Glutathione

What Glutathione Is and How It Is Studied An Unusual Tripeptide Glutathione is three residues — glutamate, cysteine, glycine — but the first bond is a gamma linkage off the glutamate side chain rather than a standard alpha peptide bond. That single structural difference is what makes the molecule stable enough to accumulate intracellularly at millimolar concentrations, and it is why glutathione cannot be made by the ribosome like an ordinary peptide. How It Is Synthesised Two ATP-dependent enzymes build it. Glutamate-cysteine ligase joins the first two residues and is the rate-limiting step, controlled by feedback inhibition from the finished product. Glutathione synthetase adds the glycine. Both steps happen in the cytosol, and cysteine availability is generally the limiting input. The Redox Couple The free thiol on the cysteine residue is the reactive part. Two glutathione molecules oxidise to a disulfide-linked dimer, GSSG, and glutathione reductase converts them back using NADPH. The ratio of reduced to oxidised form is one of the standard measures of cellular redox state, and glutathione peroxidases and S-transferases use the reduced form as their substrate. The Bioavailability Problem Worth stating plainly, because most vendor copy skips it. Administered glutathione is rapidly broken down by gamma-glutamyl transpeptidase on cell surfaces, and whether intact molecules reach the intracellular pool in any meaningful quantity has been contested for decades. The enzyme that degrades it is the same gamma linkage specialist that makes the molecule interesting in the first place.

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Glutathione was first described more than a century ago. Frederick Gowland Hopkins isolated and named it in 1921, building on an earlier observation by de Rey-Pailhade in 1888, and its central place in cell biology has not been in question since.

It is present in most cells at millimolar concentration, higher than almost any other small-molecule thiol, and the ratio between its reduced and oxidised forms is a standard laboratory readout for redox state. Glutathione peroxidases use it to reduce peroxides; glutathione S-transferases conjugate it to electrophilic compounds, which is the first step of the mercapturic acid pathway.

Those roles are textbook biochemistry and are not disputed. What is disputed is what happens when the molecule is administered from outside rather than synthesised inside the cell.

Gamma-glutamyl transpeptidase cleaves extracellular glutathione rapidly, and the question of whether administered glutathione raises intracellular levels, or simply supplies its constituent amino acids for resynthesis, has been argued in the literature for decades without resolution.

That unresolved question underlies every claim about glutathione as an administered compound, and it is why we describe the molecule’s biochemistry rather than its reported effects here.

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A defined tripeptide with settled specifications, supplied at the lowest per-milligram cost in the catalogue.

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