Quantitative determination of the main glucose metabolic fluxes in human erythrocytes by 13C- and 1H-MR spectroscopy.

Information displayed by homonuclear and heteronuclear spin-coupling patterns in 13C- and 1H-MR spectra allowed us to identify the major lactate isotopomers produced either from [1-(13)C]-glucose or from [2-(13)C]-glucose by human erythrocytes. Relative concentrations of detectable isotopomers were determined by integrating the corresponding MR signals. The interpretation of these data in terms of the fractional glucose metabolised through glycolysis and pentose phosphate pathway was performed by a computer simulation of the metabolism that took into account metabolic schemes pertaining to glycolysis and to the F-type of pentose phosphate pathway. The simulation was organised in a way to anticipate the populations of the isotopomers produced from any precursor at a priori established metabolic steady state. By the simulation, isotopomer populations were determined according to different values of pentose cycle, defined as the flux of glyceraldehyde 3-phosphate originating from pentose phosphate pathway at unitary glucose uptake. The populations of the isotopomers originating from [2-(13)C]-glucose were described by polynomials, and ratios between the polynomials were used in conjunction with 13C- and 1H-MR data to determine pentose cycle values. The knowledge of glucose uptake and of pentose cycle value allowed us to perform accurate measurement of the pentose phosphate pathway flux, of the hexokinase and phosphofructokinase fluxes as well as, indirectly, of the carbon dioxide production.     

Respirometric 13C flux analysis--Part II: in vivo flux estimation of lysine-producing Corynebacterium glutamicum

A novel method for metabolic flux studies of central metabolism which is based on respirometric (13)C flux analysis, i.e., parallel (13)C tracer studies with online CO(2) labeling measurements is applied to flux quantification of a lysine-producing mutant of Corynebacterium glutamicum. For this purpose, 3 respirometric (13)C labeling experiments with [1-(13)C(1)], [6-(13)C(1)] and [1,6-(13)C(2)] glucose were carried out in parallel. All fluxes comprising the reactions of glycolysis, of TCA cycle, of C3- and C4-metabolite interconversion and of lysine biosynthesis as well as the net reactions in the pentose phosphate pathway could be quantified solely using experimental data obtained from CO(2) labeling and extracellular rate measurements. At key branch points, 68+/-5% of glucose 6-phosphate were observed to be metabolized into pentose phosphate pathway and 48+/-1% of pyruvate into TCA cycle via pyruvate dehydrogenase. The results showed a good agreement with the previous studies using (13)C tracer cultivation and GC/MS analysis of proteinogenic amino acids. Also, respiratory quotient calculated from flux estimates using redox balance showed a high accordance with the value determined directly from the measured specific rates of O(2) consumption and CO(2) production. The results strongly support that the respirometric (13)C metabolic flux analysis is suited as an alternative to the conventional methods to study functional and regulatory activities of cells. The developed method is applicable to study growing or non-growing cells, primary and secondary metabolism and immobilized cells. Due to the non-accumulating nature of CO(2) labeling and instantaneous nature of the resulting fluxes, the method can also be used for dynamic profiling of metabolic activities. Therefore, it is complementary to conventional methods for metabolic flux analysis.     

Elucidating the role of copper in CHO cell energy metabolism using 13C metabolic flux analysis

