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PubMed1995

Chloroplast pentose-5-phosphate 3-epimerase from potato: cloning, cDNA sequence, and tissue-specific enzyme accumulation.

A cDNA clone encoding the chloroplast enzyme pentose-5-phosphate 3-epimerase (EC 5.1.3.1) in potato (Solanum tuberosum) was isolated and sequenced. The deduced sequence of 235 amino acids is similar to protein sequences of bacterial epimerases. Northern blot analysis showed the highest level of epimerase mRNA expression in potato leaves, whereas it was low in roots, tubers, and stems. Epimerase protein is mulated only in plant tissues possessing chloroplasts, i.e. in land to a lesser extent in stem. In contrast, transketolase, a sequential enzyme of epimerase in the reductive and oxidative pentose phosphate cycle, is accumulated in all plant tissues.

Amino Acid Sequence↗
PubMed1982

Mechanism and contribution of the pentose phosphate cycle to glucose metabolism in epididymal fat tissue.

1. Glucose 6-phosphate, fructose 6-phosphate and altroheptulose 7-phosphate are the major products formed non-oxidatively from ribose 5-phosphate by rat epididymal fat pad enzymes. 2. Arabinose 5-phosphate was detected among the reaction products and significant activity of the new enzyme of the L-type pentose pathway, D-glycero D-ido octulose 1,8-bisphosphate: D-altroheptulose 7-phosphotransferase was found. 3. The glucose moieties of glucose 1-phosphate, glucose 6-phosphate and glucose 1,6-bisphosphate were degraded and showed that epididymal fat pad enzymes relocate 14C from [2-14C]glucose into C-1, C-2, and C-3 of each hexose-phosphate. 4. The 14C-distribution patterns in the hexose-phosphates revealed that these intermediates were not in isotopic equilibrium and the rate of the transaldolase exchange reaction was relatively small. 5. The 14C-distribution data suggest that glucose 1-phosphate, rather than glucose 6-phosphate, is the first intermediate in the path of glycogen synthesis from glucose in this tissue. 6. The data provide the first proof of the mechanism of the pentose pathway in adipose tissue.

Adipose Tissue↗
PubMed1997

The pentose phosphate pathway and parasitic protozoa.

The pentose phosphate pathway plays a crucial role in the host-parasite relationship. It maintains a pool of NADPH, which serves to protect against oxidant stress and which generates carbohydrate intermediates used in nucleotide and other biosynthetic pathways. Deficiency in the first enzyme of the pathway, glucose-6-phosphate dehydrogenase, protects human erythrocytes from infection with Plasmodium falciparum for reasons that remain obscure. Loss of the third enzyme of the pathway, 6-phosphogluconate de-hydrogenase, is toxic, suggesting this enzyme might be a target for chemotherapy. Mike Barrett here summarizes the roles of the pentose phosphate pathway in various parasitic protozoa.

Journal Article↗
PubMed1999

Mineral induced formation of pentose-2,4-bisphosphates.

Formation of rac.-pentose-2,4-bisphosphates is demonstrated, starting from glycolaldehyde phosphate and glyceraldehyde-2-phosphate, and induced by mixed valence double layer metal hydroxide minerals. The reactions proceed from dilute aqueous reactant solutions (1.5 mM) at near neutral pH. Conditions have been established, where ribose-2,4-bisphosphate is the major product (approximately 48%) among the pentose-2,4-bisphosphates, which are formed with up to 25% yield.

Aluminum Hydroxide↗
PubMed1965

Estimation of the pentose cycle in the perfused cow's udder.

1. The distributions of (14)C have been compared in the glucose and galactose moieties of lactose obtained from cows' udders perfused with blood containing [1-(14)C]-, [2-(14)C]- and [6-(14)C]-glucose. The (14)C of the glucose moiety was found in the same position as that of the administered glucose, but in the galactose moiety the (14)C from [2-(14)C]glucose was extensively randomized into positions 1 and 3. It is concluded that the glucose moiety arose from free glucose and the galactose moiety from hexose phosphate intermediates and that the latter reflected the randomization occurring through reactions of the pentose cycle. 2. The proportion of the glucose metabolized via the pentose cycle for those cells making lactose was estimated from the distribution of (14)C in the galactose moiety and found to be about 23% in one experiment and 30% in another experiment. 3. The yield and distribution of (14)C were determined in the glycerol of fat from the tissue in experiments with [2-(14)C]- and [6-(14)C]-glucose. There was a greater randomization of (14)C in the glycerol than in C-1, C-2 and C-3 of the galactose moiety of lactose. The ratio of the yield of (14)C in the glycerol from [2-(14)C]glucose to that of [6-(14)C]glucose was very low and from this ratio it was calculated that less than 10% of the glucose was metabolized by the Embden-Meyerhof pathway and approx. 60-70% was converted into lactose. 4. [6-(14)C]Glucose and [6-(3)H]glucose were used to determine whether the (3)H at the C-6 position remained stable during its conversion into glyceride of fat from the tissue. Twenty-seven per cent of the (3)H was labilized during this conversion. Therefore it was not possible to use [2-(14)C]glucose and [6-(3)H]glucose in a single experiment to measure the relative conversion of the C-2 and C-6 positions of glucose to glycerol.

