Abstract: Superoxide dismutase (SOD) plays a role as an antioxidant by scavenging superoxide produced in organisms. Extracellular superoxide dismutase (Ec-SOD) is an important antioxidant protein in extracellular environments. However, Ec-SOD homologs in oyster plasma have been characterized to lack the SOD activity. In this study, 3 sequences that shared high similarity with dominin of the eastern oyster (Crassostrea virginica) were identified in the genome of the Pacific oyster (Crassostrea gigas) and verified by RT-PCR as real genes. The predicted mature peptide of Cg-dominin2, one of the 3 genes, was recombinantly expressed in Escherichia coli. The total antioxidant activity of the recombinant Cg-dominin 2 protein was equivalent to 0.0525 mmol/g Trolox. After challenges by zinc and cadmium ions, the relative expression of Cg-dominin2 gene was measured using qRT-PCR to be up-regulated significantly and the correspondent metal content of the oyster plasma proteins was also increased. These results suggested that the Ec-SOD homologs might play a role in defending against heavy metal stress in oysters by participating in metal metabolism.

Key words: Pacific oyster, superoxide dismutase, antioxidant, metalloprotein, environmental stress