Purine and pyrimidine metabolism in the Trypanosomatidae

Ecto-nucleotidases and Ecto-phosphatases from Leishmania and Trypanosoma parasites

Ecto-enzymes can be defined as membrane-bound proteins that have their active site facing the extracellular millieu. In trypanosomatids, the physiological roles of these enzymes remain to be completed elucidated; however, many important events have already been related to them, such as the survival of parasites during their complex life cycle and the successful establishment of host infection. This chapter focuses on two remarkable classes of ecto-enzymes: ecto-nucleotidases and ecto-phosphatases, summarizing their occurrence and possible physiological roles in Leishmania and Trypanosoma genera. Ecto-nucleotidases are characterized by their ability to hydrolyze extracellular nucleotides, playing an important role in purinergic signaling. By the action of these ecto-enzymes, parasites are capable of modulating the host immune system, which leads to a successful parasite infection. Furthermore, ecto-nucleotidases are also involved in the purine salvage pathway, acting in the generation of nucleosides that are able to cross plasma membrane via specialized transporters. Another important ecto-enzyme present in a vast number of pathogenic organisms is the ecto-phosphatase. These enzymes are able to hydrolyze extracellular phosphorylated substrates, releasing free inorganic phosphate that can be internalized by the cell, crossing the plasma membrane through a Pi-transporter. Ecto-phosphatases are also involved in the invasion and survival of parasite in the host cells. Several alternative functions have been suggested for these enzymes in parasites, such as participation in their proliferation, differentiation, nutrition and protection. In this context, the present chapter provides an overview of recent discoveries related to the occurrence of ecto-nucleotidase and ecto-phosphatase activities in Leishmania and Trypanosoma parasites.

Trypanosoma brucei (UMP synthase null mutants) are avirulent in mice, but recover virulence upon prolonged culture in vitro while retaining pyrimidine auxotrophy

African trypanosomes are capable of both de novo synthesis and salvage of pyrimidines. The last two steps in de novo synthesis are catalysed by UMP synthase (UMPS) – a bifunctional enzyme comprising orotate phosphoribosyl transferase (OPRT) and orotidine monophosphate decarboxylase (OMPDC). To investigate the essentiality of pyrimidine biosynthesis in Trypanosoma brucei, we generated a umps double knockout (DKO) line by gene replacement. The DKO was unable to grow in pyrimidine-depleted medium in vitro, unless supplemented with uracil, uridine, deoxyuridine or UMP. DKO parasites were completely resistant to 5-fluoroorotate and hypersensitive to 5-fluorouracil, consistent with loss of UMPS, but remained sensitive to pyrazofurin indicating that, unlike mammalian cells, the primary target of pyrazofurin is not OMPDC. The null mutant was unable to infect mice indicating that salvage of host pyrimidines is insufficient to support growth. However, following prolonged culture in vitro, parasites regained virulence in mice despite retaining pyrimidine auxotrophy. Unlike the wild-type, both pyrimidine auxotrophs secreted substantial quantities of orotate, significantly higher in the virulent DKO line. We propose that this may be responsible for the recovery of virulence in mice, due to host metabolism converting orotate to uridine, thereby bypassing the loss of UMPS in the parasite.

Helicobacter pylori salvages purines from extracellular host cell DNA utilizing the outer membrane-associated nuclease NucT

Helicobacter pylori is a bacterial pathogen that establishes life-long infections in humans, and its presence in the gastric epithelium is strongly associated with gastritis, peptic ulcer disease, and gastric cancer. Having evolved in this specific gastric niche for hundreds of thousands of years, this microbe has become dependent on its human host. Bioinformatic analysis reveals that H. pylori has lost several genes involved in the de novo synthesis of purine nucleotides, and without this pathway present, H. pylori must salvage purines from its environment in order to grow. While the presence and abundance of free purines in various mammalian tissues has been loosely quantified, the concentration of purines present within the gastric mucosa remains unknown. There is evidence, however, that a significant amount of extracellular DNA is present in the human gastric mucosal layer as a result of epithelial cell turnover, and this DNA has the potential to serve as an adequate purine source for gastric purine auxotrophs. In this study, we characterize the ability of H. pylori to grow utilizing only DNA as a purine source. We show that this ability is independent of the ComB DNA uptake system, and that H. pylori utilization of DNA as a purine source is largely influenced by the presence of an outer membrane-associated nuclease (NucT). A ΔnucT mutant exhibits significantly reduced extracellular nuclease activity and is deficient in growth when DNA is provided as the sole purine source in laboratory growth media. These growth defects are also evident when this nuclease mutant is grown in the presence of AGS cells or in purine-free tissue culture medium that has been conditioned by AGS cells in the absence of fetal bovine serum. Taken together, these results indicate that the salvage of purines from exogenous host cell DNA plays an important role in allowing H. pylori to meet its purine requirements for growth.

