Trypanosomal antioxidants and emerging aspects of redox regulation in the trypanosomatids

Unique thiol metabolism in trypanosomatids: Redox homeostasis and drug resistance

Trypanosomatids are mainly responsible for heterogeneous parasitic diseases: Leishmaniasis, Sleeping sickness, and Chagas disease and control of these diseases implicates serious challenges due to the emergence of drug resistance. Redox-active biomolecules are the endogenous substances in organisms, which play important role in the regulation of redox homeostasis. The redox-active substances like glutathione, trypanothione, cysteine, cysteine persulfides, etc., and other inorganic intermediates (hydrogen peroxide, nitric oxide) are very useful as defence mechanism. In the present review, the suitability of trypanothione and other essential thiol molecules of trypanosomatids as drug targets are described in Leishmania and Trypanosoma. We have explored the role of tryparedoxin, tryparedoxin peroxidase, ascorbate peroxidase, superoxide dismutase, and glutaredoxins in the anti-oxidant mechanism and drug resistance. Up-regulation of some proteins in trypanothione metabolism helps the parasites in survival against drug pressure (sodium stibogluconate, Amphotericin B, etc.) and oxidative stress. These molecules accept electrons from the reduced trypanothione and donate their electrons to other proteins, and these proteins reduce toxic molecules, neutralize reactive oxygen, or nitrogen species; and help parasites to cope with oxidative stress. Thus, a better understanding of the role of these molecules in drug resistance and redox homeostasis will help to target metabolic pathway proteins to combat Leishmaniasis and trypanosomiases.

Mitochondrial Ca2+ and Reactive Oxygen Species in Trypanosomatids

Significance: Millions of people are infected with trypanosomatids and new therapeutic approaches are needed. Trypanosomatids possess one mitochondrion per cell and its study has led to discoveries of general biological interest. These mitochondria, as in their animal counterparts, generate reactive oxygen species (ROS) and have evolved enzymatic and nonenzymatic defenses against them. Mitochondrial calcium ion (Ca2+) overload leads to generation of ROS and its study could lead to relevant information on the biology of trypanosomatids and to novel drug targets. Recent Advances: Mitochondrial Ca2+ is normally involved in maintaining the bioenergetics of trypanosomes, but when Ca2+ overload occurs, it is associated with cell death. Trypanosomes lack key players in the mechanism of cell death described in mammalian cells, although mitochondrial Ca2+ overload results in collapse of their membrane potential, production of ROS, and cytochrome c release. They are also very resistant to mitochondrial permeability transition, and cell death after mitochondrial Ca2+ overload depends on generation of ROS. Critical Issues: In this review, we consider the mechanisms of mitochondrial oxidant generation and removal and the involvement of Ca2+ in trypanosome cell death. Future Directions: More studies are required to determine the reactions involved in generation of ROS by the mitochondria of trypanosomatids, their enzymatic and nonenzymatic defenses against ROS, and the occurrence and composition of a mitochondrial permeability transition pore. Antioxid. Redox Signal. 36, 969-983.

Measurement of Acid Ecto-phosphatase Activity in Live Leishmania donovani Parasites

Acid ecto-phosphatases are enzymes that hydrolyze phosphomonoesters in the acidic pH range with their active sites facing the extacellular medium. Their activities can be measured in living cells. In bacteria and protozoan pathogens, acid ecto-phosphatases have been associated with the survival of intracellular pathogens within phagocytes through inhibition of the respiratory burst, suggesting that they act as virulence factors. Extracellular acid phosphatase activity in Leishmania (L.) donovani has been associated with the degree of promastigote virulence/infectivity. The levels of acid ecto-phosphatase activity in different Leishmania sp or even strains of the same species vary and this has been linked to their virulence. It may also be related to their ability to survive and multiply in the insect host. Acid phosphatase enzymatic activity can be measured in crude membrane fractions and in membrane fractions enriched in plasma membrane, however, in these cases, the intracellular acid phosphatases, mainly localized in lysosomes, contribute to the final result. Therefore, measuring phosphatase activity at the surface of live cells in acidic pH range is the only accurate way to measure acid ecto-phosphatase activity. This assay is performed at 25 °C or 37 °C for 30 min using as substrate the generic phosphatase substrate p-nitrophenyl phosphate (pNPP), in a citrate buffer, with or without sodium tartrate (L(+)-tartaric acid), as histidine acid phosphatases are classified according to their sensitivity to tartate inhibition. The steps of the protocol consist of pelleting cells in suspension, in this case Leishmania promastigotes, washing twice with HEPES buffer, resuspending the cells in the substrate reaction mixture and terminating the reaction by the addition of 0.5 N NaOH. The cells are removed by centrifugation and the absorbance of the reaction product (p-nitrophenolate=pNP) in the supernatant is measured at 405 nm. The enzymatic activity (A₄₀₅ values) is normalized for the mean number of cells/ml used for each independent experiment.

