Conjugations with Glutathione 506371 Glutathione and glutathione transferases

Glutathione (64, Figure 22; GSH) is a thiol-containing tripeptide of major significance in the detoxification of drugs and other xenobiotics. In the body, it exists in a redox equilibrium between the reduced form (GSH) and an oxidized form (GSSG). The metabolism of glutathione (i.e., its synthesis, redox equilibrium, and degradation) is quite complex and involves a number of enzymes.112-115

Glutathione reacts in a variety of ways, one of which is its redox capacity. Indeed, GSH can reduce peroxides (a reaction catalyzed by glutathione peroxidase) and organic nitrates; in its GSSG form, glutathione can oxidize the superoxide anion radical. Of major significance in detoxification reactions is the capacity of GSH (and other endogenous thiols including albumin) to scavenge free radicals, in particular radical oxygen species (e.g., R", HO", HOO", ROO"). As such, glutathione and other thiols have a critical role to play in cellular protection.116 The reactions involved are highly complex and can be simplified as follows:

In this chapter, we focus on the conjugation reactions of glutathione as catalyzed by glutathione transferases. The glutathione transferases (EC; GSTs) are multifunctional proteins coded by two multigene superfamilies (Table 4).117-123 They can act as enzymes as well as binding proteins. The microsomal superfamily contains the homotrimer GST enzymes, whereas the cytosolic GST enzymes are homodimers and heterodimers. Seven classes are now known in humans, e.g., the alpha class with the following dimers: A1-1, A1-2, A2-2, A3-3, A4-4, and A5-5. The

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