Entamoeba histolytica. I. Aerobic metabolism

Effects of monosaccharides including rare sugars on proliferation of Entamoeba histolytica trophozoites in vitro

Entamoeba histolytica is a parasitic protozoan with roles in pathogenicity of intestinal amoebiasis. E. histolytica trophozoites lack functional mitochondria and their energy production depends mostly on glycolysis. D-Glucose has a pivotal role in this process and trophozoites store this sugar as glycogen in glycogen granules. Rare sugars, which are defined as sugars present in nature in limited amounts, are of interest as natural low-calorie sweeteners for improving physical conditions of humans. One such rare sugar, D-allose, can be absorbed by a sodium-dependent glucose cotransporter as a substitute for D-glucose, and some rare sugars are known to inhibit growth of cancer cells, Caenorhabditis elegans and Tritrichomonas foetus. Based on these observations, we examined the effects of rare sugars on growth of E. histolytica trophozoites, together with those of D-galactose and D-fructose. The results indicate that treatment with D-allose or D-psicose (D-allulose) alone inhibits proliferation of E. histolytica trophozoites, but that these sugars enhance proliferation of trophozoites in the presence of D-glucose or D-galactose. The trophozoites could take up D-glucose and D-galactose, but not D-fructose, D-allose or D-psicose. Cell sizes of the trophozoites also differed depending on the culture medium.

Immune Response of Amebiasis and Immune Evasion by Entamoeba histolytica

Entamoeba histolytica is a protozoan parasite and the causative agent of amebiasis. It is estimated approximately 1% of humans are infected with E. histolytica, resulting in an estimate of 100,000 deaths annually. Clinical manifestations of amebic infection range widely from asymptomatic to severe symptoms, including dysentery and extra-intestinal abscesses. Like other infectious diseases, it is assumed that only ~20% of infected individuals develop symptoms, and genetic factors of both the parasite and humans as well as the environmental factors, e.g., microbiota, determine outcome of infection. There are multiple essential steps in amebic infection: degradation of and invasion into the mucosal layer, adherence to the intestinal epithelium, invasion into the tissues, and dissemination to other organs. While the mechanisms of invasion and destruction of the host tissues by the amebae during infection have been elucidated at the molecular levels, it remains largely uncharacterized how the parasite survive in the host by evading and attacking host immune system. Recently, the strategies for immune evasion by the parasite have been unraveled, including immunomodulation to suppress IFN-γ production, elimination of immune cells and soluble immune mediators, and metabolic alterations against reactive oxygen and nitrogen species to fend off the attack from immune system. In this review, we summarized the latest knowledge on immune reaction and immune evasion during amebiasis.

Entamoeba thiol-based redox metabolism: A potential target for drug development

Amebiasis is an intestinal infection widespread throughout the world caused by the human pathogen Entamoeba histolytica. Metronidazole has been a drug of choice against amebiasis for decades despite its low efficacy against asymptomatic cyst carriers and emergence of resistance in other protozoa with similar anaerobic metabolism. Therefore, identification and characterization of specific targets is urgently needed to design new therapeutics for improved treatment against amebiasis. Toward this goal, thiol-dependent redox metabolism is of particular interest. The thiol-dependent redox metabolism in E. histolytica consists of proteins including peroxiredoxin, rubrerythrin, Fe-superoxide dismutase, flavodiiron proteins, NADPH: flavin oxidoreductase, and amino acids including l-cysteine, S-methyl-l-cysteine, and thioprolines (thiazolidine-4-carboxylic acids). E. histolytica completely lacks glutathione and its metabolism, and l-cysteine is the major intracellular low molecular mass thiol. Moreover, this parasite possesses a functional thioredoxin system consisting of thioredoxin and thioredoxin reductase, which is a ubiquitous oxidoreductase system with antioxidant and redox regulatory roles. In this review, we summarize and highlight the thiol-based redox metabolism and its control mechanisms in E. histolytica, in particular, the features of the system unique to E. histolytica, and its potential use for drug development against amebiasis.

New enzymatic pathways for the reduction of reactive oxygen species in Entamoeba histolytica

BACKGROUND

Entamoeba histolytica, an intestinal parasite that is the causative agent of amoebiasis, is exposed to elevated amounts of highly toxic reactive oxygen and nitrogen species during tissue invasion. A flavodiiron protein and a rubrerythrin have been characterized in this human pathogen, although their physiological reductants have not been identified.

METHODS

The present work deals with biochemical studies performed to reach a better understanding of the kinetic and structural properties of rubredoxin reductase and two ferredoxins from E. histolytica.