¹³C‐metabolic flux analysis was used to understand copper deficiency‐related restructuring of energy metabolism, which leads to excessive lactate production in recombinant protein‐producing CHO cells. Stationary‐phase labeling experiments with U‐¹³C glucose were conducted on CHO cells grown under high and limiting copper in 3 L fed‐batch bioreactors. The resultant labeling patterns of soluble metabolites were measured by GC‐MS and used to estimate metabolic fluxes in the central carbon metabolism pathways using OpenFlux. Fluxes were evaluated 300 times from stoichiometrically feasible random guess values and their confidence intervals calculated by Monte Carlo simulations. Results from metabolic flux analysis exhibited significant carbon redistribution throughout the metabolic network in cells under Cu deficiency. Specifically, glycolytic fluxes increased (25%–79% relative to glucose uptake) whereas fluxes through the TCA and pentose phosphate pathway (PPP) were lower (15%–23% and 74%, respectively) compared with the Cu‐containing condition. Furthermore, under Cu deficiency, 33% of the flux entering TCA via the pyruvate node was redirected to lactate and malate production. Based on these results, we hypothesize that Cu deficiency disrupts the electron transport chain causing ATP deficiency, redox imbalance, and oxidative stress, which in turn drive copper‐deficient CHO cells to produce energy via aerobic glycolysis, which is associated with excessive lactate production, rather than the more efficient route of oxidative phosphorylation. © 2015 American Institute of Chemical Engineers Biotechnol. Prog., 31:1179–1186, 2015     

Metabolic flux analysis of CHO cells at growth and non-growth phases using isotopic tracers and mass spectrometry

Chinese hamster ovary (CHO) cells are the main platform for production of biotherapeutics in the biopharmaceutical industry. However, relatively little is known about the metabolism of CHO cells in cell culture. In this work, metabolism of CHO cells was studied at the growth phase and early stationary phase using isotopic tracers and mass spectrometry. CHO cells were grown in fed-batch culture over a period of six days. On days 2 and 4, [1,2-¹³C] glucose was introduced and the labeling of intracellular metabolites was measured by gas chromatography-mass spectrometry (GC–MS) at 6, 12 and 24 h following the introduction of tracer. Intracellular metabolic fluxes were quantified from measured extracellular rates and ¹³C-labeling dynamics of intracellular metabolites using non-stationary ¹³C-metabolic flux analysis (¹³C-MFA). The flux results revealed significant rewiring of intracellular metabolic fluxes in the transition from growth to non-growth, including changes in energy metabolism, redox metabolism, oxidative pentose phosphate pathway and anaplerosis. At the exponential phase, CHO cell metabolism was characterized by a high flux of glycolysis from glucose to lactate, anaplerosis from pyruvate to oxaloacetate and from glutamate to α-ketoglutarate, and cataplerosis though malic enzyme. At the stationary phase, the flux map was characterized by a reduced flux of glycolysis, net lactate uptake, oxidative pentose phosphate pathway flux, and reduced rate of anaplerosis. The fluxes of pyruvate dehydrogenase and TCA cycle were similar at the exponential and stationary phase. The results presented here provide a solid foundation for future studies of CHO cell metabolism for applications such as cell line development and medium optimization for high-titer production of recombinant proteins.     

Isotopically nonstationary 13C metabolic flux analysis in resting and activated human platelets

Platelet metabolism is linked to platelet hyper- and hypoactivity in numerous human diseases. Developing a detailed understanding of the link between metabolic shifts and platelet activation state is integral to improving human health. Here, we show the first application of isotopically nonstationary ¹³C metabolic flux analysis to quantitatively measure carbon fluxes in both resting and thrombin activated platelets. Metabolic flux analysis results show that resting platelets primarily metabolize glucose to lactate via glycolysis, while acetate is oxidized to fuel the tricarboxylic acid cycle. Upon activation with thrombin, a potent platelet agonist, platelets increase their uptake of glucose 3-fold. This results in an absolute increase in flux throughout central metabolism, but when compared to resting platelets they redistribute carbon dramatically. Activated platelets decrease relative flux to the oxidative pentose phosphate pathway and TCA cycle from glucose and increase relative flux to lactate. These results provide the first report of reaction-level carbon fluxes in platelets and allow us to distinguish metabolic fluxes with much higher resolution than previous studies.     

