What is the gamma-glutamyl cycle?

A closed sequence in which glutathione is synthesised, its gamma-glutamyl group transferred to another molecule, and the components recovered and reassembled.

Glutathione

Glutathione Proposed as a Carrier, Not Just a Buffer

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

Orlowski and Meister described the gamma-glutamyl cycle in the Proceedings of the National Academy of Sciences in 1970, proposing that its properties fulfil the requirements of an amino acid transport system and that glutathione serves a carrier function within it.

Key facts

Named by: Orlowski & Meister, 1970
Journal: PNAS, November 1970 (PMID 5274454)
Tissue studied: Rat kidney
Proposed role: Amino acid transport
Translocation step: Gamma-glutamyl transpeptidase
Recovery steps: The two ATP-dependent ligases
Later tissue work: Cornell 1976, PNAS (PMID 1755)

What the cycle is

A closed sequence in which glutathione is synthesised, its gamma-glutamyl group transferred to something else, and the components recovered and reassembled. Orlowski and Meister showed rat kidney contains the full set of enzymes for both halves — synthesis and utilisation — which is what makes it a cycle rather than a pathway running one way.

The proposal that reframes the molecule

Their conclusion was that the cycle has properties fulfilling the requirements of an amino acid transport system, and that glutathione serves a carrier function within it. That is a different account of what the molecule is for than antioxidant defence — it makes glutathione a vehicle rather than only a buffer.

Research material referenced

Glutathione 1500mg — third-party HPLC tested

View — £34.99

How transport would work

Gamma-glutamyl transpeptidase transfers the gamma-glutamyl group from glutathione onto an amino acid, forming a gamma-glutamyl amino acid. Orlowski and Meister proposed that this species is what crosses, with the transpeptidase functioning in translocation. Inside, the amino acid is released and the glutamyl portion recycled.

Why the ATP cost then makes sense

The 1970 abstract names gamma-glutamylcysteine synthetase and glutathione synthetase as catalysing energy-requiring recovery steps in transport. Those are the two ATP-dependent ligases that build glutathione. Active transport requires energy input somewhere, and in this account that is where it enters — so the expense of making glutathione outside a ribosome has a proposed purpose.

What this adds to the gamma bond story

This site already explains that the gamma-glutamyl linkage is why glutathione cannot be made by a ribosome and why proteases largely cannot cleave it. Both are consequences. Orlowski and Meister propose a reason the bond exists at all: a gamma linkage is transferable in a way an alpha peptide bond is not, which is what makes the molecule usable as a handle.

How the account stands now

The cycle itself is established biochemistry and its enzymes are well characterised. How much amino acid transport actually proceeds this way, against other transport systems, has been debated since. The paper's own title says a possible transport system — the hedge was there from the start and it is worth preserving.

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

What is the gamma-glutamyl cycle?: A closed sequence in which glutathione is synthesised, its gamma-glutamyl group transferred to another molecule, and the components recovered and reassembled.
What did Orlowski and Meister propose?: That the cycle fulfils the requirements of an amino acid transport system and that glutathione serves a carrier function within it.
Why does the gamma bond matter for that?: A gamma linkage is transferable in a way an alpha peptide bond is not, which is what makes the molecule usable as a handle.

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.

How we write and check this content Report a factual error in this article

Popular across the research hub

One flagship guide from every other research category — keep exploring.

← Back to Forum