Molecular dynamics and information on possible sites of interaction of intramyocellular metabolites in vivo from resolved dipolar couplings in localized 1H NMR spectra

The effects of residual dipolar coupling on carnosine in proton muscle spectra

Carnosine, an MR-visible dipeptide in human muscle, is well characterized by two peaks at ~8 and ~7 ppm from C2 and C4 imidazole protons. Like creatine and other metabolites, carnosine is subject to residual dipolar coupling in the anisotropic environment of muscle fibers, but the effects have not been studied extensively. Single-voxel TE 30-32 PRESS spectra from three different 3T studies were acquired from gastrocnemius medialis and soleus muscles in the human lower leg. In these studies, carnosine T2 values were measured, and spectra were obtained at three different foot angles. LCModel was used to fit the carnosine peaks with a basis set that was generated using shaped RF pulses and included a range of dipolar couplings affecting the C4 peak. A seven-parameter analytic expression was used to fit the CH2 doublets of creatine. It incorporated an optimized "effective TE" value to model the effect of shaped RF pulses. The fits confirm that the triplet C4 peak of carnosine is dipolar coupled to a pair of CH2 protons, with no need to include a contribution from a separate pool of freely rotating uncoupled carnosine. Moreover, the couplings experienced by carnosine C4 protons and creatine CH2 protons are strongly correlated (R2  =  0.88, P<0.001), exhibiting a similar 3cos2 θ - 1 dependence on the angle θ between fiber orientation and B0. T2 values for the singlet C2 peak of gastrocnemius carnosine are inversely proportional to the C4 dipolar coupling strength (R2  = 0.97, P < 0.001), which in turn is a function of foot orientation. This dependence indicates that careful positioning of the foot while acquiring lower leg muscle spectra is important to obtain reproducible carnosine concentrations. As proton magnetic resonance spectroscopy of carnosine is currently used to non-invasively estimate the muscle fiber typology, these results have important implications in sport science.

CORP: quantification of human skeletal muscle carnosine concentration by proton magnetic resonance spectroscopy

Noninvasive techniques to quantify metabolites in skeletal muscle provide unique insight into human physiology and enable the translation of research into practice. Proton magnetic resonance spectroscopy (¹H-MRS) permits the assessment of several abundant muscle metabolites in vivo, including carnosine, a dipeptide composed of the amino acids histidine and beta-alanine. Muscle carnosine loading, accomplished by chronic oral beta-alanine supplementation, improves muscle function and exercise capacity and has pathophysiological relevance in multiple diseases. Moreover, the marked difference in carnosine content between fast-twitch and slow-twitch muscle fibers has rendered carnosine an attractive candidate to estimate human muscle fiber type composition. However, the quantification of carnosine with ¹H-MRS requires technical expertise to obtain accurate and reproducible data. In this review, we describe the technical and physiological factors that impact the detection, analysis, and quantification of carnosine in muscle with ¹H-MRS. We discuss potential sources of error during the acquisition and preprocessing of the ¹H-MRS spectra and present best practices to enable the accurate, reliable, and reproducible application of this technique.

Proton magnetic resonance spectroscopy in skeletal muscle: Experts' consensus recommendations

1 H-MR spectroscopy of skeletal muscle provides insight into metabolism that is not available noninvasively by other methods. The recommendations given in this article are intended to guide those who have basic experience in general MRS to the special application of 1 H-MRS in skeletal muscle. The highly organized structure of skeletal muscle leads to effects that change spectral features far beyond simple peak heights, depending on the type and orientation of the muscle. Specific recommendations are given for the acquisition of three particular metabolites (intramyocellular lipids, carnosine and acetylcarnitine) and for preconditioning of experiments and instructions to study volunteers.

Magnetization exchange observed in human skeletal muscle by non-water-suppressed proton magnetic resonance spectroscopy

Many metabolites in the proton magnetic resonance spectrum undergo magnetization exchange with water, such as those in the downfield region (6.0-8.5 ppm) and the upfield peaks of creatine, which can be measured to reveal additional information about the molecular environment. In addition, these resonances are attenuated by conventional water suppression techniques complicating detection and quantification. To characterize these metabolites in human skeletal muscle in vivo at 3 T, metabolite cycled non-water-suppressed spectroscopy was used to conduct a water inversion transfer experiment in both the soleus and tibialis anterior muscles. Resulting median exchange-independent T1 times for the creatine methylene resonances were 1.26 and 1.15 s, and for the methyl resonances were 1.57 and 1.74 s, for soleus and tibialis anterior muscles, respectively. Magnetization transfer rates from water to the creatine methylene resonances were 0.56 and 0.28 s(-1), and for the methyl resonances were 0.39 and 0.30 s(-1), with the soleus exhibiting faster transfer rates for both resonances, allowing speculation about possible influences of either muscle fibre orientation or muscle composition on the magnetization transfer process. These water magnetization transfer rates observed without water suppression are in good agreement with earlier reports that used either postexcitation water suppression in rats, or short CHESS sequences in human brain and skeletal muscle.

