The trypanothione system

Identification of host antioxidant effectors as thioridazine targets: Impact on cardiomyocytes infection and Trypanosoma cruzi-induced acute myocarditis

Phenothiazines inhibit antioxidant enzymes in trypanosomatids. However, potential interferences with host cell antioxidant defenses are central concerns in using these drugs to treat Trypanosoma cruzi-induced infectious myocarditis. Thus, the interaction of thioridazine (TDZ) with T. cruzi and cardiomyocytes antioxidant enzymes, and its impact on cardiomyocytes and cardiac infection was investigated in vitro and in vivo. Cardiomyocytes and trypomastigotes in culture, and mice treated with TDZ and benznidazole (Bz, reference antiparasitic drug) were submitted to microstructural, biochemical and molecular analyses. TDZ was more cytotoxic and less selective against T. cruzi than Bz in vitro. TDZ-pretreated cardiomyocytes developed increased infection rate, reactive oxygen species (ROS) production, lipid and protein oxidation; similar catalase (CAT) and superoxide dismutase (SOD) activity, and reduced glutathione's (peroxidase - GPx, S-transferase - GST, and reductase - GR) activity than infected untreated cells. TDZ attenuated trypanothione reductase activity in T. cruzi, and protein antioxidant capacity in cardiomyocytes, making these cells more susceptible to H2O2-based oxidative challenge. In vivo, TDZ potentiated heart parasitism, total ROS production, myocarditis, lipid and protein oxidation; as well as reduced GPx, GR, and GST activities compared to untreated mice. Benznidazole decreased heart parasitism, total ROS production, heart inflammation, lipid and protein oxidation in T. cruzi-infected mice. Our findings indicate that TDZ simultaneously interact with enzymatic antioxidant targets in cardiomyocytes and T. cruzi, potentiating the infection by inducing antioxidant fragility and increasing cardiomyocytes and heart susceptibility to parasitism, inflammation and oxidative damage.

Computational models as catalysts for investigating redoxin systems

Thioredoxin, glutaredoxin and peroxiredoxin systems play central roles in redox regulation, signaling and metabolism in cells. In these systems, reducing equivalents from NAD(P)H are transferred by coupled thiol-disulfide exchange reactions to redoxins which then reduce a wide array of targets. However, the characterization of redoxin activity has been unclear, with redoxins regarded as enzymes in some studies and redox metabolites in others. Consequently, redoxin activities have been quantified by enzyme kinetic parameters in vitro, and redox potentials or redox ratios within cells. By analyzing all the reactions within these systems, computational models showed that many kinetic properties attributed to redoxins were due to system-level effects. Models of cellular redoxin networks have also been used to estimate intracellular hydrogen peroxide levels, analyze redox signaling and couple omic and kinetic data to understand the regulation of these networks in disease. Computational modeling has emerged as a powerful complementary tool to traditional redoxin enzyme kinetic and cellular assays that integrates data from a number of sources into a single quantitative framework to accelerate the analysis of redoxin systems.

Atypical network topologies enhance the reductive capacity of pathogen thiol antioxidant defense networks

Infectious diseases are a significant health burden for developing countries, particularly with the rise of multidrug resistance. There is an urgent need to elucidate the factors underlying the persistence of pathogens such as Mycobacterium tuberculosis, Plasmodium falciparum and Trypanosoma brucei. In contrast to host cells, these pathogens traverse multiple and varied redox environments during their infectious cycles, including exposure to high levels of host-derived reactive oxygen species. Pathogen antioxidant defenses such as the peroxiredoxin and thioredoxin systems play critical roles in the redox stress tolerance of these cells. However, many of the kinetic rate constants obtained for the pathogen peroxiredoxins are broadly similar to their mammalian homologs and therefore, their contributions to the redox tolerances within these cells are enigmatic. Using graph theoretical analysis, we show that compared to a canonical Escherichia coli redoxin network, pathogen redoxin networks contain unique network connections (motifs) between their thioredoxins and peroxiredoxins. Analysis of these motifs reveals that they increase the hydroperoxide reduction capacity of these networks and, in response to an oxidative insult, can distribute fluxes into specific thioredoxin-dependent pathways. Our results emphasize that the high oxidative stress tolerance of these pathogens depends on both the kinetic parameters for hydroperoxide reduction and the connectivity within their thioredoxin/peroxiredoxin systems.

Insights into the molecular basis of some chalcone analogues as potential inhibitors of Leishmania donovani: An integrated in silico and in vitro study

Protozoal infections caused by species belonging to Leishmania donovani complex are responsible for the most severe form of leishmaniasis, especially in Sudan and other developing countries. Drugs commonly used for the treatment of the disease show varying levels of effectiveness and also have associated side effects. Thus, the present work highlights the synthesis of some chalcones to be used as potential anti-leishmanial agents. The activity of the synthesized chalcones has been evaluated against L. donovani. The ADMET profile of the synthesized compounds were tested using various integrated web-based tools. Moreover, in order to investigate the molecular mechanism of action, the chalcone compounds were docked into L. donovani trypanothione reductase (TR) using Autodock 4.0 and molecular dynamics were studies. Eight compounds showed the highest activity against the morphological forms. Among these compounds, chalcones 15 has shown the highest inhibitory effect with IC50 value of 1.1 µM. In addition, pharmacokinetic and toxicological investigations revealed its good oral bioavailability and low toxicity. Furthermore, chalcone 15 was found to interact with high affinity (−13.7 kcal/mol) with TR, an essential enzyme for the leishmanial parasite. Thus, this promising activity against L. donovani supports the use of chalcone 15 as a potential new therapy for visceral leishmaniasis.

Nitric Oxide Resistance in Leishmania (Viannia) braziliensis Involves Regulation of Glucose Consumption, Glutathione Metabolism and Abundance of Pentose Phosphate Pathway Enzymes

In American Tegumentary Leishmaniasis production of cytokines, reactive oxygen species and nitric oxide (NO) by host macrophages normally lead to parasite death. However, some Leishmania braziliensis strains exhibit natural NO resistance. NO-resistant strains cause more lesions and are frequently more resistant to antimonial treatment than NO-susceptible ones, suggesting that NO-resistant parasites are endowed with specific mechanisms of survival and persistence. To tests this, we analyzed the effect of pro- and antioxidant molecules on the infectivity in vitro of L. braziliensis strains exhibiting polar phenotypes of resistance or susceptibility to NO. In addition, we conducted a comprehensive quantitative mass spectrometry-based proteomics analysis of those parasites. NO-resistant parasites were more infective to peritoneal macrophages, even in the presence of high levels of reactive species. Principal component analysis of protein concentration values clearly differentiated NO-resistant from NO-susceptible parasites, suggesting that there are natural intrinsic differences at molecular level among those strains. Upon NO exposure, NO-resistant parasites rapidly modulated their proteome, increasing their total protein content and glutathione (GSH) metabolism. Furthermore, NO-resistant parasites showed increased glucose analogue uptake, and increased abundance of phosphotransferase and G6PDH after nitrosative challenge, which can contribute to NADPH pool maintenance and fuel the reducing conditions for the recovery of GSH upon NO exposure. Thus, increased glucose consumption and GSH-mediated redox capability may explain the natural resistance of L. braziliensis against NO.

