Characterization of iron-dependent endogenous superoxide dismutase of Plasmodium falciparum

Identification and Molecular Characterization of Superoxide Dismutases Isolated From A Scuticociliate Parasite: Physiological Role in Oxidative Stress

Philasterides dicentrarchi is a free-living microaerophilic scuticociliate that can become a facultative parasite and cause a serious parasitic disease in farmed fish. Both the free-living and parasitic forms of this scuticociliate are exposed to oxidative stress associated with environmental factors and the host immune system. The reactive oxygen species (ROS) generated by the host are neutralized by the ciliate by means of antioxidant defences. In this study we aimed to identify metalloenzymes with superoxide dismutase (SOD) activity capable of inactivating the superoxide anion (•O2-) generated during induction of oxidative stress. P. dicentrarchi possesses the three characteristic types of SOD isoenzymes in eukaryotes: copper/zinc-SOD, manganese-SOD and iron-SOD. The Cu/Zn-SOD isoenzymes comprise three types of homodimeric proteins (CSD1-3) of molecular weight (MW) 34-44 kDa and with very different AA sequences. All Cu/Zn-SODs are sensitive to NaCN, located in the cytosol and in the alveolar sacs, and one of them (CSD2) is extracellular. Mn- and Fe-SOD transcripts encode homodimeric proteins (MSD and FSD, respectively) in their native state: a) MSD (MW 50 kDa) is insensitive to H2O2 and NaN3 and is located in the mitochondria; and b) FSD (MW 60 kDa) is sensitive to H2O2, NaN3 and the polyphenol trans-resveratrol and is located extracellularly. Expression of SOD isoenzymes increases when •O2- is induced by ultraviolet (UV) irradiation, and the increase is proportional to the dose of energy applied, indicating that these enzymes are actively involved in cellular protection against oxidative stress.

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.

Iron superoxide dismutases in eukaryotic pathogens: new insights from Apicomplexa and Trypanosoma structures

Prior studies have highlighted the potential of superoxide dismutases as drug targets in eukaryotic pathogens. This report presents the structures of three iron-dependent superoxide dismutases (FeSODs) from Trypanosoma cruzi, Leishmania major and Babesia bovis. Comparison with existing structures from Plasmodium and other trypanosome isoforms shows a very conserved overall fold with subtle differences. In particular, structural data suggest that B. bovis FeSOD may display similar resistance to peroxynitrite-mediated inactivation via an intramolecular electron-transfer pathway as previously described in T. cruzi FeSOD isoform B, thus providing valuable information for structure-based drug design. Furthermore, lysine-acetylation results in T. cruzi indicate that acetylation occurs at a position close to that responsible for the regulation of acetylation-mediated activity in the human enzyme.

Excreted Leishmania peruviana and Leishmania amazonensis iron-superoxide dismutase purification: specific antibody detection in Colombian patients with cutaneous leishmaniasis

Leishmania sp. survival in the vertebrate host depends on the host macrophage immune response as well as on the parasite's defense against free radicals. Iron-superoxide dismutase (Fe-SOD) is a key antioxidant enzyme that contributes to radical superoxide dismutation, preventing the disease from surging and propagating itself. Leishmania sp. has various Fe-SOD isoforms, one of which (Fe-SODe) is excreted into the medium and, being highly immunogenic, can be considered a very good molecular marker. In this work, we purified the Fe-SOD enzymes excreted by L. peruviana and L. amazonensis and studied them as antigens in serodiagnosis. We used ELISA and Western blot techniques to test 51 human cutaneous leishmaniasis sera from Colombia. All 51 patients presented with dermal injuries caused by unknown Leishmania species. The results observed with the purified proteins were compared with those obtained when total soluble lysate and unpurified Fe-SODe were used as the antigen fraction. Thus, we conclude that the purified enzymes are more sensitive and specific than their unpurified counterparts and that there is no cross-reactivity between them.

A secretory multifunctional serine protease, DegP of Plasmodium falciparum, plays an important role in thermo-oxidative stress, parasite growth and development

Plasmodium falciparum heat shock proteins and proteases are known for their indispensable roles in parasite virulence and survival in the host cell. They neutralize various host-derived stress responses that are deleterious for parasite growth and invasion. We report identification and functional characterization of the first DegP from an apicomplexan (P. falciparum). To determine the molecular identity and functions of the parasite-encoded DegP, we complemented the Escherichia coli degP null mutant with a putative PfdegP gene, and the results showed that PfDegP complements the growth defect of the temperature sensitive DegP-deficient mutant and imparts resistance to non-permissive temperatures and oxidative stress. Molecular interaction studies showed that PfDegP exists as a complex with parasite-encoded heat shock protein 70, iron superoxide dismutase and enolase. DegP expression is significantly induced in parasite culture upon heat shock/oxidative stress. Our data suggest that the PfDegP protein may play a role in the growth and development of P. falciparum through its ability to confer protection against thermal/oxidative stress. Antibody against DegP showed anti-plasmodial activity against blood-stage parasites in vitro, suggesting that PfDegP and its associated complex may be a potential focus for new anti-malarial therapies.

