Is glutathione a peptide?

Yes, a tripeptide of glutamic acid, cysteine and glycine. It is unusual because the glutamic acid is joined through its side chain, the gamma position, rather than through the normal backbone bond. That linkage changes which tests are informative: amino acid analysis cannot see it, and retention time against a glutathione reference standard carries more weight than for most research peptides.

What is GSSG and why might it appear on a glutathione report?

GSSG, glutathione disulfide, is two glutathione molecules joined through their cysteine sulfur atoms. It forms when reduced glutathione oxidises. PubChem lists it at 612.6 g/mol, roughly double the 307.33 g/mol of reduced glutathione. A report that lists GSSG is telling you how much of the material has oxidised, either in the vial or during analysis.

Can the testing process itself oxidise glutathione?

Yes, and published methods take steps to prevent it. One LC-MS study found that leaving out an alkylating agent created GSSG equal to 2 to 3% of the glutathione present and lowered the measured GSH. Adding N-ethylmaleimide with a strong acid stopped this. A credible report describes how thiols were protected during sample preparation.

What mass should a glutathione certificate show?

PubChem gives reduced glutathione an average molecular weight of 307.33 g/mol and a monoisotopic mass of 307.08 Da. In positive-mode mass spectrometry the main ion is usually one proton heavier. A strong signal near 612 to 613 suggests glutathione disulfide is present, which the report should quantify rather than leave unexplained.

Does a glutathione assay figure mean reduced or total glutathione?

It depends on the method, and the report should say. Some methods reduce all disulfides before measuring, which reports total glutathione and hides any oxidation. Others measure reduced GSH and GSSG separately. If you care whether the powder is still in its reduced form, ask for both figures rather than a single total.

One of the 30 compound testing profiles

Glutathione

Glutathione is a 3-amino-acid peptide (C10H17N3O6S, average mass 307.33 g/mol); a test should confirm that mass and report how much of the material is still reduced glutathione (GSH) rather than its oxidised dimer, glutathione disulfide (GSSG).

Glutathione is a tripeptide in which glutamic acid is attached through its side chain to the N-terminus of cysteinylglycine, giving the sequence gamma-Glu-Cys-Gly (PubChem).

Test your glutathione

Identity card

Formula: C10H17N3O6S
Average mass: 307.33 g/mol
Monoisotopic mass: 307.08 Da
Length: 3 residues
Modifications: Gamma-glutamyl linkage (Glu side-chain carboxyl bonded to the Cys amine); Free thiol on Cys2
Also sold as: GSH, Reduced glutathione, L-Glutathione, gamma-L-Glutamyl-L-cysteinylglycine
Sequence: gamma-Glu-Cys-Gly
Note: PubChem lists the line notation H-gGlu-Cys-Gly-OH. The oxidised dimer, glutathione disulfide (GSSG), is a separate record (CID 65359, C20H32N6O12S2, average mass 612.6 g/mol).

Source: Glutathione compound record (CID 124886), Glutathione Disulfide compound record (CID 65359).

Check a certificate's mass against these values with the mass checker, prefilled for Glutathione.

What usually goes wrong with glutathione. Documented failure modes, with sources.

Oxidation of the free thiol to GSSG

Glutathione's cysteine carries a free thiol group, and two glutathione molecules can join through that thiol to form glutathione disulfide (GSSG). PubChem lists GSSG as C20H32N6O12S2 with an average mass of 612.6 g/mol, against 307.33 g/mol for reduced glutathione (PubChem, GSSG; PubChem, glutathione). GSSG is a different molecule from the reduced form most labels describe, and only a method that measures the two forms separately shows how much of a vial has oxidised.

Oxidation created by the test itself

Glutathione is one of the clearest examples of an analyte that a careless method can change. In a validated LC-MS method, leaving out an alkylating agent during sample preparation caused both an underestimate of GSH and artefactual production of GSSG equal to 2 to 3% of the GSH present; adding N-ethylmaleimide with a strong acid prevented both problems (Steghens et al., 2003). The work was done on blood, but the free thiol behaves the same way in a dissolved vial sample, so a certificate reporting a few percent GSSG should say how thiols were protected during preparation.

Several routes to error during sample handling

A detailed study of glutathione measurement listed the pitfalls that shift results: oxidation of thiols in acidified samples, further oxidation after restoring neutral or alkaline pH, oxidation during acid deproteinisation, a shifted GSH to GSSG balance when free thiols are blocked irreversibly, and reactions of electrophiles with amino groups (Rossi et al., 2002). The authors concluded that most published blood GSSG data may be affected by artefacts. Glutathione results are method-sensitive, and the method description is part of the result.

