Novel biochemical pathways in parasitic protozoa

The coiled-coil domain of glycosomal membrane-associated Leishmania donovani PEX14: cloning, overexpression, purification and preliminary crystallographic analysis

The glycosomal membrane-associated Leishmania donovani protein PEX14, which plays a crucial role in protein import from the cytosol to the glycosomal matrix, consists of three domains: an N-terminal domain where the signalling molecule binds, a transmembrane domain and an 84-residue coiled-coil domain (CC) that is responsible for oligomerization. CCs are versatile domains that participate in a variety of functions including supramolecular assembly, cellular signalling and transport. Recombinant PEX14 CC was cloned, overexpressed, affinity-purified with in-column thrombin cleavage and further purified by size-exclusion chromatography. Crystals that diffracted to 1.98 Å resolution were obtained from a condition consisting of 1.4 M sodium citrate tribasic dihydrate, 0.1 M HEPES buffer pH 7.5. The crystals belonged to the monoclinic space group C2, with unit-cell parameters a = 143.98, b = 32.62, c = 95.62 Å, β = 94.68°. Structure determination and characterization are in progress.

IDO, COX and iNOS have an important role in the proliferation of Neospora caninum in neuron/glia co-cultures

Central nervous system (CNS) is the main site for encystment of Neospora caninum in different animal species. In this tissue, glial cells (astrocytes and microglia) modulate responses to aggression in order to preserve homeostasis and neuronal function. Previous data showed that when primary cultures of glial cells are infected with N. caninum, they develop gliosis and the immune response is characterized by the release of TNF and IL-10, followed by the control of parasite proliferation. In order to elucidate this control, three enzymatic systems involved in parasite-versus-host interactions were observed on a model of neuron/glia co/cultures obtained from rat brains. Indoleamine 2,3-dioxygenase (IDO), induced nitric oxide synthase (iNOS) responsible for the catabolism of tryptophan and arginine, respectively, and cycloxigenase (COX) were studied comparing their modulation by respective inhibitors with the number of tachyzoites or the immune response measured by the release of IL-10 and TNF. Cells were treated with the inhibitors of iNOS (1.5 mM L-NAME), IDO (1 mM 1-methyl tryptophan), COX-1 (1 μM indomethacin) and COX-2 (1 μM nimesulide) before infection with tachyzoites of N. caninum (1:1 cell: parasite). After 72 h of infection, immunocytochemistry showed astrogliosis and a significant increase in the number and length of neurites, compared with uninfected co-cultures, while an increase of IL-10 and TNF was verified. N. caninum did not change iNOS activity, but the inhibition of the basal levels of this enzyme stimulated parasite proliferation. Additionally, a significant increase of about 40% was verified in the IDO activity, whose inhibition caused 1.2-fold increase in parasitic growth. For COX-2 activity, infection of cultures stimulated a significant increase in release of PGE₂ and its inhibition by nimesulide allowed the parasitic growth. These data indicate that iNOS, IDO and COX-2 control the proliferation of N. caninum in this in vitro model. On the other hand, the release of IL-10 by glia besides modulating the inflammation also allow the continuity of parasitism.

A Molecular Window into the Biology and Epidemiology of Pneumocystis spp

Pneumocystis, a unique atypical fungus with an elusive lifestyle, has had an important medical history. It came to prominence as an opportunistic pathogen that not only can cause life-threatening pneumonia in patients with HIV infection and other immunodeficiencies but also can colonize the lungs of healthy individuals from a very early age. The genus Pneumocystis includes a group of closely related but heterogeneous organisms that have a worldwide distribution, have been detected in multiple mammalian species, are highly host species specific, inhabit the lungs almost exclusively, and have never convincingly been cultured in vitro, making Pneumocystis a fascinating but difficult-to-study organism. Improved molecular biologic methodologies have opened a new window into the biology and epidemiology of Pneumocystis. Advances include an improved taxonomic classification, identification of an extremely reduced genome and concomitant inability to metabolize and grow independent of the host lungs, insights into its transmission mode, recognition of its widespread colonization in both immunocompetent and immunodeficient hosts, and utilization of strain variation to study drug resistance, epidemiology, and outbreaks of infection among transplant patients. This review summarizes these advances and also identifies some major questions and challenges that need to be addressed to better understand Pneumocystis biology and its relevance to clinical care.

The Architecture of Thiol Antioxidant Systems among Invertebrate Parasites

The use of oxygen as the final electron acceptor in aerobic organisms results in an improvement in the energy metabolism. However, as a byproduct of the aerobic metabolism, reactive oxygen species are produced, leaving to the potential risk of an oxidative stress. To contend with such harmful compounds, living organisms have evolved antioxidant strategies. In this sense, the thiol-dependent antioxidant defense systems play a central role. In all cases, cysteine constitutes the major building block on which such systems are constructed, being present in redox substrates such as glutathione, thioredoxin, and trypanothione, as well as at the catalytic site of a variety of reductases and peroxidases. In some cases, the related selenocysteine was incorporated at selected proteins. In invertebrate parasites, antioxidant systems have evolved in a diversity of both substrates and enzymes, representing a potential area in the design of anti-parasite strategies. The present review focus on the organization of the thiol-based antioxidant systems in invertebrate parasites. Differences between these taxa and its final mammal host is stressed. An understanding of the antioxidant defense mechanisms in this kind of parasites, as well as their interactions with the specific host is crucial in the design of drugs targeting these organisms.

In silico work flow for scaffold hopping in Leishmania

BACKGROUND

Leishmaniasis,a broad spectrum of diseases caused by several sister species of protozoa belonging to family trypanosomatidae and genus leishmania , generally affects poorer sections of the populace in third world countries. With the emergence of strains resistant to traditional therapies and the high cost of second line drugs which generally have severe side effects, it becomes imperative to continue the search for alternative drugs to combat the disease. In this work, the leishmanial genomes and the human genome have been compared to identify proteins unique to the parasite and whose structures (or those of close homologues) are available in the Protein Data Bank. Subsequent to the prioritization of these proteins (based on their essentiality, virulence factor etc.), inhibitors have been identified for a subset of these prospective drug targets by means of an exhaustive literature survey. A set of three dimensional protein-ligand complexes have been assembled from the list of leishmanial drug targets by culling structures from the Protein Data Bank or by means of template based homology modeling followed by ligand docking with the GOLD software. Based on these complexes several structure based pharmacophores have been designed and used to search for alternative inhibitors in the ZINC database.

