Inhibition of human glutathione reductase by 10-arylisoalloxazines: crystalline, kinetic, and electrochemical studies

Innovative Approach for a Classic Target: Fragment Screening on Trypanothione Reductase Reveals New Opportunities for Drug Design

Trypanothione reductase (TR) is a key factor in the redox homeostasis of trypanosomatid parasites, critical for survival in the hostile oxidative environment generated by the host to fight infection. TR is considered an attractive target for the development of new trypanocidal agents as it is essential for parasite survival but has no close homolog in humans. However, the high efficiency and turnover of TR challenging targets since only potent inhibitors, with nanomolar IC50, can significantly affect parasite redox state and viability. To aid the design of effective compounds targeting TR, we performed a fragment-based crystal screening at the Diamond Light Source XChem facility using a library optimized for follow-up synthesis steps. The experiment, allowing for testing over 300 compounds, resulted in the identification of 12 new ligands binding five different sites. Interestingly, the screening revealed the existence of an allosteric pocket close to the NADPH binding site, named the “doorstop pocket” since ligands binding at this site interfere with TR activity by hampering the “opening movement” needed to allow cofactor binding. The second remarkable site, known as the Z-site, identified by the screening, is located within the large trypanothione cavity but corresponds to a region not yet exploited for inhibition. The fragments binding to this site are close to each other and have some remarkable features making them ideal for follow-up optimization as a piperazine moiety in three out of five fragments.

Druggable hot spots in trypanothione reductase: novel insights and opportunities for drug discovery revealed by DRUGpy

Assessment of target druggability guided by search and characterization of hot spots is a pivotal step in early stages of drug-discovery. The raw output of FTMap provides the data to perform this task, but it relies on manual intervention to properly combine different sets of consensus sites, therefore allowing identification of hot spots and evaluation of strength, shape and distance among them. Thus, the user's previous experience on the target and the software has a direct impact on how data generated by FTMap server can be explored. DRUGpy plugin was developed to overcome this limitation. By automatically assembling and scoring all possible combinations of consensus sites, DRUGpy plugin provides FTMap users a straight-forward method to identify and characterize hot spots in protein targets. DRUGpy is available in all operating systems that support PyMOL software. DRUGpy promptly identifies and characterizes pockets that are predicted by FTMap to bind druglike molecules with high-affinity (druggable sites) or low-affinity (borderline sites) and reveals how protein conformational flexibility impacts on the target's druggability. The use of DRUGpy on the analysis of trypanothione reductases (TR), a validated drug target against trypanosomatids, showcases the usefulness of the plugin, and led to the identification of a druggable pocket in the conserved dimer interface present in this class of proteins, opening new perspectives to the design of selective inhibitors.

A Class of Valuable (Pro-)Activity-Based Protein Profiling Probes: Application to the Redox-Active Antiplasmodial Agent, Plasmodione

Plasmodione (PD) is a potent antimalarial redox-active drug acting at low nM range concentrations on different malaria parasite stages. In this study, in order to determine the precise PD protein interactome in parasites, we developed a class of (pro-)activity-based protein profiling probes (ABPP) as precursors of photoreactive benzophenone-like probes based on the skeleton of PD metabolites (PDO) generated in a cascade of redox reactions. Under UV-photoirradiation, we clearly demonstrate that benzylic oxidation of 3-benzylmenadione 11 produces the 3-benzoylmenadione probe 7, allowing investigation of the proof-of-concept of the ABPP strategy with 3-benzoylmenadiones 7-10. The synthesized 3-benzoylmenadiones, probe 7 with an alkyne group or probe 9 with -NO2 in para position of the benzoyl chain, were found to be the most efficient photoreactive and clickable probes. In the presence of various H-donor partners, the UV-irradiation of the photoreactive ABPP probes generates different adducts, the expected "benzophenone-like" adducts (pathway 1) in addition to "benzoxanthone" adducts (via two other pathways, 2 and 3). Using both human and Plasmodium falciparum glutathione reductases, three protein ligand binding sites were identified following photolabeling with probes 7 or 9. The photoreduction of 3-benzoylmenadiones (PDO and probe 9) promoting the formation of both the corresponding benzoxanthone and the derived enone could be replaced by the glutathione reductase-catalyzed reduction step. In particular, the electrophilic character of the benzoxanthone was evidenced by its ability to alkylate heme, as a relevant event supporting the antimalarial mode of action of PD. This work provides a proof-of-principle that (pro-)ABPP probes can generate benzophenone-like metabolites enabling optimized activity-based protein profiling conditions that will be instrumental to analyze the interactome of early lead antiplasmodial 3-benzylmenadiones displaying an original and innovative mode of action.

Efficient Dimerization Disruption of Leishmania infantum Trypanothione Reductase by Triazole-phenyl-thiazoles

Inhibition of Leishmania infantum trypanothione disulfide reductase (LiTryR) by disruption of its homodimeric interface has proved to be an alternative and unexploited strategy in the search for novel antileishmanial agents. Proof of concept was first obtained by peptides and peptidomimetics. Building on previously reported dimerization disruptors containing an imidazole-phenyl-thiazole scaffold, we now report a new 1,2,3-triazole-based chemotype that yields noncompetitive, slow-binding inhibitors of LiTryR. Several compounds bearing (poly)aromatic substituents dramatically improve the ability to disrupt LiTryR dimerization relative to reference imidazoles. Molecular modeling studies identified an almost unexplored hydrophobic region at the interfacial domain as the putative binding site for these compounds. A subsequent structure-based design led to a symmetrical triazole analogue that displayed even more potent inhibitory activity over LiTryR and enhanced leishmanicidal activity. Remarkably, several of these novel triazole-bearing compounds were able to kill both extracellular and intracellular parasites in cell cultures.

