ABSTRACT
Under physiological conditions, oxygen-derived free radicals or reactive oxygen species (ROS) such as superoxide, hydrogen peroxide, and hydroxyl radicals are metabolic byproducts in all aerobic organisms, including humans. By definition, a free radical is any atom, molecule, or compound that possesses at least one unpaired electron. Owing to the high tendency of the unpaired electron to acquire a partner electron, the free radical is highly reactive in oxidative reactions, in which the free radical grabs an electron from a victim molecule. In addition to energy metabolism, oxygen-and nitrogen-centered free radicals are products of other biochemical reactions, such as those involved in cytochrome P450-mediated arachidonic acid and xenobiotic metabolisms.
