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Cu²⁺ ions and GSH molecules interact swiftly to form the complex Cu(I)–glutathione. We investigated the potential capacity of such complex to reduce molecular oxygen. The addition of SOD to a solution containing Cu(I)–glutathione led to a sustained decline of the basal oxygen level. Such effect was partially reverted by the addition of catalase. The complex was able to induce the reduction of cytochrome c and the oxidation of dyhydroethidium into 2-hydroxyethidium. Both effects were totally blocked by SOD. The ability of the complex to generate superoxide radicals was confirmed by EPR spin-trapping. Cu(I)–glutathione induces no oxidation of fluorescein, a hydroxyl radical-sensitive probe. We conclude that in solutions containing the complex, oxygen is continually reduced into superoxide, and that—in absence of interceptors—the latter radicals are quantitatively re-oxidized into molecular oxygen. We suggest …