RESULT

This process led to a list of prospective compounds which could serve as potential antileishmanials. These small molecules were also used to search the Drug Bank to identify prospective lead compounds already in use as approved drugs. Interestingly, paromomycin which is currently being used as an antileishmanial drug spontaneously appeared in the list, probably giving added confidence to the 'scaffold hopping' computational procedures adopted in this work.

CONCLUSIONS

The report thus provides the basis to experimentally verify several lead compounds for their predicted antileishmanial activity and includes several useful data bases of prospective drug targets in leishmania, their inhibitors and protein--inhibitor three dimensional complexes.

Explicit consideration of topological and parameter uncertainty gives new insights into a well-established model of glycolysis

Previous models of glycolysis in the sleeping sickness parasite Trypanosoma brucei assumed that the core part of glycolysis in this unicellular parasite is tightly compartimentalized within an organelle, the glycosome, which had previously been shown to contain most of the glycolytic enzymes. The glycosomes were assumed to be largely impermeable, and exchange of metabolites between the cytosol and the glycosome was assumed to be regulated by specific transporters in the glycosomal membrane. This tight compartmentalization was considered to be essential for parasite viability. Recently, size-specific metabolite pores were discovered in the membrane of glycosomes. These channels are proposed to allow smaller metabolites to diffuse across the membrane but not larger ones. In light of this new finding, we re-analyzed the model taking into account uncertainty about the topology of the metabolic system in T. brucei, as well as uncertainty about the values of all parameters of individual enzymatic reactions. Our analysis shows that these newly-discovered nonspecific pores are not necessarily incompatible with our current knowledge of the glycosomal metabolic system, provided that the known cytosolic activities of the glycosomal enzymes play an important role in the regulation of glycolytic fluxes and the concentration of metabolic intermediates of the pathway.

Polymorphism of the parasite lactate dehydrogenase gene from Plasmodium vivax Korean isolates

Background

Assaying for the parasitic lactate dehydrogenase (pLDH) is widely used as a rapid diagnostic test (RDT), but the efficacy of its serological effectiveness in diagnosis, that is antibody detection ability, is not known. The genetic variation of Korean isolates was analysed, and recombinant protein pLDH was evaluated as a serodiagnostic antigen for the detection of Plasmodium vivax malaria.

Methods

Genomic DNA was purified, and the pLDH gene of P. vivax was amplified from blood samples from 20 patients. The samples came from five epidemic areas: Bucheon-si, Gimpo-si, and Paju-si of Gyeonggi Province, Gangwha-gun of Incheon metropolitan city, and Cheorwon-gun of Gangwon Province, South Korea, from 2010 to 2011. The antigenicity of the recombinant protein pLDH was tested by western blot and enzyme-linked immunosorbent assay (ELISA).

Results

Sequence analysis of 20 Korean isolates of P. vivax showed that the open reading frame (ORF) of 951 nucleotides encoded a deduced protein of 316 amino acids (aa). This ORF showed 100% identity with the P. vivax Belem strain (DQ060151) and P. vivax Hainan strain (FJ527750), 89.6% homology with Plasmodium falciparum FCC1_HN (DQ825436), 90.2% homology with Plasmodium berghei (AY437808), 96.8% homology with Plasmodium knowlesi (JF958130), and 90.2% homology with Plasmodium reichenowi (AB122147). A single-nucleotide polymorphism (SNP) at nucleotide 456 (T to C) was also observed in the isolate from Bucheon, but it did not change in the amino acid sequence. The expressed recombinant protein had a molecular weight of approximately 32 kDa, as analysed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis. Of the 40 P. vivax patients, 34 (85.0%) were positive by ELISA.

Conclusions

The pLDH genes of 19 isolates of P. vivax were identical, except one for SNP at nucleotide 456. This observation indicates that this gene is relatively stable. Based on these results, the relationship between antibody production against pLDH and the pattern of disease onset should be investigated further before using pLDH for serodiagnosis.

Contribution of dietary nitrogen and purine bases to the duodenal digesta: comparison of duodenal and polyester-bag measurements

Four ewes fitted with ruminal and duodenalT-piece cannulas were given fourdietsin a 4 × 4 factorial design. Diets consisted of 700 (HF) or 400 (LF) g/day of ammonia-treated barley straw supplemented respectively with 150 or 600 g/day of concentrate made up with barley plus either soya-bean meal (SBM) or fishmeal (FM) as the protein source, offered at 2-h intervals. Duodenal flowsof digestawere estimated by the dual-phasetechniqueusing CoEDTAand Yb-acetate as markers and (15NH4)2SO4 was infusedinto the rumento label microbial N. Bacteria were isolated from the liquid (LAB) or solid (SAB) rumendigesta. Purinebases (PB) were isolated by precipitationin an acid solution of AgN03, and microbial contribution either to the duodenalnitrogen(N) or PB were determinedby 15N measurements induodenaldigesta and bacteria.Simultaneously, therumen degradation of Nand PB contained in SBM and FM was studiedby incubating supplements in polyesterbags in the rumen.PBcontent (mmol/g dry matter)and guanine: adenine(G/A) ratio of barley strawwas 2·89 and 5·23; barley grain,7·91 and 111;SBM, 18·8 and 1·26; and FM, 58·9 and 6·96, respectively. Duodenal flow ofPB(mmol/day)was significantly higher than PB intake on all diets and G/A ratio showed a meanvalue of 0·97, similarto the ratios determined in SAB(0·80) and LAB (1·04) and muchlower than diets(1·31 to 4·32). Microbial contribution to duodenal Nflow ranged from43·3% to 61·0%, beinghigherin SBM(59·0%)thanin FM(46·7%)diets. However, microbial contribution to duodenal PB was not affected by the experimentaltreatment, accounting for proportionately 0·77 of total PB at the duodenum. Rumen degradability of PB was much higher than that of total N and in both cases degradability was higher in SBM than FM. Direct measurements of non-microbialN were significantly higher than values determined by the polyester-bagmeasurements. However, once corrected forthe endogenousN (52 mgN per kg live weight)contribution, results show edan acceptable agreement. Duodenal flow of PB non-attributable to microbes (unlabelled PB) showed a mean value of 3·25 mmol/daywithouta significanteffect of dietary treatment. However, undegradablePBsupply determinedfor0·02, 0·05 and 0·08 per h fractional out flow rates were proportionately lower than 0·025 with SBM and 0·100 with FM diets of the estimated duodenalPB flow. Despite the magnitudeof the unlabelledduodenalPB, the close agreement between G/A ratios in duodenaldigesta and bacteria suggests thatthe contribution of dietary PB to the duodenalflow was low and seemsto confirm the reliability of values obtained from polyester-bag measurements.