Total glutathione reported as if it were reduced

Some assays measure glutathione only after converting disulfides back to thiols. In the Rossi study, one HPLC approach used dithiothreitol to reduce disulfides before derivatising the thiols with monobromobimane (Rossi et al., 2002). A method like that, run without a parallel non-reduced measurement, reports total glutathione: the GSH originally present plus the GSH released from GSSG. A vial that is partly oxidised can then read as if it were fully reduced. A useful report says whether its figure is reduced GSH, GSSG, or total.

Which tests matter most

LC-MSMeasures reduced GSH (307.08 Da monoisotopic) and GSSG (612.15 Da) separately, provided the thiol is protected during preparation.
HPLC-UVGives purity and, with derivatisation or suitable detection, quantifies GSH and GSSG against reference standards.
Karl Fischer titrationWater in the powder adds weight without adding glutathione, so it helps explain a low assay figure.
Appearance and solubilityA cloudy or discoloured solution is an early cue to look for degradation or other ingredients.

Reading glutathione results

Glutathione's glutamic acid is linked through its side-chain (gamma) carboxyl, not the backbone carboxyl of an ordinary peptide bond (PubChem). An alpha-linked Glu-Cys-Gly has the same formula and mass, and amino acid analysis finds glutamic acid, cysteine and glycine for either, because hydrolysis breaks both bonds. Retention time against an authentic glutathione reference standard is what supports the linkage.

The expected monoisotopic masses are 307.08 Da for reduced glutathione and 612.15 Da for GSSG (PubChem, glutathione; PubChem, GSSG). A large signal near 612 alongside 307 means disulfide is present, from the vial or from preparation.

A glutathione amount is often given as an assay percentage, so the basis matters: reduced or total glutathione, as-is or dried. The content calculator converts an assay percentage into milligrams for a given fill; use the reduced figure if the report gives one.

Public Tested.ph results

No public glutathione records yet

Each public glutathione record will appear here with its identity, HPLC purity and measured amount against the label, once the first vial is tested.

FIG 13Amount against the label

Questions about testing glutathione

Is glutathione a peptide?: Yes, a tripeptide of glutamic acid, cysteine and glycine. It is unusual because the glutamic acid is joined through its side chain, the gamma position, rather than through the normal backbone bond. That linkage changes which tests are informative: amino acid analysis cannot see it, and retention time against a glutathione reference standard carries more weight than for most research peptides.
What is GSSG and why might it appear on a glutathione report?: GSSG, glutathione disulfide, is two glutathione molecules joined through their cysteine sulfur atoms. It forms when reduced glutathione oxidises. PubChem lists it at 612.6 g/mol, roughly double the 307.33 g/mol of reduced glutathione. A report that lists GSSG is telling you how much of the material has oxidised, either in the vial or during analysis.
Can the testing process itself oxidise glutathione?: Yes, and published methods take steps to prevent it. One LC-MS study found that leaving out an alkylating agent created GSSG equal to 2 to 3% of the glutathione present and lowered the measured GSH. Adding N-ethylmaleimide with a strong acid stopped this. A credible report describes how thiols were protected during sample preparation.
What mass should a glutathione certificate show?: PubChem gives reduced glutathione an average molecular weight of 307.33 g/mol and a monoisotopic mass of 307.08 Da. In positive-mode mass spectrometry the main ion is usually one proton heavier. A strong signal near 612 to 613 suggests glutathione disulfide is present, which the report should quantify rather than leave unexplained.
Does a glutathione assay figure mean reduced or total glutathione?: It depends on the method, and the report should say. Some methods reduce all disulfides before measuring, which reports total glutathione and hides any oxidation. Others measure reduced GSH and GSSG separately. If you care whether the powder is still in its reduced form, ask for both figures rather than a single total.

Sources

Glutathione compound record (CID 124886), PubChem, US National Library of Medicine
Glutathione Disulfide compound record (CID 65359), PubChem, US National Library of Medicine
Steghens JP et al. Fast liquid chromatography-mass spectrometry glutathione measurement in whole blood: micromolar GSSG is a sample preparation artifact. J Chromatogr B. 2003;798(2):343-9, Journal of Chromatography B (PubMed)
Rossi R et al. Blood glutathione disulfide: in vivo factor or in vitro artifact? Clin Chem. 2002;48(5):742-53, Clinical Chemistry (PubMed)

Last reviewed 6 Oct 2026.