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

An adapted isotope dilution 1H-13C heteronuclear single-quantum correlation (ID-HSQC) for rapid and accurate quantification of endogenous and exogenous plasma glucose

A wide variety of methods, such as enzymatic methods, LC-MS, and LC-MS/MS, are currently available for the concentration determination of plasma glucose in studies of diabetes, obesity, exercise, etc. However, these methods rarely discriminate endogenous and exogenous glucose in plasma. A novel NMR strategy for discriminative quantification of the endogenous and exogenous glucose in plasma has been developed using an adapted isotope dilution ¹H-¹³C heteronuclear single-quantum correlation (ID-HSQC) with uniformly ¹³C-labeled glucose as a tracer of exogenous glucose. This method takes advantage of the distinct ¹H-¹³C chemical shifts of the hemiacetal group of the α-D-glucopyranose and makes use of the ¹³C-¹³C one-bond J-coupling (¹J_CC) in uniformly ¹³C-labeled glucose to differentiate the ¹H-¹³C HSQC signal of labeled glucose from that of its natural counterpart when data are acquired in high-resolution mode. The molar ratio between the endogenous and exogenous plasma glucose can then be calculated from the peak intensities of the natural and labeled glucose. The accuracy and precision of the method were evaluated using a series of standard mixtures of natural and uniformly ¹³C-labeled glucose with varied but known concentrations. Application of this method is demonstrated for the quantification of endogenous and exogenous glucose in plasma derived from healthy and diabetic cynomolgus monkeys. The results nicely agree with our previous LC-MS/MS results. Considering the natural abundance of ¹³C isotope at the level of 1.1% in endogenous glucose, comparable peak intensities of quantitatively measurable signals derived from natural and labeled glucose imply that the ID-HSQC can tolerate a significantly high ratio of isotope dilution, with labeled/natural glucose at ~ 1%. We expect that the ID-HSQC method can serve as an alternative approach to the biomedical or clinical studies of glucose metabolism.

Tracer-based assessments of hepatic anaplerotic and TCA cycle flux: practicality, stoichiometry, and hidden assumptions

Two groups recently used different tracer methods to quantify liver-specific flux rates. The studies had a similar goal, i.e., to characterize mitochondrial oxidative function. These efforts could have a direct impact on our ability to understand metabolic abnormalities that affect the pathophysiology of fatty liver and allow us to examine mechanisms surrounding potential therapeutic interventions. Briefly, one method couples the continuous infusion of [(13)C]acetate with direct real-time measurements of [(13)C]glutamate labeling in liver; the other method administers [(13)C]propionate, in combination with other tracers, and subsequently measures the (13)C labeling of plasma glucose and/or acetaminophen-glucuronide. It appears that a controversy has arisen, since the respective methods yielded different estimates of the anaplerotic/TCA flux ratio (VANA:VTCA) in "control" subjects, i.e., the [(13)C]acetate- and [(13)C]propionate-derived VANA:VTCA flux ratios appear to be ∼1.4 and ∼5, respectively. While the deep expertise in the respective groups makes it somewhat trivial for each to perform the tracer studies, the data interpretation is inherently difficult. The current perspective was undertaken to examine potential factors that could account for or contribute to the apparent differences. Attention was directed toward 1) matters of practicality, 2) issues surrounding stoichiometry, and 3) hidden assumptions. We believe that the [(13)C]acetate method has certain weaknesses that limit its utility; in contrast, the [(13)C]propionate method likely yields a more correct answer. We hope our discussion will help clarify the differences in the recent reports. Presumably this will be of interest to investigators who are considering tracer-based studies of liver metabolism.