Animals↗
PubMed1966

Glucose metabolism in the mucosa of the small intestine. Enzymes of the pentose phosphate pathway.

1. The occurrence of five enzymes of the pentose phosphate pathway in cell-free preparations of the mucosa of rat small intestine is described. These enzymes were found to be localized mainly in the supernatant fraction (6240000g-min.). 2. The properties of glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase were studied with respect to K(m) values for substrates and NADP(+), pH optima and the effects of p-chloromercuribenzoate and palmitoyl-CoA. Higher total and specific activities of these two dehydrogenases were noted in the proximal half of the small intestine of the rat than in the distal half. 3. The specific activities of glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase in the mucosa of the small intestine of the rat, cat, rabbit and guinea pig were compared. 4. In the rat the specific activities of ribose 5-phosphate isomerase, transketolase and transaldolase were higher in the supernatant fractions from the intestinal mucosa than in those from the liver. 5. The role of the pentose phosphate pathway is discussed in relation to the metabolism of hexose phosphates in the intestinal mucosa.

Animals↗
PubMed1974

Catalysis of pentose phosphate pathway reactions by cytoplasmic fractions from muscle, uterus and liver of the rat, and the presence of a reduced nicotinamide-adenine dinucleotide phosphate-triose phosphate oxidoreductase in rat muscle.

1. The enzymes of the pentose phosphate pathway were assayed in supernatant fractions from rat muscle, liver and uterus. 2. On incubation of ribose 5-phosphate with uterus and liver supernatants, triose phosphate, sedoheptulose 7-phosphate and hexose monophosphate accumulated. 3. When a muscle supernatant was used, glycerol 3-phosphate instead of triose phosphate appeared and there was a negligible accumulation of hexose monophosphate. 4. Hexose monophosphate production from ribose 5-phosphate was also followed by measuring NADP(+) reduction in the presence of an excess of phosphoglucose isomerase, glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase. 5. With a muscle supernatant, NADPH was reoxidized as rapidly as it was formed owing to the presence of a NADPH-triose phosphate oxidoreductase. 6. A modification of the pentose phosphate pathway in skeletal muscle incorporating this enzyme is proposed.

Animals↗
PubMed1974

Regulation of the pentose phosphate cycle.

1. A search was made for mechanisms which may exert a ;fine' control of the glucose 6-phosphate dehydrogenase reaction in rat liver, the rate-limiting step of the oxidative pentose phosphate cycle. 2. The glucose 6-phosphate dehydrogenase reaction is expected to go virtually to completion because the primary product (6-phosphogluconate lactone) is rapidly hydrolysed and the equilibrium of the joint dehydrogenase and lactonase reactions is in favour of virtually complete formation of phosphogluconate. However, the reaction does not go to completion, because glucose 6-phosphate dehydrogenase is inhibited by NADPH (Neglein & Haas, 1935). 3. Measurements of the inhibition (which is competitive with NADP(+)) show that at physiological concentrations of free NADP(+) and free NADPH the enzyme is almost completely inhibited. This indicates that the regulation of the enzyme activity is a matter of de-inhibition. 4. Among over 100 cell constituents tested only GSSG and AMP counteracted the inhibition by NADPH; only GSSG was highly effective at concentrations that may be taken to occur physiologically. 5. The effect of GSSG was not due to the GSSG reductase activity of liver extracts, because under the test conditions the activity of this enzyme was very weak, and complete inhibition of the reductase by Zn(2+) did not abolish the GSSG effect. 6. Preincubation of the enzyme preparation with GSSG in the presence of Mg(2+) and NADP(+) before the addition of glucose 6-phosphate and NADPH much increased the GSSG effect. 7. Dialysis of liver extracts and purification of glucose 6-phosphate dehydrogenase abolished the GSSG effect, indicating the participation of a cofactor in the action of GSSG. 8. The cofactor removed by dialysis or purification is very unstable. The cofactor could be separated from glucose 6-phosphate dehydrogenase by ultrafiltration of liver homogenates. Some properties of the cofactor are described. 9. The hypothesis that GSSG exerts a fine control of the pentose phosphate cycle by counteracting the NADPH inhibition of glucose 6-phosphate dehydrogenase is discussed.

