EXTRACELLULAR GLUTATHIONE AND GAMMA-GLUTAMYL-TRANSPEPTIDASE PREVENT H2O2-INDUCED INJURY BY 2,3-DIMETHOXY-1,4-NAPHTHOQUINONE

Quinones are intracellular H2O2 generators that have been used extensi vely in models of oxidant injury; however, their toxicity is mediated partially through direct conjugation with glutathione (GSH). To focus upon the action of extracellular GSH in preventing H2O2-mediated toxic ity, we used 2,3-dimethoxy-1,4-naphthoquinone (DMNQ), which cannot con jugate with GSH but does continuously generate H2O2 through redox cycl ing. A eukaryotic cell line (3T3-GGT) stably overexpressing gamma-glut amyl transpeptidase (GGT) activity was used to study the role of GGT i n utilizing extracellular GSH against DMNQ-induced oxidative stress. D MNQ (0 to 150 muM) caused a dose-dependent decrease of intracellular G SH and adenosine 5'-triphosphate (ATP) in both control and 3T3-GGT cel ls. The rate of H2O2 escape into the medium during DMNQ exposure was a lso the same in both cell lines. Administration of GSH helped to maint ain intracellular GSH and supported resistance to ATP depletion caused by DMNQ in 3T3-GGT cells but not in control cells. The protective eff ect of extracellular GSH was completely prevented by acivicin, an inhi bitor of GGT. Our results suggest that GGT-dependent breakdown of extr acellular GSH for subsequent intracellular resynthesis helped to maint ain cellular GSH levels and increased cellular resistance against DMNQ -induced oxidative injury.