The Relevance of Glutathione Reductase Inhibition by Fluoxetine to Human Health and Disease: Insights Derived from a Combined Kinetic and Docking Study

Glutathione reductase (GR) is a homodimeric enzyme playing an important role in the regeneration of the central antioxidant molecule reduced glutathione (GSH) from oxidized glutathione (GSSG) at the expense of a molecule of NADPH. GSH scavenges and eliminates superoxide and hydroxyl radicals non-enzymatically or serves as an electron donor for several enzymes. Fluoxetine (FLU), a selective serotonin reuptake inhibitor, is widely prescribed in the treatment of major depressive disorder. Here, using enzyme kinetic studies and molecular docking simulations, we aimed at disclosing the mechanistic and structural aspects of the interaction between GR and FLU. Affecting enzyme activity in a dose-dependent manner, FLU was shown to be a moderately potent (IC₅₀ = 0.88 mM) inhibitor of GR. When the variable substrate was GSSG, the type of inhibition was linear mixed-type competitive (K_i = 279 ± 32 μM; α = 5.48 ± 1.29). When the variable substrate was NADPH, however, the type of inhibition was non-competitive (K_i = 879 ± 82 μM). The observed difference in inhibition types was attributed to the binding of FLU in the large intermonomer cavity of GR, where it hampered catalysis and interfered with substrate binding. Overall, although it is anticipated that long-term use of FLU leads to acquired GR deficiency, the inhibitory action of FLU on GR may be therapeutically exploited in anti-cancer research.

Oxidative Stress and Apoptosis of Gaeumannomyces graminis (Get) Induced by Carabrone

Carabrone is isolated from Carpesium macrocephalum Franch. et Sav, which has good fungicidal activity, especially for Gaeumannomyces graminis (Get). According to previous studies, we speculated that carabrone targets the mitochondrial enzyme complex III of Get. To elucidate the mode of action, we used carabrone to induce oxidative stress and apoptosis in Get. Incubation with carabrone reduced the burst of reactive oxygen species (ROS) and mitochondrial membrane potential, as well as phosphatidylserine release. Carabrone caused ROS accumulation in mycelia by inhibiting the activity of antioxidase enzymes, among which inhibition of glutathione reductase (GR) activity was most obvious. The catalytic center of GR consists of l-cysteine residues that react with the α-methylene-γ-butyrolactone active site of carabrone. Additionally, a positive TUNEL reaction led to diffusion of the DNA electrophoresis band and upregulation of Ggmet1 and Ggmet2. We propose that carabrone inhibits antioxidant enzymes and promotes ROS overproduction, which causes membrane hyperpermeability, release of apoptotic factors, activation of the mitochondria-mediated apoptosis pathway, and fungal cell apoptosis.

Mechanistic and structural insights into the in vitro inhibitory action of hypericin on glutathione reductase purified from baker's yeast

This work aims at studying the interaction between glutathione reductase (GR) and hypericin. The type of inhibition was determined by measuring changes in GR activity at increasing concentrations of hypericin as well as at varying concentrations of glutathione disulfide (GSSG) and nicotinamide adenine dinucleotide phosphate (NADPH), and the binding pose of hypericin was predicted by molecular docking. Accordingly, hypericin emerges as an effective inhibitor of GR. When the variable substrate is GSSG, the type of inhibition is competitive. When the variable substrate is NADPH, however, the type of inhibition appears to be linear mixed-type competitive. Our computational analyses suggest that hypericin binds in the large intermonomer cavity of GR, and that it may interfere with the normal positioning/functioning of the redox-active disulfide center at the enzyme's active site. Overall, besides its contributory role in promoting oxidative stress via the formation of reactive oxygen species in photodynamic therapy, hypericin can also weaken cancer cells through inhibiting GR.

Flavins and flavoproteins: applications in medicine

The potential of flavoproteins as targets of pharmacological treatments is immense. In this review we present an overview of the current research progress on medical interventions based on flavoproteins with a special emphasis on cancer, infectious diseases, and neurological disorders.

Characterization of PfTrxR inhibitors using antimalarial assays and in silico techniques

Background

The compounds 1,4-napthoquinone (1,4-NQ), bis-(2,4-dinitrophenyl)sulfide (2,4-DNPS), 4-nitrobenzothiadiazole (4-NBT), 3-dimethylaminopropiophenone (3-DAP) and menadione (MD) were tested for antimalarial activity against both chloroquine (CQ)-sensitive (D6) and chloroquine (CQ)-resistant (W2) strains of Plasmodium falciparum through an in vitro assay and also for analysis of non-covalent interactions with P. falciparum thioredoxin reductase (PfTrxR) through in silico docking studies.

Results

The inhibitors of PfTrxR namely, 1,4-NQ, 4-NBT and MD displayed significant antimalarial activity with IC₅₀ values of < 20 μM and toxicity against 3T3 cell line. 2,4-DNPS was only moderately active. In silico docking analysis of these compounds with PfTrxR revealed that 2,4-DNPS, 4-NBT and MD interact non-covalently with the intersubunit region of the enzyme.

Conclusions

In this study, tools for the identification of PfTrxR inhibitors using phenotyphic screening and docking studies have been validated for their potential use for antimalarial drug discovery project.

