31P NMR saturation transfer measurements of phosphorus exchange reactions in rat heart and kidney in situ

Protein and peptide engineering for chemical exchange saturation transfer imaging in the age of synthetic biology

At the beginning of the millennium, the first chemical exchange saturation transfer (CEST) contrast agents were bio-organic molecules. However, later, metal-based CEST agents (paraCEST agents) took center stage. This did not last too long as paraCEST agents showed limited translational potential. By contrast, the CEST field gradually became dominated by metal-free CEST agents. One branch of research stemming from the original work by van Zijl and colleagues is the development of CEST agents based on polypeptides. Indeed, in the last 2 decades, tremendous progress has been achieved in this field. This includes the design of novel peptides as biosensors, genetically encoded recombinant as well as synthetic reporters. This was a result of extensive characterization and elucidation of the theoretical requirements for rational designing and engineering of such agents. Here, we provide an extensive overview of the evolution of more precise protein-based CEST agents, review the rationalization of enzyme-substrate pairs as CEST contrast enhancers, discuss the theoretical considerations to improve peptide selectivity, specificity and enhance CEST contrast. Moreover, we discuss the strong influence of synthetic biology on the development of the next generation of protein-based CEST contrast agents.

31P ParaCEST: 31P MRI-CEST Imaging Based on the Formation of a Ternary Adduct between Inorganic Phosphate and Eu-DO3A

Development of the field of magnetic resonance imaging (MRI) chemical exchange saturation transfer (CEST) contrast agents is hampered by the limited sensitivity of the technique. In water, the high proton concentration allows for an enormous amplification of the exchanging proton pool. However, the 1H CEST in water implies that the number of nuclear spins of the CEST-generating species has to be in the millimolar range. The use of nuclei other than a proton allows exploitation of signals different from that of water, thus lowering the concentration of the exchanging pool as the source of the CEST effect. In this work, we report on the detection of a 31P signal from endogenous inorganic phosphate (Pifree) as the source of CEST contrast by promoting its exchange with the Pi bound to the exogenous complex 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (Pibound). The herein-reported results demonstrate that this approach can improve the detectability threshold by 3 orders of magnitude with respect to the conventional 1H CEST detection (considered per single proton). This achievement reflects the decrease of the bulk concentration of the detected signal from 111.2 M (water) to 10 mM (Pi). This method paves the way to a number of biological studies and clinically translatable applications, herein addressed with a proof-of-concept in the field of cellular imaging.

In-cell NMR: Why and how?

NMR spectroscopy has been applied to cells and tissues analysis since its beginnings, as early as 1950. We have attempted to gather here in a didactic fashion the broad diversity of data and ideas that emerged from NMR investigations on living cells. Covering a large proportion of the periodic table, NMR spectroscopy permits scrutiny of a great variety of atomic nuclei in all living organisms non-invasively. It has thus provided quantitative information on cellular atoms and their chemical environment, dynamics, or interactions. We will show that NMR studies have generated valuable knowledge on a vast array of cellular molecules and events, from water, salts, metabolites, cell walls, proteins, nucleic acids, drugs and drug targets, to pH, redox equilibria and chemical reactions. The characterization of such a multitude of objects at the atomic scale has thus shaped our mental representation of cellular life at multiple levels, together with major techniques like mass-spectrometry or microscopies. NMR studies on cells has accompanied the developments of MRI and metabolomics, and various subfields have flourished, coined with appealing names: fluxomics, foodomics, MRI and MRS (i.e. imaging and localized spectroscopy of living tissues, respectively), whole-cell NMR, on-cell ligand-based NMR, systems NMR, cellular structural biology, in-cell NMR… All these have not grown separately, but rather by reinforcing each other like a braided trunk. Hence, we try here to provide an analytical account of a large ensemble of intricately linked approaches, whose integration has been and will be key to their success. We present extensive overviews, firstly on the various types of information provided by NMR in a cellular environment (the "why", oriented towards a broad readership), and secondly on the employed NMR techniques and setups (the "how", where we discuss the past, current and future methods). Each subsection is constructed as a historical anthology, showing how the intrinsic properties of NMR spectroscopy and its developments structured the accessible knowledge on cellular phenomena. Using this systematic approach, we sought i) to make this review accessible to the broadest audience and ii) to highlight some early techniques that may find renewed interest. Finally, we present a brief discussion on what may be potential and desirable developments in the context of integrative studies in biology.

Time-dependent effect of ethanol force-feeding on glycogen repletion: NMR evidence of a link with ATP turnover in rat liver

The purpose was to study the hepatic effects of low-dose ethanol on the links between ATP and glycogen production. Fasted male Wistar rats received a single force-feeding of glucose plus ethanol or isocaloric glucose. At different times after force-feeding (0-10 h), glycogen repletion and ATP characteristics (content, apparent catalytic time constant, mitochondrial turnover) were monitored by (13)C- or (31)P-nuclear magnetic resonance (NMR) in perfused and isolated liver. In vivo glycogen repletion after force-feeding was slower after glucose plus ethanol vs. glucose (12.04 ± 0.68 and 8.50 ± 0.86 μmol/h/g liver wet weight [ww], respectively), reaching a maximum at the 6th hour. From the 3rd to the 8th hour, glycogen content was lower after glucose plus ethanol vs. glucose. After glucose plus ethanol, the correlation between glycogen and ATP contents presented two linear steps: before and after the 3rd hour (30 and 102 μmol glycogen/g ww per μmol ATP/g ww, respectively, the latter being near the single step measured in glucose). After glucose plus ethanol, ATP turnover remained stable for 2 h, was 3-fold higher from the 3rd hour to the 8th hour, and was higher than after glucose (2.59 ± 0.45 and 1.39 ± 0.19 μmol/min/g ww, respectively). In the 1st hour, glucose plus ethanol induced a transient acidosis and an increase in the phosphomonoesters signal. In conclusion, after ethanol consumption, a large part of the ATP production was diverted to redox re-equilibrium during the first 2 h, thereby reducing the glycogen synthesis. Thereafter, the maintenance of a large oxidative phosphorylation allowed the stimulation of glycogen synthesis requiring ATP.

