13C NMR relaxation times of hepatic glycogen in vitro and in vivo

Multinuclear Magnetic Resonance Spectroscopy at Ultra-High-Field: Assessing Human Cerebral Metabolism in Healthy and Diseased States

The brain is a highly energetic organ. Although the brain can consume metabolic substrates, such as lactate, glycogen, and ketone bodies, the energy metabolism in a healthy adult brain mainly relies on glucose provided via blood. The cerebral metabolism of glucose produces energy and a wide variety of intermediate metabolites. Since cerebral metabolic alterations have been repeatedly implicated in several brain disorders, understanding changes in metabolite levels and corresponding cell-specific neurotransmitter fluxes through different substrate utilization may highlight the underlying mechanisms that can be exploited to diagnose or treat various brain disorders. Magnetic resonance spectroscopy (MRS) is a noninvasive tool to measure tissue metabolism in vivo. ¹H-MRS is widely applied in research at clinical field strengths (≤3T) to measure mostly high abundant metabolites. In addition, X-nuclei MRS including, ¹³C, ²H, ¹⁷O, and ³¹P, are also very promising. Exploiting the higher sensitivity at ultra-high-field (>4T; UHF) strengths enables obtaining unique insights into different aspects of the substrate metabolism towards measuring cell-specific metabolic fluxes in vivo. This review provides an overview about the potential role of multinuclear MRS (¹H, ¹³C, ²H, ¹⁷O, and ³¹P) at UHF to assess the cerebral metabolism and the metabolic insights obtained by applying these techniques in both healthy and diseased states.

The early days of ex vivo 1 H, 13 C, and 31 P nuclear magnetic resonance in the laboratory of Dr. Robert G. Shulman from 1975 to 1995

This paper provides a brief description of the early use of ex vivo nuclear magnetic resonance (NMR) studies of tissue and tissue extracts performed in the laboratory of Dr. Robert G. Shulman from 1975 through 1995 at Bell Laboratories, then later at Yale University. During that period, ex vivo NMR provided critical information in support of resonance assignments and the quantitation of concentrations for magnetic resonance spectroscopy studies. The period covered saw rapid advances in magnet technology, starting with studies of microorganisms in vertical bore high-resolution NMR studies, then by 1981 studies of small mammals in a horizontal bore magnet, and then studies of humans in 1984. Ex vivo NMR played a critical role in all these studies. A general strategy developed in the lab for using ex vivo NMR to support in vivo studies is presented, as well as illustrative examples.

Magnetic Resonance Imaging and Spectroscopy Methods to Study Hepatic Glucose Metabolism and Their Applications in the Healthy and Diabetic Liver

The liver plays an important role in whole-body glucose homeostasis by taking up glucose from and releasing glucose into the blood circulation. In the postprandial state, excess glucose in the blood circulation is stored in hepatocytes as glycogen. In the postabsorptive state, the liver produces glucose by breaking down glycogen and from noncarbohydrate precursors such as lactate. In metabolic diseases such as diabetes, these processes are dysregulated, resulting in abnormal blood glucose levels. Magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) are noninvasive techniques that give unique insight into different aspects of glucose metabolism, such as glycogenesis, glycogenolysis, and gluconeogenesis, in the liver in vivo. Using these techniques, liver glucose metabolism has been studied in regard to a variety of interventions, such as fasting, meal intake, and exercise. Moreover, deviations from normal hepatic glucose metabolism have been investigated in both patients with type 1 and 2 diabetes, as well as the effects of antidiabetic medications. This review provides an overview of current MR techniques to measure hepatic glucose metabolism and the insights obtained by the application of these techniques in the healthy and diabetic liver.

NMR visibility of deuterium-labeled liver glycogen in vivo

PURPOSE

Deuterium metabolic imaging (DMI) combined with [6,6'-² H₂ ]-glucose has the potential to detect glycogen synthesis in the liver. However, the similar chemical shifts of [6,6'-² H₂ ]-glucose and [6,6'-² H₂ ]-glycogen in the ² H NMR spectrum make unambiguous detection and separation difficult in vivo, in contrast to comparable approaches using ¹³ C MRS. Here the NMR visibility of ² H-labeled glycogen is investigated to better understand its potential contribution to the observed signal in liver following administration of [6,6'-² H₂ ]-glucose.

METHODS

Mice were provided drinking water containing ² H-labeled glucose. High-resolution NMR analyses was performed of isolated liver glycogen in solution, before and after the addition of the glucose-releasing enzyme amyloglucosidase.

RESULTS

² H-labeled glycogen was barely detectable in solution using ² H NMR because of the very short T₂ (<2 ms) of ² H-labeled glycogen, giving a spectral line width that is more than five times as broad as that of ¹³ C-labeled glycogen (T₂ = ~10 ms).

CONCLUSION

² H-labeled glycogen is not detectable with ² H MRS(I) under in vivo conditions, leaving ¹³ C MRS as the preferred technique for in vivo detection of glycogen.

Mechanism and quantitative assessment of saturation transfer for water-based detection of the aliphatic protons in carbohydrate polymers

PURPOSE

CEST MRI experiments of mobile macromolecules, for example, proteins, carbohydrates, and phospholipids, often show signals due to saturation transfer from aliphatic protons to water. Currently, the mechanism of this nuclear Overhauser effect (NOE)-based transfer pathway is not completely understood and could be due either to NOEs directly to bound water or NOEs relayed intramolecularly via exchangeable protons. We used glycogen as a model system to investigate this saturation transfer pathway in sugar polymer solution.

METHODS

To determine whether proton exchange affected saturation transfer, saturation spectra (Z-spectra) were measured for glycogen solutions of different pH, D₂ O/H₂ O ratio, and glycogen particle size. A theoretical model was derived to analytically describe the NOE-based signals in these spectra. Numerical simulations were performed to verify this theory, which was further tested by fitting experimental data for different exchange regimes.