Quantitative in vivo nuclear magnetic resonance studies of hybridoma metabolism

Carbon-13 nuclear magnetic resonance (NMR) spectroscopy was used to study the metabolism of a murine hybridoma cell line at two feed glutamine concentrations, 4.0 and 1.7 mM. Carbon-13 labeling patterns were used in conjunction with nutrient uptake rates to calculate the metabolic fluxes through the glycolytic pathway, the pentose shunt, the malate shunt, lipid biosynthesis, and the tricarboxylic acid (TCA) cycle. Decreasing the feed glutamine concentration significantly decreased glutamine uptake but had little effect on glucose metabolism. A significant incrase in antibody productivity occurred upon decreasing the feed glutamine level. The increased antibody productivity in concert with decreased glutamine uptake and no apparent change in glucolytic metabolism suggests that antibody production was not energy limited. Metabolic flux calculations indicate that (1) approximately 92% of the glucose consumed proceeds directly through glycolysis with 8% channeled through the pentose shunt; (2) lipid biosynthesis appears to be greater than malate shunt activity; and (3) considerable exchange occurs between TCA cycle intermediates and amino acid metabolic pools, leading to substantial loss of (13)C label from the TCA cycle. These results illustrate that (13)NMR spectroscopy is a powerfulf tool in the calculation of metabolic fluxes, particularly for exchange pathways where no net flux occurs. (c) 1994 John Wiley & Sons, Inc.     

Examination of primary metabolic pathways in a murine hybridoma with carbon-13 nuclear magnetic resonance spectroscopy

Primary metabolism of a murine hybridoma was probed with (13)C nuclear magnetic resonance (NMR) spectroscopy. Cells cultured in a hollow fiber bioreactor were serially infused with [1-(13)C] glucose, [2-(13)C] glucose, and [3-(13)C] glutamine. In vivo spectroscopy of the culture was used in conjunction with off-line spectroscopy of the medium to determine the intracellular concentration of several metabolic intermediates and to determine fluxes for primary metabolic pathways. Intracellular concentrations of pyruvate and alanine were very high relative to levels observed in normal quiescent mammalian cells. Estimates made from labeling patterns in lactate indicate that 76% of pyruvate is derived directly from glycolysis; some is also derived from the malate shunt, the pyruvate/melate shuttle associated with lipid synthesis and the pentose phosphate pathway. The rate of formation of pyruvate from the pentose phosphate pathway was estimated to be 4% of that from glycolysis; This value is a lower limit and the actual value may be higher. Incorporation of pyruvate into the tricarboxylic acid (TCA) cycle appears to occur through only pyruvate dehydrogenase; no pyruvate carboxylase activity was detected. The malate shunt rate was approximately equal to the rate of glutamine uptake. The rate of incorporation of glucosederived acetyl-CoA into lipids was 4% of the glucose uptake rate. The TCA cycle rate between isocitrate and alpha-ketoglutarate was 110% of the glutamine uptake rate. (c) 1994 John Wiley & Sons, Inc.     

Metabolic profiling by 13C-NMR spectroscopy: [1,2-13C2]glucose reveals a heterogeneous metabolism in human leukemia T cells

Metabolic profiling is defined as the simultaneous assessment of substrate fluxes within and among the different pathways of metabolite synthesis and energy production under various physiological conditions. The use of stable-isotope tracers and the analysis of the distribution of labeled carbons in various intermediates, by both mass spectrometry and NMR spectroscopy, allow the role of several metabolic processes in cell growth and death to be defined. In the present paper we describe the metabolic profiling of Jurkat cells by isotopomer analysis using (13)C-NMR spectroscopy and [1,2-(13)C(2)]glucose as the stable-isotope tracer. The isotopomer analysis of the lactate, alanine, glutamate, proline, serine, glycine, malate and ribose-5-phosphate moiety of nucleotides has allowed original integrated information regarding the pentose phosphate pathway, TCA cycle, and amino acid metabolism in proliferating human leukemia T cells to be obtained. In particular, the contribution of the glucose-6-phosphate dehydrogenase and transketolase activities to phosphoribosyl-pyrophosphate synthesis was evaluated directly by the determination of isotopomers of the [1'-(13)C], [4',5'-(13)C(2)]ribosyl moiety of nucleotides. Furthermore, the relative contribution of the glycolysis and pentose cycle to lactate production was estimated via analysis of lactate isotopomers. Interestingly, pyruvate carboxylase and pyruvate dehydrogenase flux ratios measured by glutamate isotopomers and the production of isotopomers of several metabolites showed that the metabolic processes described could not take place simultaneously in the same macrocompartments (cells). Results revealed a heterogeneous metabolism in an asynchronous cell population that may be interpreted on the basis of different metabolic phenotypes of subpopulations in relation to different cell cycle phases.     