1,4-naphthoquinones and other NADPH-dependent glutathione reductase-catalyzed redox cyclers as antimalarial agents

The homodimeric flavoenzyme glutathione reductase catalyzes NADPH-dependent glutathione disulfide reduction. This reaction is important for keeping the redox homeostasis in human cells and in the human pathogen Plasmodium falciparum. Different types of NADPH-dependent disulfide reductase inhibitors were designed in various chemical series to evaluate the impact of each inhibition mode on the propagation of the parasites. Against malaria parasites in cultures the most potent and specific effects were observed for redox-active agents acting as subversive substrates for both glutathione reductases of the Plasmodium-infected red blood cells. In their oxidized form, these redox-active compounds are reduced by NADPH-dependent flavoenzyme-catalyzed reactions in the cytosol of infected erythrocytes. In their reduced forms, these compounds can reduce molecular oxygen to reactive oxygen species, or reduce oxidants like methemoglobin, the major nutrient of the parasite, to indigestible hemoglobin. Furthermore, studies on a fluorinated suicide-substrate of the human glutathione reductase indicate that the glutathione reductase-catalyzed bioactivation of 3-benzylnaphthoquinones to the corresponding reduced 3-benzoyl metabolites is essential for the observed antimalarial activity. In conclusion, the antimalarial lead naphthoquinones are suggested to perturb the major redox equilibria of the targeted cells. These effects result in developmental arrest of the parasite and contribute to the removal of the parasitized erythrocytes by macrophages.

Characterization of a novel dithiocarbamate glutathione reductase inhibitor and its use as a tool to modulate intracellular glutathione

Thiol redox state (TRS) is an important parameter to reflect intracellular oxidative stress and is associated with various normal and abnormal biochemical processes. Agents that can be used to increase intracellular TRS will be valuable tools in TRS-related research. Glutathione reductase (GR) is a critical enzyme in the homeostasis of TRS. The enzyme catalyzes the reduction of GSSG to GSH to maintain a high GSH:GSSG ratio. Inhibition of the enzyme can be used to increase TRS. Despite the reports of various GR inhibitors, N,N-bis(2-chloroethyl)-N-nitrosourea, an anticancer drug with IC(50) = 647 microm against yeast GR, remains the most commonly used GR inhibitor in the literature. However, the toxicity caused by nonspecific interactions, as well as inhibition of DNA synthesis, complicates the use of N,N-bis(2-chloroethyl)-N-nitrosourea as a GR inhibitor. We report 2-acetylamino-3-[4-(2-acetylamino-2-carboxyethylsulfanylthiocarbonylamino)phenylthiocarbamoylsulfanyl]propionic acid (2-AAPA) as a novel irreversible GR inhibitor. 2-AAPA was prepared by one-step synthesis from commercially available reagents. The K(i) and k(inact) of 2-AAPA against yeast GR were determined to be 56 microm and 0.1 min(-1), respectively. At the concentration that produced >80% yeast GR inhibition, 2-AAPA showed no inhibition against glutamylcysteine synthetase, glutathione synthetase, catalase, and superoxide dismutase, but minimal inhibition against glutathione S-transferase and glutathione peroxidase. In CV-1 cells, 2-AAPA (0.1 mm) produced 97% GR inhibition, 25% GSH reduction, and a 5-fold increase in GSSG in 20 min. The compound can be a useful tool in TRS-related research.

Thioredoxin glutathione reductase from Schistosoma mansoni: an essential parasite enzyme and a key drug target

BACKGROUND

Schistosomiasis--infection with helminth parasites in the genus Schistosoma, including S. mansoni--is a widespread, devastating tropical disease affecting more than 200 million people. No vaccine is available, and praziquantel, the only drug extensively utilized, is currently administered more than 100 million people yearly. Because praziquantel resistance may develop it is essential to identify novel drug targets. Our goal was to investigate the potential of a unique, selenium-containing parasite enzyme thioredoxin glutathione reductase (TGR) as a drug target.

METHODS AND FINDINGS

Using RNA interference we found that TGR is essential for parasite survival; after silencing of TGR expression, in vitro parasites died within 4 d. We also found that auranofin is an efficient inhibitor of pure TGR (Ki = 10 nM), able to kill parasites rapidly in culture at physiological concentrations (5 microM), and able to partially cure infected mice (worm burden reductions of ~60%). Furthermore, two previously used antischistosomal compounds inhibited TGR activity, suggesting that TGR is a key target during therapy with those compounds.

CONCLUSIONS

Collectively, our results indicate that parasite TGR meets all the major criteria to be a key target for antischistosomal chemotherapy. To our knowledge this is the first validation of a Schistosoma drug target using a convergence of both genetic and biochemical approaches.

A Fluoro Analogue of the Menadione Derivative 6-[2‘-(3‘-Methyl)-1‘,4‘-naphthoquinolyl]hexanoic Acid Is a Suicide Substrate of Glutathione Reductase. Crystal Structure of the Alkylated Human Enzyme†

Glutathione reductase is an important housekeeping enzyme for redox homeostasis both in human cells and in the causative agent of tropical malaria, Plasmodium falciparum. Glutathione reductase inhibitors were shown to have anticancer and antimalarial activity per se and to contribute to the reversal of drug resistance. The development of menadione chemistry has led to the selection of 6-[2'-(3'-methyl)-1',4'-naphthoquinolyl]hexanoic acid, called M(5), as a potent reversible and uncompetitive inhibitor of both human and P. falciparum glutathione reductases. Here we describe the synthesis and kinetic characterization of a fluoromethyl-M(5) analogue that acts as a mechanism-based inhibitor of both enzymes. In the course of enzymatic catalysis, the suicide substrate is activated by one- or two-electron reduction, and then a highly reactive quinone methide is generated upon elimination of the fluorine. Accordingly the human enzyme was found to be irreversibly inactivated with a k(inact) value of 0.4 +/- 0.2 min(-1). The crystal structure of the alkylated enzyme was solved at 1.7 A resolution. It showed the inhibitor to bind covalently to the active site Cys58 and to interact noncovalently with His467', Arg347, Arg37, and Tyr114. On the basis of the crystal structure of the inactivated human enzyme and stopped-flow kinetic studies with two- and four-electron-reduced forms of the unreacted P. falciparum enzyme, a mechanism is proposed which explains naphthoquinone reduction at the flavin of glutathione reductase.

