| Summary: | This study shows that resistance to killing by reactive oxygen intermediates (ROI) increases during migration and development in Schistosoma mansoni. Resistance is associated with the protective role of antioxidants as shown by the increased levels of superoxide dismutase and of the glutathione system enzymes. Hydroperoxide-dependent glutathione peroxidase activity was not detectable in newly transformed schistosomula, however the activity was present in the liver stages. The antischistosomal drug oltipraz (OPZ) decreased in an irreversible manner the activity of S. mansoni glutathione S-transferase (GST), an important protective enzyme, both in vivo and in vitro. The inhibition of GST activity was not isoenzyme restricted and was non-competitive with respect to the two substrates essential for GST activity. On the other hand, OPZ treatment increased the levels of mouse (S. mansoni host) liver GST activity in an isoenzyme specific manner, with the $ mu$ class subunit induction accounting for most of the increase. However, mammalian GST activity was inhibited by OPZ in vitro. However, the inhibition of mammalian GST activity was reversible upon addition of dithiol reducing compounds. OPZ inhibited the binding of ($ sp{14}$C) N-ethylmaleimide (specifically alkylates SH groups), suggesting that OPZ interacts with SH-groups of GST to inhibit its enzymatic activity. Another SH-dependent enzyme, hexokinase, from yeast and S. mansoni was reversibly inhibited by OPZ. The oxy-analogue of OPZ, in which the thione sulphur is replaced with oxygen, did not inhibit the enzymatic activity of GST and hexokinase. Many of the biochemical effects of OPZ on S. mansoni and its mammalian hosts may be related to its ability to bind to SH groups and inactivation of the functions of many essential proteins.
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