STRUCTURED DIGITAL ABSTRACT

●PfDegP physically interacts with PfHsp70 and PfEno by anti-bait co-immunoprecipitation (View interaction) ●PfDegP physically interacts with PfEno, PfSod, PfOat, PfHsp70, PfLDH and PfGpi by anti-bait co-immunoprecipitation (View interaction) ●PfHsp-70 and PfDegP co-localize by fluorescence microscopy (View interaction) ●PfDegP physically interacts with PfOat, PfHsp70, PfEno, PfSod, PfGpi and PfLDH by surface plasmon resonance (View interaction) ●PfEno and PfDegP co-localize by fluorescence microscopy (View interaction) ●PfDegP and PfHsp70 co-localize by co-sedimentation through density gradient (View interaction).

Functional expression and characterization of an iron-containing superoxide dismutase of Acanthamoeba castellanii

Acanthamoeba spp. are free-living amoebae, but opportunistic infections of some strains of the organisms cause severe diseases such as acanthamoebic keratitis, pneumonitis, and granulomatous amoebic encephalitis in human. In this study, we identified a gene encoding iron superoxide dismutase of Acanthamoeba castellanii (AcFe-SOD) and characterized biochemical and functional properties of the recombinant enzyme. Multiple sequence alignment of the deduced amino acid sequence of AcFe-SOD with those of previously reported iron-containing SODs (Fe-SODs) from other protozoan parasites showed that AcFe-SOD shared common metal-binding residues and motifs that are conserved in Fe-SODs. The genomic length of the AcFe-SOD gene was 926 bp consisting of five exons interrupted by four introns. The recombinant AcFe-SOD showed similar biochemical characteristics with its native enzyme and shared typical biochemical properties with other characterized Fe-SODs, including molecular structure, broad pH optimum, and sensitivity to hydrogen peroxide. Immunolocalization analysis revealed that the enzyme localized in the cytosol of the trophozoites. Activity and expression level of the enzyme were significantly increased under oxidative stressed conditions. These results collectively suggest that AcFe-SOD may play essential roles in the survival of the parasite not only by protecting itself from endogenous oxidative stress but also by detoxifying oxidative killing of the parasite by host immune effector cells.

Superoxide dismutases: ancient enzymes and new insights

Superoxide dismutases (SODs) catalyze the de toxification of superoxide. SODs therefore acquired great importance as O(2) became prevalent following the evolution of oxygenic photosynthesis. Thus the three forms of SOD provide intriguing insights into the evolution of the organisms and organelles that carry them today. Although ancient organisms employed Fe-dependent SODs, oxidation of the environment made Fe less bio-available, and more dangerous. Indeed, modern lineages make greater use of homologous Mn-dependent SODs. Our studies on the Fe-substituted MnSOD of Escherichia coli, as well as redox tuning in the FeSOD of E. coli shed light on how evolution accommodated differences between Fe and Mn that would affect SOD performance, in SOD proteins whose activity is specific to one or other metal ion.

Compartmentation of redox metabolism in malaria parasites

Malaria, caused by the apicomplexan parasite Plasmodium, still represents a major threat to human health and welfare and leads to about one million human deaths annually. Plasmodium is a rapidly multiplying unicellular organism undergoing a complex developmental cycle in man and mosquito – a life style that requires rapid adaptation to various environments. In order to deal with high fluxes of reactive oxygen species and maintain redox regulatory processes and pathogenicity, Plasmodium depends upon an adequate redox balance. By systematically studying the subcellular localization of the major antioxidant and redox regulatory proteins, we obtained the first complete map of redox compartmentation in Plasmodium falciparum. We demonstrate the targeting of two plasmodial peroxiredoxins and a putative glyoxalase system to the apicoplast, a non-photosynthetic plastid. We furthermore obtained a complete picture of the compartmentation of thioredoxin- and glutaredoxin-like proteins. Notably, for the two major antioxidant redox-enzymes – glutathione reductase and thioredoxin reductase – Plasmodium makes use of alternative-translation-initiation (ATI) to achieve differential targeting. Dual localization of proteins effected by ATI is likely to occur also in other Apicomplexa and might open new avenues for therapeutic intervention.

The unicellular parasite Plasmodium falciparum is the causative agent of tropical malaria, which represents a global health burden. In order to survive in its human host and the malaria vector Anopheles, malaria parasites depend on adequate antioxidant defense and efficient redox regulation. Furthermore, as shown by glucose-6 phosphate dehydrogenase deficiency, a genetic variation protecting from malaria, redox equilibrium plays a vital role in parasite pathogenicity. Using a green fluorescent protein reporter gene, we systematically studied the subcellular compartmentation of redox networks in Plasmodium falciparum. Based on our results and data from literature, we provide the first thorough map of redox compartmentation. Most interestingly, for the two major antioxidant redox-enzymes – glutathione reductase (GR) and thioredoxin reductase (TrxR) – Plasmodium falciparum makes use of alternative translation initiation to translate protein isoforms with differing subcellular localization from one gene. Dual localization of proteins due to alternative translation initiation might occur frequently in Apicomplexa and identification of further genes that have evolved alternative translation initiation is likely to offer new therapeutic strategies against this devastating disease.

