Response of antioxidant enzymes to Cd and Pb exposure in water flea Daphnia magna: Differential metal and age - Specific patterns

To investigate oxidative stress responses to cadmium and lead, the freshwater water flea Daphnia magna was exposed to Cd and Pb for 48 h. Following treatment with sub-lethal concentrations, intracellular reactive oxygen species (ROS) levels, as well as modulation of multiple biomarker, such as superoxide dismutase (SOD) activity, glutathione (GSH) contents, glutathione S-transferase (GST) activity, antioxidant enzyme - coding genes (three GST isoforms, glutaredoxin [GRx], glutathione peroxidase [GPx], and thioredoxin [TRx]), and stress-response proteins (heat shock protein 70 [Hsp70] and Hsp90) were examined. The results showed that intracellular ROS level was not changed at 24 h, but reduced at 48 h. Levels of total GSH content were reduced by Cd, but highly induced by Pb. SOD and GST activities were stimulated 48 h after exposure to Cd and Pb. A significant modulation of oxidative stress marker genes was observed after exposure to each element with different expression patterns depending on the metal and developmental stages. In particular, the expression levels of GST-sigma, HSP70, and HSP90 genes were enhanced in Cd - and Pb - exposed neonates. These findings imply that oxidative stress markers appear to be actively involved in cellular protection against metal-induced oxidative stress in D. magna. This study would facilitate the understanding of the molecular response to Cd and Pb exposure in water fleas.