Phospholipid-hydroperoxide glutathione peroxidase (PHGPx) enzymes are associated with cellular protection by the role they play in reducing hydroperoxides of phospholipids, thereby preventing membrane lipoperoxidation. As part of their toxic effect, some pesticides stimulate peroxidation of cellular membranes. We isolated and sequenced a PHGPx gene from the cattle tick Boophilus microplus that encodes a protein of 169 amino acids, including a TGA-encoded selenocysteine at residue 46 and active site residues Gln(82) and Trp(135) that interact with the selenocysteine. The motif that directs the insertion of selenocysteine at the opal codon is found in the 3'-untranslated region. PHGPx sequences from pesticide-resistant and susceptible B. microplus ticks show nucleotide differences at eight positions among the strains, with five resulting in amino acid substitutions in the deduced protein sequence. Two distinct PHGPx alleles were identified in an organophosphate-resistant tick strain. Real-time PCR quantification of gene expression revealed increased PHGPx in two strains resistant to a single acaricide class. Strains resistant to two or more classes showed a reduction in PHGPx.
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