GSH vs GSSG: Reduced and Oxidized Glutathione in the Laboratory

GSH vs GSSG: two forms of one tripeptide

Glutathione is a tripeptide of glutamate, cysteine and glycine with one unusual bond. The enzyme that begins its synthesis, in Aquilano and colleagues’ words, “forms an unusual peptide bond between the γ-carboxyl of glutamate and the amino group of cysteine”, which is why it is written γ-Glu-Cys-Gly. Our guide to tripeptides and other short peptides covers that linkage in more detail.

Here the cysteine is what matters. Its side chain ends in a thiol (–SH), and two thiols can be oxidized to a disulfide (–S–S–). Reduced glutathione, GSH, is the tripeptide with its thiol intact: PubChem lists it as CID 124886, formula C₁₀H₁₇N₃O₆S. Oxidized glutathione, also called glutathione disulfide or GSSG, is two GSH molecules linked through their cysteine sulfur atoms: PubChem CID 65359, C₂₀H₃₂N₆O₁₂S₂. Doubling the GSH formula gives C₂₀H₃₄N₆O₁₂S₂; the disulfide accounts for the two missing hydrogens.

So GSH and GSSG are less two products than two states of one molecule, and a sample of either can contain the other. That is why the OIV specification discussed below assays reduced glutathione specifically, and why GSSG is the oxidation product that careful handling of a GSH reagent is meant to prevent.

The redox chemistry in one paragraph

Oxidizing two GSH to one GSSG releases two electrons and two protons: 2 GSH → GSSG + 2 H⁺ + 2 e⁻. Cells reverse the reaction enzymatically; GSSG is “efficiently reduced back to GSH by the NADPH-dependent catalysis of the flavoenzyme GSH reductase”, as Aquilano and colleagues put it. Because one GSSG pairs with two GSH, the couple’s reduction potential depends on the square of the GSH concentration. Schafer and Buettner give the half-cell equation at 25 °C and pH 7.0 as E_hc = −240 − (59.1/2) log([GSH]²/[GSSG]) mV, and point out that “[GSH] enters as a squared term.”

Why researchers measure the GSH:GSSG ratio

The GSH/GSSG ratio condenses a cell’s redox environment into one number. Owen and Butterfield describe reduced GSH as constituting “up to 98% of cellular GSH under normal conditions”, and Aquilano and colleagues put the reduced form at 10 to 100 times the oxidized species. A shift toward GSSG is the change researchers look for in oxidative-stress models.

The ratio also hides a subtlety. Because [GSH] enters the Nernst equation squared, the same ratio corresponds to different potentials at different total concentrations. Schafer and Buettner illustrate this with GSH that is 1% oxidized; working their equation ourselves gives about −249 mV starting from 10 mM GSH but about −219 mV starting from 1 mM, a 30 mV gap at an identical ratio of roughly 198:1. Their conclusion is that “to have full knowledge of the redox state of the GSSG/2GSH couple, absolute concentrations are needed.”

How GSH and GSSG are measured

The main methods differ less in their detectors than in how they keep the two forms apart.

Whatever the method, the reported GSSG is only as good as what happened to the sample before it reached the instrument.

The oxidation artifact and how studies avoid it

The most consequential error in glutathione analysis is GSSG that was not in the sample when it was taken. Giustarini and colleagues put it plainly: “Artifactual oxidation to GSSG of 5–15% of the GSH found in a sample can occur during deproteination of biological samples with any of the commonly used acids, with consequent marked overestimation of GSSG.” Their remedy is to derivatize GSH with the alkylating agent N-ethylmaleimide (NEM), forming GS-NEM, before the acid step, then back-extract the excess NEM with dichloromethane so that it cannot interfere with the enzymatic GSSG assay.

Small percentages matter because GSSG is the minority species. Take a hypothetical sample with a true ratio of 100:1. Oxidizing 5% of its GSH during preparation turns that into about 27:1 and multiplies the GSSG reading by 3.5; at 15%, the ratio falls to 10:1 and GSSG reads 8.5 times too high. That arithmetic is why Tomin and colleagues warn that even slight artificial oxidation can lead to an overestimation of GSSG in blood “even as high as 7–50-fold”.

The same group describes time-dependent oxidation of GSH on exposure to air as a serious source of bias, concludes that “Immediate derivatization of GSH is therefore inevitable for accurate determination of the redox state”, and recommends performing NEM alkylation near neutral pH for selectivity and reactivity. And because NEM inactivates glutathione reductase, any method that later uses the recycling assay has to remove the excess reagent first.

Keeping solid GSH reagent stable

Solid reduced glutathione is more forgiving than a solution, but the aim is the same: keep air, water and warmth away from the thiol. The most explicit published specification we found comes from the International Organisation of Vine and Wine (OIV), whose International Oenological Codex includes a monograph for glutathione used in winemaking. It describes GSH as “usually available in white crystalline powder form soluble in water”, limits the moisture of the solid to “less than or equal to 0.5%”, and says: “Store in sealed packaging in a cool (2-8 °C), dry place.” For its analytical method it adds that “The GSH solution must be prepared each day and stored at low temperature (2-4 °C) in a brown glass bottle.” The stated reason is the one this article keeps returning to: precautions are needed “to avoid oxidation and oxidised glutathione (GSSG) production.”

That is a winemaking specification, not a rule for laboratory reagents, and we make no claim about which specification governs any particular product. It is still a sensible model for the bench: keep the container sealed and cold, take out only what you need and reseal promptly, and prepare analytical solutions on the day they are needed rather than storing them. Our guide to how peptides degrade covers oxidation alongside the other degradation routes, and the peptide storage guide covers general handling, including opening cold containers.

Identity checks: expected masses

Because GSSG is a dimer, the two forms sit about 305 Da apart, and no mass measurement that reports the right ion will confuse them. Values are our calculations from the PubChem formulas (CIAAW 2024 atomic weights; NIST isotope masses).

Two details catch people out. First, the doubly charged GSSG ion falls at m/z 307.083, within 0.001 of GSH’s neutral monoisotopic mass, while GSH itself appears at m/z 308.091; a peak near 307.08 whose isotope peaks sit half a mass unit apart is GSSG. The precursor ions Tomin and colleagues used for quantification match these values: 308 for GSH, 613.2 for GSSG and 433 for GS-NEM. Second, an intact mass confirms composition, not connectivity: an α-linked Glu-Cys-Gly has the same formula as glutathione and therefore the same mass. Our guide to reading a mass spectrum explains charge states and isotope spacing.

What’s established and what isn’t

Sourcing glutathione for research in Canada

For a Canadian lab buying glutathione as a reagent, a few checks cover most of what matters. Pure Peptides Canada is itself a supplier, so apply the same checks to us.

• Confirm the form. A listing and certificate for reduced glutathione should name GSH and report identity data consistent with it: C₁₀H₁₇N₃O₆S, about 307.3 g/mol, or [M+H]⁺ near m/z 308.09. A main value near 612–613 describes GSSG.
• Look for the oxidized form. If the documentation reports impurities, GSSG is the one that tracks oxidation; if it is silent on the point, ask.
• Plan storage before the reagent arrives. Move the sealed container to cold, dry storage on receipt, and prepare analytical solutions on the day they are needed, as the OIV method does.
• Block the thiol if you measure the ratio. Add NEM before any acid step, and remove the excess before a recycling assay.
• Ask when documentation is incomplete. Questions about a product or its documentation go through our Contact page. Reduced glutathione sits in our anti-aging and cellular research reagents category.

Frequently asked questions