Identification and characterization of a mitochondrial iron-superoxide dismutase of Cryptosporidium parvum

Cryptosporidium parvum is an intracellular protozoan parasite that causes cryptosporidiosis in mammals. In this study, we identified a gene encoding mitochondrial iron-superoxide dismutase of C. parvum (Cp-mtSOD) and characterized biochemical properties of the recombinant protein. Multiple sequence alignment of the deduced amino acid sequence of Cp-mtSOD with those of previously reported iron-containing SODs (Fe-SODs) from other protozoan parasites showed that Cp-mtSOD shares common metal-binding residues and motifs that were conserved in Fe-SODs. However, the N-terminal 26-amino acid residues of Cp-mtSOD did not show sequence identities to any other Fe-SOD sequences. Further analysis of the N-terminal presequence of Cp-mtSOD suggested that it shares common physiochemical characteristics found in mitochondria targeting sequences and predicted localization of Cp-mtSOD in the mitochondria. The recombinant Cp-mtSOD showed typical biochemical properties with other characterized Fe-SODs, including molecular structure, broad pH optimum, and sensitivity to hydrogen peroxide.

Molecular characterization of iron-containing superoxide dismutases in the heterotrophic dinoflagellate Crypthecodinium cohnii

Superoxide dismutases (SODs) are a family of antioxidant enzymes that catalyse the degradation of toxic superoxide radicals in obligate and facultative aerobic organisms. Here, we report the presence of a multi-copy gene family encoding SODs in the heterotrophic dinoflagellate Crypthecodinium cohnii. All the genes identified (sod1 to sod17) have been cloned and sequenced, and shown to encode potentially functional dimeric iron-containing SOD isozymes. Our data revealed a considerable molecular heterogeneity of this enzyme in C. cohnii at both genomic and transcriptional levels. The C. cohnii SOD1, overexpressed in Escherichia coli, was active and its structure obtained by homology modeling using X-ray crystal structures of homologues exhibited the typical fold of dimeric FeSODs. Phylogenetic studies including 110 other dimeric FeSODs and closely related cambialistic dimeric SOD sequences showed that the C. cohnii SODs form a monophyletic group and have all been acquired by the same event of horizontal gene transfer. It also revealed a dichotomy within the C. cohnii SOD sequences that could be explained by an ancestral sod gene duplication followed by subsequent gene duplications within each of the two groups. Enzyme assays of SOD activity indicated the presence of two FeSOD activities in C. cohnii cell lysate whereas MnSOD and Cu/ZnSOD were not detected. These activities contrasted with the SOD repertoire previously characterized in photosynthetic dinoflagellates. To explain these differences, a hypothetical evolutionary scenario is proposed that suggests gains and losses of sod genes in dinoflagellates.

Peroxiredoxins in malaria parasites: parasitologic aspects

Malaria is one of the most debilitating and life threatening diseases in tropical regions of the world. Over 500 million clinical cases occur, and 2-3 million people die of the disease each year. Because Plasmodium lacks genuine glutathione peroxidase and catalase, the two major antioxidant enzymes in the eukaryotic cell, malaria parasites are likely to utilize members of the peroxiredoxin (Prx) family as the principal enzymes to reduce peroxides, which increase in the parasite cell due to metabolism and parasitism during parasite development. In addition to its function of protecting macromolecules from H(2)O(2), Prx has also been reported to regulate H(2)O(2) as second messenger in transmission of redox signals, which mediate cell proliferation, differentiation, and apoptosis. In the malaria parasite, several lines of experimental data have suggested that the parasite uses Prxs as multifunctional molecules to adapt themselves to asexual and sexual development. In this review, we summarize the accumulated knowledge on the Prx family with respect to their functions in mammalian cells and their possible function(s) in malaria parasites.

The use of an excreted superoxide dismutase in an ELISA and Western blotting for the diagnosis of Leishmania (Leishmania) infantum naturally infected dogs

An excreted iron superoxide dismutase of pI 3.75 and a molecular mass of approximately 25 kDa was partially purified by QAE Sephadex ion-exchange chromatography from the in vitro culture of Leishmania (Leishmania) infantum. This enzyme was detected by enzyme-linked immunosorbent assay and Western blot of anti-L. infantum antibodies in dog serum. For the determination of the sensitivity and specificity of this protein, the results using the complete-parasite antigen fraction were taken as references. For this, 39 sera were assayed in dogs from different Spanish provinces. By Western blot, at a dilution of 1:250, 82% of the sera were positive when superoxide dismutase excreted was used as the antigen, against 56.4% positivity when the complete parasite was used as the antigen. These findings support the results of a previous study, indicating that the superoxide dismutase excreted can be useful in diagnosing L. (L.) infantum.

