Identification of an Entamoeba histolytica gene encoding a protein homologous to prokaryotic disulphide oxidoreductases

Redox Pathways as Drug Targets in Microaerophilic Parasites

The microaerophilic parasites Entamoeba histolytica, Trichomonas vaginalis, and Giardia lamblia jointly cause hundreds of millions of infections in humans every year. Other microaerophilic parasites such as Tritrichomonas foetus and Spironucleus spp. pose a relevant health problem in veterinary medicine. Unfortunately, vaccines against these pathogens are unavailable, but their microaerophilic lifestyle opens opportunities for specifically developed chemotherapeutics. In particular, their high sensitivity towards oxygen can be exploited by targeting redox enzymes. This review focusses on the redox pathways of microaerophilic parasites and on drugs, either already in use or currently in the state of development, which target these pathways.

A review of the proposed role of neutrophils in rodent amebic liver abscess models

Host invasion by Entamoeba histolytica, the pathogenic agent of amebiasis, can lead to the development of amebic liver abscess (ALA). Due to the difficulty of exploring host and amebic factors involved in the pathogenesis of ALA in humans, most studies have been conducted with animal models (e.g., mice, gerbils, and hamsters). Histopathological findings reveal that the chronic phase of ALA in humans corresponds to lytic or liquefactive necrosis, whereas in rodent models there is granulomatous inflammation. However, the use of animal models has provided important information on molecules and mechanisms of the host/parasite interaction. Hence, the present review discusses the possible role of neutrophils in the effector immune response in ALA in rodents. Properly activated neutrophils are probably successful in eliminating amebas through oxidative and non-oxidative mechanisms, including neutrophil degranulation, the generation of free radicals (O₂⁻, H₂O₂, HOCl) and peroxynitrite, the activation of NADPH-oxidase and myeloperoxidase (MPO) enzymes, and the formation of neutrophil extracellular traps (NETs). On the other hand, if amebas are not eliminated in the early stages of infection, they trigger a prolonged and exaggerated inflammatory response that apparently causes ALAs. Genetic differences in animals and humans are likely to be key to a successful host immune response.

Molecular profiling and functional insights of rock bream (Oplegnathus fasciatus) thioredoxin reductase 3-like molecule: investigation of its transcriptional modulation in response to live pathogen stress

The thioredoxin (Trx) system plays a significant role in cellular antioxidative defense by dismutating the surpluses of reactive oxygen species. Thus, the role of thioredoxin reductase (TrxR) cannot be ignored, owing to its participation in initiating the Trx enzyme cascade. Here, we report the identification and molecular characterization of a teleostean TrxR (RbTrxR-3) ortholog that showed high similarity with the TrxR-3 isoforms of other vertebrates. The complete RbTrxR-3 coding sequence comprised 1800 nucleotides, encoding a 600-amino acid protein with a predicted molecular mass of ~66 kDa. RbTrxR-3 consisted of 16 exons separated by 15 introns and had a total length of 12,658 bp. In silico analysis of the RbTrxR-3 protein sequence revealed that it possesses typical TrxR domain architecture. Moreover, using multiple sequence alignment and pairwise sequence alignment strategies, we showed that RbTrxR-3 has high overall sequence similarity to other teleostean TrxR-3 proteins, including highly conserved active site residues. Phylogenetic reconstruction of RbTrxR-3 affirmed its close evolutionary relationship with fish TrxR-3 orthologs, as indicated by its clustering pattern. RbTrxR-3 transcriptional analysis, performed using quantitative polymerase chain reaction (qPCR), showed that RbTrxR-3 was ubiquitously distributed, with the highest level of mRNA expression in the blood, followed by the gill, and liver. Live bacterial and viral stimuli triggered the modulation of RbTrxR-3 basal transcription in liver tissues that correlated temporally with that of its putative substrate, rock bream thioredoxin1 under the same conditions of pathogenic stress. Finally, resembling the typical function of TrxR protein, purified recombinant RbTrxR-3 showed detectable dose-dependent thiol reductase activity against 5,5'-dithiobis (2-nitrobenzoic) acid. Taken together, these results suggest that RbTrxR-3 plays a role in the host Trx system under conditions of oxidative and pathogenic stress.