Naphthoquinones and Derivatives for Chemotherapy: Perspectives and Limitations of their Anti-trypanosomatids Activities

Chagas disease, Sleeping sickness and Leishmaniasis, caused by trypanosomatids Trypanosoma cruzi, Trypanosoma brucei and Leishmania spp., respectively, are considered neglected tropical diseases, and they especially affect impoverished populations in the developing world. The available chemotherapies are very limited, and a search for alternatives is still necessary. In folk medicine, natural naphthoquinones have been employed for the treatment of a great variety of illnesses, including parasitic infections. This review is focused on the anti-trypanosomatid activity and mechanistic analysis of naphthoquinones and derivatives. Among all the series of derivatives tested in vitro, naphthoquinone-derived 1,2,3-triazoles were very active on T. cruzi infective forms in blood bank conditions, as well as in amastigotes of Leishmania spp. naphthoquinones containing a CF₃ on a phenyl amine ring inhibited T. brucei proliferation in the nanomolar range, and naphthopterocarpanquinones stood out for their activity on a range of Leishmania species. Some of these compounds showed a promising selectivity index (SI) (30 to 1900), supporting further analysis in animal models. Indeed, high toxicity to the host and inactivation by blood components are crucial obstacles to be overcome to use naphthoquinones and/or their derivatives for chemotherapy. Multidisciplinary initiatives embracing medicinal chemistry, bioinformatics, biochemistry, and molecular and cellular biology need to be encouraged to allow the optimization of these compounds. Large scale automated tests are pivotal for the efficiency of the screening step, and subsequent evaluation of both the mechanism of action in vitro and pharmacokinetics in vivo is essential for the development of a novel, specific and safe derivative, minimizing adverse effects.

TrypOx, a Novel Eukaryotic Homolog of the Redox-Regulated Chaperone Hsp33 in Trypanosoma brucei

ATP-independent chaperones are widespread across all domains of life and serve as the first line of defense during protein unfolding stresses. One of the known crucial chaperones for bacterial survival in a hostile environment (e.g., heat and oxidative stress) is the highly conserved, redox-regulated ATP-independent bacterial chaperone Hsp33. Using a bioinformatic analysis, we describe novel eukaryotic homologs of Hsp33 identified in eukaryotic pathogens belonging to the kinetoplastids, a family responsible for lethal human diseases such as Chagas disease as caused by Trypanosoma cruzi, African sleeping sickness caused by Trypanosoma brucei spp., and leishmaniasis pathologies delivered by various Leishmania species. During their pathogenic life cycle, kinetoplastids need to cope with elevated temperatures and oxidative stress, the same conditions which convert Hsp33 into a powerful chaperone in bacteria, thus preventing aggregation of a wide range of misfolded proteins. Here, we focused on a functional characterization of the Hsp33 homolog in one of the members of the kinetoplastid family, T. brucei, (Tb927.6.2630), which we have named TrypOx. RNAi silencing of TrypOx led to a significant decrease in the survival of T. brucei under mild oxidative stress conditions, implying a protective role of TrypOx during the Trypanosomes growth. We then adopted a proteomics-driven approach to investigate the role of TrypOx in defining the oxidative stress response. Depletion of TrypOx significantly altered the abundance of proteins mediating redox homeostasis, linking TrypOx with the antioxidant system. Using biochemical approaches, we identified the redox-switch domain of TrypOx, showing its modularity and oxidation-dependent structural plasticity. Kinetoplastid parasites such as T. brucei need to cope with high levels of oxidants produced by the innate immune system, such that parasite-specific antioxidant proteins like TrypOx - which are depleted in mammals - are highly promising candidates for drug targeting.

The mitochondrial peroxiredoxin displays distinct roles in different developmental stages of African trypanosomes

Hydroperoxide reduction in African trypanosomes relies on 2-Cys-peroxiredoxins (Prxs) and glutathione peroxidase-type enzymes (Pxs) which both obtain their reducing equivalents from the trypanothione/tryparedoxin couple and thus act as tryparedoxin peroxidases. While the cytosolic forms of the peroxidases are essential, the mitochondrial mPrx and Px III appear dispensable in bloodstream Trypanosoma brucei. This led to the suggestion that in this developmental stage which is characterized by a mitochondrion that lacks an active respiratory chain, only one of the two peroxidases might be required. Here we show that bloodstream cells in which the Px III gene is deleted and mPrx is down-regulated by RNA interference, proliferate as the parental cells indicating that both mitochondrial peroxidases are dispensable. However, when we raised the culture temperature to 39 °C, mPrx-depleted cells died indicating that under conditions mimicking a fever situation in the mammalian host, the protein becomes essential. In contrast, depletion of mPrx in insect stage procyclic T. brucei causes a proliferation defect under standard conditions at 27 °C, in the absence of any stress. In the absence of mPrx, a tryparedoxin-coupled roGFP2 biosensor expressed in the mitochondrial matrix is unable to respond to antimycin A treatment. Thus mPrx reduces mitochondrial H₂O₂ with the generation of trypanothione disulfide and acts as peroxidase. However, mPrx-depleted procyclic cells neither display any alteration in the cytosolic or mitochondrial trypanothione redox state nor increased sensitivity towards exogenous oxidative stressors suggesting that the peroxidase activity is not the crucial physiological function. After prolonged mPrx-depletion, the cells almost stop proliferation and display a highly elongated shape and diminished MitoTracker Red staining. In contrast to the situation in the mammalian bloodstream T. brucei and Leishmania, mPrx appears to play a constitutive role for the morphology, mitochondrial function and proliferation of the insect stage of African trypanosomes.

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In bloodstream T. brucei, both mitochondrial tryparedoxin peroxidases are dispensable.

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Heat-stressed bloodstream cells require the mitochondrial peroxiredoxin (mPrx).

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In procyclic (PC) T. brucei, mPrx plays a constitutive role for proliferation.

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Lack of mPrx affects the structure and mitochondrial membrane potential of PC cells.

Critical functions of the polyamine putrescine for proliferation and viability of Leishmania donovani parasites

Polyamines are metabolites that play important roles in rapidly proliferating cells, and recent studies have highlighted their critical nature in Leishmania parasites. However, little is known about the function of polyamines in parasites. To address this question, we assessed the effect of polyamine depletion in Leishmania donovani mutants lacking ornithine decarboxylase (Δodc) or spermidine synthase (Δspdsyn). Intracellular putrescine levels depleted rapidly in Δodc mutants and accumulated in Δspdsyn mutants, while spermidine levels were maintained at low but stable levels in both cell lines. Putrescine depletion in the Δodc mutants led to cell rounding, immediate cessation of proliferation, and loss of viability, while putrescine-rich Δspdsyn mutants displayed an intermediate proliferation phenotype and were able to arrest in a quiescent-like state for 6 weeks. Supplementation of Δodc mutants with spermidine had little effect on cell proliferation and morphology but enabled parasites to persist for 14 weeks. Thus, putrescine is not only essential as precursor for spermidine formation but also critical for parasite proliferation, morphology, and viability.