Phosphocreatine interacts with phospholipids, affects membrane properties and exerts membrane-protective effects

A broad spectrum of beneficial effects has been ascribed to creatine (Cr), phosphocreatine (PCr) and their cyclic analogues cyclo-(cCr) and phospho-cyclocreatine (PcCr). Cr is widely used as nutritional supplement in sports and increasingly also as adjuvant treatment for pathologies such as myopathies and a plethora of neurodegenerative diseases. Additionally, Cr and its cyclic analogues have been proposed for anti-cancer treatment. The mechanisms involved in these pleiotropic effects are still controversial and far from being understood. The reversible conversion of Cr and ATP into PCr and ADP by creatine kinase, generating highly diffusible PCr energy reserves, is certainly an important element. However, some protective effects of Cr and analogues cannot be satisfactorily explained solely by effects on the cellular energy state. Here we used mainly liposome model systems to provide evidence for interaction of PCr and PcCr with different zwitterionic phospholipids by applying four independent, complementary biochemical and biophysical assays: (i) chemical binding assay, (ii) surface plasmon resonance spectroscopy (SPR), (iii) solid-state ³¹P-NMR, and (iv) differential scanning calorimetry (DSC). SPR revealed low affinity PCr/phospholipid interaction that additionally induced changes in liposome shape as indicated by NMR and SPR. Additionally, DSC revealed evidence for membrane packing effects by PCr, as seen by altered lipid phase transition. Finally, PCr efficiently protected against membrane permeabilization in two different model systems: liposome-permeabilization by the membrane-active peptide melittin, and erythrocyte hemolysis by the oxidative drug doxorubicin, hypoosmotic stress or the mild detergent saponin. These findings suggest a new molecular basis for non-energy related functions of PCr and its cyclic analogue. PCr/phospholipid interaction and alteration of membrane structure may not only protect cellular membranes against various insults, but could have more general implications for many physiological membrane-related functions that are relevant for health and disease.

Carnosine and its possible roles in nutrition and health

The dipeptide carnosine has been observed to exert antiaging activity at cellular and whole animal levels. This review discusses the possible mechanisms by which carnosine may exert antiaging action and considers whether the dipeptide could be beneficial to humans. Carnosine's possible biological activities include scavenger of reactive oxygen species (ROS) and reactive nitrogen species (RNS), chelator of zinc and copper ions, and antiglycating and anticross-linking activities. Carnosine's ability to react with deleterious aldehydes such as malondialdehyde, methylglyoxal, hydroxynonenal, and acetaldehyde may also contribute to its protective functions. Physiologically carnosine may help to suppress some secondary complications of diabetes, and the deleterious consequences of ischemic-reperfusion injury, most likely due to antioxidation and carbonyl-scavenging functions. Other, and much more speculative, possible functions of carnosine considered include transglutaminase inhibition, stimulation of proteolysis mediated via effects on proteasome activity or induction of protease and stress-protein gene expression, upregulation of corticosteroid synthesis, stimulation of protein repair, and effects on ADP-ribose metabolism associated with sirtuin and poly-ADP-ribose polymerase (PARP) activities. Evidence for carnosine's possible protective action against secondary diabetic complications, neurodegeneration, cancer, and other age-related pathologies is briefly discussed.

Carnosine as molecular probe for sensitive detection of Cu(II) ions using localized 1H NMR spectroscopy

Complex formation of carnosine (Csn) with Cu(II) is suspected to be of significant biochemical importance and can be detected by NMR via ion-induced paramagnetic relaxation of Csn signals. Here, we present quantification of the sensitivity achieved with localized (1)H NMR spectroscopy at physiological pH and high ligand-to-metal ratios. While characterizing the highly effective relaxation transfer onto a huge Csn pool due to fast ligand exchange, it is demonstrated that a metal-to-ligand ratio of approximately 100 ppm suffices to reduce Csn signals by approximately 50% in vitro, thus making the dipeptide a sensitive probe for such ions. Variation of the donor accessibility reveals that the paramagnetic effect is transferred onto a approximately 1370-fold donor abundance for a given ion concentration. A method is presented to characterize such effective ligand exchange relaxation transfer. These studies focus on the monomer formation since comparison with (1)H NMR data of human calf muscle demonstrates that the dimer complex is insignificant in vivo. Observed line broadening in living tissue yields an upper limit of ca. 195 ppm for the Csn-related copper concentration in human skeletal muscle.