The Leishmania donovani histidine acid ecto-phosphatase LdMAcP: insight into its structure and function

Acid ecto-phosphatase activity has been implicated in Leishmania donovani promastigote virulence. In the present study, we report data contributing to the molecular/structural and functional characterization of the L. donovani LdMAcP (L. donovani membrane acid phosphatase), member of the histidine acid phosphatase (HAcP) family. LdMAcP is membrane-anchored and shares high sequence identity with the major secreted L. donovani acid phosphatases (LdSAcPs). Sequence comparison of the LdMAcP orthologues in Leishmania sp. revealed strain polymorphism and species specificity for the L. donovani complex, responsible for visceral leishmaniasis (Khala azar), proposing thus a potential value of LdMAcP as an epidemiological or diagnostic tool. The extracellular orientation of the LdMAcP catalytic domain was confirmed in L. donovani promastigotes, wild-type (wt) and transgenic overexpressing a recombinant LdMAcP–mRFP1 (monomeric RFP1) chimera, as well as in transiently transfected mammalian cells expressing rLdMAcP–His. For the first time it is demonstrated in the present study that LdMAcP confers tartrate resistant acid ecto-phosphatase activity in live L. donovani promastigotes. The latter confirmed the long sought molecular identity of at least one enzyme contributing to this activity. Interestingly, the L. donovani rLdMAcP–mRFP1 promastigotes generated in this study, showed significantly higher infectivity and virulence indexes than control parasites in the infection of J774 mouse macrophages highlighting thereby a role for LdMAcP in the parasite's virulence.

Acid ecto-phosphatase activity has been linked to Leishmania donovani virulence. In the present study, we confirm the molecular identity and characterize molecular and functional properties of an enzyme contributing to this activity, the LdMAcP, an L. donovani specific membrane histidine acid phosphatase (HAcP).

Molecular mechanisms of in vitro betulin-induced apoptosis of Leishmania donovani

Although leishmanial infections of humans occur globally, the major health impact lies in developing nations, thus, leishmaniases remain "neglected" diseases for new drugs development. Multidrug resistance has been documented in most countries where leishmaniases is endemic. Betulin is a widely available and affordable natural product exerting leishmanicidal activity at micromolar concentration. In this study, the molecular mechanisms of death that contribute to the anti-leishmanial activity of betulin are investigated. In promastigotes, betulin stimulated reactive oxygen species generation at micromolar concentrations in Leishmania. Apoptosis was observed in betulin-treated promastigotes using flow cytometric analysis of treated cells stained with annexin V-FITC and propidium iodide. Furthermore, betulin treatment of promastigotes led to mitochondrial membrane damage, activation of caspase-like proteases, and DNA fragmentation in Leishmania donovani promastigotes. Betulin treatment of amastigotes cultured within macrophages, resulted in a reduced number of amastigotes, with no substantive cytotoxic damage to the host macrophage cells at leishmanicidal drug concentrations.

Ecto-nucleotidases and Ecto-phosphatases from Leishmania and Trypanosoma parasites

