Molecular characterization of the glutathione peroxidase gene of the human malaria parasite Plasmodium falciparum

Thiol-dependent peroxidases care little about homology-based assignments of function

Thiol-dependent peroxidase systems are reviewed with special emphasis on their potential use as drug targets. The basic catalytic mechanism of the two major thiol-peroxidase families, the glutathione peroxidases and the peroxiredoxins, are reasonably well understood. Sequence-based predictions of substrate specificities are still unsatisfactory. GPx-type enzymes are not generally specific for GSH but may specifically react with CXXC motifs as present in thioredoxins or tryparedoxins. Inversely, the peroxiredoxin family that was believed to be specific for CXXC-type proteins, also comprises glutathione peroxidases. Since structure-based predictions of function are also limited by small data bases, the increasing number of sequences emerging from genome projects require enzymatic characterization and genetic proof of relevance before they can be classified as drug targets.

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.

Plasmodium falciparum 2-Cys peroxiredoxin reacts with plasmoredoxin and peroxynitrite

Thioredoxin peroxidase 1 (TPx1) of the malarial parasite Plasmodium falciparum is a 2-Cys peroxiredoxin involved in the detoxification of reactive oxygen species and - as shown here - of reactive nitrogen species. As novel electron acceptor of reduced TPx1, we characterised peroxynitrite; the rate constant for ONOO- reduction by the enzyme (1 x 10(6) M(-1) s(-1) at pH 7.4 and 37 degrees C) was determined by stopped-flow measurements. As reducing substrate of TPx1, we identified - aside from thioredoxin - plasmoredoxin; this 22-kDa protein occurs only in malarial parasites. When studying the potential roles of Cys74 and Cys170 of Tpx1 in catalysis, as well as in oligomerisation behaviour, we found that replacement of Cys74 by Ala influenced neither the dimerisation nor enzymatic activity of TPx1. In the C170A mutant, however, the kcat/Km for reduced Trx as a substrate was shown to be approximately 50-fold lower and, in contrast to the wild-type enzyme, covalently linked dimers were not formed. For the catalytic cycle of TPx1, we conclude that oxidation of the peroxidatic Cys50 by the oxidising substrate is followed by the formation of an intermolecular disulfide bond between Cys50 and Cys170' of the second subunit, which is then attacked by an external electron donor such as thioredoxin or plasmoredoxin.

The thiol-based redox networks of pathogens: unexploited targets in the search for new drugs

Hydroperoxide metabolism in diverse pathogens is reviewed under consideration of involved enzymes as potential drug targets. The common denominator of the peroxidase systems of Trypanosoma, Leishmania, Plasmodium, and Mycobacterium species is the use of NAD(P)H to reduce hydroperoxides including peroxynitrite via a flavin-containing disulfide reductase, a thioredoxin (Trx)-related protein and a peroxidase that operates with thiol catalysis. In Plasmodium falciparum, thioredoxin- and glutathione dependent systems appear to be linked via glutaredoxin and plasmoredoxin to terminal thioredoxin peroxidases belonging to both, the peroxiredoxin (Prx) and glutathione peroxidase (GPx) family. In Mycobacterium tuberculosis, a catalase-type peroxidase is complemented by the typical 2-C-Prx AhpC that, in contrast to most bacteria, is reduced by TrxC, and an atypical 2-C-Prx reduced by TrxB or C. A most complex variation of the scheme is found in trypanosomatids, where the unique redox metabolite trypanothione reduces the thioredoxin-related tryparedoxin, which fuels Prx- and GPx-type peroxidases as well as ribonucleotide reductase. In Trypanosoma brucei and Leishmania donovani the system has been shown to be essential for viability and virulence by inversed genetics. It is concluded that optimum efficacy can be expected from inhibitors of the most upstream components of the redox cascades. For trypanosomatids attractive validated drug targets are trypanothione reductase and trypanothione synthetase; for mycobacteria thioredoxin reductase appears most appealing, while in Plasmodium simultaneous inhibition of both the thioredoxin and the glutathione pathway appears advisable to avoid mutual substitution in co-substrate supply to the peroxidases. Financial and organisational needs to translate the scientific progress into applicable drugs are discussed under consideration of the socio-economic impact of the addressed diseases.

