Thioredoxin and glutathione system of malaria parasite Plasmodium falciparum

Glutathione reductase: A cytoplasmic antioxidant enzyme and a potential target for phenothiazinium dyes in Neospora caninum

Neospora caninum causes heavy losses related to abortions in bovine cattle. This parasite developed a complex defense redox system, composed of enzymes as glutathione reductase (GR). Methylene blue (MB) impairs the activity of recombinant form of Plasmodium GR and inhibits the parasite proliferation in vivo and in vitro. Likewise, MB and its derivatives inhibits Neospora caninum proliferation, however, whether the MB mechanism of action is correlated to GR function remains unclear. Therefore, here, N. caninum GR (NcGR) was characterized and its potential inhibitors were determined. NcGR was found in the tachyzoite cytosol and has a similar structure and sequence compared to its homologs. We verified the in vitro activity of rNcGR (875 nM) following NADPH absorbance at 340 nM (100 mM KH2PO4, pH 7.5, 1 mM EDTA, ionic strength: 600 mM, 25 °C). rNcGR exhibited a Michaelian behavior (Km(GSSG):0.10 ± 0.02 mM; kcat(GSSG):0.076 ± 0.003 s-1; Km(NADPH):0.006 ± 0.001 mM; kcat(NADPH): 0.080 ± 0.003 s-1). The IC50 of MB,1,9-dimethyl methylene blue, new methylene blue, and toluidine blue O on rNcGR activity were 2.1 ± 0.2 μM, 11 ± 2 μM, 0.7 ± 0.1 μM, and 0.9 ± 0.2 μM, respectively. Our results suggest the importance of NcGR in N. caninum biology and antioxidant mechanisms. Moreover, data presented here strongly suggest that NcGR is an important target of phenothiazinium dyes in N. caninum proliferation inhibition.

Identification and functional study of a novel 2-cys peroxiredoxin (BmTPx-1) of Babesia microti

Babesia microti is an emerging human pathogen and the primary causative agent of human babesiosis in many regions of the world. Although the peroxiredoxins (Prxs) or thioredoxin peroxidases (TPx) enzymes of this parasite have been sequenced and annotated, their biological properties remain largely unknown. Prxs are a family of antioxidant enzymes that protect biological molecules against metabolically produced reactive oxygen species (ROS) and reduce hydrogen peroxide (H₂O₂) to water in both eukaryotes and prokaryotes. In this study, TPx-1 cDNA was cloned from B. microti (designated BmTPx-1). Recombinant BmTPx-1 (rBmTPx-1) was expressed in Escherichia coli as a histidine fusion protein and purified using Ni-NTA His bind resin. To test the defense capacity of enzymatic antioxidants against the effect of ROS, a mixed-function oxidation system was utilized with the recombinant BmTPx-1 protein. A decreased ability of rBmTPx-1 to donate electrons to the thioredoxin (Trx)/TrxR reductase system was clarified by reaction with H₂O₂. These results suggest that BmTPx-1 has a great impact on protecting parasites from oxidative stress in the erythrocytic stage.

Functional thioredoxin reductase from pathogenic and free-living Leptospira spp

Low molecular mass thiols and antioxidant enzymes have essential functions to detoxify reactive oxygen and nitrogen species maintaining cellular redox balance. The metabolic pathways for redox homeostasis in pathogenic (Leptospira interrogans) and free-living (Leptospira biflexa) leptospires species were not functionally characterized. We performed biochemical studies on recombinantly produced proteins to in depth analyze kinetic and structural properties of thioredoxin reductase (LinTrxR) and thioredoxin (LinTrx) from L. interrogans, and two TrxRs (LbiTrxR1 and LbiTrxR2) from L. biflexa. All the TrxRs were characterized as homodimeric flavoproteins, with LinTrxR and LbiTrxR1 catalyzing the NADPH dependent reduction of LinTrx and DTNB. The thioredoxin system from L. interrogans was able to use glutathione disulfide, lipoamide disulfide, cystine and bis-γ-glutamyl cysteine and homologous peroxiredoxin as substrates. Classic TrxR activity of LinTrxR2 had not been evidenced in vitro, but recombinant Escherichia coli cells overexpressing LbiTrxR2 showed high tolerance to oxidative stress. The enzymatic systems herein characterized could play a key role for the maintenance of redox homeostasis and the function of defense mechanisms against reactive oxidant species in Leptospira spp. Our results contribute to the general knowledge about redox biochemistry in these bacteria, positioning TrxR as a critical molecular target for the development of new anti-leptospiral drugs.

