fails to occur in the absence of aldolase activity. Radioautography of pentose pathway products formed by liver enzyme catalysis of [U-14C] arabinose 5-P and unlabelled ribose 5-P illustrates the incorporation of 14C into ketopentose, sedoheptulose, fructose and glucose phosphates. There is approximate congruity of the mole specific radioactivity of the pentose and hexose phosphates. These findings are consistent with the proposal that L-pentose pathway reactions constitute the non-oxidative segment of the pathway in liver.

Animals↗
PubMed1993

Pentose utilization and transport by the ruminal bacterium Prevotella ruminicola.

Plant cell wall polysaccharides are primarily composed of hexose or hexose derivatives, but a significant fraction is hemicellulose which contains pentose sugars. Prevotella ruminicola B14, a predominant ruminal bacterium, simultaneously metabolized pentoses and glucose or maltose, but the organism preferentially fermented pentoses over cellobiose and preferred xylose to sucrose. Xylose and arabinose transport at either low (2 microM) or high (1 mM) substrate concentrations were observed only in the presence of sodium and if oxygen was excluded during the harvest and assay procedures. An artificial electrical potential (delta psi) or chemical gradient of sodium (delta pNa) drove transport in anaerobically prepared membrane vesicles. Because (i) transport was electrogenic, (ii) a delta pNa drove uptake, and (iii) the number of sodium binding sites was approximately 1, it appeared that P. ruminicola possessed pentose/sodium support mechanisms for the transport of arabinose and xylose at low substrate concentrations. Pentose uptake exhibited a low affinity for xylose or arabinose (> 300 microM), and transport of xylose exhibited bi-phasic kinetics which suggested that a second sodium-dependent xylose transport system was present. Little study has been made on solute transport by Prevotella (Bacteroides) species and this work represents the first use of isolated membrane vesicles from these organisms.

Animals↗
PubMed1975

The pentose cycle and insulin release in isolated mouse pancreatic islets during starvation.

When islets from mice were incubated with 16.7 mM-glucose, previous starvation for 48 h decreased the rate of insulin release by approx. 50% and glucose utilization was decreased by approx. 35%. The maximally extractable activity of glucose 6-phosphate dehydrogenase was diminished by 28% after starvation. The formation of 14CO2 from both [1-14C]glucose was, however, higher than the rate of oxidation of [6-14C]-glucose in islets from both fed and starved mice. The fraction of glucose utilized that was oxidized (specific 14CO2 yield) ranged from one-fifth to one-third and was higher in islets from starved mice with both [1-14C]glucose and [6-14C]glucose as substrate. The contribution of pentose-cycle oxidation to total glucose metabolism was small (3% in the fed state and 4% in the starved state). The absolute rates of glucose carbon metabolism via the pentose-cycle oxidation to total glucose metabolism was small (3% in the fed state and 4% in the starved state). The absolute rates of glucose carbon metabolism via the pentose cycle and the turnover of NADPH in this pathway were identical in islets from fed and starved animals. After incubation at 16.7 mM-glucose for 30 min the contents of glucose (6-phosphate and 6-phosphogluconate were both unchanged by starvation. It is concluded that there is no correlation between the decreased sensitivity of the insulin secretory mechanism during starvation and the metabolism of glucose via the pentose cycle, the islet content of glucose 6-phosphate or 6-phosphogluconate.

Animals↗
PubMed1980

Use of [2-14C]glucose and [5-14C]glucose for evaluating the mechanism and quantitative significance of the 'liver-cell' pentose cycle.