Direct assessment of hepatic mitochondrial oxidative and anaplerotic fluxes in humans using dynamic 13C magnetic resonance spectroscopy

Despite the central role of the liver in the regulation of glucose and lipid metabolism, there are currently no methods to directly assess hepatic oxidative metabolism in humans in vivo. By using a new (13)C-labeling strategy in combination with (13)C magnetic resonance spectroscopy, we show that rates of mitochondrial oxidation and anaplerosis in human liver can be directly determined noninvasively. Using this approach, we found the mean rates of hepatic tricarboxylic acid (TCA) cycle flux (VTCA) and anaplerotic flux (VANA) to be 0.43 ± 0.04 μmol g(-1) min(-1) and 0.60 ± 0.11 μmol g(-1) min(-1), respectively, in twelve healthy, lean individuals. We also found the VANA/VTCA ratio to be 1.39 ± 0.22, which is severalfold lower than recently published estimates using an indirect approach. This method will be useful for understanding the pathogenesis of nonalcoholic fatty liver disease and type 2 diabetes, as well as for assessing the effectiveness of new therapies targeting these pathways in humans.

Gluconeogenesis is associated with high rates of tricarboxylic acid and pyruvate cycling in fasting northern elephant seals

Animals that endure prolonged periods of food deprivation preserve vital organ function by sparing protein from catabolism. Much of this protein sparing is achieved by reducing metabolic rate and suppressing gluconeogenesis while fasting. Northern elephant seals (Mirounga angustirostris) endure prolonged fasts of up to 3 mo at multiple life stages. During these fasts, elephant seals maintain high levels of activity and energy expenditure associated with breeding, reproduction, lactation, and development while maintaining rates of glucose production typical of a postabsorptive mammal. Therefore, we investigated how fasting elephant seals meet the requirements of glucose-dependent tissues while suppressing protein catabolism by measuring the contribution of glycogenolysis, glycerol, and phosphoenolpyruvate (PEP) to endogenous glucose production (EGP) during their natural 2-mo postweaning fast. Additionally, pathway flux rates associated with the tricarboxylic acid (TCA) cycle were measured specifically, flux through phosphoenolpyruvate carboxykinase (PEPCK) and pyruvate cycling. The rate of glucose production decreased during the fast (F(1,13) = 5.7, P = 0.04) but remained similar to that of postabsorptive mammals. The fractional contributions of glycogen, glycerol, and PEP did not change with fasting; PEP was the primary gluconeogenic precursor and accounted for ∼95% of EGP. This large contribution of PEP to glucose production occurred without substantial protein loss. Fluxes through the TCA cycle, PEPCK, and pyruvate cycling were higher than reported in other species and were the most energetically costly component of hepatic carbohydrate metabolism. The active pyruvate recycling fluxes detected in elephant seals may serve to rectify gluconeogeneic PEP production during restricted anaplerotic inflow in these fasting-adapted animals.

Effect of supplementation with vitamin D₃ on glucose production pathways in human subjects

SCOPE

Research reports suggest that vitamin D affects glucose and insulin metabolism; however, the exact mechanisms are unclear. ²H NMR analysis of monoacetone glucose (MAG) after tracer administration provides a non-invasive method of profiling hepatic glucose metabolism. This study examined the effects of supplementation with vitamin D₃ on contribution of glycogenolysis to glucose production.

METHODS AND RESULTS

Tracer administration and biofluid collections were performed with eight healthy females before and following a 4-wk vitamin D₃ administration period. Following an overnight fast subjects ingested deuterated water and acetaminophen. Full void urine samples were collected after 4 h. ²H NMR spectra of urinary monoacetone glucose were acquired to determine the contribution of glycogenolysis to glucose production. The mean contribution of glycogenolysis to glucose production was 60±13%. Supplementation with vitamin D₃ had no effect on hepatic glucose production. Regression analysis revealed a significant relationship between carbohydrate intake and the contribution of glycogenolysis (β=0.914, p=0.004).

CONCLUSION

In conclusion, we saw no changes in the percentage contribution of glycogenolysis following supplementation with vitamin D₃. The reproducibility of our results and the non-invasive nature of the method highlight the potential for this method in assessing mechanistic modes of action in future nutritional interventions.

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.

