Why does glutathione oxidise so readily?

Its free thiol reacts with oxygen to form a disulfide with another glutathione. No enzyme is needed and trace metals catalyse it.

Chromatography with mass detection separates GSH at 307.33 from GSSG at 612.6, or use a thiol-specific assay for free sulfhydryl.

Glutathione

Glutathione Storage and the Oxidation Problem

JMWritten & reviewed by Jack Muncaster · Founder, UK PeptidesLast reviewed 2026-08-233 cited sources

Glutathione's free thiol oxidises to the disulfide GSSG on contact with air. This makes it the one compound in this catalogue where disulfide chemistry is a practical daily concern rather than a theoretical one, and a stated purity figure does not describe the redox state.

Key facts

Main degradation route: Thiol oxidation to GSSG
Trigger: Air, and metal ion catalysis
Product: GSSG, 612.6 Da — measurable
Purity figure: Does not capture redox state
Contrast: Most peptides here have no cysteine
Solution stability: Poor once dissolved

Why this compound is the exception

The storage articles across this site repeat that disulfide chemistry is inapplicable, because KPV, Selank, TB-500, DSIP and Semax contain no cysteine. Glutathione is the compound those statements were implicitly excluding. Its thiol is both the source of its function and its principal vulnerability.

What actually happens

Two thiols meet, lose two hydrogens, and form a disulfide. Oxygen drives it and trace metal ions catalyse it. The reaction needs no enzyme and no biology — it proceeds in a vial of solution left standing, and it proceeds faster at higher pH.

Research material referenced

Glutathione 1500mg — third-party HPLC tested

View — £34.99

Why a purity figure misses it

GSSG is not an impurity in the usual sense. It is glutathione, correctly assembled, in a different oxidation state — and a purity assay may well count it as related material rather than contamination. A preparation can be 99% pure and substantially oxidised at the same time, which is a distinction worth understanding before reading a certificate.

How to tell

GSH and GSSG are separable and separately quantifiable — they differ in mass by essentially a factor of two, at 307.33 against 612.6. Chromatography with mass detection distinguishes them directly. A thiol-specific assay quantifies free sulfhydryl, which is the functional quantity.

Practical handling

Keep it lyophilised, cold and dry, which slows oxidation by removing the water the reaction needs. Minimise headspace air. Prepare solutions fresh and use them promptly rather than storing them. Where an experiment depends on the reduced form specifically, verify rather than assume — this is the one compound here where that verification is genuinely worth doing.

Extended research context

The Glutathione deep dive

Deep dive: the bond that puts a peptide outside peptide biology

Glutamate is one of only two amino acids carrying two carboxyl groups - the backbone alpha-carboxyl every residue has, plus one on its side chain. Standard peptide bonds use the alpha. Glutathione uses the gamma, and that one choice cascades. Ribosomes have exactly one chemistry, in which an incoming residue's amine attacks the growing chain's alpha-carboxyl, and no mechanism whatsoever for recruiting a side chain. So glutathione cannot be a gene product. It is assembled instead by two ATP-dependent ligases, which means the genome encodes the machinery but never the molecule - a peptide present in nearly every cell of nearly every organism, with no coding sequence anywhere. The same geometry that excludes the ribosome also excludes most peptidases, whose active sites are built around the spacing of an alpha bond. Only gamma-glutamyl transpeptidase cleaves it, which puts turnover of a millimolar-concentration metabolite under the control of a single enzyme. Protease resistance by structural mismatch is more complete than anything proline achieves in a conventional peptide.

Deep dive: the one compound here where a purity figure does not tell you what you need

Every storage article on this site says disulfide chemistry is inapplicable, because KPV, Selank, TB-500, DSIP and Semax contain no cysteine at all. Glutathione is the compound those statements were implicitly excluding, and the exception is not marginal - its thiol is simultaneously the source of its function and its principal vulnerability. Two thiols meet, lose two hydrogens, and become GSSG at 612.6 Da. Oxygen drives it, trace metals catalyse it, no enzyme is required, and it proceeds in a vial left standing. The subtle part is that GSSG is not an impurity in the ordinary sense. It is correctly assembled glutathione in a different oxidation state, and a purity assay may well score it as related material rather than contamination. A preparation can be 99% pure and substantially oxidised at once. Where an experiment depends on the reduced form, the certificate does not answer the question - chromatography separating 307.33 from 612.6, or a thiol-specific assay, does.