Ecto-enzymes can be defined as membrane-bound proteins that have their active site facing the extracellular millieu. In trypanosomatids, the physiological roles of these enzymes remain to be completed elucidated; however, many important events have already been related to them, such as the survival of parasites during their complex life cycle and the successful establishment of host infection. This chapter focuses on two remarkable classes of ecto-enzymes: ecto-nucleotidases and ecto-phosphatases, summarizing their occurrence and possible physiological roles in Leishmania and Trypanosoma genera. Ecto-nucleotidases are characterized by their ability to hydrolyze extracellular nucleotides, playing an important role in purinergic signaling. By the action of these ecto-enzymes, parasites are capable of modulating the host immune system, which leads to a successful parasite infection. Furthermore, ecto-nucleotidases are also involved in the purine salvage pathway, acting in the generation of nucleosides that are able to cross plasma membrane via specialized transporters. Another important ecto-enzyme present in a vast number of pathogenic organisms is the ecto-phosphatase. These enzymes are able to hydrolyze extracellular phosphorylated substrates, releasing free inorganic phosphate that can be internalized by the cell, crossing the plasma membrane through a Pi-transporter. Ecto-phosphatases are also involved in the invasion and survival of parasite in the host cells. Several alternative functions have been suggested for these enzymes in parasites, such as participation in their proliferation, differentiation, nutrition and protection. In this context, the present chapter provides an overview of recent discoveries related to the occurrence of ecto-nucleotidase and ecto-phosphatase activities in Leishmania and Trypanosoma parasites.

Molecular mechanism underlying antileishmanial effect of oxabicyclo[3.3.1]nonanones: inhibition of key redox enzymes of the pathogen

We report oxabicyclo[3.3.1]nonanones as inhibitors of key redox enzymes, trypanothione synthetase (TryS), and trypanothione reductase (TryR) of Leishmania. Further, detailed cellular effects of 4-(4,4,8-Trimethyl-7-oxo-3-oxabicyclo[3.3.1]non-2-yl)-benzoic acid methyl ester, a oxabicyclo[3.3.1]nonanones, on the parasite were investigated. As these compounds inhibit key redox enzymes (TryR amd TryS), treatment of these compounds resulted in increased reactive oxygen species (ROS), mitochondrial membrane damage, activation of caspase like proteases, and DNA damage that finally leads to apoptosis. Although the compound has modest IC50 value against parasite (4.9±0.4 μM), they identify a novel chemical space to design and develop drugs based on these compounds against the Leishmania parasite. This is first report of oxabicyclo[3.3.1]nonanones as antileishmanial.

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.

Reactive oxygen species-mediated immunity against Leishmania mexicana and Serratia marcescens in the sand phlebotomine fly Lutzomyia longipalpis

Phlebotomine sand flies are the vectors of medically important Leishmania. The Leishmania protozoa reside in the sand fly gut, but the nature of the immune response to the presence of Leishmania is unknown. Reactive oxygen species (ROS) are a major component of insect innate immune pathways regulating gut-microbe homeostasis. Here we show that the concentration of ROS increased in sand fly midguts after they fed on the insect pathogen Serratia marcescens but not after feeding on the Leishmania that uses the sand fly as a vector. Moreover, the Leishmania is sensitive to ROS either by oral administration of ROS to the infected fly or by silencing a gene that expresses a sand fly ROS-scavenging enzyme. Finally, the treatment of sand flies with an exogenous ROS scavenger (uric acid) altered the gut microbial homeostasis, led to an increased commensal gut microbiota, and reduced insect survival after oral infection with S. marcescens. Our study demonstrates a differential response of the sand fly ROS system to gut microbiota, an insect pathogen, and the Leishmania that utilize the sand fly as a vehicle for transmission between mammalian hosts.

Metabolic pathways required for the intracellular survival of Leishmania

Leishmania spp. are sandfly-transmitted parasitic protozoa that cause a spectrum of important diseases and lifelong chronic infections in humans. In the mammalian host, these parasites proliferate within acidified vacuoles in several phagocytic host cells, including macrophages, dendritic cells, and neutrophils. In this review, we discuss recent progress that has been made in defining the nutrient composition of the Leishmania parasitophorous vacuole, as well as metabolic pathways required by these parasites for virulence. Analysis of the virulence phenotype of Leishmania mutants has been particularly useful in defining carbon sources and nutrient salvage pathways that are essential for parasite persistence and/or induction of pathology. We also review data suggesting that intracellular parasite stages modulate metabolic processes in their host cells in order to generate a more permissive niche.

