Probing alanine transaminase catalysis with hyperpolarized 13CD3-pyruvate

Hyperpolarized 15N-labeled, deuterated tris (2-pyridylmethyl)amine as an MRI sensor of freely available Zn2

Dynamic nuclear polarization (DNP) coupled with ¹⁵N magnetic resonance imaging (MRI) provides an opportunity to image quantitative levels of biologically important metal ions such as Zn²⁺, Mg²⁺ or Ca²⁺ using appropriately designed ¹⁵N enriched probes. For example, a Zn-specific probe could prove particularly valuable for imaging the tissue distribution of freely available Zn²⁺ ions, an important known metal ion biomarker in the pancreas, in prostate cancer, and in several neurodegenerative diseases. In the present study, we prepare the cell-permeable, ¹⁵N-enriched, d₆-deuterated version of the well-known Zn²⁺ chelator, tris(2-pyridylmethyl)amine (TPA) and demonstrate that the polarized ligand had favorable T₁ and linewidth characteristics for ¹⁵N MRI. Examples of how polarized TPA can be used to quantify freely available Zn²⁺ in homogenized human prostate tissue and intact cells are presented.

Acquisition strategies for spatially resolved magnetic resonance detection of hyperpolarized nuclei

Hyperpolarization is an emerging method in magnetic resonance imaging that allows nuclear spin polarization of gases or liquids to be temporarily enhanced by up to five or six orders of magnitude at clinically relevant field strengths and administered at high concentration to a subject at the time of measurement. This transient gain in signal has enabled the non-invasive detection and imaging of gas ventilation and diffusion in the lungs, perfusion in blood vessels and tissues, and metabolic conversion in cells, animals, and patients. The rapid development of this method is based on advances in polarizer technology, the availability of suitable probe isotopes and molecules, improved MRI hardware and pulse sequence development. Acquisition strategies for hyperpolarized nuclei are not yet standardized and are set up individually at most sites depending on the specific requirements of the probe, the object of interest, and the MRI hardware. This review provides a detailed introduction to spatially resolved detection of hyperpolarized nuclei and summarizes novel and previously established acquisition strategies for different key areas of application.

Reaction Energetics and 13C Fractionation of Alanine Transamination in the Aqueous and Gas Phases

The alanine transaminase enzyme catalyzes the transfer of an amino group from alanine to α-ketoglutarate to produce pyruvate and glutamate. Isotope fractionation factors (IFFs) for the reaction ⁺H₃NCH(CH₃)COO^- + ^-OOCCH₂CH₂C(O)COO^- ↔ CH₃C(O)COO^- + ⁺H₃NCH(CH₂CH₂COO^-)COO^- (zwitterionic neutral alanine + doubly deprotonated α-ketoglutarate ↔ pyruvate + zwitterionic glutamate anion) were calculated from the partition functions of explicitly and implicitly solvated molecules at 298 K. Calculations were done for alanine (noncharge separated, zwitterion, deprotonated), pyruvic acid (neutral, deprotonated), glutamic acid (noncharge separated, zwitterion, deprotonated, doubly deprotonated), and α-ketoglutaric acid (neutral, deprotonated, doubly deprotonated). The computational results, calculated from gas phase- and aqueous-optimized clusters with explicit H₂O molecules at the MP2/aug-cc-pVDZ and MP2/aug-cc-pVDZ/COSMO levels, respectively, predict that substitution of ¹³C at the C2 position of alanine and pyruvic acid and their various forms leads to the C2 position of pyruvic acid/pyruvate being enriched in ¹³C/¹²C ratio by 9‰. Simpler approaches that estimate the IFFs based solely on changes in the zero-point energies (ZPEs) are consistent with the higher-level model. ZPE-based IFFs calculated for simple analogues formaldehyde and methylamine (analogous to the C2 positions of pyruvate and alanine, respectively) predict a ¹³C enrichment in formaldehyde of 7-8‰ at the MP2/aug-cc-pVDZ and aug-cc-pVTZ levels. A simple predictive model using canonical functional group frequencies and reduced masses for ¹³C exchange between R₂C═O and R₂CH-NH₂ predicted enrichment in R₂C═O that is too large by a factor of two but is qualitatively accurate compared with the more sophisticated models. Our models are all in agreement with the expectation that pyruvate and formaldehyde will be preferentially enriched in ¹³C because of the strength of their >C═O bond relative to that of ≡C-NH₂ in alanine and methylamine. ¹³C/¹²C substitution is also modeled at the methyl and carboxylic acid sites of alanine and pyruvic acid, respectively.

