ABSTRACT
Free radicals are formed from molecules by bond breaking such that each fragment keeps one electron (free radicals may also be formed by collision of the nonradical species by a reaction between a radical and a molecule, which must then result in a radical, as the total number of electrons is odd), by cleavage of a radical to give another radical and by oxidation or reduction reactions. Aerobic organisms, including man, animals, and plants, are constantly challenged by reactive nitrogen species (RNS) and reactive oxygen species (ROS). These are either synthesized endogenously, e.g., in energy
metabolism and by the immune defense system of the body, or produced as reactions to exogenous exposures such as cigarette smoking, imbalanced diet, exhaustive exercise, environmental pollutants, and food contaminants (1-3). Reactive nitrogen species is a collective term that includes nitric oxide (NO) and nitrogen dioxide (NO2
) radicals, as well as nonradicals such as peroxynitrite (OONO), nitrous acid (HNO2), and dinitrogen trioxide (N2O3). Reactive oxygen species is also a collective term that includes both oxygen radicals, e.g., peroxyl (ROO), superoxide (O2
–), and hydroxyl (OH) radicals and the nonradicals, e.g., singlet oxygen (1O2), hypochlorous acid (HOCl), and hydrogen peroxide (H2O2). Intricate and diverse defense systems exist in vivo (2-5). For example, antioxidant protection is afforded by the antioxidant enzymes such as superoxide dismutases, catalase, and glutathione peroxidase, as well as by low molecular weight antioxidants such as glutathione, NADH, carnosine, uric acid, melatonin, a-lipoic acid, bilirubin, some of them are endogenously produced and some are provided through dietary intake (ascorbic acid, tocopherols, ergothioneine, carotenoids, quinones, phenolics). Links continue to be made between oxidative stress and increased risk of chronic diseases such as cancer, cardiovascular diseases, eye diseases such as cataract, and agerelated macular degeneration, immune and neurodegenerative disorders, as well as for the general ageing process (2,6,7).
II. NEURODEGENERATIVE DISORDERS
