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Control of cellular redox homeostasis is a central issue for all living organisms. Glutathione and thioredoxin enzymatic redox systems are the usual mean used to achieve such a control. However, parasitic platyhelminths studied to date possess a nicotinamide adenine dinucleotide phosphate-dependent thioredoxin-glutathione reductase (TGR) as the sole redox control system. Thus, TGR is considered as a potential therapeutic target of parasitic platyhelminths, and based on this assumption, the gold compound auranofin is a potent inhibitor of TGR. The aim of this research was to investigate the effect of auranofin on metacestode (cysticerci) of Taenia crassiceps in culture. Accordingly, the time course for viability and respiration of cysticerci in culture was evaluated in the presence of this compound. After 4 h at 10 microM auranofin, 90% of cysticerci were alive, but respiration activity had declined by 50%. After 12 h, neither survivors nor respiration was detected; a LD(50) for auranofin of 3.8 microM was calculated. Interestingly, crude extracts of cysticerci pretreated with 3 microM auranofin nearly nil TGR activity (IC(50) = 0.6 microM). Zymography for TGR in polyacrylamide gel electrophoresis was conducted because the previously mentioned extracts clearly showed a dose-response inactivation of TGR toward auranofin. The killing of cysticerci by this gold compound is most likely related with TGR inactivation. Therefore, further research on the suitability of auranofin as a therapeutic tool in the treatment of cysticercosis in animals and humans is sustained.

Glucose 6-phosphate dehydrogenase (EC 1.1.1.49) was purified to homogeneity from the soluble fraction of larval Taenia crassiceps (Eucestoda: Cyclophyllidea) by a three-step protocol. Specific activity of the pure enzyme was 33.8 +/- 2.1 U mg(-1) at 25 degrees C and pH 7.8 with D: -glucose 6-phosphate and NADP+ as substrates. The activity increases to 67.6 +/- 3.9 U mg(-1) at 39 degrees C, a more physiological temperature in the intermediary host. Enzyme activity was maximal between pH 6.7 and 7.8. Km values were 14 +/- 1.7 microM and 1.3 +/- 0.4 microM for glucose 6-phosphate and NADP+, respectively. The enzyme showed absolute specificity for its sugar substrate. NAD+ was also a substrate but with a low catalytic efficiency (207 M(-1) s(-1)). No essential requirement for Mg++ or Ca++ was observed. Relative molecular mass of the native enzyme was 134,000 +/- 17,200, while a value of 61,000 +/- 1,700 was obtained for the enzyme subunit. Thus, glucose 6-phosphate dehydrogenase from T. crassiceps exists as a dimeric protein. The enzyme's isoelectric point was 4.5. The enzyme's activity dependence on temperature was complex, resulting in a biphasic Arrhenius plot. Activation energies of 9.91 +/- 0.51 and 7.94 +/- 0.45 kcal mol(-1) were obtained. Initial velocity patterns complemented with inhibition studies by product and substrate's analogues support a random bi bi sequential mechanism in rapid equilibrium. The low Ki value of 1.95 microM found for NADPH suggests a potential regulatory role for this nucleotide.

Mitochondrial thioredoxin-glutathione reductase was purified from larval Taenia crassiceps (cysticerci). The preparation showed NADPH-dependent reductase activity with either thioredoxin or GSSG, and was able to perform thiol/disulfide exchange reactions. At 25°C specific activities were 437 ± 27 mU mg−1 and 840 ± 49 mU mg−1 with thioredoxin and GSSG, respectively. Apparent Km values were 0.87 ± 0.04 μM, 41 ± 6 μM and 19 ± 10 μM for thioredoxin, GSSG and NADPH, respectively. Thioredoxin from eukaryotic sources was accepted as substrate. The enzyme reduced H2O2 in a NADPH-dependent manner, although with low catalytic efficiency. In the presence of thioredoxin, mitochondrial TGR showed a thioredoxin peroxidase-like activity. All disulfide reductase activities were inhibited by auranofin, suggesting mTGR is dependent on selenocysteine. The reductase activity with GSSG showed a higher dependence on temperature as compared with the DTNB reductase activity. The variation of the GSSG- and DTNB reductase activities on pH was dependent on the disulfide substrate. Like the cytosolic isoform, mTGR showed a hysteretic kinetic behavior at moderate or high GSSG concentrations, but it was less sensitive to calcium. The enzyme was able to protect glutamine synthetase from oxidative inactivation, suggesting that mTGR is competent to contend with oxidative stress.