Measurement of gluconeogenesis and pyruvate recycling in the rat liver: a simple analysis of glucose and glutamate isotopomers during metabolism of [1,2,3-(13)C3]propionate

Adiabatic JHSQC for 13C isotopomer analysis

A previous implementation of JHSQC enhanced the detection limits for 13C isotopomer analysis, but owing to experimental instabilities required extensive calibration prior to data collection. The addition of adiabatic pulses as well as a double spin-echo during the heteronuclear transfer periods has dramatically increased the precision of the experiment and done away with the need for calibration prior to running each sample. This new experiment allows the sensitivity gains from inverse detection to be realized in a time-efficient manner for the experimentalist.

Characterizing phenotype with tracer based metabolomics

In the post-genomic era, a pressing challenge to biological scientists is to understand the organization of gene functions, the interaction between gene and nutrient environment, and the genesis of phenotypes. Metabolomics, the quantitation of low molecular weight compounds, has been used to provide a phenotypic description of a cell or tissue by a set of metabolites. Gene function is hypothesized from its correlation with the corresponding set of macromolecules by transcriptomics or proteomics. Another approach to genotype-phenotype correlation is by the reconstruction of genome-scale metabolic maps. The utilization of specific pathways as predicted by reaction network analysis provides the phenotypic characterization of a cell, which can be plotted on a phenotypic phase plane. Tracer based metabolomics is the experimental approach to reaction network analysis using stable isotope tracers. The redistribution of the isotope tracer among metabolic intermediates is used to identify a finite number of pathways, the utilization of which is characteristic of the phenotypic behavior of cells. In this paper, we review tracer based metabolomic methods for the construction of phenotypic phase plane plots, and discuss the functional implications of phenotypic phase plane analysis. Examples of phenotypic changes in response to differentiation, inhibition of signaling pathways and perturbation in nutrient environment are provided.

Cellular to tissue informatics: approaches to optimizing cellular function of engineered tissue

Tissue engineering is a rapidly expanding, multi-disciplinary field in biomedicine. It provides the ability to manipulate living cells and biomaterials for the purpose of restoring, maintaining, and enhancing tissue and organ function. Scientists have engineered various tissues in the body, from skin substitutes to artificial nerves to heart tissues, with varying degrees of success. Although the field of tissue engineering has come a long way since its first successful demonstration by Bisceglie in the 1930s, methods of coaxing them into functional tissues have been predominantly empirical to date. To successfully develop tissue-engineered organs, it is important to understand how to maintain the cells under conditions that maximize their ability to perform their physiological roles, regardless of their environment. In that context, a methodology that combines empirical data with mathematical and statistical techniques, such as metabolic engineering and cellular informatics, to systematically determine the optimal (1) type of cell to use, (2) scaffold properties and the corresponding processing conditions to achieve those properties, and (3) the required types and levels of environmental factors and the operating conditions needed in the bioreactor, will enable the design of viable and functional tissues tailored to the specific requirements of individual situations.

Effect of murine strain on metabolic pathways of glucose production after brief or prolonged fasting

Background strain is known to influence the way a genetic manipulation affects mouse phenotypes. Despite data that demonstrate variations in the primary phenotype of basic inbred strains of mice, there is limited data available about specific metabolic fluxes in vivo that may be responsible for the differences in strain phenotypes. In this study, a simple stable isotope tracer/NMR spectroscopic protocol has been used to compare metabolic fluxes in ICR, FVB/N (FVB), C57BL/6J (B6), and 129S1/SvImJ (129) mouse strains. After a short-term fast in these mice, there were no detectable differences in the pathway fluxes that contribute to glucose synthesis. However, after a 24-h fast, B6 mice retain some residual glycogenolysis compared with other strains. FVB mice also had a 30% higher in vivo phosphoenolpyruvate carboxykinase flux and total glucose production from the level of the TCA cycle compared with B6 and 129 strains, while total body glucose production in the 129 strain was approximately 30% lower than in either FVB or B6 mice. These data indicate that there are inherent differences in several pathways involving glucose metabolism of inbred strains of mice that may contribute to a phenotype after genetic manipulation in these animals. The techniques used here are amenable to use as a secondary or tertiary tool for studying mouse models with disruptions of intermediary metabolism.