Characterization of Deinococcus radiophilus thioredoxin reductase active with both NADH and NADPH

Thioredoxin reductase (TrxR, EC 1.6.4.5) of Deinococcus radiophilus was purified by steps of sonication, ammonium sulfate fractionation, 2'5' ADP Sepharose 4B affinity chromatography, and Sephadex G-100 gel filtration. The purified TrxR, which was active with both NADPH and NADH, gave a 368 U/mg protein of specific activity with 478-fold purification and 18% recovery from the cell-free extract. An isoelectric point of the purified enzymes was ca. 4.5. The molecular weights of the purified TrxR estimated by PAGE and gel filtration were about 63.1 and 72.2 kDa, respectively. The molecular mass of a TrxR subunit is 37 kDa. This suggests that TrxR definitely belongs to low molecular weight TrxR (L-TrxR). The Km and Vmax of TrxR for NADPH are 12.5 μM and 25 μM/min, whereas those for NADH are 30.2 μM and 192 μM/min. The Km and Vmax for 5, 5'-dithio-bis-2-nitrobenzoic acid (DTNB, a substituted substrate for thioredoxin) are 463 μM and 756 μM/min, respectively. The presence of FAD in TrxR was confirmed with the absorbance peaks at 385 and 460 nm. The purified TrxR was quite stable from pH 3 to 9, and was thermo-stable up to 70°C. TrxR activity was drastically reduced (ca. 70%) by Cu(2+), Zn(2+), Hg(2+), and Cd(2+), but moderately reduced (ca. 50%) by Ag(+). A significant inhibition of TrxR by N-ethylmaleimide suggests an occurrence of cysteine at its active sites. Amino acid sequences at the N-terminus of purified TrxR are H(2)N-Ser-Glu-Gln-Ala-Gln-Met-Tyr-Asp-Val-Ile-Ile-Val-Gly-Gly-Gly-Pro-Ala-Gly-Leu-Thr-Ala-COOH. These sequences show high similarity with TrxRs reported in Archaea, such as Methanosarcina mazei, Archaeoglobus fulgidus etc.

The immunopathogenesis of Entamoeba histolytica

Amebiasis is the disease caused by the enteric dwelling protozoan parasite Entamoeba histolytica. The WHO considers amebiasis as one of the major health problems in developing countries; it is surpassed by only malaria and schistosomiasis for death caused by parasitic infection. E. histolytica primarily lives in the colon as a harmless commensal, but is capable of causing devastating dysentery, colitis and liver abscess. What triggers the switch to a pathogenic phenotype and the onset of disease is unknown. We are becoming increasingly aware of the complexity of the host-parasite interaction. During chronic stages of amebiasis, the host develops an immune response that is incapable of eliminating tissue resident parasites, while the parasite actively immunosuppresses the host. However, most individuals with symptomatic infections succumb only to an episode of dysentery. Why most halt invasion and a minority progress to chronic disease remains poorly understood. This review presents a current understanding of the immune processes that shape the outcome of E. histolytica infections during its different stages.

Nitroimidazole action in Entamoeba histolytica: a central role for thioredoxin reductase

Metronidazole, a 5-nitroimidazole drug, has been the gold standard for several decades in the treatment of infections with microaerophilic protist parasites, including Entamoeba histolytica. For activation, the drug must be chemically reduced, but little is known about the targets of the active metabolites. Applying two-dimensional gel electrophoresis and mass spectrometry, we searched for protein targets in E. histolytica. Of all proteins visualized, only five were found to form adducts with metronidazole metabolites: thioredoxin, thioredoxin reductase, superoxide dismutase, purine nucleoside phosphorylase, and a previously unknown protein. Recombinant thioredoxin reductase carrying the modification displayed reduced enzymatic activity. In treated cells, essential non-protein thiols such as free cysteine were also affected by covalent adduct formation, their levels being drastically reduced. Accordingly, addition of cysteine allowed E. histolytica to survive in the presence of otherwise lethal metronidazole concentrations and reduced protein adduct formation. Finally, we discovered that thioredoxin reductase reduces metronidazole and other nitro compounds, suggesting a new model of metronidazole activation in E. histolytica with a central role for thioredoxin reductase. By reducing metronidazole, the enzyme renders itself and associated thiol-containing proteins vulnerable to adduct formation. Because thioredoxin reductase is a ubiquitous enzyme, similar processes could occur in other eukaryotic or prokaryotic organisms.

