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PubMed1979

Effect of sugars on D-arabitol production and glucose metabolism in Saccharomyces rouxii.

The effect of sugars on the production of d-arabitol and on the glucose catabolic pathways was investigated in the osmotrophic yeast Saccharomyces rouxii. The activity of d-arabitol dehydrogenase, which served as a measure of total d-arabitol production, increased when cells were grown in the presence of increasing glucose concentrations. Growth in sucrose had no effect on the enzyme activity. A high intracellular concentration of d-arabitol could be demonstrated when the cells were grown in a 60% glucose medium and could be eliminated by anaerobic growth or growth in the presence of 4 mg of chloramphenicol per ml. A mutant was isolated that would not grow in 60% glucose; although the regulation of d-arabitol dehydrogenase was altered in this strain, the production of d-arabitol was not eliminated. The activity of d-arabitol dehydrogenase followed the growth phases of the parent strain when the cells were preadapted to 30% glucose. If the cells were adapting from 1 to 30% glucose, a large increase in enzyme activity was detected before growth occurred. Protein synthesis was found to be involved in this increase in activity. There was an increased participation of the pentose phosphate pathway when the cells were grown in the presence of increasing glucose concentrations. The mutant strain had only an 11% pentose phosphate pathway participation compared with 20% for the parent strain in glucose. The results suggest that the active pentose phosphate pathway is involved in glucose tolerance by providing a plentiful supply of reduced nicotinamide adenine dinucleotide phosphate which is necessary for cell survival.

Adaptation, Physiological↗

PubMed1995

Biosynthetic origin of mycobacterial cell wall arabinosyl residues.

Designing new drugs that inhibit the biosynthesis of the D-arabinan moiety of the mycobacterial cell wall arabinogalactan is one important basic approach for treatment of mycobacterial diseases. However, the biosynthetic origin of the D-arabinosyl monosaccharide residues themselves is not known. To obtain information on this issue, mycobacteria growing in culture were fed glucose labeled with 14C or 3H in specific positions. The resulting radiolabeled cell walls were isolated and hydrolyzed, the arabinose and galactose were separated by high-pressure liquid chromatography, and the radioactivity in each sugar was determined. [U-14C]glucose, [6-3H]glucose, [6-14C]glucose, and [1-14C]glucose were all converted to cell wall arabinosyl residues with equal retention of radioactivity. The positions of the labeled atoms in the arabinose made from [1-14C]glucose and [6-3H]glucose were shown to be C-1 and H-5, respectively. These results demonstrated that the arabinose carbon skeleton is formed via the nonoxidative pentose shunt and not via hexose decarboxylation or via triose condensations. Since the pentose shunt product, ribulose-5-phosphate, is converted to arabinose-5-phosphate as the first step in 3-keto-D-manno-octulosonic acid biosynthesis by gram-negative bacteria, such a conversion was then searched for in mycobacteria. However, cell-free enzymatic analysis using both phosphorous nuclear magnetic resonance spectrometry and colorimetric methods failed to detect the conversion. Thus, the conversion of the pentose shunt intermediates to the D-arabino stereochemistry is not via the expected isomerase but rather must occur via novel metabolic transformations.

Arabinose↗

PubMed2004

Pyruvate formate lyase and acetate kinase are essential for anaerobic growth of Escherichia coli on xylose.