RESULTS

We complemented the characterization of two different metabolic pathways for O2 and H2O2 detoxification in E. histolytica. We characterized a novel amoebic protein with rubredoxin reductase activity that is able to catalyze the NAD(P)H-dependent reduction of heterologous rubredoxins, amoebic rubrerythrin and flavodiiron protein but not ferredoxins. In addition, the protein exhibited an NAD(P)H oxidase activity, which generates hydrogen peroxide from molecular oxygen. We describe how different ferredoxins were also efficient reducing substrates for both flavodiiron protein and rubrerythrin.

CONCLUSIONS

The enzymatic systems herein characterized could contribute to the in vivo detoxification of O2 and H2O2, playing a key role for the parasite defense against reactive oxidant species.

GENERAL SIGNIFICANCE

To the best of our knowledge this is the first characterization of a eukaryotic rubredoxin reductase, including a novel kinetic study on ferredoxin-dependent reduction of flavodiiron and rubrerythrin proteins.

Molecular and biochemical characterization of Entamoeba histolytica fructokinase

Entamoeba histolytica is the causative agent of amoebic dysentery and liver abscess. The medium for its axenic culture contains glucose as energy source, and we addressed the question whether E. histolytica can also use fructose instead. As the amoebic hexokinases do not phosphorylate fructose, a separate fructokinase is essential. The genome project revealed a single candidate gene encoding an E. histolytica homolog of bacterial fructokinases. This gene was cloned, and the recombinant enzyme had a magnesium-dependent fructose 6-kinase activity (EC 2.7.1.4) with a K_m for fructose of 0.156 mM and a V_max of 131 U/mg protein. Recombinant fructokinase also showed a much weaker mannokinase activity, but no activity with glucose or galactose. The amoebae could be switched from glucose to fructose medium without any detectable consequence on doubling time. Fructokinase messenger RNA (mRNA) was modestly but significantly upregulated in amoebae switched to fructose medium as well as in fructose-adapted E. histolytica.

Mass spectrometric analysis of L-cysteine metabolism: physiological role and fate of L-cysteine in the enteric protozoan parasite Entamoeba histolytica

l-Cysteine is essential for virtually all living organisms, from bacteria to higher eukaryotes. Besides having a role in the synthesis of virtually all proteins and of taurine, cysteamine, glutathione, and other redox-regulating proteins, l-cysteine has important functions under anaerobic/microaerophilic conditions. In anaerobic or microaerophilic protozoan parasites, such as Entamoeba histolytica, l-cysteine has been implicated in growth, attachment, survival, and protection from oxidative stress. However, a specific role of this amino acid or related metabolic intermediates is not well understood. In this study, using stable-isotope-labeled l-cysteine and capillary electrophoresis-time of flight mass spectrometry, we investigated the metabolism of l-cysteine in E. histolytica. [U-¹³C3, ¹⁵N]l-cysteine was rapidly metabolized into three unknown metabolites, besides l-cystine and l-alanine. These metabolites were identified as thiazolidine-4-carboxylic acid (T4C), 2-methyl thiazolidine-4-carboxylic acid (MT4C), and 2-ethyl-thiazolidine-4-carboxylic acid (ET4C), the condensation products of l-cysteine with aldehydes. We demonstrated that these 2-(R)-thiazolidine-4-carboxylic acids serve for storage of l-cysteine. Liberation of l-cysteine occurred when T4C was incubated with amebic lysates, suggesting enzymatic degradation of these l-cysteine derivatives. Furthermore, T4C and MT4C significantly enhanced trophozoite growth and reduced intracellular reactive oxygen species (ROS) levels when it was added to cultures, suggesting that 2-(R)-thiazolidine-4-carboxylic acids are involved in the defense against oxidative stress.

Amebiasis is a human parasitic disease caused by the protozoan parasite Entamoeba histolytica. In this parasite, l-cysteine is the principal low-molecular-weight thiol and is assumed to play a significant role in supplying the amino acid during trophozoite invasion, particularly when the parasites move from the anaerobic intestinal lumen to highly oxygenated tissues in the intestine and the liver. It is well known that E. histolytica needs a comparatively high concentration of l-cysteine for its axenic cultivation. However, the reason for and the metabolic fate of l-cysteine in this parasite are not well understood. Here, using a metabolomic and stable-isotope-labeled approach, we investigated the metabolic fate of this amino acid in these parasites. We found that l-cysteine inside the cell rapidly reacts with aldehydes to form 2-(R)-thiazolidine-4-carboxylic acid. We showed that these 2-(R)-thiazolidine-4-carboxylic derivatives serve as an l-cysteine source, promote growth, and protect cells against oxidative stress by scavenging aldehydes and reducing the ROS level. Our findings represent the first demonstration of 2-(R)-thiazolidine-4-carboxylic acids and their roles in protozoan parasites.