Metabolic flux analysis of hybridoma cells in different culture media using mass balances

The estimation of the intracellular fluxes of mammalian cells using only the mass balances of the relevant metabolites is not possible because the set of linear equations defined by these mass balances is underdetermined. Either additional experimental flux data or additional theoretical constraints are required to find one unique flux distribution out of the solution space that is bound by the mass balances. Here, a method is developed using the latter approach. The uptake and production rates of amino acids, glucose, lactate, O(2), CO(2), NH(4), MAB, and the intracellular amino acid pools have been determined for two different steady-states. The cellular composition {total protein and protein composition, total lipids and fatty acid distribution, total carbohydrates, DNA and RNA} has been measured to calculate the requirements for biosynthesis. It is shown to be essential to determine the uptake/production rates of ammonia and either carbon dioxide or oxygen. In mammalian cells these are cometabolites of cyclic metabolic pathways. The flux distribution that is found using the Euclidean minimum norm as the additional theoretical constraint and taking either the CO(2) or the NAD(P)H mass balance into account is shown to be in agreement with the measured O(2) and CO(2) metabolic rates.The metabolic fluxes in hybridoma cells in continuous culture at a specific growth rate of 0.83 day(-1) are estimated for a medium with (optimal medium) and without (suboptimal medium) Primatone RL, an enzymatic hydrolysate of animal tissue that causes a more than twofold increase in cell density. It is concluded that (i)The majority of the consumed glucose (>90%) is channeled through the pentose-phosphate pathway in rapidly proliferating cells.(ii)Pyruvate oxidation and tricarboxylic acid (TCA) cycle activity are relatively low, i.e., 8% of the glucose uptake in suboptimal and 14% in optimal medium, respectively. Under both conditions, only a small fraction of pyruvate is further oxidized to CO(2).(iii)The flux from glutamate to alpha-ketoglutarate (catalyzed by glutamate dehydrogenase) is almost zero in medium with and even slightly reversed in medium without Primatone RL. Almost all glutamate enters the TCA cycle due to the action of transaminases.(iv)Transhydrogenation plays a significant role in hybridoma cells under our experimental conditions. NADPH is produced at relatively high rates (11 x 10(-12) to 13 x 10(-12) mol . cell(-1) . day(-1)) compared to other fluxes in both culture media. (c) 1996 John Wiley & Sons, Inc.     

Comparative study on central metabolic fluxes of <Emphasis Type="Italic">Bacillus megaterium</Emphasis> strains in continuous culture using <Superscript>13</Superscript>C labelled substrates

Fluxes of central carbon metabolism [glycolysis, pentose phosphate pathway (PPP), tricarboxylic acid cycle (TCA cycle), biomass formation] were determined for several Bacillus megaterium strains (DSM319, WH320, WH323, MS941) in C- and N-limited chemostat cultures by ¹³C labelling experiments. The labelling patterns of proteinogenic amino acids were analysed by GC/MS and therefrom flux ratios at important nodes within the metabolic network could be calculated. On the basis of a stoichiometric metabolic model flux distributions were estimated for the different B. megaterium strains used at various cultivation conditions. Generally all strains exhibited similar metabolic flux distributions, however, several significant changes were found in (1) the glucose flux entering the PPP via the oxidative branch, (2) the reversibilities within the PPP, (3) the relative fluxes of pyruvate and acetyl-CoA fed to the TCA cycle, (4) the fluxes around the pyruvate node involving a futile cycle.     

