Alterations in substrate utilization in the reperfused myocardium: a direct analysis by 13C NMR

Analysis of steady-state carbon tracer experiments using akaike information criteria

INTRODUCTION

Carbon isotope tracers have been used to determine relative rates of tricarboxylic acid cycle (TCA) cycle pathways since the 1950s. Steady-state experimental data are typically fit to a single mathematical model of metabolism to determine metabolic fluxes. Whether the chosen model is appropriate for the biological system has generally not been evaluated systematically. An overly-simple model omits known pathways while an overly-complex model may produce incorrect results due to overfitting.

OBJECTIVES

The objectives were to develop and study a method that systematically evaluates multiple TCA cycle mathematical models as part of the fitting process.

METHODS

The problem of choosing overly-simple or overly-complex models was approached by developing software that automatically explores all possible combinations of flux through pyruvate dehydrogenase, pyruvate kinase, pyruvate carboxylase and anaplerosis at propionyl-CoA carboxylase, and equivalent pathways, all relative to TCA cycle flux. Typical TCA cycle metabolic tracer experiments that use 13C nuclear magnetic resonance for detection and quantification of 13C-enriched glutamate products were simulated and analyzed. By evaluating the multiple model fits with both the conventional sum-of-squares residual error (SSRE) and the Akaike Information Criterion (AIC), the software helps the investigator understand the interaction between model complexity and goodness of fit.

RESULTS

When fitting alternative models of the TCA cycle metabolism, the SSRE may identify more than one model that fits the data well. Among those models, the AIC provides guidance as to which is the simplest of the candidate models is sufficient to describe the observed data. However under some conditions, AIC used alone inappropriately discriminates against necessary metabolic complexity.

CONCLUSION

In combination, the SSRE and AIC help the investigator identify the model that best describes the metabolism of a biological system.

tcaSIM: A Simulation Program for Optimal Design of 13C Tracer Experiments for Analysis of Metabolic Flux by NMR and Mass Spectroscopy

Increasingly sophisticated instrumentation for chemical separations and identification has facilitated rapid advancements in our understanding of the metabolome. Since many analyses are performed using either mass spectroscopy (MS) or nuclear magnetic resonance (NMR) spectroscopy, the spin ½ stable ¹³C isotope is now widely used as a metabolic tracer. There is strong interest in quantitative analysis of metabolic flux through pathways in vivo, particularly in human patients. Although instrumentation advances and scientific interests in metabolism are increasing in parallel, a practical and rational design of a ¹³C tracer study can be challenging. Prior to planning the details of a tracer experiment, is it important to consider whether the analytical results will be sensitive to flux through the pathways of interest. Here, we briefly summarize the various approaches that have been used to design carbon tracer experiments, outline the sources of complexity, and illustrate the use of a software tool, tcaSIM, to aid in the experimental design of both MS and NMR data in complex systems including patients.

A novel inhibitor of pyruvate dehydrogenase kinase stimulates myocardial carbohydrate oxidation in diet-induced obesity

The pyruvate dehydrogenase complex (PDC) is a key control point of energy metabolism and is subject to regulation by multiple mechanisms, including posttranslational phosphorylation by pyruvate dehydrogenase kinase (PDK). Pharmacological modulation of PDC activity could provide a new treatment for diabetic cardiomyopathy, as dysregulated substrate selection is concomitant with decreased heart function. Dichloroacetate (DCA), a classic PDK inhibitor, has been used to treat diabetic cardiomyopathy, but the lack of specificity and side effects of DCA indicate a more specific inhibitor of PDK is needed. This study was designed to determine the effects of a novel and highly selective PDK inhibitor, 2((2,4-dihydroxyphenyl)sulfonyl) isoindoline-4,6-diol (designated PS10), on pyruvate oxidation in diet-induced obese (DIO) mouse hearts compared with DCA-treated hearts. Four groups of mice were studied: lean control, DIO, DIO + DCA, and DIO + PS10. Both DCA and PS10 improved glucose tolerance in the intact animal. Pyruvate metabolism was studied in perfused hearts supplied with physiological mixtures of long chain fatty acids, lactate, and pyruvate. Analysis was performed using conventional ¹H and ¹³C isotopomer methods in combination with hyperpolarized [1-¹³C]pyruvate in the same hearts. PS10 and DCA both stimulated flux through PDC as measured by the appearance of hyperpolarized [¹³C]bicarbonate. DCA but not PS10 increased hyperpolarized [1-¹³C]lactate production. Total carbohydrate oxidation was reduced in DIO mouse hearts but increased by DCA and PS10, the latter doing so without increasing lactate production. The present results suggest that PS10 is a more suitable PDK inhibitor for treatment of diabetic cardiomyopathy.

