When do metal complexes protect the biological system from superoxide toxicity and when do they enhance it?

Many copper and iron complexes can be reduced by O2- as well as by H2O2. According to the rates of reduction and the concentration of O2- and H2O2, the metal complexes may serve either as catalyst of O2- dismutation or as catalysts of the reaction between O2- and H2O2 to form OH. radical (Haber-Weiss reaction). Various factors which influence whether metal complexes protect the biological systems from superoxide toxicity or enhance it are discussed.