A new thioredoxin reductase with additional glutathione reductase activity in Haemonchus contortus

We report, herein, the purification to homogeneity and the biochemical and kinetic characterization of HcTrxR3, a new isoform of thioredoxin reductase (TrxR) from Haemonchus contortus. HcTrxR3 was found to have a relative molecular weight of 134,000, while the corresponding value per subunit obtained under denaturing conditions, was of 67,000. By peptide mass spectrophotometric analysis, HcTrxR3 was determined to have 99% identity with the H. contortus HcTrxR1 although, and most importantly, they are different in their amino acid sequence in two amino acid positions: 48 (isoleucine instead of leucine) and 460 (leucine instead of proline). The enzyme catalyzes NADPH-dependent reduction of DTNB and, unexpectedly, it follows the pattern of glutathione reductases (GR) performing the reduction of oxidized glutathione (GSSG) to reduced glutathione using NADPH as the reducing cofactor. Hence, it is important to highlight this enzyme's new and unexpected condition that makes so special and one our main finding. Enzyme K_cat values for DTNB, GSSG and NADPH were 12, 3 and 8 s^-1, respectively. HcTrxR3 developed, into specific TrxR substrates: ebselen and sodium selenite, with activity at 0.5 and 0.068 (U/mg), respectively; and 0.044 (U/mg) for S-nitrosoglutathione through its GR activity. The enzyme was inhibited by gold compound auranofin (AU), a selective inhibitor of thiol-dependent flavoreductases. Although HcTrxR3 has both TrxR and GR activity as thioredoxin glutathione reductase (TGR) does, it is a TrxR because it has no glutaredoxin domain and it does not develop any hysteretic behavior as does TGR. The importance of this new enzyme is potential to further clarify the detoxification and haemostasis redox mechanism in H. contortus. Likewise, this enzyme could also be a protein model to recognize more differences between TrxR and GR.