Comparative genomics of metabolic networks of free-living and parasitic eukaryotes

Background

Obligate endoparasites often lack particular metabolic pathways as compared to free-living organisms. This phenomenon comprises anabolic as well as catabolic reactions. Presumably, the corresponding enzymes were lost in adaptation to parasitism. Here we compare the predicted core metabolic graphs of obligate endoparasites and non-parasites (free living organisms and facultative parasites) in order to analyze how the parasites' metabolic networks shrunk in the course of evolution.

Results

Core metabolic graphs comprising biochemical reactions present in the presumed ancestor of parasites and non-parasites were reconstructed from the Kyoto Encyclopedia of Genes and Genomes. While the parasites' networks had fewer nodes (metabolites) and edges (reactions), other parameters such as average connectivity, network diameter and number of isolated edges were similar in parasites and non-parasites. The parasites' networks contained a higher percentage of ATP-consuming reactions and a lower percentage of NAD-requiring reactions. Control networks, shrunk to the size of the parasites' by random deletion of edges, were scale-free but exhibited smaller diameters and more isolated edges.

Conclusions

The parasites' networks were smaller than those of the non-parasites regarding number of nodes or edges, but not regarding network diameters. Network integrity but not scale-freeness has acted as a selective principle during the evolutionary reduction of parasite metabolism. ATP-requiring reactions in particular have been retained in the parasites' core metabolism while NADH- or NADPH-requiring reactions were lost preferentially.

Polyamine metabolism in Trypanosoma cruzi: studies on the expression and regulation of heterologous genes involved in polyamine biosynthesis

Biochemical studies have shown that Trypanosoma cruzi and Toxoplasma gondii are the only eukaryotic organisms so far described which are auxotrophic for polyamines. Both parasites are unable to carry out the de novo biosynthesis of putrescine, and therefore they need the addition of exogenous polyamines to the culture medium for their normal proliferation. Further investigations at the molecular level have demonstrated that the wild-type T. cruzi genome does not contain ornithine or arginine decarboxylase-like nucleic acid sequences, and that the corresponding genes have been presumably lost during evolution. Since T. cruzi behaves as a deletion mutant for ornithine decarboxylase (ODC) and arginine decarboxylase (ADC) genes, this parasite has been selected to study the regulation of the expression of heterologous genes involved in polyamine biosynthesis in other organisms. The resulting transgenic parasites have been useful tools to analyze the different stages of gene expression after transformation, as well as the mechanisms of drug resistance induction and the post-translational processing of enzyme precursors.

Leishmania donovani peroxin 14 undergoes a marked conformational change following association with peroxin 5

The import of PTS1 proteins into the glycosome or peroxisome requires binding of a PTS1-laden PEX5 receptor to the membrane-associated protein PEX14 to facilitate translocation of PTS1 proteins into the lumen of these organelles. Quaternary structure analysis of protozoan parasite Leishmania donovani PEX14 (LdPEX14) revealed that this protein forms a homomeric complex with a size > 670 kDa. Moreover, deletion mapping indicated that disruption of LdPEX14 oligomerization correlated with the elimination of the hydrophobic region and coiled-coil motif present in LdPEX14. Analysis of the LdPEX5-LdPEX14 interaction by isothermal titration calorimetry revealed a molar binding stoichiometry of 1:4 (LdPEX5: LdPEX14) and an in-solution dissociation constant (K(d)) of approximately 74 nm. Calorimetry, circular dichroism, intrinsic fluorescence, and analytical ultracentrifugation experiments showed that binding of LdPEX5 resulted in a dramatic conformational change in the LdPEX14 oligomeric complex that involved the reorganization of the hydrophobic segment in LdPEX14. Finally, limited tryptic proteolysis assays established that in the presence of LdPEX5, LdPEX14 became more susceptible to proteolytic degradation consistent with this protein interaction triggering a significant conformational change in the recombinant and native LdPEX14 structures. These structural changes provide essential clues to how LdPEX14 functions in the translocation of folded proteins across the glycosomal membrane.

Stress response pathways in protozoan parasites

Diseases caused by protozoan parasites have a dramatic impact on world health. Emerging drug resistance and a general lack of experimental understanding has created a void in the medicine cabinet used to treat these widespread infections. A novel therapeutic idea that is receiving more attention is centred on targeting the microbe's response to the multitude of environmental stresses it encounters. Protozoan pathogens have complex life cycles, often having to transition from one host to another, or survive in a cyst form in the environment until a new host arrives. The need to respond to environmental cues and stress, and endure in less than optimal conditions, is paramount to their viability and successful progression through their life cycle. This review summarizes the research on parasitic stress responses for Apicomplexa, kinetoplastids and anaerobic protozoa, with an eye towards how these processes may be exploited therapeutically.

Treatment of experimental chronic chagas disease with trifluralin

We tested trifluralin against Trypanosoma cruzi in a model of chronic Chagas disease in mice. CF1 mice (n=148) were intraperitoneally infected with 10(5) trypomastigotes of T. cruzi, H510C8C3 clone. One hundred mice were partially treated with benznidazole. Mortality was 100% at day 41 in the control group (n=48). At day 90 of the chronic disease (74% survival) mice were divided into three groups and treated orally with trifluralin (50 mg/kg/day, n=26), benznidazole (50 mg/kg/day, n=25) and vehicle (peanut oil; control group, n=23) for 60 days. Electrocardiography (under pentobarbital anaesthesia, 30 mg/kg/dose), serologic immunofluorescence and microstrout were performed at the beginning and at the end of the treatment. Mice were sacrificed at day 10 after treatment; cardiac tissue was studied histopathologically and polymerase chain reaction (PCR) was performed. Spontaneous mortality was 30.43%, 3.85% and 4% in the control, trifluralin and benznidazole groups, respectively (significant survival, P=0.03). Microstrouts were negative in all three groups. Negative immunofluorescence titers were 0%, 16% (P=0.05) and 29% (P<0.02) in the control, trifluralin and benznidazole groups, respectively. The prevailing electrocardiographic disorder was prolongation of the PR interval in the control group, which was not significantly altered in trifluralin- and benznidazole-treated mice, suggesting that trifluralin and benznidazole improve or even stop the damage caused by the disease on the conduction system. Trifluralin- and benznidazole-treated animals showed similar histologic patterns of myocarditis. PCR results were negative for benznidazole and trifluralin (100% and 70.8%, respectively). These results show the therapeutic potential of trifluralin in the treatment of chronic Chagas disease.