RESULTS

Signal intensities of aliphatic NOEs in Z-spectra of glycogen in D₂ O solution were influenced by hydroxyl proton exchange rates, whereas those in H₂ O were not. This indicates that the primary transfer pathway is an exchange-relayed NOE from these aliphatic protons to neighboring hydroxyl protons, followed by the exchange to water protons. Experimental data for glycogen solutions in D₂ O and H₂ O could be analyzed successfully using an analytical theory derived for such relayed NOE transfer, which was further validated using numerical simulations with the Bloch equations.

CONCLUSION

The predominant mechanism underlying aliphatic signals in Z-spectra of mobile carbohydrate polymers is intramolecular relayed NOE transfer followed by proton exchange.

Magnetic resonance imaging of glycogen using its magnetic coupling with water

Glycogen plays a central role in glucose homeostasis and is abundant in several types of tissue. We report an MRI method for imaging glycogen noninvasively with enhanced detection sensitivity and high specificity, using the magnetic coupling between glycogen and water protons through the nuclear Overhauser enhancement (NOE). We show in vitro that the glycogen NOE (glycoNOE) signal is correlated linearly with glycogen concentration, while pH and temperature have little effect on its intensity. For validation, we imaged glycoNOE signal changes in mouse liver, both before and after fasting and during glucagon infusion. The glycoNOE signal was reduced by 88 ± 16% (n = 5) after 24 h of fasting and by 76 ± 22% (n = 5) at 1 h after intraperitoneal (i.p.) injection of glucagon, which is known to rapidly deplete hepatic glycogen. The ability to noninvasively image glycogen should allow assessment of diseases in which glucose metabolism or storage is altered, for instance, diabetes, cardiac disease, muscular disorders, cancer, and glycogen storage diseases.

Methodological and physiological test-retest reliability of (13) C-MRS glycogen measurements in liver and in skeletal muscle of patients with type 1 diabetes and matched healthy controls

Glycogen is a major substrate in energy metabolism and particularly important to prevent hypoglycemia in pathologies of glucose homeostasis such as type 1 diabetes mellitus (T1DM). (13) C-MRS is increasingly used to determine glycogen in skeletal muscle and liver non-invasively; however, the low signal-to-noise ratio leads to long acquisition times, particularly when glycogen levels are determined before and after interventions. In order to ease the requirements for the subjects and to avoid systematic effects of the lengthy examination, we evaluated if a standardized preparation period would allow us to shift the baseline (pre-intervention) experiments to a preceding day. Based on natural abundance (13) C-MRS on a clinical 3 T MR system the present study investigated the test-retest reliability of glycogen measurements in patients with T1DM and matched controls (n = 10 each group) in quadriceps muscle and liver. Prior to the MR examination, participants followed a standardized diet and avoided strenuous exercise for two days. The average coefficient of variation (CV) of myocellular glycogen levels was 9.7% in patients with T1DM compared with 6.6% in controls after a 2 week period, while hepatic glycogen variability was 13.3% in patients with T1DM and 14.6% in controls. For comparison, a single-session test-retest variability in four healthy volunteers resulted in 9.5% for skeletal muscle and 14.3% for liver. Glycogen levels in muscle and liver were not statistically different between test and retest, except for hepatic glycogen, which decreased in T1DM patients in the retest examination, but without an increase of the group distribution. Since the CVs of glycogen levels determined in a "single session" versus "within weeks" are comparable, we conclude that the major source of uncertainty is the methodological error and that physiological variations can be minimized by a pre-study standardization. For hepatic glycogen examinations, familiarization sessions (MR and potentially strenuous interventions) are recommended. Copyright © 2016 John Wiley & Sons, Ltd.

Reproducibility and absolute quantification of muscle glycogen in patients with glycogen storage disease by 13C NMR spectroscopy at 7 Tesla

Carbon-13 magnetic resonance spectroscopy (13C MRS) offers a noninvasive method to assess glycogen levels in skeletal muscle and to identify excess glycogen accumulation in patients with glycogen storage disease (GSD). Despite the clinical potential of the method, it is currently not widely used for diagnosis or for follow-up of treatment. While it is possible to perform acceptable 13C MRS at lower fields, the low natural abundance of 13C and the inherently low signal-to-noise ratio of 13C MRS makes it desirable to utilize the advantage of increased signal strength offered by ultra-high fields for more accurate measurements. Concomitant with this advantage, however, ultra-high fields present unique technical challenges that need to be addressed when studying glycogen. In particular, the question of measurement reproducibility needs to be answered so as to give investigators insight into meaningful inter-subject glycogen differences. We measured muscle glycogen levels in vivo in the calf muscle in three patients with McArdle disease (MD), one patient with phosphofructokinase deficiency (PFKD) and four healthy controls by performing 13C MRS at 7T. Absolute quantification of the MRS signal was achieved by using a reference phantom with known concentration of metabolites. Muscle glycogen concentration was increased in GSD patients (31.5±2.9 g/kg w. w.) compared with controls (12.4±2.2 g/kg w. w.). In three GSD patients glycogen was also determined biochemically in muscle homogenates from needle biopsies and showed a similar 2.5-fold increase in muscle glycogen concentration in GSD patients compared with controls. Repeated inter-subject glycogen measurements yield a coefficient of variability of 5.18%, while repeated phantom measurements yield a lower 3.2% system variability. We conclude that noninvasive ultra-high field 13C MRS provides a valuable, highly reproducible tool for quantitative assessment of glycogen levels in health and disease.

Combination of two fat saturation pulses improves detectability of glucose signals in carbon-13 MR spectroscopy

In order to improve the fat suppression performance of in vivo (13)C-MRS operating at 3.0 Tesla, a phantom model study was conducted using a combination of two fat suppression techniques; a set of pulses for frequency (chemical shift) selective suppression (CHESS), and spatial saturation (SAT). By optimizing the slab thickness for SAT and the irradiation bandwidth for CHESS, the signals of the -(13)CH(3) peak at 49 ppm and the -(13)CH(2)- peak at 26 ppm simulating fat components were suppressed to 5% and 19%, respectively. Combination of these two fat suppression pulses achieved a 53% increase of the height ratio of the glucose C1β peak compared with the sum of all other peaks, indicating better sensitivity for glucose signal detection. This method will be applicable for in vivo (13)C-MRS by additional adjustment with the in vivo relaxation times of the metabolites.

