Copper-induced changes in antioxidant enzymes activities in fronds of duckweed (Lemna minor)

Changes in activity of antioxidant enzymes caused by copper sulfate were ex amined in Fronds of duckweed (Lemna minor) during time- and concentration-d ependent exposure to the metal salt, Glutathione S-transferase and glutathi one reductase activities were inhibited by the high CuSO4 concentrations wh ile a strong stimulation of catalase, guaiacol peroxidase and pyrogallol pe roxidase activities were observed from the lowest concentration tested (0.2 5 mu M). Ascorbate peroxidase was transiently and weakly stimulated by low CuSO4 treatments while longer exposure led to an inhibition of its activity up to 40% after a 96-h exposure to 1.6 mu M of CuSO4. Our results showed t hat inducibility of catalase and guaiacol peroxidase was highly significant since the highest CuSO4 concentrations (5-10 mu M) were responsible for 24 0-280% increases in their activity. This stimulation was transient reaching maximum values between 12 and 36 h. The simultaneousness of enzymatic resp onses suggested that induction of these various enzymes is part of a genera l defense strategy to cope with overproduction of reactive oxygen species, The rapid inducibility of some of these enzymes are useful early and sensit ive indicators of heavy metal toxicity. (C) 2000 Elsevier Science Ireland L td. All rights reserved.