Assessing Cardiac Metabolism: A Scientific Statement From the American Heart Association

In a complex system of interrelated reactions, the heart converts chemical energy to mechanical energy. Energy transfer is achieved through coordinated activation of enzymes, ion channels, and contractile elements, as well as structural and membrane proteins. The heart's needs for energy are difficult to overestimate. At a time when the cardiovascular research community is discovering a plethora of new molecular methods to assess cardiac metabolism, the methods remain scattered in the literature. The present statement on "Assessing Cardiac Metabolism" seeks to provide a collective and curated resource on methods and models used to investigate established and emerging aspects of cardiac metabolism. Some of those methods are refinements of classic biochemical tools, whereas most others are recent additions from the powerful tools of molecular biology. The aim of this statement is to be useful to many and to do justice to a dynamic field of great complexity.

Production of hyperpolarized 13CO2 from [1-13C]pyruvate in perfused liver does reflect total anaplerosis but is not a reliable biomarker of glucose production

In liver, 13CO2 can be generated from [1-13C] pyruvate via pyruvate dehydrogenase or anaplerotic entry of pyruvate into the TCA cycle followed by decarboxylation at phosphoenolpyruvate carboxykinase (PEPCK), the malic enzyme, isocitrate dehydrogenase, or α-ketoglutarate dehydrogenase. The purpose of this study was to determine the relative importance of these pathways in production of hyperpolarized (HP) 13CO2 after administration of hyper-polarized pyruvate in livers supplied with a fatty acid plus substrates for gluconeogenesis. Isolated mouse livers were perfused with a mixture of thermally-polarized 13C-enriched pyruvate, lactate and octanoate in various combinations prior to exposure to HP pyruvate. Under all perfusion conditions, HP malate, aspartate and fumarate were detected within ~ 3 s showing that HP [1-13C]pyruvate is rapidly converted to [1-13C]oxaloacetate which can subsequently produce HP 13CO2 via decarboxylation at PEPCK. Measurements using HP [2-13C]pyruvate allowed the exclusion of reactions related to TCA cycle turnover as sources of HP 13CO2. Direct measures of O2 consumption, ketone production, and glucose production by the intact liver combined with 13C isotopomer analyses of tissue extracts yielded a comprehensive profile of metabolic flux in perfused liver. Together, these data show that, even though the majority of HP 13CO2 derived from HP [1-13C]pyruvate in livers exposed to fatty acids reflects decarboxylation of [4-13C]oxaloacetate (PEPCK) or [4-13C]malate (malic enzyme), the intensity of the HP 13CO2 signal is not proportional to glucose production because the amount of pyruvate returned to the TCA cycle via PEPCK and pyruvate kinase is variable, depending upon available substrates.

Acetate is a bioenergetic substrate for human glioblastoma and brain metastases

Glioblastomas and brain metastases are highly proliferative brain tumors with short survival times. Previously, using (13)C-NMR analysis of brain tumors resected from patients during infusion of (13)C-glucose, we demonstrated that there is robust oxidation of glucose in the citric acid cycle, yet glucose contributes less than 50% of the carbons to the acetyl-CoA pool. Here, we show that primary and metastatic mouse orthotopic brain tumors have the capacity to oxidize [1,2-(13)C]acetate and can do so while simultaneously oxidizing [1,6-(13)C]glucose. The tumors do not oxidize [U-(13)C]glutamine. In vivo oxidation of [1,2-(13)C]acetate was validated in brain tumor patients and was correlated with expression of acetyl-CoA synthetase enzyme 2, ACSS2. Together, the data demonstrate a strikingly common metabolic phenotype in diverse brain tumors that includes the ability to oxidize acetate in the citric acid cycle. This adaptation may be important for meeting the high biosynthetic and bioenergetic demands of malignant growth.

The ratio of acetate-to-glucose oxidation in astrocytes from a single 13C NMR spectrum of cerebral cortex

The (13) C-labeling patterns in glutamate and glutamine from brain tissue are quite different after infusion of a mixture of (13) C-enriched glucose and acetate. Two processes contribute to this observation, oxidation of acetate by astrocytes but not neurons, and preferential incorporation of α-ketoglutarate into glutamate in neurons, and incorporation of α-ketoglutarate into glutamine in astrocytes. The acetate:glucose ratio, introduced previously for analysis of a single (13) C NMR spectrum, provides a useful index of acetate and glucose oxidation in the brain tissue. However, quantitation of relative substrate oxidation at the cell compartment level has not been reported. A simple mathematical method is presented to quantify the ratio of acetate-to-glucose oxidation in astrocytes, based on the standard assumption that neurons do not oxidize acetate. Mice were infused with [1,2-(13) C]acetate and [1,6-(13) C]glucose, and proton decoupled (13) C NMR spectra of cortex extracts were acquired. A fit of those spectra to the model indicated that (13) C-labeled acetate and glucose contributed approximately equally to acetyl-CoA (0.96) in astrocytes. As this method relies on a single (13) C NMR spectrum, it can be readily applied to multiple physiologic and pathologic conditions. Differences in (13) C labeling of brain glutamate and glutamine have been attributed to metabolic compartmentation. The acetate:glucose ratio, introduced for description of a (13) C NMR (nuclear magnetic resonance) spectrum, is an index of glucose and acetate oxidation in brain tissue. A simple mathematical method is presented to quantify the ratio of acetate-to-glucose oxidation in astrocytes from a single NMR spectrum. As kinetic analysis is not required, the method is readily applicable to analysis of tissue extracts. α-KG = alpha-ketoglutarate; CAC = citric acid cycle; GLN = glutamine; GLU = glutamate.