The protist parasites Entamoeba histolytica, Trichomonas vaginalis, and Giardia intestinalis grow in environments with low oxygen concentration. Infections with these parasites are commonly treated with metronidazole, a nitroimidazole drug that must be reduced for activation, resulting in several toxic metabolites. We examined the soluble proteome of metronidazole-treated E. histolytica cells for target proteins of these metabolites, applying two-dimensional gel electrophoresis and mass spectrometry. Of about 1,500 proteins visualized, only five formed covalent adducts with metronidazole metabolites, including thioredoxin, thioredoxin reductase, and superoxide dismutase. Metronidazole-bound thioredoxin reductase displayed diminished activity. In addition to these proteins, small thiol molecules, including cysteine, formed adducts with metronidazole. Supplementation with cysteine allowed the cells to survive otherwise lethal metronidazole concentrations. Finally, we discovered that one of the modified proteins, thioredoxin reductase, reduces metronidazole, suggesting a central role for this enzyme with regard to metronidazole toxicity. Taken together, our work reveals a new area of molecular interactions of activated metronidazole with cellular components. Because thioredoxin reductase is a ubiquitous enzyme, similar processes could also occur in other eukaryotic or prokaryotic organisms.

Metronidazole is used for treatment of infections with microaerophilic protist parasites. Here, a new model of metronidazole activation is proposed, with a central role for thioredoxin reductase.

The 29-kilodalton thiol-dependent peroxidase of Entamoeba histolytica is a factor involved in pathogenesis and survival of the parasite during oxidative stress

The 29-kDa surface antigen (thiol-dependent peroxidase; Eh29) of Entamoeba histolytica exhibits peroxidative and protective antioxidant activities. During tissue invasion, the trophozoites are exposed to oxidative stress and need to deal with highly toxic reactive oxygen species (ROS). In this investigation, attempts have been made to understand the role of the 29-kDa peroxidase gene in parasite survival and pathogenesis. Inhibition of eh29 gene expression by antisense RNA technology has shown approximately 55% inhibition in eh29 expression, maximum ROS accumulation, and significantly lower viability in 29-kDa downregulated trophozoites during oxidative stress. The cytopathic and cytotoxic activities were also found to decrease effectively in the 29-kDa downregulated trophozoites. Size of liver abscesses was substantially lower in hamsters inoculated with 29-kDa downregulated trophozoites compared to the normal HM1:IMSS. These findings clearly suggest that the 29-kDa protein of E. histolytica has a role in both survival of trophozoites in the presence of ROS and pathogenesis of amoebiasis.

Genes induced by a high-oxygen environment in Entamoeba histolytica

Entamoeba histolytica, although a microaerophilic protozoan parasite, encounters a high-oxygen environment, during invasive amoebiasis. The parasite requires specific regulation of certain proteins to maintain its physiological functions to survive in the more oxygenated condition. Our endeavor was to know how does amoeba adapt itself in a high-oxygen environment. Reactive oxygen species (ROS) was found to accumulate in an increasing concentration within the stressed trophozoites in a time-dependent manner. Increased cytopathic activity was detected at 2h in high-oxygen-exposed E. histolytica lysate compared to lysate of normal E. histolytica trophozoites by Ussing chamber assay. The differential display and semi-quantitative polymerase chain reaction showed overexpression in the mRNA levels of thiol-dependent peroxidase (Eh29), superoxide dismutase (SOD), EhCP5, G protein, HSP70, and peptidylprolyl isomerase at different time periods of oxidative stressed trophozoites compared to normally cultured E. histolytica. Analyses of the up-regulated genes that are associated with stress response, viz., signal transduction, tissue destruction, and oxidative stress management, including enhanced expression of a 29-kDa Eh29, suggest that this organism has several protective mechanisms to deal with oxidative stress during invasion.

The diversity and evolution of thioredoxin reductase: new perspectives

The thioredoxin system is a major line of cellular defence against oxygen damage. Two distinct thioredoxin reductases found in eukaryotes have different catalytic mechanisms and a mutually exclusive distribution reflecting a complex evolutionary history. Most eukaryotes, including several important parasites, contain a low molecular weight thioredoxin reductase, apparently of bacterial origin. By contrast, animals and apicomplexan protozoa, including Plasmodium, appear to have lost this enzyme. Instead, they contain a high molecular weight thioredoxin reductase, which shares common ancestry with glutathione reductase. This article reviews these fundamental differences between the thioredoxin reductases of some parasites and their hosts, discusses their phylogenetic relationships and considers the potential of the enzymes as therapeutic targets.