During anaerobic growth of bacteria, organic intermediates of metabolism, such as pyruvate or its derivatives, serve as electron acceptors to maintain the overall redox balance. Under these conditions, the ATP needed for cell growth is derived from substrate-level phosphorylation. In Escherichia coli, conversion of glucose to pyruvate yields 2 net ATPs, while metabolism of a pentose, such as xylose, to pyruvate only yields 0.67 net ATP per xylose due to the need for one (each) ATP for xylose transport and xylulose phosphorylation. During fermentative growth, E. coli produces equimolar amounts of acetate and ethanol from two pyruvates, and these reactions generate one additional ATP from two pyruvates (one hexose equivalent) while still maintaining the overall redox balance. Conversion of xylose to acetate and ethanol increases the net ATP yield from 0.67 to 1.5 per xylose. An E. coli pfl mutant lacking pyruvate formate lyase cannot convert pyruvate to acetyl coenzyme A, the required precursor for acetate and ethanol production, and could not produce this additional ATP. E. coli pfl mutants failed to grow under anaerobic conditions in xylose minimal medium without any negative effect on their survival or aerobic growth. An ackA mutant, lacking the ability to generate ATP from acetyl phosphate, also failed to grow in xylose minimal medium under anaerobic conditions, confirming the need for the ATP produced by acetate kinase for anaerobic growth on xylose. Since arabinose transport by AraE, the low-affinity, high-capacity, arabinose/H+ symport, conserves the ATP expended in pentose transport by the ABC transporter, both pfl and ackA mutants grew anaerobically with arabinose. AraE-based xylose transport, achieved after constitutively expressing araE, also supported the growth of the pfl mutant in xylose minimal medium. These results suggest that a net ATP yield of 0.67 per pentose is only enough to provide for maintenance energy but not enough to support growth of E. coli in minimal medium. Thus, pyruvate formate lyase and acetate kinase are essential for anaerobic growth of E. coli on xylose due to energetic constraints.

Acetate Kinase↗

PubMed1964

2-KETOGLUCONATE FERMENTATION BY STREPTOCOCCUS FAECALIS.

Goddard, J. L. (University of Oklahoma School of Medicine, Oklahoma City), and J. R. Sokatch. 2-Ketogluconate fermentation by Streptococcus faecalis. J. Bacteriol. 87:844-851. 1964.-Streptococcus faecalis 10Cl did not grow with 2-ketogluconate alone as an energy source, but did grow when gluconate was added. More growth was obtained than could be accounted for by the gluconate alone. The requirement for gluconate in the stimulation of growth on 2-ketogluconate was found to be stoichiometric, not catalytic. Glucose did not replace gluconate in this phenomenon, apparently owing to the repression of the 2-ketogluconate pathway by glucose. Resting cells grown on a combination of gluconate and 2-ketogluconate did ferment 2-ketogluconate without added gluconate. Fermentation balance studies with resting cells detected the following products in moles (per mole of 2-ketogluconate): carbon dioxide, 0.98; lactic acid, 0.19; formic acid, 1.42; acetic acid, 0.70; and ethanol, 0.42. 2-Ketogluconate-1-C(14) and -2-C(14) were prepared and fermented. The data were interpreted to show that 90% of the substrate was decarboxylated to carbon dioxide and pentose phosphate. Pentose phosphate was then fermented to pyruvate through the sedoheptulose diphosphate variation of the pentose phosphate pathway found in this organism. The other 10% of the substrate was converted to pyruvate by way of the Entner-Doudoroff pathway. Calculations of the energy available by the above combination of pathways indicated that about 2.3 moles of adenosine triphosphate per mole of 2-ketogluconate could be obtained if the energy available in acetate formation is conserved through the acetokinase reaction.

Acetates↗

PubMed1967

Lipopolysaccharides of Salmonella T mutants.

The composition of lipopolysaccharides derived from various Salmonella T forms was studied. All T1-form lipopolysaccharides examined contained 14 to 22% each of both d-galactose and pentose in addition to 4 to 9% each of ketodeoxyoctonic acid, heptose, d-glucosamine, and d-glucose. The pentose was identified as d-ribose. The T2-form lipopolysaccharide examined did not contain a significant amount of pentose, nor more than the usual amounts of d-galactose. Periodate oxidation of T1 (lipo) polysaccharides followed by NaBH(4) reduction revealed that ribose was almost quantitatively protected, galactose was destroyed, and threitol and mannose were newly formed. The latter two products probably originated from 4-linked galactose and heptose, respectively. Ribose and galactose were found in specific precipitates of T1 lipopolysaccharide with anti-T1 antiserum but were not found in specific precipitates of alkali-treated T1 lipopolysaccharide and of Freeman degraded polysaccharide with anti-T1 serum Ribose and galactose are present in these degraded preparations in the form of nondialyzable polymers. The T1-form mutant lipopolysaccharides lacked the O-specific sugars constituting the side-chains in the wild-type antigens. They did not produce the soluble O-specific haptenic polysaccharide known to be accumulated in RI strains. With these properties, T1 lipopolysaccharides resemble RII lipopolysaccharides. Like RII degraded polysaccharides, T1-degraded polysaccharides also contained glucosamine. Furthermore, strong cross-reactions were found to exist between T1 and RII lipopolysaccharides in both hemagglutination inhibition assays and in precipitation tests. It is proposed that T1 lipopolysaccharides represent RII lipopolysaccharides to which polymers consisting of ribose and galactose are attached.