Selenocysteine in mammalian thioredoxin reductase and application of ebselen as a therapeutic

Thioredoxin system is a ubiquitous disulfide reductase system evolutionarily conserved through all living organisms. It contains thioredoxin (Trx), thioredoxin reductase (TrxR) and NADPH. TrxR can use NADPH to reduce Trx which passes the reducing equivalent to its downstream substrates involved in various biomedical events, such as ribonucleotide reductase for deoxyribonucleotide and DNA synthesis, or peroxiredoxins for counteracting oxidative stress. Obviously, TrxR stays in the center of the system to maintain the electron flow. Mammalian TrxR contains a selenocysteine (Sec) in its active site, which is not present in the low molecular weight prokaryotic TrxRs. Due to the special property of Sec, mammalian TrxR employs a different catalytic mechanism from prokaryotic TrxRs and has a broader substrate-spectrum. On the other hand, Sec is easily targeted by electrophilic compounds which inhibits the TrxR activity and may turn TrxR into an NADPH oxidase. Ebselen, a synthetic seleno-compound containing selenazol, has been tested in several clinical studies. In mammalian cells, ebselen works as a GSH peroxidase mimic and mainly as a peroxiredoxin mimic via Trx and TrxR to scavenge hydrogen peroxide and peroxynitrite. In prokaryotic cells, ebselen is an inhibitor of TrxR and leads to elevation of reactive oxygen species (ROS). Recent studies have made use of the difference and developed ebselen as a potential antibiotic, especially in combination with silver which enables ebselen to kill multi-drug resistant Gram-negative bacteria. Collectively, Sec is important for the biological functions of mammalian TrxR and distinguishes it from prokaryotic TrxRs, therefore it is a promising drug target.

Identification of chalcone-based antileishmanial agents targeting trypanothione reductase

All currently used first-line and second-line drugs for the treatment of leishmaniasis exhibit several drawbacks including toxicity, high costs and route of administration. Furthermore, some drugs are associated with the emergence of drug resistance. Thus, the development of new treatments for leishmaniasis is a priority in the field of neglected tropical diseases. The present work highlights the use of natural derived products, i.e. chalcones, as potential source of antileishmanial agents. Thirty-one novel chalcone compounds have been synthesized and their activity has been evaluated against promastigotes of Leishmania donovani; 16 compounds resulted active against L. donovani in a range from 3.0 to 21.5 μM, showing low toxicity against mammalian cells. Among these molecules, 6 and 16 showed good inhibitory activity on both promastigotes and intracellular amastigotes, coupled with an high selectivity index. Furthermore, compounds 6 and 16 inhibited the promastigote growth of other leishmanial species, including L. tropica, L. major and L. infantum. Finally, 6 and 16 interacted with high affinity with trypanothione reductase (TR), an essential enzyme for the leishmanial parasite and compound 6 inhibited TR with sub-micromolar potency. Thus, the effective inhibitory activity against Leishmania, the lack of toxicity on mammalian cells and the ability to block a crucial parasite's enzyme, highlight the potential for compound 6 to be optimized as novel drug candidate against leishmaniasis.

Novel β-carboline-quinazolinone hybrids disrupt Leishmania donovani redox homeostasis and show promising antileishmanial activity

Visceral Leishmaniasis is a deadly parasitic disease caused by Leishmania donovani. Paucity exists in the discovery of novel chemotherapeutics against Leishmaniasis. In this study, we synthesized a natural product inspired Diversity Oriented Synthesis library of L. donovani Trypanothione reductase (LdTR) inhibitor β-carboline-quinazolinone hybrids, which are different in stereochemical architecture and diverse in the bioactive chemical space. It is noteworthy that chirality affects drug-to-protein binding affinity since proteins in any living system are present only in one of the chiral forms. Upon evaluation of the hybrids, one of the chiral forms i.e. Compound 1 showed profound cytotoxic effect in micromolar range as compared to its other chiral form i.e. Compound 2. In-silico docking studies confirmed high binding efficiency of Compound 1 with the catalytic pocket of LdTR. Treatment of L. donovani parasites with Compound 1 inhibits LdTR activity, induces imbalance in redox homeostasis by enhancing ROS, disrupts the mitochondrial membrane potential, modifies actin polymerization and alters the surface topology and architecture. All these cellular modifications eventually led to apoptosis-like death of promastigotes. Furthermore, we synthesized the analogues of Compound 1 and found that these compounds show profound antileishmanial activity in the nanomolar range both in promastigotes and intracellular amastigotes. The enhanced inhibitory potential of these compounds was further supported by in-silico analysis of protein-ligand interactions which revealed high binding efficiency towards the catalytic pocket of LdTR. Taken together, this study reports the serendipitous discovery of β-carboline-quinazolinone hybrids with enhanced antileishmanial activity along with the in-depth structure-activity relationships and mechanism of action of these analogues.

Analogues of Marine Guanidine Alkaloids Are in Vitro Effective against Trypanosoma cruzi and Selectively Eliminate Leishmania (L.) infantum Intracellular Amastigotes

Synthetic analogues of marine sponge guanidine alkaloids showed in vitro antiparasitic activity against Leishmania (L.) infantum and Trypanosoma cruzi. Guanidines 10 and 11 presented the highest selectivity index when tested against Leishmania. The antiparasitic activity of 10 and 11 was investigated in host cells and in parasites. Both compounds induced depolarization of mitochondrial membrane potential, upregulation of reactive oxygen species levels, and increased plasma membrane permeability in Leishmania parasites. Immunomodulatory assays suggested an NO-independent effect of guanidines 10 and 11 on macrophages. The same compounds also promoted anti-inflammatory activity in L. (L.) infantum-infected macrophages cocultived with splenocytes, reducing the production of cytokines MCP-1 and IFN-γ. Guanidines 10 and 11 affect the bioenergetic metabolism of Leishmania, with selective elimination of parasites via a host-independent mechanism.