Insights into the central metabolism of Spodoptera frugiperda (Sf-9) and Trichoplusia ni BTI-Tn-5B1-4 (Tn-5) insect cells by radiolabeling studies

The insect cell baculovirus expression vector system (BEVS) is one of the most commonly used expression systems for recombinant protein production. This system is also widely used for the production of recombinant virus and virus-like particles. Although several published reports exist on recombinant protein expression using insect cells, information dealing with their metabolism in vitro is relatively scarce. In this work we have analyzed the metabolism of glucose and glutamine, the main carbon and/or energy compounds, of the two most commonly used insect cell lines, Spodoptera frugiperda (Sf-9) and the Trichoplusia ni BTI-Tn-5B1-4 (Tn-5). Radiolabeled substrates have been used to determine the flux of glucose carbon entering the tricarboxylic acid cycle (TCA) and the pentose phosphate (PP) pathway by direct measurement of 14CO2 produced. The percentage of total glucose metabolized to CO2 via the TCA cycle was higher in the case of the Sf-9 (2.7%) compared to Tn-5 (0.6%) cells, while the percentage of glucose that is metabolized via the PP pathway was comparable at 14% and 16% for the two cell lines, respectively. For both cell lines, the remaining 83% of glucose is metabolized through other pathways generating, for example, lactate, alanine, etc. The percentage of glutamine oxidized in the TCA cycle was approximately 5-fold higher in the case of the Tn-5 (26.1%) as compared to the Sf-9 cells (4.6%). Furthermore, the changes in the metabolic fluxes of glucose and glutamine in Tn-5-PYC cells, which have been engineered to express a cytosolic pyruvate carboxylase, have been studied and compared to the unmodified cells Tn-5. As a result of this metabolic engineering, significant increase in the percentage of glucose oxidized in the TCA cycle (3.2%) as well as in the flux through the PP pathway (34%) of the Tn-5-PYC were observed.     

Metabolic flux and metabolic network analysis of Penicillium chrysogenum using 2D [13C, 1H] COSY NMR measurements and cumulative bondomer simulation

At present two alternative methods are available for analyzing the fluxes in a metabolic network: (1) combining measurements of net conversion rates with a set of metabolite balances including the cofactor balances, or (2) leaving out the cofactor balances and fitting the resulting free fluxes to measured (13)C-labeling data. In this study these two approaches are applied to the fluxes in the glycolysis and pentose phosphate pathway of Penicillium chrysogenum growing on either ammonia or nitrate as the nitrogen source, which is expected to give different pentose phosphate pathway fluxes. The presented flux analyses are based on extensive sets of 2D [(13)C, (1)H] COSY data. A new concept is applied for simulation of this type of (13)C-labeling data: cumulative bondomer modeling. The outcomes of the (13)C-labeling based flux analysis substantially differ from those of the pure metabolite balancing approach. The fluxes that are determined using (13)C-labeling data are shown to be highly dependent on the chosen metabolic network. Extending the traditional nonoxidative pentose phosphate pathway with additional transketolase and transaldolase reactions, extending the glycolysis with a fructose 6-phosphate aldolase/dihydroxyacetone kinase reaction sequence or adding a phosphoenolpyruvate carboxykinase reaction to the model considerably improves the fit of the measured and the simulated NMR data. The results obtained using the extended version of the nonoxidative pentose phosphate pathway model show that the transketolase and transaldolase reactions need not be assumed reversible to get a good fit of the (13)C-labeling data. Strict statistical testing of the outcomes of (13)C-labeling based flux analysis using realistic measurement errors is demonstrated to be of prime importance for verifying the assumed metabolic model.     