Structures of purine nucleosidase from Trypanosoma brucei bound to isozyme-specific trypanocidals and a novel metalorganic inhibitor

Sleeping sickness is a deadly disease that primarily affects sub-Saharan Africa and is caused by protozoan parasites of the Trypanosoma genus. Trypanosomes are purine auxotrophs and their uptake pathway has long been appreciated as an attractive target for drug design. Recently, one tight-binding competitive inhibitor of the trypanosomal purine-specific nucleoside hydrolase (IAGNH) showed remarkable trypanocidal activity in a murine model of infection. Here, the enzymatic characterization of T. brucei brucei IAGNH is presented, together with its high-resolution structures in the unliganded form and in complexes with different inhibitors, including the trypanocidal compound UAMC-00363. A description of the crucial contacts that account for the high-affinity inhibition of IAGNH by iminoribitol-based compounds is provided and the molecular mechanism underlying the conformational change necessary for enzymatic catalysis is identified. It is demonstrated for the first time that metalorganic complexes can compete for binding at the active site of nucleoside hydrolase enzymes, mimicking the positively charged transition state of the enzymatic reaction. Moreover, we show that divalent metal ions can act as noncompetitive IAGNH inhibitors, stabilizing a nonproductive conformation of the catalytic loop. These results open a path for rational improvement of the potency and the selectivity of existing compounds and suggest new scaffolds that may be used as blueprints for the design of novel antitrypanosomal compounds.

Whole-organism high-throughput screening against Trypanosoma brucei brucei

INTRODUCTION

Human African trypanosomiasis (HAT) occurs as a result of infection with the protozoan parasites Trypanosoma brucei gambiense and T.b. rhodesiense and is nearly always fatal without treatment. However, current therapeutic options are severely limited and there is a desperate need for new compounds to treat the disease. Whole-cell high-throughput screening (HTS) is a technique frequently used to identify compounds with trypanocidal activity.

AREAS COVERED

The authors examine the development of whole-organism HTS assays for T.b. brucei. The authors describe the successes achieved through HTS and discuss the advantages and disadvantages of whole-organism HTS.

EXPERT OPINION

Despite hundreds of trypanocidal molecules being identified by whole-organism HTS, very few have progressed into preclinical development. The failure of molecules identified by HTS to progress along the drug development pathway is due to a multitude of factors including undrug-like molecules and molecules having poor pharmacodynamics/kinetic properties. Future studies should focus on screening libraries that contain drug-like molecules that possess some of the properties required in the final compound.

Cordycepin (3'-deoxyadenosine) pentostatin (deoxycoformycin) combination treatment of mice experimentally infected with Trypanosoma evansi

The aim of this study was to evaluate the anti-trypanosomal effect of treatment with 3'-deoxyadenosine (cordycepin) combined with deoxycoformycin (pentostatin: inhibitor of the enzyme adenosine deaminase) in vitro by using mice experimentally infected with Trypanosoma evansi. In vitro, a dose-dependent trypanocidal effect of cordycepin was observed against the parasite. In the in vivo trials, the two drugs were used individually and in combination of different doses. The drugs when used individually had no curative effect on infected mice. However, the combination of cordycepin (2 mg kg-1) and pentostatin (2 mg kg-1) was 100% effective in the T. evansi-infected groups. There was an increase in levels of some biochemical parameters, especially on liver enzymes, which were accompanied by histological lesions in the liver and kidneys. Based on these results we conclude that treatment using the combination of 3'-deoxyadenosine with deoxycoformycin has a curative effect on mice infected with T. evansi. However, the therapeutic protocol tested led to liver and kidney damage, manifested by hepatotoxicity and nephrotoxicity.

