Glutathione, an Antioxidant Tripeptide: Dual Roles in Carcinogenesis and Chemoprevention

Glutathione (GSH or reduced glutathione) is a tripeptide of gamma-Glutamyl-cysteinylglycine and the predominant intracellular antioxidant in many organisms including humans. GSH and associated enzymes are controlled by a transcription factor-nuclear factor-2 related erythroid factor-2 (Nrf2). In cellular milieu, GSH protects the cells essentially against a wide variety of free radicals including reactive oxygen species, lipid hydroperoxides, xenobiotic toxicants, and heavy metals. It has two forms, the reduced form or reduced glutathione (GSH) and oxidized form (GSSG), where two GSH moieties combine by sulfhydryl bonds. Glutathione peroxidase (GPx) and glutathione-s-transferase (GST) essentially perform the detoxification reactions using GSH, converting it into GSSG. Glutathione reductase (GR) operates the salvage pathway by converting GSSG to GSH with the expense of NADPH and restores the cellular GSH pool. Hence, GSH and GSH-dependent enzymes are necessary for maintaining the normal redox balance in the body and help in cell survival under stress conditions. In addition, GST removes various carcinogenic compounds offering a chemopreventive property, whereas the GSH system plays a significant role in regulating the cellular survival by offering redox stability in a variety of cancers including prostate, lung, breast, and colon cancer. Studies have also indicated that GSH inhibitors, such as buthionine sulfoximine, improve the chemo-sensitivity in cancer cells. In addition, GSH and dependent enzymes provide a survival advantage for cancer cells against chemotherapeutic drugs and radiotherapy.