Glucose metabolism via the pentose phosphate pathway, glycolysis and Krebs cycle in an orthotopic mouse model of human brain tumors

It has been hypothesized that increased flux through the pentose phosphate pathway (PPP) is required to support the metabolic demands of rapid malignant cell growth. Using orthotopic mouse models of human glioblastoma (GBM) and renal cell carcinoma metastatic to brain, we estimated the activity of the PPP relative to glycolysis by infusing [1,2-(13) C(2) ]glucose. The [3-(13) C]lactate/[2,3-(13) C(2) ]lactate ratio was similar for both the GBM and brain metastasis and their respective surrounding brains (GBM, 0.197 ± 0.011 and 0.195 ± 0.033, respectively (p = 1); metastasis: 0.126 and 0.119 ± 0.033, respectively). This suggests that the rate of glycolysis is significantly greater than the PPP flux in these tumors, and that the PPP flux into the lactate pool is similar in both tumors. Remarkably, (13) C-(13) C coupling was observed in molecules derived from Krebs cycle intermediates in both tumor types, denoting glucose oxidation. In the renal cell carcinoma, in contrast with GBM, (13) C multiplets of γ-aminobutyric acid (GABA) differed from its precursor glutamate, suggesting that GABA did not derive from a common glutamate precursor pool. In addition, the orthotopic renal tumor, the patient's primary renal mass and brain metastasis were all strongly immunopositive for the 67-kDa isoform of glutamate decarboxylase, as were 84% of tumors on a renal cell carcinoma tissue microarray of the same histology, suggesting that GABA synthesis is cell autonomous in at least a subset of renal cell carcinomas. Taken together, these data demonstrate that (13) C-labeled glucose can be used in orthotopic mouse models to study tumor metabolism in vivo and to ascertain new metabolic targets for cancer diagnosis and therapy.

Substrate selection in hearts subjected to ischemia/reperfusion: role of cardioplegic solutions and gender

In conditions of ischemia/reperfusion (I/R), the relative use of all available substrates by the heart has a significant effect on the recovery of the organ. This substrate preference in perfused hearts is influenced by ischemia. We followed the metabolic fate of [U-(13) C]glucose and [3-(13) C]lactate in hearts preserved in Celsior (Cs) and histidine buffer solution (HBS) for 4 or 6 h and subsequently perfused with a Krebs-Henseleit solution (KH) containing [U-(13) C]glucose and [3-(13) C]lactate. We also assessed gender-specific metabolic modulation in our I/R experimental conditions. Hearts from male and female Wistar rats (6-8 weeks) were subjected to moderate (0-240 min) or prolonged (240-360 min) cold ischemia whilst immersed in Cs and HBS, and perfused for 30 min with KH containing [U-(13) C]glucose and [3-(13) C]lactate. After perfusion, hearts were freeze-clamped and metabolites were extracted for (13) C NMR isotopomer analysis. In control conditions, there were no differences with regard to lactate origin in hearts from males and females. After 6 h of preservation in Cs, lactate origin was mostly from [U-(13) C]glucose in hearts from males and from [3-(13) C]lactate in hearts from females. During the 6 h of organ preservation in HBS, the lactate pool showed a strong contribution from unenriched sources in male hearts and from [U-(13) C]glucose in female hearts. The glutamate C2/C4 ratio was stable or increased in hearts from females after I/R, and the alanine index increased in hearts from both males and females. Octanoate was, as predicted, the preferential substrate during perfusion. Glucose and lactate suffer a distinct metabolic fate in our I/R conditions, which is related to the cardioplegic solution used during organ storage, and the gender. Hearts from females appear to be less sensitive to I/R injury, and heart preservation in HBS proved to be effective in enhancing anaplerosis during perfusion, especially in hearts from females.