Adenosine Monophosphate↗
PubMed1978

Regulation of the oxidative phase of the pentose phosphate cycle in mussels.

1. The mechanisms that control the oxidative phase of the pentose phosphate cycle in mussel hepatopancreas were investigated. 2. The effects of GSSG (oxidized glutathione) on the inhibition of glucose 6-phosphate dehydrogenase by NADPH [Eggleston & Krebs (1974) Biochem. J. 138, 425-435] extend to 6-phosphogluconate dehydrogenase. 3. The effect of GSSG on both enzymes increases as the [NADP+1]/[NADPH] ratio decreases; greater percentage deinhibition always was obtained for 6-phosphogluconate dehydrogenase. 4. Increasing concentration of GSSG increased the percentage deinhibition. This effect is more pronounced with 6-phosphogluconate dehydrogenase. 5. We confirmed the apparent imbalance between the activities of the two enzymes [sapag-Hagar, Lagunas & Sols (1973) Biochem. Biophys. Res. Commun, 50, 179-185] in the presence of 10mM-Mg2+. 6. The imbalance practically disappears when the substrate concentrations are less than saturating and Mg2+ approaches physiological concentrations. 7. The addition of GSSG at physiological concentrations allows the activities of both enzymes to be measured at high [NADPH]/[NADP+] ratios ratios and the co-operative action of GSSG and Mg2+ on the imbalance between the two enzymes to be verified. 8. The control of the activity of the two enzymes of the pentose cycle could be carried out by deinhibition of the two dehydrogenases and by the intracellular concentrations of substrates and inorganic ions.

Animals↗
PubMed1983

Quantitative analysis of flux along the gluconeogenic, glycolytic and pentose phosphate pathways under reducing conditions in hepatocytes isolated from fed rats.

Hepatocytes were isolated from the livers of fed rats and incubated with a mixture of glucose (10 mM), ribose (1 mM), mannose (4 mM), glycerol (3 mM), acetate (1.25 mM), and ethanol (5 mM) with one substrate labelled with 14C in any given incubation. Incorporation of label into CO2, glucose, glycogen, lipid glycerol and fatty acids, acetate and C-1 of glucose was measured at 20 and 40 min after the start of the incubation. The data (about 48 measurements for each interval) were used in conjunction with a single-compartment model of the reactions of the gluconeogenic, glycolytic and pentose phosphate pathways and a simplified model of the relevant mitochondrial reactions. An improved method of computer analysis of the equations describing the flow of label through each carbon atom of each metabolite under steady-state conditions was used to compute values for the 34 independent flux parameters in this model. A good fit to the data was obtained, thereby permitting good estimates of most of the fluxes in the pathways under consideration. The data show that: net flux above the level of the triose phosphates is gluconeogenic; label in the hexose phosphates is fully equilibrated by the second 20 min interval; the triose phosphate isomerase step does not equilibrate label between the triose phosphates; substrate cycles are operating at the glucose-glucose 6-phosphate, fructose 6-phosphate-fructose 1,6-bisphosphate and phosphoenolpyruvate-pyruvate-oxaloacetate cycles; and, although net flux through the enzymes catalysing the non-oxidative steps of the pentose phosphate pathway is small, bidirectional fluxes are large.

Acetates↗
PubMed1986

Quantitative determination of the pentose phosphate pathway in preimplantation mouse embryos.

A quantitative calculation was made of the pentose phosphate pathway (PPP) activity in preimplantation mouse embryos from the 2-cell through the late blastocyst stage. This activity varied with development and showed repeated high and low values. Peak activities occurred at both the 2-cell (15.8%) and compacted morula (13.6%) stages, with lowest activity at the development of the late blastocyst (3.2%). The metabolic effectors dimitrophenol (DNP) and phenazine ethosulfate (PES) had opposite effects on PPP activity. Dinitrophenol, although stimulating total CO2 production, virtually eliminated PPP activity while PES stimulated the pentose cycle activity 6-fold. These results indicated that the PPP was under metabolic control and that the embryos had a potential for much larger PPP activities. There was no correlation between the C-1/C-6 ratio obtained from the metabolism of [1-14C] and [6-14C] glucose and calculated PPP activities. A metabolic incubation chamber was devised for these experiments that exhibited certain unique features, including continuous collection of 14CO2 and 3H2O. Single embryos were placed in the chamber and sampled momentarily for metabolic activity. Subsequently, such embryos were successfully transferred to pseudopregnant recipients.