The Role of Heme and Reactive Oxygen Species in Proliferation and Survival of Trypanosoma cruzi

Trypanosoma cruzi, the protozoan responsible for Chagas disease, has a complex life cycle comprehending two distinct hosts and a series of morphological and functional transformations. Hemoglobin degradation inside the insect vector releases high amounts of heme, and this molecule is known to exert a number of physiological functions. Moreover, the absence of its complete biosynthetic pathway in T. cruzi indicates heme as an essential molecule for this trypanosomatid survival. Within the hosts, T. cruzi has to cope with sudden environmental changes especially in the redox status and heme is able to increase the basal production of reactive oxygen species (ROS) which can be also produced as byproducts of the parasite aerobic metabolism. In this regard, ROS sensing is likely to be an important mechanism for the adaptation and interaction of these organisms with their hosts. In this paper we discuss the main features of heme and ROS susceptibility in T. cruzi biology.

Ecto-phosphatases in protozoan parasites: possible roles in nutrition, growth and ROS sensing

The cellular plasma membrane contains enzymes whose active sites face the external medium rather than the cytoplasm. The activities of these enzymes, referred to as ecto-enzymes, can be measured using living cells. Ecto-phosphatases are ecto-enzymes that presumably hydrolyze extracellular phosphorylated substrates, releasing free inorganic phosphate. Although, several alternative functions have been suggested for these enzymes, such as participation in proliferation, differentiation, adhesion, virulence, and infection, little is known about the physiological roles of these enzymes in protozoa parasites. In this review, we discuss the principal features of ecto-phosphatases in protozoan parasites that are causative agents of important diseases such as Chagas' disease, leishmaniasis, amoebiasis, giardiasis, trichomoniasis and, sleeping sickness.

Modulation of Trypanosoma rangeli ecto-phosphatase activity by hydrogen peroxide

As a protozoan parasite of hematophagous insects, Trypanosoma rangeli epimastigotes are exposed to reactive oxygen species during development in hosts. In this work, we investigated the role of H(2)O(2) as a modulator of the ecto-phosphatase activity present in living T. rangeli. We observed that H(2)O(2) inhibits ecto-phosphatase activities in the short and long epimastigote forms of T. rangeli. Ecto-phosphatase activity found in the short form was more sensitive than that found in the long form. Moreover, H(2)O(2) inhibited ecto-phosphatase activity of the short form in a dose-dependent manner and this inhibition was reversible after H(2)O(2) removal. This effect was not observed for T. rangeli ecto-ATPase, another ecto-enzyme present on the external surface of T. rangeli. Cysteine, beta-mercaptoethanol, and reduced glutathione were able to revert the enzyme inhibition promoted by H(2)O(2). Catalase and glutathione peroxidase stimulated this ecto-phosphatase activity, whereas superoxide dismutase was not able to modulate this activity. The ecto-phosphatase activity was also activated by FCCP and inhibited by oligomycin. It seems that H(2)O(2) plays a fundamental role in the regulation of cellular processes of these organisms. We showed, for the first time, that these parasites can produce H(2)O(2), and it is able to regulate ecto-phosphatase activity.

Peroxidases of trypanosomatids

This article provides an overview about the recent advances in the dissection of the peroxide metabolism of Trypanosomatidae. This family of protozoan organisms comprises the medically relevant parasites Trypanosoma brucei, Trypanosoma cruzi, and Leishmania spp. Over the past 10 years, three major families of peroxidases have been identified in these organisms: (a) 2-cysteine peroxiredoxins, (b) nonselenium glutathione peroxidases, and (c) ascorbate peroxidases. In trypanosomatids, these enzymes display the unique feature of using reducing equivalents derived from trypanothione, a dithiol found exclusively in these protozoa. The electron transfer between trypanothione and the peroxidases is mediated by a redox shuttle, which can either be tryparedoxin, ascorbate, or even glutathione. The preference for the intermediate molecule differs among each peroxidase and so does the specificity for the peroxide substrate. These observations, added to the fact that these peroxidases are distributed throughout different subcellular compartments, point to the existence of an elaborate peroxide metabolism in trypanosomatids. With the completion of the trypanosomatids genome, other molecules displaying peroxidase activity might be added to this list in the future.

