Characterization and mRNA expression of Mn-SOD and physiological responses to stresses in the Pacific oyster<i>Crassostrea gigas</i>

Abstract

Abstract Superoxide dismutases (SODs) are metalloenzymes that play an important role in mollusc immune defence systems by eliminating oxidative stress to reactive oxygen species. We investigated physiological changes in the Pacific oyster Crassostrea gigas caused by exposure to pollutants (cadmium, tributyltin) and acute water temperature change. We analysed mRNA expression of Mn-SOD in gills using a quantitative polymerase chain reaction (QPCR), and measured the glutamate oxaloacetate transaminase (GOT), glutamate pyruvate transaminase (GPT) and hydrogen peroxide (H2O2) levels in haemolymph based on time- and dose-related effects of pollutants and acute water temperature treatments. We cloned a cgMn-SOD full-length cDNA that included an open reading frame of 675 nucleotides that was predicted to encode proteins of 225 amino acids. BLAST analysis of other species indicated that residues essential to the enzymatic functions of Mn-SOD proteins are highly conserved. Levels of Mn-SOD mRNA expression, GOT and GPT were gradually increased and then decreased during the experimental periods. On the other hand, H2O2 levels increased continuously during the exposure periods. These results suggest that Mn-SOD plays an important role in the physiological changes related to metabolism and cell protection that occur in C. gigas when exposed to oxidative stress by pollutants and fluctuations in water temperature.