Paul H. Gamache

The old adage “moderation in all things” is perhaps best demonstrated by the toxicity of pure oxygen. Although it is obvious that oxygen is essential for life (at least for aerobic organisms), breathing pure oxygen at atmospheric pressure for longer than 48 hours will lead to respiratory distress and death (Huber and Drath, 1981). This toxicity is not due entirely to oxygen per se but more to the production of a variety of reactive oxygen species (ROS). Several ROS exist. Diatomic molecular oxygen (O₂) readily reacts to form partially reduced species, which are generally short-lived and highly reactive and include the superoxide anion (O₂), the hydroperoxyl radical (HO₂’) as well as the most reactive species, the hydroxyl free radical (HO’). ROS are produced as part of normal metabolism (e.g. superoxide is formed when xanthine is converted to uric acid by xanthine oxidase) and are also extremely useful (e.g. superoxide and hydrogen peroxide are the major “microbicides” produced by circulating phagocytic leukocytes) (Johnston et al., 1975; Babior, 1978; Colton and Gilbert, 1987; Green et al., 1987). Furthermore, the effectiveness of certain anticancer drugs is also dependent on their ability to produce free radicals (Kehrer, 1993).