Comparison of [3,4-13C2]glucose to [6,6-2H2]glucose as a tracer for glucose turnover by nuclear magnetic resonance

A recently introduced tracer, [3,4-(13)C(2)]glucose, was compared to the widely used tracer, [6,6-(2)H(2)]glucose, for measurement of whole-body glucose turnover. The rate of glucose production (GP) was measured in rats after primed infusions of [3,4-(13)C(2)]glucose, [6,6-(2)H(2)]glucose, or both tracers simultaneously followed by a constant infusion of tracer(s) over 90 min. Blood glucose was purified and converted into monoacetone glucose for analysis by (13)C NMR (for [3,4-(13)C(2)]glucose) or (1)H and (2)H NMR (for [6,6-(2)H(2)]glucose). The values of GP measured during infusion of each single tracer were not significantly different. In rats infused with both tracers simultaneously, GP was identical as reported by each tracer, 42 +/- 4 micromol/kg/min. Since (2)H and (13)C enrichment in glucose is typically much less than 2% for in vivo studies, [3,4-(13)C(2)]glucose does not interfere with measurements of (13)C or (2)H enrichment patterns and therefore is valuable when multiple metabolic pathways are being evaluated simultaneously.

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.

MR techniques for in vivo molecular and cellular imaging

MR-based molecular imaging is a science in infancy. Current clinical contrast agents are often geared toward the assessment of gross physiologic function, rather than targeting specific biochemical pathways. The development of specific targeted smart contrast agents for Food and Drug Administration approval or clinical trials has only begun. The fact that MR imaging can obtain images of extremely high resolution, coupled with its ability to simultaneously assess structure and function through the use of targeted contrast agents indicates that MR will play a pivotal role in clinical molecular imaging of the future. Many of the challenges that face MR imaging and spectroscopy are inherent to all modalities in the rapidly growing field of molecular imaging. The development of smart contrast agents to report on receptor function, and to monitor gene expression or the results of gene therapy in humans is paramount. These compounds need to undergo rigorous testing to be approved for clinical use: the assessment of acute toxicity, pharmacokinetics, long-term accumulation, and subsequent chronic effects. For receptor-targeted contrast agents, the degree of receptor occupancy and the intrinsic agonist or antagonist properties of the probe that may affect normal cellular function need to be determined to avoid undesired side effects. The particular problems that face MR imaging, those of sensitivity and target specificity, need to be overcome. Signal amplification achieved through high relaxivity contrast agents containing multiple paramagnetic centers, or of larger superparamagnetic particles, is the first step in this direction. The modulation of relaxivity through oligomerization, or other modifications that cause restriction of rotational motions, shows great promise for improving the discriminative powers of MR imaging, and may permit multiple targets to be assessed simultaneously. Moreover, the introduction of smart indicators that lead to changes in spectroscopic properties will allow further discrimination to be achieved through the implementation of chemical shift or spectroscopic imaging. The growing number of MR imaging applications in this rapidly expanding field point to a bright future for MR imaging in molecular imaging.

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.

Metabolic engineering: advances in modeling and intervention in health and disease

The field of metabolic engineering encompasses a powerful set of tools that can be divided into (a) methods to model complex metabolic pathways and (b) techniques to manipulate these pathways for a desired metabolic outcome. These tools have recently seen increased utility in the medical arena, and this paper aims to review significant accomplishments made using these approaches. The modeling of metabolic pathways has been applied to better understand disease-state physiology in a variety of cellar, subcellular, and organ systems, including the liver, heart, mitochondria, and cancerous cells. Metabolic pathway engineering has been used to generate cells with novel biochemical functions for therapeutic use, and specific examples are provided in the areas of glycosylation engineering and dopamine-replacement therapy. In order to document the potential of applying both metabolic modeling and pathway manipulation, we describe pertinent advances in the field of diabetes research. Undoubtedly, as the field of metabolic engineering matures and is applied to a wider array of problems, new advances and therapeutic strategies will follow.

