The Role of Reactive Oxygen Species and Antioxidants in Oxidative Stress

Under normal conditions, the physiologically important intracellular levels of reactive oxygen species (ROS) are maintained at low levels by various antioxidant systems in the body but at high concentrations, they produce adverse modifications to cell components such as lipids, proteins and DNA. In humans, oxidative stress is involved in the development of cancer, parkinson's disease, alzheimer's disease, atherosclerosis, heart failure, myocardial infarction, fragile X syndrome, sickle cell disease, lichen planus, vitiligo, autism, infection, chronic fatigue syndrome and depression. However, reactive oxygen species can be beneficial, as they are used by the immune system as a way to attack and kill pathogens, also short-term oxidative stress is important in the prevention of aging by induction of mitohormesis. ROS can be from endogenous or exogenous sources and they are produced from molecular oxygen as a result of normal cellular metabolism, they are divided into free radicals and non-radicals. The major endogenous sources of ROS of physiological importance are superoxide anion, hydroxyl radical, hydrogen peroxide, hypochlorous acid, peroxyl radical and hydroperoxyl radical while the exogenous sources of ROS include cigarette smoke, ozone exposure, hyperoxia, ionizing radiation and heavy metal ions. Aerobic organisms have integrated antioxidant systems, which include enzymatic and non-enzymatic antioxidant that are usually effective in blocking harmful effects of reactive oxygen species. Enzymatic defense is a system of enzymes which includes glutathione peroxidases, superoxide dismutases and catalase, which decrease the concentration of the most harmful ROS whereas non-enzymatic antioxidants include low molecular weight compounds such as vitamins C and E, β-carotene, uric acid and glutathione. Humans have a complex antioxidant protection system, which functions interactively and synergistically to neutralize free radicals and knowledge as presented in this review will be important in the management of oxidative stress and its associated