Animals↗
PubMed1992

Evidence for Activation of the Oxidative Pentose Phosphate Pathway during Photosynthetic Assimilation of NO(3) but Not NH(4) by a Green Alga.

Addition of NO(3) (-) to N-limited Selenastrum minutum during photosynthesis resulted in an immediate drop in the NADPH/NADP ratio and a slower increase of the NADH/NAD ratio. These changes were accompanied by a rapid decrease in glucose-6-phosphate and increase in 6-phosphogluconate, indicating activation of glucose-6-phosphate dehydrogenase and a role for the oxidation pentose phosphate pathway during photosynthetic NO(3) (-) assimilation. In contrast, the short-term changes in pyridine nucleotides and metabolites during photosynthetic assimilation of NH(4) (+) were not consistent with a stimulation of the oxidative pentose phosphate pathway.

Journal Article↗
PubMed1975

Development and intracellular distribution of enzymes of the oxidative pentose phosphate cycle in radish cotyledons.

Developmental and compartmentation studies were used to evaluate the relative roles of the oxidative pentose phosphate cycle, the Calvin cycle, and the glycolysis in cotyledons of radish (Raphanus sativus L.).Glucose-6-P dehydrogenase, 6-P-gluconate dehydrogenase, glucose-6-P isomerase, and the NAD-dependent glyceraldehyde-3-P dehydrogenase were present in high activity in ungerminated seeds, increased about 2-fold during germination in the dark, and were slightly enhanced by light. In contrast, NADP-dependent glyceraldehyde-3-P dehydrogenase was developed to only a small degree in the dark, but increased severalfold in continuous white or far red light. The activity of phosphofructokinase was low throughout germination.The separation of cell compartment-specific isoenzymes showed that, except in ungerminated seeds, the plastid enzyme accounted for 40 to 45% of the total activity of 6-P-gluconate dehydrogenase and for 15 to 20% of glucose-6-P isomerase. The remaining activity was due to the cytosolic isoenzymes. The presence of glucose-6-P dehydrogenase and 6-P-gluconate dehydrogenase in plastids was also established by their presence in the isolated organelle. The NAD-dependent glyceraldehyde-3-P dehydrogenase was mostly due to the cytosolic isoenzyme, whereas the NAD-dependent activity associated with the NADP-dependent glyceraldehyde-3-P dehydrogenase was very small.The data indicate that the enzymes of the oxidative pentose phosphate cycle are present in the cytosol throughout germination. In the plastids these enzymes already became fully developed during early germination in the dark, whereas enzymes of the Calvin cycle increased only in the light. Glycolysis seemed to be of minor importance.

Journal Article↗
PubMed1977

Enzymes of the Glycolytic and Pentose Phosphate Pathways in Proplastids from the Developing Endosperm of Ricinus communis L.

The metabolism of sucrose to long chain fatty acids in the endosperm of developing castor bean (Ricinus communis L.) seeds requires a combination of cytosolic and proplastid enzymes. The total activity and the subcellular distribution of the intermediate enzymic steps responsible for the conversion of sucrose to pyruvate have been determined. Hexose phosphate synthesis from sucrose occurs in the cytosol along with the first oxidative step in the pentose phosphate pathway, glucose-6-phosphate dehydrogenase. The proplastids contain the necessary complement of glycolytic enzymes to account for the in vivo rates of acetate synthesis from glucose 6-phosphate. These organelles also contain the majority of the cellular 6-phosphogluconate dehydrogenase, transketolase, and transaldolase activities.The consequence of these enzyme distributions is that glucose 6-phosphate or 6-phosphogluconate produced in the cytosol must be transported into the proplastids where conversion to pyruvate occurs. The unique segregation of the two oxidative steps in the pentose phosphate pathway may be required to meet the metabolic needs of these fat-storing seeds. Compartmentation of glucose-6-phosphate dehydrogenase in the cytosol and 6-phosphogluconate dehydrogenase in the proplastids is discussed in light of the NADPH requirements for fatty acid synthesis in these subcellular locations.

Journal Article↗
PubMed1981

Studies in Wild Oat Seed Dormancy: II. ACTIVITIES OF PENTOSE PHOSPHATE PATHWAY DEHYDROGENASES.

