Substrate specificity of the mitochondrial thioredoxin reductase of the parasitic nematode Haemonchus contortus.

Thioredoxin reductase (TrxR) is an evolutionary diverse enzyme. In higher eukaryotes, TrxR contains a selenocysteine in the active site which confers high activity and a broad substrate range. The parasitic nematode Haemonchus contortus contains two TrxRs, a cytoplasmic enzyme HcTrxR1 with a selenocysteine in the active site (Gly-Cys-SeCys-Gly), similar to the mammalian TrxR, and a mitochondrial enzyme HcTrxR2 with a Gly-Cyc-Cys-Gly active site which is unique to nematodes. Both enzymes were cloned and expressed and their activity compared to rat TrxR. Although the expressed HcTrxR1 contained a cysteine to replace the selenocycteine, it showed broad activity with thioredoxins from Escherichia coli, sheep, and H. contortus similar to the rat selenoenzyme. However, HcTrxR2 had high activity only with the mitochondrial H. contortus thioredoxin. HcTrxR2 was also active with the commonly used substrate 5,5'-dithiobis(2-nitrobenzoate) and sodium selenite and was inhibited by auranofin and the natural product compounds curcumin and theaflavin. This demonstrates that the non-seleno-enzyme HcTrxR2 is as active as the rat seleno-containing enzyme when reacting with the H. contortus thioredoxin.