So is it a peptide or not?

It is. Three amino acids, two amide bonds. The unusual part is that one bond is gamma-linked.

Why can't ribosomes make it?

Ribosomes form alpha-peptide bonds only. Glutathione's glutamate is attached through its side chain, so it must be built enzymatically.

How does this compare with NAD+?

NAD+ is not a peptide at all — no amino acids, no peptide bonds. Glutathione genuinely is one, just an atypical one.

Glutathione

Is Glutathione a Peptide?

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

Yes. Glutathione is three amino acids joined by amide bonds, which meets the definition. But the glutamate-cysteine bond is gamma-linked rather than alpha-linked, and that difference removes it from most of what is generally true of peptides.

Key facts

Amino acids: Three — Glu, Cys, Gly
Peptide bonds: Two
Standard bonds: One (Cys-Gly)
Atypical bond: One (γ-Glu-Cys)
Ribosomally made: No
Compare with NAD+: Not a peptide at all

The definition is met

A peptide is amino acids joined by amide bonds. Glutathione has three amino acids and two amide bonds. It qualifies, and that distinguishes it from NAD+, which sits in the same catalogue with no amino acids whatsoever.

Where it departs

A standard peptide bond joins one residue's alpha-carboxyl to the next residue's amine. In glutathione, glutamate contributes its side-chain carboxyl instead — the gamma position. The cysteine-glycine bond is entirely normal; the glutamate-cysteine bond is not.

Research material referenced

Glutathione 1500mg — third-party HPLC tested

View — £34.99

What that costs it

Ribosomal synthesis. The ribosome forms alpha-peptide bonds and nothing else, so no gene encodes glutathione and no mRNA is transcribed for it. It is assembled instead by two ATP-dependent ligases working in sequence. A peptide with no gene is an unusual object.

What that buys it

Protease resistance, and substantially. Peptidases evolved to recognise alpha-peptide bonds, so a gamma linkage is largely invisible to them. Only gamma-glutamyl transpeptidase cleaves it, which means glutathione's turnover is controlled by one specific enzyme rather than by general proteolysis. For a molecule at millimolar concentrations, that control matters.

Why the distinction has practical weight

Because the peptide guidance on this site assumes alpha bonds throughout. Protease susceptibility, the logic of stepwise solid-phase synthesis, sequence verification — all of it is written for ribosomally-encodable peptides. Glutathione is a peptide by definition and an outlier in behaviour, which is a more interesting position than either NAD+'s or a conventional peptide's.

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

So is it a peptide or not?: It is. Three amino acids, two amide bonds. The unusual part is that one bond is gamma-linked.
Why can't ribosomes make it?: Ribosomes form alpha-peptide bonds only. Glutathione's glutamate is attached through its side chain, so it must be built enzymatically.
How does this compare with NAD+?: NAD+ is not a peptide at all — no amino acids, no peptide bonds. Glutathione genuinely is one, just an atypical one.

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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