MRI detection of glycogen in vivo by using chemical exchange saturation transfer imaging (glycoCEST)

Detection of glycogen in vivo would have utility in the study of normal physiology and many disorders. Presently, the only magnetic resonance (MR) method available to study glycogen metabolism in vivo is (13)C MR spectroscopy, but this technology is not routinely available on standard clinical scanners. Here, we show that glycogen can be detected indirectly through the water signal by using selective radio frequency (RF) saturation of the hydroxyl protons in the 0.5- to 1.5-ppm frequency range downfield from water. The resulting saturated spins are rapidly transferred to water protons via chemical exchange, leading to partial saturation of the water signal, a process now known as chemical exchange saturation transfer. This effect is demonstrated in glycogen phantoms at magnetic field strengths of 4.7 and 9.4 T, showing improved detection at higher field in adherence with MR exchange theory. Difference images obtained during RF irradiation at 1.0 ppm upfield and downfield of the water signal showed that glycogen metabolism could be followed in isolated, perfused mouse livers at 4.7 T before and after administration of glucagon. Glycogen breakdown was confirmed by measuring effluent glucose and, in separate experiments, by (13)C NMR spectroscopy. This approach opens the way to image the distribution of tissue glycogen in vivo and to monitor its metabolism rapidly and noninvasively with MRI.

Evaluation of muscle glycogen content by 13C NMR spectroscopy in adult-onset acid maltase deficiency

Muscle glycogen storage was measured by in vivo, natural abundance 13C nuclear magnetic resonance spectroscopy in distal and proximal lower limb segments of patients suffering from adult-onset acid maltase deficiency. Interleaved T1-weighted acquisitions of glycogen and creatine served to quantify glycogen excess. For acid maltase deficient patients (n=11), glycogen:creatine was higher than controls (n=12), (1.20+/-0.39 vs. 0.83+/-0.18, P=0.0005). Glycogen storage was above the normal 95% confidence limits in at least one site for 7/11 patients. The intra-individual coefficient of reproducibility was 12%. This totally atraumatic measurement of glycogen allows repeated measurement at different muscle sites of acid maltase deficient patients, despite selective fatty replacement of tissue. This could provide an additional parameter to follow the development of disease in individual patients, including in the perspective of forthcoming therapeutic trials. It may also offer an appropriate tool to study the role of glycogen accumulation in progression of the pathology.

Quantification of the glycogen 13C-1 NMR signal during glycogen synthesis in perfused rat liver

We studied glycogen synthesis from glucose in perfused livers of fed (n = 4) and 24 h starved (n = 7) rats. Glycogenolysis was inhibited by BAY R3401 (150 microM) and proglycosyn (100 microM). After 60 min, we replaced 99% (13)C-1 glucose by natural abundance glucose. This pulse-chase design allowed us to recognize residual ongoing futile glycogen turnover from the release of initially deposited (13)C-label, into the (13)C-free chase medium. Net residual turnover was less than 2 +/- 0.7% and 0.6 +/- 0.2% of 1-(13)C glycogen deposition rates of 0.31 +/- 0.04 and 0.99 +/- 0.04 micromol glucose g(-1) min(-1), in starved and fed livers, respectively. The 1-(13)C glycogen signal was monitored throughout the experiment with proton-decoupled (13)C NMR spectroscopy and analyzed in the time domain using AMARES. We noticed progressive line-broadening in any single experiment in the chase phase. One or a sum of two to three overlapping Lorentzians, with different exponential damping factors, were fitted to the signal. When the S/N was better than 40, the fit always delivered a small and a broad component. In the chase phase, the fit with a single Lorentzian resulted in a decline of glycogen signal by about 15 +/- 4 and 12 +/- 2% in starved and fed rats, respectively. This apparent decline in 1-(13)C glycogen signal could not be accounted for by the appearance of equivalent amounts of (13)C-labeled metabolites in the perfusate. The fit with a sum of two Lorentzians resulted in a decline of glycogen signal intensity of 7 +/- 5 and 5 +/- 3% in starved and fed rats, respectively, which reduced the apparent turnover to 8 +/- 9% and 6 +/- 4%, respectively. Quantification of the growing (13)C-1 glycogen signal requires a model function that accommodates changes in line shape throughout the period under study.

Is in vivo nuclear magnetic resonance spectroscopy currently a quantitative method for whole-body carbohydrate metabolism?

In vivo nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for noninvasive metabolic research. NMR studies of tissue glycogen metabolism and glucose utilization have generated results with major implications for normal glucose homeostasis and the pathophysiology of type 2 diabetes mellitus. A key question for clinicians and physiologists reading these highly technical studies is: How accurate for whole-body carbohydrate metabolism is NMR spectroscopy? We review this topic and discuss technical, metabolic, and interpretive factors that may limit quantitative accuracy of this modality. We conclude that seeing is not yet believing regarding in vivo NMR spectroscopy; there are still important limitations to quantification of whole-body carbohydrate metabolism.

Simultaneous in vivo monitoring of hepatic glucose and glucose-6-phosphate by (13)C-NMR spectroscopy

Hepatic glucose-6-phosphate (G6P) was monitored non-invasively in rat liver by in vivo (13)C NMR spectroscopy after infusion of [1-(13)C] glucose. The phosphorylation of glucose to G6P yields small but characteristic displacements for all of its (13)C-NMR resonances relative to those of glucose. It is demonstrated that in vivo (13)C-NMR spectroscopy at 7 Tesla provides the spectral sensitivity and resolution to detect hepatic G6P present at sub-millimolar concentration as partially resolved low-field shoulders of the glucose C1 resonances at 96.86 ppm (C1beta) and 93. 02 ppm (C1alpha). Upon (13)C-labeling, the intracellular conversion of [1-(13)C] glucose to [1-(13)C] G6P could be monitored, which allowed the hepatic glucose-G6P substrate cycle to be assessed in situ. The close correlation found for the (13)C labeling patterns of glucose and G6P supports the concept of an active substrate cycle whose rate exceeds that of net hepatic glucose metabolism. High-resolution (13)C-NMR spectroscopy and biochemical analyses of tissue biopsies collected at the end of the experiments confirmed qualitatively the findings obtained in vivo.