Carbohydrates↗

PubMed1985

Cerebral glucose use measured with [14C]glucose labeled in the 1, 2, or 6 position.

The efficacy of [14C]glucose molecules labeled in various positions as tracers of regional cerebral glucose utilization (rCMRGlc) was examined in rats. Arteriovenous differences of different [14C]-glucose species and 14CO2 were measured across brain to determine the relative rates of 14CO2 loss. As anticipated, 14CO2 evolution decreased in the order: [U-14C]glucose greater than [2-14C]glucose greater than [1-14C]glucose greater than [6-14C]glucose. Release of 14CO2 from [6-14C]glucose was undetectable at 5 min and barely detectable at 10 min, and release from [1-14C]glucose, which includes the pentose phosphate pathway, was only slightly greater. rCMRGlc was measured with [1-14C]-,[2-14C]-, or [6-14C]glucose in 5-min experiments. The results of [1-14C]- and [6-14C]glucose were indistinguishable; no difference due to the activity of the pentose phosphate pathway was found. Both [1-14C]- and [6-14C]-glucose gave values similar to, but on the whole slightly higher than, [2-14C]glucose. It was concluded that when knowledge of total rCMRGlc is required, [6-14C]glucose is the labeled substrate of choice. When the experimental objective is measurement of energy metabolism, use of [1-14C]glucose avoids inclusion of the nonenergy-yielding pentose phosphate pathway.

Animals↗

PubMed1986

Ribose metabolism and nucleic acid synthesis in normal and glucose-6-phosphate dehydrogenase-deficient human erythrocytes infected with Plasmodium falciparum.

The metabolism of pentose-phosphate was investigated in Plasmodium falciparum-infected normal and glucose-6-phosphate dehydrogenase (G6PD)-deficient human red blood cells in vitro. 5'-Phosphoribosyl-1-pyrophosphate (PRPP) content of infected normal red blood cells was increased 50-60-fold at the parasite trophozoite growth stage over that of uninfected cells. The PRPP increment in infected G6PD-deficient cells at comparable stage and parasitemia was only 40% of the value in normal infected cells. Red blood cell PRPP synthetase activity did not change during the growth cycle of the parasite and was similar in both normal and G6PD-deficient cells. Reduced glutathione (GSH) content of G6PD-deficient cells under conditions of culture fell to low or undetectable levels. These low levels of GSH were shown to inhibit the function of red blood cell PRPP synthetase, which requires GSH for full activity. Measurements of the incorporation of 1-14C or 6-14C selectively labeled glucose into parasite nucleic acids revealed that in normal infected red cells, approximately 20% of the pentose was produced via the oxidation of glucose-6-phosphate, whereas in infected G6PD-deficient cells (Mediterranean type), none of the pentose was produced via the oxidative pathway. It is concluded that the low level of reduced GSH found in G6PD deficiency and the resultant partial inhibition of PRPP synthetase together with the missing oxidative pathway for ribose phosphate production can account fully for the reduced parasite growth rate in G6PD-deficient red blood cells described previously. Of these two mechanisms, the predominant one is the impaired PRPP synthetase activity due to low GSH levels in enzyme-deficient red blood cells. The contribution to the ribose-phosphate pool by the hexose monophosphate shunt is relatively minor. A co-existing oxidative stress (which is often hypothesized to mediate the destruction of parasitized red blood cells) is not required to explain growth inhibition in this scheme and does not represent the most straight-forward explanation of the data described in this report.