Regulation of purine biosynthesis by a eukaryotic-type kinase in Streptococcus agalactiae

Group B streptococci (GBS) are the principal causal agents of human neonatal pneumonia, sepsis and meningitis. We had previously described the existence of a eukaryotic-type serine/threonine kinase (Stk1) and phosphatase (Stp1) in GBS that regulate growth and virulence of the pathogen. Our previous results also demonstrated that these enzymes reversibly phosphorylated an inorganic pyrophosphatase. To understand the role of these eukaryotic-type enzymes on growth of GBS, we assessed the stk1-mutants for auxotrophic requirements. In this report, we describe that in the absence of the kinase (Stk1), GBS are attenuated for de novo purine biosynthesis and are consequently growth arrested. During growth in media lacking purines, the intracellular G nucleotide pools (GTP, GDP and GMP) are significantly reduced in the Stk1-deficient strains, while levels of A nucleotides (ATP, ADP and AMP) are marginally increased when compared with the isogenic wild-type strain. We provide evidence that the reduced pools of G nucleotides result from altered activity of the IMP utilizing enzymes, adenylosuccinate synthetase (PurA) and IMP dehydrogenase (GuaB) in these strains. We also demonstrate that Stk1 and Stp1 reversibly phosphorylate and consequently regulate PurA activity in GBS. Collectively, these data indicate the novel role of eukaryotic-type kinases in regulation of metabolic processes such as purine biosynthesis.

A validated HPLC method for determining residues of a dual active ingredient anti-malarial drug on manufacturing equipment surfaces

Analytical method validation, determining the recovery rate from the equipment surface and the stability of a potential contaminant are important steps of a cleaning validation process. A rapid, sensitive and reproducible reversed-phase high-performance liquid chromatographic method was developed for the determination of pyrimethamine (PYR) and sulfadoxine (SUL) in cleaning validation swab samples. The active compounds can be selectively quantified in a sample matrix containing detergent and swab material as low as 0.12 microg/ml. The swabbing procedure used on stainless steel coupons was validated and the stability of PYR and SUL in the swab samples was assessed. The calculated limit of contamination values for PYR (4.99 microg/cm2) and SUL (19.14 microg/cm2) were not exceeded during four consecutive equipment cleaning trials. This confirms that the desired level of cleanliness is achieved with the current cleaning procedures, which are consequently validated.

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.

Modulation of the Leishmania donovani peroxin 5 quaternary structure by peroxisomal targeting signal 1 ligands

The import of proteins containing the peroxisomal targeting signal 1 (PTS1) into the Leishmania glycosome is dependent on the docking of the PTS1-loaded LdPEX5 cytosolic receptor with LdPEX14 on the glycosome surface. Here we show that, in the absence of PTS1, LdPEX5 is a tetramer that is stabilized by two distinct interaction domains; the first is a coiled-coil motif encompassing residues 277 to 310, whereas the second domain is localized to residues 1 to 202. By using microcalorimetry, surface plasmon resonance, and size exclusion chromatography techniques, we show that PTS1 peptide binding to LdPEX5 tetramers promotes their dissociation into dimeric structures, which are stabilized by a coiled-coil interaction. Moreover, we demonstrated that the resulting LdPEX5-PTS1 complex is remarkably stable and exhibits extremely slow dissociation kinetics. However, binding of LdPEX14 to LdPEX5 modulates the LdPEX5-PTS1 affinity as it decreases the thermodynamic dissociation constant for this latter complex by 10-fold. These changes in the oligomeric state of LdPEX5 and in its affinity for PTS1 ligand upon LdPEX14 binding may explain how, under physiological conditions, LdPEX5 can function to deliver and unload its cargo to the protein translocation machinery on the glycosomal membrane.

Identification of Babesia bovis l-lactate dehydrogenase as a potential chemotherapeutical target against bovine babesiosis

In this study, we characterized a novel Babesia bovis cDNA clone obtained by immunoscreening the cDNA expression phage library with B. bovis-infected bovine serum. The genetic analyses showed that it contained an open reading frame of 993 bp, which was considered to encode B. bovis L-lactate dehydrogenase (BbLDH: E.C. 1.1.1.27) because of the strikingly high amino acid identities of its gene product to the LDHs of Plasmodium falciparum and Toxoplasma gondii. Immunological analyses with the anti-recombinant BbLDH mouse serum showed that 36 kDa of the native BbLDH was expressed not only in the cytoplasm of intra- and extraerythrocytic parasites but also along the membrane of infected erythrocytes. The kinetic properties of recombinant BbLDH proved a certain enzymatic activity of LDH, and the activity was significantly inhibited by the addition of gossypol, a competitive inhibitor of protozoan LDHs. Moreover, 100 microM of the gossypol irretrievably arrested the in vitro growth of B. bovis. The results demonstrated that BbLDH provides a suitable drug target for the design of novel babesial chemotherapeutics.

Arginase plays a pivotal role in polyamine precursor metabolism in Leishmania. Characterization of gene deletion mutants

The polyamine pathway of protozoan parasites has been successfully targeted in anti-parasitic therapies and is significantly different from that of the mammalian host. To gain knowledge into the metabolic routes by which parasites synthesize polyamines and their precursors, the arginase gene was cloned from Leishmania mexicana, and Deltaarg null mutants were created by double targeted gene replacement and characterized. The ARG sequence exhibited significant homology to ARG proteins from other organisms and predicted a peroxisomal targeting signal (PTS-1) that steers proteins to the glycosome, an organelle unique to Leishmania and related parasites. ARG was subsequently demonstrated to be present in the glycosome, whereas the polyamine biosynthetic enzymes, in contrast, were shown to be cytosolic. The Deltaarg knockouts expressed no ARG activity, lacked an intracellular ornithine pool, and were auxotrophic for ornithine or polyamines. The ability of the Deltaarg null mutants to proliferate could be restored by pharmacological supplementation, either with low putrescine or high ornithine or spermidine concentrations, or by complementation with an arginase episome. Transfection of an arg construct lacking the PTS-1 directed the synthesis of an arg that mislocalized to the cytosol and notably also complemented the genetic lesion and restored polyamine prototrophy to the Deltaarg parasites. This molecular, biochemical, and genetic dissection of ARG function in L. mexicana promastigotes establishes: (i) that the enzyme is essential for parasite viability; (ii) that Leishmania, unlike mammalian cells, expresses only one ARG activity; (iii) that the sole vital function of ARG is to provide polyamine precursors for the parasite; and (iv) that ARG is present in the glycosome, but this subcellular milieu is not essential for its role in polyamine biosynthesis.