[13C6,D8]2-deoxyglucose phosphorylation by hexokinase shows selectivity for the β-anomer

A non-radioactive 2-deoxyglucose (2DG) analog has been developed here for hyperpolarized magnetic resonance investigations. The analog, [13C6,D8]2DG, showed 13% polarization in solution (27,000-fold signal enhancement at the C1 site), following a dissolution-DNP hyperpolarization process. The phosphorylation of this analog by yeast hexokinase (yHK) was monitored in real-time with a temporal resolution of 1 s. We show that yHK selectively utilizes the β anomer of the 2DG analog, thus revealing a surprising anomeric specificity of this reaction. Such anomeric selectivity was not observed for the reaction of yHK or bacterial glucokinase with a hyperpolarized glucose analog. yHK is highly similar to the human HK-2, which is overexpressed in malignancy. Thus, the current finding may shed a new light on a fundamental enzyme activity which is utilized in the most widespread molecular imaging technology for cancer detection - positron-emission tomography with 18F-2DG.

Effects of deuteration on transamination and oxidation of hyperpolarized 13C-Pyruvate in the isolated heart

This study was designed to determine the effects of deuteration in pyruvate on exchange reactions in alanine aminotransferase (ALT), lactate dehydrogenase (LDH) and flux through pyruvate dehydrogenase (PDH). Although deuteration of a ¹³C enriched substrate is commonly used to increase the lifetime of a probe for hyperpolarization experiments, the potential impact of kinetic isotope effects on such substitutions has not been studied in detail. Metabolism of deuterated pyruvate was investigated in isolated rat hearts. Hearts were perfused with a 1:1 mixture of [U-¹³C₃]pyruvate and [2-¹³C₁]pyruvate or a 1:1 mixture of [U-¹³C₃]pyruvate plus [2-¹³C₁, U-²H₃]pyruvate for 30 min before being freeze clamped. Another set of hearts received [2-¹³C₁, U-²H₃]pyruvate and was freeze-clamped at 3 min or 6 min. Tissue extracts were analyzed by ¹H and ¹³C{¹H} NMR spectroscopy. The chemical shift isotope effect of ²H was monitored in the ¹³C NMR spectra of the C2 resonance of lactate and alanine plus the C5 of glutamate. There was little kinetic isotope effect of ²H in pyruvate on flux through PDH, LDH or ALT as detected by the distribution of ¹³C, but the distribution of ²H differed markedly between alanine and lactate. At steady-state, alanine was a mixture of deuterated species, while lactate was largely perdeuterated. Consistent with results at steady-state, hearts freeze-clamped at 3 min or 6 min showed rapid removal of deuterium in alanine but not in lactate. Metabolism of hyperpolarized [1-¹³C₁]pyruvate was compared to [1-¹³C₁,U-²H₃]pyruvate in isolated hearts. Consistent with the results from tissue extracts, there was little effect of deuteration on the kinetics of appearance of lactate, alanine or bicarbonate, but there was a small, time-dependent upfield chemical shift in the HP[1-¹³C₁]alanine signal reflecting exchange of methyl deuterons with water protons. Together, these results demonstrate that (1) the kinetics of pyruvate metabolism in hearts detected by ¹³C NMR are not affected by replacement of the pyruvate methyl protons with deuterons and (2) that the loss of deuterium from the methyl position occurs rapidly during the conversion of pyruvate to alanine. The majority of the deuterium atoms are lost on the time-scale of a hyperpolarization experiment.

Late-stage deuteration of 13C-enriched substrates for T1 prolongation in hyperpolarized 13C MRI

A robust and selective late-stage deuteration methodology was applied to 13C-enriched amino and alpha hydroxy acids to increase spin-lattice relaxation constant T1 for hyperpolarized 13C magnetic resonance imaging. For the five substrates with 13C-labeling on the C1-position ([1-13C]alanine, [1-13C]serine, [1-13C]lactate, [1-13C]glycine, and [1-13C]valine), significant increase of their T1 was observed at 3 T with deuterium labeling (+26%, 22%, +16%, +25% and +29%, respectively). Remarkably, in the case of [2-13C]alanine, [2-13C]serine and [2-13C]lactate, deuterium labeling led to a greater than four fold increase in T1. [1-13C,2-2H]alanine, produced using this method, was applied to in vitro enzyme assays with alanine aminotransferase, demonstrating a kinetic isotope effect.