Differing mechanisms of hepatic glucose overproduction in triiodothyronine-treated rats vs. Zucker diabetic fatty rats by NMR analysis of plasma glucose

The metabolic mechanism of hepatic glucose overproduction was investigated in 3,3'-5-triiodo-l-thyronine (T3)-treated rats and Zucker diabetic fatty (ZDF) rats (fa/fa) after a 24-h fast. 2H2O and [U-13C3]propionate were administered intraperitoneally, and [3,4-13C2]glucose was administered as a primed infusion for 90 min under ketamine-xylazine anesthesia. 13C NMR analysis of monoacetone glucose derived from plasma glucose indicated that hepatic glucose production was twofold higher in both T3-treated rats and ZDF rats compared with controls, yet the sources of glucose overproduction differed significantly in the two models by 2H NMR analysis. In T3-treated rats, the hepatic glycogen content and hence the contribution of glycogenolysis to glucose production was essentially zero; in this case, excess glucose production was due to a dramatic increase in gluconeogenesis from TCA cycle intermediates. 13C NMR analysis also revealed increased phosphoenolpyruvate carboxykinase flux (4x), increased pyruvate cycling flux (4x), and increased TCA flux (5x) in T3-treated animals. ZDF rats had substantial glycogen stores after a 24-h fast, and consequently nearly 50% of plasma glucose originated from glycogenolysis; other fluxes related to the TCA cycle were not different from controls. The differing mechanisms of excess glucose production in these models were easily distinguished by integrated 2H and 13C NMR analysis of plasma glucose.

Glucose production, gluconeogenesis, and hepatic tricarboxylic acid cycle fluxes measured by nuclear magnetic resonance analysis of a single glucose derivative

A triple-tracer method was developed to provide absolute fluxes contributing to endogenous glucose production and hepatic tricarboxylic acid (TCA) cycle fluxes in 24-h-fasted rats by (2)H and (13)C nuclear magnetic resonance (NMR) analysis of a single glucose derivative. A primed, intravenous [3,4-(13)C(2)]glucose infusion was used to measure endogenous glucose production; intraperitoneal (2)H(2)O (to enrich total body water) was used to quantify sources of glucose (TCA cycle, glycerol, and glycogen), and intraperitoneal [U-(13)C(3)] propionate was used to quantify hepatic anaplerosis, pyruvate cycling, and TCA cycle flux. Plasma glucose was converted to monoacetone glucose (MAG), and a single (2)H and (13)C NMR spectrum of MAG provided the following metabolic data (all in units of micromol/kg/min; n = 6): endogenous glucose production (40.4+/-2.9), gluconeogenesis from glycerol (11.5+/-3.5), gluconeogenesis from the TCA cycle (67.3+/-5.6), glycogenolysis (1.0+/-0.8), pyruvate cycling (154.4+/-43.4), PEPCK flux (221.7+/-47.6), and TCA cycle flux (49.1+/-16.8). In a separate group of rats, glucose production was not different in the absence of (2)H(2)O and [U-(13)C]propionate, demonstrating that these tracers do not alter the measurement of glucose turnover.

Impaired tricarboxylic acid cycle activity in mouse livers lacking cytosolic phosphoenolpyruvate carboxykinase

Liver-specific phosphoenolpyruvate carboxykinase (PEPCK) null mice, when fasted, maintain normal whole body glucose kinetics but develop dramatic hepatic steatosis. To identify the abnormalities of hepatic energy generation that lead to steatosis during fasting, we studied metabolic fluxes in livers lacking hepatic cytosolic PEPCK by NMR using 2H and 13C tracers. After a 4-h fast, glucose production from glycogenolysis and conversion of glycerol to glucose remains normal, whereas gluconeogenesis from tricarboxylic acid (TCA) cycle intermediates was nearly absent. Upon an extended 24-h fast, livers that lack PEPCK exhibit both 2-fold lower glucose production and oxygen consumption, compared with the controls, with all glucose production being derived only from glycerol. The mitochondrial reduction-oxidation (red-ox) state, as indicated by the NADH/NAD+ ratio, is 5-fold higher, and hepatic TCA cycle intermediate concentrations are dramatically increased in the PEPCK null livers. Consistent with this, flux through the TCA cycle and pyruvate cycling pathways is 10- and 40-fold lower, respectively. Disruption of hepatic cataplerosis due to loss of PEPCK leads to the accumulation of TCA cycle intermediates and a nearly complete blockage of gluconeogenesis from amino acids and lactate (an energy demanding process) but intact gluconeogenesis from glycerol (which contributes to net NADH production). Inhibition of the TCA cycle and fatty acid oxidation due to increased TCA cycle intermediate concentrations and reduced mitochondrial red-ox state lead to the development of steatosis.