Determination of tricarboxylic acid cycle acids and other related substances in cultured mammalian cells by gradient ion-exchange chromatography with suppressed conductivity detection

An ion-exchange chromatography method was established for simultaneously analyzing the tricarboxylic acid (TCA) cycle acids and other related substances in cultured mammalian cells, including citrate, cis-aconitate, isocitrate, alpha-ketoglutarate, succinate, malate, fumarate, oxaloacetate, trans-aconitate, phosphate, lactate and pyruvate. A Dionex 600 ion chromatograph with an ion suppressor and a conductivity detector, and an IonPac AS11-HC analytical column were employed. An NaOH gradient elution containing 13.5% methanol contributed to sufficient separation of target substances. The stability of carboxylic acids was investigated and oxaloacetate was found to be extremely unstable especially at pH 3. TCA cycle acids and other related substances in Chinese hamster ovary (CHO) cells were separated completely, and lactate, malate, phosphate, citrate and cis-aconitate were quantified due to their higher concentrations. In the quantification of the five substances, detection limits (S/N=3) ranged from 0.12 to 0.48 microM, the correlation coefficients from 0.9982 to 1.0000 in their linear ranges of concentration, and the recoveries from 87 to 95%. The metabolic status of CHO cells was analyzed on the basis of the intracellular concentrations of TCA cycle acids.

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.

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.

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.

Metabolic flux analysis of cultured hepatocytes exposed to plasma

Hepatic metabolism can be investigated using metabolic flux analysis (MFA), which provides a comprehensive overview of the intracellular metabolic flux distribution. The characterization of intermediary metabolism in hepatocytes is important for all biotechnological applications involving liver cells, including the development of bioartificial liver (BAL) devices. During BAL operation, hepatocytes are exposed to plasma or blood from the patient, at which time they are prone to accumulate intracellular lipids and exhibit poor liver-specific functions. In a prior study, we found that preconditioning the primary rat hepatocytes in culture medium containing physiological levels of insulin, as opposed to the typical supraphysiological levels found in standard hepatocyte culture media, reduced lipid accumulation during subsequent plasma exposure. Furthermore, supplementing the plasma with amino acids restored hepatospecific functions. In the current study, we used MFA to quantify the changes in intracellular pathway fluxes of primary rat hepatocytes in response to low-insulin preconditioning and amino acid supplementation. We found that culturing hepatocytes in medium containing lower physiological levels of insulin decreased the clearance of glucose and glycerol with a concomitant decrease in glycolysis. These findings are consistent with the general notion that low insulin, especially in the presence of high glucagon levels, downregulates glycolysis in favor of gluconeogenesis in hepatocytes. The MFA model shows that, during subsequent plasma exposure, low-insulin preconditioning upregulated gluconeogenesis, with lactate as the primary precursor in unsupplemented plasma, with a greater contribution from deaminated amino acids in amino acid-supplemented plasma. Concomitantly, low-insulin preconditioning increased fatty acid oxidation, an effect that was further enhanced by amino acid supplementation to the plasma. The increase in fatty acid oxidation reduced intracellular triglyceride accumulation. Overall, these findings are consistent with the notion that the insulin level in medium culture presets the metabolic machinery of hepatocytes such that it directly impacts on their metabolic behavior during subsequent plasma culture.

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.

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.

Activation of direct and indirect pathways of glycogen synthesis by hepatic overexpression of protein targeting to glycogen

Glycogen-targeting subunits of protein phosphatase-1, such as protein targeting to glycogen (PTG), direct the phosphatase to the glycogen particle, where it stimulates glycogenesis. We have investigated the metabolic impact of overexpressing PTG in liver of normal rats. After administration of PTG cDNA in a recombinant adenovirus, animals were fasted or allowed to continue feeding for 24 hours. Liver glycogen was nearly completely depleted in fasted control animals, whereas glycogen levels in fasted or fed PTG-overexpressing animals were 70% higher than in fed controls. Nevertheless, transgenic animals regulated plasma glucose, triglycerides, FFAs, ketones, and insulin normally in the fasted and fed states. Fasted PTG-overexpressing animals receiving an oral bolus of [U-(13)C]glucose exhibited a large increase in hepatic glycogen content and a 70% increase in incorporation of [(13)C]glucose into glycogen. However, incorporation of labeled glucose accounted for only a small portion of the glycogen synthesized in PTG-overexpressing animals, consistent with our earlier finding that PTG promotes glycogen synthesis from gluconeogenic precursors. We conclude that hepatic PTG overexpression activates both direct and indirect pathways of glycogen synthesis. Because of its ability to enhance glucose storage without affecting other metabolic indicators, the glycogen-targeting subunit may prove valuable in controlling blood glucose levels in diabetes.