Metabolomic analysis of Entamoeba: applications and implications

Entamoeba histolytica is an enteric protozoan parasite that causes hemorrhagic dysentery and extraintestinal abscesses in millions of inhabitants of endemic areas. The genome of E. histolytica has already been sequenced and used to predict the metabolic potential of the organism. Since nearly 56% of the E. histolytica genes remain unannotated, correlative 'omics' analyses of genomics, transcriptomics, proteomics, and biochemical metabolic profiling are essential in uncovering new, or poorly understood metabolic pathways. Metabolomics aims at understanding biology by comprehensive metabolite profiling. In this review, we discuss recent metabolomics approaches to elucidate unidentified metabolic systems of this pathogen and also discuss future applications of metabolomics to understand the biology and pathogenesis of E. histolytica.

Dramatic increase in glycerol biosynthesis upon oxidative stress in the anaerobic protozoan parasite Entamoeba histolytica

Entamoeba histolytica, a microaerophilic enteric protozoan parasite, causes amebic colitis and extra intestinal abscesses in millions of inhabitants of endemic areas. Trophozoites of E. histolytica are exposed to a variety of reactive oxygen and nitrogen species during infection. Since E. histolytica lacks key components of canonical eukaryotic anti-oxidative defense systems, such as catalase and glutathione system, alternative not-yet-identified anti-oxidative defense strategies have been postulated to be operating in E. histolytica. In the present study, we investigated global metabolic responses in E. histolytica in response to H₂O₂- and paraquat-mediated oxidative stress by measuring charged metabolites on capillary electrophoresis and time-of-flight mass spectrometry. We found that oxidative stress caused drastic modulation of metabolites involved in glycolysis, chitin biosynthesis, and nucleotide and amino acid metabolism. Oxidative stress resulted in the inhibition of glycolysis as a result of inactivation of several key enzymes, leading to the redirection of metabolic flux towards glycerol production, chitin biosynthesis, and the non-oxidative branch of the pentose phosphate pathway. As a result of the repression of glycolysis as evidenced by the accumulation of glycolytic intermediates upstream of pyruvate, and reduced ethanol production, the levels of nucleoside triphosphates were decreased. We also showed for the first time the presence of functional glycerol biosynthetic pathway in E. histolytica as demonstrated by the increased production of glycerol 3-phosphate and glycerol upon oxidative stress. We proposed the significance of the glycerol biosynthetic pathway as a metabolic anti-oxidative defense system in E. histolytica.

During the course of infection, trophozoites of E. histolytica need to cope with the oxidative stress in order to survive under the oxidative environment of its host. As a result of the absence of the key eukaryotic anti-oxidative defense system, it needs to employ novel defense strategies. Several studies such as transcriptomic profiling of trophozoites exposed to oxidative stress, and biochemical and functional analysis of individual proteins has been done in the past. Since, oxidative stress damages several metabolic enzymes, and modulate expression of many genes, it is important to analyze the detailed metabolomic response of E. histolytica upon oxidative stress to understand the role of metabolism in combating oxidative stress. In the present study, we demonstrated that oxidative stress causes glycolytic inhibition and redirection of metabolic flux towards glycerol production, chitin biosynthesis, and the non-oxidative branch of the pentose phosphate pathway.

Metabolome analysis revealed increase in S-methylcysteine and phosphatidylisopropanolamine synthesis upon L-cysteine deprivation in the anaerobic protozoan parasite Entamoeba histolytica

L-cysteine is ubiquitous in all living organisms and is involved in a variety of functions, including the synthesis of iron-sulfur clusters and glutathione and the regulation of the structure, stability, and catalysis of proteins. In the protozoan parasite Entamoeba histolytica, the causative agent of amebiasis, L-cysteine plays an essential role in proliferation, adherence, and defense against oxidative stress; however, the essentiality of this amino acid in the pathways it regulates is not well understood. In the present study, we applied capillary electrophoresis time-of-flight mass spectrometry to quantitate charged metabolites modulated in response to L-cysteine deprivation in E. histolytica, which was selected as a model for examining the biological roles of L-cysteine. L-cysteine deprivation had profound effects on glycolysis, amino acid, and phospholipid metabolism, with sharp decreases in the levels of L-cysteine, L-cystine, and S-adenosylmethionine and a dramatic accumulation of O-acetylserine and S-methylcysteine. We further demonstrated that S-methylcysteine is synthesized from methanethiol and O-acetylserine by cysteine synthase, which was previously considered to be involved in sulfur-assimilatory L-cysteine biosynthesis. In addition, L-cysteine depletion repressed glycolysis and energy generation, as it reduced acetyl-CoA, ethanol, and the major nucleotide di- and triphosphates, and led to the accumulation of glycolytic intermediates. Interestingly, L-cysteine depletion increased the synthesis of isopropanolamine and phosphatidylisopropanolamine, and it was confirmed that their increment was not a result of oxidative stress but was a specific response to L-cysteine depletion. We also identified a pathway in which isopropanolamine is synthesized from methylglyoxal via aminoacetone. To date, this study represents the first case where L-cysteine deprivation leads to drastic changes in core metabolic pathways, including energy, amino acid, and phospholipid metabolism.

