ABSTRACT
Human cells, as part of their normal metabolism, generate free radicals in the natural process of producing energy. The production of cellular energy is essential for the normal functioning of the body. Even the smallest physical activity that we take for granted, such as lifting our arm, breathing, and blinking our eyes, requires the generation of ATP by the cells. In the process, a lot of free radicals are generated as part of the body’s metabolism and defensive processes. The free radicals that can be formed within the body include the superoxide anion ( )O2− , the hydroxyl radical (OH⋅), singlet oxygen ( ),1 2O and hydrogen peroxide (H2O2). Superoxide anions are formed when oxygen (O2) acquires an additional electron, leaving the molecule with only one unpaired electron. Within the mitochondria, O2− is continually formed, and the rate of formation depends on the amount of oxygen owing through the mitochondria at any given time. Hydroxyl radicals are short-lived, but are the most damaging radicals within the body. They can be formed from O2− and H2O2 via the Haber-Weiss reaction.1 The oxidation of carbohydrates, fats, and proteins to produce energy through both aerobic and anaerobic processes can also lead to the production of free radicals. The progression of aerobic metabolic processes, such as respiration and photosynthesis, inevitably leads to the production of reactive oxygen species (ROS) in mitochondria, chloroplasts, and peroxisomes. The mitochondrial respiratory chain, under hypoxic conditions, also produces nitric oxide (NO), which can regenerate reactive nitrogen species (RNS). Excessive lipid peroxidation can also lead to the formation of reactive aldehydes such as malondialdehydes and 4-hydroxynonenal. Other reactive species that may be injurious to the body include reactive molecules generated from the metabolism of carbohydrates, fats, and proteins.
