The small thiol present in highest concentration in the cytoplasm is glutathione (GSH), a tripeptide glycine-cysteineglutamic acid ( Figure 2 ): the -SH group of its cysteine is extremely sensitive to oxidation, mainly by peroxides. Its importance is supported by the existence of a molecular machinery that makes it particularly effective. In fact, a scavenger, including any thiol antioxidant such as the common laboratory reagent beta-mercaptoethanol, would normally act as a suicidal decoy, being oxidized and thus becoming useless. However, when GSH is oxidized, it forms GSH disulfide (GSSG), and this can be re-reduced by a specific enzyme, glutathione reductase. Not only can GSH be enzymatically regenerated from GSSG, but also the reaction of GSH with the ROS (a peroxide), which already takes place with high reactivity, is catalyzed by GSH peroxidases that facilitate the inactivation of a wide range of peroxides ( Figure 3 ).