Two atypical L-cysteine-regulated NADPH-dependent oxidoreductases involved in redox maintenance, L-cystine and iron reduction, and metronidazole activation in the enteric protozoan Entamoeba histolytica

We discovered novel catalytic activities of two atypical NADPH-dependent oxidoreductases (EhNO1/2) from the enteric protozoan parasite Entamoeba histolytica. EhNO1/2 were previously annotated as the small subunit of glutamate synthase (glutamine:2-oxoglutarate amidotransferase) based on similarity to authentic bacterial homologs. As E. histolytica lacks the large subunit of glutamate synthase, EhNO1/2 were presumed to play an unknown role other than glutamine/glutamate conversion. Transcriptomic and quantitative reverse PCR analyses revealed that supplementation or deprivation of extracellular L-cysteine caused dramatic up- or down-regulation, respectively, of EhNO2, but not EhNO1 expression. Biochemical analysis showed that these FAD- and 2[4Fe-4S]-containing enzymes do not act as glutamate synthases, a conclusion which was supported by phylogenetic analyses. Rather, they catalyze the NADPH-dependent reduction of oxygen to hydrogen peroxide and L-cystine to L-cysteine and also function as ferric and ferredoxin-NADP(+) reductases. EhNO1/2 showed notable differences in substrate specificity and catalytic efficiency; EhNO1 had lower K(m) and higher k(cat)/K(m) values for ferric ion and ferredoxin than EhNO2, whereas EhNO2 preferred L-cystine as a substrate. In accordance with these properties, only EhNO1 was observed to physically interact with intrinsic ferredoxin. Interestingly, EhNO1/2 also reduced metronidazole, and E. histolytica transformants overexpressing either of these proteins were more sensitive to metronidazole, suggesting that EhNO1/2 are targets of this anti-amebic drug. To date, this is the first report to demonstrate that small subunit-like proteins of glutamate synthase could play an important role in redox maintenance, L-cysteine/L-cystine homeostasis, iron reduction, and the activation of metronidazole.

Structure and content of the Entamoeba histolytica genome

The intestinal parasite Entamoeba histolytica is one of the first protists for which a draft genome sequence has been published. Although the genome is still incomplete, it is unlikely that many genes are missing from the list of those already identified. In this chapter we summarise the features of the genome as they are currently understood and provide previously unpublished analyses of many of the genes.

Current therapeutics, their problems, and sulfur-containing-amino-acid metabolism as a novel target against infections by "amitochondriate" protozoan parasites

The "amitochondriate" protozoan parasites of humans Entamoeba histolytica, Giardia intestinalis, and Trichomonas vaginalis share many biochemical features, e.g., energy and amino acid metabolism, a spectrum of drugs for their treatment, and the occurrence of drug resistance. These parasites possess metabolic pathways that are divergent from those of their mammalian hosts and are often considered to be good targets for drug development. Sulfur-containing-amino-acid metabolism represents one such divergent metabolic pathway, namely, the cysteine biosynthetic pathway and methionine gamma-lyase-mediated catabolism of sulfur-containing amino acids, which are present in T. vaginalis and E. histolytica but absent in G. intestinalis. These pathways are potentially exploitable for development of drugs against amoebiasis and trichomoniasis. For instance, L-trifluoromethionine, which is catalyzed by methionine gamma-lyase and produces a toxic product, is effective against T. vaginalis and E. histolytica parasites in vitro and in vivo and may represent a good lead compound. In this review, we summarize the biology of these microaerophilic parasites, their clinical manifestation and epidemiology of disease, chemotherapeutics, the modes of action of representative drugs, and problems related to these drugs, including drug resistance. We further discuss our approach to exploit unique sulfur-containing-amino-acid metabolism, focusing on development of drugs against E. histolytica.

Sulfur-Containing Amino Acid Metabolism in Parasitic Protozoa

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

The protistan parasite Perkinsus marinus is resistant to selected reactive oxygen species

The parasite Perkinsus marinus has devastated natural and farmed oyster populations along the Atlantic and Gulf coasts of North America. When viable P. marinus trophozoites are engulfed by oyster hemocytes, the typical accumulation of reactive oxygen species (ROS) normally associated with phagocyte activity is not observed. One hypothesis to explain this is that the parasite rapidly removes ROS. A manifestation of efficient ROS removal should be a high level of resistance to exogenous ROS. We investigated the in vitro susceptibility of P. marinus to ROS as compared to the estuarine bacterium Vibrio splendidus. We find that P. marinus is markedly less susceptible than V. splendidus to superoxide and hydrogen peroxide (H(2)O(2)), but equally sensitive to hypochlorite. Viable P. marinus trophozoites degrade H(2)O(2) in vitro, but lack detectable catalase activity. However, extracts contain an ascorbate dependent peroxidase activity that may contribute to H(2)O(2) removal in vitro and in vivo.

