Figure 2.

The dual role of ROS. There are many reactive oxygen species (ROS) sources in the cell. Although mitochondria are known to produce around 90% of the cellular ROS under physiological conditions, there are other notable sources too. These include NADPH oxidases (NOX), endoplasmic reticulum (ER) and peroxisomes. The superoxide radical (O₂·⁻) produced in the cell is converted by superoxide dismutase (SOD) to hydrogen peroxide (H₂O₂). H₂O₂ can be converted to highly reactive and cytotoxic hydroxyl radical (HO·), via the Haber–Weiss (Equation (1)) or Fenton reactions (Equation (2)). Although ROS can cause oxidative damage to biomolecules, at strictly regulated levels they are required to maintain the redox homeostasis of the cell and are involved in adaptive signalling to overcome various stresses. If the antioxidant system fails to keep ROS under control, high ROS concentrations can initiate malignant signalling or cell death. Since H₂O₂ is relatively stable and can cross biological membranes, it is considered to be the most important redox signalling molecule.