Antioxidant strategies in the epididymis

Spermatozoa are very specialized cells, dedicated to fertilization of the oocyte. The attainment of this biological role is partly due to the fusogenic properties of the sperm plasma membrane, which is particularly rich in polyunsaturated fatty acids (PUFA). This predominance of PUFA renders spermatozoa highly susceptible to lipid peroxidation due to attacks from reactive oxygen species (ROS). These attacks ultimately lead to the impairment of sperm function through oxidative stress. Despite such disruptive effects, it should be also emphasized that these molecules also play an important positive, physiological role in the regulation of sperm physiology through their participation in apoptosis and the signal transduction cascades that control sperm maturation and capacitation. In this article, the different sources of ROS are examined and then the antioxidant strategies that protect these cells during epididymal transit are reviewed. While the major focus is on the involvement of glutathione peroxidase in this process, consideration will also be given to a range of additional antioxidant enzymes (catalase, indolamine dioxygenase and superoxide dismutase) that have evolved to protect spermatozoa during this extremely vulnerable phase in their life history. Besides the classical enzymatic roles of these enzymes in recycling ROS, additional features are discussed in the light of contraceptive development.

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

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

Prooxidant diet provides protection during murine infection with Toxoplasma gondii

Toxoplasmosis, particularly toxoplasmic encephalitis, has emerged as a major cause of morbidity and mortality in patients with acquired immunodeficiency syndrome. Patients infected with human immunodeficiency virus typically experience chronic oxidative stress, and concurrent infection with the intracellular parasite Toxoplasma gondii would be expected to further exacerbate this condition. The present study was conducted to determine whether vitamin E and selenium supplementation might be beneficial in a murine model of toxoplasmosis. To investigate the effect of these antioxidants on the severity of parasitic infection. Swiss Webster (SW) or C57Bl/6J mice infected with oocysts of the ME49 strain of T. gondii were maintained on diets containing no vitamin E or selenium, no vitamin E and 8 ppm selenium, 400 IU/kg vitamin E plus 8 ppm selenium, or vitamin E and selenium at the levels present in standard rodent chow (16 IU/kg and 0.2 ppm, respectively). The results of the study showed that increased dietary supplementation with vitamin E and selenium resulted in trends toward increased tissue cyst number, tissue pathology, and weight loss during infection. In contrast, both resistant SW and susceptible C57Bl/6J mice fed a deficient diet (complete absence of vitamin E and selenium) showed the lowest mean numbers of tissue cysts and very little evidence of tissue pathology during chronic infection.

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

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

A putative glutathione peroxidase of Drosophila encodes a thioredoxin peroxidase that provides resistance against oxidative stress but fails to complement a lack of catalase activity

Cellular defense systems against reactive oxygen species (ROS) include thioredoxin reductase (TrxR) and glutathione reductase (GR). They generate sulfhydryl-reducing systems which are coupled to antioxidant enzymes, the thioredoxin and glutathione peroxidases (TPx and GPx). The fruit fly Drosophila lacks a functional GR, suggesting that the thioredoxin system is the major source for recycling glutathione. Whole genome in silico analysis identified two non-selenium containing putative GPx genes. We examined the biochemical characteristics of one of these gene products and found that it lacks GPx activity and functions as a TPx. Transgene-dependent overexpression of the newly identified Glutathione peroxidase homolog with thioredoxin peroxidase activity (Gtpx-1) gene increases resistance to experimentally induced oxidative stress, but does not compensate for the loss of catalase, an enzyme which, like GTPx-1, functions to eliminate hydrogen peroxide. The results suggest that GTPx-1 is part of the Drosophila Trx antioxidant defense system but acts in a genetically distinct pathway or in a different cellular compartment than catalase.

