Evidence that the reactions of cadmium in the presence of metallothionein can produce hydroxyl radicals

A variety of Cu reconstituted metallothioneins (MTs) containing different amounts of copper ions together with Cd7-MT free of copper were prepared and used in spin trapping experiments designed to show cadmium is not a Fenton active metal. A significant increase of the DMPO/.OH adduct was observed, with increased concentrations of the copper containing MTs, H2O2 enhanced DMPO/.OH adduct formation, catalase and the Cu (I) specific chelating agent bathocuproine, reduced DMPO/.OH adduct formation. These results suggest that Cu (I) and H2O2 both have important roles in the production of active species in these systems and cause DMPO/.OH formation. However, Cd-7-MT showed no ability to cause generation of DMPO adducts with H2O2 seeming to indicate cadmium is not a Fenton metal. To test this hypothesis further trapping studies were run with added sulphite and lipid peroxide using both commercial MT and Cd-7-MT since cadmium causes peroxidation in vivo. Commercial MT generates radicals with added sulphite and peroxide, Cd-7-MT does not, demonstrating that cadmium is not a Fenton metal. These results help to explain the oxidative damage to DNA observed in the presence of MT and cadmium in vitro.