An aqueous glutathione sample is not one substance. It is a redox pair: reduced glutathione (GSH) and its oxidised dimer, glutathione disulfide (GSSG), present together in a ratio that moves with dissolved oxygen, trace metals, pH and time. So a single percentage printed next to the word "glutathione" says very little until the method names which species it counted. This piece sets out the two identities, the chemistry that shifts the balance between them, and what the standard assays actually count.

Two substances, two registry records

Reduced glutathione and glutathione disulfide are not two grades of one article. They are separate substances with separate registry entries, formulas and masses, read here from PubChem as of September 23, 2026. FDA states the point directly in a footnote to its own 2022 advisory committee briefing:

However, glutathione disulfide (GSSG) is not a salt of glutathione (it is a dimer of glutathione) and GSSG is not the same substance as glutathione.FDA Briefing Information, Pharmacy Compounding Advisory Committee, June 8, 2022

The same footnote assigns each its own FDA substance code: "The UNII Code for glutathione is GAN16C9B8O, while the UNII Code for glutathione disulfide is ULW86O013H."

The names in the top row cause most of the confusion in search. L-glutathione, reduced glutathione and plain glutathione all resolve to one PubChem record, CID 124886: the L describes stereochemistry, reduced describes redox state, and neither changes the substance. Oxidized glutathione and glutathione disulfide do change it, and resolve to CID 65359.

The mass relationship is how mass spectrometry tells the two apart. GSSG is two GSH molecules joined by a disulfide bond between their cysteine sulfurs, each losing a hydrogen. Twice 307.33 is 614.66; the disulfide comes in at 612.6, two hydrogen atoms lighter, which is the signature of the bond that formed.

Glutathione's first amide bond runs from the side-chain gamma-carboxyl of glutamate rather than the usual alpha position, which is why the systematic name is gamma-L-glutamyl-L-cysteinylglycine and why it behaves unlike an ordinary tripeptide. That identity question is covered in NAD+, 5-amino-1MQ and glutathione: which are peptides?.

Why an aqueous glutathione sample changes with time

The usual shorthand, that glutathione "oxidises in air", is not what the literature says. In a 2016 study in Chemistry (Weinheim), the authors followed free-thiol consumption spectrophotometrically and reported that "GSH is not auto-oxidized by oxygen in the absence of a catalyst". Oxygen alone is slow. A trace transition metal makes the reaction go, and copper(II) is the best-characterised case.

The same paper describes two timescales: fast formation of a copper-glutathione complex through the cysteine thiol, then, more slowly, the copper-catalysed conversion of GSH to GSSG. The rate law they derive is worth stating in words. The rate of thiol consumption goes with the concentration of the copper-glutathione complex, with the square root of dissolved oxygen, and inversely with the square root of hydrogen peroxide, the half-order in oxygen being read as evidence of a pre-equilibrium ahead of the rate-determining step.

Two consequences run against intuition. The reaction is inhibited by hydrogen peroxide rather than driven by it, and the rate falls as glutathione concentration rises, which the authors attribute to "the competing formation of an inactive form of the copper-glutathione complex (binding to glutamic and/or glycine moieties)".

pH enters through the thiol. Winterbourn and Metodiewa, comparing biologically important thiols against superoxide and hydrogen peroxide, found that "the relative reactivities of the different thiols with both oxidants were inversely related to the pK of the thiol group", which is another way of saying the species doing the chemistry is the deprotonated thiolate anion. As pH climbs toward and past the thiol's pKa, the thiolate fraction climbs with it and oxidation accelerates. We publish no number for that pKa: no primary value was retrieved for this piece, and a figure without a source is not worth printing.

For bulk behaviour over days, FDA collected the figures in the same 2022 briefing document:

As a solid, glutathione is stable at room temperature when carefully kept away from oxygen... Only 65 - 80% of glutathione remains unchanged in aqueous solutions with various pH values after 7 days at room temperature. The instability of glutathione in solutions may be due to the rapid oxidation of the thiol group into a disulfide group (Harbin et al. 2004).FDA Briefing Information, Pharmacy Compounding Advisory Committee, June 8, 2022

The same document records the opposite case from the same source: a reduced glutathione solution held at 5 degrees Celsius in a pH 6.4 buffer for 112 days showed no decrease in concentration. Both figures are real, and the gap between them is the point. Oxygen exposure, buffer, pH and temperature decide which one a given container resembles. The general handling chemistry for oxidation-prone residues is set out in Freeze-thaw, aliquots and the residues that oxidize.

What does a glutathione assay actually measure?

Three method families dominate, and they do not count the same thing.

The Ellman (DTNB) method

Ellman's 1959 method reacts thiols with 5,5'-dithiobis-(2-nitrobenzoic acid) and reads the yellow TNB dianion it releases at 412 nm. Eyer and colleagues reassessed the molar absorption coefficient in 2003 and recommend "14.15 x 10(3)M(-1)cm(-1) at 25 degrees C and 13.8 x 10(3)M(-1)cm(-1) at 37 degrees C" in place of Ellman's original 13.6 x 10^3, a difference of a few percent that propagates straight into any concentration calculated from it.