New trypanocidal hybrid compounds from the association of hydrazone moieties and benzofuroxan heterocycle

Hybrid compounds containing hydrazones and benzofuroxan pharmacophores were designed as potential Trypanosoma cruzi-enzyme inhibitors. The majority of the designed compounds was successfully synthesized and biologically evaluated displaying remarkable in vitro activity against different strains of T. cruzi. Unspecific cytotoxicity was evaluated using mouse macrophages, displaying isothiosemicarbazone 10 and thiosemicarbazone 12 selectivity indexes (macrophage/parasite) of 21 and 27, respectively. In addition, the mode of anti-trypanosomal action of the derivatives was investigated. Some of these derivatives were moderate inhibitors of cysteinyl active site enzymes of T. cruzi, cruzipain and trypanothione reductase. ESR experiments using T. cruzi microsomal fraction suggest that the main mechanism of action of the trypanocidal effects is the production of oxidative stress into the parasite.

Nucleoredoxin, a novel thioredoxin family member involved in cell growth and differentiation

Thioredoxin (TRX) family proteins are involved in various biologic processes by regulating the response to oxidative stress. Nucleoredoxin (NRX), a relatively uncharacterized member of the TRX family protein, has recently been reported to regulate the Wnt/beta-catenin pathway, which itself regulates cell fate and early development, in a redox-dependent manner. In this review, we describe the TRX family proteins and discuss in detail the similarities and differences between NRX and other TRX family proteins. Although NRX possesses a conserved TRX domain and a catalytic motif for oxidoreductase activity, its sequence homology to TRX is not as high as that of the close relatives of TRX. The sequence of NRX is more similar to that of tryparedoxin (TryX), a TRX family member originally identified in parasite trypanosomes. We also discuss the reported properties and potential physiologic roles of NRX.

Malaria circumsporozoite protein inhibits the respiratory burst in Kupffer cells

After transmission by infected mosquitoes, malaria sporozoites rapidly travel to the liver. To infect hepatocytes, sporozoites traverse Kupffer cells, but surprisingly, the parasites are not killed by these resident macrophages of the liver. Here we show that Plasmodium sporozoites and recombinant circumsporozoite protein (CSP) suppress the respiratory burst in Kupffer cells. Sporozoites and CSP increased the intracellular concentration of cyclic adenosyl mono-phosphate (cAMP) and inositol 1,4,5-triphosphate in Kupffer cells, but not in hepatocytes or liver endothelia. Preincubation with cAMP analogues or inhibition of phosphodiesterase also inhibited the respiratory burst. By contrast, adenylyl cyclase inhibition abrogated the suppressive effect of sporozoites. Selective protein kinase A (PKA) inhibitors failed to reverse the CSP-mediated blockage and stimulation of the exchange protein directly activated by cAMP (EPAC), but not PKA inhibited the respiratory burst. Both blockage of the low-density lipoprotein receptor-related protein (LRP-1) with receptor-associated protein and elimination of cell surface proteoglycans inhibited the cAMP increase in Kupffer cells. We propose that by binding of CSP to LRP-1 and cell surface proteoglycans, malaria sporozoites induce a cAMP/EPAC-dependent, but PKA-independent signal transduction pathway that suppresses defence mechanisms in Kupffer cells. This allows the sporozoites to safely pass through these professional phagocytes and to develop inside neighbouring hepatocytes.

Mode of action of natural and synthetic drugs against Trypanosoma cruzi and their interaction with the mammalian host

Current knowledge of the biochemistry of Trypanosoma cruzi has led to the development of new drugs and the understanding of their mode of action. Some trypanocidal drugs such as nifurtimox and benznidazole act through free radical generation during their metabolism. T. cruzi is very susceptible to the cell damage induced by these metabolites because enzymes scavenging free radicals are absent or have very low activities in the parasite. Another potential target is the biosynthetic pathway of glutathione and trypanothione, the low molecular weight thiol found exclusively in trypanosomatids. These thiols scavenge free radicals and participate in the conjugation and detoxication of numerous drugs. Inhibition of this key pathway could render the parasite much more susceptible to the toxic action of drugs such as nifurtimox and benznidazole without affecting the host significantly. Other drugs such as allopurinol and purine analogs inhibit purine transport in T. cruzi, which cannot synthesize purines de novo. Nitroimidazole derivatives such as itraconazole inhibit sterol metabolism. The parasite's respiratory chain is another potential therapeutic target because of its many differences with the host enzyme complexes. The pharmacological modulation of the host's immune response against T. cruzi infection as a possible chemotherapeutic target is discussed. A large set of chemicals of plant origin and a few animal metabolites active against T. cruzi are enumerated and their likely modes of action are briefly discussed.