Glutathione-deficient Plasmodium berghei parasites exhibit growth delay and nuclear DNA damage

Plasmodium parasites are exposed to endogenous and exogenous oxidative stress during their complex life cycle. To minimize oxidative damage, the parasites use glutathione (GSH) and thioredoxin (Trx) as primary antioxidants. We previously showed that disruption of the Plasmodium berghei gamma-glutamylcysteine synthetase (pbggcs-ko) or the glutathione reductase (pbgr-ko) genes resulted in a significant reduction of GSH in intraerythrocytic stages, and a defect in growth in the pbggcs-ko parasites. In this report, time course experiments of parasite intraerythrocytic development and morphological studies showed a growth delay during the ring to schizont progression. Morphological analysis shows a significant reduction in size (diameter) of trophozoites and schizonts with increased number of cytoplasmic vacuoles in the pbggcs-ko parasites in comparison to the wild type (WT). Furthermore, the pbggcs-ko mutants exhibited an impaired response to oxidative stress and increased levels of nuclear DNA (nDNA) damage. Reduced GSH levels did not result in mitochondrial DNA (mtDNA) damage or protein carbonylations in neither pbggcs-ko nor pbgr-ko parasites. In addition, the pbggcs-ko mutant parasites showed an increase in mRNA expression of genes involved in oxidative stress detoxification and DNA synthesis, suggesting a potential compensatory mechanism to allow for parasite proliferation. These results reveal that low GSH levels affect parasite development through the impairment of oxidative stress reduction systems and damage to the nDNA. Our studies provide new insights into the role of the GSH antioxidant system in the intraerythrocytic development of Plasmodium parasites, with potential translation into novel pharmacological interventions.

Preliminary LC-MS Based Screening for Inhibitors of Plasmodium falciparum Thioredoxin Reductase (PfTrxR) among a Set of Antimalarials from the Malaria Box

Plasmodium falciparum thioredoxin reductase (PfTrxR) has been identified as a potential drug target to combat growing antimalarial drug resistance. Medicines for Malaria Venture (MMV) has pre-screened and identified a set of 400 antimalarial compounds called the Malaria Box. From those, we have evaluated their mechanisms of action through inhibition of PfTrxR and found new active chemical scaffolds. Five compounds with significant PfTrxR inhibitory activity, with IC₅₀ values ranging from 0.9–7.5 µM against the target enzyme, were found out of the Malaria Box.

Plasmodium falciparum Thioredoxin Reductase (PfTrxR) and Its Role as a Target for New Antimalarial Discovery

The growing resistance to current antimalarial drugs is a major concern for global public health. The pressing need for new antimalarials has led to an increase in research focused on the Plasmodium parasites that cause human malaria. Thioredoxin reductase (TrxR), an enzyme needed to maintain redox equilibrium in Plasmodium species, is a promising target for new antimalarials. This review paper provides an overview of the structure and function of TrxR, discusses similarities and differences between the thioredoxin reductases (TrxRs) of different Plasmodium species and the human forms of the enzyme, gives an overview of modeling Plasmodium infections in animals, and suggests the role of Trx functions in antimalarial drug resistance. TrxR of Plasmodium falciparum is a central focus of this paper since it is the only Plasmodium TrxR that has been crystallized and P. falciparum is the species that causes most malaria cases. It is anticipated that the information summarized here will give insight and stimulate new directions in which research might be most beneficial.