The unusual transhydrogenase of Entamoeba histolytica

We have expressed and purified a protein fragment from Entamoeba histolytica. It catalyses transhydrogenation between analogues of NAD(H) and NADP(H). The characteristics of this reaction resemble those of the reaction catalysed by a complex of the NAD(H)- and NADP(H)-binding subunits of proton-translocating transhydrogenases from bacteria and mammals. It is concluded that the complete En. histolytica protein, which, along with similar proteins from other protozoan parasites, has an unusual subunit organisation, is also a proton-translocating transhydrogenase. The function of the transhydrogenase, thought to be located in organelles which do not have the enzymes of oxidative phosphorylation, is not clear.

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.

Analysis of expressed sequence tags (ESTs) of the parasitic protozoa Entamoeba histolytica

A directional cDNA library constructed from mRNA of the trophozoite of Entamoeba histolytica HM-1:IMSS strain was used for the generation of expressed sequence tags (ESTs). From 5' ends of the distinct cDNA clones, 105 ESTs were obtained. Of these, 30 clones (29%) were previously known E. histolytica genes. Forty-five clones (42%) had matches with entries for other organisms in the databases. These new E. histolytica genes constituted a broad range of transcripts distributed among cytoplasmic structural and regulatory proteins, enzymes, nuclear and other proteins, and proteins of unknown function. Thirty clones (29%) had no significant database matches and thus potentially represent E. histolytica-specific genes. These data of E. histolytica genes identified by nucleotide sequencing indicate the value of the adoption of genome sequencing strategies for the rapid increase in knowledge of organisms causing dysentery and liver abscess.

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.

Oxido-reductive functions of Entamoeba histolytica in relation to virulence

Entamoeba histolytica can reduce nitro-blue tetrazolium (NBT) in Hank's balanced salt solution to almost the same extent as in Eagle's minimal medium. Further, this reduction was stimulated only to a minor degree by glucose, pyruvate and DL-serine, substrates known to support respiratory activity (O2 uptake) in E. histolytica. However, both NADH and NADPH increased NBT reduction several-fold, the effect being greater with NADPH. A sizeable proportion of this endogenous dye-reducing capability (in Hank's solution) was associated with low-speed sediments obtainable from amoebic homogenates, which also shared the bulk of 125I labelling (when the homogenates were prepared after surface labelling with Na 125I). Conversion of the dye to formazan was strongly inhibited by -SH blocking agents, but was not influenced by rotenone and antimycin A. The activity was also inhibited by H2O2, but stimulated by catalase. Superoxide dismutase only slightly curtailed NBT reduction in intact cells, but inhibited it in homogenates in a concentration-dependent manner to a maximal extent of 33%. Almost the same degree of curtailment of this activity was induced by anaerobic conditions. Both concanavalin A (Con A) and phorbol myristate acetate stimulated the activity in intact cells, though the effect of Con A was nullified by alpha-methyl mannoside. Both NBT-reducing capability and alcohol dehydrogenase activities were higher in the more virulent IP:106 strain, and they increased with time in cultures of NIH:200 in a cholesterol-enriched environment.

Novel biochemical pathways in parasitic protozoa

Throughout evolution, enzymes and their metabolites have been highly conserved. Parasites are no exception to this and differ most markedly by the absence of metabolic pathways that are present in the mammalian host. In general, parasites are metabolically lazy and rely on the metabolism of the host both for a supply of prefabricated components such as purines, fatty acids, sterols and amino acids and for the removal of end-products. Nonetheless, parasites are metabolically highly sophisticated in that (1) they retain the genetic capacity to induce many pathways, when needed, and (2) they have developed complex mechanisms for their survival in the host. Certain unique features of the metabolism of trypanosomes, leishmania, malaria and anaerobic protozoa will be discussed. This will include (1) glycolysis and electron transport with reference to the unique organelles: the glycosome and the hydrogenosome, (2) purine salvage, pyrimidine biosynthesis and folic acid metabolism and (3) polyamine and thiol metabolism with special reference to the role of the unique metabolite of trypanosomes and leishmanias, trypanothione.

