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Abdulla ZK. Taxonomy and biology of <i>Cladosporium endophyticum</i> as the first record in Iraq. STUDIES IN FUNGI 2023. [DOI: 10.48130/sif-2023-0005] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
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2
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Theillet FX, Luchinat E. In-cell NMR: Why and how? PROGRESS IN NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY 2022; 132-133:1-112. [PMID: 36496255 DOI: 10.1016/j.pnmrs.2022.04.002] [Citation(s) in RCA: 19] [Impact Index Per Article: 9.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/15/2021] [Revised: 04/19/2022] [Accepted: 04/27/2022] [Indexed: 06/17/2023]
Abstract
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.
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Affiliation(s)
- Francois-Xavier Theillet
- Université Paris-Saclay, CEA, CNRS, Institute for Integrative Biology of the Cell (I2BC), 91198 Gif-sur-Yvette, France.
| | - Enrico Luchinat
- Dipartimento di Scienze e Tecnologie Agro-Alimentari, Alma Mater Studiorum - Università di Bologna, Piazza Goidanich 60, 47521 Cesena, Italy; CERM - Magnetic Resonance Center, and Neurofarba Department, Università degli Studi di Firenze, 50019 Sesto Fiorentino, Italy
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Wu Y, Judge MT, Edison AS, Arnold J. Uncovering in vivo biochemical patterns from time-series metabolic dynamics. PLoS One 2022; 17:e0268394. [PMID: 35550643 PMCID: PMC9098013 DOI: 10.1371/journal.pone.0268394] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/10/2021] [Accepted: 04/28/2022] [Indexed: 11/19/2022] Open
Abstract
System biology relies on holistic biomolecule measurements, and untangling biochemical networks requires time-series metabolomics profiling. With current metabolomic approaches, time-series measurements can be taken for hundreds of metabolic features, which decode underlying metabolic regulation. Such a metabolomic dataset is untargeted with most features unannotated and inaccessible to statistical analysis and computational modeling. The high dimensionality of the metabolic space also causes mechanistic modeling to be rather cumbersome computationally. We implemented a faster exploratory workflow to visualize and extract chemical and biochemical dependencies. Time-series metabolic features (about 300 for each dataset) were extracted by Ridge Tracking-based Extract (RTExtract) on measurements from continuous in vivo monitoring of metabolism by NMR (CIVM-NMR) in Neurospora crassa under different conditions. The metabolic profiles were then smoothed and projected into lower dimensions, enabling a comparison of metabolic trends in the cultures. Next, we expanded incomplete metabolite annotation using a correlation network. Lastly, we uncovered meaningful metabolic clusters by estimating dependencies between smoothed metabolic profiles. We thus sidestepped the processes of time-consuming mechanistic modeling, difficult global optimization, and labor-intensive annotation. Multiple clusters guided insights into central energy metabolism and membrane synthesis. Dense connections with glucose 1-phosphate indicated its central position in metabolism in N. crassa. Our approach was benchmarked on simulated random network dynamics and provides a novel exploratory approach to analyzing high-dimensional metabolic dynamics.