Ursodeoxycholic acid treatment of hepatic steatosis: a (13)C NMR metabolic study

Ursodeoxycholic acid (UDCA) is commonly used for the treatment of hepatobiliary disorders. In this study, we tested whether a 4-week treatment with this bile acid (12-15 mg/kg/day) could improve hepatic fatty acid oxidation in obese Zucker rats - a model for nonalcoholic fatty liver disease and steatosis. After 24 h of fasting, livers were perfused with physiological concentrations of [U-(13) C]nonesterified fatty acids and [3-(13) C]lactate/[3-(13) C]pyruvate. Steatosis was associated with abundant intracellular glucose, lactate, alanine and methionine, and low concentrations of choline and betaine. Steatotic livers also showed the highest output of glucose and lactate. Glucose and glycolytic products were mostly unlabeled, indicating active glycogenolysis and glycolysis after 24 h of fasting. UDCA treatment resulted in a general amelioration of liver metabolic abnormalities with a decrease in intracellular glucose and lactate, as well as their output. Hepatic betaine and methionine were also normalized after UDCA treatment, suggesting the amelioration of anti-oxidative defenses. Choline levels were not affected by the bile acid, which may indicate a deficient synthesis of very-low-density lipoproteins. The percentage contribution of [U-(13) C]nonesterified fatty acids to acetyl-coenzyme A entering the tricarboxylic acid (TCA) cycle was significantly lower in livers from Zucker obese rats relative to control rats: 23.1 ± 4.9% versus 44.1 ± 2.7% (p < 0.01). UDCA treatment did not alter significantly fatty acid oxidation in control rats, but improved significantly oxidation in Zucker obese rats to 46.0 ± 6.1% (p > 0.05), comparable with control group values. The TCA cycle activity subsequent to fatty acid oxidation was reduced in steatotic livers and improved when UDCA was administered (0.24 ± 0.04 versus 0.37 ± 0.05, p = 0.05). We further suggest that the mechanism of action of UDCA is either related to the activity of the farnesoid receptor, or to the amelioration of the anti-oxidative defenses and cell nicotinamide adenine dinucleotide (NAD(+) /NADH) ratio, favoring TCA cycle activity and β-oxidation.

Cardiovascular Applications of Hyperpolarized MRI

Many applications of MRI are limited by an inherently low sensitivity. Previous attempts to overcome this insensitivity have focused on the use of MRI systems with stronger magnetic fields. However, the gains that can be achieved in this way are relatively small and increasing the magnetic field invariably leads to greater technical challenges. More recently, the development of a range of techniques, which can be gathered under the umbrella term of "hyperpolarization," has offered potential solutions to the low sensitivity. Hyperpolarization techniques have been demonstrated to temporarily increase the signal available in an MRI experiment by as much as 100,000-fold. This article outlines the main hyperpolarization techniques that have been proposed and explains how they can increase MRI signals. With particular emphasis on the emerging technique of dynamic nuclear polarization, the existing preclinical cardiovascular applications are reviewed and the potential for clinical translation is discussed.

Competition of pyruvate with physiological substrates for oxidation by the heart: implications for studies with hyperpolarized [1-13C]pyruvate

Carbon 13 nuclear magnetic resonance (NMR) isotopomer analysis was used to measure the rates of oxidation of long-chain fatty acids, ketones, and pyruvate to determine the minimum pyruvate concentration ([pyruvate]) needed to suppress oxidation of these alternative substrates. Substrate mixtures were chosen to represent either the fed or fasted state. At physiological [pyruvate], fatty acids and ketones supplied the overwhelming majority of acetyl-CoA. Under conditions mimicking the fed state, 3 mM pyruvate provided approximately 80% of acetyl-CoA, but under fasting conditions 6 mM pyruvate contributed only 33% of acetyl-CoA. Higher [pyruvate], 10-25 mM, was associated with transient reduced cardiac output, but overall hemodynamic performance was unchanged after equilibration. These observations suggested that 3-6 mM pyruvate in the coronary arteries would be an appropriate target for studies with hyperpolarized [1-(13)C]pyruvate. However, the metabolic products of 3 mM hyperpolarized [1-(13)C]pyruvate could not be detected in the isolated heart during perfusion with a physiological mixture of substrates including 3% albumin. In the presence of albumin even at high concentrations of pyruvate, 20 mM, hyperpolarized H(13)CO(3)(-) could be detected only in the absence of competing substrates. Highly purified albumin (but not albumin from plasma) substantially reduced the longitudinal relaxation time of [1-(13)C]pyruvate. In conclusion, studies of cardiac metabolism using hyperpolarized [1-(13)C]pyruvate are sensitive to the effects of competing substrates on pyruvate oxidation.

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.

(13)C-NMR isotopomer distribution analysis: a method for measuring metabolic fluxes in condensation biosynthesis

(13)C NMR spectroscopy associated with the use of (13)C-enriched substrates is a powerful tool to investigate intracellular metabolism because of the wealth of information contained in the distribution of isotopes in key metabolites. A new method of using (13)C label distribution measurements in carbon skeletons of metabolites to estimate metabolic fluxes through biochemical reaction networks is presented here. This method can be applied to metabolite synthesis occurring by condensation reactions of the type nA --> B, where n is the number of precursor A molecules needed to synthesize one molecule of product B. NMR isotopomer distribution analysis (NMR-IDA) involves the introduction of a (13)C-enriched precursor, and measurements of the (13)C positional enrichments at just one carbon atom position of the product B via (13)C NMR spectroscopy. Information on isotopomer distribution is obtained, and data are analyzed according to a mathematical model based on multinomial probability expressions to obtain the best fit between theoretical and experimental (13)C label distribution. The use of the NMR-IDA method allows for estimation of two key parameters representing the fractional flux of (13)C-enriched tracer A molecules to total precursor A pool and the fraction of product B synthesized in the presence of a (13)C-enriched source, respectively. A practical example of NMR-IDA application to fatty acid synthesis from [(1,2 (13)C(2))acetyl]-L-carnitine in cultured primary astrocytes is also presented.