Single-shot, three-dimensional "non-echo" localization method for in vivo NMR spectroscopy

Metabolite signals with short T(1) or T(2) are difficult to localize with full sensitivity. This limitation was overcome with the development and implementation of a single-shot, complete three-dimensional "non-echo" localization method with reduced sensitivity to spatial B(1) variation, which is suitable for measuring signals with very short T(1) or T(2), e.g., the (13)C NMR signals of glycogen. The proposed method is based on a T(1)-optimized outer volume suppression scheme using pulses of the hyperbolic secant type applied at different power levels, which is robust over a fivefold range of T(1). Strong lipid, muscle glycogen, and glucose signals originating outside the rat brain were suppressed. Signals of glycogen, aspartate, glutathione, GABA C4, N-acetyl aspartate as well as the C3 and C4 signals of glutamate and glutamine with resolved homonuclear (13)C-(13)C coupling were fully resolved in vivo at 9.4 Tesla using higher-order shimming. The method can be extended to other nuclei and to localized MRS of humans.

NMR of glycogen in exercise

Natural-abundance 13C NMR spectroscopy is a non-invasive technique that enables in vivo assessments of muscle and/or liver glycogen concentrations. Over the last several years, 13C NMR has been developed and used to obtain information about human glycogen metabolism with diet and exercise. Since NMR is non-invasive, more data points are available over a specified time course, dramatically improving the time resolution. This improved time resolution has enabled the documentation of subtleties of muscle glycogen re-synthesis following severe glycogen depletion that were not previously observed. Muscle and liver glycogen concentrations have been tracked in several different human populations under conditions that include: (1) muscle glycogen recovery from intense localized exercise with normal insulin and with insulin suppressed; (2) muscle glycogen recovery in an insulin-resistant population; (3) muscle glycogen depletion during prolonged low-intensity exercise; (4) effect of a mixed meal on postprandial muscle and liver glycogen synthesis. The present review focuses on basic 13C NMR and gives results from selected studies.

Noninvasive measurements of [1-(13)C]glycogen concentrations and metabolism in rat brain in vivo

Using a specific 13C NMR localization method, 13C label incorporation into the glycogen C1 resonance was measured while infusing [1-(13)C]glucose in intact rats. The maximal concentration of [1-(13)C]glycogen was 5.1 +/- 0.6 micromol g(-1) (mean +/- SE, n = 8). During the first 60 min of acute hyperglycemia, the rate of 13C label incorporation (synthase flux) was 2.3 +/- 0.7 micromol g(-1) h(-1) (mean +/- SE, n = 9 rats), which was higher (p < 0.01) than the rate of 0.49 +/- 0.14 micromol g(-1) h(-1) measured > or = 2 h later. To assess whether the incorporation of 13C label was due to turnover or net synthesis, the infusion was continued in seven rats with unlabeled glucose. The rate of 13C label decline (phosphorylase flux) was lower (0.33 +/- 0.10 micromol g(-1) h(-1)) than the initial rate of label incorporation (p < 0.01) and appeared to be independent of the duration of the preceding infusion of [1-(13)C]glucose (p > 0.05 for correlation). The results implied that net glycogen synthesis of approximately 3 micromol g(-1) had occurred, similar to previous reports. When infusing unlabeled glucose before [1-(13)C]glucose in three studies, the rate of glycogen C1 accumulation was 0.46 +/- 0.08 micromol g(-1) h(-1). The results suggest that steady-state glycogen turnover rates during hyperglycemia are approximately 1% of glucose consumption.

1H NMR spectroscopy study of the dynamic properties of glycogen in solution by steady-state magnetisation measurement with off-resonance irradiation

The dynamics of size-selected fractions of glycogen in solution have been investigated by proton NMR spectroscopy, using a recently described relaxation study method which relies on strong offresonance irradiation. The dependence of the steady-state magnetisation on angle and intensity of the effective radio-frequency field was measured and compared to theoretical curves derived from different models of motion. Absence or presence of contributions to relaxation from molecular motions on the microsecond time scale can be tested with this method, without having to resort to models. We found that glycogen dipolar relaxation did not result from isotropic Brownian rotation, and despite some contribution from slow motion (> 1 microsecond) to relaxation in glycogen alpha-particles extracted from rat liver, bulk movement of the molecules did not appear to participate in averaging the dipolar term to zero. Whereas hepatic glycogen rat beta-particles and commercial oyster glycogen displayed very similar relaxation properties, alpha-particles showed significantly different behaviour. However, all results were compatible with a diversity of movements within the molecule, ranging from freely rotating pyranoside rings through collective chain motion and possibly to bulk movement of the beta sub-units within the alpha-particle.

High-field NMR as a technique for the determination of polysaccharide structures

NMR spectroscopy has played a developing role in the study of polysaccharide structures for over 30 years. Many new bacterial polysaccharide repeat unit structures have recently been published as a result of the application of modern NMR techniques. NMR can also be used to elucidate the structures of both regular and heterogeneous polysaccharides from fungal and plant sources, as well as complex glycosaminoglycans of animal origin. In addition to covalent structure, conformation and dynamics of polysaccharides are susceptible to NMR analysis, both in solution and in the solid state. Improvements in NMR technology with potential applications to polysaccharide studies hold promise for the future.

Muscle glycogen recovery after exercise during glucose and fructose intake monitored by 13C-NMR

The purpose of this study was to examine muscle glycogen recovery with glucose feeding (GF) compared with fructose feeding (FF) during the first 8 h after partial glycogen depletion using 13C-nuclear magnetic resonance (NMR) on a clinical 1.5-TNMR system. After measurement of the glycogen concentration of the vastus lateralis (VL) muscle in seven male subjects, glycogen stores of the VL were depleted by bicycle exercise. During 8 h after completion of exercise, subjects were orally given either GF or FF while the glycogen content of the VL was monitored by 13C-NMR spectroscopy every second hour. The muscular glycogen concentration was expressed as percentage of the glycogen concentration measured before exercise. The glycogen recovery rate during GF (4.2 +/- 0.2%/h) was significantly higher (P < 0.05) compared with values during FF (2.2 +/- 0.3%/h). This study shows that 1) muscle glycogen levels are perceptible by 13 C-NMR spectroscopy at 1.5 T and 2) the glycogen restoration rate is higher after GF compared with after FF.

