RESEARCH ARTICLES 1-6.pmd

Molecular oxygen (O 2 ) is essential for the survival of all aerobic organisms. Reactive oxygen species (ROS), such as superoxide anion (O 2 ), hydrogen peroxide (H 2 O 2 ), and hydroxyl radical (HO•), consist of radical and non-radical oxygen species formed by the partial reduction of oxygen. The radicals derived from oxygen represent the most abundant class of radical species generated in living system. There are exogenous and endogenous sources of cellular ROS. Exogenous sources include irradiation (UV, Xand γ-rays) and chemicals, such as redox-active metals (such as Fe and Cu) that enhance the Fenton/Haber– Weiss reaction (Trachootham et al. 2008). ROS are also generated through multiple sources inside the cells, such as the electron transport chain in mitochondria and through enzymes producing superoxide anion such as phagocytic and nonphagocytic NADPH oxidase, lipoxygenases and cycloxygenases. Two other oxidant species that are physiologically relevant are HOCl produced by the myeloperoxidase from neutrophils, and singlet oxygen (O 2 ) generated upon photosensitization and UVA irradiation (Roos et al. 2003). ROS can inflict damage to nucleic acids, proteins, and lipids and have been implicated in carcinogenesis, neurodegeneration, atherosclerosis, diabetes and aging (Ray et al. 2012). When ROS overwhelm the cellular antioxidant defense system, the cellular redox status is perturbed resulting in oxidative stress.