Absolute quantification of carnosine in human calf muscle by proton magnetic resonance spectroscopy

Carnosine has been shown to be present in the skeletal muscle and in the brain of a variety of animals and humans. Despite the various physiological functions assigned to this metabolite, its exact role remains unclear. It has been suggested that carnosine plays a role in buffering in the intracellular physiological pHi range in skeletal muscle as a result of accepting hydrogen ions released in the development of fatigue during intensive exercise. It is thus postulated that the concentration of carnosine is an indicator for the extent of the buffering capacity. However, the determination of the concentration of this metabolite has only been performed by means of muscle biopsy, which is an invasive procedure. In this paper, we utilized proton magnetic resonance spectroscopy (1H MRS) in order to perform absolute quantification of carnosine in vivo non-invasively. The method was verified by phantom experiments and in vivo measurements in the calf muscles of athletes and untrained volunteers. The measured mean concentrations in the soleus and the gastrocnemius muscles were found to be 2.81 +/- 0.57/4.8 +/- 1.59 mM (mean +/- SD) for athletes and 2.58 +/- 0.65/3.3 +/- 0.32 mM for untrained volunteers, respectively. These values are in agreement with previously reported biopsy-based results. Our results suggest that 1H MRS can provide an alternative method for non-invasively determining carnosine concentration in human calf muscle in vivo.

β-Alanine supplementation augments muscle carnosine content and attenuates fatigue during repeated isokinetic contraction bouts in trained sprinters

Carnosine (β-alanyl-l-histidine) is present in high concentrations in human skeletal muscle. The ingestion of β-alanine, the rate-limiting precursor of carnosine, has been shown to elevate the muscle carnosine content. We aimed to investigate, using proton magnetic resonance spectroscopy (proton MRS), whether oral supplementation with β-alanine during 4 wk would elevate the calf muscle carnosine content and affect exercise performance in 400-m sprint-trained competitive athletes. Fifteen male athletes participated in a placebo-controlled, double-blind study and were supplemented orally for 4 wk with either 4.8 g/day β-alanine or placebo. Muscle carnosine concentration was quantified in soleus and gastrocnemius by proton MRS. Performance was evaluated by isokinetic testing during five bouts of 30 maximal voluntary knee extensions, by endurance during isometric contraction at 45% maximal voluntary contraction, and by the indoor 400-m running time. β-Alanine supplementation significantly increased the carnosine content in both the soleus (+47%) and gastrocnemius (+37%). In placebo, carnosine remained stable in soleus, while a small and significant increase of +16% occurred in gastrocnemius. Dynamic knee extension torque during the fourth and fifth bout was significantly improved with β-alanine but not with placebo. Isometric endurance and 400-m race time were not affected by treatment. In conclusion, 1) proton MRS can be used to noninvasively quantify human muscle carnosine content; 2) muscle carnosine is increased by oral β-alanine supplementation in sprint-trained athletes; 3) carnosine loading slightly but significantly attenuated fatigue in repeated bouts of exhaustive dynamic contractions; and 4) the increase in muscle carnosine did not improve isometric endurance or 400-m race time.

[Hyperfine structure analysis in magnetic resonance spectroscopy: from astrophysical measurements towards endogenous biosensors in human tissue]

The hyperfine interaction of two spins is a well studied effect in atomic systems. Magnetic resonance experiments demonstrate that the detectable dipole transitions are determined by the magnetic moments of the constituents and the external magnetic field. Transferring the corresponding quantum mechanics to molecular bound nuclear spins allows for precise prediction of NMR spectra obtained from metabolites in human tissue. This molecular hyperfine structure has been neglected so far in in vivo NMR spectroscopy but contains useful information, especially when studying molecular dynamics. This contribution represents a review of the concept of applying the Breit-Rabi formalism to coupled nuclear spins and discusses the immobilization of different metabolites in anisotropic tissue revealed by 1H NMR spectra of carnosine, phosphocreatine and taurine. Comparison of atomic and molecular spin systems allows for statements on the biological constraints for direct spin-spin interactions. Moreover, the relevance of hyperfine effects on the line shapes of multiplets of indirectly-coupled spin systems with more than two constituents can be predicted by analyzing quantum mechanical parameters. As an example, the superposition of eigenstates of the A MX system of adenosine 5'-triphosphate and its application for better quantification of 31P-NMR spectra will be discussed.