DNA Replication↗

PubMed2003

Defective RNA ribose synthesis in fibroblasts from patients with thiamine-responsive megaloblastic anemia (TRMA).

Fibroblasts from patients with thiamine-responsive megaloblastic anemia (TRMA) syndrome with diabetes and deafness undergo apoptotic cell death in the absence of supplemental thiamine in their cultures. The basis of megaloblastosis in these patients has not been determined. Here we use the stable [1,2-13C2]glucose isotope-based dynamic metabolic profiling technique to demonstrate that defective high-affinity thiamine transport primarily affects the synthesis of nucleic acid ribose via the nonoxidative branch of the pentose cycle. RNA ribose isolated from TRMA fibroblasts in thiamine-depleted cultures shows a time-dependent decrease in the fraction of ribose derived via transketolase, a thiamine-dependent enzyme in the pentose cycle. The fractional rate of de novo ribose synthesis from glucose is decreased several fold 2 to 4 days after removal of thiamine from the culture medium. No such metabolic changes are observed in wild-type fibroblasts or in TRMA mutant cells in thiamine-containing medium. Fluxes through glycolysis are similar in TRMA versus control fibroblasts in the pentose and TCA cycles. We conclude that reduced nucleic acid production through impaired transketolase catalysis is the underlying biochemical disturbance that likely induces cell cycle arrest or apoptosis in bone marrow cells and leads to the TRMA syndrome in patients with defective high-affinity thiamine transport.

Anemia, Megaloblastic↗

PubMed2006

Alkaline fluid circulation in ultramafic rocks and formation of nucleotide constituents: a hypothesis.

Seawater is constantly circulating through oceanic basement as a low-temperature hydrothermal fluid (<150 degrees C). In cases when ultramafic rocks are exposed to the fluids, for instance during the initial phase of subduction, ferromagnesian minerals are altered in contact with the water, leading to high pH and formation of secondary magnesium hydroxide, among other--brucite, that may scavenge borate and phosphate from seawater. The high pH may promote abiotic formation of pentoses, particularly ribose. Pentoses are stabilized by borate, since cyclic pentoses form a less reactive complex with borate. Analyses have shown that borate occupies the 2' and 3' positions of ribose, thus leaving the 5' position available for reactions like phosphorylation. The purine coding elements (adenine, in particular) of RNA may be formed in the same general hydrothermal environments of the seafloor.

Journal Article↗

PubMed2006

Co-utilization of L-arabinose and D-xylose by laboratory and industrial Saccharomyces cerevisiae strains.

BACKGROUND: Fermentation of lignocellulosic biomass is an attractive alternative for the production of bioethanol. Traditionally, the yeast Saccharomyces cerevisiae is used in industrial ethanol fermentations. However, S. cerevisiae is naturally not able to ferment the pentose sugars D-xylose and L-arabinose, which are present in high amounts in lignocellulosic raw materials. RESULTS: We describe the engineering of laboratory and industrial S. cerevisiae strains to co-ferment the pentose sugars D-xylose and L-arabinose. Introduction of a fungal xylose and a bacterial arabinose pathway resulted in strains able to grow on both pentose sugars. Introduction of a xylose pathway into an arabinose-fermenting laboratory strain resulted in nearly complete conversion of arabinose into arabitol due to the L-arabinose reductase activity of the xylose reductase. The industrial strain displayed lower arabitol yield and increased ethanol yield from xylose and arabinose. CONCLUSION: Our work demonstrates simultaneous co-utilization of xylose and arabinose in recombinant strains of S. cerevisiae. In addition, the co-utilization of arabinose together with xylose significantly reduced formation of the by-product xylitol, which contributed to improved ethanol production.

Journal Article↗

PubMed1977

Mechanisms of decreased insulin responsiveness of large adipocytes.