Flavin-Catalyzed Oxidation of Amines and Sulfides with Molecular Oxygen: Biomimetic Green Oxidation

Flavin-catalyzed green oxidation of heteroatom compounds such as sulfides and amines with molecular oxygen and even air in the presence of hydrazine monohydrate in a fluorous solvent such as 2,2,2-trifluoroethanol at room temperature gives the corresponding oxidation products highly efficiently and selectively along with water and molecular nitrogen, which are environmentally benign by-products. The proposed reaction mechanism is based on the kinetics, solvent effect, and redox properties of flavin catalysts.

2-Acetylamino-3-[4-(2-acetylamino-2-carboxyethylsulfanylcarbonylamino)- phenylcarbamoylsulfanyl]propionic acid and its derivatives as a novel class of glutathione reductase inhibitors

Glutathione reductase (GR) catalyzes the reduction of oxidized glutathione to reduced glutathione. The enzyme is an attractive target for the development of antimalarial agents, agents to decrease malarial drug resistance and anticancer agents. In addition, inhibition of the enzyme has been employed as a tool in research for various purposes. In this paper, we present a rational design of 2-acetylamino-3-[4-(2-acetylamino-2-carboxyethylsulfanylcarbonylamino)phenylcarbamoylsulfanyl]propionic acid and its derivatives as irreversible GR inhibitors. The K(i) and k(inact) values of 2-acetylamino-3-[4-(2-acetylamino-2-carboxyethylsulfanylcarbonylamino)phenylcarbamoylsulfanyl]propionic acid, the most potent derivative of the series, are 88 muM and 0.1 min(-1), respectively. Although the K(i) value of the inhibitor is in the micromolar range, it is more potent than N,N-bis(2-chloroethyl)-N-nitrosourea, which is currently the most commonly employed irreversible GR inhibitor with a reported IC(50) value of 646 microM. Additional attractive features of the inhibitor include its ready availability through a one-step synthesis and good solubility in both organic and aqueous solutions.

Dithiol Proteins as Guardians of the Intracellular Redox Milieu in Parasites: Old and New Drug Targets in Trypanosomes and Malaria-Causing Plasmodia

Parasitic diseases such as sleeping sickness, Chagas' heart disease, and malaria are major health problems in poverty-stricken areas. Antiparasitic drugs that are not only active but also affordable and readily available are urgently required. One approach to finding new drugs and rediscovering old ones is based on enzyme inhibitors that paralyze antioxidant systems in the pathogens. These antioxidant ensembles are essential to the parasites as they are attacked in the human host by strong oxidants such as peroxynitrite, hypochlorite, and H2O2. The pathogen-protecting system consists of some 20 thiol and dithiol proteins, which buffer the intraparasitic redox milieu at a potential of -250 mV. In trypanosomes and leishmania the network is centered around the unique dithiol trypanothione (N1,N8-bis(glutathionyl)spermidine). In contrast, malaria parasites have a more conservative dual antioxidative system based on glutathione and thioredoxin. Inhibitors of antioxidant enzymes such as trypanothione reductase are, indeed, parasiticidal but they can also delay or prevent resistance against a number of other antiparasitic drugs.

5-Substituted Tetrazoles as Bioisosteres of Carboxylic Acids. Bioisosterism and Mechanistic Studies on Glutathione Reductase Inhibitors as Antimalarials

Plasmodium parasites are exposed to elevated fluxes of reactive oxygen species during intraerythrocytic life. The most important antioxidative systems are based on the glutathione reductases of the malarial parasite Plasmodium falciparum and the host erythrocyte. The development of menadione chemistry has led to the selection of the carboxylic acid 6-[2'-(3'-methyl)-1',4'-naphthoquinolyl] hexanoic acid M(5) as an inhibitor of the parasitic enzyme. As reported here, revisiting the mechanism of M(5) action revealed an uncompetitive inhibition type with respect to both NADPH and glutathione disulfide. Masking the polarity of the acidic function of M(5) by ester or amide bonds improved antiplasmodial activity. Bioisosteric replacement of the carboxylic function by tetrazole to increase bioavailability and to maintain comparable acidity led to improved antimalarial properties as well, but only with the cyanoethyl-protected tetrazoles. Using computed ab initio quantum methods, detailed analyses of the electronic profiles and the molecular properties evidenced the similarity of M(5) and the bioisoteric tetrazole T(4). The potential binding site of these molecules is discussed in light of the recently solved crystallographic structure of P. falciparum enzyme.

Double-drug development against antioxidant enzymes from Plasmodium falciparum

New drugs against malaria are urgently and continuously needed. Plasmodium parasites are exposed to higher fluxes of reactive oxygen species and need high activities of intracellular antioxidant systems. A most important antioxidative system consists of (di)thiols which are recycled by disulfide reductases (DR), namely both glutathione reductases (GR) of the malarial parasite Plasmodium falciparum and man, and the thioredoxin reductase (TrxR) of P. falciparum. The aim of our interdisciplinary research is to substantiate DR inhibitors as antimalarial agents. Such compounds are active per se but, in addition, they can reverse thiol-based resistance against other drugs in parasites. Reversal of drug resistance by DR inhibitors is currently investigated for the commonly used antimalarial drug chloroquine (CQ). Our recent strategy is based on the synthesis of inhibitors of the glutathione reductases from parasite and host erythrocyte. With the expectation of a synergistic or additive effect, double-headed prodrugs were designed to be directed against two different and essential functions of the malarial parasite P. falciparum, namely glutathione regeneration and heme detoxification. The prodrugs were prepared by linking bioreversibly a GR inhibitor to a 4-aminoquinoline moiety which is known to concentrate in the acidic food vacuole of parasites. Drug-enzyme interaction was correlated with antiparasitic action in vitro on strains resistant towards CQ and in vivo in Plasmodium berghei-infected mice as well as absence of cytotoxicity towards human cells. Because TrxR of P. falciparum was recently shown to be responsible for the residual glutathione disulfide-reducing capacity observed after GR inhibition in P. falciparum, future development of antimalarial drug-candidates that act by perturbing the redox equilibrium of parasites is based on the design of new double-drugs based on TrxR inhibitors as potential antimalarial drug candidates.

