Cloning and characterization of two glutathione peroxidase cDNAs from southern bluefin tuna (Thunnus maccoyii)

Glutathione peroxidases (GPXs; EC 1.11.19) are key enzymes in the antioxidant defence systems of living organisms, including fish. Here we report the cloning of two GPX cDNAs from southern bluefin tuna (SBT, Thunnus maccoyii). One encodes a GPX1 or classical GPX and the other encodes a GPX4 or phospholipid hydroperoxide GPX. The 898 base-pair (bp) SBT GPX1 cDNA (GenBank accession no. EF452497) includes the complete protein coding region plus 35. bp of the 5'-untranslated region (5'-UTR) and 296. bp of the 3'-untranslated region (3'-UTR). The 974. bp SBT GPX4 cDNA (GenBank accession no. EF452498) includes the complete protein coding region plus 101. bp of the 5'-UTR and 306. bp of the 3'-UTR. Both cDNAs contain a selenocysteine (Sec) codon in the protein coding region and a selenocysteine insertion sequence (SECIS) element in the 3'-UTR. The SBT GPX4 cDNA may encode a mitochondrial targeting sequence. A deduced amino acid sequence comparison revealed that the SBT GPX1 sequence was 67% identical to a human GPX1 sequence (GenBank accession no. X13709) and 69-83% identical to other fish GPX1 sequences. Similarly, the SBT GPX4 sequence was 63% identical to a human GPX4 sequence (GenBank accession no. BC046163) and 72-93% identical to other fish GPX4 sequences. GPX1 and GPX4 gene expressions and total GPX enzyme activity were investigated in SBT liver, muscle, kidney, spleen, heart, stomach, gill and pyloric caeca. Expression of the two genes varied from one tissue to the next and roughly paralleled the inter-tissue variation in GPX enzyme activity. The results are discussed in relation to possible roles for GPX1 and GPX4 genes in antioxidant defense in SBT.