Pathogenesis of intestinal amebiasis: from molecules to disease

In spite of a wealth of knowledge on the biochemistry and cellular and molecular biology of Entamoeba histolytica, little has been done to apply these advances to our understanding of the lesions observed in patients with intestinal amebiasis. In this review, the pathological and histological findings in acute amebic colitis are related to the molecular mechanisms of E. histolytica pathogenicity described to date. Infection of the human colon by E. histolytica produces focal ulceration of the intestinal mucosa, resulting in dysentery (diarrhea with blood and mucus). Although a complete picture has not yet been achieved, the basic mechanisms involved in the production of focal lytic lesions include complex multifactorial processes in which lectins facilitate adhesion, proteases degrade extracellular matrix components, porins help nourish the parasite and may also kill incoming polymorphonuclear leukocytes and macrophages, and motility is used by the parasite to invade deeper layers of the colon. In addition, E. histolytica has developed mechanisms to modulate the immune response during acute infection. Nevertheless, much still needs to be unraveled to understand how this microscopic parasite has earned its well-deserved histolytic name.

Pathogenesis of intestinal amebiasis: from molecules to disease

In spite of a wealth of knowledge on the biochemistry and cellular and molecular biology of Entamoeba histolytica, little has been done to apply these advances to our understanding of the lesions observed in patients with intestinal amebiasis. In this review, the pathological and histological findings in acute amebic colitis are related to the molecular mechanisms of E. histolytica pathogenicity described to date. Infection of the human colon by E. histolytica produces focal ulceration of the intestinal mucosa, resulting in dysentery (diarrhea with blood and mucus). Although a complete picture has not yet been achieved, the basic mechanisms involved in the production of focal lytic lesions include complex multifactorial processes in which lectins facilitate adhesion, proteases degrade extracellular matrix components, porins help nourish the parasite and may also kill incoming polymorphonuclear leukocytes and macrophages, and motility is used by the parasite to invade deeper layers of the colon. In addition, E. histolytica has developed mechanisms to modulate the immune response during acute infection. Nevertheless, much still needs to be unraveled to understand how this microscopic parasite has earned its well-deserved histolytic name.

Characterization of the gene encoding serine acetyltransferase, a regulated enzyme of cysteine biosynthesis from the protist parasites Entamoeba histolytica and Entamoeba dispar. Regulation and possible function of the cysteine biosynthetic pathway in Entamoeba

The enteric protist parasites Entamoeba histolytica and Entamoeba dispar possess a cysteine biosynthetic pathway, unlike their mammalian host, and are capable of de novo production of L-cysteine. We cloned and characterized cDNAs that encode the regulated enzyme serine acetyltransferase (SAT) in this pathway from these amoebae by genetic complementation of a cysteine-auxotrophic Escherichia coli strain with the amoebic cDNA libraries. The deduced amino acid sequences of the amoebic SATs exhibited, within the most conserved region, 36-52% identities with the bacterial and plant SATs. The amoebic SATs contain a unique insertion of eight amino acids, also found in the corresponding region of a plasmid-encoded SAT from Synechococcus sp., which showed the highest overall identities to the amoebic SATs. Phylogenetic reconstruction also revealed a close kinship of the amoebic SATs with cyanobacterial SATs. Biochemical characterization of the recombinant E. histolytica SAT revealed several enzymatic features that distinguished the amoebic enzyme from the bacterial and plant enzymes: 1) inhibition by L-cysteine in a competitive manner with L-serine; 2) inhibition by L-cystine; and 3) no association with cysteine synthase. Genetically engineered amoeba strains that overproduced cysteine synthase and SAT were created. The cysteine synthase-overproducing amoebae had a higher level of cysteine synthase activity and total thiol content and revealed increased resistance to hydrogen peroxide. These results indicate that the cysteine biosynthetic pathway plays an important role in antioxidative defense of these enteric parasites.

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.