Effect of thioredoxin peroxidase-1 gene disruption on the liver stages of the rodent malaria parasite Plasmodium berghei

Phenotypic observation of thioredoxin peroxidase-1 (TPx-1) gene-disrupted Plasmodium berghei (TPx-1 KO) in the liver-stage was performed with an in vitro infection system in order to investigate defective liver-stage development in a mouse infection model. Indirect immunofluorescence microscopy assay with anti-circumsporozoite protein antibody revealed that in the liver schizont stage, TPx-1 KO parasite cells were significantly smaller than cells of the wild-type parent strain (WT). Indirect immunofluorescence microscopy assay with anti-merozoite surface protein-1 antibody, which was used to evaluate late schizont-stage development, indicated that TPx-1 KO schizont development was similar to WT strain development towards the merozoite-forming stage (mature schizont). However, fewer merozoites were produced in the mature TPx-1 KO schizont than in the mature WT schizont. Taken together, the results suggest that TPx-1 may be involved in merozoite formation during liver schizont development.

Characterization of PfTrxR inhibitors using antimalarial assays and in silico techniques

Background

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

Results

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

Conclusions

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

Targeting the vitamin biosynthesis pathways for the treatment of malaria

The most severe form of malaria is Malaria tropica, caused by Plasmodium falciparum. There are more than 1 billion people that are exposed to malaria parasites leading to more than 500,000 deaths annually. Vaccines are not available and the increasing drug resistance of the parasite prioritizes the need for novel drug targets and chemotherapeutics, which should be ideally designed to target selectively the parasite. In this sense, parasite-specific pathways, such as the vitamin biosyntheses, represent perfect drug-target characteristics because they are absent in humans. In the past, the vitamin B9 (folate) metabolism has been exploited by antifolates to treat infections caused by malaria parasites. Recently, two further vitamin biosynthesis pathways - for the vitamins B6 (pyridoxine) and B1 (thiamine) - have been identified in Plasmodium and analyzed for their suitability to discover new drugs. In this review, the current status of the druggability of plasmodial vitamin biosynthesis pathways is summarized.

Methylene blue as an antimalarial agent

Methylene blue has intrinsic antimalarial activity and it can act as a chloroquine sensitizer. In addition, methylene blue must be considered for preventing methemoglobinemia, a serious complication of malarial anemia. As an antiparasitic agent, methylene blue is pleiotropic: it interferes with hemoglobin and heme metabolism in digestive organelles, and it is a selective inhibitor of Plasmodium falciparum glutathione reductase. The latter effect results in glutathione depletion which sensitizes the parasite for chloroquine action. At the Centre de Recherche en Santé de Nouna in Burkina Faso, we study the combination of chloroquine with methylene blue (BlueCQ) as a possible medication for malaria in endemic regions. A pilot study with glucose-6-phosphate dehydrogenase-sufficient adult patients has been conducted recently.

Oxidative stress in malaria

Malaria is a significant public health problem in more than 100 countries and causes an estimated 200 million new infections every year. Despite the significant effort to eradicate this dangerous disease, lack of complete knowledge of its physiopathology compromises the success in this enterprise. In this paper we review oxidative stress mechanisms involved in the disease and discuss the potential benefits of antioxidant supplementation as an adjuvant antimalarial strategy.

Dissecting the role of glutathione biosynthesis in Plasmodium falciparum

Glutathione (γ-glutamylcysteinyl-glycine, GSH) has vital functions as thiol redox buffer and cofactor of antioxidant and detoxification enzymes. Plasmodium falciparum possesses a functional GSH biosynthesis pathway and contains mM concentrations of the tripeptide. It was impossible to delete in P. falciparum the genes encoding γ-glutamylcysteine synthetase (γGCS) or glutathione synthetase (GS), the two enzymes synthesizing GSH, although both gene loci were not refractory to recombination. Our data show that the parasites cannot compensate for the loss of GSH biosynthesis via GSH uptake. This suggests an important if not essential function of GSH biosynthesis pathway for the parasites. Treatment with the irreversible inhibitor of γGCS L-buthionine sulfoximine (BSO) reduced intracellular GSH levels in P. falciparum and was lethal for their intra-erythrocytic development, corroborating the suggestion that GSH biosynthesis is important for parasite survival. Episomal expression of γgcs in P. falciparum increased tolerance to BSO attributable to increased levels of γGCS. Concomitantly expression of glutathione reductase was reduced leading to an increased GSH efflux. Together these data indicate that GSH levels are tightly regulated by a functional GSH biosynthesis and the reduction of GSSG.