Bonded Cumomer Analysis of Human Melanoma Metabolism Monitored by 13C NMR Spectroscopy of Perfused Tumor Cells

A network model for the determination of tumor metabolic fluxes from ¹³C NMR kinetic isotopomer data has been developed and validated with perfused human DB-1 melanoma cells carrying the BRAF V600E mutation, which promotes oxidative metabolism. The model generated in the bonded cumomer formalism describes key pathways of tumor intermediary metabolism and yields dynamic curves for positional isotopic enrichment and spin-spin multiplets. Cells attached to microcarrier beads were perfused with 26 mm [1,6-¹³C₂]glucose under normoxic conditions at 37 °C and monitored by ¹³C NMR spectroscopy. Excellent agreement between model-predicted and experimentally measured values of the rates of oxygen and glucose consumption, lactate production, and glutamate pool size validated the model. ATP production by glycolytic and oxidative metabolism were compared under hyperglycemic normoxic conditions; 51% of the energy came from oxidative phosphorylation and 49% came from glycolysis. Even though the rate of glutamine uptake was ∼50% of the tricarboxylic acid cycle flux, the rate of ATP production from glutamine was essentially zero (no glutaminolysis). De novo fatty acid production was ∼6% of the tricarboxylic acid cycle flux. The oxidative pentose phosphate pathway flux was 3.6% of glycolysis, and three non-oxidative pentose phosphate pathway exchange fluxes were calculated. Mass spectrometry was then used to compare fluxes through various pathways under hyperglycemic (26 mm) and euglycemic (5 mm) conditions. Under euglycemic conditions glutamine uptake doubled, but ATP production from glutamine did not significantly change. A new parameter measuring the Warburg effect (the ratio of lactate production flux to pyruvate influx through the mitochondrial pyruvate carrier) was calculated to be 21, close to upper limit of oxidative metabolism.     

Parallel labeling experiments with [U-(13)C]glucose validate E. coli metabolic network model for (13)C metabolic flux analysis

(13)C-metabolic flux analysis (MFA) is a widely used method for measuring intracellular metabolic fluxes in living cells. (13)C MFA relies on several key assumptions: (1) the assumed metabolic network model is complete, in that it accounts for all significant enzymatic and transport reactions; (2) (13)C-labeling measurements are accurate and precise; and (3) enzymes and transporters do not discriminate between (12)C- and (13)C-labeled metabolites. In this study, we tested these inherent assumptions of (13)C MFA for wild-type E. coli by parallel labeling experiments with [U-(13)C]glucose as tracer. Cells were grown in six parallel cultures in custom-constructed mini-bioreactors, starting from the same inoculum, on medium containing different mixtures of natural glucose and fully labeled [U-(13)C]glucose, ranging from 0% to 100% [U-(13)C]glucose. Macroscopic growth characteristics of E. coli showed no observable kinetic isotope effect. The cells grew equally well on natural glucose, 100% [U-(13)C]glucose, and mixtures thereof. (13)C MFA was then used to determine intracellular metabolic fluxes for several metabolic network models: an initial network model from literature; and extended network models that accounted for potential dilution effects of isotopic labeling. The initial network model did not give statistically acceptable fits and produced inconsistent flux results for the parallel labeling experiments. In contrast, an extended network model that accounted for dilution of intracellular CO(2) by exchange with extracellular CO(2) produced statistically acceptable fits, and the estimated metabolic fluxes were consistent for the parallel cultures. This study illustrates the importance of model validation for (13)C MFA. We show that an incomplete network model can produce statistically unacceptable fits, as determined by a chi-square test for goodness-of-fit, and return biased metabolic fluxes. The validated metabolic network model for E. coli from this study can be used in future investigations for unbiased metabolic flux measurements.     

Glucose and glutamine metabolism in rat macrophages: enhanced glycolysis and unaltered glutaminolysis in spontaneously diabetic BB rats.