The crystal structure of superoxide dismutase from Plasmodium falciparum

BACKGROUND

Superoxide dismutases (SODs) are important enzymes in defence against oxidative stress. In Plasmodium falciparum, they may be expected to have special significance since part of the parasite life cycle is spent in red blood cells where the formation of reactive oxygen species is likely to be promoted by the products of haemoglobin breakdown. Thus, inhibitors of P. falciparum SODs have potential as anti-malarial compounds. As a step towards their development we have determined the crystal structure of the parasite's cytosolic iron superoxide dismutase.

RESULTS

The cytosolic iron superoxide dismutase from P. falciparum (PfFeSOD) has been overexpressed in E. coli in a catalytically active form. Its crystal structure has been solved by molecular replacement and refined against data extending to 2.5 A resolution. The structure reveals a two-domain organisation and an iron centre in which the metal is coordinated by three histidines, an aspartate and a solvent molecule. Consistent with ultracentrifugation analysis the enzyme is a dimer in which a hydrogen bonding lattice links the two active centres.

CONCLUSION

The tertiary structure of PfFeSOD is very similar to those of a number of other iron-and manganese-dependent superoxide dismutases, moreover the active site residues are conserved suggesting a common mechanism of action. Comparison of the dimer interfaces of PfFeSOD with the human manganese-dependent superoxide dismutase reveals a number of differences, which may underpin the design of parasite-selective superoxide dismutase inhibitors.

Identification of excreted iron superoxide dismutase for the diagnosis of Phtytomonas

An excreted iron superoxide dismutase (FeSODe) of pI 3.6 with a molecular weight of 28-30 kDa was detected in the in vitro culture of Phytomonas isolated from Euphorbia characias (SODeCHA) and from Lycopersicon esculentum (SODeTOM), in Grace's medium without serum. These FeSODe excreted into the medium had immunogenic capacity: the positivity of the anti-SODeCHA serum persisted to a dilution of 1/30,000, and for the anti-SODeTOM to 1/10,000 by Western blot. In addition, cross reaction was detected between the anti-SODe serum of Phytomonas isolated from E. characias against SODeTOM, and the anti-SODe serum from L. esculentum with SODeCHA. This characteristic offers the possibility of its use to diagnose plant trypanosomatids. The validation of the test was confirmed by experimental inoculation of tomato fruits with Phytomonas isolated from L. esculentum. At 7, 10, 15, and 21 days post infection, it was possible to detect the presence of the parasites with the anti-SODe serum of Phytomonas isolated from L. esculentum at a dilution of 1/250. These serological results were confirmed by visualization of the parasites by optical microscopy. The data of this study confirm that the SOD is sufficient to identify a trypanosomatid isolated from plants as belonging to the genus Phytomonas.

Functional characterisation of the iron superoxide dismutase gene repertoire in Trypanosoma brucei

Superoxide dismutases (SOD) are a family of antioxidant enzymes that function by removing superoxide anions from the cellular environment. Here, we show that the African trypanosome, Trypanosoma brucei, expresses four SOD isoforms, three of which we have validated biochemically as iron dependent, a feature normally associated with prokaryotic SODs. Localisation studies reveal that two of the enzymes are found predominantly in a parasite-specific organelle, the glycosome (TbSODB1 and TbSODB2), while the other two are targeted to the mitochondrion (TbSODA and TbSODC). Functional analysis of the SOD repertoire in bloodstream form parasites was performed using an inducible RNA interference (RNAi) approach. Down-regulation of the glycosomal SOD transcripts corresponded with a significant reduction in the corresponding proteins and a dramatic level of cell death within the population. The importance of one of the mitochondrial enzymes (TbSODA) only became apparent when parasites were exposed to the superoxide-generating agent paraquat following induction of RNAi. These experiments therefore identify essential components of the superoxide metabolising arm of the T. brucei oxidative defence system and validate these enzymes as parasite-specific targets for drug design.

Identification of a mitochondrial superoxide dismutase with an unusual targeting sequence in Plasmodium falciparum

The intraerythrocytic stages of Plasmodium falciparum are exposed to oxidative stress and require functional anti-oxidant systems to survive. In addition to the parasite's known iron-dependent superoxide dismutase PfSOD1, a second SOD gene (PfSOD2) interrupted by 8 introns was identified on chromosome 6. Molecular modelling shows that the structure of PfSOD2 is similar to other iron-dependent SODs and phylogenetic analysis suggests PfSOD1 and PfSOD2 are the result of an ancestral gene duplication. The deduced amino acid sequence of PfSOD2 is similar to PfSOD1 but has a long N-terminal extension. Immunofluorescence studies show that PfSOD1 is cytosolic, whereas the N-terminal extension of PfSOD2 targets a green fluorescent protein fusion into the parasite's mitochondrion. Both SOD genes are transcribed during the erythrocytic cycle with PfSOD1 mRNA levels up to 35-fold higher than those of PfSOD2. Northern blots demonstrated that the mRNA levels of both SOD genes are up-regulated upon exposure to oxidative stress.

