Superoxide dismutase (SOD) is an antioxidant enzyme involved in the defense system against reactive oxygen species (ROS). SOD catalyzes the dismutation reaction of superoxide radical anion (O2-) to hydrogen peroxide, which is then catalyzed to innocuous O2 and H2O by glutathione peroxidase and catalase. Several classes of SOD have been identified. : Cu/Zn SOD (SOD1) is localized in cytosol, Mn SOD (SOD2) in mitochondria, and ecSOD (SOD3) in extracellular space. Cu/Zn SOD (SOD1) is a homodimer containing copper and zinc that is found almost exclusively in intracellular cytoplasmic spaces. The level of SOD1 elevates in response to a wide array of mechanical, chemical and biological messengers such as heat shock, shear stress, and UVB- and X-irradiation. Decreased levels of SOD1 expression can also be triggered by activation of the AP2 transcription factor. A down-regulation of SOD1 has been shown in alveolar type II epithelial cells and lung fibroblasts after exposure to hypoxia.