Glucose production pathways by2H and13C NMR in patients with HIV-associated lipoatrophy

Patients with HIV taking protease inhibitors were selected for the presence (five subjects) or absence (five subjects) of lipoatrophy. Following an overnight fast, subjects were given oral (2)H(2)O in divided doses (5 mL/kg body water), [U-(13)C(3)] propionate (10 mg/kg), and acetaminophen (1000 mg). Glucose (from plasma) or acetaminophen glucuronide (from urine) were converted to monoacetone glucose for (2)H NMR and (13)C NMR analysis. The fraction of plasma glucose derived from gluconeogenesis was not significantly different between groups. However, flux from glycerol into gluconeogenesis relative to glucose production was increased from 0.20 +/- 0.13 among subjects without lipoatrophy to 0.42 +/- 0.12 (P < 0.05) among subjects with lipoatrophy, and the TCA cycle contribution was reduced. Lipoatrophy was associated with an abnormal profile of glucose production as assessed by (13)C and (2)H NMR of plasma and urine.

Octanoate oxidation measured by 13C-NMR spectroscopy in rat skeletal muscle, heart, and liver

Contribution of octanoate to the oxidative metabolism of the major sites of fatty acid oxidation (heart, liver, and resting and contracting skeletal muscle) was assessed in the intact rat with 13C-NMR spectroscopy. Under inhalation anesthesia, [2,4,6,8-13C4]octanoate was infused into the jugular vein and the sciatic nerve of one limb was stimulated for 1 h. Octanoate was a principal contributor to the acetyl-CoA pool in all tissues examined, with highest oxidation occurring in heart and soleus muscle followed by predominantly red portion of gastrocnemius muscle (RG), liver, and then white portion of gastrocnemius muscle (WG). Fractional contribution of 13C-labeled octanoate to the acetyl-CoA pool (Fc2) was 0.563 +/- 0.066 for heart and 0.367 +/- 0.054 for liver. Significant differences were observed between each of the muscle types during both rest and contraction. In muscle, Fc2 was highest in soleus (0.565 +/- 0.089 rested, 0.564 +/- 0.096 contracted), followed by RG (0.470 +/- 0.092 rested, 0.438 +/- 0.072 contracted), and lowest in WG (0.340 +/- 0.081 rested, 0.272 +/- 0.065 contracted). Our findings demonstrate that the fractional contribution of octanoate to oxidative metabolism correlates with oxidative capacity of the tissue and that octanoate metabolism increases in contracted muscle in proportion to the overall increase in oxidative rate.

Energy contribution of octanoate to intact rat brain metabolism measured by 13C nuclear magnetic resonance spectroscopy

Glucose is the dominant oxidative fuel for brain, but studies have indicated that fatty acids are used by brain as well. We postulated that fatty acid oxidation in brain could contribute significantly to overall energy usage and account for non-glucose-derived energy production. [2,4,6,8-13C4]octanoate oxidation in intact rats was determined by nuclear magnetic resonance spectroscopy. We found that oxidation of 13C-octanoate in brain is avid and contributes approximately 20% to total brain oxidative energy production. Labeling patterns of glutamate and glutamine were distinct, and analysis of these metabolites indicated compartmentalized oxidation of octanoate in brain. Examination of liver and blood spectra revealed that label from 13C-octanoate was incorporated into glucose and ketones, which enabled calculation of its overall energy contribution to brain metabolism: glucose (predominantly unlabeled) and 13C-labeled octanoate can account for the entire oxidative metabolism of brain. Additionally, flux through anaplerotic pathways relative to tricarboxylic acid cycle flux (Y) was calculated to be 0.08 +/- 0.039 in brain, indicating that anaplerotic flux is significant and should be considered when assessing brain metabolism. Y was associated with the glutamine synthesis compartment, consistent with the view that anaplerotic flux occurs primarily in astrocytes.

Mechanisms by which liver-specific PEPCK knockout mice preserve euglycemia during starvation

Liver-specific PEPCK knockout mice, which are viable despite markedly abnormal lipid metabolism, exhibit mild hyperglycemia in response to fasting. We used isotopic tracer methods, biochemical measurements, and nuclear magnetic resonance spectroscopy to show that in mice lacking hepatic PEPCK, 1) whole-body glucose turnover is only slightly decreased; 2) whole-body gluconeogenesis from phosphoenolpyruvate, but not from glycerol, is moderately decreased; 3) tricarboxylic acid cycle activity is globally increased, even though pyruvate cycling and anaplerosis are decreased; 4) the liver is unable to synthesize glucose from lactate/pyruvate and produces only a minimal amount of glucose; and 5) glycogen synthesis in both the liver and muscle is impaired. Thus, although mice without hepatic PEPCK have markedly impaired hepatic gluconeogenesis, they are able to maintain a near-normal blood glucose concentration while fasting by increasing extrahepatic gluconeogenesis coupled with diminishing whole-body glucose utilization.

