Many functions have been ascribed to glutathione, a tripeptide that has been known for almost a century.* Glutathione is found in virtually all mammalian cells, usually in substantial concentrations (0.5-10 mM). The concentration of glutathione in extracellular fluids such as blood plasma is in the micromolar range. The intracellular concentration of glutathione exceeds by far that of glutathione disulphide, which is effectively converted into glutathione by the NADPH-utilizing enzyme glutathione reductase. The functions of glutathione are related to its thiol group and its y-glutamyl moiety. The intracellular stability of glutathione is favoured by its y-glutamyl linkage, which is not split by a-peptidases, and which protects the thiol group from rapid oxidation. The C-terminal glycine residue of glutathione protects the tripeptide from the action of y-glutamyl cyclotransferase.

There is good evidence that glutathione functions in several different ways. It serves as an intracellular reductant. It functions in the destruction of free radicals and of hydrogen peroxide and other peroxides. Glutathione forms S-substituted derivatives that are involved in detoxification of foreign compounds and also in various phases of endogenous metabolism. Glutathione also plays a role in a variety of other metabolic phenomena and is a coenzyme for certain enzymic reactions. Glutathione functions in one of the systems involved in the transport of amino acids, and glutathione itself is transported out of many cells. The intracellular concentrations of glutathione are much greater than those of cysteine; it appears that glutathione functions as a storage form of cysteine. Glutathione also plays a role in the inter-organ transport of amino acid sulphur.