Glutathione and related enzymes in multidrug resistance.

INTRODUCTION THE DEVELOPMENT of resistance to chemotherapeutic agents, at concentrations which were once effective for treatment, is a major obstacle in the clinical treatment of cancers. Several mechanisms have been described that mediate such resistance: (i) those which afford increased rates of drug efflux from the cell, so that the drug interacts with its target to a reduced extent (e.g. P-glycoprotein [l], multidrug resistance-associated protein (MRP) [2]); (ii) d ecreased drug sensitivity, either by elevating levels of the target to overcome drug doses, or mutating the target, thus rendering the drug ineffective (e.g. dihydrofolate reductase) [3]; (“‘) 111 increased DNA repair mechanisms as a means to reverse cytotoxicity (e.g. 06-alkylguanine transferase) [4]; and (iv) altered expression of metabolic and detoxification processes that protect the cell against such damage. This latter mechanism may be mediated by the glutathioneiglutathione-S-transferase (GSHIGST) detoxification system and is the focus of the discussion below. Many reports have shown that resistance to alkylating agents is associated with increased GSH levels and GST activity. GSH and its associated enzymes serve a protective role within the cell. GSH is an important intracellular antioxidant and is the most abundant non-protein thiol present in the cell. GSH’s conjugation with a compound, either spontaneously, or when catalysed by GST, renders the compound less toxic against cellular targets, and more hydrophilic and thus more readily excretable. The GSH tripeptide (yglutamylcysteinylglycine) is synthesised by cells de nova via the y-glutamyl cycle, salvaged by y-glutamyl transpeptidase (?/GT), or recycled through the sequential action of glyoxalase I (gly I) and glyoxalase II (gly II) (Figure 1). In de novo synthesis, y-glutamylcysteine synthase (?GCS), the rate limiting enzyme, catalyses the peptide linkage between the y-carboxyl group of glutamate and the amino group of cysteine. GSH synthase then catalyses the condensation of the carboxyl group of cysteine with the amino group of glycine to form the tripepride. GSH salvage through plasma membrane-bound yGT occurs through the transfer of y-glutamyl group of extracellular GSH to an o-amino group of an acceptor amino acid, releasing cysteinyl-glycine into the cell cytosol while the y-glutamyl amino acid remains on the extracellular face of the plasma membrane. The cysteinyl-glycine dipeptide can be used to synthesise GSH. Gly I and gly II are integral to the