UNLABELLED: We have studied glucose metabolism using large adipocytes isolated from older, fatter rats (greater than 12 months old, greater than 550 g), and smaller cells obtained from younger, leaner animals (4-5 weeks old, 126-160 g). 2-Deoxyglucose uptake was equal in large and small adipocytes, while insulin mediated oxidation of [1(-14)C]glucose was greatly diminished (7-fold) in large cells. Thus, the defect in oxidation of the number one carbon atom of glucose (pentose pathway oxidation) is distal to the 2-deoxyglucose uptake system. However, this intracellular defect is not present in all pathways of glucose oxidation as demonstrated by the finding that [6(-14)C]glucose oxidation was comparable in small and large adipocytes. Thus, the number six carbon atom of glucose is oxidized normally indicating that glycolytic and Krebs cycle activity is intact in the large adipocyte. Furthermore, in large adipocytes conversion of glucose to total lipid was normal in the basal state and moderately decreased at high glucose concentrations in the presence of insulin (up to 35%). When the radioactivity in total lipids was fractionated, a severe decrease in glucose incorporation into fatty acids was found in the large cells. Total glucose uptake was also measured, and found to be 10-50% decreased in large cells, suggesting that the decreases in pentose pathway glucose metabolism and conversion to fatty acids lead to accumulation of free intracellular glucose with glucose efflux and a decrease in net glucose uptake. Comparing the 2-deoxyglucose uptake and glucose oxidation data showed that insulin promotes [6(-14)C]glucose oxidation by stimulating the processes responsible for 2-deoxyglucose uptake whereas insulin promotes [1(-14)C]glucose oxidation both by increasing these processes and by increasing the activity of the C-1 oxidative pathway. IN CONCLUSION: 1) the 2-deoxyglucose uptake system of the large adipocyte is basically intact, 2) [1(-14)C]glucose oxidation is markedly decreased in large adipocytes, while [6(-14)C]glucose oxidation is normal, and 3) in comparing small and large adipocytes, it appears that it is the ability of insulin to enhance glucose oxidation via the pentose pathway and to promote glucose incorporation into fatty acids which is most impaired in large adipocytes.

Adipose Tissue↗

PubMed1996

Epiandrosterone and dehydroepiandrosterone affect glucose oxidation and interleukin-1 beta effects in pancreatic islets.

Isolated rat islets or RINm5F insulinoma cells treated with interleukin-1 beta (IL-1 beta) for 18 h show reduced glucose-sensitive insulin release and increased nitrite formation as a result of nitric oxide synthase induction. Although a phosphodiesterase inhibitor, isobutylmethylxanthine, potentiated insulin release in response to glucose stimulation, the secretory response was not restored to normal in IL-1 beta-treated islets. Islets that were cultured for 18 h in the presence of IL-1 beta and epiandrosterone (EA) or dehydroepiandrosterone (DHEA) and then washed responded with a concentration-dependent reversal of the effects of IL-1 beta on insulin release in the presence of a glucose or glucose plus isobutylmethylxanthine stimulus. In contrast, when EA and DHEA were not washed from the islets before determination of insulin release, the presence of EA or DHEA inhibited insulin release in both freshly isolated and cultured islets. Nitrite formation in islets and RINm5F cells in response to IL-1 beta was also significantly reduced during culture with EA or DHEA, although nitrite levels were still elevated above control values. Neither steroid affected cell growth or DNA or protein content. Pyrrolidine dithiocarbamate also reduced IL-1 beta-induced nitrite formation. EA and DHEA inhibited [U-14C]glucose oxidation in islets and RINm5F cells. Comparison of [1-14C]glucose and [6-14C]glucose oxidation in islets and RINm5F cells when EA was present during culture and metabolic determination indicated that EA inhibited glycolysis and the pentose shunt contribution to glucose utilization. Neither IL-1 beta in islets nor DHEA in RINm5F cells inhibited pentose shunt activity, although total glucose oxidation and utilization were inhibited. The effects of DHEA and EA on glucose oxidation were rapidly reversible. EA and DHEA reduced glucose-6-phosphate dehydrogenase activity only when added directly to tissue homogenates. Thus, EA and DHEA antagonize the effects of IL-1 beta on beta-cells. Inhibition of glucose metabolism and pentose shunt activity may protect the cells from nitric oxide synthase activation and related toxicities.

Androsterone↗

PubMed2002

Performance testing of Zymomonas mobilis metabolically engineered for cofermentation of glucose, xylose, and arabinose.

