Fig. 2. The generation of ROS by mitochondria. Electrons released from mitochondria reduce oxygen molecules, thereby producing such ROS as superoxides (O₂^-). Superoxide dismutase (SOD) catalyzes H₂O₂ formation from superoxides. H₂O₂ might be deactivated by catalase (CAT). However, when H₂O₂ reacts with iron or copper ions, hydroxyl radicals (OH•), the most reactive form of ROS, are produced. Excessive antioxidants (AO) can inhibit production of O₂^- and other ROS.