Antioxidant defense in Plasmodium falciparum--data mining of the transcriptome

The intraerythrocytic malaria parasite is under constant oxidative stress originating both from endogenous and exogenous processes. The parasite is endowed with a complete network of enzymes and proteins that protect it from those threats, but also uses redox activities to regulate enzyme activities. In the present analysis, the transcription of the genes coding for the antioxidant defense elements are viewed in the time-frame of the intraerythrocytic cycle. Time-dependent transcription data were taken from the transcriptome of the human malaria parasite Plasmodium falciparum. Whereas for several processes the transcription of the many participating genes is coordinated, in the present case there are some outstanding deviations where gene products that utilize glutathione or thioredoxin are transcribed before the genes coding for elements that control the levels of those substrates are transcribed. Such insights may hint to novel, non-classical pathways that necessitate further investigations.

Purification and characterization of two iron superoxide dismutases ofPhytomonassp. isolated fromEuphorbia characias(plant trypanosomatids)

Two superoxide dismutases (SODI and SODII) have been purified by differential centrifugation, fractionation with ammonium sulphate followed by chromatographic separation (ionic exchange and affinity), from a plant trypanosomatid isolated fromEuphorbia characias, and then characterized for several biochemical properties. Both enzymes were insensitive to cyanide but sensitive to hydrogen peroxide, properties characteristic of iron-containing superoxide dismutase. SODI had a molecular mass of approximately 66 kDa, whereas the molecular mass of SODII was approximately 22 kDa, both enzymes showing single bands. The isoelectric points of SODI and SODII were 6·8 and 3·6, respectively. The enzymatic stability persisted at least for 6 months when the sample was lyophilized and preserved at −80 °C. Digitonin titration and subcellular fractionation showed that both enzymes were in the cytoplasmic fraction, although part of SODII isoenzyme was also associated with glycosomes. We assayed these activities (SOD) in 18 trypanosomatid isolates on isoelectric focusing gels, and have demonstrated that the SOD is a biochemical marker sufficient to identify a trypanosomatid isolated from a plant as belonging to the genusPhytomonasand to distinguish between a truePhytomonasand other trypanosomatids that are capable of causing transient infections in plants.

Cloning, expression, and characterization of iron-containing superoxide dismutase from Neospora caninum

A gene encoding superoxide dismutase (SOD) from Neospora caninum, a causative agent of neosporosis, has been cloned and its gene product functionally expressed and characterized. The gene had an open reading frame of 606 bp and deduced 201 amino acids. Sequence analysis showed that the gene had conserved metal-binding residues and conserved amino acid residues that were found in Fe-SODs. Comparison of the deduced amino acid sequence of the enzyme with previously reported Fe-SOD amino acid sequences of the other parasitic protozoans revealed significant high homology. The coding region of the N. caninum Fe-SOD was cloned and functionally expressed in Escherichia coli. Enzyme activity of the expressed protein was inhibited by hydrogen peroxide but not by sodium azide and potassium cyanide, and the enzyme showed similar biochemical properties with typical Fe-SODs of other parasitic protozoans. Southern blot analysis showed that the SOD gene appears to be present as a single-copy gene in N. caninum genome. Semiquantitative reverse transcription-polymerase chain reaction and immunoblot using antiserum raised against the purified recombinant protein showed that Fe-SOD is expressed in both developmental stages of N. caninum, i.e., in bradyzoites and tachyzoites. In an immunofluorescence assay, the enzyme was localized on the cell surface of N. caninum tachyzoites. These results suggest that Fe-SOD might be essential for the intracellular survival of N. caninum and may play an important role in the pathogenesis of the parasite by protecting the parasite from oxidative killing.

Oxidative stress and antioxidant defenses: a target for the treatment of diseases caused by parasitic protozoa

Parasitic protozoa cause several diseases, affecting hundreds of millions, particularly in underdeveloped countries. Although these organisms are eukaryotic cells, some of them present major differences with their mammalian host in selected metabolic pathways. These differences may be exploited as targets for developing better pharmacological agents for the treatment of specific parasitic diseases. This review describes some of the differences in terms of antioxidant defenses between these organisms and their mammalian host, which may provide useful targets for the treatment of these diseases. Some of the potential targets are: (i). iron metabolism in Plasmodium, (ii). the presence of a Fe-containing form of superoxide dismutase in trypanosomatids and malaria-causing parasites, (iii). the unique trypanothione-dependent antioxidant metabolism in trypanosomatids, (iv). the ascorbate peroxidase found in Trypanosoma cruzi and perhaps present in other trypanosomatids.

Mutation of an unusual mitochondrial targeting sequence of SODB2 produces multiple targeting fates in Toxoplasma gondii

Proteins destined for the mitochondria travel an intricate pathway through two membranes, each with its own receptors and channels. These proteins interact with receptors via N-terminal presequences that form amphipathic helices. Generally, these helices contain abundant positive charges on one face and hydrophobic residues on the other, but share little primary sequence homology. While extensive research on mitochondrial import has been done in yeast and mammalian cells, little is known about import or contents of the single mitochondrion of Toxoplasma gondii, a parasite in the phylum Apicomplexa. We describe here the characterization of TgSODB2, a novel, mitochondrial superoxide dismutase in T. gondii with an unusual targeting sequence consisting of a hydrophobic segment resembling a signal peptide, followed by a presequence. We show that although the hydrophobic segment is competent to target a reporter protein to the secretory system, it is prevented from directing ER translocation when coupled with the presequence. When we mutated the only charged residue in the hydrophobic sequence, ER translocation is restored and the reporter targeted to the apicoplast, a chloroplast-like organelle found in most apicomplexans. The presequence that follows is predicted to form an amphipathic helix, but targeted the cytoplasm when the hydrophobic peptide is removed. In addition to having an unusual targeting sequence, TgSODB2 is only the second mitochondrially imported, iron-containing SOD to be described.

