In all vertebrates studied so far, antioxidant enzymes (AE) seem to have similar structural and functional properties. The most relevant inter-species difference is the quantitative distribution of AE in different tissues and at sub- cellular levels. Mitochondria are the most important sub-cellular sites of primary superoxide anion (O2·-) production and subsequent reactive oxygen species (ROS) formation, and therefore red muscle mitochondria are a major source of ROS in true endotherms such as most mammals and birds. In the majority of fish, amphibians and reptiles, red muscles are relatively scarce, and other tissues such as liver, blood, heart, kidney, roe, and lens are more important in this regard. Active marine fish species display, in general, higher AE activity in the liver and blood, comparing to sedentary or bottom-dwelling species. Antioxidant status in marine fish tissues may be related to the tissue O2 consumption and of the whole organism. By contrast, in freshwater fish, antioxidant status is better related to physical and chemical characteristics of the environment than to the fish activity level. In this context, it can be expected that antioxidants of fish and other termoconformers depend directly on O2 consumption and/or O2 availability, supported by the fact that O2·- production in mitochondria is almost linearly related to O2 tensions. A different strategy was apparently adopted by true endotherms such as most birds and mammals in which a relatively low rate of mitochondrial O2·- generation is not always related with their O2 consumption. The relatively high antioxidant levels that typify fish may safeguard water- breathing organisms, considering the consequences of temperature changes and therefore metabolic changes. In addition, seasonal and daily antioxidant adaptations seem to be common in fish, in freeze-tolerant amphibians and reptiles, as well as in mammals that are subject to hibernation or torpor. Hypoxia and hyperoxia provokes different antioxidant responses in vertebrates. Some ontogenetic antioxidant differences in fish, reptiles, and mammals are reported in the literature, and are probably present in other vertebrates. This paper deals with comparative aspects of vertebrate enzymatic and non- enzymatic antioxidants with emphasis on fish and mammals, considering data available in the literature related to normal” conditions and the adaptations to different internal or external determinants such as temperature, O2 tensions, and ontogenetic and seasonal variations. No attempts have been made to cover the bulk of publications on pathological events, exercise, environmental contaminants, effects of diets, or antioxidant supplementation.
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