Metronidazole resistance in the protozoan parasite Entamoeba histolytica is associated with increased expression of iron-containing superoxide dismutase and peroxiredoxin and decreased expression of ferredoxin 1 and flavin reductase

To obtain insight into the mechanism of metronidazole resistance in the protozoan parasite Entamoeba histolytica, amoeba trophozoites were selected in vitro by stepwise exposures to increasing amounts of metronidazole, starting with sublethal doses of 4 microM. Subsequently, amoebae made resistant were able to continuously multiply in the presence of a 40 microM concentration of the drug. In contrast to mechanisms of metronidazole resistance in other protozoan parasites, resistant amoebae did not substantially down-regulate pyruvate:ferredoxin oxidoreductase or up-regulate P-glycoproteins, but exhibited increased expression of iron-containing superoxide dismutase (Fe-SOD) and peroxiredoxin and decreased expression of flavin reductase and ferredoxin 1. Episomal transfection and overexpression of the various antioxidant enzymes revealed significant reduction in susceptibility to metronidazole only in those cells overexpressing Fe-SOD. Reduction was highest in transfected cells simultaneously overexpressing Fe-SOD and peroxiredoxin. Although induced overexpression of Fe-SOD did not confer metronidazole resistance to the extent found in drug-selected cells, transfected cells quickly adapted to constant exposures of otherwise lethal metronidazole concentrations. Moreover, metronidazole selection of transfected amoebae favored retention of the Fe-SOD-containing plasmid. These results strongly suggest that peroxiredoxin and, in particular, Fe-SOD together with ferredoxin 1 are important components involved in the mechanism of metronidazole resistance in E. histolytica.

Cloning and bacterial expression of adenosine-5'-triphosphate sulfurylase from the enteric protozoan parasite Entamoeba histolytica

A gene encoding adenosine-5'-triphosphate sulfurylase (AS) was cloned from the enteric protozoan parasite Entamoeba histolytica by polymerase chain reaction using degenerate oligonucleotide primers corresponding to conserved regions of the protein from a variety of organisms. The deduced amino acid sequence of E. histolytica AS revealed a calculated molecular mass of 47925 Da and an unusual basic pI of 9.38. The amebic protein sequence showed 23-48% identities with AS from bacteria, yeasts, fungi, plants, and animals with the highest identities being to Synechocystis sp. and Bacillus subtilis (48 and 44%, respectively). Four conserved blocks including putative sulfate-binding and phosphate-binding regions were highly conserved in the E. histolytica AS. The upstream region of the AS gene contained three conserved elements reported for other E. histolytica genes. A recombinant E. histolytica AS revealed enzymatic activity, measured in both the forward and reverse directions. Expression of the E. histolytica AS complemented cysteine auxotrophy of the AS-deficient Escherichia coli strains. Genomic hybridization revealed that the AS gene exists as a single copy gene. In the literature, this is the first description of an AS gene in Protozoa.

Molecular cloning and characterization of the genes encoding two isoforms of cysteine synthase in the enteric protozoan parasite Entamoeba histolytica

The enteric protozoan parasite Entamoeba histolytica was shown to possess cysteine synthase (CS) activity. The cDNA and genomic clones that encode two isoforms of the E. histolytica CS were isolated and characterized from a clonal strain of E. histolytica by genetic complementation of the cysteine-auxotrophic Escherichia coli NK3 with an E. histolytica cDNA library. The two types of the E. histolytica CS genes differed from each other by three nucleotides, two of which resulted in amino acid substitution. Deduced amino acid sequences of the E. histolytica CS, with a calculated molecular mass of 36721 Da and an isoelectric point of 6.39, exhibited 38-48% identity with CS of bacterial and plant origins. The absence of the amino-terminal transit peptide in the deduced protein sequences and the presence of the CS protein mainly in the supernatant fraction of the amoebic lysate after cellular fractionation suggested that the identified E. histolytica CS genes encoded cytosolic isoforms. Substrate specificity of the recombinant E. histolytica CS was similar to that of plant CS. Phylogenetic analysis indicates that the amoebic CS, first described in Protozoa, does not belong to any families of the CS superfamily, and represents a new family.

