Biochemistry. A tripeptide of glycine, cystine, and glutamic acid that is widely distributed in animal and plant tissues. It is involved in oxidation-reduction reactions, in the formation and maintenance of disulfide bonds in proteins, in transport of amino acids across cell membranes, and is a cofactor for some enzymes. It exists in both the reduced thiol form (GSH) and the oxidized disulfide form (GSSG). In erythrocytes, these reactions prevent oxidative damage, GSH is oxidized by H2O2 to glutathione disulfide (GSSG) catalyzed by glutathione peroxidase.