Effect of Echinococcus multilocularis on the origin of acetyl-coA entering the tricarboxylic acid cycle in host liver

Carbon-13 nuclear magnetic resonance (NMR) spectroscopy was employed to investigate alterations in hepatic carbohydrate metabolism in Meriones unguiculatus infected with Echinococcus multilocularis. Following portal vein injections of an equimolar mixture of [1,2-13C2]acetate and [3-13C]lactate, perchloric acid extracts of the livers were prepared and NMR spectra obtained. Isotopomer analysis using glutamate resonances in these spectra showed that the relative contributions of endogenous and exogenous substrates to the acetyl-CoA entering the tricarboxylic acid cycle differed significantly between infected and control groups. The mole fraction of acetyl-CoA that was derived from endogenous, unlabelled sources (F(U)) was 0.50 +/- 0.10 in controls compared to 0.34 +/- 0.04 in infected animals. However, the fraction of acetyl-CoA derived from [3-13C]lactate (FLL) was larger in livers of infected animals than those from controls with values of 0.27 +/- 0.04 and 0.18 +/- 0.04, respectively. Similarly, the fraction of acetyl-CoA derived from [1,2-13C2]acetate (FLA) was larger in livers of infected animals compared to those in controls; the fractions were 0.38 +/- 0.01 and 0.32 +/- 0.07, respectively. The ratio of FLA:FLL was significantly smaller in the infected group with a value of 1.42 +/- 0.18 compared to 1.74 +/- 0.09 for the controls. These results indicate that alveolar hydatid disease has a pronounced effect on the partitioning of substrates within the pathways of carbohydrate metabolism in the host liver.

Metabolic engineering

Metabolic engineering is the science that combines systematic analysis of metabolic and other pathways with molecular biological techniques to improve cellular properties by designing and implementing rational genetic modifications. As such, metabolic engineering deals with the measurement of metabolic fluxes and elucidation of their control as determinants of metabolic function and cell physiology. A novel aspect of metabolic engineering is that it departs from the traditional reductionist paradigm of cellular metabolism, taking instead a holistic view. In this sense, metabolic engineering is well suited as a framework for the analysis of genome-wide differential gene expression data, in combination with data on protein content and in vivo metabolic fluxes. The insights of the integrated view of metabolism generated by metabolic engineering will have profound implications in biotechnological applications, as well as in devising rational strategies for target selection for screening candidate drugs or designing gene therapies. In this article we review basic concepts of metabolic engineering and provide examples of applications in the production of primary and secondary metabolites, improving cellular properties, and biomedical engineering.

Aldose reductase inhibition improves altered glucose metabolism of isolated diabetic rat hearts

Alterations in glucose metabolism have been implicated in the cardiovascular complications of diabetes. Previous work in this laboratory demonstrated that hearts from diabetic animals have an elevated cytosolic redox ratio (NADH/NAD+) and that this redox imbalance is probably due to elevated polyol pathway flux. We therefore hypothesized that 1) the elevated cytosolic redox ratio of diabetic hearts could result in inhibition of glycolytic enzymes sensitive to the redox state, 2) polyol pathway inhibition could restore the abnormal glucose metabolism of diabetic hearts, and 3) the relative incorporation of mixed substrates into hearts from diabetic animals would demonstrate less glycolytic and more fatty acid oxidation. Hearts from diabetic (BB/W) and nondiabetic control rats were perfused with buffers containing 13C-labeled substrates, and the metabolism of these hearts was analyzed using 13C NMR spectroscopy. Tissue samples were analyzed for metabolite levels using biochemical assay. Compared with controls, diabetic hearts had glyceraldeyde 3-phosphate levels that were four times greater than nondiabetic hearts and exhibited 91% less 13C labeling of lactate and 92% less 13C labeling of glutamate (P < 0.03). Aldose reductase inhibition with zopolrestat restored the metabolite labeling of diabetic hearts. Diabetic hearts perfused with a mixture of substrates used 53% more acetate than nondiabetic control hearts (P < 0.05), and aldose reductase inhibition lowered the acetate utilization of diabetic hearts by 9% (P < 0.05). These data suggest that glycolytic flux in diabetic hearts is inhibited at glyceraldehyde-3-phosphate dehydrogenase and that inhibition of the polyol pathway with zopolrestat increases glycolytic flux in these hearts. Furthermore, hearts from diabetic animals showed a marked dependence on fatty acids for substrate utilization compared with nondiabetic controls, consistent with inhibition of the pyruvate dehydrogenase complex in diabetic hearts.

