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This study evaluated the response of antioxidant enzymes, such as Catalase (CAT) and Glutathione-S-transferase (GST) in freshwater Nile tilapia fish Oreochromis niloticus (O. niloticus) exposed to heavy metals (HMs) including copper (Cu), lead (Pb) and cadmium (Cd). Fish were expose to various concentrations of Cu2+, Pb2+ (0, 0.02, 0.05, 0.2 mg/l) and Cd2+ (0, 0.005, 0.01, 0.05 mg/l) for 15, 30, 45 and 60 days. The results indicated that enzyme activity was different according to the exposure time, concentration and type of heavy metals. CAT activity increased significantly beginning at day 45 of HMs exposure. After 60 days of exposure, CAT activity was steady with Cu and Pb but inhibited by Cd. While, GST induction was earlier observed from day 15 of HMs exposure. The increase of GST activity was found with the increase of exposure time in the treatment with Cu and Cd but not with Pb. Interestingly, GST activity was inhibited by Pb at longer exposure (45 and 60 days). Among tested metals, Cu showed the weaker effect on the activity of CAT and GST in comparison with Pb and Cd suggesting that these enzymes were less sensitive to Cu than other tested metals.
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