Glutathione is a tripeptide, -glutamylcysteinylglycine, which protects against two kinds of metabolic stress:

1. It can nonenzymatically reduce substances, such as peroxides or free radicals, which accumulate in cells under oxidizing conditions. By maintaining an intracellular reducing environment, glutathione prevents intracellular protein thiols from oxidizing to disulfides.

2. In conjunction with glutathione S-transferases, glutathione participates in detoxification of many substances, such as organic halides, fatty acid peroxides derived from lipid oxidation, and products derived from radiation-damaged DNA. Glutathione reacts with compounds as shown here.

In fertilized eggs, the sulfur amino acid called ovothiol (see here) plays a role comparable to that of glutathione. That is, ovothiol protects the egg against oxidative damage by peroxides produced at the egg surface early in fertilization. Oxidized ovothiol is in turn reduced by glutathione.