Quantitative NMR-Based Metabolomics on Tissue Biomarkers and Its Translation into In Vivo Magnetic Resonance Spectroscop
Nuclear magnetic resonance (NMR) spectroscopy is an established analytical platform for analyzing metabolic profiles of cells, tissues, and body fluids. There are several advantages in introducing an NMR-based study design into metabolomics studies, inclu
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Nuclear magnetic resonance (NMR) spectroscopy is one of the two principal analytical techniques in metabolomics (along with mass spectrometry (MS)-based platforms). The advantages of NMR-based metabolomics include its quantitative and nondestructive nature and high reproducibility, as well as the ability to detect all metabolites simultaneously in one snapshot. The disadvantages of NMR are related to low sensitivity (NMR detects only high-abundant metabolites in the micromole to millimole range) and signal overlap between individual metabolites, particularly on Angelo D’Alessandro (ed.), High-Throughput Metabolomics: Methods and Protocols, Methods in Molecular Biology, vol. 1978, https://doi.org/10.1007/978-1-4939-9236-2_23, © Springer Science+Business Media, LLC, part of Springer Nature 2019
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one-dimensional proton NMR spectra. Proton NMR (1H-NMR) is the major NMR application to quantify endogenous metabolites and to elucidate chemical structures of unknown metabolites. 1HNMR can identify and quantify up to 150 endogenous metabolites from a biological sample [1–4] (Table 1). Metabolomics applications with other nuclei (31P-, 13C-, and 19F-NMR, mostly) remain limited due to low sensitivity or low natural abundance. Fortunately, the recent progress in the development of high-field magnets (750 MHz and above), cold probe (CryoProbe) technologies, and the use of dynamic nuclear polarization for hyperpolarized 13CNMR has dramatically improved the low limit of detection (LLOD) for multinuclear NMR spectroscopy. The basic principle of NMR lies on the fact that each spinning proton will behave as a “mini-magnet” when placed into a strong magnetic field. The strong external magnetic field aligns the protons’ magnetic moments parallel and antiparallel to the magnetic field (Fig. 1a). The population of spins in the parallel orientation (which are of a slightly lower energy level) is larger than in the antiparallel orientation as described by the Boltzmann distribution. This results in a net magnetic moment in the sample aligned with the direction of the external magnetic field (Fig. 1b). When a radio-frequency pulse is applied to the sample, the magnetization orientation is changed, and after the pulse is gone, the system relaxes to its original status (Fig. 1b). Hydrogen nuclei in different tissues have different relaxation properties, which can be detected by radio-frequency MR receivers. MR tissue relaxation characteristics following a radio-frequency pulse reveal information about the concentration, mobility, and chemical bonding of hydrogen and, less frequently, other tissue elements. The strongest NMR signals in a living system arise from water and fat protons due to their metabolic abundance. Other endogenous and exogenous metabolites also give signals on 1H-NMR but with much weaker signal-tonoise ratios due to lower concentrations. A significant number of endogenous metabolite signals are usually obscured by the water signal (both ex vivo and in vivo), and their detection requires
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