Determination of acetyl-CoA enrichment in rat heart and skeletal muscle by 1H nuclear magnetic resonance analysis of glutamate in tissue extracts

The contribution of a 13C-enriched substrate to the acetyl-CoA pool in animal tissues is typically measured by analysis of glutamate enrichment from tissue extracts. 13C NMR analysis offers the advantages of minimal sample processing and high information content, but has a low analytical sensitivity compared to other methods of tracer analysis such as GC/MS. We present a sensitive, simple, and direct 1H NMR measurement of glutamate C4 enrichment from tissue extracts. The method is demonstrated with heart and hindlimb muscle tissue extracts of rats infused with [2,4,6,8-13C4]-octanoate, a source of [2-13C]acetyl-CoA. Glutamate C4 enrichment in extracts of individual hindlimb soleus muscles weighing approximately 150 mg and containing approximately 0.3 mumol of glutamate was quantified by 1H NMR within about 40 min. Glutamate C4 enrichment measurements by 1H NMR in heart and gastrocnemius muscle were also highly correlated with independent measurements obtained from 13C NMR isotopomer analysis.

Detection of 13C-labeled metabolites in the in vivo canine heart by B1 insensitive heteronuclear coherent polarization transfer and comparison of signal enhancement with NOE

A recently developed adiabatic coherent polarization transfer enhancement technique [H. Merkle, H. Wei, M. Garwood, K. Uğurbil. J. Magn. Reson, 99, 480-494 (1992)] was employed to perform 13C spectroscopy in the intact canine heart in vivo during [2-13C]-acetate infusion into the left descending coronary artery, the results were compared with 13C spectra obtained with conventionally employed nuclear Overhauser enhancement. The results demonstrate that both methods can be performed by using surface coils to obtain in vivo 13C spectra and that coherent polarization transfer provides better enhancement than NOE for [2-13C]-acetate but not for short T2 compounds.

Isotopic methods for probing organization of cellular metabolism

These examples serve to illustrate that it is now possible to investigate metabolism in intact tissues using a variety of biophysical methods. While we have concentrated on NMR methods, reflecting our own interests in using 13C as a metabolic tracer, GC-mass spectroscopy can often provide similar metabolic information and has the advantage of increased sensitivity over NMR. Combining either or both of these technologies with cleaver 'chemical biopsy' methods offers new opportunities to examine what may seem to be old metabolic questions in a much more relevant environment, the native state.

Substrate selection early after reperfusion of ischemic regions in the working rabbit heart

Several substrates are available in vivo for oxidation by the myocardium. Although substrate selection has been studied extensively in normoxic myocardium, relatively little is known about substrate preference very early during reperfusion after ischemia. Carbon-13 isotopomer analysis was used to study substrate usage by nonischemic and reperfused-ischemic myocardium in a working heart that was subjected to 15 min or regional ischemia and reperfused for 5 min. Compared with nonischemic myocardium, the contribution of acetoacetate to acetyl coenzyme A was increased in the reperfused-ischemic region, and the contribution of exogenous lactate was decreased. Free fatty acid oxidation, however, was not different in the two regions. These results indicate that (1) early during reperfusion, ketone body oxidation may be more significant than has been emphasized, (2) the relative contribution of fatty acids to acetyl coenzyme A is not sensitive to ischemia followed by reperfusion, and (3) Carbon-13 magnetic resonance spectroscopy methods may be used for analysis of spatial heterogeneity of metabolism in the heart.

Effects of different oxidative insults on intermediary metabolism in isolated perfused rat hearts

13C and 31P NMR were used to evaluate exogenous substrate utilization and endogenous phosphate metabolites in perfused rat hearts exposed to tert-butylhydroperoxide (tert-BOOH) and hydrogen peroxide (H2O2). Both reagents caused a reduction in developed pressure compared to controls and, in agreement with previous 31P NMR data, had different effects on intracellular high-energy phosphates and glycolysis. 13C Isotopomer analysis of tissue extracts showed that H2O2 and tert-BOOH also had significantly different effects on substrate utilization by the citric acid cycle. The contribution of exogenous lactate and glucose to acetyl-CoA was 43% in controls and increased to over 80% in the presence of either oxidant. With tert-BOOH, exogenous glucose and lactate were both significant contributors to acetyl-CoA (44 +/- 2 and 41 +/- 3%). However, with H2O2, exogenous lactate supplied a much higher fraction of acetyl-CoA (72 +/- 2%) than glucose (9 +/- 1%). Also, when [2-(13)C] glucose was supplied, accumulation of [2-(13)C] and [5-(13)C] fructose 1,6-bisphosphate was observed in the presence of H2O2, indicating inhibition of glyceraldehyde-3-phosphate dehydrogenase. These results indicate that despite this glycolytic inhibition, H2O2 increased the utilization of pyruvate precursors when lactate was present as an alternative carbohydrate substrate.

