A major threat to cellular homeostasis of aerobic organisms arises from normal metabolic processes that are essential to the cell. Although aerobic lifestyle provides many advantages, utilisation of oxygen by cells results in production of reactive oxygen species (ROS), which can initiate free radical chain reactions eventually leading to oxidative damage of DNA, proteins, carbohydrates, and lipids.

To neutralise the oxidative effects of reactive metabolites of oxygen the evolutionary survival processes have provided aerobic organisms with a system of biochemical defences, which include both low molecular weight free radical scavengers and complex enzyme systems.

When ROS generation exceeds the antioxidant capacity of cells, a situation of oxidative stress ensues that leads to excessive molecular damage and tissue injury. Thus the integrity and survival of an organism depend on the ability of tissues to cope with damaging free radical reactions. Comparative studies among different species could help to establish whether during evolution such an ability has undergone particular modifications and what factors should have determined them. At present, a firm answer to the above questions cannot be given. In fact, there is poor information on overall antioxidant capacity of animal tissues and it is difficult to draw conclusions regarding the functional consequences of differences in the levels of single antioxidants. However, available data seem to indicate that antioxidant defences in tissues of vertebrate species are adapted to ROS production associated to mitochondrial basal respiration. Moreover, some studies have shown constitutive and induced modifications in the cellular antioxidant systems which allow to deal with increased ROS production due to either organism activity or environmental changes.