Rates of Chemical Reactions Embedded in a Metabolic Network by Dissolution Dynamic Nuclear Polarisation NMR

The isomerisation of 6-phosphogluconolactones and their hydrolyses into 6-phosphogluconic acid form a non enzymatic side cycle of the pentose-phosphate pathway (PPP) in cells. Dissolution dynamic nuclear polarisation can be used for determining the kinetic rates of the involved transformations in real time. It is found that the hydrolysis of both lactones is significantly slower than the isomerisation process, thereby shedding new light onto this subtle chemical process.

The rate of lactate production from glucose in hearts is not altered by per-deuteration of glucose

This study was designed to determine whether perdeuterated glucose experiences a kinetic isotope effect (KIE) as glucose passes through glycolysis and is further oxidized in the tricarboxylic acid (TCA) cycle. Metabolism of deuterated glucose was investigated in two groups of perfused rat hearts. The control group was supplied with a 1:1 mixture of [U-¹³C₆]glucose and [1,6-¹³C₂]glucose, while the experimental group received [U-¹³C₆,U-²H₇]glucose and [1,6-¹³C₂]glucose. Tissue extracts were analyzed by ¹H, ²H and proton-decoupled ¹³C NMR spectroscopy. Extensive ²H-¹³C scalar coupling plus chemical shift isotope effects were observed in the proton-decoupled ¹³C NMR spectra of lactate, alanine and glutamate. A small but measureable (∼8%) difference in the rate of conversion of [U-¹³C₆]glucose vs. [1,6-¹³C₂]glucose to lactate, likely reflecting rates of CC bond breakage in the aldolase reaction, but conversion of [U-¹³C₆]glucose versus [U-¹³C₆,U-²H₇]glucose to lactate did not differ. This shows that the presence of deuterium in glucose does not alter glycolytic flux. However, there were two distinct effects of deuteration on metabolism of glucose to alanine and oxidation of glucose in the TCA. First, alanine undergoes extensive exchange of methyl deuterons with solvent protons in the alanine amino transferase reaction. Second, there is a substantial kinetic isotope effect in metabolism of [U-¹³C₆,U-²H₇]glucose to alanine and glutamate. In the presence of [U-¹³C₆,U-²H₇]glucose, alanine and lactate are not in rapid exchange with the same pool of pyruvate. These studies indicate that the appearance of hyperpolarized ¹³C-lactate from hyperpolarized [U-¹³C₆,U-²H₇]glucose is not substantially influenced by a deuterium kinetic isotope effect.

Dynamic nuclear polarization facilitates monitoring of pyruvate metabolism in Trypanosoma brucei

Dynamic nuclear polarization provides sensitivity improvements that make NMR a viable method for following metabolic conversions in real time. There are now many in vivo applications to animal systems and even to diagnosis of human disease. However, application to microbial systems is rare. Here we demonstrate its application to the pathogenic protozoan, Trypanosoma brucei, using hyperpolarized ¹³C1 pyruvate as a substrate and compare the parasite metabolism with that of commonly cultured mammalian cell lines, HEK-293 and Hep-G2. Metabolic differences between insect and bloodstream forms of T. brucei were also investigated. Significant differences are noted with respect to lactate, alanine, and CO₂ production. Conversion of pyruvate to CO₂ in the T. brucei bloodstream form provides new support for the presence of an active pyruvate dehydrogenase in this stage.

Advances in Magnetic Resonance Imaging Contrast Agents for Biomarker Detection

Recent advances in magnetic resonance imaging (MRI) contrast agents have provided new capabilities for biomarker detection through molecular imaging. MRI contrast agents based on the T2 exchange mechanism have more recently expanded the armamentarium of agents for molecular imaging. Compared with T1 and T2* agents, T2 exchange agents have a slower chemical exchange rate, which improves the ability to design these MRI contrast agents with greater specificity for detecting the intended biomarker. MRI contrast agents that are detected through chemical exchange saturation transfer (CEST) have even slower chemical exchange rates. Another emerging class of MRI contrast agents uses hyperpolarized (13)C to detect the agent with outstanding sensitivity. These hyperpolarized (13)C agents can be used to track metabolism and monitor characteristics of the tissue microenvironment. Together, these various MRI contrast agents provide excellent opportunities to develop molecular imaging for biomarker detection.