Trypanothione reductase: a target protein for a combined in vitro and in silico screening approach

With the goal to identify novel trypanothione reductase (TR) inhibitors, we performed a combination of in vitro and in silico screening approaches. Starting from a highly diverse compound set of 2,816 compounds, 21 novel TR inhibiting compounds could be identified in the initial in vitro screening campaign against T. cruzi TR. All 21 in vitro hits were used in a subsequent similarity search-based in silico screening on a database containing 200,000 physically available compounds. The similarity search resulted in a data set containing 1,204 potential TR inhibitors, which was subjected to a second in vitro screening campaign leading to 61 additional active compounds. This corresponds to an approximately 10-fold enrichment compared to the initial pure in vitro screening. In total, 82 novel TR inhibitors with activities down to the nM range could be identified proving the validity of our combined in vitro/in silico approach. Moreover, the four most active compounds, showing IC50 values of <1 μM, were selected for determining the inhibitor constant. In first on parasites assays, three compounds inhibited the proliferation of bloodstream T. brucei cell line 449 with EC50 values down to 2 μM.

The double-edged sword in pathogenic trypanosomatids: the pivotal role of mitochondria in oxidative stress and bioenergetics

The pathogenic trypanosomatids Trypanosoma brucei, Trypanosoma cruzi, and Leishmania spp. are the causative agents of African trypanosomiasis, Chagas disease, and leishmaniasis, respectively. These diseases are considered to be neglected tropical illnesses that persist under conditions of poverty and are concentrated in impoverished populations in the developing world. Novel efficient and nontoxic drugs are urgently needed as substitutes for the currently limited chemotherapy. Trypanosomatids display a single mitochondrion with several peculiar features, such as the presence of different energetic and antioxidant enzymes and a specific arrangement of mitochondrial DNA (kinetoplast DNA). Due to mitochondrial differences between mammals and trypanosomatids, this organelle is an excellent candidate for drug intervention. Additionally, during trypanosomatids' life cycle, the shape and functional plasticity of their single mitochondrion undergo profound alterations, reflecting adaptation to different environments. In an uncoupling situation, the organelle produces high amounts of reactive oxygen species. However, these species role in parasite biology is still controversial, involving parasite death, cell signalling, or even proliferation. Novel perspectives on trypanosomatid-targeting chemotherapy could be developed based on better comprehension of mitochondrial oxidative regulation processes.

Metabolic reprogramming during purine stress in the protozoan pathogen Leishmania donovani

The ability of Leishmania to survive in their insect or mammalian host is dependent upon an ability to sense and adapt to changes in the microenvironment. However, little is known about the molecular mechanisms underlying the parasite response to environmental changes, such as nutrient availability. To elucidate nutrient stress response pathways in Leishmania donovani, we have used purine starvation as the paradigm. The salvage of purines from the host milieu is obligatory for parasite replication; nevertheless, purine-starved parasites can persist in culture without supplementary purine for over three months, indicating that the response to purine starvation is robust and engenders parasite survival under conditions of extreme scarcity. To understand metabolic reprogramming during purine starvation we have employed global approaches. Whole proteome comparisons between purine-starved and purine-replete parasites over a 6–48 h span have revealed a temporal and coordinated response to purine starvation. Purine transporters and enzymes involved in acquisition at the cell surface are upregulated within a few hours of purine removal from the media, while other key purine salvage components are upregulated later in the time-course and more modestly. After 48 h, the proteome of purine-starved parasites is extensively remodeled and adaptations to purine stress appear tailored to deal with both purine deprivation and general stress. To probe the molecular mechanisms affecting proteome remodeling in response to purine starvation, comparative RNA-seq analyses, qRT-PCR, and luciferase reporter assays were performed on purine-starved versus purine-replete parasites. While the regulation of a minority of proteins tracked with changes at the mRNA level, for many regulated proteins it appears that proteome remodeling during purine stress occurs primarily via translational and/or post-translational mechanisms.

Leishmania, the cause of a deadly spectrum of diseases in humans, surmounts a number of environmental challenges, including changes in the availability of salvageable nutrients, to successfully colonize its host. Adaptation to environmental stress is clearly of significance in parasite biology, but the underlying mechanisms are not well understood. To simulate the response to periodic nutrient scarcity in vivo, we have induced purine starvation in vitro. Purines are essential for growth and viability, and serve as the major energy currency of cells. Leishmania cannot synthesize purines and must salvage them from the surroundings. Extracellular purine depletion in culture induces a robust survival response in Leishmania, whereby growth arrests, but parasites persist for months. To profile the events that enable endurance of purine starvation, we used shotgun proteomics. Our data suggest that purine starvation induces extensive proteome remodeling, tailored to enhance purine capture and recycling, reduce energy expenditures, and maintain viability of the metabolically active, non-dividing population. Through global and targeted approaches, we reveal that proteome remodeling is multifaceted, and occurs through an array of responses at the mRNA, translational, and post-translational level. Our data provide one of the most inclusive views of adaptation to microenvironmental stress in Leishmania.

Characterization of a Novel Putative S-Adenosylmethionine Decarboxylase-Like Protein from Leishmania donovani

In addition to the S-adenosylmethionine decarboxylase (AD) present in all organisms, trypanosomatids including Leishmania spp. possess an additional copy, annotated as the putative S-adenosylmethionine decarboxylase-like proenzyme (ADL). Phylogenetic analysis confirms that ADL is unique to trypanosomatids and has several unique features such as lack of autocatalytic cleavage and a distinct evolutionary lineage, even from trypanosomatid ADs. In Trypanosoma ADL was found to be enzymaticaly dead but plays an essential regulatory role by forming a heterodimer complex with AD. However, no structural or functional information is available about ADL from Leishmania spp. Here, in this study, we report the cloning, expression, purification, structural and functional characterization of Leishmania donovani (L. donovani) ADL using biophysical, biochemical and computational techniques. Biophysical studies show that, L. donovani ADL binds S-adenosylmethionine (SAM) and putrescine which are natural substrates of AD. Computational modeling and docking studies showed that in comparison to the ADs of other organisms including human, residues involved in putrescine binding are partially conserved while the SAM binding residues are significantly different. In silico protein-protein interaction study reveals that L. donovani ADL can interact with AD. These results indicate that L. donovani ADL posses a novel substrate binding property and may play an essential role in polyamine biosynthesis with a different mode of function from known proteins of the S-adenosylmethionine decarboxylase super family.

Molecular dynamics reveal binding mode of glutathionylspermidine by trypanothione synthetase

The trypanothione synthetase (TryS) catalyses the two-step biosynthesis of trypanothione from spermidine and glutathione and is an attractive new drug target for the development of trypanocidal and antileishmanial drugs, especially since the structural information of TryS from Leishmania major has become available. Unfortunately, the TryS structure was solved without any of the substrates and lacks loop regions that are mechanistically important. This contribution describes docking and molecular dynamics simulations that led to further insights into trypanothione biosynthesis and, in particular, explains the binding modes of substrates for the second catalytic step. The structural model essentially confirm previously proposed binding sites for glutathione, ATP and two Mg²⁺ ions, which appear identical for both catalytic steps. The analysis of an unsolved loop region near the proposed spermidine binding site revealed a new pocket that was demonstrated to bind glutathionylspermidine in an inverted orientation. For the second step of trypanothione synthesis glutathionylspermidine is bound in a way that preferentially allows N¹-glutathionylation of N⁸-glutathionylspermidine, classifying N⁸-glutathionylspermidine as the favoured substrate. By inhibitor docking, the binding site for N⁸-glutathionylspermidine was characterised as druggable.