Selective identification of new therapeutic targets of Mycobacterium tuberculosis by IVIAT approach

The in vivo induced antigen technology (IVIAT)(1) has been used for the identification of open reading frames (ORFs) which could be possible therapeutic targets. A recombinant lambdagt11:: Mycobacterium tuberculosis H37Rv expression library was screened with pooled TB patient sera preabsorbed with in vitro grown M. tuberculosis H37Rv. Preabsorption of pooled TB patient sera allowed identification of antigens specifically expressed or upregulated during infection and growth in vivo. Six ORFs were identified, of which four (rv0287, rv2402, rv3878 and rv1045) were of hypothetical functions. Rv0287 is a probable regulatory protein. Rv3878 is present uniquely in M. tuberculosis H37Rv and is a part of RDI deletion region of M. bovis BCG, which includes esat 6 region. This could be exploited as a tool for diagnosis. Two ORFs were assigned function solely on the basis of homology, dnaQ (rv3711c) and lpdA (rv3303c). dnaQ codes for the epsilon subunit of DNA polymerase III, which is responsible for the proofreading activity of the complex. lpdA codes for dihydrolipoamide dehydrogenase, which is a part of many multienzyme complexes such as pyruvate dehydrogenase, keto-acid dehydrogenase and alpha-ketoglutarate dehydrogenase. These two enzymes appear to be potential targets for drug development.

Metabolic pathway analysis in trypanosomes and malaria parasites

Identification of novel drug targets is required for the development of new classes of drugs to overcome drug resistance and replace less efficacious treatments. In theory, knowledge of the entire genome of a pathogen identifies every potential drug target in any given microbe. In practice, the sheer complexity and the inadequate or inaccurate annotation of genomic information makes target identification and selection somewhat more difficult. Analysis of metabolic pathways provides a useful conceptual framework for the identification of potential drug targets and also for improving our understanding of microbial responses to nutritional, chemical and other environmental stresses. A number of metabolic databases are available as tools for such analyses. The strengths and weaknesses of this approach are discussed.

Novel thiols of prokaryotes

Glutathione metabolism is associated with oxygenic cyanobacteria and the oxygen-utilizing purple bacteria, but is absent in many other prokaryotes. This review focuses on novel thiols found in those bacteria lacking glutathione. Included are glutathione amide and its perthiol, produced by phototrophic purple sulfur bacteria and apparently involved in their sulfide metabolism. Among archaebacteria, coenzyme M (2-mercaptoethanesulfonic acid) and coenzyme B (7-mercaptoheptanoylthreonine phosphate) play central roles in the anaerobic production of CH4 and associated energy conversion by methanogens, whereas the major thiol in the aerobic phototrophic halobacteria is gamma-glutamylcysteine. The highly aerobic actinomycetes produce mycothiol, a conjugate of N-acetylcysteine with a pseudodisaccharide of glucosamine and myo-inositol, AcCys-GlcNalpha(1 --> 1)Ins, which appears to play an antioxidant role similar to glutathione. Ergothioneine, also produced by actinomycetes, remains a mystery despite many years of study. Available data on the biosynthesis and metabolism of these and other novel thiols is summarized and key areas for additional study are identified.

Structure and function of a novel purine specific nucleoside hydrolase from Trypanosoma vivax

The purine salvage pathway of parasitic protozoa is currently considered as a target for drug development because these organisms cannot synthesize purines de novo. Insight into the structure and mechanism of the involved enzymes can aid in the development of potent inhibitors, leading to new curative drugs. Nucleoside hydrolases are key enzymes in the purine salvage pathway of Trypanosomatidae, and they are especially attractive because they have no equivalent in mammalian cells. We cloned, expressed and purified a nucleoside hydrolase from Trypanosoma vivax. The substrate activity profile establishes the enzyme to be a member of the inosine-adenosine-guanosine-preferring nucleoside hydrolases (IAG-NH). We solved the crystal structure of the enzyme at 1.6 A resolution using MAD techniques. The complex of the enzyme with the substrate analogue 3-deaza-adenosine is presented. These are the first structures of an IAG-NH reported in the literature. The T. vivax IAG-NH is a homodimer, with each subunit consisting of ten beta-strands, 12 alpha-helices and three small 3(10)-helices. Six of the eight strands of the central beta-sheet form a motif resembling the Rossmann fold. Superposition of the active sites of this IAG-NH and the inosine-uridine-preferring nucleoside hydrolase (IU-NH) of Crithidia fasciculata shows the molecular basis of the different substrate specificity distinguishing these two classes of nucleoside hydrolases. An "aromatic stacking network" in the active site of the IAG-NH, absent from the IU-NH, imposes the purine specificity. Asp10 is the proposed general base in the reaction mechanism, abstracting a proton from a nucleophilic water molecule. Asp40 (replaced by Asn39 in the IU-NH) is positioned appropriately to act as a general acid and to protonate the purine leaving group. The second general acid, needed for full enzymatic activity, is probably part of a flexible loop located in the vicinity of the active site.

Cloning of a pyruvate phosphate dikinase from Trypanosoma cruzi

We have cloned and characterised a gene that encodes a putative pyruvate phosphate dikinase (PPDK) from Trypanosoma cruzi, an enzyme that catalyses the reversible conversion of phosphoenolpyruvate to pyruvate. PPDK is absent in mammalian cells, but has been found in a wide variety of other organisms, including plants and bacteria. In T. cruzi, two genes (PPDK1 and PPDK2) are present in a tandem array localised on a 1 Mbp chromosome. Northern and Western blot analyses indicates that PPDK is expressed as a 100-kDa protein in epimastigote, amastigote and trypomastigote forms. PPDK1 and PPDK2 encode an identical protein of 100.8 kDa with a C-terminal extension ending with the sequence AKL, a signal for glycosomal import. Both T. cruzi and T. brucei enzymes possess a 23-residue insertion, that is absent in other PPDKs. A three-dimensional alignment with the crystal structure of the enzyme from Clostridium symbiosum predicts that this insertion is located on the surface of the nucleotide-binding domain. Phylogenetic studies indicate that bacterial and protist PPDKs cluster as a separate group from those of plants. The evolutionary implications and possible role of this enzyme in T. cruzi is discussed.

Characterisation of the antitrypanosomal activity of peptidyl alpha-aminoalkyl phosphonate diphenyl esters

Two groups of irreversible serine peptidase inhibitors, peptidyl chloromethyl ketones and peptidyl phosphonate diphenyl esters, were examined for antitrypanosomal activity against the bloodstream form of Trypanosoma brucei brucei. Both peptidyl chloromethyl ketones and peptidyl phosphonate diphenyl esters inhibited trypsin-like peptidases of the parasites and exhibited antitrypanosomal activity at micromolar concentrations. In live T. b. brucei, labelled analogues of both of these groups of inhibitors primarily targeted an 80-kDa peptidase, possibly a serine oligopeptidase known as oligopeptidase B. In an in vivo mouse model of infection, one of these inhibitors, carbobenzyloxyglycyl-4-amidinophenylglycine phosphonate diphenyl ester, was curative at 5 mg kg(-1) day(-1) but appeared toxic at higher doses. There was no significant correlation between the inhibitory potency (as evaluated against purified T. b. brucei oligopeptidase B) and the in vitro antitrypanosomal efficacy of either group of inhibitors, suggesting that these inhibitors were acting on multiple targets within the parasites, or had different cell permeability properties. These findings suggest that serine peptidases may represent novel chemotherapeutic targets in African trypanosomes.