Illumination of the malaria parasite Plasmodium falciparum alters intracellular pH. Implications for live cell imaging

Live cell fluorescence microscopy has been widely used to study physiological processes in the human malarial parasite Plasmodium falciparum, including pH homeostasis, Ca(2+) signaling and protein targeting. However, the reproducibility of the data is often poor. Controversial statements exist regarding cytosolic and vacuolar baseline pH, as well as regarding the subcellular localization of some of the fluorochromes used. When trying to reproduce published baseline values, we observed an unexpected light sensitivity of P. falciparum, which manifests itself in the form of a strong cytoplasmic acidification. Even short exposure times with moderate to low light intensities caused the parasite cytosol to acidify. We show that this effect arises from the selective disruption of the parasite's acidic food vacuole, brought about by lipid peroxidation initiated by light-induced generation of hydroxyl radicals. Our data suggest that heme serves as a photosensitizer in this process. Our findings have major implications for the use of live cell microscopy in P. falciparum and add a cautionary note to previous studies where live cell fluorometry has been used to determine physiological parameters in P. falciparum.

Homology modeling and docking mechanism of the mercaptosuccinate and methotrexate to P. falciparum 1-Cys peroxiredoxin: a preliminary molecular study

A three-dimensional (3-D) model of 1-Cys peroxiredoxin from P. falciparum (Pf-Prx) has been constructed by homology modeling. The model building was based on a structural alignment with the human 1-Cys peroxiredoxin ray structure. First, mercaptosuccinate was docked by Molecular and Quantum Mechanics at the active site in both isozymes, evidencing the role of different residues in the ligand-protein interaction. Stable conformation of the inhibitor in the active site was obtained from the conformational analysis by molecular dynamics. Next, The complex was reoptimized by semiempirical molecular orbital AM1 method. Conformational and frontier orbitals analyses of the ligand-protein complex were carried out in an attempt to obtain structural insight into the inhibition mechanism. Finally, the docking study of the methotrexate (MTX), an anticancer drug also used as an antimalarial inhibitor, into the modes binding site was performed. From the resulting stable complex structure, it was found that the glutamate ring of MTX fits the active site with high complementarity. The glutamate ring formed two hydrogen bonds to the imidazol group of His41 and the amino groups of Arg129. The side-chain of glutamate was in close proximity to the sulfur atom of the catalytic residue, Cys47. This binding mode suggests a possible inhibition mechanism, whereby the cysteine residue is covered with the glutamate ring of the MTX inhibitor, forming an enzyme-ligand adduct. In addition, the higher interaction energies and the molecular orbitals localization between the Pf-Prx active site and the inhibitors alluded to the probable binding sites of the ligand nucleophilic ring.

A selenoprotein in the plant kingdom. Mass spectrometry confirms that an opal codon (UGA) encodes selenocysteine in Chlamydomonas reinhardtii gluththione peroxidase

Selenoproteins that contain the rare amino acid selenocysteine in their primary structure have been identified in diverse organisms such as viruses, bacteria, archea, and mammals, but so far not in yeast or plants. Among the most thoroughly investigated families of selenoenzymes are the animal glutathione peroxidases (GPXs). In the last few years, genes encoding GPX-like homologues from Chlamydomonas and higher plants have been isolated, but, unlike the animal ones, all of them have cysteine (rather than selenocysteine) residues in their catalytic site. In all organisms investigated that contain selenoproteins, selenocysteine is encoded by a UGA opal codon, which is usually a stop codon. We report here that, in Chlamydomonas reinhardtii, the cDNA-cloned sequence of a GPX homologue contains an internal TGA codon in frame to the ATG. Specific mRNA expression, protein production, and enzyme activity are selenium-dependent. Sequence analysis of the peptides produced by proteolytic digestion, performed by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS), confirmed the presence of a selenocysteine residue at the predicted site and suggest its location in the mitochondria. Thus, our data present the first direct proof that a UGA opal codon is decoded in the plant kingdom to incorporate selenocysteine.