Low-molecular-mass antioxidants in parasites

SIGNIFICANCE

Parasitic infections continue to be a major problem for global human health. Vaccines are practically not available and chemotherapy is highly unsatisfactory. One approach toward a novel antiparasitic drug development is to unravel pathways that may be suited as future targets. Parasitic organisms show a remarkable diversity with respect to the nature and functions of their main low-molecular-mass antioxidants and many of them developed pathways that do not have a counterpart in their mammalian hosts.

RECENT ADVANCES

Work of the last years disclosed the individual antioxidants employed by parasites and their distinct pathways. Entamoeba, Trichomonas, and Giardia directly use cysteine as main low-molecular-mass thiol but have divergent cysteine metabolisms. Malarial parasites rely exclusively on cysteine uptake and generate glutathione (GSH) as main free thiol as do metazoan parasites. Trypanosomes and Leishmania have a unique trypanothione-based thiol metabolism but employ individual mechanisms for their cysteine supply. In addition, some trypanosomatids synthesize ovothiol A and/or ascorbate. Various essential parasite enzymes such as trypanothione synthetase and trypanothione reductase in Trypanosomatids and the Schistosoma thioredoxin GSH reductase are currently intensively explored as drug target molecules.

CRITICAL ISSUES

Essentiality is a prerequisite but not a sufficient property of an enzyme to become a suited drug target. The availability of an appropriate in vivo screening system and many other factors are equally important.

FUTURE DIRECTIONS

The current organism-wide RNA-interference and proteome analyses are supposed to reveal many more interesting candidates for future drug development approaches directed against the parasite antioxidant defense systems.

Peroxiredoxins in parasites

SIGNIFICANCE

Parasite survival and virulence relies on effective defenses against reactive oxygen and nitrogen species produced by the host immune system. Peroxiredoxins (Prxs) are ubiquitous enzymes now thought to be central to such defenses and, as such, have potential value as drug targets and vaccine antigens.

RECENT ADVANCES

Plasmodial and kinetoplastid Prx systems are the most extensively studied, yet remain inadequately understood. For many other parasites our knowledge is even less well developed. Through parasite genome sequencing efforts, however, the key players are being discovered and characterized. Here we describe what is known about the biochemistry, regulation, and cell biology of Prxs in parasitic protozoa, helminths, and fungi. At least one Prx is found in each parasite with a sequenced genome, and a notable theme is the common patterns of expression, localization, and functionality among sequence-similar Prxs in related species.

CRITICAL ISSUES

The nomenclature of Prxs from parasites is in a state of disarray, causing confusion and making comparative inferences difficult. Here we introduce a systematic Prx naming convention that is consistent between organisms and informative about structural and evolutionary relationships.

FUTURE DIRECTIONS

The new nomenclature should stimulate the crossfertilization of ideas among parasitologists and with the broader redox research community. The diverse parasite developmental stages and host environments present complex systems in which to explore the variety of roles played by Prxs, with a view toward parlaying what is learned into novel therapies and vaccines that are urgently needed.

A tryparedoxin-dependent peroxidase protects African trypanosomes from membrane damage

Hydroperoxide detoxification in African trypanosomes is achieved by 2-Cys-peroxiredoxin (TXNPx)- and non-selenium glutathione peroxidase (Px)-type enzymes which both obtain their reducing equivalents from the unique trypanothione/tryparedoxin system. Previous RNA interference approaches revealed that the cytosolic TXNPx and the Px-type enzymes are essential for Trypanosoma brucei. Because of partially overlapping in vitro substrate specificities and subcellular localisation the physiological function of the individual enzymes was not yet clear. As shown here, TXNPx and Px are expressed at comparable levels and in their active reduced state. Px-overexpressing parasites were less sensitive toward linoleic acid hydroperoxide but not hydrogen peroxide. Kinetic studies confirmed that Px-but not TXNPx-reduces lipophilic hydroperoxides including phospholipids with high efficiency. Most interestingly, the severe proliferation defect of Px-depleted bloodstream cells could be rescued by Trolox, but not by hydrophilic antioxidants, in the medium. This allowed us to knock-out the three Px genes individually and thus to distinguish their in vivo role. Deletion of the cytosolic Px I and II resulted in extremely fast membrane peroxidation followed by cell lysis. Cells lacking specifically the mitochondrial Px III showed a transient growth retardation and cardiolipin peroxidation but adapted within 24h to normal proliferation.

A comparison of thiol peroxidase mechanisms

Thiol peroxidases comprise glutathione peroxidases (GPx) and peroxiredoxins (Prx). The enzymes of both families reduce hydroperoxides with thiols by enzyme-substitution mechanisms. H(2)O(2) and organic hydroperoxides are reduced by all thiol peroxidases, most efficiently by SecGPxs, whereas fast peroxynitrite reduction is more common in Prxs. Reduction of lipid hydroperoxides is the domain of monomeric GPx4-type enzymes and of some Prxs. The catalysis starts with oxidation of an active-site selenocysteine (U(P)) or cysteine (C(P)). Activation of Cys (Sec) for hydroperoxide reduction in the GPx family is achieved by a typical tetrad composed of Cys (Sec), Asn, Gln, and Trp, whereas a triad of Cys Thr (or Ser) and Arg is the signature of Prx. In many of the CysGPxs and Prxs, a second Cys (C(R)) is required. In these 2-CysGPxs and 2-CysPrxs, the C(P) oxidized to a sulfenic acid forms an intra- or intermolecular disulfide (typical 2-CysPrx) with C(R), before a stepwise regeneration of ground-state enzyme by redoxin-type proteins can proceed. In SecGPxs and sporadically in Prxs, GSH is used as the reductant. Diversity combined with structural variability predestines thiol peroxidases for redox regulation via ROOH sensing and direct or indirect transduction of oxidant signals to specific protein targets.

Leishmania-macrophage interactions: insights into the redox biology

Leishmaniasis is a neglected tropical disease that affects about 350 million individuals worldwide. The protozoan parasite has a relatively simple life cycle with two principal stages: the flagellated mobile promastigote living in the gut of the sandfly vector and the intracellular amastigote within phagolysosomal vesicles of the vertebrate host macrophage. This review presents a state-of-the-art overview of the redox biology at the parasite-macrophage interface. Although Leishmania species are susceptible in vitro to exogenous superoxide radical, hydrogen peroxide, nitric oxide, and peroxynitrite, they manage to survive the endogenous oxidative burst during phagocytosis and the subsequent elevated nitric oxide production in the macrophage. The parasite adopts various defense mechanisms to cope with oxidative stress: the lipophosphoglycan membrane decreases superoxide radical production by inhibiting NADPH oxidase assembly and the parasite also protects itself by expressing antioxidant enzymes and proteins. Some of these enzymes could be considered potential drug targets because they are not expressed in mammals. In respect to antileishmanial therapy, the effects of current drugs on parasite-macrophage redox biology and its involvement in the development of drug resistance and treatment failure are presented.