Nucleotide sequence analysis of an Entamoeba histolytica ferredoxin gene

A cDNA clone (subclone B) previously isolated from the human parasite Entamoeba histolytica was characterized. DNA sequence analysis of subclone B identified the DNA as that encoding apoferredoxin. E. histolytica ferredoxin cDNA contains unusually short 5' and 3' noncoding regions of 9 and 25 nucleotides, respectively. A genomic ferredoxin clone was isolated from E. histolytica DNA, and comparison of genomic and cDNA sequences revealed that the ferredoxin gene is unspliced. The deduced amino acid sequence of E. histolytica ferredoxin resembles clostridial type of ferredoxins, and shows an arrangement of cysteines characteristic for the coordination of 2[4Fe-4S] centres. Of interest is the absence of an aromatic amino acid in the N-terminal region of the protein, a feature which is conserved in clostridial ferredoxins. Southern blot analysis of three different E. histolytica strains (200:NIH, Rahman and HM-1:IMSS) demonstrated the presence of a family of at least two ferredoxin genes. One of these genes is marked by restriction length polymorphisms in different strains of E. histolytica.

Reducing agents and Entamoeba histolytica

Entamoeba histolytica remains an important but enigmatic parasite. It displays both non-pathogenic and invasive pathogenic types, which can be distinguished clinically and by isoenzyme markers. Yet as debated in Parasitology Today last year(1), the relationship between these two forms remains unclear. Bacterial associates and reducing agents are known to play on important role in the culture of E. histolytica, and possibly in its differentiation and invasive mechanisms. This article briefly reviews available information on the role o f reducing agents, and explores the possibility that bacteria may play a role in reduction o f toxic oxygen product - thereby promoting the virulence of E. histolytica. The review is not definitive, but should help to stimulate further research in this neglected area.

Calcium transport and catabolism of adenosine triphosphate in the protozoan parasite Giardia lamblia

1. Calcium uptake by washed trophozoites of Giardia lamblia was dependent on inorganic orthophosphate and stimulated by glucose. Uptake was both rapid and substantial: 224 +/- 73 nmoles Ca2+/mg protein/min. 2. Known inhibitors of Ca2+ uptake in mammalian cells also impeded Ca2+ influx into G. lamblia. 3. The inhibitor studies indicated that Ca2+ transport in G. lamblia was an active process. Energy for such a process could be provided by the action of ATPases. 4. Two types of ATPases were found in the parasite; one, a membrane-associated enzyme activated by Ca2+; the other, a soluble, cytosolic enzyme activated by Mg2+. 5. These enzymes differed not only in their intracellular distribution and divalent cation requirements, but also in their sensitivity to calmodulin antagonists. The particulate enzyme was sensitive to these inhibitors whereas the soluble ATPase was not. 6. Our data indicate that Ca2+ transport in G. lamblia is mediated by a membrane-bound, calmodulin-regulated, Ca2+-ATPase.

Ameba-bacterium relationship in amebiasis

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Susceptibility of Entamoeba histolytica to oxidants

The effect of hydrogen peroxide and hypochlorite on culture forms of Entamoeba histolytica trophozoites was examined by using two strains of E. histolytica, virulent (IP:0682:1) and nonvirulent (DKB). The amoebae were incubated with various concentrations of hydrogen peroxide and hypochlorite, and their viability was determined at different times after incubation. When the viability of the virulent and nonvirulent strains was compared to different oxidant strengths, it became apparent that the virulent strain was less susceptible than the nonvirulent one to the cytotoxic effect of hydrogen peroxide and hypochlorite. Our studies further showed that the toxic effect was both time and dose dependent. To confirm that the killing of amoebae in this system was associated with the presence of hydrogen peroxide, amoebae were incubated with hydrogen peroxide and catalase. Catalase reduced the killing effect of hydrogen peroxide to the control level. These data confirmed previous observations of the susceptibility of amoebic trophozoites to hydrogen peroxide and also demonstrated susceptibility to hypochlorite.

[Electron-microscopic studies on fine structure and enzyme activity in the axenic and conventional strains of Entamoeba histolytica]