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.

The PmSOD1 gene of the protistan parasite Perkinsus marinus complements the sod2Delta mutant of Saccharomyces cerevisiae, and directs an iron superoxide dismutase to mitochondria

The facultative intracellular oyster parasite, Perkinsus marinus, taxonomically related to both dinoflagellates and apicomplexan parasites, possesses at least two distinct genes (PmSOD1 and PmSOD2) predicted to encode iron-containing superoxide dismutases (Fe-SOD). The present study demonstrates that PmSOD1 complements a Saccharomyces cerevisiae mutant lacking the mitochondrial manganese-containing SOD (Mn-SOD), whereas PmSOD2 complements an Escherichia coli mutant lacking genes for cytosolic SOD activities. Mitochondria isolated from complemented yeast contain an SOD activity susceptible to inhibition by hydrogen peroxide, but resistant to cyanide, both characteristics of Fe-SODs. In cultured P. marinus trophozoites, indirect immunofluorescence using anti-PmSOD1 antibodies shows colocalization of PmSOD1 product with the mitochondrial marker MitoTracker Red. Further analysis of the leader sequence of the predicted PmSOD1 product revealed similarities to a mitochondrial targeting domain, an unusual observation for Fe-SODs, which are typically localized in the cytoplasm. These results suggest that PmSOD1 encodes a mitochondrial Fe-SOD, which may contribute to P. marinus resistance to exogenous oxidative damage in host phagocytes. The present study constitutes the first report of an endogenous Fe-SOD that is directed to the mitochondria, and suggests that mitochondria targeting sequences have been conserved among diverse branches of the eukaryotes, including the early protista. It also illustrates the potential of complementation-based approaches for further gene discovery and characterization in P. marinus.

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.

Recent advances in the search for new anti-coccidial drugs

Coccidia provide a rich hunting ground for drug-designers, as there are significant biochemical differences between the parasites and their hosts. Recent years have brought the discovery of the plastid and its possible metabolic machinery, characterisation of acidocalcisomes, reports on the apparent absence from some coccidia of a typical mitochondrion, and the discovery of the mannitol cycle and shikimate pathway in the parasites. Moreover, modern technologies such as genomics and proteomics are bringing new insights into the biochemistry of coccidia and highlighting possible drug targets in abundance. A major issue for would-be drug discoverers is to decide upon the targets to prioritise. This review provides an update on recent findings on how coccidia differ biochemically from vertebrates. It includes discoveries within coccidian parasites themselves but also uses findings in Plasmodium to provide an overview of biochemical features that may be characteristics of many apicomplexan parasites and so potential targets for broad-spectrum drugs.

Biochemical and electron paramagnetic resonance study of the iron superoxide dismutase from Plasmodium falciparum

Recombinant iron-containing superoxide dismutase (Fe-SOD) from Plasmodium falciparum was produced in a SOD-deficient strain of Escherichia coli, purified and characterised. The enzyme is a dimer, which contains 1.7 Fe equivalents and is sensitive to hydrogen peroxide (H(2)O(2)). Electron paramagnetic resonance (EPR) analysis showed two different signals, reflecting the presence of two different types of high-spin Fe sites with different symmetries. The role of the W71 residue during inactivation by H(2)O(2) of the P. falciparum Fe-SOD was studied by site-directed mutagenesis. First, the W71V mutation led to a change in the relative proportion of the two Fe-based EPR signals. Second, the mutant protein was almost as active as the wild-type (WT) protein but more sensitive to heat inactivation. Third, resistance to H(2)O(2) was only slightly increased indicating that W71 was marginally responsible for the sensitivity of Fe-SOD to H(2)O(2). A molecular model of the subunit was designed to assist in interpretation of the results. The fact that the parasite SOD does not belong to classes of SOD present in humans may provide a novel approach for the design of antimalarial drugs.