Oxidative stress in malaria parasite-infected erythrocytes: host–parasite interactions

Experimenta naturae, like the glucose-6-phosphate dehydrogenase deficiency, indicate that malaria parasites are highly susceptible to alterations in the redox equilibrium. This offers a great potential for the development of urgently required novel chemotherapeutic strategies. However, the relationship between the redox status of malarial parasites and that of their host is complex. In this review article we summarise the presently available knowledge on sources and detoxification pathways of reactive oxygen species in malaria parasite-infected red cells, on clinical aspects of redox metabolism and redox-related mechanisms of drug action as well as future prospects for drug development. As delineated below, alterations in redox status contribute to disease manifestation including sequestration, cerebral pathology, anaemia, respiratory distress, and placental malaria. Studying haemoglobinopathies, like thalassemias and sickle cell disease, and other red cell defects that provide protection against malaria allows insights into this fine balance of redox interactions. The host immune response to malaria involves phagocytosis as well as the production of nitric oxide and oxygen radicals that form part of the host defence system and also contribute to the pathology of the disease. Haemoglobin degradation by the malarial parasite produces the redox active by-products, free haem and H(2)O(2), conferring oxidative insult on the host cell. However, the parasite also supplies antioxidant moieties to the host and possesses an efficient enzymatic antioxidant defence system including glutathione- and thioredoxin-dependent proteins. Mechanistic and structural work on these enzymes might provide a basis for targeting the parasite. Indeed, a number of currently used drugs, especially the endoperoxide antimalarials, appear to act by increasing oxidant stress, and novel drugs such as peroxidic compounds and anthroquinones are being developed.

Flavoprotein Disulfide Reductases: Advances in Chemistry and Function

The flavoprotein disulfide reductases represent a family of enzymes that show high sequence and structural homology. They catalyze the pyridine-nucleotide-dependent reduction of a variety of substrates, including disulfide-bonded substrates (lipoamide dehydrogenase, glutathione reductase and functional homologues, thioredoxin reductase, and alkylhydroperoxide reductase), mercuric ion (mercuric ion reductase), hydrogen peroxide (NADH peroxidase), molecular oxygen (NADH oxidase), and the reductive cleavage of a carbonyl-activated carbon-sulfur bond followed by carboxylation (2-ketopropyl-coenzyme-M carboxylase?oxidoreductase). They use at least one nonflavin redox center to transfer electrons from reduced pyridine nucleotide to their substrate through flavin adenine dinucleotide. The nature of the nonflavin redox center located adjacent to the flavin varies and three types have been identified: an enzymic disulfide (most commonly), an enzymic cysteine sulfenic acid (NADH peroxidase and NADH oxidase), and a mixed Cys-S-S-CoA disulfide (coenzyme A disulfide reductase). Selection of the particular nonflavin redox center and utilization of a second, or even a third, nonflavin redox center in some cases presumably represents the most efficient strategy for reduction of the individual substrate.

Thioredoxin reductase from the malaria mosquito Anopheles gambiae

The mosquito, Anopheles gambiae, is an important vector of Plasmodium falciparum malaria. Full genome analysis revealed that, as in Drosophila melanogaster, the enzyme glutathione reductase is absent in A. gambiae and functionally substituted by the thioredoxin system. The key enzyme of this system is thioredoxin reductase-1, a homodimeric FAD-containing protein of 55.3 kDa per subunit, which catalyses the reaction NADPH + H+ + thioredoxin disulfide-->NADP+ + thioredoxin dithiol. The A. gambiae trxr gene is located on chromosome X as a single copy; it represents three splice variants coding for two cytosolic and one mitochondrial variant. The predominant isoform, A. gambiae thioredoxin reductase-1, was recombinantly expressed in Escherichia coli and functionally compared with the wild-type enzyme isolated in a final yield of 1.4 U.ml(-1) of packed insect cells. In redox titrations, the substrate A. gambiae thioredoxin-1 (Km=8.5 microm, kcat=15.4 s(-1) at pH 7.4 and 25 degrees C) was unable to oxidize NADPH-reduced A. gambiae thioredoxin reductase-1 to the fully oxidized state. This indicates that, in contrast to other disulfide reductases, A. gambiae thioredoxin reductase-1 oscillates during catalysis between the four-electron reduced state and a two-electron reduced state. The thioredoxin reductases of the malaria system were compared. A. gambiae thioredoxin reductase-1 shares 52% and 45% sequence identity with its orthologues from humans and P. falciparum, respectively. A major difference among the three enzymes is the structure of the C-terminal redox centre, reflected in the varying resistance of catalytic intermediates to autoxidation. The relevant sequences of this centre are Thr-Cys-Cys-SerOH in A. gambiae thioredoxin reductase, Gly-Cys-selenocysteine-GlyOH in human thioredoxin reductase, and Cys-X-X-X-X-Cys-GlyOH in the P. falciparum enzyme. These differences offer an interesting approach to the design of species-specific inhibitors. Notably, A. gambiae thioredoxin reductase-1 is not a selenoenzyme but instead contains a highly unusual redox-active Cys-Cys sequence.