Metabolism of [3-13C]pyruvate and [3-13C]propionate in normal and ischaemic rat heart in vivo: 1H- and 13C-NMR studies

The oxidation of [3-13C]pyruvate and [3-13C]propionate was studied in vivo in infused rats. The infused [3-13C]pyruvate was quickly converted to [3-13C]lactate in the blood, and the [3-13C]lactate formed was well metabolized in both normoxic and ischaemic hearts. Large differences (200-600%) in the 13C enrichment of alanine (C-3) and acetyl-CoA (C-2) compared with lactate (C-3) were found in both normoxic and ischaemic hearts, suggesting that the extracellular [3-13C]lactate preferentially entered a region of the cytoplasm which specifically transfers the labelled pyruvate (formed from [3-13C]lactate) to the mitochondria. The highly enriched mitochondrial pyruvate gave high enrichment in alanine and acetyl-CoA, which was detected by 1H- and 13C-NMR spectroscopy. Ischaemia increased 13C incorporation into the main cytoplasmic lactate pool and decreased 13C incorporation into citric acid cycle intermediates, mainly decreasing the pyruvate anaplerosis. Isoprenaline-induced ischaemia of the heart caused only a slight decrease in pyruvate oxidation. In contrast to the decreased anaplerosis of pyruvate, the anaplerosis of propionate (and propionyl-carnitine) increased significantly in ischaemic hearts, which may contribute to the protective effect of propionyl-carnitine seen in ischaemia. In addition, we found that [3-13C]propionate preferentially labelled aspartate C-3 in rat heart, suggesting incomplete randomization of label in the succinyl-CoA-malate span of the citric acid cycle. These data show that proton observed 13C edited spectroscopic methods, i.e. heteronuclear spin-echo and the one-dimensional heteronuclear multiple quantum coherence sequence, can be successfully used to study heart metabolism in vivo.

A 13C isotopomer n.m.r. method for monitoring incomplete beta-oxidation of fatty acids in intact tissue

An n.m.r. method is presented for monitoring the extent to which fatty acids undergo beta-oxidation without release of shorter-chain intermediates. It is based upon a 13C isotopomer analysis of glutamate from tissue presented with a mixture of [2,4,6,8-13C]octanoate and [1,2,3,4-13C]octanoate. The method does not require steady-state metabolic or isotopic conditions, so it may be applied during a variety of metabolic circumstances, including perfused tissue under stress and in vivo. We have tested the method in perfused rat hearts during anoxia, a model where previous work has shown that beta-oxidation of palmitate is incomplete and shorter-chain intermediates are released [Rabinowitz and Hercker (1974) Arch. Biochem. Biophys. 161, 621-627]. Indeed, n.m.r. spectra of freeze-clamped, acid-extracted tissue show that octanoate undergoes complete beta-oxidation in control normoxic rat hearts, but not in anoxic hearts. Complete beta-oxidation of octanoate was observed under a number of other metabolic conditions in perfused rat hearts, including low-pressure-induced ischaemia, KCl arrest and in the presence of high concentrations of competing substrates. We also demonstrate that the technique is applicable in intact tissue by taking direct measurements in perfused rat hearts using a recently published [13C]homonuclear decoupling technique and in in vivo heart and liver removed from rats after an intravenous infusion of a mixture of [2,4,6,8-13C]octanoate and [1,2,3,4-13C]octanoate.

Effects of oxidant exposure on substrate utilization and high-energy phosphates in isolated rat hearts

The effects of a xanthine oxidase-mediated free radical-generating system containing purine and iron-loaded transferrin or solutions containing hydrogen peroxide and iron-loaded transferrin on substrate utilization and high-energy phosphates were evaluated by nuclear magnetic resonance (NMR) spectroscopy in isolated perfused rat hearts. Hearts were supplied with lactate, acetate, and glucose, and the contribution of each substrate to acetyl coenzyme A was measured in control hearts and in the presence of a free radical-generating system. Perfused hearts were monitored by 31P NMR, and tissue extracts were analyzed by 13C NMR. Free radicals decreased the phosphocreatine and beta-ATP peak areas and reduced contractile function. Under control conditions, lactate, acetate, and endogenous sources were the major contributors of acetyl coenzyme A units, with only 5% originating from glucose. In the presence of a xanthine oxidase-mediated free radical-generating system, the glucose contribution increased to 54%, while contributions from acetate and endogenous sources were significantly reduced. Both 13C and 31P NMR analyses showed no significant accumulation of glycolytic sugar phosphates, suggesting little inhibition of glyceraldehyde-3-phosphate dehydrogenase. The increased contribution of glucose to the tricarboxylic acid cycle relative to acetate and endogenous sources is consistent with activation of pyruvate dehydrogenase. In contrast, hearts exposed to a hydrogen peroxide-based free radical-generating system showed an increase in lactate utilization, a decrease in acetate utilization, and no change in glucose utilization compared with control hearts. Glycolytic sugar phosphates were found to accumulate, suggesting possible inhibition of glyceraldehyde-3-phosphate. Thus, different radicals or their metabolites may have varying effects on myocardial metabolism.