The metabolism of Entamoeba histolytica would be affected by various environmental factors, and alteration of the environment was known to affect the fine structure of E. histolytica. The present study was designed electronmicroscopically to investigate the ultrastructure and enzyme activities in the axenic and conventional strains of E. histolytica. The trophozoites of axenically cultivated HK-9 strain and conventional YS-27 and YS-49 strains of E. histolytica were collected and fixed with 4 percent paraformaldehyde/0.1 M cacodylate buffer (pH 7.4). After washing them by centrifugation, 1 percent warm agar was added in the sediment. Solidified agar with the trophozoites was cut into 1 mm(3) cubes, and incubated in the various substrates to observe enzyme activities. Then, the specimen was post-fixed with 3 percent glutaraldehyde/0.1 M cacodylate buffer (pH 7.4) and 1 percent osmium tetroxide/0.1 M cacodylate buffer (pH 7.4), dehydrated in ascending ethanol series and embedded in epoxy resin. These were sectioned on an ultramicrotome and observed with a transmission electron microscope. The procedures for the observation of the fine structure were same as the above, except for the incubation in the substrate. The sections were stained with uranyl scetate and lead citrate. For the observation of the surface of the amoebae, scanning electron microscopy was carried out. The results obtained in the present study are summarized as follows: 1. The fuzzy coat around double-layered plasma membrane of E. histolytica was more irregularly and densely distributed in the conventional strains (YS-27, YS-49 strains) than in the axenic strain (HK-9 strain). 2. The endosomes, button bodies and chromatin material were surrounded by a double-layered nuclear membrane having scattered nuclear pores. The paranuclear body, mono- or double-layered vacuoles, vacuolar membrane whorls, rosette-like cylindrical bodies, aggregation of cylindrical bodies and helical bodies were found in the cytoplasm of the amoebae. Helical bodies and glycogen granules were generally abundant, while a few smooth endoplasmic reticula were observed in the cytoplasm. 3. Alkaline phosphatase activity was mainly demonstrated in the plasma membrane, limiting membranes of vacuoles and smooth endoplasmic reticula. ATPase activity was observed in the nucleus, limiting membranes of vacuoles and vacuolar membrane whorls. 4. Acid phosphatase activity was commonly demonstrated in the limiting membranes an contents of vacuoles, lysosome-like organelles, plasma membrane and the button bodies in the nucleus. The activity was more weakly demonstrated in the HK-9 strain than in the other conventional strains of E. histolytica. No peroxidase activity was observed in the amoeba strains employed in the present study. 5. With a scanning electron microscope, no distinct structural differences were observed between the amoeba strains. All the trophozoite forms of the amoebae showed crater-like depressions and rugged features on the outer surface.

Entamoeba histolytica: inhibition in vitro by bithionol of respiratory activity and growth

Endogenous and 2-propanol-supported respiration of intact trophozoites of of Entamoeba histolytica (stain HM-1:IMSS) were inhibited by bithionol, an effective chemotherapeutic agent for some trematode and cestode infections in humans. Dichlorophene and hexachlorophene also inhibited 2-propanol-supported respiration of the parasite. In contrast, ethanol formation by E. histolytica extract in the presence of N2 was scarcely inhibited by bithionol. The compound also inhibited in vitro growth of axenic (HM-1 strain) and polyxenic (strain HJ-1:KEIO) amoebae in culture. It took less than 24 hr to kill and disrupt virtually all amoebae of either strain with 0.28 mM bithionol. Omission of bovine serum from BI-S-33 medium resulted in considerably less disruption of HM-1 strain amoebae by the compound. However, organisms that looked undisrupted were strained with trypan blue. Moreover, the number of amoebae incubated for 10 min in the serum-free BI-S-33 medium containing 0.14 mM bithionol did not increase, even after incubation for 24 hr following replacement of the experimental culture fluid with fresh complete BI-S-33 medium free of the compound. These findings suggest that, although serum appears to diminish the antiamoebic action, some halogenated bisphenols (in particular bithionol) may be useful for treatment of amoebiasis.

Virulence of Entamoeba histolytica trophozoites. Effects of bacteria, microaerobic conditions, and metronidazole

The association of axenically grown trophozoites of Entamoeba histolytica strains HK-9 or HM-1:IMSS with various types of gram-negative bacteria for relatively short periods markedly increased their virulence, as evidenced by their ability to destroy monolayers of tissue-cultured cells. Interaction of trophozoites with bacteria that were heat inactivated, glutaraldehyde fixed, or disrupted by sonication, or bacteria treated with inhibitors of protein synthesis, did not augment amebic virulence. Lethally irradiated bacteria, however, retained their stimulative properties and trophozoites that ingested bacteria were protected from the toxic effects of added hydrogen peroxide. An increase in virulent properties of amebae was also found in experiments carried out under microaerobic conditions (5% O2, 10% CO2). The augmentation of amebic virulence due to association with bacteria was specifically blocked by metronidazole, but not by tetracycline or aminoglycosides, and the rate of metronidazole uptake in stimulated trophozoites was two to three times higher. The results obtained suggest that virulence of axenically grown E. histolytica trophozoites may depend to a considerable extent on the cell's reducing power. Both microaerobic conditions and the association with bacteria apparently stimulate the electron transport system of the ameba. Bacteria may function as broad range scavengers for oxidized molecules and metabolites through the contribution of enzymatic systems, components, or products.

Susceptibility of Entamoeba histolytica to oxygen intermediates

To explore the susceptibility of the extracellular protozoan, Entamoeba histolytica, to toxic oxygen intermediates, trophozoites were exposed to fluxes of O2, H2O2, and OH. generated enzymatically by the glucose oxidase and xanthine oxidase reactions. HM-1 trophozoites were resistant to O2, but were readily killed by H2O2 alone. OH. and 1O2 were not required for effective amebicidal activity. The addition of peroxidase and halide enhanced trophozoite killing by H2O2. Sonicates of amebae contained virtually no catalase and little glutathione peroxidase activity which may contribute to susceptibility to H2O2. Coupled with our previous studies with Toxoplasma gondii and Leishmania spp. these observations indicate that there is a broad spectrum of susceptibility of intra- and extracellular pathogenic protozoa to killing by oxygen intermediates.

