It cleaves glutathione's gamma-glutamyl bond — the only enzyme that does — initiating the breakdown that recovers its amino acids.

Why is it on liver panels?

It is abundant in liver and biliary epithelium, so damage or enzyme induction there raises the amount reaching circulation.

Why is a raised GGT non-specific?

Alcohol, many medicines, biliary obstruction and several unrelated conditions all raise it, so it narrows little on its own.

Glutathione

A Routine Liver Test That Measures a Glutathione Enzyme

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

Gamma-glutamyl transferase, the enzyme measured on routine liver blood panels as GGT, is the same enzyme that cleaves glutathione's gamma-glutamyl bond. Its presence on a standard test is a direct connection between glutathione biochemistry and ordinary clinical practice.

Key facts

Enzyme: Gamma-glutamyl transferase (GGT)
Also called: Gamma-glutamyl transpeptidase
Substrate: Glutathione's gamma-glutamyl bond
Location: Outer cell membrane surfaces
Definitive review: Whitfield 2001 (PMID 11563810)
Liver disease patterns: Xing 2022 (PMID 35029891)

The connection most people never make

GGT appears on liver function panels alongside ALT, AST and alkaline phosphatase, and is generally interpreted as a marker of liver or biliary disturbance. It is also the only enzyme that cleaves glutathione's gamma-glutamyl bond. Anyone who has had liver bloods taken has had a glutathione-pathway enzyme measured.

Why it sits on the outside of cells

Its active site faces the extracellular space. That placement is what allows it to act on glutathione that has been exported from cells, initiating the breakdown that recovers its constituent amino acids — and it is also why it is the gatekeeper of glutathione turnover rather than one degradative route among several.

Research material referenced

Glutathione 1500mg — third-party HPLC tested

View — £34.99

Why it rises in liver disease

GGT is abundant in liver and biliary epithelium, so damage or induction there increases the amount reaching circulation. Xing and colleagues examined peripheral blood GGT characteristics across different liver diseases in 2022, which is the kind of work that turns a single number into a differential pattern.

Why it is also an oxidative stress marker

Because its substrate is glutathione. Conditions increasing glutathione turnover increase the enzyme handling it, so GGT reflects something about redox demand as well as about hepatobiliary integrity. Whitfield's review in Critical Reviews in Clinical Laboratory Sciences remains the standard account of that dual character.

Why it is a non-specific test

Induction by alcohol and by many medicines raises it, as does biliary obstruction, as do several conditions unrelated to the liver. A raised GGT narrows very little on its own, which is why it is interpreted alongside other results rather than read in isolation — a general property of markers sitting at the junction of several processes.

What this does not concern

Anything about a person's own results, which belong with a clinician. This describes what an enzyme does and why it appears on a panel. Nothing supplied on this site affects liver function or any blood test, and no claim is made about health in any person.

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 does GGT do?: It cleaves glutathione's gamma-glutamyl bond — the only enzyme that does — initiating the breakdown that recovers its amino acids.
Why is it on liver panels?: It is abundant in liver and biliary epithelium, so damage or enzyme induction there raises the amount reaching circulation.
Why is a raised GGT non-specific?: Alcohol, many medicines, biliary obstruction and several unrelated conditions all raise it, so it narrows little on its own.

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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