ABSTRACT
Free radicals and other reactive oxygen species (ROS) are formed as side products of oxygen metabolism in every aerobic organism. Acceptance of a single electron by an oxygen molecule forms the superoxide radical, 0 2~, which has an unpaired electron. Superoxide is formed in vivo in a variety of ways. A major source is the mitochondrial electron transport chain. The electrons passing through this chain are captured by 0 2 leading to water as the end-product. Because 0 2 accepts one electron at a time, 0 2~ is formed. Efficient antioxidant systems have been developed to prevent uncontrolled free radical formation before they can cause damage to cellular structures (Fig. 1). Superoxide dismutase is involved in detoxi fication by dismutation of 0 2“ to hydrogen peroxide (H20 2), which is rapidly converted to water by the reducing enzymes catalase or glutathione peroxidase (1-3). When the balance between the production of oxygen-derived species, such as 0 2“ and H20 2, and antioxidant defenses against them is disturbed, competing mechanisms can lead to abnormal levels of these molecules. H20 2 can react with ferrous ion (Fe2+) to undergo Fenton-type activation, giving rise to the highly cytotoxic hydroxyl radical (OH’), as schematized in Figure 1.