Resveratrol mainly stimulates the glycolytic ATP synthesis flux and not the mitochondrial one: a saturation transfer NMR study in perfused and isolated rat liver

Our aim was to monitor the effects of resveratrol (RSV) on the respective contribution of glycolysis and oxidative phosphorylation on the unidirectional flux of ATP synthesis in whole isolated rat liver perfused with Krebs-Henseleit Buffer (KHB). The rate of tissular ATP supply was measured directly by monitoring the chemical exchange Pi toward ATP with saturation transfer (ST) (31)P nuclear magnetic resonance, a method applied for the first time for studying the effects of RSV. ST allows the measurement of the total cellular Pi→ATP chemical exchange; after specific inhibition of glycolysis with iodacetate, ST could provide the Pi→ATP flux issued from mitochondria. This latter was compared to mitochondrial ATP turn-over evaluated after chemical ischemia (CI), performed with specific inhibition (KCN) of oxidative phosphorylation, and measured by standard (31)P NMR spectroscopy. In controls (KHB alone), the apparent time constant (ks) of Pi exchange toward ATP as measured by ST was 0.48±0.04s(-1) leading to a total ATP synthesis rate of 37±3.9μmolmin(-1)g(-1). KHB+RSV perfusion increased ks (+52%; p=0.0009 vs. KHB) leading to an enhanced rate of total ATP synthesis (+52%; p=0.01 vs. KHB). When glycolysis was previously inhibited in KHB, both ks and ATP synthesis flux dramatically decreased (-87% and -86%, respectively, p<0.0001 vs. KHB without inhibition), evidencing a collapse of Pi-to-ATP exchange. However, glycolysis inhibition in KHB+RSV reduced to less extent ks (-41%, p=0.0005 vs. KHB+RSV without inhibition) and ATP synthesis flux (-18%). Using the CI method in KHB and KHB+RSV, KCN addition after glycolysis inhibition induced a rapid fall to zero of the ATP content. The mitochondrial ATP turnover R(t0) and its time constant kd mito were similar in KHB (1.18±0.19μmolmin(-1)g(-1) and 0.91±0.13min(-1)) and KHB+RSV (1.36±0.26μmolmin(-1)g(-1) and 0.77±0.18min(-1)). Since mitochondrial ATP turnover was not increased by RSV, the stimulation of Pi-to-ATP exchange by RSV mainly reflected an increase in glycolytic ATP synthesis flux. Moreover, the maintenance by RSV of a high level of Pi-to-ATP exchange after glycolysis inhibition evidenced a protective effect of the polyphenol, in agreement with our previous hypothesis of a stimulation of substrate flux throughout the glycolysis 3-carbon step.

Early development of arterial spin labeling to measure regional brain blood flow by MRI

Two major avenues of work converged in the late 1980's and early 1990's to give rise to brain perfusion MRI. The development of anatomical brain MRI quickly had as a major goal the generation of angiograms using tricks to label flowing blood in macroscopic vessels. These ideas were aimed at getting information about microcirculatory flow as well. Over the same time course the development of in vivo magnetic resonance spectroscopy had as its primary goal the assessment of tissue function and in particular, tissue energetics. For this the measurement of the delivery of water to tissue was critical for assessing tissue oxygenation and viability. The measurement of the washin/washout of "freely" diffusible tracers by spectroscopic based techniques pointed the way for quantitative approaches to measure regional blood flow by MRI. These two avenues came together in the development of arterial spin labeling (ASL) MRI techniques to measure regional cerebral blood flow. The early use of ASL to measure brain activation to help verify BOLD fMRI led to a rapid development of ASL based perfusion MRI. Today development and applications of regional brain blood flow measurements with ASL continues to be a major area of activity.

Measurement of creatine kinase reaction rate in human brain using magnetization transfer image-selected in vivo spectroscopy (MT-ISIS) and a volume ³¹P/¹H radiofrequency coil in a clinical 3-T MRI system

High-energy phosphate metabolism, which allows the synthesis and regeneration of adenosine triphosphate (ATP), is a vital process for neuronal survival and activity. In particular, creatine kinase (CK) serves as an energy reservoir for the rapid buffering of ATP levels. Altered CK enzyme activity, reflecting compromised high-energy phosphate metabolism or mitochondrial dysfunction in the brain, can be assessed using magnetization transfer (MT) MRS. MT (31)P MRS has been used to measure the forward CK reaction rate in animal and human brain, employing a surface radiofrequency coil. However, long acquisition times and excessive radiofrequency irradiation prevent these methods from being used routinely for clinical evaluations. In this article, a new MT (31)P MRS method is presented, which can be practically used to measure the CK forward reaction rate constant in a clinical MRI system employing a volume head (31)P coil for spatial localization, without contamination from the scalp muscle, and an acquisition time of 30 min. Other advantages associated with the method include radiofrequency homogeneity within the regions of interest of the brain using a volume coil with image-selected in vivo spectroscopy localization, and reduction of the specific absorption rate using nonadiabatic radiofrequency pulses for MT saturation. The mean value of k(f) was measured as 0.320 ± 0.075 s(-1) from 10 healthy volunteers with an age range of 18-40 years. These values are consistent with those obtained using earlier methods, and the technique may be used routinely to evaluate energetic processes in the brain on a clinical MRI system.