In vivo NMR studies of the glutamate neurotransmitter flux and neuroenergetics: implications for brain function

Until very recently, non-invasive measurement of the glutamate-glutamine cycle in the intact mammalian brain had not been possible. In this review, we describe some studies that have led to quantitative assessment of the glutamate-glutamine cycle (Vcyc), as well as other important metabolic fluxes (e.g., glucose oxidation, CMRglc(ox)), with (13)C magnetic resonance spectroscopy (MRS) in vivo. These (13)C MRS studies clearly demonstrate that glutamate released from presynaptic neurons is taken up by the astrocyte for subsequent glutamine synthesis. Contrary to the earlier concept of a small, metabolically inactive neurotransmitter pool, in vivo (13)C MRS studies demonstrate that glutamate release and recycling is a major metabolic pathway that cannot be distinguished from its actions of neurotransmission. Furthermore, the in vivo (13)C MRS studies demonstrate in the rat cerebral cortex that increases in Vcyc and neuronal CMRglc(ox) are linearly related with a close to 1:1 slope. Measurements in human cerebral cortex are in agreement with this result. This relationship is consistent with more than two thirds of the energy yielded by glucose oxidation being used to support events associated with glutamate neurotransmission, and it supports a molecular model of a stoichiometric coupling between glutamate neurotransmission and functional glucose oxidation. (13)C MRS measurements of resting human cerebral cortex have found a high level of glutamate-glutamine cycling. This high resting neuronal activity, which is subtracted away in brain mapping studies by positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), has significant implications for the interpretations of functional imaging data. Here we review and discuss the importance of neurotransmission and neuroenergetics as measured by (13)C MRS for understanding brain function and interpreting fMRI.

Integration of [U-13C]glucose and 2H2O for quantification of hepatic glucose production and gluconeogenesis

Glucose metabolism in five healthy subjects fasted for 16 h was measured with a combination of [U-13C]glucose and 2H2O tracers. Phenylbutyric acid was also provided to sample hepatic glutamine for the presence of 13C-isotopomers derived from the incorporation of [U-13C]glucose products into the hepatic Krebs cycle. Glucose production (GP) was quantified by 13C NMR analysis of the monoacetone derivative of plasma glucose following a primed infusion of [U-13C]glucose and provided reasonable estimates (1.90 +/- 0.19 mg/kg/min with a range of 1.60-2.15 mg/kg/min). The same derivative yielded measurements of plasma glucose 2H-enrichment from 2H2O by 2H NMR from which the contribution of glycogenolytic and gluconeogenic fluxes to GP was obtained (0.87 +/- 0.14 and 1.03 +/- 0.10 mg/kg/min, respectively). Hepatic glutamine 13C-isotopomers representing multiply-enriched oxaloacetate and [U-13C]acetyl-CoA were identified as multiplets in the 13C NMR signals of the glutamine moiety of urinary phenylacetylglutamine, demonstrating entry of the [U-13C]glucose tracer into both oxidative and anaplerotic pathways of the hepatic Krebs cycle. These isotopomers contributed 0.1-0.2% excess enrichment to carbons 2 and 3 and approximately 0.05% to carbon 4 of glutamine.

Relative rates of anaplerotic flux in rested and contracted rat skeletal muscle measured by 13C NMR spectroscopy

Flux through anaplerotic pathways in skeletal muscle has not been evaluated quantitatively during both rest and contraction, nor have fibre type-specific rates of anaplerotic flux been studied. Steady-state analysis using 13C NMR spectroscopy enables calculation of Y (flux rate through anaplerotic pathways relative to tricarboxylic acid (TCA) cycle flux). Under inhalation anaesthesia, [2,4,6,8-13C4]octanoate was infused into the jugular vein of the intact rat (n = 10) and the sciatic nerve of one limb was stimulated at the voltage required to elicit maximal force output at 0.5 Hz. In resting muscle, Y was higher in soleus (0.41 +/- 0.22) versus white gastrocnemius (WG) (0.18 +/- 0.11). Y was 0.29 +/- 0.06 in the predominantly red portion of the gastrocnemius (RG) during rest. During contraction, Y was similar to the resting value in soleus (0.34 +/- 0.14), RG (0.20 +/- 0.04) and WG (0.15 +/- 0.08); Y was higher in soleus versus both RG and WG during contraction. These results demonstrate: (1) relative flux through anaplerotic pathways is 15-41 % of TCA cycle flux at rest and during muscle contraction, (2) higher relative anaplerotic flux in oxidative (soleus) versus glycolytic muscle (WG) during rest and in slow-twitch (soleus) versus fast-twitch (RG and WG) muscle during contraction, and (3) relative flux through anaplerotic pathways is maintained in all muscle fibre types during contraction, which indicates that absolute rates of anaplerotic flux rise in proportion to increased oxidation rates during contraction. These results are consistent with a sustained increase in substrate entry into and exit from the TCA cycle through anaplerotic pathways during contraction.

