Effects of heavy metals (Cd, Cu, Cr, Pb, Zn) on fish glutathione metabolism.

The glutathione metabolism contains crucial antioxidant molecules to defend the organisms against oxidants. Thus, the aim of this study was to investigate the response of the glutathione metabolism in the liver of freshwater fish Oreochromis niloticus exposed to metals (Cu, Cd, Cr, Pb, Zn) in different periods. Fish were exposed to metals (as 1 µg/mL) individually for 1, 7, and 14 days and subsequently antioxidant enzymes (glutathione peroxidase, GPX; glutathione reductase, GR and glutathione S-transferase, GST) and glutathione levels (total glutathione, tGSH; reduced glutathione, rGSH; oxidized glutathione, GSSG and GSH/GSSG ratios) in the liver were measured. There was no fish mortality during the experiments, except Cu exposure. The antioxidant enzymes responded differently to metal exposures depending on metal types and exposure durations. GPX activity increased only after Cd exposure, while GST activity increased following 7 days of all metal exposures. However, GR activity did not alter in most cases. Total GSH and GSH/GSSG levels generally decreased, especially after 7 days. Data showed that metal exposures significantly altered the response of antioxidant system parameters, particularly at day 7 and some recovery occurred after 14 days. This study suggests that the response of antioxidant system could help to predict metal toxicity in the aquatic environments and be useful as an "early warning tool" in natural monitoring studies.

Effects of fish size on the response of antioxidant systems of Oreochromis niloticus following metal exposures.

The size of a fish is an important factor in its physiology, and metal uptake is affected by animal physiology. In this study, small and large tilapias (Oreochromis niloticus) differing approximately twofold in length and fivefold in weight were compared for their antioxidant response. Both groups were exposed to Cu or Cr (1.0 µg/mL) in a freshwater (-80 mg CaCO3/L, conductivity 1.77 mS/cm) using 2 exposure protocols (20 µM for 48 h and 10 µM for 6 days). Following the exposures, the antioxidant enzyme activities (superoxide dismutase, SOD; catalase, CAT; glutathione peroxidase, GPX; glutathione reductase, GR and glutathione S-transferase, GST) and glutathione (GSH) levels were measured in the liver of fish. Results showed that small fish was affected from exposure conditions much more than large ones as their antioxidant parameters significantly decreased even in controls. Metal exposures of small fish caused significant increases in SOD and CAT activity in acute Cu or Cr exposures. Subchronic Cr exposure of small fish also caused significant increases in CAT, GPx and GST activities, while there was no significant change in Cu-exposed ones. Large fish, however, showed different antioxidant responses as their levels mostly decreased. This study demonstrated that the response of antioxidant system in the liver of tilapia varied in relation to fish sizes and emphasized using different size groups in environmental monitoring and also in evaluation of fish biomarkers.