The important limitation is structural, not numerical. DTNB reports free thiols. It is not specific to glutathione, so any other thiol in the sample is counted too, and it is blind to GSSG, whose sulfurs are already bonded to each other. A plain DTNB figure is a free-thiol figure that happens to be reported in glutathione equivalents.

The enzymatic recycling method

Tietze's 1969 method is the origin of the phrase "total glutathione". Glutathione reductase and NADPH are added alongside DTNB, so every GSSG in the sample is converted back into two GSH and read. Rahman and colleagues state the mechanism plainly in their 2006 description: "The glutathione disulfide (GSSG) formed can be recycled to GSH by glutathione reductase in the presence of NADPH... the detection of total glutathione (GSH and GSSG)." They give a lower detection limit of 0.103 nM in a 96-well plate.

Reductase is exactly what makes the number a sum. A recycling assay cannot distinguish a sample that is entirely reduced from one that is half oxidised, because it converts the second into the first before reading it. Reduced glutathione on its own requires either a read with no recycling step or a blocking step that takes GSH out of play first.

Chromatography

HPLC separates the species before detecting them, so it reports GSH and GSSG as distinct peaks rather than as the difference between two readings. An isocratic system with electrochemical detection can resolve GSH, GSSG and S-nitrosoglutathione in one run, the latter two needing a detection potential above 700 mV. A 2017 critical review in Free Radical Biology and Medicine surveys the method space and its limits.

Why "total" is not "reduced", and how the oxidised figure gets inflated

There is a documented way to manufacture GSSG in the laboratory without any of it having existed in the sample. Acid deproteination, the standard first step for a biological sample, oxidises part of the reduced pool as it proceeds. Giustarini and colleagues quantified it:

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. This can be prevented by derivatizing GSH with the alkylating agent N-ethylmaleimide (NEM) to form GS-NEM before acid deproteination.Giustarini D et al., Analysis of GSH and GSSG after derivatization with N-ethylmaleimide, Nature Protocols, 2013

Because GSH is normally the large pool and GSSG the small one, a few percent shifted from one to the other is a rounding error on the reduced figure and a large proportional error on the oxidised one. The same group measured that distortion in human blood:

When methodologic artifacts were prevented by pretreatment with NEM, GSSG results increased up to 3-fold over the basal concentrations.Giustarini D et al., Oxidized forms of glutathione in peripheral blood as biomarkers of oxidative stress, Clinical Chemistry, 2006

A threefold difference in a reported GSSG figure can therefore come entirely from sample preparation. The lesson generalises: two laboratories can run defensible methods on one lot and report different numbers for reasons that live in the handling, which is the subject of Why two laboratories get different numbers on the same lot.

What to ask of a certificate that says "glutathione"

The reading rule is short: the word glutathione on its own is ambiguous, and the ambiguity is resolved only by naming the method and the species.

Four questions cover it. Which species was quantified, reduced, oxidised or total. Which method produced the figure. Whether an alkylating block was used before any acid step. And when the sample was drawn relative to when the solution was made, since for an aqueous sample the answer has a shelf life that a solid does not.

None of this is a purity question. A sample can be chemically pure glutathione-derived material and still be a mixture of two substances, which is the distinction drawn in Purity is not potency. Our glutathione is a lyophilized solid, where the redox split is far more static than in water; the glutathione liquid spray is an aqueous solution, where every variable above is live from the moment it is filled.

Sources

• Glutathione, PubChem CID 124886, National Library of Medicine, accessed 2026-09-23
• Glutathione disulfide, PubChem CID 65359, National Library of Medicine, accessed 2026-09-23
• Evaluation of Glutathione for Inclusion on the 503A Bulk Drug Substances List, FDA Briefing Information, Pharmacy Compounding Advisory Committee, June 8, 2022
• The Copper(II)-Catalyzed Oxidation of Glutathione, Chemistry (Weinheim an der Bergstrasse, Germany), 2016
• Winterbourn CC, Metodiewa D, Reactivity of biologically important thiol compounds with superoxide and hydrogen peroxide, Free Radical Biology and Medicine, 1999
• Ellman GL, Tissue sulfhydryl groups, Archives of Biochemistry and Biophysics, 1959
• Eyer P et al., Molar absorption coefficients for the reduced Ellman reagent: reassessment, Analytical Biochemistry, 2003
• Tietze F, Enzymic method for quantitative determination of nanogram amounts of total and oxidized glutathione, Analytical Biochemistry, 1969
• Rahman I, Kode A, Biswas SK, Assay for quantitative determination of glutathione and glutathione disulfide levels using enzymatic recycling method, Nature Protocols, 2006
• Giustarini D et al., Analysis of GSH and GSSG after derivatization with N-ethylmaleimide, Nature Protocols, 2013
• Giustarini D et al., Oxidized forms of glutathione in peripheral blood as biomarkers of oxidative stress, Clinical Chemistry, 2006
• Giustarini D et al., Assessment of glutathione/glutathione disulphide ratio and S-glutathionylated proteins in human blood, solid tissues, and cultured cells, Free Radical Biology and Medicine, 2017