13C-NMR relaxation in glycogen

This study is the first report on the multiexponential T2 relaxation of the 13C-1 carbon of glycogen. In contrast to T1 relaxation, which does not display observable multiexponential decay behavior, T2 relaxation is described by a continuous distribution of T2 times. Changes in molecular weight and sample viscosity, which affect the overall mobility of the glycogen particle have little influence on T1 and T2 relaxation times. This is in contradiction with earlier results that T2 is dominated by the overall motion of the glycogen particles [L.-H. Zang Biochemistry 29, 6815-6820 (1990)]. T1 depends strongly on the external field Bo and is almost temperature independent in the range 23-37 degrees C whereas T2 is field independent and varies appreciably with temperature. The experimental T1 and T2 relaxation data are shown to be consistent with existing theoretical models for relaxation, suitably modified to include a distribution of correlation times for the internal motions. The presence of fast decaying components (short T2) in the FID implies broad line components in the frequency spectrum and the corresponding need to appropriately set the integration limits for the quantification of the glycogen peak.

Validation of 13C NMR measurements of liver glycogen in vivo

The natural abundance 13C NMR intensity of the glycogen C1 resonance was measured in the surgically exposed liver of rabbits in vivo (n = 17) by integration from 98 to 104 ppm and compared double blindedly to the subsequent biochemical measurement. Coil loading was measured each time from a reference sphere at the coil center and the NMR intensity was normalized accordingly. For quantification, the normalized NMR intensity was calibrated using aqueous glycogen solutions ranging from 110 to 1100 mumol glucosyl units/g (n = 14). An in vivo range from 110 to 800 mumol glucosyl units/g wet weight was measured with a highly linear correlation with concentration (r = 0.85, P < 0.001). The in vivo NMR concentration was 0.95 +/- 0.05 (mean +/- standard error, n = 17) of the concomitant enzymatic measurement of glycogen content. We conclude that the 13C NMR signal of liver glycogen C1 is essentially 100% visible in vivo and that natural abundance 13C NMR spectroscopy can provide reliable noninvasive estimates of in vivo glycogen content over the physiological range of liver glycogen concentrations when using adequate localization and integration procedures.

Proton NMR observation of glycogen in vivo

Our previous solution studies of the proton relaxation properties of glycogen H1 have shown significant dipolar cross-relaxation with intra-ring H2 and inter-ring H4' protons characterized by a correlation time tau c = 2.7 x 10(-9) s. This leads to a significant negative Nuclear Overhauser Enhancement (NOE) of glycogen H1 following either transient or steady state perturbations of the longitudinal magnetization of dipolar coupled protons, especially H2 and H4'. Here we use the NOE to edit selectively the H1 resonance of glycogen in the rat liver in vivo using a surface coil probe. The approach shows the possibility of measuring glycogen in vivo with high sensitivity using 1H NMR.

Non-cooperative effects of glucose and 2-deoxyglucose on their metabolism in Saccharomyces cerevisiae studied by 1H-NMR and 13C-NMR spectroscopy

The reciprocal effects of 2-deoxy-D-glucose (dGlc) and glucose (Glc) on the aerobic metabolism of Glc and dGlc in Glc-grown repressed Saccharomyces cerevisiae were studied at 30 degrees C in a standard pyrophosphate medium containing about 4.5 x 10(7) cells/ml. 1H-, 13C-NMR and biochemical techniques were used for quantitative evaluation of Glc and dGlc metabolites. The detection of intracellular dGlc and the determination of the intracellular dGlc6P/dGlc ratio were realised using [1-13C]dGlc and 13C-NMR spectroscopy. The rates of Glc consumption in the absence and in the presence of 5 mM dGlc and 20 mM dGlc were 29 +/- 0.03 mM/min (n = 3), 16 +/- 0.02 mM/min (n = 3), and 0.08 +/- 0.01 mM/min (n = 3), respectively. This means that the Glc consumption is reduced about 41% and 70% in the presence of 5 mM and 20 mM dGlc, respectively. When dGlc is the unique carbon source, only alpha and beta anomers of dGlc6P were formed. Their quantities are equivalent and reach the maximum values within 1 h of incubation and then decrease gradually. By contrast, Glc favours the consumption of dGlc and the synthesis of dGlc6P, dideoxy-trehalose (dGlc-dGlc), deoxy-trehalose (dGlc-Glc). In the presence of Glc, dGlc6P reaches a plateau after 1 h or 2 h of incubation while the quantities of trehalose (Glc-Glc), dGlc-dGlc, dGlc-Glc, which are small at 1 h, rapidly increase with time. The reasons why dGlc and Glc exert opposite effects on their metabolism are discussed. The production of Glc-Glc decreases with increasing the external dGlc concentration or the dGlc/Glc ratio. The effect of dGlc on the biosynthesis of Glc-Glc can be explained by the competition of dGlc and Glc with respect to hexokinase. Although Glc favours the synthesis of dGlc6P, the maximum concentration of dGlc6P shows little dependence on the external dGlc concentration as long as glucose is not exhausted. The internal dGlc6P/dGlc ratio at equilibrium, about 4.7 +/- 0.7, is also found to be independent of the dGlc concentration in the suspension. Only a small fraction of dGlc6P disappears to give rise to the formation of dGlc-dGlc and dGlc-Glc. At equilibrium the inverse reaction from dGlc6P to dGlc may be important to compensate for the fast reaction of dGlc phosphorylation by hexokinase. At least nine series of experiments were conducted and showed that, in pyrophosphate media and for incubation times less than 4 h, dGlc-dGlc was not observed in the absence of Glc.(ABSTRACT TRUNCATED AT 400 WORDS)