Relationship between chloroquine toxicity and iron acquisition in Saccharomyces cerevisiae

Chloroquine is one of the most effective antimalarials, but resistance to it is becoming widespread. However, we do not fully understand either the drug's mode of action or the mechanism of resistance. In an effort to expand our understanding of the mechanism of action and resistance associated with chloroquine, we used Saccharomyces cerevisiae as a model eukaryotic system. To aid in the discovery of potential drug targets we applied the transcriptional profiling method to identify genes transcriptionally responsive to chloroquine treatment in S. cerevisiae. Among the genes that were differentially expressed with chloroquine treatment were a number of metal transporters involved in iron acquisition (SIT1, ARN2, ARN4, and SMF2). These genes exhibit similar expression patterns, and several are known to be regulated by AFT1, a DNA binding protein, which responds to iron levels in the cell. We investigated the role of chloroquine in iron metabolism by using a variety of approaches, including pharmacological, genetic, and biochemical techniques. For these experiments, we utilized yeast lacking the major iron uptake pathways (FET3 and FET4) and yeast deficient in SIT1, encoding the major up-regulated iron siderophore transporter. Our experiments show that yeast genetically or environmentally limited in iron availability has increased sensitivity to chloroquine in pharmacological assays and that the addition of iron rescues these cells from chloroquine killing. 55FeCl3 accumulation was inhibited in the presence of chloroquine, and kinetic analysis demonstrated that inhibition was competitive. These results are consistent with deprivation of iron as a mechanism of chloroquine killing in yeast.

Identification of a developmentally regulated iron superoxide dismutase of Trypanosoma brucei

An iron superoxide dismutase (FeSOD) gene of the protozoan parasite Trypanosoma brucei has been cloned and its gene product functionally characterized. The gene encodes a protein of 198 residues which shows 80% identity with FeSODs from other trypanosomatids. Inhibitor studies with purified recombinant FeSOD expressed in Escherichia coli confirmed that the enzyme is an iron-containing SOD. The FeSOD is developmentally regulated in the parasite, expression being lowest in the cell-cycle-arrested, short stumpy bloodstream forms. Differential expression of the FeSOD protein contrasts with only minor quantitative changes in the FeSOD mRNA, indicating post-transcriptional regulation of the enzyme. As the level of FeSOD increases during differentiation of cell-cycle-arrested short stumpy into dividing procyclic forms, it is suggested that the enzyme is only required in proliferating stages of the parasite for the elimination of superoxide radicals which are released during the generation of the iron-tyrosyl free-radical centre in the small subunit of ribonucleotide reductase.

Molecular characterization of a 2-Cys peroxiredoxin from the human malaria parasite Plasmodium falciparum

We have identified the 2-Cys peroxiredoxin (PfPrx-1) from the human malaria parasite Plasmodium falciparum. The PfPrx-1 showed the highest identity at amino acid level to the type II Prx among the currently known six subfamilies of mammalian Prx. The sequence identity between the PfPrx-1 and the previously reported 1-Cys Prx of P. falciparum (PfPrx-2), which corresponded to mammalian type VI Prx, was 25%. This suggests that the parasite possesses two Prx subfamilies. The PfPrx-1 showed significant sequence similarities with those of 2-Cys peroxiredoxins of plants in the BLASTX search. This may reflect the consequences of a genetic transfer from an algal endosymbiont to the parasite nucleus during evolution. The recombinant PfPrx-1 protein (rPfPrx-1) was expressed as a histidine fusion protein in Escherichia coli and purified with Ni chromatography. The rPfPrx-1 existed as dimers under non-reducing conditions and dissociated into monomers in the presence of dithiothreitol. The PfPrx-1 protein also exists as a dimer in the parasites themselves. The reduction of the oxidized enzyme by the donation of electrons from E. coli thioredoxin (Trx)/Trx reductase system was demonstrated in its reaction with H(2)O(2), using the rPfPrx-1 protein. These results suggested that the PfPrx-1 can act as a terminal peroxidase of the parasite Trx system. An elevated expression of the PfPrx-1 protein seen in the trophozoite, the stage with active metabolism, suggests an association of the parasite Trx system with its intracellular redox control.

Hemoglobin metabolism in the malaria parasite Plasmodium falciparum

Hemoglobin degradation in intraerythrocytic malaria parasites is a vast process that occurs in an acidic digestive vacuole. Proteases that participate in this catabolic pathway have been defined. Studies of protease biosynthesis have revealed unusual targeting and activation mechanisms. Oxygen radicals and heme are released during proteolysis and must be detoxified by dismutation and polymerization, respectively. The quinoline antimalarials appear to act by preventing sequestration of this toxic heme. Understanding the disposition of hemoglobin has allowed identification of essential processes and metabolic weakpoints that can be exploited to combat this scourge of mankind.

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.

Purification and characterization of iron superoxide dismutase and copper-zinc superoxide dismutase from Acanthamoeba castellanii

Two superoxide dismutases (SOD I and SOD II) were purified from Acanthamoeba castellanii and characterized for several biochemical properties. Analysis of the primary structure and inhibition studies revealed that SOD I is iron SOD (Fe-SOD), with a molecular mass of 50 kDa, and SOD II is copper-zinc SOD (Cu,Zn-SOD), with a molecular mass of 38 kDa. Both enzymes have a homodimeric structure consisting of 2 identical subunits, each with a molecular mass of 26 and 19 kDa for SOD I and SOD II, respectively. The isoelectric points of SOD I and SOD II were 6.4 and 3.5, respectively, and there were no isoenzyme forms detected. Both enzymes show a broad optimal pH of 7.0-11.0. Because no differences were observed in the apparent molecular weight of SOD I after addition of the reducing agent 2-mercaptoethanol, the subunits do not appear to be linked covalently by disulfide bonds. However, the subunits of SOD II were covalently linked by intra- and interdisulfide bonds. Western blot analyses showed that the 2 enzymes have different antigenicity. Both enzymes occur as cytoplasmic and detergent-extractable fractions. These enzymes may be potential virulence factors of A. castellanii by acting both as antioxidants and antiinflammatory agents. These enzymes may be attractive targets for chemotherapy and immunodiagnosis of acanthamoebiasis.