A selected strain of wild oat (Avena fatua L.) seed has been shown to lose dormancy rapidly during moist soil incubation at 25 C, whereas seed kept similarly at 5 C maintained a high level of dormancy.The activities of cytosolic dehydrogenase enzymes of the pentose phosphate pathway were assayed throughout a period of moist soil incubation at these two temperatures. A distinction was made between extractable dehydrogenases from the embryo and the endosperm regions of the caryopsis.Dehydrogenase activities monitored in seeds incubated at 25 C gradually increased over the course of the investigation. The largest increases in activity occurred during incubation at 5 C, the situation in which dormancy is maintained. No obvious connection could be found between dormancy breakage and increased activity of the pentose phosphate pathway dehydrogenases.

Journal Article↗
PubMed1986

Enzymes of Glucose Oxidation in Leaf Tissues : The Distribution of the Enzymes of Glycolysis and the Oxidative Pentose Phosphate Pathway between Epidermal and Mesophyll Tissues of C(3)-Plants and Epidermal, Mesophyll, and Bundle Sheath Tissues of C(4)-Plants.

The distribution of the glycolytic enzymes, phosphofructokinase, aldolase, triosephosphate isomerase, phosphoglycerate kinase, pyruvate kinase, and the oxidative pentose phosphate pathway enzymes, glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, was determined in the leaf tissues of two C(3)-plants, pea and leek, and two C(4)-plants, maize and sorghum. All enzymes examined were found in epidermal tissue. In pea, maize, and sorghum leaves, the specific activities of these enzymes were usually higher in the nonphotosynthetic epidermal tissue than in the photosynthetic tissues of the leaves. In leek leaves, which were etiolated, specific activities were similar in both epidermal and mesophyll tissue. The distribution of the rate limiting enzymes of glycolysis and the oxidative pentose phosphate pathways probably reflects the capacity of each tissue to generate NADH, NADPH, and ATP from the oxidation of glucose. This capacity appears to be greater in leaf tissues unable to generate reducing equivalents and ATP by photosynthesis, that is, in epidermal tissues and etiolated mesophyll tissue.

Journal Article↗
PubMed1987

Changes in Levels of Intermediates of the C(4) Cycle and Reductive Pentose Phosphate Pathway under Various Concentrations of CO(2) in Maize Leaves.

The rate of CO(2) assimilation and levels of metabolites of the C(4) cycle and reductive pentose phosphate pathway in an attached leaf of maize (Zea mays L) were measured over a range of intercellular CO(2) concentration (Ci) of 10 to 190 microliters per liter. The CO(2) assimilation rate was saturated at a Ci of around 175 microliters per liter. The levels of ribulose 1,5-bisphosphate and fructose 1,6-bisphosphate decreased substantially with increasing Ci. The levels of 3-phosphoglycerate, phosphoenolpyruvate (PEP), and pyruvate increased with increasing Ci. The level of dihydroxyacetone phosphate increased moderately from Ci of 10 microliters per liter to 20 to 50 microliters per liter and stayed almost constant over the rest of the range of Ci investigated. The levels of fructose 6-phosphate did not show any significant changes over the range of Ci. The levels of glucose 6-phosphate decreased slightly with increasing Ci. Although photosynthetically inactive pools of malate, asparate, and alanine could mask real changes in levels of the photosynthetically active pools of these compounds, the apparent levels of these compounds and the total amount of intermediates in the C(4) cycle (malate, aspartate, pyruvate, PEP, and alanine) increased with increasing Ci. The results suggest that there is carbon input into the C(4) cycle from the reductive pentose phosphate pathway which increases the level of total intermediates of the C(4) cycle with increasing Ci.

Journal Article↗
PubMed1997

Early Events Induced by the Elicitor Cryptogein in Tobacco Cells: Involvement of a Plasma Membrane NADPH Oxidase and Activation of Glycolysis and the Pentose Phosphate Pathway.

Application of the elicitor cryptogein to tobacco (cv Xanthi) is known to evoke external medium alkalinization, active oxygen species production, and phytoalexin synthesis. These are all dependent on an influx of calcium. We show here that cryptogein also induces calcium-dependent plasma membrane depolarization, chloride efflux, cytoplasm acidification, and NADPH oxidation without changes in NAD+ and ATP levels, indicating that the elicitor-activated redox system, responsible for active oxygen species production, uses NADPH in vivo. NADPH oxidation activates the functioning of the pentose phosphate pathway, leading to a decrease in glucose 6-phosphate and to the accumulation of glyceraldehyde 3-phosphate, 3- and 2-phosphoglyceric acid, and phosphoenolpyruvate. By inhibiting the pentose phosphate pathway, we demonstrate that the activation of the plasma membrane NADPH oxidase is responsible for active oxygen species production, external alkalinization, and acidification of the cytoplasm. A model is proposed for the organization of the cryptogein responses measured to date.

Journal Article↗