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Affiliation(s)
- Yue Wu
- Institute of Bioinformatics, University of Georgia, Athens, GA, United States of America
| | - Michael T. Judge
- Department of Genetics, University of Georgia, Athens, GA, United States of America
| | - Arthur S. Edison
- Institute of Bioinformatics, University of Georgia, Athens, GA, United States of America
- Department of Genetics, University of Georgia, Athens, GA, United States of America
- Complex Carbohydrate Research Center, University of Georgia, Athens, GA, United States of America
- Department of Biochemistry and Molecular Biology, University of Georgia, Athens, GA, United States of America
- * E-mail: (ASE); (JA)
| | - Jonathan Arnold
- Institute of Bioinformatics, University of Georgia, Athens, GA, United States of America
- Department of Genetics, University of Georgia, Athens, GA, United States of America
- Department of Statistics, University of Georgia, Athens, GA, United States of America
- Department of Physics and Astronomy, University of Georgia, Athens, GA, United States of America
- * E-mail: (ASE); (JA)
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Judge MT, Wu Y, Tayyari F, Hattori A, Glushka J, Ito T, Arnold J, Edison AS. Continuous in vivo Metabolism by NMR. Front Mol Biosci 2019; 6:26. [PMID: 31114791 PMCID: PMC6502900 DOI: 10.3389/fmolb.2019.00026] [Citation(s) in RCA: 29] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/20/2019] [Accepted: 04/04/2019] [Indexed: 01/10/2023] Open
Abstract
Dense time-series metabolomics data are essential for unraveling the underlying dynamic properties of metabolism. Here we extend high-resolution-magic angle spinning (HR-MAS) to enable continuous in vivo monitoring of metabolism by NMR (CIVM-NMR) and provide analysis tools for these data. First, we reproduced a result in human chronic lymphoid leukemia cells by using isotope-edited CIVM-NMR to rapidly and unambiguously demonstrate unidirectional flux in branched-chain amino acid metabolism. We then collected untargeted CIVM-NMR datasets for Neurospora crassa, a classic multicellular model organism, and uncovered dynamics between central carbon metabolism, amino acid metabolism, energy storage molecules, and lipid and cell wall precursors. Virtually no sample preparation was required to yield a dynamic metabolic fingerprint over hours to days at ~4-min temporal resolution with little noise. CIVM-NMR is simple and readily adapted to different types of cells and microorganisms, offering an experimental complement to kinetic models of metabolism for diverse biological systems.
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Affiliation(s)
- Michael T. Judge
- Department of Genetics, University of Georgia, Athens, GA, United States
| | - Yue Wu
- Institute of Bioinformatics, University of Georgia, Athens, GA, United States
| | - Fariba Tayyari
- Department of Biochemistry and Molecular Biology, University of Georgia, Athens, GA, United States
- Complex Carbohydrate Research Center, University of Georgia, Athens, GA, United States
| | - Ayuna Hattori
- Department of Biochemistry and Molecular Biology, University of Georgia, Athens, GA, United States
- Division of Hematological Malignancy, National Cancer Center Research Institute, Tokyo, Japan
| | - John Glushka
- Department of Biochemistry and Molecular Biology, University of Georgia, Athens, GA, United States
- Complex Carbohydrate Research Center, University of Georgia, Athens, GA, United States
| | - Takahiro Ito
- Department of Biochemistry and Molecular Biology, University of Georgia, Athens, GA, United States
| | - Jonathan Arnold
- Department of Genetics, University of Georgia, Athens, GA, United States
- Institute of Bioinformatics, University of Georgia, Athens, GA, United States
| | - Arthur S. Edison
- Department of Genetics, University of Georgia, Athens, GA, United States
- Institute of Bioinformatics, University of Georgia, Athens, GA, United States
- Department of Biochemistry and Molecular Biology, University of Georgia, Athens, GA, United States
- Complex Carbohydrate Research Center, University of Georgia, Athens, GA, United States
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Use of 1H nuclear magnetic resonance to measure intracellular metabolite levels during growth and asexual sporulation in Neurospora crassa. EUKARYOTIC CELL 2011; 10:820-31. [PMID: 21460191 DOI: 10.1128/ec.00231-10] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/13/2023]
Abstract
Conidiation is an asexual sporulation pathway that is a response to adverse conditions and is the main mode of dispersal utilized by filamentous fungal pathogens for reestablishment in a more favorable environment. Heterotrimeric G proteins (consisting of α, β, and γ subunits) have been shown to regulate conidiation in diverse fungi. Previous work has demonstrated that all three of the Gα subunits in the filamentous fungus Neurospora crassa affect the accumulation of mass on poor carbon sources and that loss of gna-3 leads to the most dramatic effects on conidiation. In this study, we used (1)H nuclear magnetic resonance (NMR) to profile the metabolome of N. crassa in extracts isolated from vegetative hyphae and conidia from cultures grown under conditions of high or low sucrose. We compared wild-type and Δgna-3 strains to determine whether lack of gna-3 causes a significant difference in the global metabolite profile. The results demonstrate that the global metabolome of wild-type hyphae is influenced by carbon availability. The metabolome of the Δgna-3 strain cultured on both high and low sucrose is similar to that of the wild type grown on high sucrose, suggesting an overall defect in nutrient sensing in the mutant. However, analysis of individual metabolites revealed differences in wild-type and Δgna-3 strains cultured under conditions of low and high sucrose.