Toxoplasma gondii catalase: are there peroxisomes in toxoplasma?

The intracellular protozoan parasite Toxoplasma gondii, like all members of the phylum Apicomplexa, is known to possess many organelles: in addition to mitochondria and the compartments of the secretory pathway, there is a reduced chloroplast (the apicoplast) and the phylum-specific components of the apical complex: dense granules, micronemes and rhoptries. Conspicuously missing so far are microbodies, organelles that can be found in nearly all eukaryotic organisms. Microbodies show a large variation with regard to their size, number and contents, depending on the organism and cell type. One marker enzyme of this single membrane-bound organelle is catalase, which is responsible for the degradation of hydrogen peroxide to water and oxygen. The EST project in T. gondii revealed the existence of two overlapping clones which showed similarity with catalase, and these were used to clone the corresponding gene. The predicted sequence of T. gondii catalase has -AKM at the C terminus, which falls within the consensus of the PTS1 peroxisomal targeting signal. Southern blot analysis confirmed the presence of a single copy gene. Northern and western blot analyses showed that the catalase gene is transcribed and translated. Immunofluorescence assays using an antibody raised against a catalase peptide identified a distinct structure towards the apical end, but other catalase-specific antibodies failed to confirm this localisation. Cell fractionations indicated that the majority of the enzyme was in the cytosol. The fusion of the C-terminal twelve amino acids, including AKM, or the canonical peroxisomal targeting signal, -SKL, to GFP resulted in predominantly cytosolic localization in T. gondii. There was therefore no evidence for membrane-bound peroxisomes in Toxoplasma.

Molecular cloning, expression analysis and iron metal cofactor characterisation of a superoxide dismutase from Toxoplasma gondii

A genomic region of 12 kb encompassing the gene encoding the superoxide dismutase (SOD) of Toxoplasma gondii has been cloned. The gene contains four exons of 121, 42, 381 and 59 bp which are separated by three introns of 321, 202, and 577 bp, respectively. The open reading frame can be translated into a protein of 201 amino acids with a molecular mass of 22.6 kDa. Alignment indicated that it is a FeSOD, a type only found in bacteria, protozoa and chloroplast of higher plants. Recombinant SOD was expressed in a Escherichia coli double mutant lacking both MnFeSOD and FeSODs. The presence of iron as metal cofactor was confirmed by measurements of iron by absorption mass spectrometry and electron paramagnetic resonance studies. Semi-quantitative reverse transcribed polymerase chain reaction experiments showed a similar amount of SOD transcripts in two developmental stages of T. gondii. Antibodies raised against the purified recombinant protein detected SOD protein in both bradyzoite and tachyzoite forms suggesting this SOD might be essential for the intracellular growth of both developmental stages. Southern blot analysis indicated that SOD occured as a single copy gene in T. gondii genome.

Analysis of genetic diversity at the iron-containing superoxide dismutase locus in Plasmodium falciparum wild isolates

In order to investigate the genetic diversity of iron-containing superoxide dismutase (FeSOD) from Plasmodium falciparum, a potential anti-malarial therapeutic target, we cloned and sequenced Plasmodium FeSOD from 26 blood samples from non-infected patients. Fifteen clones had the same nucleotide sequence as that of the FeSOD gene of the P. falciparum strain HB3 cultivated in vitro. The other 11 clones presented mutations responsible for punctual amino acid changes which did not modify key residues for the function or the structure of the enzyme. The high sequence conservation between FeSOD from the isolates confirms that this enzyme could represent a therapeutic target.

The role of superoxide dismutation in malaria parasites

Oxidant stress is associated with the generation of reactive oxygen species that are responsible for the damage of a variety of cellular components. The prevention of such biological damage can be achieved by dismutation of superoxide to H2O2 which in turn is removed by catalase and GSH peroxidase. However, redox-active iron released during the development of plasmodia in the erythrocyte can mediate the conversion of H2O2 to hydroxyl radical which is more reactive. The roles of SOD and the nitroxide SOD mimic 4-OH,2,2,6,6,tetramethyl piperidine-N-oxyl (Tempol) were examined in P. falciparum grown in vitro. Both compounds did not prevent the interference with growth inflicted by various inducers of oxidant stress. Moreover, Tempol inhibited parasite growth, in agreement with previous experiments depicting accelerated mortality in SOD overexpressing mouse model of malaria. Probably, effective defense against ROS requires balanced increments in antioxidant enzymes and is not necessarily improved by an increase in the activity of one enzyme.

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.