Deep dive: the same question NAD+ raises, with better evidence and a less obvious answer

Both categories on this site face one structural question: does supplying the finished molecule work, or does it succeed only by being degraded to something the cell can actually use? For NAD+ the answer is fairly clearly the latter - 663 Da with two negative charges cannot cross a membrane, and CD38 degrades it outside the cell. For glutathione it is genuinely open, and the evidence is better. Richie and colleagues published a randomised controlled trial on body stores in the European Journal of Nutrition in 2015, reporting increases. But an increase in stores admits two readings: intact absorption and distribution, or degradation to glutamate, cysteine and glycine followed by resynthesis inside cells - in which case the useful contribution is essentially the cysteine, and the tripeptide is an expensive delivery vehicle for it. Since cysteine availability is what normally limits synthesis, and since gamma-glutamyl transpeptidase sits on intestinal surfaces waiting for exactly this substrate, the second reading is not a sceptical stretch. A store measurement alone cannot distinguish them.

Research applications

▸Cellular redox state measurement via GSH/GSSG ratio
▸Glutathione peroxidase and S-transferase enzyme assays
▸Oxidative stress model systems
▸Gamma-glutamyl transpeptidase activity studies
▸Thiol chemistry and disulfide exchange research
▸Melanin synthesis pathway investigation

Handling checklist

✓Verify against CID 124886, 307.33 Da, C10H17N3O6S
✓Check the oxidised form separately - GSSG is CID 65359 at 612.6 Da
✓Do not treat a purity figure as a statement about redox state
✓Store lyophilised, cold, dry; minimise headspace air
✓Prepare solutions fresh - thiol oxidation proceeds without any enzyme
✓Where the reduced form matters, assay free thiol rather than assuming

Common research-handling mistakes

Learnt from thousands of researcher orders across our UK labs.

✗ Assuming a high purity figure means the material is reduced

Fix: GSSG is correctly assembled glutathione in a different oxidation state. A purity assay may score it as related material, not contamination.

✗ Treating glutathione like the other peptides on this site

Fix: Its gamma bond makes it protease-resistant and non-ribosomal, and it is the only compound here with a reactive thiol. Most generalisations do not apply.

✗ Reading increased body stores as proof of intact absorption

Fix: Degradation to amino acids followed by intracellular resynthesis produces the same measurement. The trial endpoint cannot distinguish them.

✗ Citing the large biochemistry literature as evidence about supplementation

Fix: What glutathione does inside cells is settled. What supplementing it accomplishes is a separate and contested question.

✗ Repeating systematic review subject matter as a product claim

Fix: Describing what a literature examined and claiming a product does it are different acts. Only the first is permissible.

Continue researching

Peer-reviewed guides, comparators and matched reference materials.

Frequently asked questions

Why does glutathione oxidise so readily?: Its free thiol reacts with oxygen to form a disulfide with another glutathione. No enzyme is needed and trace metals catalyse it.
Does a purity figure tell me the redox state?: No. GSSG is correctly assembled glutathione in a different oxidation state, and a purity assay may not count it as an impurity at all.
How do I check?: Chromatography with mass detection separates GSH at 307.33 from GSSG at 612.6, or use a thiol-specific assay for free sulfhydryl.

Primary sources & clinical trials

Peer-reviewed research and registered trials from PubMed, ClinicalTrials.gov, PubChem, FDA and NIH. All links open in a new tab and point to the primary source, so every claim can be verified at origin.

Written and reviewed by

Jack Muncaster · Founder, UK Peptides

Jack founded UK Peptides in Manchester after repeatedly receiving research compounds with missing or recycled paperwork. He is responsible for supplier selection, batch release decisions and the content published in this research library. Every article here is sourced to primary literature and every product page to a signed third-party certificate.

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