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

Changes in activity of antioxidant enzymes caused by copper sulfate were examined in Fronds of duckweed (Lemna minor) during time- and concentration-dependent exposure to the metal salt, Glutathione S-transferase and glutathione reductase activities were inhibited by the high CuSO4 concentrations while a strong stimulation of catalase, guaiacol peroxidase and pyrogallol peroxidase activities were observed from the lowest concentration tested (0.25 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 that inducibility of catalase and guaiacol peroxidase was highly significant since the highest CuSO4 concentrations (5-10 mu M) were responsible for 240-280% increases in their activity. This stimulation was transient reaching maximum values between 12 and 36 h. The simultaneousness of enzymatic responses suggested that induction of these various enzymes is part of a general defense strategy to cope with overproduction of reactive oxygen species, The rapid inducibility of some of these enzymes are useful early and sensitive indicators of heavy metal toxicity. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.