The trypanothione system and the opportunities it offers to create drugs for the neglected kinetoplast diseases

Parasitic trypanosomatids (Kinetoplastida) are the causative agents of devastating and hard-to-treat diseases such as African sleeping sickness, Chagas disease and various forms of Leishmaniasis. Altogether they affect > 30 Million patients, account for half a million fatalities p.a. and cause substantial economical problems in the Third World due to human morbidity and life stock losses. The design of efficacious and safe drugs is expected from inhibition of metabolic pathways that are unique and essential to the parasite and absent in the host. In this respect, the trypanothione system first detected in the insect-pathogenic trypanosomatid Crithidia fasciculata qualified as an attractive drug target area. The existence of the system in pathogenic relatives was established by homology cloning and PCR. The vital importance of the system was verified in Trypanosoma brucei by dsRNA technology or knock-out in other trypanosomatids, respectively, and is explained by its pivotal role in the parasite's antioxidant defense and DNA synthesis. The key system component is the bis-glutathionyl derivative of spermidine, trypanothione. It is the proximal reductant of tryparedoxin which substitutes for thioredoxin-, glutaredoxin- and glutathione-dependent reactions. Heterologous expression, functional characterization and crystallization of recombinant system components finally enable structure-based rational inhibitor design.

Targeting essential pathways in trypanosomatids gives insights into protozoan mechanisms of cell death

Apoptosis is a normal component of the development and health of multicellular organisms. However, apoptosis is now considered a prerogative of unicellular organisms, including the trypanosomatids of the genera Trypanosoma spp. and Leishmania spp., causative agents of some of the most important neglected human diseases. Trypanosomatids show typical hallmarks of apoptosis, although they lack some of the key molecules contributing to this process in metazoans, like caspase genes, Bcl-2 family genes and the TNF-related family of receptors. Despite the lack of these molecules, trypanosomatids appear to have the basic machinery to commit suicide. The components of the apoptotic execution machinery of these parasites are slowly coming into light, by targeting essential processes and pathways with different apoptogenic agents and inhibitors. This review will be confined to the events known to drive trypanosomatid parasites to apoptosis.

The dithiol glutaredoxins of african trypanosomes have distinct roles and are closely linked to the unique trypanothione metabolism

Trypanosoma brucei, the causative agent of African sleeping sickness, possesses two dithiol glutaredoxins (Grx1 and Grx2). Grx1 occurs in the cytosol and catalyzes protein deglutathionylations with k(cat)/K(m)-values of up to 2 × 10(5) M(-1) S(-1). It accelerates the reduction of ribonucleotide reductase by trypanothione although less efficiently than the parasite tryparedoxin and has low insulin disulfide reductase activity. Despite its classical CPYC active site, Grx1 forms dimeric iron-sulfur complexes with GSH, glutathionylspermidine, or trypanothione as non-protein ligands. Thus, contrary to the generally accepted assumption, replacement of the Pro is not a prerequisite for cluster formation. T. brucei Grx2 shows an unusual CQFC active site, and orthologues occur exclusively in trypanosomatids. Grx2 is enriched in mitoplasts, and fractionated digitonin lysis resulted in a co-elution with cytochrome c, suggesting localization in the mitochondrial intermembrane space. Grx2 catalyzes the reduction of insulin disulfide but not of ribonucleotide reductase and exerts deglutathionylation activity 10-fold lower than that of Grx1. RNA interference against Grx2 caused a growth retardation of procyclic cells consistent with an essential role. Grx1 and Grx2 are constitutively expressed with cellular concentrations of about 2 μM and 200 nM, respectively, in both the mammalian bloodstream and insect procyclic forms. Trypanothione reduces the disulfide form of both proteins with apparent rate constants that are 3 orders of magnitude higher than those with glutathione. Grx1 and, less efficiently, also Grx2 catalyze the reduction of GSSG by trypanothione. Thus, the Grxs play exclusive roles in the trypanothione-based thiol redox metabolism of African trypanosomes.

Quiescin sulfhydryl oxidase from Trypanosoma brucei: catalytic activity and mechanism of a QSOX family member with a single thioredoxin domain

Quiescin sulfhydryl oxidase (QSOX) flavoenzymes catalyze the direct, facile, insertion of disulfide bonds into reduced unfolded proteins with the reduction of oxygen to hydrogen peroxide. To date, only QSOXs from vertebrates have been characterized enzymatically. These metazoan sulfhydryl oxidases have four recognizable domains: a redox-active thioredoxin (Trx) domain containing the first of three CxxC motifs (C(I)-C(II)), a second Trx domain with no obvious redox-active disulfide, a helix-rich domain, and then an Erv/ALR domain. This last domain contains the FAD moiety, a proximal C(III)-C(IV) disulfide, and a third CxxC of unknown function (C(V)-C(VI)). Plant and protist QSOXs lack the second Trx domain but otherwise appear to contain the same complement of redox centers. This work presents the first characterization of a single-Trx QSOX. Trypanosoma brucei QSOX was expressed in Escherichia coli using a synthetic gene and found to be a stable, monomeric, FAD-containing protein. Although evidently lacking an entire domain, TbQSOX shows catalytic activity and substrate specificity similar to the vertebrate QSOXs examined previously. Unfolded reduced proteins are more than 200-fold more effective substrates on a per thiol basis than glutathione and some 10-fold better than the parasite bisglutathione analogue, trypanothione. These data are consistent with a role for the protist QSOX in oxidative protein folding. Site-directed mutagenesis of each of the six cysteine residues (to serines) shows that the CxxC motif in the single-Trx domain is crucial for efficient catalysis of the oxidation of both reduced RNase and the model substrate dithiothreitol. As expected, the proximal disulfide C(III)-C(IV), which interacts with the flavin, is catalytically crucial. However, as observed with human QSOX1, the third CxxC motif shows no obvious catalytic role during the in vitro oxidation of reduced RNase or dithiothreitol. Pre-steady-state kinetics demonstrates that turnover in TbQSOX is limited by an internal redox step leading to 2-electron reduction of the FAD cofactor. In sum, the single-Trx domain QSOX studied here shows a striking similarity in enzymatic behavior to its double-Trx metazoan counterparts.

Mode of action of nifurtimox and N-oxide-containing heterocycles against Trypanosoma cruzi: is oxidative stress involved?