NMR studies of 1H NOEs in glycogen

We have examined the cross-relaxation behavior among the protons of oyster glycogen using nuclear Overhauser enhancement (NOE). Steady-state and transient NOEs were generated using low-power CW irradiation and frequency-selective inversions. In D2O, saturation of glycogen H2 and H4' at 3.64 ppm gave a strong negative NOE (eta = -0.74) at H1. The NOE was similar to the value predicted by the correlation time (tau c) calculated from the T1 and T2 of glycogen H1 in D2O assuming an isotropic rigid motor dipole-dipole model. Selective inversion of H2 and H4' gave a transient NOE at H1. In D2O, selective inversion of H1 also led to negative transient NOEs in the H2 + H4', H3, and H5 resonances. The magnitude and rates of appearance of the NOEs in H3 and H5 were too large to arise from direct H1-H3 and H1-H5 dipolar interactions, but were consistent with very efficient cross-relaxation leading to large second-order NOEs. The glycogen H1 NOE in H2O was also studied. Replacement of D2O with H2O as solvent significantly reduced the steady-state NOE at H1 following saturation of H2 + H4'. Saturation of the water resonance caused a large negative NOE at H1 (eta = -0.55) consistent with our earlier study which indicated that there was no direct dipolar interaction between H1 and free H2O.

In situ 13C NMR quantification of hepatic glycogen

We report on the 13C NMR visibility of the C-1 glycosidic carbon of alpha-particulate glycogen in perfused rat liver. We used rats fed ad libitum, animals refed after a 48 h fast with a sucrose supplement with or without glucocorticoid treatment, and gsd/gsd rats with a hepatic glycogen storage disease due to phosphorylase kinase deficiency. Thus we studied a wide range of glycogen levels (25-140 mg/g liver). All livers were perfused with 15 mM glucose, to maintain constant glycogen levels. Failure to activate glycogen phosphorylase ensures stable glycogen levels in gsd/gsd livers. Natural abundance 13C NMR signals were calibrated against a phantom containing a fixed amount of glycogen. Accumulated free induction decays were analysed after Fourier transformation by numerical integration, or by direct analysis of the signal in the time domain using a non-iterative method based on singular value decomposition. NMR quantification of the glycogen correlated well with the chemical determination over the whole concentration range. However, the precision (reproducibility) of glycogen determinations (be it by chemical methods or by NMR spectroscopy) may pose problems. Authors should be encouraged to report systematically on the precision of their methods.

Nuclear magnetic resonance relaxation of glycogen H1 in solution

The NMR relaxation properties of the H1 proton of oyster glycogen in D2O and H2O solutions have been studied using nonselective, semiselective, and selective inversion recovery and Hahn spin-echo pulse sequences. The data were analyzed in terms of an isotropic, rigid-rotor dipole-dipole model including cross-relaxation. At 8.4 T in D2O, p = 5.4 +/- 0.4 s-1 and sigma = -4.5 +/- 0.4 s-1. The large, negative sigma value is consistent with strong cross-relaxation and a long correlation time. The relaxation data can be explained by a single correlation time, tau c = 2.7 x 10(-9) s, indicating significant internal mobility. With this value of tau c, and assuming that the structure of the glucose moieties was the same as in alpha-D-glucose crystals, the dipole sum contributing to T1 relaxation was calculated. The intra-ring relaxation was dominated by dipole fields from the H2 proton, but these only accounted for approximately 18% of the total relaxation. Most of the relaxation comes from inter-glucose relaxation. From modeling, this is dominated by the H4' across the alpha-1,4-glycosidic bond. The H1 longitudinal relaxation rates were significantly enhanced in H2O compared with D2O. This enhancement is not due to direct dipolar interaction between H1 and bulk water. Transverse relaxation rates were not significantly enhanced in H2O.

Comparison of backbone and tryptophan side-chain dynamics of reduced and oxidized Escherichia coli thioredoxin using 15N NMR relaxation measurements

The backbone and tryptophan side-chain dynamics of both the reduced and oxidized forms of uniformly 15N-labeled Escherichia coli thioredoxin have been characterized using inverse-detected two-dimensional 1H-15N NMR spectroscopy. Longitudinal (T1) and transverse (T2) 15N relaxation time constants and steady-state (1H)-15N NOEs were measured for more than 90% of the protonated backbone nitrogen atoms and for the protonated indole nitrogen atoms of the two tryptophan residues. These data were analyzed by using a model free dynamics formalism to determine the generalized order parameter (S2), the effective correlation time for internal motions (tau e), and 15N exchange broadening contributions (Rex) for each residue, as well as the overall molecular rotational correlation time (tau m). The reduced and oxidized forms exhibit almost identical dynamic behavior on the picosecond to nanosecond time scale. The W31 side chain is significantly more mobile than the W28 side chain, consistent with the positions of W31 on the protein surface and W28 buried in the hydrophobic core. Backbone regions which are significantly more mobile than the average include the N-terminus, which is constrained in the crystal structure of oxidized thioredoxin by specific contacts with a Cu2+ ion, the C-terminus, residues 20-22, which constitute a linker region between the first alpha-helix and the second beta-strand, and residues 73-75 and 93-94, which are located adjacent to the active site. In contrast, on the microsecond to millisecond time scale, reduced thioredoxin exhibits considerable dynamic mobility in the residue 73-75 region, while oxidized thioredoxin exhibits no significant mobility in this region. The possible functional implications of the dynamics results are discussed.

