Metabolic Pathways Of Glutathione Conjugates

Although there are parallels in cleavage reactions of xenobiotic GS-conjugates and the catabolism of glutathione in animals and plants, there is considerable evidence that at least some of the enzymes of gSh metabolism are not identical with those for the breakdown of xenobiotic conjugates (Hubbell and Casida 1977, Frear et al. 1985, Anderson 1990, Lamoureux et al. 1991). In plants, the cleavage of glycine from the tripeptide has been addressed as the first step of GSH degradation (Figure 6). It is catalysed by a specific GSH-carboxypeptidase (Wolf et al. 1996). The remaining dipeptide, is further degraded to cysteine and glutamate via 5-oxo-proline. This pathway does not depend on the activity of a y-glutamyl-transpeptidase as it does in animals, but on a y-glutamylcyclotransferase. An 5-oxo-prolinase would, in the last step, catalyse the energy dependent formation of glutamate from oxoproline (Hubbell and Casida 1977).

Clear-cut evidence from numerous studies shows that glutathione conjugates are metabolized within hours to the corresponding dipeptide and subsequently cysteine conjugates (Lamoureux and Rusness 1980, 1983, 1989, 1993, Schröder et al. 1990). Contrary to animals, and in line with the breakdown of GSH itself, higher plants metabolize xenobiotic glutathione conju gates in a first step to y-glutamylcysteine conjugates. The formation of y-glutamy Icy steine in animals (Bakke and Davidson 1994), as well as the production of cysteinylglycine conjugates in plants (see Table 1, Lamoureux and Rusness 1989, Riechers et al. 1996, Ezra and Stevenson 1986) seems to be a side reaction of minor importance. The resulting dipeptidyl conjugates are in both, animals and plants as well, cleaved to form cysteinyl conjugates. In animals this occurs via catalysis by a dipeptidase, but the nature of the respective enzyme in plants is obscure. The cysteine conjugates occupy a key position in the metabolic pathway. Only rarely have they been reported to be end points of the metabolism. In most cases they were in part or completely metabolized to an array of other products such as malonylcysteine conjugates, S-thiolactic acid derivatives, S-thioacetic acid conjugates and S-methyl-derivatives. A good example for this type of metabolism and the central role of the cysteine conjugate has been presented for the metabolism of fluorodifen in spruce cells (Schröder et al. 1990, Lamoureux et al. 1991, 1993, Figure 6).

Also S-malonyl-cysteine conjugates and their sulphoxides have long been thought to be final products of the metabolism of glutathione conjugates. They were hypothesized to occupy the same position in plant metabolism as mercapturic acids in animals. In analogy to the formation of malonylglucosyl conjugate formation it had been suggested that malonylation was the mechanism utilized to block further metabolism and a tag for tonoplast transport (Lamoureux and Rusness 1989). However, as malonylation is rather selective and not occurring on every cysteine conjugate (Lamoureux and Rusness 1983), it must be concluded that it is one of the more prominent phase III reactions in plants among others. In some cases, the liberation of the thio-group is prerequisite of subsequent glucosyl conjugation. Lamoureux and coworkers (Lamoureux and Rusness 1983, 1986, Schröder et al. 1990) were able to show that this cleavage reaction is catalysed by a C-S lyase which competes for the substrate with a malonyltransferase (see also Sandermann et al. 1997). Although numerous metabolism studies have been documented, the intracellular localization of the cleavage processes has not yet been elucidated.

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