Metabolic imaging of atrophic muscle tissue using appropriate markers in 1H and 31P NMR spectroscopy

INTRODUCTION

The purpose of this feasibility study was to demonstrate non-invasive metabolic imaging of human muscular atrophy using significant changes of NMR signals that are related directly or indirectly to fiber necrosis.

METHODS

Single-voxel (1)H NMR spectroscopy and two-dimensional (31)P spectroscopic imaging on a 1.5-T whole-body scanner were used for in vivo mapping of areas of muscle damage in two cases of differently localized and pronounced atrophy. Spectral patterns affiliated with severe and intermediate stages of degeneration were compared to data of healthy control tissue to derive appropriate metabolic markers related to lipid infiltration or high-energy (31)P metabolism.

RESULTS

Reliable detection of atrophic tissue was achieved by the following parameters: (1) liposclerotic turnover is related to a drastic reduction in the water/lipid (1)H signal intensity ratio (up to a factor of 74 compared to adjacent healthy tissue); (2) the (31)P resonance of phosphocreatine (PCr) is an adequate marker for differentiation of intact myocells with high-energy metabolism from regions dominated by terminal fiber necrosis (PCr signal vanished nearly completely or intensity was reduced by a factor of 3 in affected muscles). Metabolic images based on this signal allowed accurate non-invasive localization of atrophic tissue.

CONCLUSION

The molecular information provided by NMR spectroscopy--previously only used with poor localization in atrophy studies--enables access to both the myocell-specific high-energy metabolism and the result of lipid infiltration allowing non-invasive mapping of degenerate tissue. The ability to investigate the results of these advanced levels of atrophy would also be useful for studies of more subtle degrees of denervation.

On the mechanisms of ageing suppression by dietary restriction-is persistent glycolysis the problem?

The mechanism(s) by which dietary restriction (DR) suppresses ageing and onset of age-related pathologies are discussed in relation to frequency of glycolysis, and the reactivity of glycolytic intermediates. Most glycolytic intermediates are potentially toxic and readily modify (i.e. glycate) proteins and other macromolecules non-enzymically. Attention is drawn to the reactivity of methyglyoxal (MG) which is formed predominantly from the glycolytic intermediates dihydroxyacetone- and glyceraldehyde-3-phosphates. MG rapidly glycates proteins, damages mitochondria and induces a pro-oxidant state, similar to that observed in aged cells. It is suggested that because DR animals' energy metabolism is less glycolytic than in those fed ad libitum, intracellular MG levels are lowered by DR The decreased glycolysis during DR may delay senescence by lowering intracellular MG concentration compared to ad libitum-fed animals. Because of the reactivity MG and glycolytic intermediates, occasional glycolysis could be hormetic where glyoxalase, carnosine synthetase and ornithine decarboxylase are upregulated to control cellular MG concentration. It is suggested that in ad libitum-fed animals persistent glycolysis permanently raises MG levels which progressively overwhelm protective processes, particularly in non-mitotic tissues, to create the senescent state earlier than in DR animals. The possible impact of diet and intracellular glycating agents on age-related mitochondrial dysfunction is also discussed.

Glycation, ageing and carnosine: Are carnivorous diets beneficial?

Non-enzymic protein glycosylation (glycation) plays important roles in ageing and in diabetes and its secondary complications. Dietary constituents may play important roles in accelerating or suppressing glycation. It is suggested that carnivorous diets contain a potential anti-glycating agent, carnosine (beta-alanyl-histidine), whilst vegetarians may lack intake of the dipeptide. The possible beneficial effects of carnosine and related structures on protein carbonyl stress, AGE formation, secondary diabetic complications and age-related neuropathology are discussed.

Phase modulation in dipolar-coupled A2 spin systems: effect of maximum state mixing in 1H NMR in vivo

Coupling constants of nuclear spin systems can be determined from phase modulation of multiplet resonances. Strongly coupled systems such as citrate in prostatic tissue exhibit a more complex modulation than AX connectivities, because of substantial mixing of quantum states. An extreme limit is the coupling of n isochronous spins (An system). It is observable only for directly connected spins like the methylene protons of creatine and phosphocreatine which experience residual dipolar coupling in intact muscle tissue in vivo. We will demonstrate that phase modulation of this "pseudo-strong" system is quite simple compared to those of AB systems. Theory predicts that the spin-echo experiment yields conditions as in the case of weak interactions, in particular, the phase modulation depends linearly on the line splitting and the echo time.