Validation of 13C NMR measurement of human skeletal muscle glycogen by direct biochemical assay of needle biopsy samples

Recent developments in 13C nuclear magnetic resonance (NMR) spectroscopy have permitted noninvasive assessment of glycogen concentration in human skeletal muscle. Before these indirect measurements could be accepted as accurate, it was essential that validation should be carried out by comparing the widely used method of muscle biopsy and direct biochemical assay for glycogen concentration with measurement by NMR. Eight normal subjects underwent six NMR scans of gastrocnemius and three biopsies of the same muscle on the same day. The overall mean for muscle glycogen concentration was 87.4 mM by NMR and 88.3 mM by biopsy. There was a close correlation between the pairs of observations on each subject (R = 0.95; P less than 0.0001). The mean coefficient of variation for NMR measurement was 4.3 +/- 2.1% and that for biopsy was 9.3 +/- 5.9%. The performance of the muscle biopsies was accompanied by a small but significant rise in plasma-free fatty acids (529 +/- 157 to 667 +/- 250; P less than 0.01), epinephrine (17 +/- 6 to 25 +/- 8 pg/ml; P less than 0.02), and norepinephrine (318 +/- 119 to 400 +/- 140 pg/ml; P less than 0.02) but no change in plasma glucose, plasma insulin, nor muscle glycogen concentration assessed by NMR. The study demonstrates that in vivo 13C NMR measurement of human muscle glycogen can be regarded as accurate, and the technique is associated with a higher precision that biopsy with direct biochemical assessment.

Backbone dynamics of calcium-loaded calbindin D9k studied by two-dimensional proton-detected 15N NMR spectroscopy

Backbone dynamics of calcium-loaded calbindin D9k have been investigated by two-dimensional proton-detected heteronuclear nuclear magnetic resonance spectroscopy, using a uniformly 15N enriched protein sample. Spin-lattice relaxation rate constants, spin-spin relaxation rate constants, and steady-state [1H]-15N nuclear Overhauser effects were determined for 71 of the 72 backbone amide 15N nuclei. The relaxation parameters were analyzed using a model-free formalism that incorporates the overall rotational correlation time of the molecule, and a generalized order parameter (S2) and an effective internal correlation time for each amide group. Calbindin D9k contains two helix-loop-helix motifs joined by a linker loop at one end of the protein and a beta-type interaction between the two calcium-binding loops at the other end. The amplitude of motions for the calcium-binding loops and the helices are similar, as judged from the average S2 values of 0.83 +/- 0.05 and 0.85 +/- 0.04, respectively. The linker region joining the two calcium-binding subdomains of the molecule has a significantly higher flexibility, as indicated by a substantially lower average S2 value of 0.59 +/- 0.23. For residues in the linker loop and at the C-terminus, the order parameter is further decomposed into separate order parameters for motional processes on two distinct time scales. The effective correlation times are significantly longer for helices I and IV than for helices II and III or for the calcium-binding loops. Residue by residue comparisons reveal correlations of the order parameters with both the crystallographic B-factors and amide proton exchange rates, despite vast differences in the time scales to which these properties are sensitive. The order parameters are also utilized to distinguish regions of the NMR-derived three-dimensional structure of calbindin D9k that are poorly defined due to inherently high flexibility, from poorly defined regions with average flexibility but a low density of structural constraints.

Backbone dynamics of the Bacillus subtilis glucose permease IIA domain determined from 15N NMR relaxation measurements

The backbone dynamics of the uniformly 15N-labeled IIA domain of the glucose permease of Bacillus subtilis have been characterized using inverse-detected two-dimensional 1H-15N NMR spectroscopy. Longitudinal (T1) and transverse (T2) 15N relaxation time constants and steady-state (1H)-15N NOEs were measured, at a spectrometer proton frequency of 500 MHz, for 137 (91%) of the 151 protonated backbone nitrogens. These data were analyzed by using a model-free dynamics formalism to determine the generalized order parameter (S2), the effective correlation time for internal motions (tau e), and 15N exchange broadening contributions (Rex) for each residue, as well as the overall molecular rotational correlation time (tau m). The T1 and T2 values for most residues were in the ranges 0.45-0.55 and 0.11-0.15 s, respectively; however, a small number of residues exhibited significantly slower relaxation. Similarly, (1H)-15N NOE values for most residues were in the range 0.72-0.80, but a few residues had much smaller positive NOEs and some exhibited negative NOEs. The molecular rotational correlation time was 6.24 +/- 0.01 ns; most residues had order parameters in the range 0.75-0.90 and tau e values of less than ca. 25 ps. Residues found to be more mobile than the average were concentrated in three areas: the N-terminal residues (1-13), which were observed to be highly disordered; the loop from P25 to D41, the apex of which is situated adjacent to the active site and may have a role in binding to other proteins; and the region from A146 to S149. All mobile residues occurred in regions close to termini, in loops, or in irregular secondary structure.

Excitation characteristics of adiabatic half-passage RF pulses used in surface coil MR spectroscopy. Application to 13C detection of glycogen in the rat liver

Properties of sech/tanh and sin/cos half-passage RF pulses are discussed in view of their use in surface coil MR spectroscopy. We focus on the use of these pulses in a regime which is partially adiabatic, i.e. not strictly adiabatic off-resonance, while on-resonance the adiabaticity condition is fulfilled. It is shown that the frequencies of the singular points of the excitation profiles, as well as their number, depend on the B1 field. This leads to a signal intensity reduction from off-resonance spectral regions over much broader ranges than generally believed. We show in particular that with surface coil, sin/cos RF pulses may perform particularly well, providing optimal excitation on resonance and a desired attenuation over a broad spectral range off-resonance. This feature is applied for the in vivo detection of rat liver glycogen by means of 13C MR spectroscopy. Under suitable RF power conditions, a remarkable attenuation of the signals from the saturated carbons of the subcutaneous fat can be achieved.

13C NMR studies of glucose metabolism in human leukemic CEM-C7 and CEM-C1 cells

Glucose metabolism of human leukemic cell lines CEM-C7 and CEM-C1 was investigated in vivo by 13C NMR using 13C-labeled glucose. Exact knowledge of glucose concentration, cell count, and cell viability of the cell suspensions made it possible to analyze glucose metabolism in detail. In both cell lines aerobic glycolysis accounts for virtually all glucose consumption. The use of D-[13C2]glucose provided a simple method to measure the glucose flux through the pentose phosphate pathway as 9% (CEM-C1) and 11% (CEM-C7) of glucose channeled into glycolysis. The dexamethasone-sensitive CEM-C7 cells consume glucose at a rate about 50% higher than the dexamethasone-resistant CEM-C1 cells. It is shown that this higher consumption correlates with a larger size of the CEM-C7 cells. Therefore in CEM cells the development of drug resistance does not seem to involve related changes in cell energetics.