IOGEN Corporation of Ottawa, Canada, has recently built a 40t/d biomass-to-ethanol demonstration plant adjacent to its enzyme production facility. It has partnered with the University of Toronto to test the C6/C5 cofermenta-tion performance characteristics of the National Renewable Energy Labora-tory's metabolically engineered Zymomonas mobilis using various biomass hydrolysates. IOGEN's feedstocks are primarily agricultural wastes such as corn stover and wheat straw. Integrated recombinant Z. mobilis strain AX101 grows on D-xylose and/or L-arabinose as the sole carbon/energy sources and ferments these pentose sugars to ethanol in high yield. Strain AX101 lacks the tetracycline resistance gene that was a common feature of other recombinant Zm constructs. Genomic integration provides reliable cofermentation performance in the absence of antibiotics, another characteristic making strain AX101 attractive for industrial cellulosic ethanol production. In this work, IOGEN's biomass hydrolysate was simulated by a pure sugar medium containing 6% (w/v) glucose, 3% xylose, and 0.35% arabinose. At a level of 3 g/L (dry solids), corn steep liquor with inorganic nitrogen (0.8 g/L of ammonium chloride or 1.2 g/L of diammonium phosphate) was a cost-effective nutritional supplement. In the absence of acetic acid, the maximum volumetric ethanol productivity of a continuous fermentation at pH 5.0 was 3.54 g/L x h. During prolonged continuous fermentation, the efficiency of sugar-to-ethanol conversion (based on total sugar load) was maintained at >85%. At a level of 0.25% (w/v) acetic acid, the productivity decreased to 1.17 g/L x h at pH 5.5. Unlike integrated, xylose-utilizing rec Zm strain C25, strain AX101 produces less lactic acid as byproduct, owing to the fact that the Escherichia coli arabinose genes are inserted into a region of the host chromosome tentatively assigned to the gene for D-lactic acid dehydrogenase. In pH-controlled batch fermentations with sugar mixtures, the order of sugar exhaustion from the medium was glucose followed by xylose and arabinose. Both the total sugar load and the sugar ratio were shown to be important determinants for efficient cofermentation. Ethanol at a level of 3% (w/v) was implicated as both inhibitory to pentose fermentation and as a potentiator of acetic acid inhibition of pentose fermentation at pH 5.5. The effect of ethanol may have been underestimated in other assessments of acetic acid sensitivity. This work underscores the importance of employing similar assay conditions in making comparative assessments of biocatalyst fermentation performance.

Arabinose↗

PubMed1977

6-Phosphogluconate/glucose-6-phosphate ratio in rat pancreatic islets during inhibition of insulin release by exogenous insulin.

Inhibition of glucose-stimulated insulin release by exogenous insulin has been demonstrated in pancreatic islets to be associated with a decrease of the NADPH/NADP ratio and the pentose-phosphate cycle activity. Batches of five islets were incubated for 15 and 90 minutes in 1 ml. of KRB buffer with 2 per cent albumin containing 3 mg./ml. glucose and 0, 200, 400, or 800 microU./ml. of rat insulin, and the glucose-6-phosphate (G6P) and 6-phosphogluconate (6PG) contents were determined by enzymatic cycling. In response to a rise in the concentration of insulin, the 6PG/G6P ratio decreased. A close relationship was observed between this decrease of 6PG/G6P ratio and the net insulin release, the absolute rate of glucose oxidation via the pentose phosphate cycle, and the NADPH/NADP ratios measured under similar conditions. The results suggest that exogenous insulin, directly or indirectly, regulates the pentose cycle activity in the pancreatic islets at the G6P dehydrogenase step.

Animals↗

PubMed1990

Nutritional implications and metabolizable energy value of D-xylose and L-arabinose in chicks.

