Glutathione

Glutathione (GSH) is the tripeptide gamma-L-glutamyl-L-cysteinyl-glycine, a ubiquitous low-molecular-weight thiol nucleophile and reductant that represents the central redox agent of most aerobic organisms. ^[1] GSH is the most abundant low-molecular-weight thiol synthesized in cells and plays critical roles in protecting cells from oxidative damage and from the toxicity of xenobiotic electrophiles, and in maintaining redox homeostasis. ^[2] Beyond antioxidant protection, GSH functions in detoxification, cell signaling, iron homeostasis, DNA synthesis, gene expression, cysteine and protein metabolism, and cell death pathways including apoptosis and ferroptosis. ^[1] Because GSH inactivates reactive oxygen, nitrogen and sulfur species and its proper balance is considered essential for health, it has been described as a universal biomarker relevant to some age-related disorders and to the wider biology of oxidative stress. ^[3] GSH is found in the cytosol of cells in the range of 1 to 10 mM, while in plasma it is present only in the micromolar range. ^[2]

GSH is synthesized in the cytosol by two ATP-dependent enzymes: glutamate cysteine ligase (GCL), which joins cysteine and glutamate to form gamma-glutamylcysteine, and glutathione synthase, which adds glycine. ^[1] GCL is rate-limiting for GSH synthesis, as is the availability of the precursor amino acid cysteine, which may be supplemented as N-acetylcysteine (NAC). ^[1] Although GSH has direct scavenging effects, its antioxidant function is largely accomplished through glutathione peroxidase-catalyzed reductive removal of hydrogen peroxide, lipid hydroperoxides and peroxynitrite, while its detoxifying function is exerted with glutathione transferases. ^[1] Oxidized glutathione disulfide (GSSG) is regenerated to GSH by glutathione reductase, and exported GSH is degraded extracellularly by gamma-glutamyl transferase (GGT), which makes its component amino acids available to cells. ^{[1] [4]} GSH is synthesized mainly in the cytoplasm and delivered to mitochondria, where mitochondrial GSH metabolizes hydrogen peroxide, and a long-term imbalance between mitochondrial reactive oxygen species and mitochondrial GSH can cause apoptosis, necroptosis and ferroptosis.

The predominant cellular forms of glutathione to measure are reduced GSH and GSSG, with nitrosoglutathione and protein mixed disulfides as additional forms. ^[2] In the classical recycling assay, GSH reacts with DTNB and glutathione reductase recycles GSSG so that total GSH (GSH plus GSSG) is measured, while measuring GSSG requires first modifying the GSH present with N-ethylmaleimide or vinylpyridine. ^[2] High performance liquid chromatography (HPLC) is now a more commonly used procedure for measuring GSH and GSSG. ^[2] An increase in oxidized thiols in the cytosol is transient even during oxidative stress, so thiol oxidation can be very difficult to measure, particularly when it occurs in signal transduction. ^[2] The distribution of GSH and GSSG within cells is highly compartmentalized, which complicates a comprehensive view of redox homeostasis from any single pool. ^[7] In healthy adults, GSH levels in human plasma were estimated at 2 to 5 μM with a much lower GSSG concentration of 0.14 μM, and the liver is the major source of circulating GSH. ^[7]