Action of Antioxidants Against Oxidative Stress

Abstract

The active oxygen species and free radicals are formed in vivo endogenously by several normal metabolic processes and abnormal events. In addition, exogenous sources of oxidants such as cigarette smoke and UV light may also increase oxidative stress in vivo. These oxidants attack DNA, lipids, sugars and proteins to cause oxidative damage, which eventually leads to various diseases, cancer and aging. It has been estimated that the number of oxidative hits to DNA per cell per day is about 105 in the rat and roughly 10 times fewer in the human (Ames et al. 1993). Although there is no definitive evidence that free radical involvement is obligatory in carcinogenisis, mutation and transformation, it is clear that free radicals in biological systems could lead to mutation, transformation, and ultimately cancer. Free radicals may attack DNA directly and DNA may also be modified through oxidation of lipids, sugars and proteins. The aerobic organisms are protected from such oxidative stress by an array of defense system. Various kinds of antioxidants with different functions act in the defense system in vivo (Table 1). From the type of functions, the antioxidants may be classified into the following three groups: (1) preventive antioxidants which suppress the formation of free radicals and active oxygens species, (2) radical-scavenging antioxidants which inhibit chain initiation and break chain propagation, and (3) repair and de-novo antioxidant enzymes. These antioxidants act as the first, second and third defense line, respectively. Furthermore, there is another function in which the appropriate antioxidant is formed at the right time and transfer it to the right place in a right concentration. This “adaptation mechanism” may be regarded as the fourth defense line.