cDNA encoding glutathione peroxidase (GPx) mRNA of the white shrimp Litopenaeus vannamei was obtained from haemocytes by a reverse-transcription polymerase chain reaction (RT-PCR) and rapid amplification of cDNA (RACE) using oligonucleotide primers based on the GPx sequence of Homo sapiens (NM002083), Mus musculus (NM008160), Arabidopsis thaliana (U94495), Bos taurus (NM174770), and Capsicum chinense (AJ973135). The 727-bp cDNA contained an open reading frame (ORF) of 567 bp, a 101-bp 5'-untranslated region, and a 59-bp 3'-untranslated region containing the poly A tail. The molecular mass of the deduced amino acid (aa) sequence (189 aa) was 19.25 kDa long with an estimated pI of 8.39. It contains a putative selenocysteine residue which is encoded by the unusual stop codon, TGA, and forms the active site with residues Glu(75) and Trp(153). Comparison of amino acid sequences showed that white shrimp GPx is more closely related to GPx1 and GPx2 than to GPx3 and GPx4 of various animals. The GPx cDNA was synthesized in haemocytes, gills, the hepatopancreas, intestines, and muscles. The respiratory bursts of shrimp increased significantly after a Vibrio alginolyticus injection in order to kill the pathogen, and then induced increases in the activities of SOD and GPx to protect cells against damage from oxidation. However, GPx activity increased as a result of upregulated expression of GPx mRNA which was induced by the increase in H(2)O(2).