Chagas disease is caused by the trypanosomatid parasite Trypanosoma cruzi and threatens millions of lives in South America. As other neglected diseases there is almost no research and development effort by the pharmaceutical industry and the treatment relies on two drugs, Nifurtimox and Benznidazole, discovered empirically more than three decades ago. Nifurtimox, a nitrofurane derivative, is believed to exert its biological activity through the bioreduction of the nitro-group to a nitro-anion radical which undergoes redox-cycling with molecular oxygen. This hypothesis is generally accepted, although arguments against it have been presented. In the present work we studied the ability of Nifurtimox and five N-oxide-containing heterocycles to induce oxidative stress in T. cruzi. N-Oxide-containing heterocycles represent a promising group of new trypanosomicidal agents and their mode of action is not completely elucidated. The results here obtained argue against the oxidative stress hypothesis almost for all the studied compounds, including Nifurtimox. A significant reduction in the level of parasitic low-molecular-weight thiols was observed after Nifurtimox treatment; however, it was not linked to the production of reactive oxidant species. Besides, redox-cycling is only observed at high Nifurtimox concentrations (>400microM), two orders of magnitude higher than the concentration required for anti-proliferative activity (5microM). Our results indicate that an increase in oxidative stress is not the main mechanism of action of Nifurtimox. Among the studied N-oxide-containing heterocycles, benzofuroxan derivatives strongly inhibited parasite dehydrogenase activity and affected mitochondrial membrane potential. The indazole derivative raised intracellular oxidants production, but it was the least effective as anti-T. cruzi.

Vitamin B6: a molecule for human health?

Vitamin B₆ is an intriguing molecule that is involved in a wide range of metabolic, physiological and developmental processes. Based on its water solubility and high reactivity when phosphorylated, it is a suitable co-factor for many biochemical processes. Furthermore the vitamin is a potent antioxidant, rivaling carotenoids or tocopherols in its ability to quench reactive oxygen species. It is therefore not surprising that the vitamin is essential and unquestionably important for the cellular metabolism and well-being of all living organisms. The review briefly summarizes the biosynthetic pathways of vitamin B₆ in pro- and eukaryotes and its diverse roles in enzymatic reactions. Finally, because in recent years the vitamin has often been considered beneficial for human health, the review will also sum up and critically reflect on current knowledge how human health can profit from vitamin B₆.

Role of a differentially expressed cAMP phosphodiesterase in regulating the induction of resistance against oxidative damage in Leishmania donovani

Differentiation-coupled induction of resistance of Leishmania parasites to macrophage oxidative damage was shown to be associated with an increased cAMP response. This study explores the significance of the cAMP response in the parasite by identifying a differentially expressed cAMP phosphodiesterase (LdPDEA) and deciphering its role in regulating antioxidant machineries in the parasite. LdPDEA, a high K(M) class I cytosolic cAMP phosphodiesterase, was expressed maximally in log-phase promastigotes, but was significantly reduced in stationary-phase promastigotes and amastigotes. Chemical inhibition or silencing of PDEA conferred enhanced resistance to pro-oxidants in these cells and this led to studies on trypanothione biosynthesis and utilization, as trypanothione is one of the major modulators of antioxidant defense in kinetoplastidae. Despite enhanced arginase and ornithine decarboxylase activity, trypanothione biosynthesis seemed to be unaffected by PDEA blockage, whereas significant elevations in the expression of tryparedoxin peroxidase, ascorbate peroxidase, and tryparedoxin were detected, suggesting a definite shift of trypanothione-pool utilization bias toward antioxidant defense. Moreover, parasites that overexpressed PDEA showed reduced resistance to oxidative damage and reduced infectivity toward activated macrophages. This study reveals the significance of a cAMP phosphodiesterase in the infectivity of Leishmania parasites.

PTR1-dependent synthesis of tetrahydrobiopterin contributes to oxidant susceptibility in the trypanosomatid protozoan parasite Leishmania major

Leishmania must survive oxidative stress, but lack many classical antioxidant enzymes and rely heavily on trypanothione-dependent pathways. We used forward genetic screens to recover loci mediating oxidant resistance via overexpression in Leishmania major, which identified pteridine reductase 1 (PTR1). Comparisons of isogenic lines showed ptr1 (-) null mutants were 18-fold more sensitive to H(2)O(2) than PTR1-overproducing lines, and significant three- to fivefold differences were seen with a broad panel of oxidant-inducing agents. The toxicities of simple nitric oxide generators and other drug classes (except antifolates) were unaffected by PTR1 levels. H(2)O(2) susceptibility could be modulated by exogenous biopterin but not folate, in a PTR1- but not dihydrofolate reductase-dependent manner, implicating H(4)B metabolism specifically. Neither H(2)O(2) consumption nor the level of intracellular oxidative stress was affected by PTR1 levels. Coupled with the fact that reduced pteridines are at least 100-fold less abundant than cellular thiols, these data argue strongly that reduced pteridines act through a mechanism other than scavenging. The ability of unconjugated pteridines to counter oxidative stress has implications to infectivity and response to chemotherapy. Since the intracellular pteridine levels of Leishmania can be readily manipulated, these organisms offer a powerful setting for the dissection of pteridine-dependent oxidant susceptibility in higher eukaryotes.

Trypanocidal activity of 8-methyl-5'-{[(Z)-4-aminobut-2-enyl]-(methylamino)}adenosine (Genz-644131), an adenosylmethionine decarboxylase inhibitor

Genzyme 644131, 8-methyl-5'-{[(Z)-4-aminobut-2-enyl](methylamino)}adenosine, is an analog of the enzyme activated S-adenosylmethionine decarboxylase (AdoMetDC) inhibitor and the trypanocidal agent MDL-7381, 5-{[(Z)-4-aminobut-2-enyl](methylamino)}adenosine. The analog differs from the parent in having an 8-methyl group on the purine ring that bestows favorable pharmacokinetic, biochemical, and trypanocidal activities. The compound was curative in acute Trypanosoma brucei brucei and drug-resistant Trypanosoma brucei rhodesiense model infections, with single-dose activity in the 1- to 5-mg/kg/day daily dose range for 4 days against T. brucei brucei and 25- to 50-mg/kg twice-daily dosing against T. brucei rhodesiense infections. The compound was not curative in the TREU 667 central nervous system model infection but cleared blood parasitemia and extended time to recrudescence in several groups. This study shows that AdoMetDC remains an attractive chemotherapeutic target in African trypanosomes and that chemical changes in AdoMetDC inhibitors can produce more favorable drug characteristics than the lead compound.