Glutathione reductase of the malarial parasite Plasmodium falciparum: crystal structure and inhibitor development

The malarial parasite Plasmodium falciparum is known to be sensitive to oxidative stress, and thus the antioxidant enzyme glutathione reductase (GR; NADPH+GSSG+H(+) <==> NADP(+)+2 GSH) has become an attractive drug target for antimalarial drug development. Here, we report the 2.6A resolution crystal structure of P.falciparum GR. The homodimeric flavoenzyme is compared to the related human GR with focus on structural aspects relevant for drug design. The most pronounced differences between the two enzymes concern the shape and electrostatics of a large (450A(3)) cavity at the dimer interface. This cavity binds numerous non-competitive inhibitors and is a target for selective drug design. A 34-residue insertion specific for the GRs of malarial parasites shows no density, implying that it is disordered. The precise location of this insertion along the sequence allows us to explain the deleterious effects of a mutant in this region and suggests new functional studies. To complement the structural comparisons, we report the relative susceptibility of human and plasmodial GRs to a series of tricyclic inhibitors as well as to peptides designed to interfere with protein folding and dimerization. Enzyme-kinetic studies on GRs from chloroquine-resistant and chloroquine-sensitive parasite strains were performed and indicate that the structure reported here represents GR of P.falciparum strains in general and thus is a highly relevant target for drug development.

Glutathione--functions and metabolism in the malarial parasite Plasmodium falciparum

When present as a trophozoite in human erythrocytes, the malarial parasite Plasmodium falciparum exhibits an intense glutathione metabolism. Glutathione plays a role not only in antioxidative defense and in maintaining the reducing environment of the cytosol. Many of the known glutathione-dependent processes are directly related to the specific lifestyle of the parasite. Reduced glutathione (GSH) supports rapid cell growth by providing electrons for deoxyribonucleotide synthesis and it takes part in detoxifying heme, a product of hemoglobin digestion. Free radicals generated in the parasite can be scavenged in reaction sequences involving the thiyl radical GS* as well as the thiolate GS-. As a substrate of glutathione S-transferase, glutathione is conjugated to non-degradable compounds including antimalarial drugs. Furthermore, it is the coenzyme of the glyoxalase system which detoxifies methylglyoxal, a byproduct of the intense glycolysis taking place in the trophozoite. Proteins involved in GSH-dependent processes include glutathione reductase, glutaredoxins, glyoxalase I and II, glutathione S-transferases, and thioredoxins. These proteins, as well as the ATP-dependent enzymes of glutathione synthesis, are studied as factors in the pathophysiology of malaria but also as potential drug targets. Methylene blue, an inhibitor of the structurally known P. falciparum glutathione reductase, appears to be a promising antimalarial medication when given in combination with chloroquine.

The thioredoxin system?From science to clinic

The thioredoxin system-formed by thioredoxin reductase and its characteristic substrate thioredoxin-is an important constituent of the intracellular redox milieu. Interactions with many different metabolic pathways such as DNA-synthesis, selenium metabolism, and the antioxidative network as well as significant species differences render this system an attractive target for chemotherapeutic approaches in many fields of medicine-ranging from infectious diseases to cancer therapy. In this review we will present and evaluate the preclinical and clinical results available today. Current trends in drug development are emphasized.

Coupling of a competitive and an irreversible ligand generates mixed type inhibitors of Trypanosoma cruzi trypanothione reductase

9-Aminoacridines and (terpyridine)platinum(II) complexes are competitive and irreversible inhibitors, respectively, of trypanothione reductase from Trypanosoma cruzi, the causative agent of Chagas' disease. Four chimeric compounds in which 2-methoxy-6-chloro-9-aminoacridine was covalently linked to the (2-hydroxyethanethiolate)(2,2':6',2' '-terpyridine)platinum(II) complex were synthesized and studied as inhibitors of the parasite enzyme. The derivatives differed by the nature and/or the length of the spacer connecting the two aromatic systems. All four compounds were effective mixed type inhibitors of trypanothione reductase with K(i) and K(i)' values of 0.3-4 and 2-11 microM, respectively. The most potent inhibitor had an ethylthioether linkage between the two aromatic ring systems, and the other compounds contained an alkyl ether group with 4-6 methylene groups. In contrast to the parasite enzyme, human glutathione reductase, the closest related host enzyme was not inhibited by these compounds. The finding that the conjugation of a competitive and an irreversible inhibitor can give rise to reversible mixed type inhibitors underlines the difficulties associated with inhibitor design based on the three-dimensional structure of trypanothione reductase.

A prodrug form of a Plasmodium falciparum glutathione reductase inhibitor conjugated with a 4-anilinoquinoline

Glutathione (GSH), which is known to guard Plasmodium falciparum from oxidative damage, may have an additional protective role by promoting heme catabolism. An elevation of GSH content in parasites leads to increased resistance to chloroquine (CQ), while GSH depletion in resistant P. falciparum strains is expected to restore the sensitivity to CQ. High intracellular GSH levels depend inter alia on the efficient reduction of GSSG by glutathione reductase (GR). On the basis of this hypothesis, we have developed a new strategy for overcoming glutathione-dependent 4-aminoquinoline resistance. To direct both a 4-aminoquinoline and a GR inhibitor to the parasite, double-drugs were designed and synthesized. Quinoline-based alcohols (with known antimalarial activity) were combined with a GR inhibitor via a metabolically labile ester bond to give double-headed prodrugs. The biochemically most active double-drug 7 of this series was then evaluated as a growth inhibitor against six Plasmodium falciparum strains that differed in their degree of resistance to CQ; the ED(50) values for CQ ranged from 14 to 183 nM. While the inhibitory activity of the original 4-aminoquinoline-based alcohol followed that of CQ in these tests, the double-drug exhibited similar efficiency against all strains, the ED(50) being as low as 28 nM. For the ester 7, a dose-dependent decrease in glutathione content and GR activity and an increase in glutathione-S-transferase activity were determined in treated parasites. The drug was subsequently tested for its antimalarial action in vivo using murine malaria models infected with P. berghei. A 178% excess mean survival time was determined for the animals treated with 40 mg/kg 7 for 4 days. No cytotoxicity due to this compound was observed. Work is in progress to extend and validate the strategy outlined here.