The genome of the African trypanosome Trypanosoma brucei

African trypanosomes cause human sleeping sickness and livestock trypanosomiasis in sub-Saharan Africa. We present the sequence and analysis of the 11 megabase-sized chromosomes of Trypanosoma brucei. The 26-megabase genome contains 9068 predicted genes, including approximately 900 pseudogenes and approximately 1700 T. brucei-specific genes. Large subtelomeric arrays contain an archive of 806 variant surface glycoprotein (VSG) genes used by the parasite to evade the mammalian immune system. Most VSG genes are pseudogenes, which may be used to generate expressed mosaic genes by ectopic recombination. Comparisons of the cytoskeleton and endocytic trafficking systems with those of humans and other eukaryotic organisms reveal major differences. A comparison of metabolic pathways encoded by the genomes of T. brucei, T. cruzi, and Leishmania major reveals the least overall metabolic capability in T. brucei and the greatest in L. major. Horizontal transfer of genes of bacterial origin has contributed to some of the metabolic differences in these parasites, and a number of novel potential drug targets have been identified.

Sulfur-Containing Amino Acid Metabolism in Parasitic Protozoa

Sulfur-containing amino acids play indispensable roles in a wide variety of biological activities including protein synthesis, methylation, and biosynthesis of polyamines and glutathione. Biosynthesis and catabolism of these amino acids need to be carefully regulated to achieve the requirement of the above-mentioned activities and also to eliminate toxicity attributable to the amino acids. Genome-wide analyses of enzymes involved in the metabolic pathways of sulfur-containing amino acids, including transsulfuration, sulfur assimilatory de novo cysteine biosynthesis, methionine cycle, and degradation, using genome databases available from a variety of parasitic protozoa, reveal remarkable diversity between protozoan parasites and their mammalian hosts. Thus, the sulfur-containing amino acid metabolic pathways are a rational target for the development of novel chemotherapeutic and prophylactic agents against diseases caused by protozoan parasites. These pathways also demonstrate notable heterogeneity among parasites, suggesting that the metabolism of sulfur-containing amino acids reflects the diversity of parasitism among parasite species, and probably influences their biology and pathophysiology such as virulence competence and stress defense.

Redox potential regulates binding of universal minicircle sequence binding protein at the kinetoplast DNA replication origin

Kinetoplast DNA, the mitochondrial DNA of the trypanosomatid Crithidia fasciculata, is a remarkable structure containing 5,000 topologically linked DNA minicircles. Their replication is initiated at two conserved sequences, a dodecamer, known as the universal minicircle sequence (UMS), and a hexamer, which are located at the replication origins of the minicircle L- and H-strands, respectively. A UMS-binding protein (UMSBP), binds specifically the conserved origin sequences in their single stranded conformation. The five CCHC-type zinc knuckle motifs, predicted in UMSBP, fold into zinc-dependent structures capable of binding a single-stranded nucleic acid ligand. Zinc knuckles that are involved in the binding of DNA differ from those mediating protein-protein interactions that lead to the dimerization of UMSBP. Both UMSBP DNA binding and its dimerization are sensitive to redox potential. Oxidation of UMSBP results in the protein dimerization, mediated through its N-terminal domain, with a concomitant inhibition of its DNA-binding activity. UMSBP reduction yields monomers that are active in the binding of DNA through the protein C-terminal region. C. fasciculata trypanothione-dependent tryparedoxin activates the binding of UMSBP to UMS DNA in vitro. The possibility that UMSBP binding at the minicircle replication origin is regulated in vivo by a redox potential-based mechanism is discussed.

