A T-glutamyl transpeptidase-glutathione (GGT-GSH) system induces transition metaldependent lipid peroxidation (LPO). The role of the transition metals iron and copper in this system was studied by determination of LPO rates, the rates of Fe 3+ reduction, and the steady state concentration of Fe 2÷ as function of concentration of o-phenanthroline or citrate. Optimum curves were obtained,compatible with the idea that Fe 2+ chelated by an entity other than o-phenanthroline or citrate is important in thioi-driven LPO. Cu enhanced LPO at low concentrations, inhibited LPO at high ones, and catalytically elevated the steady state concentration of Fe 2+. Relating the steady states of Fe 2+ at various chelator concentrations with those of LPO rates, indicate that a Fe2÷-O-O-Fe3+ complex may not be the principal oxidizing entity. The above, and the resistance of LPO to catalase, superoxide dismutase and mannitol are compatible with the notion that the Fe which participates in redox cycles is chelated to an entity that may be refractory to the action of these antioxidants. BIOCHEMISTRY and MOLECULAR BIOLOGY INTERNATIONALGSH-GGT-mediated LPO rates in rat liver microsomes, and the levels of the reduction of Fe 3÷, indicate that a complex other than Fe 2+-O-O-Fe a+ (perhaps thiol-Fe) complex is important for the induction of LPO by thiols, and that Cu enhances LPO by increasing the steady state of ferrous iron.MATERIALS AND METHODS Materials. Glutathione (GSH), dithiothreitol (DTY), glycylglycine, thiobarbituric acid (TBA), acivicin, butylated hydroxyanisol (BHA), catarase (CAT, 11,000 U/mg) and superoxide dismutase (SOD, 2,670 U/rag) were from Sigma, Saint Louis. o-phenanthroline (OP) was from Aldrich, Milwaukee. Malondialdehyde-bis-dimethylacetal was from Eastman-Kodak, Rochester. Cysteinyl-bisglycine was from Bachem, Torrance, CA. Rat liver and kidney microsomes were prepared from Wistar rats weighing 250 g. Organs were homogenized in 5 ml/g tissue of ice cold 50 mM sodium phosphate, 150 mM NaC! buffer pH 7.4 and were centrifuged at 9,000 x g for 20 min. The upper lipid layer was removed with filter paper, and the supernatant was centrifuged at 43,000 x g for 30 min, and the pellets (microsomes) were washed twice and resuspended in the same buffer, aliquoted, frozen and stored at -80°C. GGT activity was assayed with 1 ram L-3,-glutamyl-p-nitroanilide and 20 raM glycylglycine in 50 mM HEPES buffer pH 7.4 at 25°C. GGT activity was not affected by iron, copper, BHA, mannitol, OP, citrate or antioxidant enzymes at the concentrations used here. Protein concentration was determined with the Bradford reagent (Bio-Rad) with bovine serum albumin as a standard. The specific activities of GGT in liver and kidney microsomes were 20 mU and 25 U/rag protein, respectively. Thiobarbiturie acid-reactive material was assayed essentially as described (12) with a TBA-TCA-HC! reagent containing 3 mM BHA. A 500/zl sample of reaction mixture was mixed with 1.5 ml of the TBA-TCA-HCI reagent and was incubated at 85°C for 15 rain, followed by centri...