Recombinant expression and biochemical characterization of an NADPH:flavin oxidoreductase from Entamoeba histolytica

The gene encoding a putative NADPH:flavin oxidoreductase of the protozoan parasite Entamoeba histolytica (Eh34) was recombinantly expressed in Escherichia coli. The purified recombinant protein (recEh34) has a molecular mass of about 35 kDa upon SDS/PAGE analysis, exhibits a flavoprotein-like absorption spectrum and contains 1 mol of non-covalently bound FMN per mol of protein. RecEh34 reveals two different enzymic activities. It catalyses the NADPH-dependent reduction of oxygen to hydrogen peroxide (H2O2), as well as of disulphides such as 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) and cystine. The disulphide reductase but not the H2O2-forming NADPH oxidase activity is inhibitable by sulphydryl-active compounds, indicating that a thiol component is part of the active site for the disulphide reductase activity, whereas for the H2O2-forming NADPH oxidase activity only the flavin is required. Compared with the recombinant protein, similar activities are present in amoebic extracts. Native Eh34 is active in a monomeric as well as in a dimeric state. In contrast to recEh34, no flavin was associated with the native protein. However, both NADPH oxidase as well as DTNB reductase activity were found to be dependent on the addition of FAD or FMN.

Regulation of expression of ribosomal protein L-21 genes of Entamoeba histolytica and E. dispar is at the post-transcriptional level

Two genes, EhgLE3 and Ehg34, encoding the ribosomal protein L21 (rp-L21) were identified and characterized from Entamoeba histolytica. Their coding regions are highly conserved, but their flanking regions differ significantly. Analogous genes (EdgLE3 and Edg34) were characterized in E. dispar. The two rp-L21 copies are transcribed at similar levels in the two parasites. However, their relative binding to the polyribosomal complex during active translation is different. In E. histolytica, binding of EhgLE3 transcripts to the polyribosomes is significantly higher in comparison with that of Ehg34 transcripts, whereas in E. dispar the binding pattern is inverse. The importance of each of the rp-L21 flanking regions to gene translation was investigated by constructing hybrid plasmids containing the CAT reporter gene flanked by rp-L21 flanking regions. The plasmids were stably transfected into E. histolytica and E. dispar, and CAT mRNA and enzymatic activity levels were determined. All plasmids promoted transcription of CAT. Yet, in E. histolytica, high levels of CAT activity were observed only when gLE3 upstream regions flanked CAT. In contrast, in E. dispar, high levels of CAT activity were observed when g34 upstream regions flanked CAT. The downstream regions showed no significant effect on CAT translation.

Anaerobic bacterial metabolism in the ancient eukaryote Giardia duodenalis

The protozoan parasite, Giardia duodenalis, shares many metabolic and genetic attributes of the bacteria, including fermentative energy metabolism which relies heavily on pyrophosphate rather than adenosine triphosphate and as a result contains two typically bacterial glycolytic enzymes which are pyrophosphate dependent. Pyruvate decarboxylation and subsequent electron transport to as yet unidentified anaerobic electron acceptors relies on a eubacterial-like pyruvate:ferredoxin oxidoreductase and an archaebacterial/eubacterial-like ferredoxin. The presence of another 2-ketoacid oxidoreductase (with a preference for alpha-ketobutyrate) and multiple ferredoxins in Giardia is also a trait shared with the anaerobic bacteria. Giardia pyruvate:ferredoxin oxidoreductase is distinct from the pyruvate dehydrogenase multienzyme complex invariably found in mitochondria. This is consistent with a lack of mitochondria, citric acid cycle, oxidative phosphorylation and glutathione in Giardia. Giardia duodenalis actively consumes oxygen and yet lacks the conventional mechanisms of oxidative stress management, including superoxide dismutase, catalase, peroxidase, and glutathione cycling, which are present in most eukaryotes. In their place Giardia contains a prokaryotic H2O-producing NADH oxidase, a membrane-associated NADH peroxidase, a broad-range prokaryotic thioredoxin reductase-like disulphide reductase and the low molecular weight thiols, cysteine, thioglycolate, sulphite and coenzyme A. NADH oxidase is a major component of the electron transport pathway of Giardia which, in conjunction with disulphide reductase, protects oxygen-labile proteins such as ferredoxin and pyruvate:ferredoxin oxidoreductase against oxidative stress by maintaining a reduced intracellular environment. As the terminal oxidase, NADH oxidase provides a means of removing excess H+, thereby enabling continued pyruvate decarboxylation and the resultant production of acetate and adenosine triphosphate. A further example of the bacterial-like metabolism of Giardia is the utilisation of the amino acid arginine as an energy source. Giardia contain the arginine dihydrolase pathway, which occurs in a number of anaerobic prokaryotes, but not in other eukaryotes apart from trichomonads and Chlamydomonas reinhardtii. The pathway includes substrate level phosphorylation and is sufficiently active to make a major contribution to adenosine triphosphate production. Two enzymes of the pathway, arginine deiminase and carbamate kinase, are rare in eukaryotes and do not occur in higher animals. Arginine is transported into the trophozoite via a bacterial-like arginine:ornithine antiport. Together these metabolic pathways in Giardia provide a wide range of potential drug targets for future consideration.

