The glutathione S-transferases comprise a group of multi-functional enzymes involved in the biotransformation/detoxication of a broad spectrum of hydrophobic compounds bearing an electrophilic center. The enzymes facilitate the nucleophilic attack of the -SH group of reduced glutathione (GSH) on the electrophilic centers of appropriate substrates which is the first step in the physiological formation of mercapturic acids. In mammals, GSH-Tr activity has been demonstrated in the cytosolic and, in some cases, microsomal fractions of the cells of various organs. In the liver, these enzymes constitute from 3-10% of the cytosolic proteins. With the possible exception of the erythrocyte, GSH-Tr exists in cells in multiple molecular forms. It has been reported that the isozymes GSH-Tr can be distinguished from one another by their substrate specificities.