The influence of oxidative bursts of phagocytes on red blood cell oxidation in anemic cattle infected withTheileria sergenti

The primary clinical symptom of Japanese bovine theileriosis, caused by the intraerythrocytic protozoan Theileria sergenti, is anemia, but the underlying mechanism of this anemia remains unknown. To elucidate the pathogenesis of anemia developing in bovine theileriosis, we investigated the relationship between oxidative bursts of peripheral blood phagocytes (neutrophils and monocytes) and the oxidation of red blood cells (RBC) to the development of anemia in cattle experimentally infected with T. sergenti. The levels of methemoglobin (MetHb) and malondialdehyde (MDA), as a parameter of intracellular and membrane oxidative damage in RBC and of production of hydrogen peroxide (H2O2) in phagocytes, were low before the onset of anemia; these parameters began to increase remarkably with decreasing packed cell volume and increasing parasitemia during the course of the anemia, which returned to initial levels during convalescence from anemia. A positive correlation between H2O2 production of phagocytes and each of the oxidative indices of MetHb and MDA was also noted during the onset of anemia. The levels of antioxidants, namely reduced glutathione and glucose-6-phosphate dehydrogenase, in RBC also decreased during the progression of anemia. These results suggest that oxidative damage of RBC has a close relationship with the onset of anemia in bovine theileriosis, and that oxidative bursts of phagocytes may play a part in the pathogenesis of anemia in infected cattle.

Glutathione reductase-null malaria parasites have normal blood stage growth but arrest during development in the mosquito

Malaria parasites contain a complete glutathione (GSH) redox system, and several enzymes of this system are considered potential targets for antimalarial drugs. Through generation of a γ-glutamylcysteine synthetase (γ-GCS)-null mutant of the rodent parasite Plasmodium berghei, we previously showed that de novo GSH synthesis is not critical for blood stage multiplication but is essential for oocyst development. In this study, phenotype analyses of mutant parasites lacking expression of glutathione reductase (GR) confirmed that GSH metabolism is critical for the mosquito oocyst stage. Similar to what was found for γ-GCS, GR is not essential for blood stage growth. GR-null parasites showed the same sensitivity to methylene blue and eosin B as wild type parasites, demonstrating that these compounds target molecules other than GR in Plasmodium. Attempts to generate parasites lacking both GR and γ-GCS by simultaneous disruption of gr and γ-gcs were unsuccessful. This demonstrates that the maintenance of total GSH levels required for blood stage survival is dependent on either de novo GSH synthesis or glutathione disulfide (GSSG) reduction by Plasmodium GR. Our studies provide new insights into the role of the GSH system in malaria parasites with implications for the development of drugs targeting GSH metabolism.

The glutathione biosynthetic pathway of Plasmodium is essential for mosquito transmission

Infection of red blood cells (RBC) subjects the malaria parasite to oxidative stress. Therefore, efficient antioxidant and redox systems are required to prevent damage by reactive oxygen species. Plasmodium spp. have thioredoxin and glutathione (GSH) systems that are thought to play a major role as antioxidants during blood stage infection. In this report, we analyzed a critical component of the GSH biosynthesis pathway using reverse genetics. Plasmodium berghei parasites lacking expression of gamma-glutamylcysteine synthetase (γ-GCS), the rate limiting enzyme in de novo synthesis of GSH, were generated through targeted gene disruption thus demonstrating, quite unexpectedly, that γ-GCS is not essential for blood stage development. Despite a significant reduction in GSH levels, blood stage forms of pbggcs⁻ parasites showed only a defect in growth as compared to wild type. In contrast, a dramatic effect on development of the parasites in the mosquito was observed. Infection of mosquitoes with pbggcs⁻ parasites resulted in reduced numbers of stunted oocysts that did not produce sporozoites. These results have important implications for the design of drugs aiming at interfering with the GSH redox-system in blood stages and demonstrate that de novo synthesis of GSH is pivotal for development of Plasmodium in the mosquito.