1. Expressions are derived for the steady-state measurement of the quantitative contribution of the liver-type pentose phosphate cycle to glucose metabolism by tissues. One method requires the metabolism of [5-14C]glucose followed by the isolation and degradation of glucose 6-phosphate. The second procedure involves the metabolism of [2-14C]glucose and the isolation and degradation of a triose phosphate derivative, usually lactate or glycerol. 2. Measurements of 14C in C-2 and C-5 of glucose 6-phosphate are required and the values of the C-2/C-5 ratios can be used to calculate the quantitative contribution of the L-type pentose cycle in all tissues. 3. The measurement of 14C in C-1, C-2 and C-3 of triose phosphate derivatives can be used to calculate the quantitative contribution of the L-type pentose cycle relative to glycolysis. 4. The effect of transaldolase and transketolase exchange reactions, reactions of gluconeogenesis and non-oxidative formation of pentose 5-phosphate, isotopic equilibration of triose phosphate pools and isotopic equilibration of fructose 6-phosphate and glucose 6-phosphate, which could interfere with a clear interpretation of the data using [2-14C]- and [5-14C]glucose are discussed.

Carbon Radioisotopes↗
PubMed1976

Thiamin deficiency and the pentose phosphate cycle in rats: intracerebral mechanisms.

Previous studies have shown that transketolase activity is decreased in the brains of thiamin deficient rats. This study assesses the effect of decreased transketolase levels on the activity of the pentose phosphate cycle in murine thiamin deficient cortex and brainstem. Thiamin deficiency was produced in newborn and adult rats by either pyrithiamin administration or by feeding a low thiamin diet. Newborn rats were killed at 22 days of age, and adults were killed at the onset of moderate to severe nurological signs. Cortices and brainstems from thiamin deficient and control rats were analyzed for activity of the two regulatory enzymes of the pentose phosphate cycle, glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase. Flux through the pathway was measured by the differentially labeled glucose technique in the brainstems of deficient and control adult rats. In both the brainstem and cortex of thiamin dificient rats, areas in which transketolase activity was decreased up to 65%, the activities of the two regulatory enzymes, glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, were unaltered. Further, flux through the pentose phosphate cycle was not decreased as compared to pair-fed control rats. These data do not support the hypothesis that in thiamin dificient rats, a decrease in cerebral transketolase activity leads to a diminished pentose phosphate cycle activity.

Age Factors↗
PubMed1971

The pentose phosphate pathway in relation to fat synthesis in the developing castor oil seed.

Slices of 25- to 28-day-old developing castor bean endosperm were incubated with various (14)C- and (3)H-labeled substrates to determine the amount of glucose dissimilated in the pentose phosphate pathway and to determine the use of the reduced nucleotides so produced in fatty acid synthesis. Ten to 12% of the metabolized glucose traversed the pentose phosphate pathway, and reduced nicotinamide adenine dinucleotide phosphate (NADPH) production would be sufficient to supply 51 to 68% of the reducing equivalents required for fat synthesis. However, using (3)H-NADPH produced from 3-(3)H-glucose as a tracer, it was found that only 40% of the NADPH produced in the pentose phosphate pathway was used in fat synthesis. Thus the actual contribution of the reducing equivalents generated from the pentose phosphate pathway to fat synthesis was 20 to 27% of that required. Because of the methods and assumptions, this value represents a minimal estimate of NADPH used in fat synthesis, and the actual contribution may be somewhat higher. However, tritium from (3)H-NADH generated from 1-(3)H-ethanol was incorporated into fatty acids, and it is contended that NADH may supply a large proportion of the reducing equivalents necessary for fat synthesis in this tissue.

Journal Article↗
PubMed1990

Activities of the pentose phosphate pathway and enzymes of proline metabolism in legume root nodules.

Based on localization and high activities of pyrroline-5-carboxylate reductase and proline dehydrogenase activities in soybean nodules, we previously suggested two major roles for pyrroline-5-carboxylate reductase in addition to the production of the considerable quantity of proline needed for biosynthesis; namely, transfer of energy to the location of biological N(2) fixation, and production of NADP(+) to drive the pentose phosphate pathway. The latter produces ribose-5-phosphate which can be used in de novo purine synthesis required for synthesis of ureides, the major form in which biologically fixed N(2) is transported from soybean root nodules to the plant shoot. In this paper, we report rapid induction (in soybean nodules) and exceptionally high activities (in nodules of eight species of N(2)-fixing plants) of pentose phosphate pathway and pyrroline-5-carboxylate reductase. There was a marked increase in proline dehydrogenase activity during soybean (Glycine max) ontogeny. The magnitude of proline dehydrogenase activity in bacteroids of soybean nodules was sufficiently high during most of the time course to supply a significant fraction of the energy requirement for N(2) fixation. Proline dehydrogenase activity in bacteroids from nodules of other species was also high. These observations support the above hypothesis. However, comparison of pentose phosphate pathway and pyrroline-5-carboxylate reductase activities of ureide versus amide-exporting nodules offers no support. The hypothesis predicts that pyrroline-5-carboxylate and pentose phosphate pathway activities should be higher in ureide-exporting nodules than in amide-exporting nodules. This predicted distinction was not observed in the results of in vitro assays of these activities.