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Schaap PJ, Müller Y, Baars JJ, Op den Camp HJ, Sonnenberg AS, van Griensven LJ, Visser J. Nucleotide sequence and expression of the gene encoding NADP+- dependent glutamate dehydrogenase (gdhA) from Agaricus bisporus. MOLECULAR & GENERAL GENETICS : MGG 1996; 250:339-47. [PMID: 8602149 DOI: 10.1007/bf02174392] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
The gene encoding NADP+-dependent glutamate dehydrogenase (gdhA) was isolated from an Agaricus bisporus recombinant phage lambda library. The deduced amino acid sequence would specify a 457-amino acid protein that is highly homologous in sequence to those derived from previously isolated and characterized genes coding for microbial NADP+-GDH. The open reading frame is interrupted by six introns. None of the introns is located at either one of the positions of the two introns conserved in the corresponding open reading frames of the ascomycete fungi Aspergillus nidulans and Neurospora crassa. Northern analysis suggests that the A. bisporus gdhA gene is transcriptionally regulated and that, unlike the case in ascomycetes, transcription of this gene is repressed upon the addition of ammonium to the culture.
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Affiliation(s)
- P J Schaap
- Section Molecular Genetics of Industrial Microorganisms, Wageningen Agricultural University, The Netherlands
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Baars JJ, Op den Camp HJ, van der Drift C, Joordens JJ, Wijmenga SS, van Griensven LJ, Vogels GD. 15N-NMR study of ammonium assimilation in Agaricus bisporus. BIOCHIMICA ET BIOPHYSICA ACTA 1996; 1310:74-80. [PMID: 9244178 DOI: 10.1016/0167-4889(95)00157-3] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
Ammonium assimilation was studied by feeding [15N]ammonium to actively growing mycelium of Agaricus bisporus. Products of ammonium assimilation were analysed using 15N-NMR. Participation of glutamine synthetase, glutamate synthase and NADP-dependent glutamate dehydrogenase was determined by inhibiting glutamine synthetase with phosphinothricin and glutamate synthase with azaserine. Our results clearly indicate that, under the conditions used, ammonium assimilation is mainly catalysed by the enzymes of the glutamine synthetase/glutamate synthase pathway. No indications were found for participation of NADP-dependent glutamate dehydrogenase. Furthermore, 15N-labelling shows that transamination of glutamate with pyruvate to yield alanine is a major route in nitrogen metabolism. Another major route is the formation of N-acetylglucosamine. Compared to the formation of N-acetylglucosamine there was only a limited formation of arginine.
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Affiliation(s)
- J J Baars
- Department of Microbiology, Faculty of Science, University of Nijmegen, The Netherlands
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Kuesel AC, Kuhn W, Sianoudis J, Grimme LH, Leibfritz D, Mayer A. N-15 in vivo NMR spectroscopic investigation of nitrogen deprived cell suspensions of the green alga Chlorella fusca. Arch Microbiol 1989. [DOI: 10.1007/bf00416603] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022]
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10
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Kanamori K, Weiss RL, Roberts JD. Ammonia assimilation in Bacillus polymyxa. 15N NMR and enzymatic studies. J Biol Chem 1987. [DOI: 10.1016/s0021-9258(18)60923-8] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
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11
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Choi BS, Roberts MF. 15N-NMR studies of Methanobacterium thermoautotrophicum: comparison of assimilation of different nitrogen sources. BIOCHIMICA ET BIOPHYSICA ACTA 1987; 928:259-65. [PMID: 3567230 DOI: 10.1016/0167-4889(87)90184-4] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Abstract
Methanobacterium thermoautotrophicum can utilize glutamine and urea as well as ammonia as the sole nitrogen source during growth on H2 and CO2. High-field 15N-NMR has been used to compare the assimilation of these different nitrogen sources by this organism. The 15N-NMR spectra of extracts of cells grown in media containing [delta-15N]glutamine as the nitrogen source show that the glutamine amide nitrogen is rapidly converted to glutamate. The 15N-NMR spectra of cell extracts from cells grown on [15N]urea show a marked increase in the labeling of the alpha-NH2 of glutamate concurrent with a decrease in the urea resonance. These two nitrogen sources do not show the metabolic shift to alanine as the major resonance in stationary phase as is seen with 15NH4Cl. This behavior is discussed in terms of the enzymes of nitrogen metabolism.