An experiment was conducted to examine the effects of graded levels (2.5, 5.0, 7.5, 10.0, and 15.0%) of dietary D-xylose or L-arabinose on chick performance. As reference, D-glucose was included in the experiment. A second experiment was performed to determine the AMEn of D-xylose and L-arabinose. Results of Experiment 1 showed a significant linear decrease (P less than .05) in weight gain and efficiency of feed utilization when the dietary level of either D-xylose or L-arabinose was increased. The same was true for daily feed intake of the D-xylose treatments. Water intake was linearly (P less than .05) increased as dietary level of both pentose sugars increased, and, as a result, dry matter content of the droppings decreased. Results of Experiment 2 showed that the AMEn value of either pentose sugar was dose related. The AMEn values for D-xylose at 5 and 10% dietary inclusion were 2,660 and 2,020 kcal/kg, respectively. Those for L-arabinose at these inclusion levels were 2,300 and 1,360 kcal/kg, respectively. Feeding equal dietary levels of either pentose sugar resulted in higher concentrations of xylose than of arabinose in blood plasma. Concentration of glucose in blood was not affected by feeding either D-xylose or L-arabinose. Cecal length and weight were markedly increased by feeding L-arabinose and intermediately by D-xylose.

Analysis of Variance↗

PubMed1976

Behavior of transaldolase (EC 2.2.1.2) and transketolase (EC 2.2.1.1) Activities in normal, neoplastic, differentiating, and regenerating liver.

The objective of this investigation was to throw light on the biological behavior and metabolic regulation of hepatic enzymes of the nonoxidative branch of the pentose phosphate pathway. The activities of transaldolase (EC 2.2.1.2) and trasketolase (EC 2.2.1.1) Were compared in biological conditions that involve modulation of gene expression such as in starvation, in differentiation, after partial hepatectomy, and in a spectrum of hepatomas of different growth rates. The enzyme activities were determined under optimal kinetic conditions by spectrophotometric methods in the 100,000 X g supernatant fluids prepared from tissue homogenates. The kinetic properties of transaldolase and transketolase were similar in normal liver and in rapidly growing hepatoma 3924A. For transaldolase, apparent Km values of 0.13 mM (normal liver) and 0.17 mM (hepatoma) were observed for erythrose 4-phosphate and of 0.30 to 0.35 mM for fructose 6-phosphate. The pH optima in liver and hepatoma were at approximately 6.9 to 7.2. For the transketolase substrates, ribose 5-phosphate and xylulose 5-phosphate, the apparent Km values were 0.3 and 0.5 mM, respectively, in both liver and hepatoma. A broad pH optimum around 7.6 was observed in both tissues. In organ distribution studies, enzyme activities were measured in liver, intestinal mucosa, thymus, kidney, spleen, brain, adipose tissue, lung, heart, and skeletal muscle. Taking the specific activity of liver as 100%, transaldolase activity was the highest in intestinal mucosa (316%) and in thymus (219%); it was the lowest in heart (53%) and in skeletal muscle (21%). Transketolase activity was highest in kidney (155%) and lowest in heart (26%) and skeletal muscle (23%). Starvation decreased transaldolase and transketolase activities in 6 days to 69 and 74%, respectively, of those of the liver of the normal, fed rat. This was in the same range as the decrease in the protein concentration (66%y. In the liver tumors, transaldolase activity was increased 1.5- to 3.4-fold over the activities observed in normal control rat liver. Transketolase activity showed no relationship to tumor proliferation rate. In the regenerating liver at 24 hr after partial hepatectomy, the activity of both pentose phosphate pathway enzymes was in the same range as that of the sham-operated controls. In differentiation at the postnatal age of 5, 12, 23, and 32 days, hepatic transaldolase activities were 33, 44, 55, and 72%, respectively, of the activities observed in the 60-day-old, adult male rat. During the same period, transketolase activ-ties were 18, 21, 26, and 55% of the activities observed in liver of adult rat. The demonstration of increased transaldolase activity in hepatomas, irrespective of the degree of tumor malignancy, differentiation, or growth rate, suggests that the reprogramming of gene expression in malignant transformation is linked with an increase in the expression of this pentose phosphate pathway enzyme...

Animals↗

PubMed2004

[Preliminary proteome analysis for Saccharomyces cerevisiae under different culturing conditions].