Peroxiredoxins

Present knowledge on peroxiredoxins is reviewed with special emphasis on catalytic principles, specificities and biological function. Peroxiredoxins are low efficiency peroxidases using thiols as reductants. They appear to be fairly promiscuous with respect to the hydroperoxide substrate; the specificities for the donor substrate vary considerably between the subfamilies, comprising GSH, thioredoxin, tryparedoxin and the analogous CXXC motifs in bacterial AhpF proteins. Peroxiredoxins are definitely responsible for antioxidant defense in bacteria (AhpC), yeast (thioredoxin peroxidase) and trypanosomatids (tryparedoxin peroxidase). They are considered to determine virulence of mycobacteria and trypanosomatids. In higher plants they are involved in balancing hydroperoxide production during photosynthesis. In higher animals peroxiredoxins appear to be involved in the redox-regulation of cellular signaling and differentiation, displaying in part opposite effects.

Plasmodium falciparum possesses a classical glutaredoxin and a second, glutaredoxin-like protein with a PICOT homology domain

The genes coding for two different proteins with homologies to glutaredoxins have been identified in the genome of the malarial parasite Plasmodium falciparum. Both genes were amplified from a gametocytic cDNA and overexpressed in Escherichia coli. The smaller protein (named PfGrx-1) with 12.4 kDa in size exhibits the typical glutaredoxin active site motif "CPYC," shows glutathione-dependent glutaredoxin activity in the beta-hydroxyethyl disulfide (HEDS) assay, and reduces Trypanosoma brucei ribonucleotide reductase. Glutathione:HEDS transhydrogenase activity (approximately 60 milliunits/mg of protein) was clearly detectable in trophozoite extracts from eight different P. falciparum strains and did not differ between chloroquine-resistant and -sensitive parasites. Five different antimalarial drugs at 100 microm did not significantly influence isolated PfGrx-1 activity. In contrast, the second protein (deduced mass 19.9 kDa) with homology to glutaredoxins (31% identity to Schizosaccharomyces pombe in a 140-amino acid overlap) was not active in the HEDS assay; however, its general dithiol reducing activity was demonstrated in the insulin assay in the presence of dithiothreitol. Interestingly, the sequence contains a PICOT (for protein kinase C-interacting cousin of thioredoxin) homology domain, which might suggest regulatory functions of the protein. We named this protein PfGLP-1, for P. falciparum 1-Cys-glutaredoxin-like protein-1. In contrast to glutaredoxins, PfGLP-1 could not be reduced by glutathione. This is the first report on glutaredoxin-like proteins in the family of Plasmodia.

The putative glutathione peroxidase gene of Plasmodium falciparum codes for a thioredoxin peroxidase

A putative glutathione peroxidase gene (Swiss-Prot accession number Z 68200) of Plasmodium falciparum, the causative agent of tropical malaria, was expressed in Escherichia coli and purified to electrophoretic homogeneity. Like phospholipid hydroperoxide glutathione peroxidase of mammals, it proved to be monomeric. It was active with H(2)O(2) and organic hydroperoxides but, unlike phospholipid hydroperoxide glutathione peroxidase, not with phosphatidylcholine hydroperoxide. With glutathione peroxidases it shares the ping-pong mechanism with infinite V(max) and K(m) when analyzed with GSH as substrate. As a homologue with selenocysteine replaced by cysteine, its reactions with hydroperoxides and GSH are 3 orders of magnitude slower than those of the selenoperoxidases. Unexpectedly, the plasmodial enzyme proved to react faster with thioredoxins than with GSH and most efficiently with thioredoxin of P. falciparum (Swiss-Prot accession number 202664). It is therefore reclassified as thioredoxin peroxidase. With plasmodial thioredoxin, the enzyme also displays ping-pong kinetics, yet with a limiting K(m) of 10 microm and a k(1)' of 0.55 s(-)1. The apparent k(1)' for oxidation with cumene, t-butyl, and hydrogen peroxides are 2.0 x 10(4) m(-1) s(-1), 3.3 x 10(3) m(-1) s(-1), and 2.5 x 10(3) m (-1) s(-1), respectively. k(2)' for reduction by autologous thioredoxin is 5.4 x 10(4) m(-1) s(-1) (21.2 m(-1) s(-1) for GSH). The newly discovered enzymatic function of the plasmodial gene product suggests a reconsideration of its presumed role in parasitic antioxidant defense.

