Glutathione in the Cell: The GSH/GSSG Ratio and Why Researchers Measure It

LynxLab · 24 October 2026 · 8 min read

How glutathione works inside a cell: the GSH/GSSG redox couple, glutathione peroxidase, why it is the main intracellular antioxidant and how labs measure it.

Photo: Malcolm Choong, Unsplash

Glutathione is three amino acids long and present in almost every cell at concentrations most signalling molecules never reach. Biochemists have been writing about it since the 1920s, and the phrase "the body's master antioxidant" has followed it into supplement marketing. The chemistry underneath that phrase is specific and measurable, and it comes down to one ratio.

This explainer covers what glutathione actually does inside a cell: the reduced and oxidised forms, the enzyme that spends it, the reason it is called the main intracellular antioxidant, and the assays researchers use to measure it. It stays inside the cell. The question of whether swallowed or infused glutathione reaches the cell is a separate one, covered in the bioavailability question.

What glutathione is and where it sits in the cell

Glutathione is gamma-L-glutamyl-L-cysteinylglycine: glutamate, cysteine and glycine, joined with an unusual bond between the glutamate side chain and the cysteine. That gamma linkage is why most peptidases ignore it, and why it survives inside cells for hours rather than minutes.

Shelly Lu's 2013 review in Biochimica et Biophysica Acta puts the concentration at 1 to 10 millimolar in mammalian tissues, with the highest levels in the liver, which makes it the most abundant non-protein thiol in the cell (Lu, Biochim Biophys Acta, 2013). For comparison, most hormones and second messengers work in the nanomolar to micromolar range, a thousand to a million times lower.

Cells make it themselves in two steps, each costing one ATP. Glutamate cysteine ligase joins glutamate to cysteine, then glutathione synthetase adds glycine. The first step is rate-limiting and is held back by the product itself, so a cell with plenty of glutathione slows its own production.

GSH and GSSG: the redox couple that buffers the cell

The working part of the molecule is the thiol group on cysteine. In its reduced form, written GSH, that sulphur carries a hydrogen it can give away. When it does, two glutathione molecules link through their sulphurs to form glutathione disulphide, written GSSG.

In a healthy cell the balance sits heavily on the reduced side. Freya Schafer and Garry Buettner's 2001 review in Free Radical Biology and Medicine describes the GSSG/2GSH pair as the most abundant redox couple in the cell and treats it as the cell's principal redox buffer (Schafer and Buettner, Free Radic Biol Med, 2001).

Their central finding was that the half-cell reduction potential of this couple tracks what the cell is doing. They estimated roughly -240 millivolts in proliferating cells, around -200 millivolts in differentiating cells and about -170 millivolts in cells undergoing apoptosis. As the pool oxidises, the potential climbs and the cell's fate changes with it.

Restoring GSSG to GSH is the job of glutathione reductase, which takes electrons from NADPH. That links the glutathione pool to the pentose phosphate pathway and to the cell's energy metabolism. Schafer and Buettner describe NADPH as the thermodynamic driving force behind the whole system.

Glutathione peroxidase: the enzyme that spends it

Glutathione does not sit still waiting for free radicals. Much of its antioxidant work is done through an enzyme. In 1957 Gordon Mills at the University of Texas described a heat-labile factor in red blood cells that used reduced glutathione to protect haemoglobin from breaking down in the presence of hydrogen peroxide. He named it glutathione peroxidase (Mills, Journal of Biological Chemistry, 1957).

The enzyme takes a peroxide, hydrogen peroxide or a lipid hydroperoxide, and reduces it to water or an alcohol. In exchange it oxidises two GSH to one GSSG. Every peroxide neutralised costs the cell glutathione, which the reductase must then pay back with NADPH.

Regina Brigelius-Flohé and Matilde Maiorino's 2013 review counts eight glutathione peroxidases in mammals, GPx1 to GPx8. Five of them, GPx1 to GPx4 and GPx6, carry selenium in the form of selenocysteine at the active site, which is why selenium status affects the whole system (Brigelius-Flohé and Maiorino, Biochim Biophys Acta, 2013). The same review argues that these enzymes do more than detoxify: GPx1 shapes hydrogen peroxide signalling in the insulin pathway and GPx4 regulates a form of cell death.

Why glutathione is called the main intracellular antioxidant

Three properties combine to earn the title. The first is sheer quantity: millimolar concentrations mean there is enough of it to absorb a burst of oxidants without running out.

