FIGURE 2: Toxic ROS generation and enzymatic antioxidant defenses. Cells encompass a group of enzymes which help counterbalancing the potential detrimental effects of ROS production upon toxic levels. O₂^–, generated as a by-product from the electron transport chain in mitochondria, is converted to H₂O₂ by superoxide dismutases (SOD), which are the first and major line of defense against ROS. H₂O₂ accumulation is highly toxic for cells, and it is subsequently transformed to H₂O and molecular oxygen by the catalase enzyme (CAT). Glutathione peroxidases (GPx) and thioredoxin reductases (TrxR) also aid in the conversion from H₂O₂ to water. Independently, H₂O₂ is also converted to OH^–.