Thioredoxin peroxidases of the malarial parasite Plasmodium falciparum

The open reading frames of two different proteins with homologies to 2-Cys peroxiredoxins have been identified in the P. falciparum genome. Both genes, with a length of 585 and 648 bp, respectively, were amplified from a gametocyte cDNA and overexpressed in Escherichia coli. The gene products (deduced m 21.8 and 24.6 kDa) with an overall identity of 51.8% were found to be active in the glutamine synthetase protector assay. The smaller protein (named Pf-thioredoxin peroxidase 1; PfTPx1) is reduced by P. falciparum thioredoxin (PfTrx) and accepts H(2)O(2), t-butylhydroperoxide, and cumene hydroperoxide as substrates, the respective k(cat) values for the N-terminally His-tagged protein in the presence of 10 microM PfTrx and 200 microM substrate being 67, 56, and 41 min(-1) at 25 degrees C. As described for many peroxiredoxins, PfTPx1 does not follow saturation kinetics. Furthermore, in oxidizing milieu both proteins are converted to another protein species migrating faster in SDS gel electrophoresis. For PfTPx1 also this second species was found to be active, however, with different kinetic properties which might indicate a mechanism of enzyme regulation in vivo.

The malaria parasite Plasmodium falciparum possesses a functional thioredoxin system

The thioredoxin system consists of the NADPH dependent disulphide oxidoreductase thioredoxin reductase (TrxR) which catalyses the reduction of the small protein thioredoxin. This system is involved in a variety of biological reactions including the reduction of deoxyribonucleotides, transcription factors and hydrogen peroxide. In recent years the TrxR of the malaria parasite Plasmodium falciparum was isolated and characterised using model substrates like 5,5'-dithiobis (2-nitrobenzoic acid) (DTNB) and Escherichia coli thioredoxin. Here we report on the isolation of a cDNA encoding for P. falciparum thioredoxin (PfTrx) and the expression and characterisation of the recombinant protein, the natural substrate of PfTrxR. The deduced amino acid sequence of PfTrx encodes for a polypeptide of 11715 Da and possesses the typical thioredoxin active site motif CysGlyProCys. Both cysteine residues are essential for catalytic activity of the protein, as shown by mutational analyses. Steady state kinetic analyses with PfTrxR and PfTrx in several coupled assay systems resulted in K(m)-values for PfTrx in the range of 0.8--2.1 microM which is about 250-fold lower than for the model substrate E. coli thioredoxin. Since the turnover of both substrates is similar, the catalytic efficiency of PfTrxR to reduce the isolated PfTrx is at least 250-fold higher than to reduce E. coli thioredoxin. PfTrx contains a cysteine residue in position 43 in addition to the active-site cysteine residues, which is partially responsible for dimer formation of the protein as demonstrated by changing this amino acid into an alanine residue. Using DTNB we showed that all three cysteine residues present in PfTrx are accessible to modification by this compound. Surprisingly the first cysteine residue of the active site motif (Cys30) is less accessible than the second cysteine (Cys33), which is highly prone to the modification. These results suggest a difference in the structure and reaction mechanism of PfTrx compared to other known thioredoxins.