Noninvasive evaluation of liver metabolism by 2H and 13C NMR isotopomer analysis of human urine

Mammalian liver disposes of acetaminophen and other ingested xenobiotics by forming soluble glucuronides that are subsequently removed via renal filtration. When given in combination with the stable isotopes 2H and 13C, the glucuronide of acetaminophen isolated from urine provides a convenient "chemical biopsy" for evaluating intermediary metabolism in the liver. Here, we describe isolation and purification of urinary acetaminophen glucuronide and its conversion to monoacetone glucose (MAG). Subsequent 2H and 13C NMR analysis of MAG from normal volunteers after ingestion of 2H2O and [U-13C3]propionate allowed a noninvasive profiling of hepatic gluconeogenic pathways. The method should find use in metabolic studies of infants and other populations where blood sampling is either limited or problematic.

Astroglial contribution to brain energy metabolism in humans revealed by 13C nuclear magnetic resonance spectroscopy: elucidation of the dominant pathway for neurotransmitter glutamate repletion and measurement of astrocytic oxidative metabolism

Increasing evidence supports a crucial role for glial metabolism in maintaining proper synaptic function and in the etiology of neurological disease. However, the study of glial metabolism in humans has been hampered by the lack of noninvasive methods. To specifically measure the contribution of astroglia to brain energy metabolism in humans, we used a novel noninvasive nuclear magnetic resonance spectroscopic approach. We measured carbon 13 incorporation into brain glutamate and glutamine in eight volunteers during an intravenous infusion of [2-13C] acetate, which has been shown in animal models to be metabolized specifically in astroglia. Mathematical modeling of the three established pathways for neurotransmitter glutamate repletion indicates that the glutamate/glutamine neurotransmitter cycle between astroglia and neurons (0.32 +/- 0.07 micromol x gm(-1) x min(-1)) is the major pathway for neuronal glutamate repletion and that the astroglial TCA cycle flux (0.14 +/- 0.06 micromol x gm(-1) x min(-1)) accounts for approximately 14% of brain oxygen consumption. Up to 30% of the glutamine transferred to the neurons by the cycle may derive from replacement of oxidized glutamate by anaplerosis. The further application of this approach could potentially enlighten the role of astroglia in supporting brain glutamatergic activity and in neurological and psychiatric disease.

13C NMR isotopomer analysis reveals a connection between pyruvate cycling and glucose-stimulated insulin secretion (GSIS)

Cellular metabolism of glucose is required for stimulation of insulin secretion from pancreatic beta cells, but the precise metabolic coupling factors involved in this process are not known. In an effort to better understand mechanisms of fuel-mediated insulin secretion, we have adapted 13C NMR and isotopomer methods to measure influx of metabolic fuels into the tricarboxylic acid (TCA) cycle in insulinoma cells. Mitochondrial metabolism of [U-13C3]pyruvate, derived from [U-13C6]glucose, was compared in four clonal rat insulinoma cell 1-derived cell lines with varying degrees of glucose responsiveness. A 13C isotopomer analysis of glutamate isolated from these cells showed that the fraction of acetyl-CoA derived from [U-13C6]glucose was the same in all four cell lines (44 +/- 5%, 70 +/- 3%, and 84 +/- 4% with 3, 6, or 12 mM glucose, respectively). The 13C NMR spectra also demonstrated the existence of two compartmental pools of pyruvate, one that exchanges with TCA cycle intermediates and a second pool derived from [U-13C6]glucose that feeds acetyl-CoA into the TCA cycle. The 13C NMR spectra were consistent with a metabolic model where the two pyruvate pools do not randomly mix. Flux between the mitochondrial intermediates and the first pool of pyruvate (pyruvate cycling) varied in proportion to glucose responsiveness in the four cell lines. Furthermore, stimulation of pyruvate cycling with dimethylmalate or its inhibition with phenylacetic acid led to proportional changes in insulin secretion. These findings indicate that exchange of pyruvate with TCA cycle intermediates, rather than oxidation of pyruvate via acetyl-CoA, correlates with glucose-stimulated insulin secretion.