Journal Article↗
PubMed1982

Hemolytic anemia in hereditary pyrimidine 5'-nucleotidase deficiency: nucleotide inhibition of G6PD and the pentose phosphate shunt.

We evaluated the erythrocytes of two patients with hereditary pyrimidine 5'-nucleotidase deficiency. Significant findings included an increased reduced glutathione content, increased incubated Heinz body formation, a positive ascorbate cyanide test, and decreased intraerythrocytic pH. The pentose phosphate shunt activity of the patients' red cells as measured by the release of 14CO2 from 14C-1-glucose was decreased compared to high reticulocyte controls. Glucose-6-phosphate dehydrogenase (G6PD) activity in hemolysates from control erythrocytes was inhibited 43% by 5.5 mM cytidine 5'-triphosphate (CTP) and 50% by 5.5 mM in uridine 5'-triphosphate (UTP) at pH 7.1. CTP was a competitive inhibitor for G6P (Ki = 1.7 mM) and a noncompetitive inhibitor for NADP+ (Ki = 7.8 mM). Glutathione peroxidase, glutathione reductase, and 6-phosphogluconate dehydrogenase were not affected by these compounds. Pentose phosphate shunt activity in control red cell hemolysate at pH 7.1 was inhibited to a similar degree by 5.5 mM CTP or UTP. Since the intracellular concentrations of G6P and NADP+ are below their KmS for G6PD, these data suggest that high concentrations of pyrimidine 5'-nucleotides depress pentose phosphate shunt activity in pyrimidin 5'-nucleotidase deficiency. Thus, this impairment of the pentose phosphate pathway appears to contribute to the pathogenesis of hemolysis in pyrimidine 5'-nucleotidase deficiency hemolytic anemia.

5'-Nucleotidase↗
PubMed1984

Inhibition of the pentose phosphate shunt by lead: a potential mechanism for hemolysis in lead poisoning.

Recent investigations have disclosed a decrease in pentose phosphate shunt activity in hereditary pyrimidine 5'-nucleotidase deficiency. Clinical lead poisoning is associated with an acquired decrease in pyrimidine 5'-nucleotidase activity. The current investigations were undertaken (1) to determine if pentose shunt activity was decreased in erythrocytes exposed to lead, and (2) to compare the mechanism of inhibition to that seen in hereditary pyrimidine 5'-nucleotidase deficiency. Normal erythrocytes incubated with lead acetate in vitro demonstrated increased Heinz body formation, decreased reduced glutathione, a positive ascorbate cyanide test, and a reversible suppression of pentose shunt activity in the intact erythrocyte. Lead acetate added to normal red cell hemolysates markedly inhibited the activities of glucose-6-phosphate dehydrogenase (G6PD) and phosphofructokinase. The mean Kis of lead for glucose-6-phosphate and nicotinamide adenine dinucleotide phosphate (NADP) for G6PD were 1.5 microM and 2.1 microM, respectively, which is within the range of intraerythrocytic lead concentrations found in clinical lead poisoning. Magnesium enhanced the ability of lead to inhibit G6PD. Thus, the shortened erythrocyte survival in lead poisoning appears to be due, in part, to increased oxidant sensitivity secondary to inhibition of G6PD and the pentose shunt. The mechanism of shunt inhibition is, in part, similar to that seen in hereditary pyrimidine 5'-nucleotidase deficiency.