31P saturation transfer spectroscopy predicts differential intracellular macromolecular association of ATP and ADP in skeletal muscle

The kinetics of phosphoryl exchange involving ATP and ADP have been investigated successfully by in vivo (31)P magnetic resonance spectroscopy using magnetization transfer. However, magnetization transfer effects seen on the signals of ATP also could arise from intramolecular cross-relaxation. This relaxation process carries information on the association state of ATP in the cell. To disentangle contributions of chemical exchange and cross-relaxation to magnetization transfer effects seen in (31)P magnetic resonance spectroscopy of skeletal muscle, we performed saturation transfer experiments on wild type and double-mutant mice lacking the cytosolic muscle creatine kinase and adenylate kinase isoforms. We find that cross-relaxation, observed as nuclear Overhauser effects (NOEs), is responsible for magnetization transfer between ATP phosphates both in wild type and in mutant mice. Analysis of (31)P relaxation properties identifies these effects as transferred NOEs, i.e. underlying this process is an exchange between free cellular ATP and ATP bound to slowly rotating macromolecules. This explains the β-ATP signal decrease upon saturation of the γ-ATP resonance. Although this usually is attributed to β-ADP ↔ β-ATP phosphoryl exchange, we did not detect an effect of this exchange on the β-ATP signal as expected for free [ADP], derived from the creatine kinase equilibrium reaction. This indicates that in resting muscle, conditions prevail that prevent saturation of β-ADP spins and puts into question the derivation of free [ADP] from the creatine kinase equilibrium. We present a model, matching the experimental result, for ADP ↔ ATP exchange, in which ADP is only transiently present in the cytosol.

Kinetic analysis of hyaluronidase activity using a bioactive MRI contrast agent

One of the attractions of molecular imaging using 'smart' bioactive contrast agents is the ability to provide non-invasive data on the spatial and temporal changes in the distribution and expression patterns of specific enzymes. The tools developed for that aim could potentially also be developed for functional imaging of enzyme activity itself, through quantitative analysis of the rapid dynamics of enzymatic conversion of these contrast agents. High molecular weight hyaluronan, the natural substrate of hyaluronidase, is a major antiangiogenic constituent of the extracellular matrix. Degradation by hyaluronidase yields low molecular weight fragments, which are proangiogenic. A novel contrast material, HA-GdDTPA-beads, was designed to provide a substrate analog of hyaluronidase in which relaxivity changes are induced by enzymatic degradation. We show here a first-order kinetic analysis of the time-dependent increase in R(2) as a result of hyaluronidase activity. The changes in R(2) and the measured relaxivity of intact HA-GdDTPA-beads (r(2B)) and HA-GdDTPA fragments (r(2D)) were utilized for derivation of the temporal drop in concentration of GdDTPA in HA-GdDTPA-beads as the consequence of the release of HA-GdDTPA fragments. The rate of dissociation of HA-GdDTPA from the beads showed typical bell-shaped temperature dependence between 7 and 36 degrees C with peak activity at 25 degrees C. The tools developed here for quantitative dynamic analysis of hyaluronidase activity by MRI would allow the use of activation of HA-GdDTPA-beads for the determination of the role of hyaluronidase in altering the angiogenic microenvironment of tumor micro metastases.

CK flux or direct ATP transfer: versatility of energy transfer pathways evidenced by NMR in the perfused heart

How the myocardium is able to permanently coordinate its intracellular fluxes of ATP synthesis, transfer and utilization is difficult to investigate in the whole organ due to the cellular complexity. The adult myocardium represents a paradigm of an energetically compartmented cell since 50% of total CK activity is bound in the vicinity of other enzymes (myofibrillar sarcolemmal and sarcoplasmic reticulum ATPases as well as mitochondrial adenine nucleotide translocator, ANT). Such vicinity of enzymes is well known in vitro as well as in preparations of skinned fibers to influence the kinetic properties of these enzymes and thus the functioning of the subcellular organelles. Intracellular compartmentation has often been neglected in the NMR analysis of CK kinetics in the whole organ. It is indeed a methodological challenge to reveal subcellular kinetics in a working organ by a global approach such as NMR. To get insight in the energy transfer pathway in the perfused rat heart, we developed a combined analysis of several protocols of magnetization transfer associated with biochemical data and quantitatively evaluated which scheme of energetic exchange best describes the NMR data. This allows to show the kinetic compartmentation of subcellular CKs and to quantify their fluxes. Interestingly, we could show that the energy transfer pathway shifts from the phosphocreatine shuttle in the oxygenated perfused heart to a direct ATP diffusion from mitochondria to cytosol under moderate inhibition of ATP synthesis. Furthermore using NMR measured fluxes and the known kinetic properties of the enzymes, it is possible to model the system, estimate local ADP concentrations and propose hypothesis for the versatility of energy transfer pathway. In the normoxic heart, a 3-fold ADP gradient was found between mitochondrial intermembrane space, cytosol and ADP in the vicinity of ATPases. The shift from PCr to ATP transport observed when ATP synthesis decreases might result from a balance in the activity of two populations of ANT, either coupled or uncoupled to CK. We believe this NMR approach could be a valuable tool to reinvestigate the control of respiration by ADP in the whole heart reconciling the biochemical knowledge of mitochondrial obtained in vitro or in skinned fibers with data on the whole heart as well as to identify the implication of bioenergetics in the pathological heart.

Measurement of unidirectional Pi to ATP flux in human visual cortex at 7 T by using in vivo 31P magnetic resonance spectroscopy

Taking advantage of the high NMR detection sensitivity and the large chemical shift dispersion offered by ultra-high field strength of 7 T, the effect of magnetization transfer on inorganic phosphate (Pi) resonance during saturation of gamma-ATP resonance, mediated by the ATP synthesis reaction, was observed noninvasively in the human primary visual cortex by using in vivo 31P magnetic resonance spectroscopy. The unidirectional flux from Pi to ATP was measured by using progressive saturation transfer experiments. The cerebral ATP synthesis rate in the human primary visual cortex measured by 31P magnetic resonance spectroscopy in this study was 12.1 +/- 2.8 micromol ATP/g per min, which agreed well with the value that was calculated indirectly from the cerebral metabolic rate of glucose consumption reported previously.