Removal of hydrogen peroxide by the 29 kDa protein of Entamoeba histolytica

The 29 kDa protein of Entamoeba histolytica (Eh29), as well as a truncated variant of this protein, which lacks a cysteine-rich N-terminal region of 40 amino acid residues (Eh29mut), were recombinantly expressed in Escherichia coli and purified to homogeneity. Both recombinant proteins (recEh29, recEh29mut) were found to have hydrogen peroxide (H2O2)-removing activity, but recEh29 was twice as active as recEh29mut. For the consumption of exogenous H2O2, activity was dependent on the presence of reducing equivalents, such as dithiothreitol (DTT), indicating that Eh29 constitutes a thiol-dependent peroxidase. DTT was not required to remove H2O2 by recEh29 or recEh29mut when H2O2 was generated enzymically by the E. histolytica NADPH:flavin oxidoreductase. This enzyme produces H2O2 under aerobic conditions and simultaneously serves as a hydrogen donor for Eh29. Peroxidase activity of the recombinant proteins was further supported by complementation of an E. coli strain that lacks the entire alkyl hydroperoxide reductase locus. The high sensitivity of these bacteria against cumene hydroperoxide was significantly reduced by the introduction of the genes encoding recEh29 or recEh29mut. Using antisera raised against the recombinant proteins, native Eh29 was localized within the cytoplasm of the amoebae. In addition, the antisera reacted with proteins of E. histolytica lysates with apparent molecular masses of 35 kDa and 160-300 kDa. All of them exhibited thiol-peroxidase activity.

Evidence for the bacterial origin of genes encoding fermentation enzymes of the amitochondriate protozoan parasite Entamoeba histolytica

Entamoeba histolytica is an amitochondriate protozoan parasite with numerous bacterium-like fermentation enzymes including the pyruvate:ferredoxin oxidoreductase (POR), ferredoxin (FD), and alcohol dehydrogenase E (ADHE). The goal of this study was to determine whether the genes encoding these cytosolic E. histolytica fermentation enzymes might derive from a bacterium by horizontal transfer, as has previously been suggested for E. histolytica genes encoding heat shock protein 60, nicotinamide nucleotide transhydrogenase, and superoxide dismutase. In this study, the E. histolytica por gene and the adhE gene of a second amitochondriate protozoan parasite, Giardia lamblia, were sequenced, and their phylogenetic positions were estimated in relation to POR, ADHE, and FD cloned from eukaryotic and eubacterial organisms. The E. histolytica por gene encodes a 1,620-amino-acid peptide that contained conserved iron-sulfur- and thiamine pyrophosphate-binding sites. The predicted E. histolytica POR showed fewer positional identities to the POR of G. lamblia (34%) than to the POR of the enterobacterium Klebsiella pneumoniae (49%), the cyanobacterium Anabaena sp. (44%), and the protozoan Trichomonas vaginalis (46%), which targets its POR to anaerobic organelles called hydrogenosomes. Maximum-likelihood, neighbor-joining, and parsimony analyses also suggested as less likely E. histolytica POR sharing more recent common ancestry with G. lamblia POR than with POR of bacteria and the T. vaginalis hydrogenosome. The G. lamblia adhE encodes an 888-amino-acid fusion peptide with an aldehyde dehydrogenase at its amino half and an iron-dependent (class 3) ADH at its carboxy half. The predicted G. lamblia ADHE showed extensive positional identities to ADHE of Escherichia coli (49%), Clostridium acetobutylicum (44%), and E. histolytica (43%) and lesser identities to the class 3 ADH of eubacteria and yeast (19 to 36%). Phylogenetic analyses inferred a closer relationship of the E. histolytica ADHE to bacterial ADHE than to the G. lamblia ADHE. The 6-kDa FD of E. histolytica and G. lamblia were most similar to those of the archaebacterium Methanosarcina barkeri and the delta-purple bacterium Desulfovibrio desulfuricans, respectively, while the 12-kDa FD of the T. vaginalis hydrogenosome was most similar to the 12-kDa FD of gamma-purple bacterium Pseudomonas putida. E. histolytica genes (and probably G. lamblia genes) encoding fermentation enzymes therefore likely derive from bacteria by horizontal transfer, although it is not clear from which bacteria these amebic genes derive. These are the first nonorganellar fermentation enzymes of eukaryotes implicated to have derived from bacteria.