5'-Nucleotidase↗
PubMed1998

Mass isotopomer study of the nonoxidative pathways of the pentose cycle with [1,2-13C2]glucose.

We present a single-tracer method for the study of the pentose phosphate pathway (PPP) using [1,2-13C2]glucose and mass isotopomer analysis. The metabolism of [1,2-13C2]glucose by the glucose-6-phosphate dehydrogenase, transketolase (TK), and transaldolase (TA) reactions results in unique pentose and lactate isotopomers with either one or two 13C substitutions. The distribution of these isotopomers was used to estimate parameters of the PPP using the model of Katz and Rognstad (J. Katz and R. Rognstad. Biochemistry 6: 2227-2247, 1967). Mass and position isotopomers of ribose, and lactate and palmitate (products from triose phosphate) from human hepatoma cells (Hep G2) incubated with 30% enriched [1,2-13C2]glucose were determined using gas chromatography-mass spectrometry. After 24-72 h incubation, 1.9% of lactate molecules in the medium contained one 13C substitution (m1) and 10% contained two 13C substitutions (m2). A similar m1-to-m2 ratio was found in palmitate as expected. Pentose cycle (PC) activity determined from incubation with [1,2-13C2]glucose was 5.73 +/- 0.52% of the glucose flux, which was identical to the value of PC (5.55 +/- 0.73%) determined by separate incubations with [1-13C] and [6-13C]glucose, 13C was found to be distributed in four ribose isotopomers ([1-13C]-, [5-13C]-, [1,2-13C2]-, and [4,5-13C2]ribose). The observed ribose isotopomer distribution was best matched with that provided from simulation by substituting 0.032 for TK and 0.85 for TA activity relative to glucose uptake into the model of Katz and Rognstad. The use of [1,2-13C2]glucose not only permits the determination of PC but also allows estimation of relative rates through the TK and TA reactions.

Carbon Isotopes↗
PubMed1985

Increased Heinz body formation and impaired erythrocyte pentose phosphate shunt function during pregnancy.

The erythrocytes of 90 pregnant women were evaluated for the presence of in vivo or in vitro oxidant damage. The reduced glutathione (P less than 0.005) and the membrane reduced sulfhydryl (P less than 0.001) concentrations were decreased in fresh erythrocytes. Following incubation with acetylphenylhydrazine, Heinz body formation was significantly increased (P less than 0.001). Both the increase in Heinz body formation and the reduction in membrane reduced sulfhydryl content correlated strongly with duration of pregnancy. Glucose consumption was significantly decreased before, but not after, new methylene blue stimulation. Pentose phosphate shunt activity was impaired both before (P less than 0.05) and after (P less than 0.001) stimulation. No changes were observed in pentose phosphate recycling. The only alteration observed in the activity of the enzymes of the pentose shunt was an elevation of 6-phosphogluconate dehydrogenase activity. Although the clinical significance of these findings remains to be determined, medications with an oxidant potential should be used judiciously during gestation.

Erythrocyte Aging↗
PubMed1999

In vivo dynamics of the pentose phosphate pathway in Saccharomyces cerevisiae.

The in vivo dynamics of the pentose phosphate pathway has been studied with transient experiments in continuous culture of Saccharomyces cerevisiae. Rapid sampling was performed with a special sampling device after disturbing the steady state with a pulse of glucose. The time span of observation was 120 s after the pulse. During this short time period the dynamic effect of protein biosynthesis can be neglected. The metabolites of interest (glucose 6-phosphate, NADP, NADPH, 6-phosphogluconate, and MgATP2-) we determined with enzymatic assays and HPLC. The experimental observations were then used for the identification of kinetic rate equations and parameters under in vivo conditions. In accordance with results from in vitro studies the in vivo diagnosis supports an ordered Bi-Bi mechanism with noncompetitive inhibition by MgATP2- for the enzyme glucose-6-phosphate dehydrogenase. In the case of 6-phosphogluconate dehydrogenase an ordered Bi-Ter mechanism with a competitive inhibition by MgATP2- has been found. Because the MgATP2- concentration decreases abruptly after the pulse of glucose the inhibitory effect vanishes and the flux through the pentose phosphate pathway increases. This regulation phenomenon guarantees the balance of fluxes through glycolysis and pentose phosphate pathway during the dynamic time period.