31P NMR detection of subcellular creatine kinase fluxes in the perfused rat heart: contractility modifies energy transfer pathways

The subcellular fluxes of exchange of ATP and phosphocreatine (PCr) between mitochondria, cytosol, and ATPases were assessed by (31)P NMR spectroscopy to investigate the pathways of energy transfer in a steady state beating heart. Using a combined analysis of four protocols of inversion magnetization transfer associated with biochemical data, three different creatine kinase (CK) activities were resolved in the rat heart perfused in isovolumic control conditions: (i) a cytosolic CK functioning at equilibrium (forward, F(f) = PCr --> ATP, and reverse flux, F(r) = ATP --> PCr = 3.3 mm.s(-1)), (ii) a CK localized in the vicinity of ATPases (MM-CK bound isoform) favoring ATP synthesis (F(f) = 1.7 x F(r)), and (iii) a mitochondrial CK displaced toward PCr synthesis (F(f) = 0.3 and F(r) = 2.6 mm.s(-1)). This study thus provides the first experimental evidence that the energy is carried from mitochondria to ATPases by PCr (i.e. CK shuttle) in the whole heart. In contrast, a single CK functioning at equilibrium was sufficient to describe the data when ATP synthesis was partly inhibited by cyanide (0.15 mm). In this case, ATP was directly transferred from mitochondria to cytosol suggesting that cardiac activity modified energy transfer pathways. Bioenergetic implications of the localization and activity of enzymes within myocardial cells are discussed.

Four-angle saturation transfer (FAST) method for measuring creatine kinase reaction rates in vivo

A new fast method of measuring kinetic reaction rates for two-site chemical exchange is described. The method employs saturation transfer magnetic resonance spectroscopy (MRS) and acquisition of only four spectra under partially saturated, high signal-to-noise ratio (SNR) conditions. In two acquisitions one of the exchanging species is saturated; the other two employ a control saturation. Each pair of acquisitions is applied with two different flip angles, and the equilibrium magnetization, relaxation times, and reaction rates are calculated therefrom. This four-angle saturation transfer (FAST) method is validated theoretically using the Bloch equations modified for two-state chemical exchange. Potential errors in the rate measurements due to the effects of exchange are evaluated for creatine kinase (CK) metabolism modeled for skeletal and heart muscle, and are found to be < 5% for forward CK flux rates of 0.05 < or = k(f) < or = 1.0 s(-1), and up to a 90% depletion of phosphocreatine (PCr). The effect of too much or too little saturating irradiation on FAST appears to be comparable to that of the conventional saturation transfer method, although the relative performance deteriorates when spillover irradiation cuts the PCr signal by 50% or more. "FASTer" and " FASTest" protocols are introduced for dynamic CK studies wherein [PCr] and/or k(f) changes. These protocols permit the omission of one or two of the four acquisitions in repeat experiments, and the missing information is recreated from initial data via a new iterative algorithm. The FAST method is validated empirically in phosphorus ((31)P) MRS studies of human calf muscle at 1.5 T. FAST measurements of 10 normal volunteers yielded the same CK reaction rates measured by the conventional method (0.29 +/- 0.06 s(-1)) in the same subjects, but an average of seven times faster. Application of the FASTer algorithm to these data correctly restored missing information within seven iterations. Finally, the FAST method was combined with 1D spatially localized (31)P MRS in a study of six volunteers, yielding the same k(f) values independent of depth, in total acquisition times of 17-39 min. These timesaving FAST methods are enabling because they permit localized measurements of metabolic flux, which were previously impractical due to intolerably long scan times.

Discrimination of cardiac subcellular creatine kinase fluxes by NMR spectroscopy: a new method of analysis

A challenge in the understanding of creatine kinase (CK) fluxes reflected by NMR magnetization transfer in the perfused rat heart is the choice of a kinetic model of analysis. The complexity of the energetic pathways, due to the presence of adenosine triphosphate (ATP)-inorganic phosphate (Pi) exchange, of metabolite compartmentation and of subcellular localization of CK isozymes cannot be resolve from the sole information obtained from a single NMR protocol. To analyze multicompartment exchanges, we propose a new strategy based on the simultaneous analysis of four inversion transfer protocols. The time course of ATP and Phosphocreatine (PCr) magnetizations computed from the McConnell equations were adjusted to their experimental value for exchange networks of increasing complexity (up to six metabolite pools). Exchange schemes were selected by the quality of their fit and their consistency with data from other sources: the size of mitochondrial pools and the ATP synthesis flux. The consideration of ATP-Pi exchange and of ATP compartmentation were insufficient to describe the data. The most appropriate exchange scheme in our normoxic heart involved the discrimination of three specific CK activities (cytosolic, mitochondrial, and close to ATPases). At the present level of heart contractility, the energy is transferred from mitochondria to myofibrils mainly by PCr.