Metabolism of glutamine (Gln, 2 mM) and glucose (5 mM) was studied in vitro in isolated resident peritoneal macrophages from both normal (BBn) and spontaneously diabetic BB (BBd) rats. The major products from Gln were ammonia, glutamate, CO2 and to a lesser extent aspartate. Glucose decreased (P less than 0.01) the production of ammonia, CO2 and aspartate from Gln by 34-60%, but had no effect on the amount of glutamate accumulated. The major products from glucose were lactate and to a much lesser extent pyruvate and CO2. Gln decreased (P less than 0.01) 14CO2 production from [U-14C]glucose by 19-28%, increased (P less than 0.01) pyruvate production by 35-49%, but had no effect on lactate production. The fraction of glucose metabolized via the pentose phosphate pathway (PC) was less than 5%. There were no significant differences in Gln metabolism between BBn and BBd macrophages. The production of lactate and pyruvate and the flux from glucose into the PC were increased (P less than 0.01) by 2.4, 1.8 and 1.5-fold, respectively, in BBd cells. Increased macrophage glucose metabolism was also observed in diabetes-prone BB (BBdp) rats at 75-80 days but not at 50 days of age. In the presence of both Gln and glucose, potential ATP production from glucose was 2- and 4-times that from Gln, respectively, in BBn and BBd cells. Lactate production was the major pathway for glucose-derived ATP generation. These results demonstrate (a) glycolysis and flux from glucose through the pentose phosphate pathway are enhanced with no alteration in glutaminolysis in BBd macrophages; and (b) glucose may be a more important fuel than Gln for macrophages, particularly in BBd rats. The increased glucose metabolism may be associated with functional activation of the macrophages that have been proposed to be involved in beta-cell destruction and the development of diabetes.     

Metabolic flux analysis for a ppc mutant Escherichia coli based on 13C-labelling experiments together with enzyme activity assays and intracellular metabolite measurements

The physiology and central metabolism of a ppc mutant Escherichia coli were investigated based on the metabolic flux distribution obtained by (13)C-labelling experiments using gas chromatography-mass spectrometry (GC-MS) and 2-dimensional nuclear magnetic resonance (2D NMR) strategies together with enzyme activity assays and intracellular metabolite concentration measurements. Compared to the wild type, its ppc mutant excreted little acetate and produced less carbon dioxide at the expense of a slower growth rate and a lower glucose uptake rate. Consequently, an improvement of the biomass yield on glucose was observed in the ppc mutant. Enzyme activity measurements revealed that isocitrate lyase activity increased by more than 3-fold in the ppc mutant. Some TCA cycle enzymes such as citrate synthase, aconitase and malate dehydrogenase were also upregulated, but enzymes of glycolysis and the pentose phosphate pathway were downregulated. The intracellular intermediates in the glycolysis and the pentose phosphate pathway, therefore, accumulated, while acetyl coenzyme A and oxaloacetate concentrations decreased in the ppc mutant. The intracellular metabolic flux analysis uncovered that deletion of ppc resulted in the appearance of the glyoxylate shunt, with 18.9% of the carbon flux being channeled via the glyoxylate shunt. However, the flux of the pentose phosphate pathway significantly decreased in the ppc mutant.     

Rates of flux through the pentose cycle in perfused rat liver. A procedure for the calculation of rates of substrate flux from 14CO2 production from [1-14C]glucose

A method for the determination of substrate flux through the pentose cycle was developed employing [1-14C]glucose in experiments with perfused rat livers. The method consists first of a kinetic analysis which differentiates between the production of 14CO2 from [1-14C]glucose via the pentose cycle and via the citrate cycle and, second of a calculation of the specific radioactivity of the hexose monophosphate pool from measured rates of glycolysis and the specific radioactivity of lactate released into the perfusate. The method was validated by experiments comparing the results of tracer infusions with [1-14C]glucose, [6-14C]glucose and [3-14C]pyruvate. In livers from fed rats perfused with 10 mM glucose, the rate of substrate flux through the pentose cycle was around 0.2 mumol X min-1 X g-1; it was about 20% of the substrate flux via glycolysis. The kinetic data were inconsistent with the existence of an L-type pentose cycle in liver.     