Flagellar elongation induced by glucose limitation is preadaptive for Trypanosoma cruzi differentiation

Trypanosomes must sense and respond to environmental change in order to progress through their life cycles. The American trypanosome, Trypanosoma cruzi, differentiates from the noninfective epimastigote form to the infective metacyclic form spontaneously in axenic culture. Here, we investigate the initial stimulus for that change and demonstrate that T. cruzi epimastigotes sense limitation of glucose in the medium and respond by undergoing significant morphological and biochemical change. As part of this change, the mean flagellar length of the population triples, which is correlated with an increased ability to maintain interactions with hydrophobic substrates, a requirement for differentiation to the next life cycle stage.

Enzymes of glucose and glycerol catabolism in in vitro-propagated Theileria parva schizonts

Theileria parva schizonts propagated in vitro in peripheral blood lymphocytes were purified and assayed for key enzymes of glucose and glycerol catabolism and the citric acid cycle. The activities of glycolytic enzymes were in the range of 21-100 nmol/min/mg protein. Glycerol kinase and alpha -glycerophosphate dehydrogenase activities were more than 16 times lower than the activities of other enzymes catalysing the oxidation of the triose phosphates to lactate. It was suggested that the catabolism of glycerol is negligible and that glucose is catabolized to lactate via the Embden-Meyerhof pathway. The activities of the enzymes catalysing the section of the citric acid cycle that involves the formation of citrate to succinyl-CoA were consistently very low (less than 2.0 nmol/min/mg protein), indicating that this part of the cycle plays a minor role in this parasite. Enzyme activities of the cycle catalysing the formation of succinate from oxaloacetate were relatively higher than those catalysing other sections of the citric acid cycle, suggesting that this section of the cycle could be important to the parasite. Pyruvate carboxylase activity was more than 10 times that of phosphoenolpyruvate carboxykinase. It was suggested that pyruvate could be carboxylated to oxaloacetate. Taken together, these results suggest that the catabolism of glucose in Theileria parva schizonts is mainly via the Embden-Meyerhof pathway and that the citric acid cycle plays a minor role in energy production.

Entamoeba histolytica lacks trypanothione metabolism

Entamoeba histolytica lacks glutathione reductase activity and the ability to synthesise glutathione de novo. However, a recent report suggested that exogenous glutathione can be taken up and conjugated to spermidine to form trypanothione, a metabolite found so far only in trypanosomatids. Given the therapeutic implications of this observation, we have carefully analysed E. histolytica for evidence of trypanothione metabolism. Using a sensitive fluorescence-based HPLC detection system we could confirm previous reports that cysteine and hydrogen sulphide are the principal low molecular mass thiols. However, we were unable to detect trypanothione or its precursor N1-glutathionylspermidine [ < 0.01 nmol (10(6) cells)(-1) or < 1.7 microM]. In contrast, Trypanosoma cruzi epimastigotes (grown in a polyamine-supplemented medium) and Leishmania donovani promastigotes contained intracellular concentrations of trypanothione two to three orders of magnitude greater than the limits of detection. Likewise, trypanothione reductase activity was not detectable in E. histolytica [ < 0.003 U (mg protein)(-1)] and therefore at least 100-fold less than trypanosomatids. Moreover, although E. histolytica were found to contain trace amounts of glutathione (approximately 20 microM), glutathione reductase activity was below the limits of detection [ < 0.005 U (mg protein)(-1)]. These findings argue against the existence of trypanothione metabolism in E. histolytica.

Molecular epidemiology of Plasmodium falciparum antifolate resistance in Vietnam: genotyping for resistance variants of dihydropteroate synthase and dihydrofolate reductase

Using PCR techniques, we analysed the dihydropteroate synthase (DHPS) mutations associated with sulphonamide resistance and the dihydrofolate reductase (DHFR) mutations associated with resistance to pyrimethamine and cycloguanil in samples from Plasmodium falciparum-infected Vietnamese patients. Of the 40 samples analysed, 39 had DHFR mutations associated with high level resistance to pyrimethamine, whereas only three had mutations at position 164, which is linked to cross resistance to both DHFR inhibitors. The DHPS, 437Gly variant associated with very mild resistance to sulphadoxine was found in 38 out of the 40 samples. Of seven samples resistant to Fansidar in vivo, only two were fully explained by the currently documented DHPS mutations. The treatment failure could be due to a high level of pyrimethamine resistance caused by the detected mutations. Most patients, however, were cured with a single dose of Fansidar in spite of the high number of resistance mutations found.

Antisense oligonucleotides targeting malarial aldolase inhibit the asexual erythrocytic stages of Plasmodium falciparum

A major obstacle in the global effort to control malaria is the paucity of anti-malarial drugs. This is compounded by the continuing emergence and spread of resistance to old and new anti-malarial drugs in the malarial parasites. Here we describe the anti-malarial effect of phosphorothioate antisense (AS) oligodeoxynucleotides (ODNs) targeting the aldolase enzyme of Plasmodium falciparum, using the asexual blood stages of the parasite grown in vitro. The blood stages of P. falciparum depend almost entirely on the energy produced by their own glycolysis. Aldolase, the fourth enzyme of the glycolytic pathway, is highly upregulated during the malarial 48-h life cycle. We found that the mRNA of this enzyme can be inhibited, in a sequence specific manner, using AS-ODN to the splice sites on the pre-mRNA of malarial aldolase. At the enzyme level, both specific AS-ODNs for the splice sites, as well as for the translation initiation site on mature mRNA, can inhibit aldolase enzyme activity within the trophozoites of P. falciparum. Furthermore, this downregulation of the malarial aldolase results in a reduction in the production of ATP within the parasite. Finally, the treatment reduces parasitemia. In summary, AS-ODNs targeting the aldolase gene of P. falciparum can interfere with the blood-stage life cycle of this parasite in vitro by inhibiting the expression of the enzyme aldolase which results in decreased malarial glycolysis and energy production. Thus, we conclude that blockade of the expression of malarial glycolytic enzymes using specific AS-ODNs has the potential of a new anti-malarial strategy.