Antiplasmodial activity of nitroaromatic and quinoidal compounds: redox potential vs. inhibition of erythrocyte glutathione reductase

Prooxidant nitroaromatic and quinoidal compounds possess antimalarial activity, which might be attributed either to their formation of reactive oxygen species or to their inhibition of antioxidant enzyme glutathione reductase (GR, EC 1.6.4.2). We have examined the activity in vitro against Plasmodium falciparum of 24 prooxidant compounds of different structure (nitrobenzenes, nitrofurans, quinones, 1,1'-dibenzyl-4,4'-bipyridinium, and methylene blue), which possess a broad range of single-electron reduction potentials (E(1)(7)) and erythrocyte glutathione reductase inhibition constants (K(i(GR))). For a series of homologous derivatives of 2-(5'-nitrofurylvinyl)quinoline-4-carbonic acid, the relationship between compound K(i(GR)) and concentration causing 50% parasite growth inhibition (IC(50)) was absent. For all the compounds examined in this study, the dependence of IC(50) on their K(i(GR)) was insignificant. In contrast, IC(50) decreased with an increase in E(1)(7) and positive electrostatic charge of aromatic part of molecule (Z): log IC(50) (microM) = -(0.9846 +/- 0.3525) - (7.2850 +/- 1.2340) E(1)(7) (V) - (1.1034 +/- 0.1832) Z (r(2) = 0.8015). The redox cycling activity of nitroaromatic and quinoidal compounds in ferredoxin:NADP(+) reductase-catalyzed reaction and the rate of oxyhemoglobin oxidation in lysed erythrocytes increased with an increase in their E(1)(7) value. Our findings imply that the antiplasmodial activity of nitroaromatic and quinoidal compounds is mainly influenced by their ability to form reactive oxygen species, and much less significantly by the GR inhibition.

2- and 3-substituted 1,4-naphthoquinone derivatives as subversive substrates of trypanothione reductase and lipoamide dehydrogenase from Trypanosoma cruzi: synthesis and correlation between redox cycling activities and in vitro cytotoxicity

Trypanothione reductase (TR) is both a valid and an attractive target for the design of new trypanocidal drugs. Starting from menadione, plumbagin, and juglone, three distinct series of 1,4-naphthoquinones (NQ) were synthesized as potential inhibitors of TR from Trypanosoma cruzi (TcTR). The three parent molecules were functionalized at carbons 2 and/or 3 by various polyamine chains. Optimization of TcTR inhibition and TcTR specificity versus human disulfide reductases was achieved with the 3,3'-[polyaminobis(carbonylalkyl)]bis(1,4-NQ) series 19-20, in which an optimum chain length was determined for inhibition of the trypanothione disulfide reduction. The most active derivatives against trypanosomes in cultures were also studied as subversive substrates of TcTR and lipoamide dehydrogenase (TcLipDH). The activities were measured by following NAD(P)H oxidation as well as coupling the reactions to the reduction of cytochrome c which permits the detection of one-electron transfer. For TcTR, 20(4-c) proved to be a potent subversive substrate and an effective uncompetitive inhibitor versus trypanothione disulfide and NADPH. Molecular modeling studies based on the known X-ray structures of TcTR and hGR were conducted in order to compare the structural features, dimensions, and accessibility of the cavity at the dimer interface of TcTR with that of hGR, as one of the putative NQ binding sites. TcLipDH reduced the plumbagin derivatives by an order of magnitude faster than the corresponding menadione derivatives. Such differences were not observed with the pig heart enzyme. The most efficient and specific subversive substrates of TcTR and TcLipDH exhibited potent antitrypanosomal activity in in vitro T. brucei and T. cruzi cultures. The results obtained here confirm that reduction of NQs by parasitic flavoenzymes is a promising strategy for the development of new trypanocidal drugs.

Inhibition of Trypanosoma cruzi trypanothione reductase by acridines: kinetic studies and structure-activity relationships

Series of 9-amino and 9-thioacridines have been synthesized and studied as inhibitors of trypanothione reductase (TR) from Trypanosoma cruzi, the causative agent of Chagas' disease. The compounds are structural analogues of the acridine drug mepacrine (quinacrine), which is a competitive inhibitor of the parasite enzyme, but not of human glutathione reductase, the closest related host enzyme. The 9-aminoacridines yielded apparent K(i) values for competitive inhibition between 5 and 43 microM. The most effective inhibitors were those with the methoxy and chlorine substituents of mepacrine and NH(2) or NHCH(CH(3))(CH(2))(4)N(Et)(2) at C9. Detailed kinetic analyses revealed that in the case of 9-aminoacridines more than one inhibitor molecule can bind to the enzyme. In contrast, the 9-thioacridine derivatives inhibit TR with mixed-type kinetics. The kinetic data are discussed in light of the three-dimensional structure of the TR-mepacrine complex. The conclusion that structurally very similar acridine compounds can give rise to completely different inhibition patterns renders modelling studies and quantitative structure-activity relationships difficult.