Intracellular flux analysis in hybridomas using mass balances and in vitro (13)C nmr

Intracellular fluxes are important in defining cellular physiology and its changes in response to environmental variations. Stoichiometric balances combined with extra cellular metabolite measurements were applied to the estimation of intracellular fluxes and the study of energy metabolism in the hybridoma cell line ATCC CRL 1606. Redundant measurements allowed the evaluation of the consistency of the stoichiometry, measurements, and pseudo-steady-state assumption leading to refinement of the assumed biochemistry and identification of measurement errors. To validate the flux estimates, two batch experiments were performed with glucose labeled in the 1 position with (13)C. The distribution of (13)C in secreted lactate was measured via nuclear magnetic resonance spectroscopy (NMR) and compared to that predicted from the estimated intracellular fluxes. There was good agreement between the measured and estimated isotope distributions, demonstrating the validity of the flux estimates obtained from stoichiometric balances. (c) 1995 John Wiley & Sons, Inc.     

Fluxes and enzyme activities in central metabolism of myeloma cells grown in chemostat culture

Activities of enzymes in glycolysis, the pentose phosphate pathway, the tricarboxylic acid cycle, and glutaminolysis have been determined in the mouse myeloma SP2/0.Ag14. Cells were grown on IMDM medium with 5% serum in steady-state chemostat culture at a fixed dilution rate of 0.03 h-1. Three culture conditions, which differed in supply of glucose and oxygen, were chosen so as to change catabolic fluxes in the central metabolism, while keeping anabolic fluxes constant. In the three steady-state situations, the ratio between specific rates of glucose and glutamine consumption differed by more than twentyfold. The specific rates of glucose consumption and lactate production were highest at low oxygen supply, whereas the specific rate of glutamine consumption was highest in the culture fed with low amounts of glucose. Under low oxygen conditions, the specific production of ammonia increased and the consumption pattern of amino acids showed large changes compared with the other two cultures. For the three steady states, activities of key enzymes in glycolysis, the pentose phosphate pathway, glutaminolysis, and the TCA cycle were measured. The differences in the in vivo fluxes were only partially reflected in changes in enzyme levels. The largest differences were observed in the levels of glycolytic enzymes, which were elevated under conditions of low oxygen supply. High activities of phosphoenolpyruvate carboxykinase (E.C. 4.1.1.32) in all cultures suggest an important role for this enzyme as a link between glutaminolysis and glycolysis. For all enzymes, in vitro activities were found that could accommodate the estimated maximum in vivo fluxes. These results show that the regulation of fluxes in central metabolism of mammalian cells occurs mainly through modulation of enzyme activity and, to a much lesser extent, by enzyme synthesis.     

Effects of deflazacort and the L-6485 metabolite on epiphyseal cartilage carbohydrate metabolism: comparison with prednisone

Male Sprague-Dawley rats were injected with 1 mg/100 g bw/day of prednisone, 1.25 mg/1--g bw/day of deflazacort, or its metabolite, for a period of 20 days. Epiphyseal cartilage slices were incubated in a modified Krebs Ringer bicarbonate buffer, at 37 degrees C for 60 min, with either 14C-1- or 14C-6-glucose to quantitate both the absolute and relative rates of pentose shunt versus aerobic and anaerobic glycolytic activity, respectively. Measurements of both total and radioactive glucose uptake, lactate production and 14CO2 generation were expressed as either mumoles or DPM/mg cellular DNA/hr, respectively. This study demonstrated: (1) chronic prednisone administration decreased anaerobic glycolysis (glucose uptake and lactate production) 3-fold (P less than 0.01); (2) prednisone on a chronic basis produced no measured alteration in either the pentose shunt or Kreb's cycle activity; (3) both deflazacort and the deflazacort metabolite significantly stimulated (P less than 0.02) anaerobic glycolytic activity in epiphyseal cartilage tissue. In contrast to prednisone, the administration of either deflazacort or its L-6485 metabolite did not inhibit the glycolytic pathway of metabolism so necessary for epiphyseal cartilage growth and mineralization.