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Thomas CM, Baxter RL. Analysis of mutational lesions of acetate metabolism in Neurospora crassa by 13C nuclear magnetic resonance. J Bacteriol 1987; 169:359-66. [PMID: 2947898 PMCID: PMC211775 DOI: 10.1128/jb.169.1.359-366.1987] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023] Open
Abstract
The adaptation of Neurospora crassa mycelium to growth on acetate as the sole carbon source was examined by using 13C nuclear magnetic resonance. Extracts were examined by nuclear magnetic resonance at various times after transfer of the mycelium from medium containing sucrose to medium containing [2-13C]acetate as the sole carbon source. The label was initially seen to enter the alanine, glutamate, and glutamine pools, and after 6 h 13C-enriched trehalose was evident, indicating that gluconeogenesis was occurring. Analysis of the isotopomer ratios in the alanine and glutamate-glutamine pools indicated that substantial glyoxylate cycle activity became evident between 2 and 4 h after transfer. Immediately after transfer of the mycelium to acetate medium, the alanine pool increased to about four times its previous level, only a small fraction of which was enriched with 13C. The quantity of 13C-enriched alanine remained almost constant between 2 and 7.5 h after the transfer, whereas the overall alanine pool decreased to its original level. The selective catabolism of the unenriched alanine leads us to suggest that the alanine pool is partitioned into two compartments during adaptation. Two acetate-nonutilizing mutants were also studied by this technique. An acu-3 strain, deficient for isocitrate lyase (EC 4.1.3.1) activity, showed metabolic changes consistent with this lesion. An acp strain, previously thought to be deficient in an inducible acetate permease, took up [2-13C]acetate but showed no evidence of glyoxylate cycle activity despite synthesizing the necessary enzymes; the lesion was therefore reinterpreted.
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von Philipsborn W, Müller R. 15N-NMR-Spektroskopie — neue Methoden und ihre Anwendung. Angew Chem Int Ed Engl 1986. [DOI: 10.1002/ange.19860980504] [Citation(s) in RCA: 52] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
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Martin F, Stewart GR, Genetet I, LE Tacon F. ASSIMILATION OF 15 NH 4 + BY BEECH (FAGUS SYLVATICA L.) ECTOMYCORRHIZAS. THE NEW PHYTOLOGIST 1986; 102:85-94. [PMID: 33873880 DOI: 10.1111/j.1469-8137.1986.tb00800.x] [Citation(s) in RCA: 33] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/12/2023]
Abstract
Ammonia assimilation has been followed in ectomycorrhizal roots of Fagus sylvatica. The absorption of ammonium ions was associated with a rapid synthesis of free amino acids in mycorrhizal tissues, glutamine being the most prominent. In the presence of [15 N]ammonium, glutamate, glutamine and alanine became the most strongly labelled metabolites of ectomycor-rhizas. Nitrogen-15 nuclear magnetic resonance spectroscopy demonstrated that the glutamine amide-N was the most highly enriched component of the extracts. Methionine sulphoximine and albizine, inhibitors of glutamine synthetase and glutamate synthase, almost completely blocked the incorporation of 15 N label into amino acids and induced an accumulation of NH4 + . These observations suggest that in the ammonia-fed beech ectomycorrhizas, ammonia assimilation occurs mainly via the glutamine synthetase/glutamate synthase pathway, and that glutamate dehydrogenase plays little, if any, part in this process. Alternative models for the nitrogen assimilation pathways in fungal and host tissues are presented.