Thioredoxin and glutathione system of malaria parasite Plasmodium falciparum

Plasmodium falciparum is the causative agent of malaria tropica. Due to the increasing resistance towards the commonly used plasmodicidal drugs there is an urgent need to identify and assess new targets for the chemotherapeutic intervention of parasite development in the human host. It is established that P. falciparum-infected erythrocytes are vulnerable to oxidative stress, and therefore efficient antioxidative systems are required to ensure parasite development within the host cell. The thioredoxin and glutathione redox systems represent two powerful means to detoxify reactive oxygen species and this article summarizes some of the recent work which has led to a better understanding of these systems in the parasite and will help to assess them as potential targets for the development of new chemotherapeutics of malaria.

Cloning of Plasmodium falciparum protein disulfide isomerase homologue by affinity purification using the antiplasmodial inhibitor 1,4-bis[3-[N-(cyclohexyl methyl)amino]propyl]piperazine

A series of 10 1,4-bis(3-aminopropyl)piperazine compounds was found to display antiplasmodial activity with 50% growth inhibition between 30 and 250 nM, on three Plasmodium falciparum strains differently sensitive to chloroquine. By affinity chromatography using one of these compounds, a 52-kDa protein was isolated from P. falciparum, microsequenced and cloned. It corresponded to a single copy gene encoding a 453 amino acid protein displaying the typical features of protein disulfide isomerases, a thiol metabolizing enzyme belonging to the thiol: disulfide oxidoreductase superfamily, which was not previously described in malarial species.

Glutathione and trypanothione in parasitic hydroperoxide metabolism

Thiol-dependent hydroperoxide metabolism in parasites is reviewed in respect to potential therapeutic strategies. The hydroperoxide metabolism of Crithidia fasciculata has been characterized to comprise a cascade of three enzymes, trypanothione reductase, tryparedoxin, and tryparedoxin peroxidase, plus two supportive enzymes to synthesize the redox mediator trypanothione from glutathione and spermidine. The essentiality of the system in respect to parasite vitality and virulence has been verified by genetic approaches. The system appears to be common to all genera of the Kinetoplastida. The terminal peroxidase of the system belongs to the protein family of peroxiredoxins which is also represented in Entamoeba and a variety of metazoan parasites. Plasmodial hydroperoxide metabolism displays similarities to the mammalian system in comprising glutathione biosynthesis, glutathione reductase, and at least one glutathione peroxidase homolog having the active site selenocysteine replaced by cysteine. Nothing precise is known about the antioxidant defence systems of Giardia, Toxoplasma, and Trichomonas species. Also, the role of ovothiols and mycothiols reportedly present in several parasites remains to be established. Scrutinizing known enzymes of parasitic antioxidant defence for suitability as drug targets leaves only those of the trypanosomatid system as directly or indirectly validated. By generally accepted criteria of target selection and feasibility considerations tryparedoxin and tryparedoxin peroxidase can at present be rated as the most appealing target structures for the development of antiparasitic drugs.

Cytoplasmic localization of the trypanothione peroxidase system in Crithidia fasciculata

Tryparedoxin I (TXNI) and tryparedoxin peroxidase (TXNPx), novel proteins isolated from Crithidia fasciculata, have been reported to reconstitute a trypanothione peroxidase activity in vitro (Nogoceke, E.; Gommel, D. U.; Kiess, M.; Kalisz, H. M.; Flohé, L. Biol. Chem. 378:827-836; 1997). Combined with trypanothione reductase, they may form an NADPH-fueled trypanothione-mediated defense system against hydroperoxides in the trypanosomatids. In situ confocal microscopy of antibody-stained TXNI and TXNPx and electron microscopy of the immunogold labeled proteins revealed their colocalization in the cytosol. Insignificant amounts of the enzymes were detected in the nucleus and vesicular structures, whereas the kinetoplast and the mitochondrion are virtually free of any label. Comparison of the PCR product sequences obtained with genomic and cDNA templates rules out any editing typical of kinetoplast mRNA. Sequence similarities with any of the established maxicircle genes of trypanosomatids were not detectable. It is concluded that both, TXNI as well as TXNPx are encoded by nuclear DNA and predominantly, if not exclusively localized in the cytosol. Working in concert with trypanothione reductase, they can function as an enzymatic system that reduces hydroperoxides at the expense of NADPH without any impairment of the flux of reduction equivalents by cellular compartmentation.

