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The thiol-based glutathione reductase (GR) and thioredoxin reductase (TrxR) are the major antioxidant enzymes present in various organisms that maintain the internal redox homeostasis. The thioredoxin system has attracted the attention of researchers from diverse investigation fields of biological sciences. Apart from redox regulation, this system is thought to be the major regulator of various biological processes including transcription, apoptosis, etc. Identification and physicobiochemical characterization of the reductase enzyme i.e. Thioredoxin reductase (TrxR) revealed the potency of it to become a promising target. Novel therapeutic interventions by selective targeting of TrxR in parasitic organisms as well as in the cancer cells have now become a usual treatment approach. However, different isoforms and their variation in the penultimate amino acid (Selenocysteine or cysteine) present in the catalytic site of the enzyme have made this enzyme to respond differently towards various drugs and synthetic and/or natural compounds. Therefore, the present article seeks to highlight the importance and the detailed molecular mechanism, functional perspective underlying the TrxR inhibition in various parasitic protozoans, helminthes as well as in cancer cells for devising suitable anti-TrxR candidates.

A kinetic study of thioredoxin-glutathione reductase (TGR) fromTaenia crassicepsmetacestode (cysticerci) was carried out. The results obtained from both initial velocity and product inhibition experiments suggest the enzyme follows a two-site ping-pong bi bi kinetic mechanism, in which both substrates and products are bound in rapid equilibrium fashion. The substrate GSSG exerts inhibition at moderate or high concentrations, which is concomitant with the observation of hysteretic-like progress curves. The effect of NADPH on the apparent hysteretic behavior of TGR was also studied. At low concentrations of NADPH in the presence of moderate concentrations of GSSG, atypical time progress curves were observed, consisting of an initial burst-like stage, followed by a lag whose amplitude and duration depended on the concentration of both NADPH and GSSG. Based on all the kinetic and structural evidence available on TGR, a mechanism-based model was developed. The model assumes a noncompetitive mode of inhibition by GSSG in which the disulfide behaves as an affinity label-like reagent through its binding and reduction at an alternative site, leading the enzyme into an inactive state. The critical points of the model are the persistence of residual GSSG reductase activity in the inhibited GSSG-enzyme complexes and the regeneration of the active form of the enzyme by GSH. Hence, the hysteretic-like progress curves of GSSG reduction by TGR are the result of a continuous competition between GSH and GSSG for driving the enzyme into active or inactive states, respectively. By using an arbitrary but consistent set of rate constants, the experimental full progress curves were successfully reproducedin silico.

Curcuma is a traditional ingredient of some Eastern cuisines, and the spice is heralded for its antitumoral and antiparasitic properties. In this report, we examine the effect of the curcuminoides which include curcumin, demethoxycurcumin (DMC) and bis-demethoxycurcumin (BDMC), as well as curcumin degradation products on thioredoxin glutathione reductase from Taenia crassiceps cysticerci Results revealed that both DMC and BDMC were inhibitors of TGR activity in the micromolar concentration range. By contrast, the inhibitory ability of curcumin was a time-dependent process. Kinetic and spectroscopical evidence suggests that an intermediary compound of curcumin oxidation, probably spiroepoxide, is responsible. Preincubation of curcumin in the presence of NADPH, but not glutathione disulfide (GSSG), resulted in the loss of its inhibitory ability, suggesting a reductive stabilizing effect. Similarly, preincubation of curcumin with sulfhydryl compounds fully protected the enzyme from inhibition. Degradation products were tested for their inhibitory potential, and 4-vinylguaiacol was the best inhibitor (IC50 = 12.9 μM), followed by feruloylmethane (IC50 = 122 μM), vanillin (IC50 = 127 μM), and ferulic aldehyde (IC50 = 180 μM). The acid derivatives ferulic acid (IC50 = 465 μM) and vanillic acid (IC50 = 657 μM) were poor inhibitors. On the other hand, results from docking analysis revealed a common binding site on the enzyme for all the compounds, albeit interacting with different amino acid residues. Dissociation constants obtained from the docking were in accord with the inhibitory efficiency of the curcumin degradation products.

The use of oxygen as the final electron acceptor in aerobic organisms results in an improvement in the energy metabolism. However, as a byproduct of the aerobic metabolism, reactive oxygen species are produced, leaving to the potential risk of an oxidative stress. To contend with such harmful compounds, living organisms have evolved antioxidant strategies. In this sense, the thiol-dependent antioxidant defense systems play a central role. In all cases, cysteine constitutes the major building block on which such systems are constructed, being present in redox substrates such as glutathione, thioredoxin, and trypanothione, as well as at the catalytic site of a variety of reductases and peroxidases. In some cases, the related selenocysteine was incorporated at selected proteins. In invertebrate parasites, antioxidant systems have evolved in a diversity of both substrates and enzymes, representing a potential area in the design of anti-parasite strategies. The present review focus on the organization of the thiol-based antioxidant systems in invertebrate parasites. Differences between these taxa and its final mammal host is stressed. An understanding of the antioxidant defense mechanisms in this kind of parasites, as well as their interactions with the specific host is crucial in the design of drugs targeting these organisms.

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.

Previously, we have studied the effect of the gold-compound auranofin (AF) on both thioredoxin-glutathione reductasa (TGR) activity and viability of Taenia crassiceps cysticerci. It was demonstrated that micromolar concentrations of AF were high enough to fully inhibit TGR and kill the parasites. In this work, the dynamics of changes in the glutathione pool of T. crassiceps cysticerci following the addition of AF, was analyzed. A dose-dependent decrease in the internal glutathione concentration, concomitant with an increase in ROS production was observed. These changes were simultaneous with the formation of glutathione-protein complexes and the export of glutathione disulfide (GSSG) to the culture medium. Incubation of cysticerci in the presence of both AF and N-acetyl cysteine (NAC) prevents all the above changes, maintaining cysticerci viability. By contrast, the presence of both AF and buthionine sulfoximine (BSO) resulted in a potentiation of the effects of the gold compound, jeopardizing cysticerci viability. These results suggest the lethal effect of AF on T. crassiceps cysticerci, observed at micromolar concentrations, can be explained as a consequence of major changes in the glutathione status, which results in a significant increase in the oxidative stress of the parasites.

Thioredoxin (Trx) is an oxidoreductase central to redox homeostasis in cells and is involved in the regulation of protein activity through thiol/disulfide exchanges. Based on these facts, our goal was to purify and characterize cytosolic thioredoxin from Taenia crassiceps cysticerci, as well as to study its behavior as a substrate of thioredoxin-glutathione reductase (TGR). The enzyme was purified >133-fold with a total yield of 9.7%. A molecular mass of 11.7kDa and a pI of 4.84 were measured. Native electrophoresis was used to identify the oxidized and reduced forms of the monomer as well as the presence of a homodimer. In addition to the catalytic site cysteines, cysticerci thioredoxin contains Cys28 and Cys65 residues conserved in previously sequenced cestode thioredoxins. The following kinetic parameters were obtained for the substrate of TGR: a Km of 3.1μM, a kcat of 10s(-1) and a catalytic efficiency of 3.2×10(6)M(-1)s(-1). The negative patch around the α3-helix of Trx is involved in the interaction with TGR and suggests variable specificity and catalytic efficiency of the reductase toward thioredoxins of different origins.