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Affiliation(s)
- F Martin
- Laboratoire de Microbiologie Forestière, INRA-CNRF, Champenoux 54280, Seichamps, France
| | - G R Stewart
- Department of Botany, Birkbeck College, University of London, London WC1E 7HX, UK
| | - I Genetet
- Laboratoire de Microbiologie Forestière, INRA-CNRF, Champenoux 54280, Seichamps, France
| | - F LE Tacon
- Laboratoire de Microbiologie Forestière, INRA-CNRF, Champenoux 54280, Seichamps, France
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Juretschke HP. 15N-NMR studies of living systems. ARCHIVES INTERNATIONALES DE PHYSIOLOGIE ET DE BIOCHIMIE 1985; 93:141-50. [PMID: 2424384 DOI: 10.3109/13813458509080634] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
15N-NMR spectroscopy has been applied to the study in vivo of nitrogen uptake during primary and secondary metabolism in Streptomyces parvulus. The nitrogen metabolism of Saccharomyces cerevisiae has been studied in a series of experiments in an effort to elucidate the flow of nitrogen along the various competing pathways as well as its dependence on culture conditions. The low NMR sensitivity of the 15N nucleus required, in spite of high isotopic enrichment, quite long acquisition times (10-20 min). Therefore, an indirect detection method using double quantum 1H-NMR spectroscopy was introduced allowing the selective detection of 15N-bound protons with excellent S/N-ratio in less than a minute.
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Botton B, Es-Sgaouri A. Morphogenesis and free amino acid composition of the ascomyceteSphaerostilbe repensas influenced by nitrogen and calcium. FEMS Microbiol Lett 1985. [DOI: 10.1111/j.1574-6968.1985.tb00781.x] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022] Open
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Legerton TL, Weiss RL. Mobilization of vacuolar arginine in Neurospora crassa. Mechanism and role of glutamine. J Biol Chem 1984. [DOI: 10.1016/s0021-9258(17)47235-8] [Citation(s) in RCA: 19] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022] Open
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Legerton TL, Kanamori K, Weiss RL, Roberts JD. Measurements of cytoplasmic and vacuolar pH in Neurospora using nitrogen-15 nuclear magnetic resonance spectroscopy. Biochemistry 1983; 22:899-903. [PMID: 6220738 DOI: 10.1021/bi00273a029] [Citation(s) in RCA: 49] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
Abstract
The nitrogen-15 chemical shift of the N1 (tau)-nitrogen of 15N-labeled histidine and the half-height line widths of proton-coupled resonances of the delta- and omega,omega'-nitrogens of 15N-labeled arginine and of the alpha-nitrogens of 15N-labeled alanine and proline were measured in intact mycelia of Neurospora crassa to obtain to estimates of intracellular pH. For intracellular 15N-labeled histidine, the N1 (tau)-nitrogen chemical shift was 200.2 ppm. In vitro measurements showed that the chemical shift was slightly affected by the presence of phosphate, with which the basic amino acids may be associated in vivo. These considerations indicate a pH of 5.7-6.0 for the environment of intracellular histidine. The half-height line widths of the delta- and omega,omega'-nitrogens of [15N]arginine were 15 and 26 Hz, respectively. In vitro studies showed that these line widths also are influenced by the presence of phosphate, and, after suitable allowance for this, the line widths indicate pH 6.1-6.5 for intracellular arginine. The half-height line widths for intracellular alanine and proline were 17 and 12 Hz, respectively, which are consistent with an intracellular pH of 7.1-7.2. Pools of histidine and arginine are found principally in the vacuole of Neurospora, most likely in association with polyphosphates. Proline and alanine are cytoplasmic. The results reported here are consistent with these localizations and indicate that the vacuolar pH is 6.1 +/- 0.4 while the cytoplasmic pH is 7.15 +/- 0.10. Comparisons of these estimates with those obtained by other techniques and their implications for vacuolar function are discussed.
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