Cardiac creatine kinase metabolite compartments revealed by NMR magnetization transfer spectroscopy and subcellular fractionation

In the perfused rat heart NMR inversion transfer revealed the existence of a compartment of ATP not exchanging through creatine kinase (CK), as demonstrated by an apparent discrepancy between the forward (F(f)) and reverse (F(r)) CK flux if this compartment was neglected in the analysis [Joubert et al. (2000) Biophys. J. 79, 1-13]. To localize this compartment, CK fluxes were measured by inversion of PCr (inv-PCr) or gamma ATP (inv-ATP), and the distribution of metabolites between mitochondria and cytosol was studied by subcellular fractionation. Physiological conditions were designed to modify the concentration and distribution of CK metabolites (control, adenylate depletion, inhibition of respiration, KCl arrest). Depending on cardiac activity, mitochondrial ATP (mito-ATP) assessed by fractionation varied from 11% to 30% of total ATP. In addition, the apparent flux discrepancy increased together with mito-ATP (F(f)/F(r) ranged from 0.85 to 0.50 in inv-PCr and from 1.13 to 1.88 in inv-ATP). Under conditions masking the influence of the ATP-P(i) exchange on CK flux, the ATP compartment could be directly quantified by the apparent flux discrepancy; its size was similar to that of mito-ATP measured by fractionation. Thus NMR inversion technique is a potential tool to assess metabolite compartmentation in the whole organ.

Lipoic acid reduces ischemia-reperfusion injury in animal models

Hypoxia and reoxygenation were studied in rat hearts and ischemia and reperfusion in rat hindlimbs. Free radicals are known to be generated through these events and to propagate complications. In order to reduce hypoxic/ischemic and especially reoxygenation/reperfusion injury the (re)perfusion conditions were ameliorated including the treatment with antioxidants (lipoate or dihydrolipoate). In isolated working rat hearts cardiac and mitochondrial parameters are impaired during hypoxia and partially recover in reoxygenation. Dihydrolipoate, if added into the perfusion buffer at 0.3 microM concentration, keeps the pH higher (7. 15) during hypoxia as compared to controls (6.98). The compound accelerates the recovery of the aortic flow and stabilizes it during reoxygenation. With dihydrolipoate, ATPase activity is reduced, ATP synthesis is increased and phosphocreatine contents are higher than in controls. Creatine kinase activity is maintained during reoxygenation in the dihydrolipoate series. Isolated rat hindlimbs were stored for 4 h in a moist chamber at 18 degrees C. Controls were perfused for 30 min with a modified Krebs-Henseleit buffer at 60 mmHg followed by 30 min Krebs-Henseleit perfusion at 100 mmHg. The dihydrolipoate group contained 8.3 microM in the modified reperfusate (controlled reperfusion). With dihydrolipoate, recovery of the contractile function was 49% (vs. 34% in controls) and muscle flexibility was maintained whereas it decreased by 15% in the controls. Release of creatine kinase was significantly lower with dihydrolipoate treatment. Dihydrolipoate effectively reduces reoxygenation injury in isolated working rat hearts. Controlled reperfusion, including lipoate, prevents reperfusion syndrome after extended ischemia in exarticulated rat hindlimbs and in an in vivo pig hindlimbs model.

Evidence for myocardial ATP compartmentation from NMR inversion transfer analysis of creatine kinase fluxes

The interpretation of creatine kinase (CK) flux measured by (31)P NMR magnetization transfer in vivo is complex because of the presence of competing reactions, metabolite compartmentation, and CK isozyme localization. In the isovolumic perfused rat heart, we considered the influence of both ATP compartmentation and ATP-P(i) exchange on the forward (F(f): PCr --> ATP) and reverse (F(r)) CK fluxes derived from complete analysis of inversion transfer. Although F(f) should equal F(r) because of the steady state, in both protocols when PCr (inv-PCr) or ATP (inv-ATP) was inverted and the contribution of ATP-P(i) was masked by saturation of P(i) (sat-P(i)), F(f)/F(r) significantly differed from 1 (0.80 +/- 0.06 or 1.32 +/- 0.06, respectively, n = 5). These discrepancies could be explained by a compartment of ATP (f(ATP)) not involved in CK. Consistently, neglecting ATP compartmentation in the analysis of CK in vitro results in an underestimation of F(f)/F(r) for inv-PCr and its overestimation for inv-ATP. Both protocols gave access to f(ATP) if the system was adequately analyzed. The fraction of ATP not involved in CK reaction in a heart performing medium work amounts to 20-33% of cellular ATP. Finally, the data suggest that the effect of sat-P(i) might not result only from the masking of ATP-P(i) exchange.

Molecular aspects of magnetic resonance imaging and spectroscopy

Magnetic resonance imaging (MRI) is a well known diagnostic tool in radiology that produces unsurpassed images of the human body, in particular of soft tissue. However, the medical community is often not aware that MRI is an important yet limited segment of magnetic resonance (MR) or nuclear magnetic resonance (NMR) as this method is called in basic science. The tremendous morphological information of MR images sometimes conceal the fact that MR signals in general contain much more information, especially on processes on the molecular level. NMR is successfully used in physics, chemistry, and biology to explore and characterize chemical reactions, molecular conformations, biochemical pathways, solid state material, and many other applications that elucidate invisible characteristics of matter and tissue. In medical applications, knowledge of the molecular background of MRI and in particular MR spectroscopy (MRS) is an inevitable basis to understand molecular phenomenon leading to macroscopic effects visible in diagnostic images or spectra. This review shall provide the necessary background to comprehend molecular aspects of magnetic resonance applications in medicine. An introduction into the physical basics aims at an understanding of some of the molecular mechanisms without extended mathematical treatment. The MR typical terminology is explained such that reading of original MR publications could be facilitated for non-MR experts. Applications in MRI and MRS are intended to illustrate the consequences of molecular effects on images and spectra.

31P NMR saturation transfer study of the creatine kinase reaction in human skeletal muscle at rest and during exercise

The creatine kinase reaction has been studied by 31P NMR in exercising human calf muscle. Quantitative analysis of high energy phosphates and saturation transfer study of the creatine kinase flux in the direction of ATP synthesis (Vfor) were performed at rest and during exercise. As expected, exercise induced a [PCr] decrease (from 28.5 +/- 0.9 to 21.9 +/- 1.5 mM, P < 0.01) matched by a Pi increase (from 4.5 +/- 0.2 to 8.9 +/- 1.8 mM, P = 0.06). pHi and [ATP] remained unchanged. Vfor did not change from rest (12.4 +/- 0.9 mM s(-1)) to moderate exercise and decreased at the highest exercise level (8.4 +/- 1.4 mM s(-1), P = 0.006). This observation differs from the prediction of the creatine kinase rate equation, showing an increase in the flux with exercise intensity. Computations suggest that this discrepancy arises from metabolite compartmentalization and/or from the reaction kinetics of a dead end complex stabilized by planar anions.

