Characterization and in vitro functional analysis of thioredoxin glutathione reductase from the liver fluke Opisthorchis viverrini

The carcinogenic liver fluke Opisthorchis viverrini causes several hepatobiliary diseases including a bile duct cancer-cholangiocarcinoma (CCA), which is a major public health problem in many countries in the Greater Mekong Sub-region. Praziquantel is the main drug against this parasite, however, reduced drug efficacy has been observed in some endemic areas. Therefore, alternative drugs are needed to prepare for praziquantel resistance in the future. The selenoprotein thioredoxin glutathione reductase (TGR) enzyme, which plays a crucial role in cellular redox balance of parasitic flatworms, has been shown as a potential drug target against these parasites. Hence, this study aimed to investigate the TGR of O. viverrini and assess its potential as a drug target. An open reading frame (ORF) that encodes O. viverrini TGR (Ov-TGR) was cloned from an O. viverrini cDNA library and the nucleotide were sequenced. The 1,812 nucleotides of the Ov-TGR full ORF encoded a polypeptide of 603 amino acid residues with a predicted molecular mass of 66 kDa. The putative amino acid sequence shared 55-96.8% similarities with TGRs from other helminths and mammals. Phylogenetic analysis revealed a close relationship of Ov-TGR with that of other trematodes. The ORF of Ov-TGR was inserted into pABC2 plasmid and transformed into Escherichia coli strain C321.Delta A to facilitate selenocysteine incorporation. The recombinant OvTGR (rOv-TGR-SEC) was expressed as a soluble protein and detected as a dimer form in the non-reducing sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Its thioredoxin reductase (TrxR) and glutathione reductase (GR) activities were detected using DTNB, Trx and GSSG substrates with the Michaelis constant (Km) of 292.6 +/- 52.3 mu M, 8.09 +/- 1.91 mu M and 13.74 +/- 1.2 mu M, respectively. The TGR enzyme activities were effectively inhibited by a well-known inhibitor, auranofin in a dose-dependent manner. Moreover, auranofin expressed a lethal toxic effect on both newly excysted juveniles (NEJs) and adult worms of O. viverrini in vitro. Taken together, these results indicated that Ov-TGR is crucial for O. viverrini survival and maybe a potential target for the development of novel agents against opisthorschiasis.