Influence of season and pollution on the antioxidant defenses of the cichlid fish acará (Geophagus brasiliensis)

The livers of Geophagus brasiliensis collected from both a non-polluted site and a polluted site were analyzed for different antioxidant defenses, O2 consumption, thiobarbituric acid-reactive substance (TBARS) levels, and histological damage. Compared to controls (116.6 + or - 26.1 nmol g-1), TBARS levels were enhanced at the polluted site (284.2 + or - 25.6 nmol g-1), as also was oxygen consumption (86.6 + or - 11.3 and 128.5 + or - 9.8 µmol O2 min-1 g-1, respectively). With respect to enzymatic antioxidants, increased catalase activities (8.7 + or - 1.3 and 29.2 + or - 2.4 mmol min-1 g-1, respectively), unchanged superoxide dismutase activities (767.2 + or - 113.3 and 563.3 + or - 70.2 U g-1, respectively), and diminished glutathione S-transferase activities (29.0 + or - 3.2 and 14.9 + or - 3.2 µmol min-1 g-1, respectively) were detected. Reduced glutathione (1.91 + or - 0.17 and 1.37 + or - 0.25 mM, respectively), oxidized glutathione (1.50 + or - 0.20 and 0.73 + or - 0.17 mM, respectively), and total glutathione (3.40 + or - 0.26 and 2.07 + or - 0.27 mM, respectively) concentrations were also below control values at the polluted site. Nevertheless, the observed ethoxyresorufin-O-deethylase activities (1.34 + or - 0.11 and 16.7 + or - 0.21 pmol min-1 mg-1, respectively) showed enhanced values at the polluted site. The main histological damage observed in the hepatocytes from fish collected at the polluted site was characterized by heavy lipid infiltration. Fish collected at the end of spring showed higher O2 consumption, higher superoxide dismutase and glutathione S-transferase activities, and higher total and oxidized glutathione concentrations compared to the beginning of autumn. No seasonal changes were observed in catalase activities, glutathione or TBARS levels. Fish chronically exposed to relatively high pollution levels seem to be unable to set up adequate antioxidant defenses, probably due to severe injury to their hepatocytes. The higher antioxidant defenses found at the end of spring are probably related to the enhanced activities during high temperature periods in thermoconforming organisms.

Fish antioxidant defenses: a comparative approach

There are few comparative studies of vertebrate antioxidant defenses (AD) in the literature. Enzymatic (superoxide dismutase, SOD, and catalase, CAT) and non-enzymatic alpha-tocopherol, beta-carotene, ubiquinol(10) and blood glutathione) antioxidant defenses were investigated in the liver and blood of 37 fish species, 15 marine species of the southeastern Brazilian coast and 22 freshwater species from the Central Amazon basin. More active marine species display in general higher concentrations of SOD and CAT in the liver and blood, compared to those of sedentary or bottom-dwelling species. AD status in marine fish may be related to the oxygen consumption of the tissues and of the whole organism, while in freshwater fish AD may be related to physical and chemical characteristics of the environment rather than to activity level. As thermoconformer organisms, most fish must routinely cope with environmental temperature changes and, consequently, with changes in their metabolic rates. The relatively high antioxidant defense levels that typify fishes, even when compared to endotherms such as birds and mammals, may protect aquatic organisms against the consequences of temperature oscillations.

Gill diffusion as a physiological mechanism for hydrogen peroxide elimination by fish

Hydrogen peroxide is metabolized by the specific enzymatic action of catalase and glutathione peroxidase in animal tissues. The relatively low catalase and glutathione peroxidase activities found in the blood of fish may be related to the ability of gills to eliminate hydrogen peroxide into the aquatic environment. Poecilia vellifera releases hydrogen peroxide apparently by gill diffusion into the environment, resulting in a steady-state H2O2 concentration of about 0.6 µM in the surrounding water. This physiological mechanism resembles ammonia excretion by teleost fish