A thioredoxin reductase-class of disulphide reductase in the protozoan parasite Giardia duodenalis

We describe the purification and characterisation of a thioredoxin reductase-like disulphide reductase from the ancient protozoan parasite, Giardia duodenalis. This dimeric flavoprotein contains 1 mol FAD per subunit and had an apparent subunit molecular mass of 35 kDa. The purified enzyme catalysed the NADPH-dependent (Km = 8 microM) reduction of 5,5'-dithio-bis(2-nitrobenzoic acid) to thionitrobenzoate and was unable to utilise NADH as an electron donor. The sulphydryl-active compounds, N-ethylmaleimide, sodium arsenite and Zn2+ ions, strongly inhibited the enzyme suggesting that a thiol component forms part of the active site. Purified enzyme was able to utilise a variety of substrates, including cystine and oxidised glutathione, which suggests that it is a broad-range disulphide reductase, probably accounting for the majority of thiol cycling activity in this organism. While the G. duodenalis enzyme does not require an intermediate electron transport protein, analogous to thioredoxin, for activity, we have identified a candidate carrier protein which enhances DTNB turnover six fold, therefore implying that Giardia contains a thioredoxin-like system. Physical, enzymatic and spectral properties of the G. duodenalis disulphide reductase are also consistent with it being a member of the thioredoxin reductase-class of disulphide reductases. Furthermore, the internal amino acid sequence of a tryptic peptide generated from the purified protein was highly homologous with thioredoxin reductases from other sources. This is the first report of a disulphide reductase to be purified from the anaerobic protozoa and explains the so called "glutathione-induced thiol-reductase activity' previously observed in G. duodenalis.

Antioxidant defense mechanisms in parasitic protozoa

Many of the parasitic protozoa, such as Entamoeba histolytica, Giardia, Trypanosoma, Leishmania, and Plasmodium, are considered to be anaerobes because they can be grown in vitro only under conditions of reduced oxygen tension. However, these parasitic protozoa have been found to be aerotolerant or microaerophilic, and also to consume oxygen to a certain extent. Furthermore, these organisms are highly susceptible to exogenous reactive oxygen species, such as hydrogen peroxide. They must, therefore, detoxify both oxygen and free radical products of enzymatic reactions. However, they lack some or all of the usual antioxidant defense mechanisms present in aerobic or other aerotolerant cells, such as catalase, superoxide dismutase, reduced glutathione, and the glutathione-recycling enzymes glutathione peroxidase and glutathione reductase. Instead, they possess alternative mechanisms for detoxification similar to those known to exist in certain prokaryotes. Although the functional aspects of these alternative mechanisms are yet to be understood completely, they could provide new insights into the biochemical peculiarities of these enigmatic pathogens.

Analysis of the 170-kDa lectin gene promoter of Entamoeba histolytica

The promoter region driving the gene for the 170-kDa heavy subunit of the Entamoeba histolytica galactose-inhibitable lectin was analysed by transient transfection using the chloramphenicol acetyltransferase gene as reporter. S1 mapping confirmed our previous notion that the promoter is located within a 1.35-kb intergenic sequence preceding the structural lectin gene. Transcripts derived from the chloramphenicol acetyltransferase gene of transfected trophozoites were found to be polyadenylated and the transcriptional start mapped to a position similar to that of the wild-type lectin gene. By deletion analysis the entire promoter was restricted to a fragment covering about 550 bp upstream from the start of transcription. On the other hand, residual promoter activity required a sequence of about 140 bp only, encompassing a newly identified CCAAT-box like element around position -100, as well as the amebic specific TATA-box. This 140-bp fragment as well as a stretch of 15 bp, which is located some 100 nt further upstream, were found to be conserved within the 5' noncoding region of a second E. histolytica lectin gene. Point-mutation analyses indicated that the 15-bp fragment, the likely CCAAT-box, as well as the TATA-box are required for full promoter activity.