The antioxidant systems of malaria parasites (Plasmodium spp.) are potential targets for the development of antimalarials. The glutathione (GSH) redox system constitutes one of the Plasmodium primary lines of defense against damage caused by reactive oxygen species and other forms of chemical stress. GSH is synthesized de novo by the sequential action of gamma-glutamylcysteine synthase (γ-GCS) and GSH synthase (GS). Biochemical studies have suggested that parasite survival depends on functional de novo GSH synthesis. Using reverse genetics we interrupted the GSH biosynthetic pathway in the rodent malaria Plasmodium berghei by disrupting the pbggcs gene. The mutation caused minor changes in parasite growth in the mammalian host but development in the mosquito was completely arrested at the oocyst stage. These results suggest that the GSH biosynthetic pathway, while essential for mosquito stage development, is not an appropriate target for antimalarials against blood stages of the parasite.

Interactions of methylene blue with human disulfide reductases and their orthologues from Plasmodium falciparum

Methylene blue (MB) has experienced a renaissance mainly as a component of drug combinations against Plasmodium falciparum malaria. Here, we report biochemically relevant pharmacological data on MB such as rate constants for the uncatalyzed reaction of MB at pH 7.4 with cellular reductants like NAD(P)H (k = 4 M(-1) s(-1)), thioredoxins (k = 8.5 to 26 M(-1) s(-1)), dihydrolipoamide (k = 53 M(-1) s(-1)), and slowly reacting glutathione. As the disulfide reductases are prominent targets of MB, optical tests for enzymes reducing MB at the expense of NAD(P)H under aerobic conditions were developed. The product leucomethylene blue (leucoMB) is auto-oxidized back to MB at pH 7 but can be stabilized by enzymes at pH 5.0, which makes this colorless compound an interesting drug candidate. MB was found to be an inhibitor and/or a redox-cycling substrate of mammalian and P. falciparum disulfide reductases, with the kcat values ranging from 0.03 s(-1) to 10 s(-1) at 25 degrees C. Kinetic spectroscopy of mutagenized glutathione reductase indicates that MB reduction is conducted by enzyme-bound reduced flavin rather than by the active-site dithiol Cys58/Cys63. The enzyme-catalyzed reduction of MB and subsequent auto-oxidation of the product leucoMB mean that MB is a redox-cycling agent which produces H2O2 at the expense of O2 and of NAD(P)H in each cycle, turning the antioxidant disulfide reductases into pro-oxidant enzymes. This explains the terms subversive substrate or turncoat inhibitor for MB. The results are discussed in cell-pathological and clinical contexts.

Melanogenesis and associated cytotoxic reactions: applications to insect innate immunity

Insects transmit the causative agents for such debilitating diseases as malaria, lymphatic filariases, sleeping sickness, Chagas' disease, leishmaniasis, river blindness, Dengue, and yellow fever. The persistence of these diseases provides testimony to the genetic capacity of parasites to evolve strategies that ensure their successful development in two genetically diverse host species: insects and mammals. Current efforts to address the problems posed by insect-borne diseases benefit from a growing understanding of insect and mammalian immunity. Of considerable interest are recent genomic investigations that show several similarities in the innate immune effector responses and associated regulatory mechanisms manifested by insects and mammals. One notable exception, however, is the nearly universal presence of a brown-black pigment accompanying cellular innate immunity in insects. This response, which is unique to arthropods and certain other invertebrates, has focused attention on the elements involved in pigment synthesis as causing or contributing to the death of the parasite, and has even prompted speculation that the enzyme cascade mediating melanogenesis constitutes an ill-defined recognition mechanism. Experimental evidence defining the role of melanin and its precursors in insect innate immunity is severely lacking. A great deal of what is known about melanogenesis comes from studies of the process occurring in mammalian systems, where the pigment is synthesized by such diverse cells as those comprising portions of the skin, hair, inner ear, brain, and retinal epithelium. Fortunately, many of the components in the metabolic pathways leading to the formation of melanin have been found to be common to both insects and mammals. This review examines some of the factors that influence enzyme-mediated melanogenic responses, and how these responses likely contribute to blood cell-mediated, target-specific cytotoxicity in immune challenged insects.