13C isotopomer analyses in intact tissue using [13C]homonuclear decoupling

Entry of 13C-enriched acetyl-CoA into the citric acid cycle results in scrambling of 13C into the various carbon positions of all intermediate pools. The eventual result is that the 13C resonances of all detectable intermediates or molecules exchanging with those intermediates appear as multiplets due to nearest neighbor spin-spin couplings. We have previously shown that an isotopomer analysis of the glutamate 13C multiplets provides a history of 13C flow through the cycle pools and that relative substrate utilization and relative anaplerotic flux can be quantitated (C.R. Malloy, A.D. Sherry, and F.M.H. Jeffrey, Am. J. Physiol. 259, H987-H995 (1990)). A major limitation of the method for in vivo applications is spectral resolution of multiline resonances required for a complete isotopomer analysis. We now show that [13C]homonuclear decoupling of the glutamate C3 resonance collapses nine-line C4 and C2 resonances into three-line multiplets. We demonstrate that these three-line 13C multiplets are well resolved in isolated, perfused rat hearts and present steady-state equations that allow an isotopomer analysis from data obtained in intact tissue. This advancement offers for the first time the possibility of extending 13C isotopomer methods to complex metabolic conditions in vivo.

Lipoamide influences substrate selection in post-ischaemic perfused rat hearts

We investigated whether lipoamide and diacetyl-lipoamide are able to change the substrate selection in post-ischaemic myocardium. This can be important, because shifting heart metabolism from fatty acid to carbohydrate oxidation can decrease ischaemic injury. Studying the metabolism of [1,2-13C]diacetyl-lipoamide in situ in perfused rat heart by 13C n.m.r., we found intense 13C labelling in glutamate and aspartate, showing that acetyl groups from diacetyl-lipoamide are effectively transferred to CoA and metabolized in heart tissue. From analysis of glutamate C-3 and C-4 isotopomers, we determined the [1,2-13C]acetate/[3-13C]lactate utilization ratio in normoxic and post-ischaemic hearts, where under our experimental conditions the acetate/lactate utilization ratios were 1.2 +/- 0.2 and 2.4 +/- 0.3 in normoxic and post-ischaemic hearts respectively. When 0.25 mM lipoamide was added to the perfusate the acetate/lactate utilization ratio decreased to 1.4 +/- 0.1, which is almost equal to that found for normoxic hearts, showing that lipoamide increased the lactate utilization. In accordance with these data, we found that lipoamide activated pyruvate dehydrogenase by 50% in post-ischaemic myocardium. Competition between [3-13C]lactate and unlabelled octanoate was also studied in post-ischaemic hearts, and we found that lipoamide increased lactate utilization by 100% and increased the rate of the tricarboxylic acid cycle by 64%. Under the same experimental conditions, lipoamide significantly promoted the recovery of post-ischaemic unpaced hearts, showing the positive effect of increased lactate oxidation in post-ischaemic myocardium.

Effects of amino acids on substrate selection, anaplerosis, and left ventricular function in the ischemic reperfused rat heart

The effect of aspartate and glutamate on myocardial function during reperfusion is controversial. A beneficial effect has been attributed to altered delivery of carbon into the citric acid cycle via substrate oxidation or by stimulation of anaplerosis, but these hypotheses have not been directly tested. 13C isotopomer analysis is well suited to the study of myocardial metabolism, particularly where isotopic and metabolic steady state cannot be established. This technique was used to evaluate the effects of aspartate and glutamate (amino acids, AA) on anaplerosis and substrate selection in the isolated rat heart after 25 min of ischemia followed by 30 or 45 min of reperfusion. Five groups of hearts (n = 8) provided with a mixture of [1,2-13C]acetate, [3-13C]lactate, and unlabeled glucose were studied: control, control plus AA, ischemia followed by 30 min of reperfusion, ischemia plus AA followed by 30 min of reperfusion, and ischemia followed by 45 min of reperfusion. The contribution of lactate to acetyl-CoA was decreased in postischemic myocardium (with a significant increase in acetate), and anaplerosis was stimulated. Metabolism of 13C-labeled aspartate or glutamate could not be detected, however, and there was no effect of AA on functional recovery, substrate selection, or anaplerosis. Thus, in contrast to earlier reports, aspartate and glutamate have no effect on either functional recovery from ischemia or on metabolic pathways feeding the citric acid cycle.