Kinetics of ATP to ADP beta-phosphoryl conversion in contracting skeletal muscle by in vivo 31P NMR magnetization transfer

The rate constant of the beta-adenosine triphosphate to beta-adenosine diphosphate conversion was measured using 31P nuclear magnetic resonance magnetization transfer in resting and contracting in vivo rat skeletal muscle. Theoretically, the rate constant should be the sum of the rate constants of the reactions catalyzing ATP-ADP exchange. In resting muscle, the conversion rate constant was 0.4 s-1 and beta-ATP intrinsic T1 was 1.7 s. The velocity of conversion was 3.8 mM s-1. During stimulation, phosphocreatine fell to 36% and ATP to 82% of initial values. The rate constant and velocity of beta-phosphoryl conversion increased to 0.8 s-1 and 6.3 mM s-1, respectively, but did not reach expected levels, i.e. the product of the ATP concentration with the sum of pseudo first-rate constants of the individual reactions. These conversion velocities should be higher than reverse creatine kinase velocities, previously measured to be 10 mM s-1 in resting muscle and 7.5 mM s-1 in contacting muscle and confirmed in this work. The discrepancy between expected and observed data could be due either to compartmentation of part of the beta-ATP in pools exchanging slowly with the bulk of cellular ATP, or to ADP binding to macromolecules thus preventing full ADP saturation during magnetization transfer.

pH and temperature effects on kinetics of creatine kinase in aqueous solution and in isovolumic perfused heart. A 31P nuclear magnetization transfer study

Phosphorylated metabolites concentrations and creatine kinase kinetics are measured by 31P NMR in solution and in isovolumic perfused rat hearts submitted to hypo- and hyperthermia as well as to acidosis (37 degrees C). In the organ, temperature variation from 40 to 25 degrees C induces an increase of phosphocreatine (PCr) stores, a decrease of Pi and ADP concentrations, but does not affect the ATP pool. Creatine kinase forward flux (Vfor) is gradually reduced when the temperature is lowered both in vitro and in perfused heart. In normothermic and hypothermic conditions, a relationship is found between cardiac performance (rate pressure product, RPP), Vfor and ATP synthesis estimated through the myocardial oxygen consumption rate (MVO2). At 40 degrees C however, the RPP is reduced although both Vfor and MVO2 increase. In vitro experiments show an optimum pH of 7.7 for the forward creatine kinase reaction. In perfused heart submitted to acidosis, a decrease of PCr concentration is observed, whereas ATP and ADP contents remain unchanged. Heart creatine kinase flux increased as in hyperthermia. These high fluxes are attributed to the coupling of the creatine kinase reaction with energy consuming or producing reactions: the increase of energy demand related to non-contractile processes could explain the high MVO2 and Vfor observed in those conditions.

Identification of glycerophosphorylcholine in mussel ovarian extracts by two-dimensional nuclear magnetic resonance

The abundance of the "phosphodiester" peak in differentiating or proliferating tissues, including reproductive organs and tumors, warrants further investigations of its metabolic role(s), which would require a rigorous confirmation of its identity. The assignment of this peak to glycerophosphorylcholine in 31P NMR spectra of biological samples has been largely based on chemical shift, which can result in ambiguities. We employed a combination of two-dimensional 31P-1H heteronuclear shift correlation and 1H total correlation spectroscopies to trace the spin connectivities of glycerophosphorylcholine and thus to identify its structure directly from crude ovarian extracts of mussels without ambiguities and the need for extensive purification. This approach can be applied generally to the identification of molecules containing heteroatoms in crude tissue extracts.

Intracellular compartmentation, structure and function of creatine kinase isoenzymes in tissues with high and fluctuating energy demands: the 'phosphocreatine circuit' for cellular energy homeostasis

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Combination of 31P-NMR magnetization transfer and radioisotope exchange methods for assessment of an enzyme reaction mechanism: rate-determining steps of the creatine kinase reaction

The theoretical analysis of a reversible enzyme reaction performed in this work shows that the 31P-NMR magnetization (saturation) transfer technique combined with a radioisotope exchange method may potentially provide information on the position of rate-determining step(s). It depends on chemical shifts of NMR signals of nuclei of interest in free and enzyme-bound forms of substrate(s) and product(s) of the reaction. The creatine kinase reaction (MgATP + creatine----MgADP + P-creatine) has been used as a model. Chemical shifts of 31P in binary, ternary and transitional state substrate-enzyme complexes have been estimated by the variable frequency saturation transfer (VFST) method. This method is based on selective irradiation of numerous points in the spectrum and observation of changes in the intensity of visible line(s) which occur due to chemical exchange between it and lines which are not visible in the routine spectrum. Also, dissociation rate constants of MgADP-containing complexes were determined. Magnetization exchange rates, P-creatine----[gamma-P]MgATP and [beta-P]MgADP----[beta-P]MgATP, were compared with radioisotope exchange rates, [gamma-32P-MgATP----P-creatine and [3H]MgADP----MgATP at different [P-creatine]/[creatine] ratios and at different temperatures. All these exchange rates were close to each other at 30-37 degrees C and [PCr]/[Cr] ratios lower than 2. It is concluded that phosphoryl group transfer is the rate-determining step of the overall creatine kinase reaction under these conditions. However, at lower temperatures (below 25 degrees C) or at high [PCr]/[Cr] ratios ([ADP] less than 20 microM) the rate-determining step seems to be shifted toward dissociation of nucleotide substrates from enzyme-substrate complexes, since exchange rates became significantly different. This approach is useful for analysis of mechanism of enzymatic reactions and also can be applied to non-enzymatic reactions and evaluation of small rapidly exchangeable metabolite pools.

