Glutathione (Reduced)

Glutathione is the gamma-glutamyl-cysteinyl-glycine tripeptide and the dominant intracellular redox buffer, present at millimolar concentrations in most cells. Its function depends entirely on the free cysteine thiol, and that thiol oxidises to the disulfide dimer GSSG on contact with atmospheric oxygen.

Which is exactly why a plain purity figure is close to meaningless for this compound. A lot can be 99 % pure glutathione and be substantially oxidised, and GSSG is a different reagent with different properties. We report the reduced-to-oxidised ratio as an explicit assay line, package under inert gas, and ship cold. Note the unusual gamma-linkage between glutamate and cysteine — it is through the glutamate side-chain carboxyl, not the alpha-carboxyl, which is why ordinary peptidases do not cleave it.