Pathways of As(III) detoxification in Saccharomyces cerevisiae

Saccharomyces cerevisiae has two independent transport systems for the removal of arsenite from the cytosol. Acr3p is a plasma membrane transporter that confers resistance to arsenite, presumably by arsenite extrusion from the cells. Ycf1p, a member of the ABC transporter superfamily, catalyzes the ATP-driven uptake of As(III) into the vacuole, also producing resistance to arsenite. Vacuolar accumulation requires a reductant such as glutathione, suggesting that the substrate is the glutathione conjugate, As(GS)3. Disruption of either the ACR3 or YCF1 gene results in sensitivity to arsenite and disruption of both genes produces additive hypersensitivity. Thus, Acr3p and Ycf1p represent separate pathways for the detoxification of arsenite in yeast.

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.

Coenzyme A disulfide reductase, the primary low molecular weight disulfide reductase from Staphylococcus aureus. Purification and characterization of the native enzyme

The human pathogen Staphylococcus aureus does not utilize the glutathione thiol/disulfide redox system employed by eukaryotes and many bacteria. Instead, this organism produces CoA as its major low molecular weight thiol. We report the identification and purification of the disulfide reductase component of this thiol/disulfide redox system. Coenzyme A disulfide reductase (CoADR) catalyzes the specific reduction of CoA disulfide by NADPH. CoADR has a pH optimum of 7.5-8.0 and is a dimer of identical subunits of Mr 49,000 each. The visible absorbance spectrum is indicative of a flavoprotein with a lambdamax = 452 nm. The liberated flavin from thermally denatured enzyme was identified as flavin adenine dinucleotide. Steady-state kinetic analysis revealed that CoADR catalyzes the reduction of CoA disulfide by NADPH at pH 7.8 with a Km for NADPH of 2 muM and for CoA disulfide of 11 muM. In addition to CoA disulfide CoADR reduces 4,4'-diphosphopantethine but has no measurable ability to reduce oxidized glutathione, cystine, pantethine, or H2O2. CoADR demonstrates a sequential kinetic mechanism and employs a single active site cysteine residue that forms a stable mixed disulfide with CoA during catalysis. These data suggest that S. aureus employs a thiol/disulfide redox system based on CoA/CoA-disulfide and CoADR, an unorthodox new member of the pyridine nucleotide-disulfide reductase superfamily.

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.

Parasite sulphur amino acid metabolism

This paper reviews current knowledge regarding the metabolism of the sulphur-containing amino acids methionine and cysteine in parasitic protozoa and helminths. Particular emphasis is placed on the unusual aspects of parasite biochemistry which may present targets for rational design of antiparasite drugs. In general, the basic pathways of sulphur amino acid metabolism in most parasites resemble those of their mammalian hosts, since the enzymes involved in (a) the methionine cycle and S-adenosylmethionine metabolism, (b) the trans-sulphuration sequence, (c) the transminative catabolism of methionine, (d) the oxidative catabolism of cysteine and (e) glutathione synthesis have been demonstrated variously in several helminth and protozoan species. Despite these common pathways, there also exist numerous differences between parasite and mammalian metabolism. Some of these differences are relatively subtle. For example, the biochemical properties (and primary amino acid structures) of certain parasite methionine cycle enzymes and S-adenosylmethionine decarboxylases differ from those of the corresponding mammalian enzymes, and nematodes and trichomonads possess a novel, non-mammalian form of the trans-sulphuration enzyme cystathionine beta-synthase. The most profound differences between parasite and mammalian biochemistry relate to a number of unusual enzymes and thiol metabolites found in parasitic protozoa. In certain protozoa the pathway for methionine recycling from 5'-methylthioadenosine differs markedly from the mammalian route, and involves 2 exclusively microbial enzymes. Trypanosomatid protozoa contain the non-mammalian antioxidant thiol compounds ovothiol A and trypanothione, together with unique trypanothione-linked enzymes. Specific anaerobic protozoa possess another exclusively microbial enzyme, methionine gamma-lyase, which catabolises methionine (and homocysteine); the physiological significance of these non-mammalian activities is not fully understood. These unusual features offer opportunities for chemotherapeutic exploitation, and in some cases represent metabolic similarities with bacteria. Additionally, some anaerobic protozoa contain unidentified thiols and this implies the presence of further unusual enzymes/pathways in these organisms. So far, no truly unique targets for chemotherapy have been found in helminth sulphur amino acid metabolism, and to some degree this reflects the relative lack of detailed study in the area.

New spermine and spermidine derivatives as potent inhibitors of Trypanosoma cruzi trypanothione reductase

Several spermine and spermidine derivatives containing 2-amino diphenylsulfide substituents were prepared and tested for their inhibiting effects on Trypanosoma cruzi trypanothione reductase. IC50 values were assessed between 0.3 and 3 microM. Compound 32 (Ki = 0.4 microM) is the most potent TR inhibitor described so far.

Characterization of the respiratory chain from cultured Crithidia fasciculata

Mitochondrial mRNAs encoding subunits of respiratory-chain complexes in kinetoplastids are post-transcriptionally edited by uridine insertion and deletion. In order to identify the proteins encoded by these mRNAs, we have analyzed respiratory-chain complexes from cultured cells of Crithidia fasciculata with the aid of 2D polyacrylamide gel electrophoresis (PAGE). The subunit composition of F0F1-ATPase (complex V), identified on the basis of its activity as an oligomycin-sensitive ATPase, is similar to that of bovine mitochondrial F0F1-ATPase. Amino acid sequence analysis, combined with binding studies using dicyclohexyldiimide and azido ATP allowed the identification of two F0 subunits (b and c) and all of the F1 subunits. The F0 b subunit has a low degree of similarity to subunit b from other organisms. The F1 alpha subunit is extremely small making the beta subunit the largest F1 subunit. Other respiratory-chain complexes were also analyzed. Interestingly, an NADH: ubiquinone oxidoreductase (complex I) appeared to be absent, as judged by electron paramagnetic resonance (EPR), enzyme activity and 2D PAGE analysis. Cytochrome c oxidase (complex IV) displayed a subunit pattern identical to that reported for the purified enzyme, whereas cytochrome c reductase (complex III) appeared to contain two extra subunits. A putative complex II was also identified. The amino acid sequences of the subunits of these complexes also show a very low degree of similarity (if any) to the corresponding sequences in other organisms. Remarkably, peptide sequences derived from mitochondrially encoded subunits were not found in spite of the fact that sequences were obtained of virtually all subunits of complex III, IV and V.