N.m.r. and conformational analysis of the capsular polysaccharide from Streptococcus pneumoniae type 4

The 1H- and 13C-n.m.r. data on the capsular polysaccharide (1) produced by Streptococcus pneumoniae type 4, the depyruvated polysaccharide (2), and a tetrasaccharide (3a) derived by Smith degradation of 2 were used as constraints on a computer-generated model of the conformation of 1 and to assess the effects of the pyruvic acetal substituent on the conformation. The dynamics of the polysaccharide systems and the influence of the pyruvic acetal were investigated using 13C-n.m.r. relaxation measurements.

Glycogen metabolism as detected by in vivo and in vitro 13C-NMR spectroscopy using [1,2-13C2]glucose as substrate

The metabolism of glucose to glycogen in the liver of fasted and well-fed rats was investigated with 13C nuclear magnetic resonance spectroscopy using [1,2-(13)C2]glucose as the main substrate. The unique spectroscopic feature of this molecule is the 13C-13C homonuclear coupling leading to characteristic doublets for the C-1 and C-2 resonances of glucose and its breakdown products as long as the two 13C nuclei remain bonded together. The doublet resonances of [1,2-(13)C2]glucose thus provide an ideal marker to follow the fate of this exogenous substrate through the metabolic pathways. [1,2-(13)C2]Glucose was injected intraperitoneally into anesthetized rats and the in vivo 13C-NMR measurements of the intact animals revealed the transformation of the injected glucose into liver glycogen. Glycogen was extracted from the liver and high resolution 13C-NMR spectra were obtained before and after hydrolysis of glycogen. Intact [1,2-13C2]glucose molecules give rise to doublet resonances, natural abundance [13C]glucose molecules produce singlet resonances. From an analysis of the doublet-to-singlet intensities the following conclusions were derived. (i) In fasted rats virtually 100% of the glycosyl units in glycogen were 13C-NMR visible. In contrast, the 13C-NMR visibility of glycogen decreased to 30-40% in well-fed rats. (ii) In fed rats a minimum of 67 +/- 7% of the exogenous [1,2-(13)C2]glucose was incorporated into the liver glycogen via the direct pathway. No contribution of the indirect pathway could be detected. (iii) In fasted rats externally supplied glucose appeared to be consumed in different metabolic processes and less [1,2-(13)C2]glucose was found to be incorporated into glycogen (13 +/- 1%). However, the observation of [5,6-(13)C2]glucose in liver glycogen provided evidence for the operation of the so-called indirect pathway of glycogen synthesis. The activity of the indirect pathway was at least 9% but not more than 30% of the direct pathway. (vi) The pentose phosphate pathway was of little significance for glucose but became detectable upon injection of [1-(13)C]ribose.

Quantitation of hepatic glycogenolysis and gluconeogenesis in fasting humans with 13C NMR

The rate of net hepatic glycogenolysis was assessed in humans by serially measuring hepatic glycogen concentration at 3- to 12-hour intervals during a 68-hour fast with 13C nuclear magnetic resonance spectroscopy. The net rate of gluconeogenesis was calculated by subtracting the rate of net hepatic glycogenolysis from the rate of glucose production in the whole body measured with tritiated glucose. Gluconeogenesis accounted for 64 +/- 5% (mean +/- standard error of the mean) of total glucose production during the first 22 hours of fasting. In the subsequent 14-hour and 18-hour periods of the fast, gluconeogenesis accounted for 82 +/- 5% and 96 +/- 1% of total glucose production, respectively. These data show that gluconeogenesis accounts for a substantial fraction of total glucose production even during the first 22 hours of a fast in humans.

13C NMR visibility of rabbit muscle glycogen in vivo

The integrated 13C NMR intensity of the glycogen C1 resonance was measured in skeletal muscle (biceps femoris region) of nine rabbits under in vivo conditions. Concurrent chemical determinations of glycogen content showed that the in vivo signal was 1.02 +/- 0.06 the intensity of analytical samples, where glycogen is known to be approximately 100% visible.

Effects of amphotericin B on the glucose metabolism in Saccharomyces cerevisiae cells. Studies by 13C-, 1H-NMR and biochemical methods

A new approach is proposed to investigate the metabolic perturbation induced by drugs in cells. The effects of various concentrations of amphotericin B on the aerobic [1-13C]glucose metabolism in glucose-grown repressed Saccharomyces cerevisiae cells were studied as a function of time using 13C-, 1H-NMR and biochemical methods. The 13C enrichment of different compounds such as ethanol, glycerol and trehalose were determined by 1H-NMR spectroscopy. In the absence of amphotericin B, glycerol diffuses slowly from the internal to the external medium, whereas in its presence this diffusion is greatly facilitated by the formation of pores in the cell membrane. Amphotericin B has been found to exert a marked influence on the glucose consumption and the production of all metabolites; for example, at 1 microM, the glucose consumption and the production of ethanol decrease while the production of glycerol and trehalose increases. The 13C relative enrichments of ethanol, glycerol and trehalose are almost the same with and without the drug. Thus it can be concluded that amphotericin B induces a large effect on the production of these compounds in the cytosol but shows no significant influence on the mechanism of their formation. Upon addition of glucose, all the amino acid concentrations decrease continuously with time; this effect is more pronounced in the presence of the drug. The ratio of the integrated resonances of glutamate (C2 + C3)/C4 reflects the activity of pyruvate carboxylase relative to citrate synthase rather than to pyruvate dehydrogenase. Without amphotericin B, this ratio (approximately 1.0) is practically constant upon addition of glucose which suggests that the activities of pyruvate carboxylase and citrate synthase are equivalent. By contrast, upon coaddition of 25 mM glucose and 1 microM amphotericin B, the glutamate C4 resonance remains virtually unchanged while that of glutamate C2 is much smaller than in its absence and continuously decreases with time. It seems likely that amphotericin B induces a reduction in the activity of pyruvate carboxylase in the mitochondria.