Thiol metabolism in the parasitic nematode Haemonchus contortus
- Publication Type:
- Thesis
- Issue Date:
- 2010
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Haemonchus contortus is an important parasitic nematode, both economically and
pathologically. The emergence of widespread drug resistance requires new drug or
vaccine targets to be identified. The requirement of aerobic organisms to control
damage caused by reactive oxygen species and, the increased necessity of parasites to
overcome the host immune response, has led to the investigation of antioxidant
systems as potential targets. This work examines the thioredoxin antioxidant system in
H. contortus, specifically the thioredoxin reductase and peroxiredoxin enzymes, to
characterise their activity and determine if they are potential targets for parasite
control.
H. contortus contains two TrxRs, a cytoplasmic enzyme HcTrxRl with a selenocysteine
in the active site, similar to the mammalian TrxR, and a mitochondrial enzyme HcTrxR2
with a nematode unique active site. HcTrxRl showed broad activity with thioredoxins
from E. coli, sheep, and H. contortus while HcTrxR2 had high activity with only the
mitochondrial H. contortus thioredoxin 1. Importantly, HcTrxRl was found to be more
sensitive to the black tea inhibitor theaflavin than the selenocysteine containing
mammalian TrxR, demonstrating the differences in the enzymes susceptibilities to
inhibitors. To determine the function of the TrxR enzymes in nematodes, knockout
(KO) strains of Caenorhabditis elegans were examined. TrxRl -/-KO worms were more
sensitive to free radical attack and also to the anthelmintic ivermectin; while TrxR2 -/-
KO eggs were highly sensitive to sodium hypochlorite. This demonstrates that
inhibition of these enzymes would sensitise the nematodes to the host's immune
attack.
H. contortus contains two peroxiredoxins, the mitochondrial HcPrxl and the
cytoplasmic HcPrx2. The activity of both peroxiredoxins was specific for the
thioredoxin system; however, both peroxiredoxins were also able to be regenerated by
the glutathione system when coupled to the nematode specific H. contortus
thioredoxin 5. Both enzymes were stable to high concentrations of hydrogen peroxide
which demonstrates different functions to their mammalian counterparts. A specific
inhibitor of these peroxiredoxins was also identified which has minimal mammalian
cytotoxicity. HcPrxl was found to be involved in drug resistance while HcPrx2 was
found to be secreted and highly immunogenic. Analysis of homologous genes in C.
elegans showed that both peroxiredoxin KO worms were sensitive to free radical
attack; however, only the cytoplasmic CePrx2 KO C. elegans were sensitive to external
oxidants.
Overall, this work adds to the knowledge of H. contortus biology and identifies the
enzymes of the thioredoxin system as potential drug or vaccine targets for parasite
control.
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