Dehydroepiandrosterone and 7-oxo-dehydroepiandrosterone in male reproductive health: Implications of differential regulation of human Sertoli cells metabolic profile

Dehydroepiandrosterone (DHEA) is a precursor of androgen synthesis whose action is partially exerted through its metabolites. 7-Oxo-dehydroepiandrosterone (7-oxo-DHEA) is a common DHEA metabolite, non-convertible to androgens, which constitutes a promising therapeutic strategy for multiple conditions. Sertoli cells (SCs) are responsible for the support of spermatogenesis, having unique metabolic characteristics strongly modulated by androgens. Consequently, disruptions in androgen synthesis compromise SCs function and hence male fertility. We aimed to evaluate the effects of DHEA and 7-oxo-DHEA in human SCs (hSCs) metabolism and oxidative profile. To do so, hSCs were exposed to increasing concentrations of DHEA and 7-oxo-DHEA (0.025, 1 and 50 μM) that revealed to be non-cytotoxic in these experimental conditions. We measured hSCs metabolites consumption/production by (1)H NMR, the protein expression levels of key players of the glycolytic pathway by Western blot as well as the levels of carbonyl groups, nitration and lipid peroxidation by Slot blot. The obtained data demonstrated that 7-oxo-DHEA is a more potent metabolic modulator than DHEA since it increased hSCs glycolytic flux. DHEA seem to redirect hSCs metabolism to the Krebs cycle, while 7-oxo-DHEA has some inhibitory effect in this path. The highest 7-oxo-DHEA concentrations (1 and 50 μM) also increased lactate production, which is of extreme relevance for the successful progression of spermatogenesis in vivo. None of these steroids altered the intracellular oxidative profile of hSCs, illustrating that, at the concentrations used they do not have pro- nor antioxidant actions in hSCs. Our study represents a further step in the establishment of safe doses of DHEA and 7-oxo-DHEA to hSCs, supporting its possible use in hormonal and non-hormonal therapies against male reproductive problems.

(13)C-labeled biochemical probes for the study of cancer metabolism with dynamic nuclear polarization-enhanced magnetic resonance imaging

In recent years, advances in metabolic imaging have become dependable tools for the diagnosis and treatment assessment in cancer. Dynamic nuclear polarization (DNP) has recently emerged as a promising technology in hyperpolarized (HP) magnetic resonance imaging (MRI) and has reached clinical relevance with the successful visualization of [1-¹³C] pyruvate as a molecular imaging probe in human prostate cancer. This review focuses on introducing representative compounds relevant to metabolism that are characteristic of cancer tissue: aerobic glycolysis and pyruvate metabolism, glutamine addiction and glutamine/glutamate metabolism, and the redox state and ascorbate/dehydroascorbate metabolism. In addition, a brief introduction of probes that can be used to trace necrosis, pH changes, and other pathways relevant to cancer is presented to demonstrate the potential that HP MRI has to revolutionize the use of molecular imaging for diagnosis and assessment of treatments in cancer.

A review of responsive MRI contrast agents: 2005-2014

This review focuses on MRI contrast agents that are responsive to a change in a physiological biomarker. The response mechanisms are dependent on six physicochemical characteristics, including the accessibility of water to the agent, tumbling time, proton exchange rate, electron spin state, MR frequency or superparamagnetism of the agent. These characteristics can be affected by changes in concentrations or activities of enzymes, proteins, nucleic acids, metabolites, or metal ions, or changes in redox state, pH, temperature, or light. A total of 117 examples are presented, including ones that employ nuclei other than (1) H, which attests to the creativity of multidisciplinary research efforts to develop responsive MRI contrast agents.

