Glutathioneresearch overview.

Overview

Glutathione is the tripeptide γ-⁠L-glutamyl-L-cysteinylglycine. The reduced form, which OzPept supplies, is usually abbreviated GSH. Forman et al. describe it as the most abundant low-molecular-weight thiol compound synthesised in cells ⁶.

In 1921, Hopkins reported an autoxidisable constituent of the cell, the work usually credited as the discovery of glutathione ¹. The tripeptide structure now accepted was settled in later work. Glutathione has since become one of the most widely studied small molecules in biochemistry, and the literature is far larger than any short overview can cover.

This summary follows four lines of research: how glutathione is made, its role as a redox couple with its disulfide form, what happens when its synthesis is blocked with an enzyme inhibitor, and what knockout mice show about where it is required ^2,3,4,5,7.

Structure and chemistry

γ-⁠Glu-Cys-Gly, supplied in its reduced form with a free cysteine thiol. Molecular formula C10H17N3O6S, molecular weight 307.3 g/mol, CAS 70-18-8.

The unusual feature is the first peptide bond. Glutamate is joined to cysteine through the carboxyl group on its side chain (the γ-⁠carboxyl), not the α-⁠carboxyl used in ordinary peptide bonds. Most peptidases do not cleave this γ-⁠glutamyl bond. It is cleaved by γ-⁠glutamyl transpeptidase, an enzyme found on the outer surface of many cell types.

The cysteine thiol (-SH) is the reactive group. Two GSH molecules can be oxidised to glutathione disulfide (GSSG), joined by a disulfide bond, and GSSG can be reduced back to GSH by glutathione reductase using NADPH. Forman et al. review this chemistry together with the methods used to measure the two forms ⁶.

Synthesis takes place in the cytosol in two ATP-dependent steps. Glutamate⁠–⁠cysteine ligase (GCL, also called γ-⁠glutamylcysteine synthetase) joins glutamate and cysteine, and glutathione synthetase then adds glycine. Lu (2013) describes cysteine availability and GCL activity as the main determinants of the rate of synthesis ⁷.

Mechanisms investigated

Glutathione does not act through a single receptor. Research has focused on its chemistry as a thiol and on what changes when its supply is altered experimentally.

• Inhibition of synthesis. Griffith and Meister (1979) reported that buthionine sulfoximine (BSO) inhibited γ-⁠glutamylcysteine synthetase about 20 times more potently than prothionine sulfoximine and at least 100 times more potently than methionine sulfoximine, without detectable inhibition of glutamine synthetase. In mice, it lowered kidney glutathione to less than 20% of control levels. The authors proposed it as a tool for inhibiting glutathione synthesis in experimental systems ².
• Redox compartments. Hwang, Sinskey and Lodish (1992) used a cell-permeant peptide probe that becomes trapped in the secretory pathway. They reported that glutathione was the main redox buffer there, with a GSH:GSSG ratio of 1:1 to 3:1, compared with 30:1 to 100:1 for the cell as a whole ³.
• Redox potential. Schafer and Buettner (2001) set out how the half-cell reduction potential of the GSSG/2GSH couple can be estimated with the Nernst equation. They described it as the most abundant redox couple in a cell, and reported an apparent correlation between more oxidising values and cell state, from proliferation through differentiation to apoptosis ⁵.
• Genetic models. Shi et al. (2000) disrupted the gene for the heavy subunit of γ-⁠glutamylcysteine synthetase in mice. Homozygous embryos failed to gastrulate and died before day 8.5, which the authors attributed to apoptosis rather than reduced cell proliferation. Cell lines derived from mutant blastocysts grew indefinitely in culture medium containing N-acetylcysteine, despite undetectable glutathione ⁴.
• Cellular roles. Forman et al. (2009) summarise two main chemical roles in cells: reduction of oxidants, and conjugation with electrophilic compounds ⁶.

Research areas

• Isolation and early characterisation (historical, in vitro) ¹
• Inhibition of synthesis by buthionine sulfoximine (in vitro and animal) ²
• Glutathione redox state in the endoplasmic reticulum (in vitro) ³
• Knockout of glutathione synthesis in mouse embryos and derived cell lines (animal and in vitro) ⁴
• Thermodynamics of the GSSG/2GSH redox couple (review) ⁵
• Biosynthesis, measurement and cellular roles (review) ^6,7

Limits of the evidence

The glutathione literature is very large. This overview cites a small selection of foundational papers and reviews and is not a systematic review.

Most mechanistic findings come from isolated enzymes, cell culture and rodent models. Shi et al. found that cultured cells could grow without glutathione while mouse embryos could not, which shows that findings in culture do not necessarily hold in a whole organism ⁴.

Measured GSH and GSSG levels, and the ratio between them, depend on sample handling and assay method ⁶. Values reported by different laboratories are not always directly comparable. Experiments that lower glutathione, for example with BSO, can also change other pathways, so an effect seen after depletion cannot always be assigned to glutathione alone.

OzPept’s glutathione is a research chemical, not a medicine, and is not for human use.

Key studies

1. 1921In vitro

   Report of an autoxidisable constituent of the cell ¹

2. 1979In vitro

   Inhibition of glutathione synthesis by BSO ²

3. 1992In vitro

   Glutathione redox state in the endoplasmic reticulum ³

4. 2000Animal

   Knockout of glutathione synthesis in mice ⁴

5. 2001Review

   Redox state of the GSSG/2GSH couple ⁵

6. 2013Review

   Regulation of glutathione synthesis ⁷

Questions

References

1. Hopkins FG. On an autoxidisable constituent of the cell. Biochem J. 1921;15(2):286-305. Source
2. Griffith OW, Meister A. Potent and specific inhibition of glutathione synthesis by buthionine sulfoximine (S-n-butyl homocysteine sulfoximine). J Biol Chem. 1979;254(16):7558-7560. Source
3. Hwang C, Sinskey AJ, Lodish HF. Oxidized redox state of glutathione in the endoplasmic reticulum. Science. 1992;257(5076):1496-1502. Source
4. Shi ZZ, et al. Glutathione synthesis is essential for mouse development but not for cell growth in culture. Proc Natl Acad Sci U S A. 2000;97(10):5101-5106. Source
5. Schafer FQ, Buettner GR. Redox environment of the cell as viewed through the redox state of the glutathione disulfide/glutathione couple. Free Radic Biol Med. 2001;30(11):1191-1212. Source
6. Forman HJ, Zhang H, Rinna A. Glutathione: overview of its protective roles, measurement, and biosynthesis. Mol Aspects Med. 2009;30(1-2):1-12. Source
7. Lu SC. Glutathione synthesis. Biochim Biophys Acta. 2013;1830(5):3143-3153. Source