These enzymes are present in most tissues, with high concentrations in the liver, intestine, kidney, testis, adrenal, and lung, where they are localized in the cytoplasm ( 95 percent) and endoplasmic reticulum ( 5 percent). The conjugation of xenobiotics with glutathione is catalyzed by a family of glutathione S-transferases. The first reaction is inhibited by buthionine- S-sulfox- imine, which can be used in vivo to decrease glutathione levels in experimental animals. At each step, ATP is hydrolyzed to ADP and inorganic phosphate. The first reaction is catalyzed by-glutamylcysteine synthetase the second by glutathione synthetase.
The synthesis of glutathione involves formation of the peptide bond between cysteine and glutamic acid, followed by peptide bond formation with glycine. Glutathione can also conjugate xenobiotics containing electrophilic heteroatoms ( O, N, and S). In contrast to the amides formed by conjugation of xenobiotics to other amino acids, glutathione conjugates are thioethers, which form by nucleophilic attack of glutathione thiolate anion (GS ) with an electrophilic carbon atom in the xenobiotic.
Substrates for glutathione conjugation include an enormous array of electrophilic xenobiotics, or xenobiotics that can be biotransformed to electrophiles. Conjugation of xenobiotics with glutathione is fundamentally different from their conjugation with other amino acids and dipeptides (Sies and Ketterer, 1988 Mantle et al., 1987). This section describes the conjugation of xenobiotics with the tripeptide glutathione, which is comprised of glycine, cysteine, and glutamic acid (the latter being linked to cysteine via the -carboxyl group, not the usual -carboxyl group, as shown in Fig. The preceding section described the conjugation of xenobiotics with certain amino acids, including some simple dipeptides, such as glycyltaurine. 6-50), and conjugation with amino acids by serylor prolyl-tRNA synthetase (Fig. 6-9), O-sulfation by sulfotransferase (Fig. 6-49), O-acetylation by N-acetyltransferase (Fig. It is now apparent that the hydroxylamines formed by the cytochrome P450–dependent N-hydroxyla- tion of aromatic amines can potentially be activated by numerous reactions, including N-glucuronidation by UDP-glucuronosyltrans- ferase (Fig. Hydroxylamines activated by aminoacyl-tRNA synthetases include N-hydroxy-4-amino- quinoline 1-oxide, which is conjugated with serine, and N-hydroxy- Trp-P-2, which is conjugated with proline.
Conjugation of hydroxylamines with amino acids is catalyzed by cytosolic aminoacyl-tRNA synthetases and requires ATP (Fig. In contrast to amino acid conjugation of carboxylic acidcontaining xenobiotics, which is a detoxication reaction, amino acid conjugation of N-hydroxy aromatic amines (hydroxylamines) is an activation reaction because it produces N-esters that can degrade to form electrophilic nitrenium and carbonium ions (Anders, 1985 Kato and Yamazoe, 1994b). Tiomers are considerably more potent than their antipodes as inhibitors of cyclooxygenase (the target of NSAID therapy).