Studies of Metabolism Using (13)C MRS of Hyperpolarized Probes

First described in 2003, the dissolution dynamic nuclear polarization (DNP) technique, combined with (13)C magnetic resonance spectroscopy (MRS), has since been used in numerous metabolic studies and has become a valuable metabolic imaging method. DNP dramatically increases the level of polarization of (13)C-labeled compounds resulting in an increase in the signal-to-noise ratio (SNR) of over 50,000 fold for the MRS spectrum of hyperpolarized compounds. The high SNR enables rapid real-time detection of metabolism in cells, tissues, and in vivo. This chapter will present a comprehensive review of the DNP approaches that have been used to monitor metabolism in living systems. First, the list of (13)C DNP probes developed to date will be presented, with a particular focus on the most commonly used probe, namely [1-(13)C] pyruvate. In the next four sections, we will then describe the different factors that need to be considered when designing (13)C DNP probes for metabolic studies, conducting in vitro or in vivo hyperpolarized experiments, as well as acquiring, analyzing, and modeling hyperpolarized (13)C data.

Toward Quantitative Measurements of Enzyme Kinetics by Dissolution Dynamic Nuclear Polarization

Dissolution dynamic nuclear polarization (D-DNP) experiments enabled us to study the kinetics of the enzymatic phosphorylation reaction of glucose to form glucose-6-phosphate (G6P) by hexokinase (HK), with or without the presence of an excess of G6P, which is known to be an inhibitor of the enzyme. Against all expectations, our observations demonstrate that the phosphorylation of both α and β glucose anomers occurs with comparable kinetics. The catalytic constant of the reaction was estimated based on a simple kinetic model tailored for hyperpolarized systems.

Detecting enzyme activities with exogenous MRI contrast agents

This review focuses on exogenous magnetic resonance imaging (MRI) contrast agents that are responsive to enzyme activity. Enzymes can catalyze a change in water access, rotational tumbling time, the proximity of a (19)F-labeled ligand, the aggregation state, the proton chemical-exchange rate between the agent and water, or the chemical shift of (19)F, (31)P, (13)C or a labile (1)H of an agent, all of which can be used to detect enzyme activity. The variety of agents attests to the creativity in developing enzyme-responsive MRI contrast agents.

Hyperpolarization of deuterated metabolites via remote cross-polarization and dissolution dynamic nuclear polarization

In deuterated molecules such as [1-(13)C]pyruvate-d3, the nuclear spin polarization of (13)C nuclei can be enhanced by combining Hartmann-Hahn cross-polarization (CP) at low temperatures (1.2 K) with dissolution dynamic nuclear polarization (D-DNP). The polarization is transferred from remote solvent protons to the (13)C spins of interest. This allows one not only to slightly reduce build-up times but also to increase polarization levels and extend the lifetimes T1((13)C) of the enhanced (13)C polarization during and after transfer from the polarizer to the NMR or MRI system. This extends time scales over which metabolic processes and chemical reactions can be monitored.

Hyperpolarized NMR probes for biological assays

During the last decade, the development of nuclear spin polarization enhanced (hyperpolarized) molecular probes has opened up new opportunities for studying the inner workings of living cells in real time. The hyperpolarized probes are produced ex situ, introduced into biological systems and detected with high sensitivity and contrast against background signals using high resolution NMR spectroscopy. A variety of natural, derivatized and designed hyperpolarized probes has emerged for diverse biological studies including assays of intracellular reaction progression, pathway kinetics, probe uptake and export, pH, redox state, reactive oxygen species, ion concentrations, drug efficacy or oncogenic signaling. These probes are readily used directly under natural conditions in biofluids and are often directly developed and optimized for cellular assays, thus leaving little doubt about their specificity and utility under biologically relevant conditions. Hyperpolarized molecular probes for biological NMR spectroscopy enable the unbiased detection of complex processes by virtue of the high spectral resolution, structural specificity and quantifiability of NMR signals. Here, we provide a survey of strategies used for the selection, design and use of hyperpolarized NMR probes in biological assays, and describe current limitations and developments.

Chemistry and biochemistry of 13C hyperpolarized magnetic resonance using dynamic nuclear polarization

The study of transient chemical phenomena by conventional NMR has proved elusive, particularly for non-(1)H nuclei. For (13)C, hyperpolarization using the dynamic nuclear polarization (DNP) technique has emerged as a powerful means to improve SNR. The recent development of rapid dissolution DNP methods has facilitated previously impossible in vitro and in vivo study of small molecules. This review presents the basics of the DNP technique, identification of appropriate DNP substrates, and approaches to increase hyperpolarized signal lifetimes. Also addressed are the biochemical events to which DNP-NMR has been applied, with descriptions of several probes that have met with in vivo success.