Biotransformation and glutathione homeostasis in rainbow trout Exposed to chemical and physical stress

Glutathione, together with biotransformation and antioxidative enzymes, plays an important role in protecting the cells against damage caused by free radicals, peroxides, oxidizing metabolites and xenobiotics introduced by environmental stress. Pollution of the environment causes many long-term physiological effects on fish. Furthermore, in mammals exercise increases oxidative stress (increase of reactive oxygen species) which overloads the defence mechanisms. Induction of the glutathione system could show whether physical stress also affects defence mechanisms of fish. In the present series of studies we have investigated the biotransformation and glutathione homeostasis of rainbow trout exposed to chemical and physical stress. Juvenile rainbow trout (Oncorhynchus mykiss) were exposed to waterborne hexachlorobenzene (HCB; 2 μg/litre) together with duck-weed (Lemna minor) using different combinations of fish, plants and HCB. In another experiment, fish were held in different water flows (none, medium, high). Hepatic mono-oxygenase (EROD) and conjugation (GST) enzyme activities, as well as antioxidant systems (GPX, GR, tGSH) were measured. In the HCB-exposed fish-plant group EROD activities in fish were suppressed compared with the uncontaminated group. It is probable that through digested plants the fish could have been exposed to possible plant-derived HCB metabolites or secondary metabolites. Fish of different physical activity showed slightly elevated EROD activities three weeks from the beginning of the experiment. After the following three-week period EROD activities had decreased to the starting level. Three weeks from the beginning GST activities were at their highest level. The elevation of mono-oxygenase activities may have caused increased production of reactive intermediates which were further metabolized through activated glutathione system. Elevated total glutathione contents in studied tissues showed the potency of these fish to resist oxidative stress.