Functional expression and characterization of an iron-containing superoxide dismutase of Acanthamoeba castellanii

Acanthamoeba spp. are free-living amoebae, but opportunistic infections of some strains of the organisms cause severe diseases such as acanthamoebic keratitis, pneumonitis, and granulomatous amoebic encephalitis in human. In this study, we identified a gene encoding iron superoxide dismutase of Acanthamoeba castellanii (AcFe-SOD) and characterized biochemical and functional properties of the recombinant enzyme. Multiple sequence alignment of the deduced amino acid sequence of AcFe-SOD with those of previously reported iron-containing SODs (Fe-SODs) from other protozoan parasites showed that AcFe-SOD shared common metal-binding residues and motifs that are conserved in Fe-SODs. The genomic length of the AcFe-SOD gene was 926 bp consisting of five exons interrupted by four introns. The recombinant AcFe-SOD showed similar biochemical characteristics with its native enzyme and shared typical biochemical properties with other characterized Fe-SODs, including molecular structure, broad pH optimum, and sensitivity to hydrogen peroxide. Immunolocalization analysis revealed that the enzyme localized in the cytosol of the trophozoites. Activity and expression level of the enzyme were significantly increased under oxidative stressed conditions. These results collectively suggest that AcFe-SOD may play essential roles in the survival of the parasite not only by protecting itself from endogenous oxidative stress but also by detoxifying oxidative killing of the parasite by host immune effector cells.