For the investigation of the metabolic regulation of Saccharomyces cerevisiae under different culture conditions, the proteins of cell utilizing various carbon sources were separated by two-dimensional electrophoresis with immobilized pH gradients as the first dimension and SDS-PAGE as the second. Samples were taken in the log phase of batch culture using glucose or lactic acid as carbon source, while another sample was taken from the broth when glucose was consumed up and ethanol accumulated in the previous phase was further metabolized. After electrophoresis, the protein spots were detected by silver-stain in a Hoefer Automated Gel Stainer with a protein silver staining kit. Silver-stained gels were scanned and digitized to create computer images. About 500 protein spots were detected by employing the 2D proteome image analysis system Image Master 2D Elite and SWISS-2DPAGE proteome database. Most of the protein expressed and involved in the glycolysis, pentose phosphate (PP) pathway, anaplerotic pathway, as well as TCA cycle were analyzed. The metabolism regulation of protein level for Saccharomyces cerevisiae under various carbon sources, as well as during different phase of growth, was studied. The expression of several glycolytic enzymes (glk, pgi, pgk, eno, pyk) was up-regulated while the expression of enzymes in oxidative pentose phosphate pathway (zwf, gnd) was down-regulated when ethanol and lactic acid were taken as carbon source. Simultaneously, frucotose 1,6-biphosphatase was found to be up-regulated due to the gluconeogenic requirement. Citrate synthase and Malate dehydrogenase do not exhibit significant difference, indicating TCA cycle is necessary when utilizing glucose, ethanol or lactic acid as carbon source. Thus, the NADPH loss due to the repressed pentose phosphate pathway could be compensated by TCA cycle in cases of ethanol and lactic acid. The expression of malic enzyme and isocitrate lyase are activated to a large extent when metabolizing ethanol, indicating glyoxylate shunt is essential in transferring ethanol to generate four carbon precursors for the biosynthesis and the NADP-dependent malic enzyme could also serve as compensation mechanism for NADPH loss in this case.

Carbon↗

PubMed1985

Quantitative estimation of the pathways followed in the conversion to glycogen of glucose administered to the fasted rat.

When [6-3H,6-14C]glucose was given in glucose loads to fasted rats, the average 3H/14C ratios in the glycogens deposited in their livers, relative to that in the glucoses administered, were 0.85 and 0.88. When [3-3H,3-14C]lactate was given in trace quantity along with unlabeled glucose loads, the average 3H/14C ratio in the glycogens deposited was 0.08. This indicates that a major fraction of the carbons of the glucose loads was converted to liver glycogen without first being converted to lactate. When [3-3H,6-14C]glucose was given in glucose loads, the 3H/14C ratios in the glycogens deposited averaged 0.44. This indicates that a significant amount of H bound to carbon 3, but not carbon 6, of glucose is removed within liver in the conversion of the carbons of the glucose to glycogen. This can occur in the pentose cycle and by cycling of glucose-6-P via triose phosphates: glucose----glucose-6-P----triose phosphates----glucose-6-P----glycogen. The contributions of these pathways were estimated by giving glucose loads labeled with [1-14C]glucose, [2-14C]glucose, [5-14C]glucose, and [6-14C]glucose and degrading the glucoses obtained by hydrolyzing the glycogens that deposited. Only a few per cent of the glucose carbons deposited in glycogen were deposited in liver via glucose-6-P conversion to triose phosphates. Between 4 and 9% of the glucose utilized by the liver was utilized in the pentose cycle. While these are relatively small percentages, since three NADP3H molecules are formed from each molecule of [3-3H]glucose-6-P utilized in the cycle, a major portion of the difference between the ratios obtained with [3-3H]glucose and with [6-3H]glucose is attributable to metabolism in the pentose cycle. Because 3H of [3-3H]glucose is extensively removed during the conversion of the glucose to glycogen within liver the extent of incorporation of the 3H into liver glycogen is not the measure of glucose's metabolism in other tissues before its carbons are deposited in liver glycogen. The distributions of 14C from the 14C-labeled glucoses into the carbons of the liver glycogens mean that at a minimum about 30% of the carbons of the glucose deposited in the glycogen were first converted to lactate or its metabolic equivalent.

Acetyl Coenzyme A↗