Adenosine Triphosphate↗
PubMed2002

Changes of pentose phosphate pathway flux in vivo in Corynebacterium glutamicum during leucine-limited batch cultivation as determined from intracellular metabolite concentration measurements.

Corynebacterium glutamicum is an important organism for the industrial production of amino acids such as lysine. In the present study time-dependent changes in the oxidative pentose phosphate pathway activity, an important site of NADPH regeneration in C. glutamicum, are investigated, whereby intracellular metabolite concentrations and specific enzyme activities in two isogenic leucine auxotrophic strains differing only in the regulation of their aspartate kinases were compared. After leucine limitation only the strain with a feedback-resistant aspartate kinase began to excrete lysine into the culture medium. Concomitantly, the intracellular NADPH to NADP concentration ratio increased from 2 to 4 in the non-producing strain, whereas it remained constant at about 1.2 in the lysine-producing strain. From these data the in'vivo flux through the pentose phosphate pathway was calculated. These results were used to approximate the total NADPH regeneration by glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase and isocitrate dehydrogenase, which agreed fairly well with the calculated demands for biomass formation and lysine biosynthesis. The analysis allowed to conclude that NADPH regeneration in the pentose phosphate pathway is essential for lysine biosynthesis in C. glutamicum.

Adenosine Diphosphate↗
PubMed1987

The NADPH consumption regulates the NADPH-producing pathways (pentose phosphate cycle and malic enzyme) in rat adipocytes.

The changes in the activity of the pentose phosphate cycle and the malic enzyme produced by the activation or inhibition of different NADPH-consuming pathways have been studied. The inhibition of the fatty acid synthesis by kynurenate produced a decrease in the flux through the pentose phosphate cycle and a diminution in the malic enzyme pathway. The incubation of the adipocytes in the presence of ter-butyl-hydroperoxide, a compound which is metabolized via a NADPH-consuming pathway, produced a big increase in the pentose phosphate cycle and the malic enzyme activities. The regulation of these NADPH-producing pathways by the NADPH/NADP ratio is discussed.

Adipose Tissue↗
PubMed1995

A model of the pentose phosphate pathway in rat liver cells.

A mathematical model based on kinetic data taken from the literature is presented for the pentose phosphate pathway in fasted rat liver steady-state. Since the oxidative and non oxidative pentose phosphate pathway can act independently, the complete (oxidative+non oxidative) and the non oxidative pentose pathway were stimulated. Sensitivity analyses are reported which show that the fluxes are mainly regulated by D-glucose-6-phosphate dehydrogenase (for the oxidative pathway) and by transketolase (for the non oxidative pathway). The most influent metabolites were the group ATP, ADP, P1 and the group NADPH, NADP+ (for the non oxidative pathway).

Animals↗
PubMed1990

Effect of 6-aminonicotinamide on pentose cycle activity in isolated hepatocytes.

1. 6-aminonicotinamide (6AN), a purported inhibitor of 6-phosphogluconate (6PG) dehydrogenase, has been regarded as an inhibitor of the pentose cycle. 2. Incubation of isolated hepatocytes with 6AN caused a time- and concentration-dependent accumulation of 6PG. 3. At 5 mM, 6AN increased the 6PG level 1000-times to values comparable to those observed in the livers of rats injected with this niacin antagonist. 4. Despite the accumulation of 6PG, neither the total rate of lipogenesis, nor the incorporation of radioactivity from [3-3H]glucose, used to estimate the activity of the pentose cycle, were impaired to a large extent. 5. The evidence presented suggests that the accumulation of 6PG is not a sufficient criterion to establish blockade of the pentose cycle.

6-Aminonicotinamide↗