We identified 2 novel genes encoding different 2-Cys peroxiredoxins (PRxs), designated CsPRx2 and CsPRx3, in Clonorchis sinensis, which invades the human hepatobiliary tracts. The CsPRx2 gene expression was temporally increased along with the parasite's development and its protein product was detected in almost all parts of adult worms including subtegument, as well as excretory-secretory products. Conversely, CsPRx3 expression was temporally maintained at a basal level and largely restricted within interior parts of various tissues/organs. The recombinant forms of CsPRx proteins exhibited reducing activity against various hydroperoxides in the presence of either thioredoxin or glutathione (GSH) as a reducing equivalent, although they preferred H2O2 and GSH as a catalytic substrate and electron donor, respectively. A steady-state kinetic study demonstrated that the CsPRx proteins followed a saturable, Michaelis-Menten-type equation with the catalytic efficiencies (kcat/Km) ranging from 103 to 104 M−1 s−1, somewhat lower than those for other PRxs studied (104–105 M−1 s−1). The expression patterns and histological distributions specific to CsPRx2 and CsPRx3 might suggest different physiological functions of the antioxidant enzymes in protecting the worms against oxidative damage.

The ability of the larval forms of Schistosoma mansoni to invade and parasitize their molluscan host, Biomphalaria glabrata, is determined by a multitude of factors. In this study we sought to elucidate the possible mechanisms by which the invading larvae are able to counteract the potentially harmful oxidative environment presented by the host upon initial miracidial infection. This was attempted by examining the gene expression profile of parasite antioxidant enzymes of the linked glutathione-(GSH) thioredoxin (Trx) redox pathway during early intramolluscan larval development. Three such enzymes, the peroxiredoxins (Prx1, Prx2 and Prx3) were examined as to their activity and sites of expression within S. mansoni miracidia and in vitro-cultured mother sporocysts. Results of these studies demonstrated that the H2O2-reducing enzymes Prx1 and 2 are upregulated during early mother sporocyst development compared to miracidia. Immunolocalization studies further indicated that Prx1 and Prx2 proteins are expressed within the apical papillae of miracidia and tegumental syncytium of sporocysts, and are released with parasite excretory-secretory proteins (ESP) during in vitro larval transformation. Removal of Prx1 and Prx2 from larval ESP by immunoabsorption significantly reduced the ability of ESP to breakdown exogenous H2O2, thereby directly linking ESP Prx proteins with H2O2-scavenging activity. Moreover, exposure of live sporocysts to exogenous H2O2 stimulated an upregulation of Prx1 and 2 gene expression suggesting the involvement of H2O2–responsive elements in regulating larval Prx gene expression. These data provide evidence that Prx1 and Prx2 may function in the protection of S. mansoni sporocysts during the early stages of infection.

A cDNA was isolated from a cDNA expression library using serum prepared against a high molecular mass fraction of Fasciola hepatica excretory–secretory products. The full-length cDNA encodes a member of the recently described peroxiredoxin antioxidant family. Peroxiredoxin could be the major hydrogen peroxide removing antioxidant in F. hepatica since this parasite does not express a catalase and expresses little glutathione peroxidase activity. This novel antioxidant may be involved in functions such as protection against reactive oxygen species (ROS) generated by metabolic processes and/or protection of the parasite against ROS released by immune effector cells.