Nitric oxide (NO) is a major messenger molecule that plays key roles in many physiological processes. NO is produced by nitric oxide synthase (NOS), which catalyzes the conversion of L-arginine to L-citruline and NO. At least three isoforms of NOS have been identified and characterized, namely, neuronal NOS (nNOS), endothelial NOS (eNOS) and inducible NOS (iNOS). Recent evidence indicates that most of the cytotoxicity attributed to NO is rather due to peroxynytrite, produced from the diffusion controlled reaction betwen NO and another free radical, the suproxide anion. Peroxynitrite interacts with protein, DNAand lipids via direct oxidative reactions or via indirect radical mediated mechanisms

Reactive products of oxygen are among the most potent and omnipresent threats faced by the living organism. Intracellular accumulation of reactive oxygen species such as superoxide anion, hydrogen peroxide, hydroxyl radical, and peroxy radical, can arise from toxic insults or normal metabolic processes. These species may perturb the cell's natural antioxidant defence systems, resulting in damage to all of the major classes of biological macromolecules, including nuclear acids, proteins, carbohydrates and lipids. Oxidative stress has been defined as a disturbance in the prooxidant-antioxidant balance, resulting in potential cell damage. It has been implicated in several biological and pathological processes like ageing, inflammation, carcinogenesis, ischemia-reperfusion and in diseases including diabetes mellitus, atherosclerosis, and/or neurodegenerative diseases

Cells continuously produce free radicals and reactive oxygen species as part of metabolic processes. These free radicals are neutralized by antioxidant defense system consisting of enzymes such as catalase, superoxide dismutase, glutathione peroxidase, and numerous non-enzymatic antioxidants, including vitamins A, E and C, glutathione, ubiquinone, and flavonoids. Antioxidants include both water and lipid soluble varieties. The lipid soluble antioxidants are located in the cellular membranes and lipoproteins, whereas the water soluble antioxidants are present in the aqueous environments, such as fluids inside cells and in the blood.