NMR studies of phosphate metabolism in the isolated perfused kidney of developing rats

During growth, the capacity for renal phosphate (Pi) reabsorption varies as a function of Pi demand. These changes occur in the absence of changes in extracellular concentration of Pi and are also observed in renal cells cultured in defined media. These findings suggest a direct regulatory effect of intracellular Pi on its transport systems. We postulate that a low intracellular Pi concentration [( Pi]i) occurs in the developing kidney as a consequence of differences in Pi metabolism between growing and mature cells and that a low [Pi]i, in turn, leads to adaptive changes in renal Pi transport. In order to assess this hypothesis, we used 31P-nuclear magnetic resonance (NMR) to measure the intracellular concentrations of NMR-visible Pi and phospho-metabolites and the rates of Pi turnover due to adenosine triphosphate (ATP) synthesis, in isolated perfused kidneys of 3- to 4-week-old and 12- to 13-week-old rats. The [Pi]i was lower (1.7 +/- 0.1 vs 2.7 +/- 0.1 mM, P less than 0.05) in kidneys of growing than of adult rats, while the ATP (2.9 +/- 0.3 vs 2.8 +/- 0.5 mM) and adenosine diphosphate (ADP) (-0.2 mM) concentrations were similar at the two ages, consistent with a high phosphorylation potential in the kidneys of the younger animals. Radiofrequency irradiation of the gamma-P of ATP resulted in reduction in the intensity of the Pi resonance of 62 +/- 5% in the newborn and 38 +/- 3% in the adult (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

31P NMR studies of ATP concentrations and Pi-ATP exchange in the rat kidney in vivo: effects of inhibiting and stimulating renal metabolism

Previous investigators found that cyanide (CN-) is a potent inhibitor of renal Na+ transport, while the uncoupling agent 2,4-dinitrophenol (DNP) and fructose (both which lower ATP levels) are weak transport inhibitors. To examine the disparate effects of these substances measurements of ATP were performed, using 31P NMR, while simultaneously monitoring renal Na+ transport. Infusion of CN-, DNP, and fructose lowered whole kidney ATP levels by about the same extent (30%) while only CN- inhibited Na+ transport. This may be due to the fact that CN- has a potent action on the thick ascending limb of Henle, while fructose and DNP may have a more proximal action. Alternatively, ATP turnover may be a more important determinant of transport than ATP concentrations. Saturation transfer experiments were performed to measure Pi-ATP flux. Unilateral nephrectomy, high protein feeding, and methylprednisolone were used to stimulate metabolism and transport. The rate of Pi-ATP flux was 20.1 mumol/min/g. However, because oxygen consumption was stimulated, the ATP/O ratio was 0.85, considerably less than the theoretical value of 3. Finally, atrial natriuretic factor, which increased Na+ transport, had no effect on Pi-ATP flux. The results raise the possibility that the saturation transfer technique does not detect all Pi-ATP flux, especially when renal metabolism is stimulated.

Observation of uridine triphosphate:glucose-1-phosphate uridylyltransferase activity in maize root tips by saturation transfer 31P-NMR. Estimation of cytoplasmic PP

Saturation transfer 31P nuclear magnetic resonance was used to estimate the unidirectional rate of phosphorus exchange between Glc-1-P and UDPGlc in maize root tips. The rate was determined to be approx. 4 mumol.min-1 per g fresh weight. This estimated rate is much higher than net rates of other reactions in glucose metabolism (e.g., more than 10-times faster than the maximal glycolytic flux in this tissue). Furthermore, exchange between Glc-1-P and UDPGlc was not significantly inhibited by the metabolic poison KCN. We conclude that the unidirectional rate of conversion of Glc-1P to UDPGlc is much faster than the net rate of UDPGlc synthesis--the UTP:Glc-1-P uridylyltransferase reaction is near-equilibrium in vivo. From the equilibrium constant for this transferase reaction and the concentrations of Glc-1-P, UTP and UDPGlc, the level of cytoplasmic PPi was estimated to be approx. 10 nmol.g-1.

Contributions of nuclear magnetic resonance to study of acute renal failure

Nuclear magnetic resonance (NMR) has contributed considerably to our understanding of experimental acute renal failure. Changes in energy metabolism which are caused by ischemia, urinary obstruction, and nephrotoxic drugs have been characterized with NMR spectroscopy. Data from our laboratory and others utilizing 31P NMR have demonstrated that levels of adenosine triphosphate fall rapidly with ischemia, and that the ability of the kidney to regenerate ATP correlates with ultimate functional recovery. Additionally, development of intracellular acidosis appears to occur early with ischemia and may, if severe enough, predict poor functional recovery. Urinary obstruction is associated with the rapid development of a large peak resonating in the phosphodiester region of the P-31 NMR kidney spectrum which is attributable to increases in urinary inorganic phosphate. Nephrotoxic acute renal failure with a variety of nephrotoxins is associated with little to no changes in high energy phosphates. Renal transplant allograft rejection is associated with energy metabolic changes similar to those seen with ischemia; however, the intracellular pH remains normal. These findings allow causes of experimental acute renal failure to be differentiated among each other in both native and transplanted kidneys. With recent advances in NMR software and hardware, the application of this methodology to human acute renal failure is now possible.