Expression of mRNAs and proteins for peroxiredoxins in Plasmodium falciparum erythrocytic stage

mRNA and protein expression profiles for three peroxiredoxins (PfTPx-1, PfTPx-2 and Pf1-Cys-Prx) and a thioredoxin (PfTrx-1) of Plasmodium falciparum during the erythrocytic stage were examined by real-time quantitative reverse transcription-PCR (RT-PCR), Western blotting and confocal laser scanning microscopy. PfTPx-1 was expressed constitutively in the parasite cytoplasm throughout the erythrocytic stage, suggesting a housekeeping role of this enzyme for control of intercellular reactive oxygen species (ROS) in the parasite. Pf1-Cys-Prx showed elevated expression during the trophozoite and early schizont stages in the parasite cytoplasm, and this profile suggested that this peroxiredoxin (Prx) detoxifies metabolism-derived ROS such as those released from heme iron. The other 2-Cys Prx, PfTPx-2, was detected in mitochondria and was expressed in both the trophozoite and schizont stages. Detection of the Prx in mitochondria is consistent with recent reports of the existence of a respiratory chain, which produces ROS, in the mitochondria of P. falciparum. PfTrx-1 showed elevated expression during the trophozoite and schizont stages in the parasite cytoplasm. Finally, expression of these antioxidant protein genes is most likely regulated at the transcriptional level because their mRNA and protein expression profiles overlapped.

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

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

Expression profiles of peroxiredoxin proteins of the rodent malaria parasite Plasmodium yoelii

Patterns of expression of the 2-Cys and 1-Cys peroxiredoxin (Prx) proteins of the rodent malaria parasite Plasmodium yoelii during its life cycle were observed by immunofluorescent antibody staining and confocal laser scanning microscopy. 2-Cys Prx was expressed in the parasite cytoplasm throughout the life cycle, and the thioredoxin (Trx)-peroxidase activity of 2-Cys Prx revealed with the recombinant protein suggested that the Prx is constitutively expressed and, thus, likely plays a housekeeping role in the parasite's intracellular redox control. In contrast, 1-Cys Prx showed stage-specific expression in blood-stage parasites. The limited expression of 1-Cys Prx in the trophozoite cytoplasm suggests that 1-Cys Prx may be involved in haemoglobin metabolism by the parasite, which generates a prooxidative haem iron and increases intracellular oxidative stress. The antioxidant activity of 1-Cys Prx was tested for its ability to protect yeast enolase against inactivation of the mixed-function oxidation system. Differential expression of the two Prx proteins during the erythrocytic and insect stages suggests the importance of these proteins in protecting parasites against oxidative stress, which is generated by the parasite's metabolism and also from the environment.

Disruption of the Plasmodium falciparum 2-Cys peroxiredoxin gene renders parasites hypersensitive to reactive oxygen and nitrogen species

In parasitism, Plasmodium falciparum is exposed to toxic reactive oxygen species and reactive nitrogen species (RNS). Peroxiredoxins (Prx) are ubiquitously distributed antioxidant enzymes. In bacteria and yeast, Prx have also been implicated in detoxifying RNS. Here, we used a gene targeting strategy to investigate the physiological role of 2-Cys Prx of P. falciparum, PfTPx-1, in living parasite cells. The PfTPx-1-null parasite line was more sensitive to paraquat (a superoxide donor) and sodium nitroprusside (a nitric oxide donor), than wildtype. These findings suggest that PfTPx-1 protects the parasite cells from oxidative and nitrosative stresses.

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.