Hepatic gluconeogenesis and Krebs cycle fluxes in a CCl4 model of acute liver failure

Acute liver failure was induced in rats by CCl4 administration and its effects on the hepatic Krebs cycle and gluconeogenic fluxes were evaluated in situ by 13C NMR isotopomer analysis of hepatic glucose following infusion of [U-13C]propionate. In fed animals, CCl4 injury caused a significant increase in relative gluconeogenic flux from 0.80+/-0.10 to 1.34 +/-0.24 times the flux through citrate synthase (p<0.01). In 24-h fasted animals, CCl4-injury also significantly increased relative gluconeogenic flux from 1.36+/-0.16 to 1.80+/-0.22 times the flux through citrate synthase (p<0.01). Recycling of PEP via pyruvate and oxaloacetate was extensive under all conditions and was not significantly altered by CCl4 injury. CCl4 injury significantly reduced hepatic glucose output by 26% (42.8+/-7.3 vs 58.1+/-2.4 micromol/kg/min, p=0.005), which was attributed to a 26% decrease in absolute gluconeogenic flux from PEP (85.6+/-14.6 vs 116+/-4.8 micromol/kg/min, p<0.01). These changes were accompanied by a 47% reduction in absolute citrate synthase flux (90.6+/-8.0 to 47.6+/-8.0 micromol/kg/min, p<0.005), indicating that oxidative Krebs cycle flux was more susceptible to CCl4 injury. The reduction in absolute fluxes indicate a significant loss of hepatic metabolic capacity, while the significant increases in relative gluconeogenic fluxes suggest a reorganization of metabolic activity towards preserving hepatic glucose output.

13C isotopomer analysis of glutamate by tandem mass spectrometry

Tandem mass spectrometry allows a compound to be isolated from the rest of the sample and dissociated into smaller fragments. We show here that fragmentation of glutamate mass isotopomers yields additional mass spectral data that significantly improve the analysis of metabolic fluxes compared to full-scan mass spectrometry. In order to validate the technique, tandem and full-scan mass spectrometry were used along with (13)C NMR to analyze glutamate from rat hearts perfused with three substrate mixtures (5 mM glucose plus 5 mM [2-(13)C]acetate, 5 mM [1-(13)C]glucose plus 5 U/L insulin, and 5 mM glucose plus 1 mM [3-(13)C]pyruvate). Analysis by tandem mass spectrometry showed that the enriched substrate contributed 98 +/- 2, 53 +/- 2, and 84 +/- 7%, respectively, of acetyl-coenzyme A while the rate of anaplerotic substrate entry was 7 +/- 3, 25 +/- 8, and 16 +/- 8%. Similar results were obtained with (13)C NMR data, while values from full-scan data had higher error. We believe that this is the first use of tandem mass spectrometry to determine pathway flux using (13)C-enriched substrates. Although analysis of the citric acid cycle by NMR is simpler (and more intuitive), tandem mass spectrometry has the potential to combine high sensitivity with the high information yield previously available only by NMR.

An integrated (2)H and (13)C NMR study of gluconeogenesis and TCA cycle flux in humans

Hepatic glucose synthesis from glycogen, glycerol, and the tricarboxylic acid (TCA) cycle was measured in five overnight-fasted subjects by (1)H, (2)H, and (13)C NMR analysis of blood glucose, urinary acetaminophen glucuronide, and urinary phenylacetylglutamine after administration of [1,6-(13)C(2)]glucose, (2)H(2)O, and [U-(13)C(3)]propionate. This combination of tracers allows three separate elements of hepatic glucose production (GP) to be probed simultaneously in a single study: 1) endogenous GP, 2) the contribution of glycogen, phosphoenolpyruvate (PEP), and glycerol to GP, and 3) flux through PEP carboxykinase, pyruvate recycling, and the TCA cycle. Isotope-dilution measurements of [1,6-(13)C(2)] glucose by (1)H and (13)C NMR indicated that GP in 16-h-fasted humans was 10.7 +/- 0.9 micromol.kg(-1).min(-1). (2)H NMR spectra of monoacetone glucose (derived from plasma glucose) provided the relative (2)H enrichment at glucose H-2, H-5, and H-6S, which, in turn, reflects the contribution of glycogen, PEP, and glycerol to total GP (5.5 +/- 0.7, 4.8 +/- 1.0, and 0.4 +/- 0.3 micromol.kg(-1).min(-1), respectively). Interestingly, (13)C NMR isotopomer analysis of phenylacetylglutamine and acetaminophen glucuronide reported different values for PEP carboxykinase flux (68.8 +/- 9.8 vs. 37.5 +/- 7.9 micromol.kg(-1).min(-1)), PEP recycling flux (59.1 +/- 9.8 vs. 27.8 +/- 6.8 micromol.kg(-1).min(-1)), and TCA cycle flux (10.9 +/- 1.4 vs. 5.4 +/- 1.4 micromol.kg(-1).min(-1)). These differences may reflect zonation of propionate metabolism in the liver.