An overview of chemotherapeutic targets for antimalarial drug discovery

The need for new antimalarials comes from the widespread resistance to those in current use. New antimalarial targets are required to allow the discovery of chemically diverse, effective drugs. The search for such new targets and new drug chemotypes will likely be helped by the advent of functional genomics and structure-based drug design. After validation of the putative targets as those capable of providing effective and safe drugs, targets can be used as the basis for screening compounds in order to identify new leads, which, in turn, will qualify for lead optimization work. The combined use of combinatorial chemistry--to generate large numbers of structurally diverse compounds--and of high throughput screening systems--to speed up the testing of compounds--hopefully will help to optimize the process. Potential chemotherapeutic targets in the malaria parasite can be broadly classified into three categories: those involved in processes occurring in the digestive vacuole, enzymes involved in macromolecular and metabolite synthesis, and those responsible for membrane processes and signalling. The processes occurring in the digestive vacuole include haemoglobin digestion, redox processes and free radical formation, and reactions accompanying haem release followed by its polymerization into haemozoin. Many enzymes in macromolecular and metabolite synthesis are promising potential targets, some of which have been established in other microorganisms, although not yet validated for Plasmodium, with very few exceptions (such as dihydrofolate reductase). Proteins responsible for membrane processes, including trafficking and drug transport and signalling, are potentially important also to identify compounds to be used in combination with antimalarial drugs to combat resistance.

The putative gamma-glutamylcysteine synthetase from Plasmodium falciparum contains large insertions and a variable tandem repeat

The tripeptide glutathione plays a pivotal role in the maintenance of the thiol redox state of the cell and for the detoxification of reactive oxygen species. Glutathione is synthesized in two consecutive reactions by y-glutamylcysteine synthetase (gamma-GCS) and glutathione synthetase, respectively. The former enzyme represents the rate limiting step of the synthetic pathway. We have cloned the cDNA and gene of a putative gamma-GCS from Plasmodium falciparum. The contiguous cDNA sequences obtained from various cDNA libraries of P. falciparum K1 and 3D7 encompass 4206 bp or 4038 bp and encode polypeptides of 1119 and 1063 amino acids, respectively. The deduced amino acid sequences show four regions of homology (identity: 31.3-43.9%) to human and Trypanosoma brucei gamma-GCS. These regions are interrupted by three large insertions between 94 and 239 amino acids. Within the first insert a variable repetitive motif was identified, which is responsible for the differing sizes of the sequences. We have analysed this phenomenon in five additional P. falciparum strains and found a high degree of variability in the number of the repeated octamer (Y/C)S(N/D)LQQ(Q/R). Therefore the predicted molecular mass of the proteins from different P. falciparum strains ranges from 124.4 to 133.2 kDa, which is almost twice that of the catalytic subunit of the human host enzyme. Isolation of three genomic clones revealed that the gene does not contain introns. P. falciparum gamma-GCS transcription peaks in trophozoites (24-30 h) suggesting that the antioxidant glutathione is predominantly produced at a time where hemoglobin degradation and the simultaneous formation of reactive oxygen species is maximal.

