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γ-Glutamylcysteine synthetase and γ-glutamyl transferase as differential enzymatic sources of γ-glutamylpeptides in mice

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Abstract

Some γ-glutamylpeptides in blood plasma are putative biomarkers for pathological conditions of the liver. γ-Glutamyltransferase (GGT) and γ-glutamylcysteine synthetase (γ-GCS) are two such potential enzymes that are responsible for the production of γ-glutamylpeptides. GGT produces γ-glutamylpeptides by transferring the γ-glutamyl moiety from glutathione to an amino acid or a peptide. γ-GCS normally catalyzes the production of γ-glutamylcysteine from glutamate and cysteine in the glutathione-synthesizing reaction, but other amino acids can also serve as an acceptor of a γ-glutamyl group, thus resulting in the formation of a variety of γ-glutamylpeptides. Based on liquid chromatography–mass spectrometry analyses, we observed differences in the distribution of γ-glutamylpeptides between the liver and kidney and were able to measure the activities of γ-GCS as well as the GGT reactions by quantifying the resulting γ-glutamylpeptides. The enzymatic characterization of γ-GCS in liver homogenates indicated that several γ-glutamylpeptides including γ-glutamyltaurine are actually produced. Cys showed the lowest Km value (0.06 mM) while other amino acids had much higher Km values (ranging from 21 to 1800 mM). The moderate Km values for these amino acids suggest that they were not the preferred amino acids in this conversion but were utilized as acceptor substrates for the production of the corresponding γ-glutamylpeptides by the γ-GCS reaction under Cys-deficient conditions. Thus, the production of these γ-glutamylpeptides by γ-GCS is directly correlated with a low Cys content, suggesting that their measurement in blood plasma could be useful for predicting the presymptomatic disease state of the liver with a defect in GSH redox balance.

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Abbreviations

GSH:

Glutathione

GPX4:

Phospholipid hydroperoxide glutathione peroxidase

γ-GCS:

γ-Glutamylcysteine synthetase

GSS:

Glutathione synthetase

Glu:

Glutamate

Cys:

Cysteine

Gly:

Glycine

Tau:

Taurine

Met:

Methionine

γ-Glu-Cys:

γ-Glutamyl-cysteine

2AB:

2-Aminobutyrate

OPT:

Ophthalmic acid

GGT:

γ-Glutamyltransferase

γ-Glu-Tau:

γ-Glutamyltaurine

LC:

Liquid chromatography

MS:

Mass spectrometry

MEM:

N-Ethylmaleimide

NMM:

N-Methylmaleimide

BSO:

Buthione sulfoximine

2ME:

2-Mercaptoethanol

DMEM:

Dulbecco’s modified Eagle medium

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Funding

This work was supported by the JSPS KAKENHI Grant-in-Aid for Young Scientists (18K15039) and Takeda Science Foundation to SK and, in part, by the YU-COE program [M30-3] and [C31-3] to SK, JF, and HK from Yamagata University.

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Authors and Affiliations

Authors

Contributions

S.K. performed the majority of the experiments. Y. I. advised on enzymological analyses. Y.S. and H.K. chemically synthesized γ-glutamylpeptides. J.F. conducted the study and wrote the paper.

Corresponding author

Correspondence to Junichi Fujii.

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The authors declare that they have no conflict of interest.

Ethical approval

This article does not contain any studies with human participants performed by any of the authors. Animal protocols were approved by the Animal Research Committee at Yamagata University.

Informed consent

Human tissues and sera were never used in this study. So that no informed consent is required.

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Handling Editor: S. W. Schaffer.

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Kobayashi, S., Ikeda, Y., Shigeno, Y. et al. γ-Glutamylcysteine synthetase and γ-glutamyl transferase as differential enzymatic sources of γ-glutamylpeptides in mice. Amino Acids 52, 555–566 (2020). https://doi.org/10.1007/s00726-020-02835-2

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  • DOI: https://doi.org/10.1007/s00726-020-02835-2

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