13C isotopomer analysis of glutamate by J-resolved heteronuclear single quantum coherence spectroscopy

13C NMR isotopomer analysis is a powerful method for measuring metabolic fluxes through pathways intersecting in the tricarboxylic acid cycle. However, the inherent insensitivity of 13C NMR spectroscopy makes application of isotopomer analysis to small tissue samples (mouse tissue, human biopsies, or cells grown in tissue culture) problematic. (1)H NMR is intrinsically more sensitive than 13C NMR and can potentially supply the same information via indirect detection of 13C providing that isotopomer information can be preserved. We report here the use of J-resolved HSQC (J-HSQC) for 13C isotopomer analysis of tissue samples. We show that J-HSQC reports isotopomer multiplet patterns identical to those reported by direct 13C detection but with improved sensitivity.

Quantitative 13C NMR studies of metabolic compartmentation in the adult mammalian brain

We review the information obtained by 13C NMR methods on the metabolic compartmentation of the adult mammalian brain with emphasis on its quantitative aspects. Classical radiotracer evidence and more recent 13C NMR results support the presence in the brain of at least two glutamate pools, small and large, associated with two kinetically different tricarboxylic acid cycles localized in glia and neurons, respectively. Neuronal and glial cycles interact closely, utilizing common substrates like glucose and oxygen and exchanging a variety of metabolites including glutamate, glutamine and GABA. A model for the cerebral metabolism of (1,2-13C2) acetate has made it possible to calculate fluxes through both cycles and evaluate the exchanges of glutamate, glutamine and GABA under different physiopathological conditions. Calculated flux values through the neuronal and glial tricarboxylic acid cycles are 1.0 and 0.4 mumol/min g, respectively. In the adult normoxic brain, the small and large glutamate pools account for approximately 10% and 90% of cerebral glutamate with estimated turnover times of 1.25 and 5.8/min, respectively. Net transfers of neuronal glutamate and GABA to the glial compartment are calculated to be 0.1 and 0.04 mumol/min g while transfer of glial glutamine to the neuronal compartment is estimated as 0.1 mumol/min g. Pyruvate recycling in the adult brain occurs mainly in the synaptic terminals with a calculated flux of 0.3 mumol/min g. These flux values are altered severely in pathological states such as hypothyroidism or ischemia.

Multiple bond 13C-13C spin-spin coupling provides complementary information in a 13C NMR isotopomer analysis of glutamate

Most 13C nuclear magnetic resonance (NMR) isotopomer analyses relate a metabolic index of interest to populations of 13C isotopomers as reported by one-bond 13C-13C spin-spin couplings. Metabolic conditions that produce highly enriched citric acid cycle intermediates often lead to 13C NMR spectra of metabolites such as glutamate that show extra multiplets due to long-range couplings. It can be demonstrated from 13C NMR spectra of hearts perfused with mixtures of acetate plus propionate that multiplets in glutamate C2 arising from 3J25 coupling provide a direct readout of acetyl-CoA fractional enrichment (FC1 and FC3), while multiplets in glutamate C5 arising from 2J35 and 3J25 couplings quantitatively reflect enrichment of the anaplerotic substrate.

Carbon-13 nuclear magnetic resonance study of metabolism of propionate by Escherichia coli

We have evaluated the use of [1,2-13C2]propionate for the analysis of propionic acid metabolism, based on the ability to distinguish between the methylcitrate and methylmalonate pathways. Studies using propionate-adapted Escherichia coli MG1655 cells were performed. Preservation of the 13C-13C-12C carbon skeleton in labeled alanine and alanine-containing peptides involved in cell wall recycling is indicative of the direct formation of pyruvate from propionate via the methylcitrate cycle, the enzymes of which have recently been demonstrated in E. coli. Additionally, formation of 13C-labeled formate from pyruvate by the action of pyruvate-formate lyase is also consistent with the labeling of pyruvate C-1. Carboxylation of the labeled pyruvate leads to formation of [1,2-13C2]oxaloacetate and to multiply labeled glutamate and succinate isotopomers, also consistent with the flux through the methylcitrate pathway, followed by the tricarboxylic acid (TCA) cycle. Additional labeling of TCA intermediates arises due to the formation of [1-13C]acetyl coenzyme A from the labeled pyruvate, formed via pyruvate-formate lyase. Labeling patterns in trehalose and glycine are also interpreted in terms of the above pathways. The information derived from the [1, 2-13C2]propionate label is contrasted with information which can be derived from singly or triply labeled propionate and shown to be more useful for distinguishing the different propionate utilization pathways via nuclear magnetic resonance analysis.