Enzymatic reduction studies of nitroheterocycles

The nitroimidazole derivative Megazol is a highly active compound used against several strains of Trypanosoma cruzi, the causative agent of Chagas' disease (American trypanomiasis). With the aim of gaining an insight into the probable mode of action, the interaction of Megazol with different redox enzymes was studied in comparison to that of Nifurtimox and Metronidazole. The three nitroaromatic compounds are reduced by L-lactate cytochrome c-reductase, adrenodoxin reductase, and NADPH:cytochrome P-450 reductase (EC 1.6.2.4), the efficiencies of the enzymatic reductions being roughly related to the reduction potentials of these pseudo-substrates. As the enzyme responsible for the reduction of Megazol within the parasite has not yet been identified, the nitroimidazole was assayed with T. cruzi lipoamide dehydrogenase and trypanothione reductase. Megazol did not inhibit the physiological reactions but proved to be a weak substrate of both flavoenzymes. The single electron reduction of the compound by NADPH:cytochrome P-450 reductase, by rat liver as well as by trypanosome microsomes was confirmed by ESR experiments. As shown here, Megazol interferes with the oxygen metabolism of the parasite, but its extra activity when compared to Nifurtimox may be related to other features not yet identified.

Ajoene is an inhibitor and subversive substrate of human glutathione reductase and Trypanosoma cruzi trypanothione reductase: crystallographic, kinetic, and spectroscopic studies

Ajoene ((E,Z)-4,5,9-trithiadodeca-1,6,11-triene 9-oxide), a garlic-derived natural compound, is a covalent inhibitor as well as a substrate of human glutathione reductase (GR) and Trypanosoma cruzi trypanothione reductase (TR). The 2.1-A resolution crystal structure of GR inhibited by (E)-ajoene revealed a mixed disulfide between the active site Cys58 and the CH2=CH-CH2-SO-CH2-CH=CH-S moiety of ajoene. The modified enzyme has a markedly increased oxidase activity when compared to free GR. GR reduces (Z)-ajoene with a kcat/Km of 6.8 x 10(3) M-1 s-1 yielding 4,5,9-trithiadodeca-1, 6,11-triene (deoxyajoene) and 4,8,9,13-tetrathiahexadeca-1,6,10, 15-tetraene as stable reaction products. The reaction leads also to the formation of single-electron reduced products and concomitantly superoxide anion radicals as shown by coupling the reaction to the reduction of cytochrome c. The interactions between the flavoenzymes and ajoene are expected to increase the oxidative stress of the respective cell. The antiparasitic and cytostatic actions of ajoene may at least in part be due to the multiple effects on key enzymes of the antioxidant thiol metabolism.

Inhibitors of glutathione reductase as potential antimalarial drugs. Kinetic cooperativity and effect of dimethyl sulphoxide on inhibition kinetics

We have developed inhibitors of glutathione reductase that improve on the inhibition of literature lead compounds by up to three orders of magnitude. Thus, analogues of Safranine O and menadione were found to be strong, reversible inhibitors of yeast glutathione reductase. Safranine O exhibited partial, uncompetitive inhibition with Ki and alpha values of 0.5 mM and 0.15, respectively. Thionine O was a partial (hyperbolic) uncompetitive inhibitor with Ki and alpha values of 0.4 microM and 0.15, respectively. LY83583 and 2-anilino-1,4-naphthoquinone also showed (hyperbolic) partial, uncompetitive inhibition with micromolar Ki values. For Nile Blue A a model for two-site binding with (parabolic) uncompetitive inhibition fitted the data with a Ki value of 11 microM and a kinetic cooperativity between the sites of 0.12, increased to 0.46 by preincubation of the enzyme and Nile Blue A in the presence of glutathione disulphide. Analysis of the effects of preincubation on the kinetics and cooperativity indicated the possibility of a slow conformational change in the homodimeric enzyme, the first such indication of kinetic cooperativity in the native enzyme to our knowledge. Further evidence of conformational changes for this enzyme came from studies of the effects of dimethyl sulphoxide which indicated that this co-solvent, which at low concentrations has no apparent effect on initial velocities under normal assay conditions, induced a slow conformational change in the enzyme. Thionine O, Nile Blue A and LY83583 were redox-cycling substrates producing superoxide ion, detectable by means of cytochrome c reduction, but leading to no loss of glutathione reductase activity, under aerobic or anaerobic conditions. The water-soluble Safranine analogues Methylene Blue, Methylene Green, Nile Blue A and Thionine O (5 mg/kg i.p. x 5) were effective antimalarial agents in vivo against P. berghei, but their effect was small and a higher dose (50 mg/kg i.p. x 1) was toxic in mice. Comparison was made with human glutathione reductase and its literature-reported interactions with several tricyclic inhibitors as studied by X-ray diffraction. It is possible that the conformational changes detected in the present study from alterations in detailed kinetic inhibition mechanisms may shed light on information transfer through the glutathione reductase molecule from the dimer interface ligand pocket to the active-site.

Recombinant Plasmodium falciparum glutathione reductase is inhibited by the antimalarial dye methylene blue

Plasmodium falciparum glutathione reductase (PfGR) has emerged as a drug target against tropical malaria. Here we report the expression of PfGR in Escherichia coli SG5(DE3) and isolation procedures for this protein. Recombinant PfGR does not differ from the authentic enzyme in its enzymic properties, the turnover number being 9900 min(-1). The dimeric flavoenzyme exhibits redox-dependent absorption spectra; the single tryptophan residue (per 57.2 kDa subunit) is strongly fluorescent. PfGR can be inhibited by the antimalarial drug methylene blue at therapeutic concentrations; the Ki for non-competitive inhibition is 6.4 microM. The sensitivity to methylene blue is observed also at high ionic strength so that, by analogy to human GR, analysis of crystalline enzyme-drug complexes can be envisaged.