The second is range. Henry Forman's 2009 overview in Molecular Aspects of Medicine lays out two separate roles: reducing oxidants, directly or via peroxidases, and conjugating electrophiles (Forman, Zhang and Rinna, Mol Aspects Med, 2009). Glutathione S-transferases attach GSH to reactive drug metabolites, carcinogens and lipid breakdown products so they can be exported and excreted. That is how paracetamol's toxic metabolite is cleared, and why an overdose that exhausts liver glutathione is dangerous.

The third is reversibility. Vitamin C and vitamin E are consumed when they act. Glutathione is recycled by the reductase, so the same molecules are used again and again. A cell with a working reductase and a supply of NADPH can keep its glutathione pool reduced for as long as the supply lasts.

How the GSH/GSSG ratio is measured in the laboratory

The classic method is the enzymatic recycling assay, standardised by Irfan Rahman and colleagues in Nature Protocols in 2006. GSH reacts with DTNB, Ellman's reagent, to produce a yellow compound read at 412 nanometres. Glutathione reductase and NADPH then recycle the GSSG formed back to GSH, so the colour keeps developing at a rate proportional to total glutathione (Rahman, Kode and Biswas, Nature Protocols, 2006).

The protocol reports a detection limit of 0.103 nanomolar in a 96-well plate and a run time of about 15 minutes. To measure GSSG on its own, the GSH in a parallel sample is masked first, usually with 2-vinylpyridine, and the ratio is then calculated from the two readings.

The catch is sample handling. Daniela Giustarini and colleagues showed in a 2013 Nature Protocols paper that 5 to 15% of the GSH in a sample oxidises to GSSG during acid deproteination, which can wildly overstate the oxidised fraction (Giustarini et al., Nature Protocols, 2013). Their fix is to block GSH with N-ethylmaleimide before the acid step. For anyone using reduced glutathione for research as a standard or a treatment in cell culture, that pre-analytical step matters as much as the assay.

Liquid chromatography with mass spectrometry is the more precise alternative and is how recent clinical trials have measured erythrocyte GSH and GSSG. It is also expensive, and the recycling assay remains the workhorse in most labs.

What the ratio does and does not tell you

Reported GSH:GSSG ratios in healthy cells run from around 100:1 upwards, and a fall in the ratio is the standard biomarker of oxidative stress. The number is convenient because the units cancel, so no one needs to know the cell volume.

Schafer and Buettner warn that this convenience hides something. Because GSH appears as a squared term in the Nernst equation for the couple, the reduction potential depends on the absolute GSH concentration as well as the ratio. Two cells with the same ratio but a tenfold difference in total glutathione differ by 59 millivolts in redox potential. Their recommendation is to report concentrations, not just the ratio.

The ratio also says nothing about where the glutathione is. Mitochondria hold a separate pool, and the endoplasmic reticulum is deliberately kept more oxidised, at about -180 millivolts, so that proteins can form disulphide bonds. A whole-cell measurement averages across all of these compartments.

For research buyers in the UK, the practical point is that any experiment using glutathione uk suppliers' material as a standard needs a known, pure reference compound. Oxidised contamination in the standard shows up directly in the GSSG reading.

LynxLab stocks glutathione 1500mg vials for research use at £38.99, tested to 99% purity with the lab reports on the product page.

Frequently asked questions

What is a normal GSH to GSSG ratio?

In healthy cells the reduced form dominates, with reported ratios of roughly 100:1 or higher. The ratio falls under oxidative stress as GSH is converted to GSSG faster than glutathione reductase can recycle it. Schafer and Buettner argue the absolute GSH concentration should be reported alongside the ratio.

Is glutathione a peptide or an antioxidant?

Both. It is a tripeptide of glutamate, cysteine and glycine, and its cysteine thiol group is what gives it antioxidant activity. The gamma-glutamyl bond protects it from most peptidases, so it persists inside cells long enough to act as a redox buffer.

What does glutathione peroxidase do?

Glutathione peroxidase reduces hydrogen peroxide and lipid peroxides to water or alcohols, oxidising two molecules of GSH to one GSSG in the process. Mammals have eight forms, GPx1 to GPx8, and five of them depend on selenium. Gordon Mills first described the enzyme in red blood cells in 1957.

How do researchers measure glutathione?

The standard method is the DTNB enzymatic recycling assay read at 412 nanometres, with a detection limit of about 0.1 nanomolar. Samples must be treated with N-ethylmaleimide before acid deproteination or 5 to 15% of GSH oxidises artificially. Labs that buy glutathione as a calibration standard need a pure, fully reduced reference for the readings to mean anything.

Sources

Research use only

Glutathione from LynxLab

1500mg vials from £38.99. ≥99% purity, lab report on the page. Same-day dispatch, UK stock.

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