Inhibition of glutathione-dependent degradation of heme by chloroquine and amodiaquine as a possible basis for their antimalarial mode of action

We propose here a new and detailed model for the antimalarial action of chloroquine (CQ), based on the its ability to inhibit degradation of heme by glutathione. Heme, which is toxic to the malaria parasite, is formed when the intraerythrocytic malaria parasite ingests and digests inside its food vacuole its host cell cytosol, which consists mainly of hemoglobin. The parasite protects itself against the toxicity of heme by polymerizing some of it to insoluble hemozoin (HZ). We show here that in Plasmodium falciparum at the trophozoite stage only ca. 30% of the heme is converted into hemozoin. We suggest that nonpolymerized heme exits the food vacuole and is subsequently degraded by glutathione, as has been shown before for uninfected erythrocytes. Marginal amounts of free heme could be detected in the membrane fraction of infected cells but nowhere else. It is well established that CQ and amodiaquine (AQ) accumulate in the parasite's food vacuole and inhibit heme polymerization, thereby increasing its efflux out of the food vacuole. We found that these drugs competitively inhibit the degradation of heme by glutathione, thus allowing heme to accumulate in membranes. Incubation of intact infected cells with CQ and AQ results in a marked increase in membrane-associated heme in a dose- and time-dependent manner, and a relationship exists between membrane heme levels and the extent of parasite killing. Heme has been shown to disrupt the barrier properties of membranes and to upset ion homeostasis in CQ-treated malaria-infected cells. In agreement with the predictions of our model, increasing the cellular levels of glutathione leads to increased resistance to CQ, whereas decreasing them results in enhanced sensitivity to the drug. These results insinuate a novel mechanism of drug resistance.

Status of Plasmodium falciparum towards catalase

The role of endogenous and internalized catalase in the protection of Plasmodium against oxidant stress was studied. Catalase activities were measured in isolated Plasmodium falciparum at different stages of intererythrocytic development. Activities measured at late schizont stages were compared to parasite markers (glutamate dehydrogenase, SOD) and to red blood cell markers (haemoglobin, Cu/Zn-SOD). The fate of the host cell catalase in the parasite digestive system was studied by immunoelectron microscopy using monoclonal antibodies. The internalized catalase appeared to be dissociated in the digestive system of the parasite and inactivated. To examine the protective role of the endogenous and internalized catalase in the parasite protection against oxidant stress, parasites were cultivated at two oxygen concentrations (5% and 20%) in inhibited catalase red blood cells. These experiments suggested that the catalases present both in red blood cell and parasite are not essential when parasites are cultivated under 5% oxygen, but are necessary to protect the parasite under 20% oxygen. Catalase may not be the main protective enzyme involved in the protection of P. falciparum in standard in vitro culture conditions, but may become critical under the higher oxygen tensions conditions encountered in vivo.

Transgenic mice with elevated level of CuZnSOD are highly susceptible to malaria infection

Copper/zinc superoxide dismutase (CuZnSOD) catalyses the conversion of O2.- into H2O2. Constitutive overexpression of CuZnSOD in cells and animals creates an indigenous oxidative stress that predisposes them to added insults. In this study, we used transgenic CuZnSOD (Tg-CuZnSOD) mice with elevated levels of CuZnSOD to determine whether overexpression of CuZnSOD affected the susceptibility of these mice to plasmodium infection. Acute malaria is associated with oxidative stress, mediated by redox-active iron released from the infected RBC. Two independently derived Tg-CuZnSOD lines showed higher sensitivity than control mice to infection by Plasmodium berghei (P. berghei), reflected by an earlier onset and increased rate of mortality. Nevertheless, while Tg-CuZnSOD mice were more vulnerable than control mice, the levels of parasitemia were comparable in both strains. Moreover, treatment of infected red blood cells (RBC) with oxidative stress inducers, such as ascorbate or paraquat, reduced the viability of parasites equally in both transgenic and control RBC. This further confirms that increased CuZnSOD does not support plasmodia development. The data are consistent with the possibility that the combination of increased redox-active iron and elevated H2O2 in the plasmodium-infected Tg-CuZnSOD mice, led to an enhanced Fenton's reaction-mediated HO. production, and the resulting oxidative injury renders the transgenic mice more vulnerable to parasite infection.