Iron and copper toxicity has been presumed to involve the formation of hydroxyl radical (.OH) from H2O2 in the Fenton reaction. The aim of this study was to verify that Fe2+-O2 and Cu+-O2 chemistry is capable of generating .OH in the quasi physiological environment of Krebs-Henseleit buffer (KH), and to compare the ability of the Fe2+-O2 system and of the Fenton system (Fe2+ + H2O2) to produce .OH. The addition of Fe2+ and Cu+ (0-20 µM) to KH resulted in a concentration-dependent increase in .OH formation, as measured by the salicylate method. While Fe3+ and Cu2+ (0-20 µM) did not result in .OH formation, these ions mediated significant .OH production in the presence of a number of reducing agents. The .OH yield from the reaction mediated by Fe2+ was increased by exogenous Fe3+ and Cu2+ and was prevented by the deoxygenation of the buffer and reduced by superoxide dismutase, catalase, and desferrioxamine. Addition of 1 µM, 5 µM or 10 µM Fe2+ to a range of H2O2 concentrations (the Fenton system) resulted in a H2O2-concentration-dependent rise in .OH formation. For each Fe2+ concentration tested, the .OH yield doubled when the ratio [H2O2]:[Fe2+] was raised from zero to one. In conclusion: (i) Fe2+-O2 and Cu+-O2 chemistry is capable of promoting .OH generation in the environment of oxygenated KH, in the absence of pre-existing superoxide and/or H2O2, and possibly through a mechanism initiated by the metal autoxidation; (ii) The process is enhanced by contaminating Fe3+ and Cu2+; (iii) In the presence of reducing agents also Fe3+ and Cu2+ promote the .OH formation; (iv) Depending on the actual [H2O2]:[Fe2+] ratio, the efficiency of the Fe2+-O2 chemistry to generate .OH is greater than or, at best, equal to that of the Fe2+-driven Fenton reaction.
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