In vivo functioning of creatine phosphokinase in human forearm muscle, studied by 31P NMR saturation transfer

31P nuclear magnetic resonance (NMR) saturation transfer has been used to measure enzymatic flux through the creatine phosphokinase reaction in the direction of ATP synthesis in the human forearm muscle flexor digitorum superficialis. Modification of the ratio method for measurement of spin-lattice relaxation (R. Freeman, H.D.W. Hill, and R. Kaptein, J. Magn. Reson. 7, 82(1972]was tested and used to appreciably shorten the duration of the measurement. Under conditions of steady state work intracellular pH decreased slightly by 0.06 units and the spin-lattice relaxation time of phosphocreatine in muscle was unchanged, while flux from phosphocreatine to ATP was 64 +/- 10% of the resting value. This is contrary to the increase in flux of 155% predicted from previous saturation transfer studies carried out in vitro on rabbit skeletal muscle creatine phosphokinase using metabolite concentrations to mimic those in vivo (E.A. Shoubridge, J.L. Bland, and G.K. Radda, Biochim. Biophys. Acta 805, 72 (1984]. This discrepancy could be accounted for by an underestimation of the ADP concentrations to which the enzyme is exposed due to inaccurate assumptions about the total metabolite concentrations, or possibly by compartmentation of creatine phosphokinase and its reactants.

NMR spectroscopy for clinical medicine. Animal models and clinical examples

Magnetic resonance spectroscopy is able to measure noninvasively a variety of important metabolites involved in cell energetics. These include phosphocreatine, ATP, inorganic phosphate, pH, and lactate. Anoxia, ischemia, and infarction produce rapid loss of high-energy phosphates and accumulation of hydrolysis products. Many animal studies have shown that MRS monitors metabolic changes in various models of human disease. The availability of large, high field magnets and the development of noninvasive localization techniques permits MRS to be performed on selected volumes within the body. It is now clear that MRS in humans will be immediately useful in several areas including studies of malignancy, ischemia, and infarction of various organs and metabolic disorders. It is expected that human MRS will be increasingly used for clinical investigation and eventually for medical diagnosis.

31P-NMR studies of phosphate transfer rates in T47D human breast cancer cells

The concentration of phosphates and the kinetics of phosphate transfer reactions were measured in the human breast cancer cell line, T47D, using 31P-NMR spectroscopy. The cells were embedded in agarose filaments and perifused with oxygenated medium during the NMR measurements. The following phosphates were identified in spectra of perifused cells and of cell extracts: phosphorylcholine (PC), phosphorylethanolamine (PE), the glycerol derivatives of PC and PE, inorganic phosphate (Pi), phosphocreatine (PCr), nucleoside triphosphate (primarily ATP) and uridine diphosphate glucose. The rates of the transfers: PC----gamma ATP (0.2 mM/s), Pi----gamma ATP (0.2 mM/s) and the conversion beta ATP----beta ADP (1.3 mM/s) were determined from analysis of data obtained in steady-state saturation transfer and inversion recovery experiments. Data from spectrophotometric assays of the specific activity of creatine kinase (approx. 0.1 mumol/min per mg protein) and adenylate kinase (approx. 0.4 mumol/min per mg protein) suggest that the beta ATP----beta ADP rate is dominated by the latter reaction. The ratio between the rate of ATP synthesis from Pi and the rate of consumption of oxygen atoms (4 X 10(-3) mM/s) was approx. 50. This high value and preliminary measurements of the rate of lactate production from glucose, indicated that aerobic glycolysis is the main pathway of ATP synthesis.

Saturation-transfer studies of ATP-Pi exchange in isolated perfused rat liver

The rate of exchange between inorganic phosphate and ATP was measured in isolated perfused rat livers in the direction of ATP synthesis using 31P NMR spectroscopy and the saturation-transfer technique. Measurement of ATP hydrolysis was not observable, even after treatment of rats with 100 micrograms T3/day per 100 g body wt. When the perfused livers were treated with iodoacetate in order to inhibit glycolysis, NMR measurable exchange between ATP and Pi was eliminated. It is concluded that the inorganic phosphate----ATP conversion detected by saturation transfer is catalyzed by enzymes of the glycolytic pathway and that the mitochondrial ATPase rate is too slow to contribute to the observed effect.

Metabolic state of the rat liver with ethanol: comparison of in vivo 31phosphorus nuclear magnetic resonance spectroscopy with freeze clamp assessment

In vivo 31phosphorus nuclear magnetic resonance spectroscopy was used to measure the hepatic metabolic state in various groups of rats given ethanol, a control liquid diet or a solid chow diet. The use of selective presaturation pulses applied to the broad phosphorus resonances of immobile phospholipids permitted reliable determination of ATP/ADP ratios by quantitation of the ATP-beta and ATP-gamma peak areas. ATP/ADP ratios were depressed by both techniques in rats chronically ingesting ethanol compared to pair-fed animals consuming the control liquid diet. These differences were observed regardless of whether ethanol feeding was continued up to the time of investigation or whether it was discontinued for 24 hr prior to study. Acute alcohol administration in chow-fed rats, not previously ingesting ethanol, did not lower hepatic ATP/ADP ratios by either methodology. In all cases, liver ATP/ADP ratios assessed by 31phosphorus nuclear magnetic resonance spectroscopy were higher than those measured by high-performance liquid chromatography. However, parallel decreases in hepatic ATP/ADP ratios were observed with chronic ethanol consumption by both 31phosphorus nuclear magnetic resonance spectroscopy and the biochemical method, confirming the utility of in vivo 31phosphorus nuclear magnetic resonance spectroscopy for assessment of the hepatic bioenergetic status. The difference in absolute ATP/ADP ratios by the two methods may to some degree be explained by binding effects of ADP with proteins or mitochondrial membranes, rendering it partially invisible to nuclear magnetic resonance or alternatively, by breakdown of high energy phosphate bonds with freeze clamp extraction.