C isotopomer analysis of glutamate by heteronuclear multiple quantum coherence-total correlation spectroscopy (HMQC-TOCSY)

13C has become an important tracer isotope for studies of intermediary metabolism. Information about relative flux through pathways is encoded by the distribution of 13C isotopomers in an intermediate pool such as glutamate. This information is commonly decoded either by mass spectrometry or by measuring relative multiplet areas in a 13C NMR spectrum. We demonstrate here that groups of glutamate 13C isotopomers may be quantified by indirect detection of protons in a 2D HMQC-TOCSY NMR spectrum and that fitting of these data to a metabolic model provides an identical measure of the 13C fractional enrichment of acetyl-CoA and relative anaplerotic flux to that given by direct 13C NMR analysis. The sensitivity gain provided by HMQC-TOCSY spectroscopy will allow an extension of 13C isotopomer analysis to tissue samples not amenable to direct 13C detection (approximately 10 mg soleus muscle) and to tissue metabolites other than glutamate that are typically present at lower concentrations.

13C NMR measurements of human gluconeogenic fluxes after ingestion of [U-13C]propionate, phenylacetate, and acetaminophen

Anaplerotic, pyruvate recycling, and gluconeogenic fluxes were measured by 13C isotopomer analysis of plasma glucose, urinary phenylacetylglutamine, and urinary glucuronide in normal, 24-h-fasted individuals after ingestion of [U-13C]propionate, phenylacetate, and acetaminophen. Plasma glucose isotopomer analysis reported a total anaplerotic flux of 5.92 +/- 1.03 (SD) relative to citrate synthase. This was not significantly different from glucuronide and phenylacetylglutamine analyses (6.08 +/- 1.16 and 7. 14 +/- 1.94, respectively). Estimates of pyruvate recycling from glucose and glucuronide isotopomer distributions were almost identical (3.55 +/- 0.99 and 3.66 +/- 1.11, respectively), whereas phenylacetylglutamine reported a significantly higher estimate (5.74 +/- 2.13). As a consequence, net gluconeogenic flux reported by phenylacetylglutamine (1.41 +/- 0.28) was significantly less than that reported by glucose (2.37 +/- 0.64) and glucuronide (2.42 +/- 0. 76). This difference in fluxes detected by analysis of phenylacetylglutamine vs. hexose is likely due to compartmentation of hepatic metabolism of propionate. Net gluconeogenic flux estimates made by use of this stable isotope method are in good agreement with recent measurements in humans with [14C]propionate.

Measurement of hepatic glucose output, krebs cycle, and gluconeogenic fluxes by NMR analysis of a single plasma glucose sample

13C and 1H NMR spectroscopy of plasma glucose was used to resolve the isotopomer contributions from tracer levels of [1,6-13C2]glucose, a novel tracer of glucose carbon skeleton turnover, and [U-13C]propionate, a tracer of hepatic citric acid cycle metabolism. This allowed simultaneous measurements of hepatic glucose production and citric acid cycle fluxes from the NMR analysis of a single plasma glucose sample in fasted animals. Glucose carbon skeleton turnover, as reported by the dilution of [1,6-13C2]glucose, was 56 +/- 2 micromol/kg/min in the presence of labeling from [U-13C]propionate and 53 +/- 4 micromol/kg/min in its absence. Therefore, as expected, the labeling contributions from [U-13C]propionate metabolism did not have a significant effect on the measurement of glucose turnover. For the group infused with both tracers, citric acid cycle flux estimates from the analysis of glucose C2 isotopomer ratios were consistent with those from our recent experiments where only [U-13C]propionate was infused, verifying that the presence of [1,6-13C2]glucose did not interfere with these measurements. This integrated analysis of hepatic glucose output and citric acid cycle fluxes from plasma glucose isotopomers yielded a noninvasive estimate of hepatic citrate synthase flux of 74 +/- 12 micromol/kg/min for 24-h fasted rats.

Amino acid metabolism and the energetics of growth

The nonessential amino acids are involved in a large number of functions that are not directly associated with protein synthesis. Recent studies using a combination of transorgan balance and stable isotopic tracers have demonstrated that a substantial portion of the extra-splanchnic flux of glutamate, glutamine, glycine and cysteine derives from tissue synthesis. A key amino acid in this respect is glutamic acid. Little glutamic acid of dietary origin escapes metabolism in the small intestinal mucosa. Furthermore, because glutamic acid is the only amino acid that can be synthesized by mammals by reductive amination of a ketoacid, it is the ultimate nitrogen donor for the synthesis of other nonessential amino acids. Because the synthesis of glutamic acid and its product glutamine involve the expenditure of adenosine triphosphate (ATP), it seems possible that nonessential amino acid synthesis might have a significant bearing on the energetics of protein synthesis and, hence, of protein deposition. This paper discusses the topic of the energy cost of protein deposition, considers the metabolic physiology of amino acid oxidation and nonessential amino acid synthesis, and attempts to combine the information to speculate on the overall impact of amino acid metabolism on the energy exchanges of animals.