A novel extracellular copper/zinc superoxide dismutase identified from Nibea albiflora and its characteristics under ammonia/nitrite stress

The cDNA of EcSOD (designated as NaEcSOD), cloned and characterized from Nibea albiflora for the first time, was 1101 bp in length including 5′-untranslated region (UTR) of 224 bp, 3′ UTR of 229 bp, and an open reading frame (ORF) of 648 bp encoding a polypeptide of 215 amino acids. Based on the results of GST pull-down assay and mass spectrometric identifications, a membrane protein (annexin A4) was found to specifically bind with NaEcSOD, indicating that the two proteins might be involved in antagonizing reactive oxygen species (ROS) together. The mRNA transcripts of NaEcSOD were detected in liver, brain, swim bladder, stomach, spleen, heart, gill, intestine, kidney, head-kidney, and muscle, and the predominant distribution was in liver. When exposed to ammonia nitrogen (ammonia-N) or nitrite nitrogen (nitrite-N), N. albiflora showed lower tolerance to ammonia-N because the 96 h LC50 was 20.23 mg/L under ammonia-N stress (0.69 mg/L in the form of NH3-N) and 99.08 mg/L under nitrite-N stress. The temporal mRNA expressions of NaEcSOD were significantly up-regulated in liver, gill, and head-kidney under the exposure to ammonia-N/nitrite-N at the concentration of 96 h LC50.