Expression and characterization of glutathione peroxidase activity in the human blood fluke Schistosoma mansoni

Antioxidants may play an important role in immune evasion by schistosome parasites. Previous studies have focused on the roles of superoxide dismutase and glutathione S-transferase. In the present study, glutathione peroxidase (GPX) activity was measured in different fractions of worm extracts from several developmental stages of Schistosoma mansoni. The enzyme activity was shown to be developmentally regulated, with higher specific activities being found in the tegument-enriched Nonidet P-40 extract of adult worms (the stage least susceptible to immune killing) than in the larval stages (which are most susceptible to immune elimination). In all extracts tested, the activity against cumene hydroperoxide, even when glutathione S-transferase activity was removed, was higher than that for hydrogen peroxide. The expression of GPX cDNA in pGEX-2T by bacteria produced a 50-kDa fusion protein and a 32-kDa truncated protein. The latter was due to termination at the internal UGA codon that codes for selenocysteine. GPX activity was detected in the recombinantly produced GPX but not with Sj26-glutathione S-transferase from the vector. Mutating the TGA codon to TGT produced a full-length product, GPXm (19 kDa), that was used to produce 19 monoclonal antibodies. Anti-GPXm monoclonal antibodies recognized a 19-kDa molecule in adult-worm extract which, upon removal by immunoprecipitation, resulted in the loss of over 90% of the GPX activity,suggesting that a single form of GPX exists in the schistosome.