Metronidazole metabolism in cultures of Entamoeba histolytica and Trichomonas vaginalis

Acetamide forms from metronidazole in cultures of either Entamoeba histolytica or Trichomonas vaginalis as it does in cultures of bacteria which are susceptible to this drug. Under aerobic conditions, the formation of acetamide is more rapid in a strain of T. vaginalis which is more susceptible to metronidazole. Thus, it appears that the formation of acetamide may be associated with the microbiocidal action of metronidazole.

Purification and properties of NADPH:flavin oxidoreductase from Entamoeba histolytica

Amebal NADPH:flavin oxidoreductase was purified to apparent homogeneity. Molecular weights of 40 000 and 38 000 were estimated by gel filtration and by sodium dodecyl sulfate polyacrylamide gel electrophoresis, respectively, indicating that the enzyme is composed of a single polypeptide chain. The enzyme does not contain firmly bound flavin. It exhibited 20-fold selectivity for NADPH over NADH. With the former donor it reduced riboflavin, galactoflavin, FMN, or FAD. Aerobically the reducing equivalents were passed from reduced flavin to oxygen to form hydrogen peroxide. Intact amebae do not produce peroxide when they respire. If the title enzyme functions to reduce flavin in the intact cells some electron carrier must intervene between reduced flavin and oxygen so that the final step produces water instead of peroxide.

Energy metabolism of the anaerobic protozoon Giardia lamblia

Cells of the aerotolerant anaerobe Giardia lamblia respire in the presence of oxygen. Endogenous respiration is stimulated by glucose but not by other carbohydrates and Krebs cycle intermediates. Endogenous and glucose-stimulated respiration are insensitive to cyanide, malonate, and 2,4-dinitrophenol, but are inhibited by atabrin and iodoacetamide. G. lamblia produces ethanol, acetate and CO2 both aerobically and anaerobically either from endogenous reserves or exogenous glucose. Molecular hydrogen is not produced. The following enzyme activities were detected in homogenates: hexokinase, fructose-biphosphate aldolase, pyruvate kinase, phosphoenolpyruvate carboxykinase, malate dehydrogenase, malate dehydrogenase (decarboxylating), pyruvate synthase, acetyl-CoA synthetase, alcohol dehydrogenase (NADP+), NADH dehydrogenase, NADPH dehydrogenase, NADPH oxidoreductase and superoxide dismutase. The enzymes of energy and carbohydrate metabolism are nonsedimentable (109 000 x g for 30 min). Activities of lactate dehydrogenase, hydrogenase, phosphate acetyltransferase, acetate kinase, citrate synthase, succinate dehydrogenase, fumarate hydratase and catalase were below the limits of detection. The results suggest the occurrence of glycolysis, energy production by substrate level phosphorylation and a flavin, iron-sulfur protein mediated electron transport system as well as the absence of cytochrome mediated oxidative phosphorylation and functional Krebs cycle.

Effects of 2,4-dinitrophenol on trichomonads and Entamoeba invadens

1. 2,4-Dinitrophenol (2,4-DNP) in substrate level concentrations (200 microM-1 mM) temporarily inhibits H2 production by Tritrichomonas foetus and Trichomonas vaginalis as well as the accumulation of metronidazole, dependent on its reduction by the two trichomonad species and by Entamoeba invadens. 2. 2,4-DNP competes for the reducing equivalents which are necessary for H2 production or for the reduction of metronidazole, thereby inhibiting these processes. 2,4-DNP is reduced to 2-amino, 4-nitrophenol. 3. 2,4-DNP in concentrations up to 800 microM has no effect on the uptake of O2 by these organisms. 4. 2,4-DNP has some toxicity for T. foetus.

Mechanism of L-serine oxidation in Entamoeba histolytica

1. The enzymatic mechanism of oxygen uptake elicited by L-serine in axenically cultivated trophozoites of Entamoeba histolytica was investigated. 2. Of 22 amino acids examined, only L-serine stimulated oxygen consumption by intact and disrupted amoebae. 3. Pyruvate, a product of serine metabolism, also stimulated oxygen consumption in the amoebae. 4. Characterization of the oxygen uptake elicited by both L-serine and pyruvate, and analysis of the products of L-serine metabolism indicate that the amino acid is first converted to pyruvate. 5. L-Serine dehydratase, which catalyzes the deamination of serine to pyruvate, was detected primarily in the soluble fraction of the amoebae. D-Serine potently inhibited the enzyme, as well as oxygen uptake in the presence of L-serine but not in the presence of pyruvate. 6. The pyruvate formed is oxidized, at least in part, by a novel pyruvate oxidase involving the uptake of molecular oxygen.