The role of reduced pterins in resistance to reactive oxygen and nitrogen intermediates in the protozoan parasite Leishmania

During its life cycle, the protozoan parasite Leishmania experiences oxidative stress when interacting with macrophages. Reduced pterins are known scavengers of reactive oxygen and nitrogen intermediates. Leishmania has a pteridine reductase, PTR1, whose main function is to provide reduced pterins. We investigated the role of PTR1 in resistance to oxidative and nitrosative stress in Leishmania tarentolae, Leishmania infantum, and Leishmania major PTR1(-/-) mutants. The PTR1(-/-) cells of the three species were more sensitive to H2O2- and NO-induced stress. Using a fluorescent probe allowing ROI quantification, we demonstrated an increase in intracellular oxidant molecules in the PTR1(-/-) mutants. The disruption of PTR1 increased metacyclogenesis in L. infantum and L. major. We purified metacyclic parasites from PTR1(-/-) mutants and control cells and tested their intracellular survival in the J774 mouse cell line and in human monocyte-derived macrophages. Our results showed that PTR1(-/-) null mutants survived less in both macrophage models compared to control cells and this decrease was more pronounced in macrophages activated for oxidant production. This study demonstrates that one physiological role of reduced pterins in Leishmania is to deal with oxidative and nitrosative species, and a decreased ability to provide reduced pterins leads to decreased intracellular survival.

Leishmania major lacking arginase (ARG) are auxotrophic for polyamines but retain infectivity to susceptible BALB/c mice

Polyamines are essential metabolites in eukaryotes participating in a variety of proliferative processes, and in trypanosomatid protozoa play an additional role in the synthesis of the critical thiol trypanothione. Whereas the polyamine biosynthesis arising from L-ornithine has been well studied in protozoa, the metabolic origin(s) of L-ornithine have received less attention. Arginase (EC 3.5.3.1) catalyzes the enzymatic hydrolysis of L-arginine to L-ornithine and urea, and we tested the role of arginase in polyamine synthesis by the generation of an arg(?) knockout in Leishmania major by double targeted gene replacement. This mutant lacked arginase activity and required the nutritional provision of polyamines or L-ornithine for growth. A complemented line (arg(?)/+ARG) expressing arginase from a multi-copy expression vector showed 30-fold elevation of arginase activity, similar polyamine and ornithine levels as the wild-type, and resistance to the inhibitors ?-difluoromethylornithine (DFMO) and N(?)-hydroxy-l-arginine (NOHA). This established that arginase is the major route of polyamine synthesis in promastigotes cultured in vitro. The arg(?) parasites retained the ability to differentiate normally to the infective metacyclic stage, and were able to induce progressive disease following inoculation into susceptible BALB/c mice, albeit less efficiently than WT parasites. These data suggest that the infective amastigote form of Leishmania, which normally resides within an acidified parasitophorous vacuole, can survive in vivo through salvage of host polyamines and/or other molecules, aided by the tendency of acidic compartments to concentrate basic metabolites. This may thus contribute to the relative resistance of Leishmania to ornithine decarboxylase (ODC) inhibitors. The availability of infective, viable, arginase-deficient parasites should prove useful in dissecting the role of l-arginine metabolism in both pro- and anti-parasitic responses involving host nitric oxide synthase, which requires L-arginine to generate NO.

Structural basis for a distinct catalytic mechanism in Trypanosoma brucei tryparedoxin peroxidase

Trypanosoma brucei, the causative agent of African sleeping sickness, encodes three cysteine homologues (Px I-III) of classical selenocysteine-containing glutathione peroxidases. The enzymes obtain their reducing equivalents from the unique trypanothione (bis(glutathionyl)spermidine)/tryparedoxin system. During catalysis, these tryparedoxin peroxidases cycle between an oxidized form with an intramolecular disulfide bond between Cys(47) and Cys(95) and the reduced peroxidase with both residues in the thiol state. Here we report on the three-dimensional structures of oxidized T. brucei Px III at 1.4A resolution obtained by x-ray crystallography and of both the oxidized and the reduced protein determined by NMR spectroscopy. Px III is a monomeric protein unlike the homologous poplar thioredoxin peroxidase (TxP). The structures of oxidized and reduced Px III are essentially identical in contrast to what was recently found for TxP. In Px III, Cys(47), Gln(82), and Trp(137) do not form the catalytic triad observed in the selenoenzymes, and related proteins and the latter two residues are unaffected by the redox state of the protein. The mutational analysis of three conserved lysine residues in the vicinity of the catalytic cysteines revealed that exchange of Lys(107) against glutamate abrogates the reduction of hydrogen peroxide, whereas Lys(97) and Lys(99) play a crucial role in the interaction with tryparedoxin.

Regulated expression of an essential allosteric activator of polyamine biosynthesis in African trypanosomes

Trypanosoma brucei is the causative agent of African sleeping sickness. The polyamine biosynthetic pathway has the distinction of being the target of the only clinically proven anti-trypanosomal drug with a known mechanism of action. Polyamines are essential for cell growth, and their metabolism is extensively regulated. However, trypanosomatids appear to lack the regulatory control mechanisms described in other eukaryotic cells. In T. brucei, S-adenosylmethionine decarboxylase (AdoMetDC) and ornithine decarboxylase (ODC) are required for the synthesis of polyamines and also for the unique redox-cofactor trypanothione. Further, trypanosomatid AdoMetDC is activated by heterodimer formation with a catalytically dead homolog termed prozyme, found only in these species. To study polyamine regulation in T. brucei, we generated inducible AdoMetDC RNAi and prozyme conditional knockouts in the mammalian blood form stage. Depletion of either protein led to a reduction in spermidine and trypanothione and to parasite death, demonstrating that prozyme activation of AdoMetDC is essential. Under typical growth conditions, prozyme concentration is limiting in comparison to AdoMetDC. However, both prozyme and ODC protein levels were significantly increased relative to stable transcript levels by knockdown of AdoMetDC or its chemical inhibition. Changes in protein stability do not appear to account for the increased steady-state protein levels, as both enzymes are stable in the presence of cycloheximide. These observations suggest that prozyme and ODC are translationally regulated in response to perturbations in the pathway. In conclusion, we describe the first evidence for regulation of polyamine biosynthesis in T. brucei and we demonstrate that the unique regulatory subunit of AdoMetDC is a key component of this regulation. The data support ODC and AdoMetDC as the key control points in the pathway and the likely rate-limiting steps in polyamine biosynthesis.

Cytotoxic interactions of methylene blue with trypanosomatid-specific disulfide reductases and their dithiol products

Methylene blue (MB) is known to have trypanocidal activity. We tested the interactions of MB with a number of trypanosomatid-specific molecules of the antioxidant metabolism. At pH 7, trypanothione and other (di)thiols were oxidized to disulfides by the phenothiazine drug. MB inhibited Trypanosoma cruzi trypanothione reductase (TR) (K(i)=1.9 microM), and served as a significant subversive substrate of this enzyme (K(M)=30 microM, k(cat)=4.9s(-1)). With lipoamide dehydrogenase, the second thiol-generating flavoenzyme of T. cruzi, the catalytic efficiency for MB reduction was found to be almost 10(6)M(-1)s(-1). When the system MB-enzyme-molecular oxygen acts as a NAD(P)H-driven redox cycler, a reactive oxygen species, H(2)O(2) or superoxide, is produced in each cycle. Since MB is an affordable, available, and accessible drug it might be tested--alone or in drug combinations--against trypanosomatid-caused diseases of animal and man.