Superoxide radicals have a protective role during H2O2 stress

Reactive oxygen species (ROS) consist of potentially toxic, partly reduced oxygen species and free radicals. After H2O2 treatment, yeast cells significantly increase superoxide radical production. Respiratory chain complex III and possibly cytochrome b function are essential for this increase. Disruption of complex III renders cells sensitive to H2O2 but not to the superoxide radical generator menadione. Of interest, the same H2O2-sensitive mutant strains have the lowest superoxide radical levels, and strains with the highest resistance to H2O2 have the highest levels of superoxide radicals. Consistent with this correlation, overexpression of superoxide dismutase increases sensitivity to H2O2, and this phenotype is partially rescued by addition of small concentrations of menadione. Small increases in levels of mitochondrially produced superoxide radicals have a protective effect during H2O2-induced stress, and in response to H2O2, the wild-type strain increases superoxide radical production to activate this defense mechanism. This provides a direct link between complex III as the main source of ROS and its role in defense against ROS. High levels of the superoxide radical are still toxic. These opposing, concentration-dependent roles of the superoxide radical comprise a form of hormesis and show one ROS having a hormetic effect on the toxicity of another.