Copper/Zinc Superoxide Dismutase from the Cladoceran Daphnia magna : Molecular Cloning and Expression in Response to Di fferent Acute Environmental Stressors
The copper/zinc superoxide dismutase (Cu/Zn- SOD) is a representative antioxidant enzyme that is responsible for the conversion of superoxide to oxygen and hydrogen peroxide in aerobic organisms. Cu/Zn-SOD mRNAs have been cloned from many species and employed as useful biomarkers of oxidative stresses. In the present study, we cloned Cu/Zn-SOD cDNA from the cladoceran Daphnia magna, analyzed its catalytic properties, and investigated mRNA expression patterns after exposure to known oxidative stressors. The full-length Cu/Zn- SOD of the D. magna (Dm-Cu/Zn-SOD) sequence consisted of 703 bp nucleotides, encoding 178 amino acids, showing wellconserved domains that were required for metal binding and several common characteristics. The deduced amino acid sequence of Dm-Cu/Zn-SOD showed that it shared high identity with Daphnia pulex (88%), Alvinella pompejana (56%), and Cristaria plicata (56%). The phylogenetic analysis indicated that Dm-Cu/Zn-SOD was highly homologous to D. pulex. The variation of Dm-Cu/Zn-SOD mRNA expression was quantified by real-time PCR, and the results indicated that the expression was up-regulated after 48-h exposure to copper, un-ionized ammonia, and low dissolved oxygen. This study shows that the Dm- Cu/Zn-SOD mRNA could be successfully employed as a biomarker of oxidative stress, which is a common mode of toxicity for many other aquatic hazardous materials.