Two linked genes of Leishmania infantum encode tryparedoxins localised to cytosol and mitochondrion

Tryparedoxins are components of the hydroperoxide detoxification cascades of Kinetoplastida, where they mediate electron transfer between trypanothione and a peroxiredoxin, which reduces hydroperoxides and possibly peroxynitrite. Tryparedoxins may also be involved in DNA synthesis, by their capacity to reduce ribonucleotide reductase. Here we report on the isolation of two tryparedoxin genes from Leishmania infantum, Li7XN1 and LiTXN2, which share the same genetic locus. These genes are both single copy and code for two active tryparedoxin enzymes, LiTXN1 and LiTXN2, with different biochemical and biological features. LiTXN1 is located to the cytosol and is upregulated in the infectious forms of the parasite, strongly suggesting that it might play an important role during infection. LiTXN2 is the first mitochondrial tryparedoxin described in Kinetoplastida. Biochemical assays performed on the purified recombinant proteins have shown that LiTXN1 preferentially reduces the cytosolic L. infantum peroxiredoxins, LicTXNPx1 and LicTXNPx2, whereas LiTXN2 has a higher specific activity for a mitochondrial peroxiredoxin, LimTXNPx. Kinetically, the two tryparedoxins follow a ping-pong mechanism and show no saturation. We suggest that LiTXN1 and LiTXN2 are part of two distinct antioxidant machineries, one cytosolic, the other mitochondrial, that complement each other to ensure effective defence from several sources of oxidants throughout the development of L. infantum.

The parasite-specific trypanothione metabolism of trypanosoma and leishmania

The bis(glutathionyl)spermidine trypanothione exclusively occurs in parasitic protozoa of the order Kinetoplastida, such as trypanosomes and leishmania, some of which are the causative agents of several tropical diseases. The dithiol is kept reduced by the flavoenzyme trypanothione reductase and the trypanothione system replaces in these parasites the nearly ubiquitous glutathione/glutathione reductase couple. Trypanothione is a reductant of thioredoxin and tryparedoxin, small dithiol proteins, which in turn deliver reducing equivalents for the synthesis of deoxyribonucleotides as well as for the detoxification of hydroperoxides by different peroxidases. Depending on the individual organism and the developmental state, the parasites also contain significant amounts of glutathione, mono-glutathionylspermidine and ovothiol, whereby all four low molecular mass thiols are directly (trypanothione and mono-glutathionylspermidine) or indirectly (glutathione and ovothiol) maintained in the reduced state by trypanothione reductase. Thus the trypanothione system is central for any thiol regeneration and trypanothione reductase has been shown to be an essential enzyme in these parasites. The absence of this pathway from the mammalian host and the sensitivity of trypanosomatids toward oxidative stress render the enzymes of the trypanothione metabolism attractive target molecules for the rational development of new drugs against African sleeping sickness, Chagas' disease and the different forms of leishmaniasis.

The metabolism of S-nitrosothiols in the trypanosomatids: the role of ovothiol A and trypanothione

It has recently been established that nitrosoglutathione is the preferred substrate of the glutathione-dependent formaldehyde dehydrogenase from divergent organisms. Trypanosomatids produce not only glutathione, but also glutathionylspermidine, trypanothione and ovothiol A. The formaldehyde dehydrogenase activity of Crithidia fasciculata was independent of these thiols and extracts possessed very low levels of nitrosothiol reductase activity with glutathione or its spermidine conjugates as the thiol component. Although ovothiol A did not form a stable nitrosothiol, it decomposed the S -nitroso groups of nitrosoglutathione (GSNO) and dinitrotrypanothione [T(SNO)(2)] with second-order rate constants of 19.12 M(-1) x s(-1) and 8.67 M(-1) x s(-1) respectively. The reaction of T(SNO)(2) with ovothiol A, however, accelerated to a rate similar to that seen with GSNO. Ovothiol A can act catalytically to decompose these nitrosothiols, although non-productive mechanisms exist. The catalytic phase of the reaction was dependent on the production of thiyl radicals, since it was abolished in the presence of 5,5-dimethyl-1-pyrroline- N -oxide and the formation of nitric oxide could be detected by means of the conversion of oxyhaemoglobin into methaemoglobin. The rate-limiting step in the catalytic process was the reduction of oxidized ovothiol species and, in this respect, T(SNO)(2) is a more efficient substrate than GSNO. Trypanothione decomposed GSNO with a second-order rate constant of 0.786 M(-1) x s(-1) and the major nitrogenous end product changed from nitrite to ammonia as the ratio of thiol to nitrosothiol increased. The results indicate that ovothiol A acts in synergy with trypanothione in the decomposition of T(SNO)(2).