An antibiotic, ascofuranone, specifically inhibits respiration and in vitro growth of long slender bloodstream forms of Trypanosoma brucei brucei

Ascofuranone, a prenylphenol antibiotic isolated from a phytopathogenic fungus, Ascochyta visiae, strongly inhibited both glucose-dependent cellular respiration and glycerol-3-phosphate-dependent mitochondrial O2 consumption of long slender bloodstream forms of Trypanosoma brucei brucei. This inhibition was suggested to be due to inhibition of the mitochondrial electron-transport system, composed of glycerol-3-phosphate dehydrogenase (EC 1.1.99.5) and plant-like alternative oxidase. Ascofuranone noncompetitively inhibited the reduced coenzyme Q1-dependent O2 uptake of the mitochondria with respect to ubiquinol (Ki = 2.38 nM). Therefore, the susceptible site is deduced to be the ubiquinone redox machinery which links the two enzyme activities. Further, ascofuranone in combination with glycerol completely blocked energy production, and potently inhibited the in vitro growth of the parasite. Our findings suggest that ascofuranone might be a promising candidate for the chemotherapeutic agents of African trypanosomiasis.

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.

Cation-pi bonding and amino-aromatic interactions in the biomolecular recognition of substituted ammonium ligands

Cation-pi bonds and amino-aromatic interactions are known to be important contributors to protein architecture and stability, and their role in ligand-protein interactions has also been reported. Many biologically active amines contain substituted ammonium moieties, and cation-pi bonding and amino-aromatic interactions often enable these molecules to associate with proteins. The role of organic cation-pi bonding and amino-aromatic interactions in the recognition of small-molecule amines and peptides by proteins is an important topic for those involved in structure-based drug design, and although the number of structures determined for proteins displaying these interactions is small, general features are beginning to emerge. This review explores the role of cation-pi bonding and amino-aromatic interactions in the biological molecular recognition of amine ligands. Perspectives on the design of ammonium-ligand-binding sites are also discussed.

6-Phosphogluconate Dehydrogenase from Trypanosoma Brucei. Kinetic Analysis and Inhibition by Trypanocidal Drugs

The kinetics of 6-phosphogluconate dehydrogenase from Trypanosoma brucei was examined and compared to those of the same enzyme from lamb's liver. Variation of kinetic parameters as a function of pH suggests a chemical mechanism similar to other 6-phosphogluconate dehydrogenases. The comparison extended to a detailed analysis of the effect on enzyme activity by several inhibitors including the trypanocidal drugs suramin, melarsoprol and analogues of these compounds. The T. brucei enzyme differs significantly from its mammalian counterpart with respect to several inhibitors, particularly the substrate analogue 6-phospho-2-deoxygluconate and the coenzyme analogue adenosine 2',5'-bisphosphate which have respectively 170-fold and 40-fold higher affinity for the parasite enzyme.

Crystal structure of the Trypanosoma cruzi trypanothione reductase.mepacrine complex

The three-dimensional structure of the complex between Trypanosoma cruzi trypanothione reductase (TR) (EC 1.6.4.8) and the antiparasitic drug mepacrine (quinacrine) has been solved at 2.9 angstoms resolution. Mepacrine is a competitive inhibitor of TR but does not affect human glutathione reductase (GR), a closely related host enzyme. Of particular importance for inhibitor binding are four amino acid residues in the disulfide substrate-binding site of TR that are not conserved in human GR, namely, Glu-18 (Ala-34 in GR), Trp-21 (Arg-37), Ser-109 (Ile-113), and Met-113 (Asn-117). The acridine ring of mepacrine is fixed at the active site close to the hydrophobic wall formed by Trp-21 and Met-113. Specific pairwise interactions between functional groups of the drug and amino acid side chains include the ring nitrogen and Met-113, the chlorine atom and Trp-21, and the oxymethyl group and Ser-109. The alkylamino chain of mepacrine points into the inner region of the active site and is held in position by a solvent-mediated hydrogen bond to Glu-18. The structure of the complex shows for the first time the atomic interactions between TR and an inhibitory ligand. This is a crucial step towards the rational design of inhibitors that might be suited as drugs against Chagas' disease.

The crystal structure of trypanothione reductase from the human pathogen Trypanosoma cruzi at 2.3 A resolution

Trypanothione reductase (TR) is an NADPH-dependent flavoprotein unique to protozoan parasites from the genera Trypanosoma and Leishmania and is an important target for the design of improved trypanocidal drugs. We present details of the structure of TR from the human pathogen Trypanosoma cruzi, the agent responsible for Chagas' disease or South American trypanosomiasis. The structure has been solved by molecular replacement, using as the starting model the structure of the enzyme from the nonpathogenic Crithidia fasciculata, and refined to an R-factor of 18.9% for 53,868 reflections with F > or = sigma F between 8.0 and 2.3 A resolution. The model comprises two subunits (968 residues), two FAD prosthetic groups, two maleate ions, and 419 water molecules. The accuracy and geometry of the enzyme model is improved with respect to the C. fasciculata enzyme model. The new structure is described and specific features of the enzyme involved in substrate interactions are compared with previous models of TR and related glutathione reductases from human and Escherichia coli. Structural differences at the edge of the active sites suggest an explanation for the differing specificities toward glutathionylspermidine disulfide.

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.

A diamidine-resistant Trypanosoma equiperdum clone contains a P2 purine transporter with reduced substrate affinity

Following the demonstration that the transport of melaminophenyl arsenical drugs in Trypanosoma brucei is dependent upon an unusual adenosine nucleoside transporter (Carter and Fairlamb, Nature 361 (1993) 173-175) we have investigated adenosine transport in the related parasite Trypanosoma equiperdum (Botat1.1) and a cloned derivative resistant to the diamidine drug berenil (diminazene aceturate) with limited cross-resistance to the melaminophenyl arsenical cymelarsen. The parental strain possesses a bipartite adenosine transport system consisting of one component which is inhibited in a dose-dependent and saturable manner with increasing concentrations of inosine and a second component which is similarly inhibited by adenine. Uptake of adenosine on this second transporter is also inhibited in a dose-dependent fashion by berenil and cymelarsen. Both transporters have high affinity for adenosine (apparent Km values of 0.60 and 0.70 mM and Vmax values of 8.4 and 6.9 pmol (s (10(8) trypanosomes))-1 at 25 degrees C, respectively). Thus T. equiperdum shares with T. brucei a system comprising two adenosine transporters named P1 and P2, respectively. The P1 transporter is similar in the sensitive and resistant T. equiperdum clones, whereas the P2 transporter has reduced transport capacity at physiological adenosine concentration and decreased affinity for adenosine in the drug-resistant clone.