Cysteine cross-linking defines the extracellular gate for the Leishmania donovani nucleoside transporter 1.1 (LdNT1.1)

Equilibrative nucleoside transporters are a unique family of proteins that enable uptake of nucleosides/nucleobases into a wide range of eukaryotes and internalize a myriad of drugs used in the treatment of cancer, heart disease, AIDs, and parasitic infections. In previous work we generated a structural model for such a transporter, the LdNT1.1 nucleoside permease from the parasitic protozoan Leishmania donovani, using ab initio computation. The model suggested that aromatic residues present in transmembrane helices 1, 2, and 7 interact to form an extracellular gate that closes the permeation pathway in the inward-open conformation. Mutation of residues Phe-48(TM1) and Trp-75(TM2) abrogated transport activity, consistent with such prediction. In this study cysteine mutagenesis and oxidative cross-linking were combined to analyze proximity relationships of helices 1, 2, and 7 in LdNT1.1. Disulfide bond formation between introduced paired cysteines at the interface of such helices (A61C(TM1)/F74C(TM2), A61C(TM1)/G350C(TM7), and F74C(TM2)/G350C(TM7)) was analyzed by transport measurement and gel mobility shifts upon oxidation with Cu (II)-(1,10-phenanthroline)(3). In all cases cross-linking inhibited transport. However, if LdNT1.1 ligands were included during cross-linking, inhibition of transport was reduced, suggesting that ligands moved the three gating helices apart. Moreover, all paired cysteine mutants exhibited a mobility shift upon oxidation, corroborating the formation of a disulfide bond. These data support the notion that helices 1, 2, and 7 constitute the extracellular gate of LdNT1.1, thus further validating the computational model and the previously demonstrated importance of F48(TM1) and Trp-75(TM2) in tethering together helices that are part of the gate.

Leishmania metacyclogenesis is promoted in the absence of purines

Leishmania parasites, the causative agent of leishmaniasis, are transmitted through the bite of an infected sand fly. Leishmania parasites present two basic forms known as promastigote and amastigote which, respectively, parasitizes the vector and the mammalian hosts. Infection of the vertebrate host is dependent on the development, in the vector, of metacyclic promastigotes, however, little is known about the factors that trigger metacyclogenesis in Leishmania parasites. It has been generally stated that "stressful conditions" will lead to development of metacyclic forms, and with the exception of a few studies no detailed analysis of the molecular nature of the stress factor has been performed. Here we show that presence/absence of nucleosides, especially adenosine, controls metacyclogenesis both in vitro and in vivo. We found that addition of an adenosine-receptor antagonist to in vitro cultures of Leishmania amazonensis significantly increases metacyclogenesis, an effect that can be reversed by the presence of specific purine nucleosides or nucleobases. Furthermore, our results show that proliferation and metacyclogenesis are independently regulated and that addition of adenosine to culture medium is sufficient to recover proliferative characteristics for purified metacyclic promastigotes. More importantly, we show that metacyclogenesis was inhibited in sand flies infected with Leishmania infantum chagasi that were fed a mixture of sucrose and adenosine. Our results fill a gap in the life cycle of Leishmania parasites by demonstrating how metacyclogenesis, a key point in the propagation of the parasite to the mammalian host, can be controlled by the presence of specific purines.

Efficiency of purine utilization by Helicobacter pylori: roles for adenosine deaminase and a NupC homolog

The ability to synthesize and salvage purines is crucial for colonization by a variety of human bacterial pathogens. Helicobacter pylori colonizes the gastric epithelium of humans, yet its specific purine requirements are poorly understood, and the transport mechanisms underlying purine uptake remain unknown. Using a fully defined synthetic growth medium, we determined that H. pylori 26695 possesses a complete salvage pathway that allows for growth on any biological purine nucleobase or nucleoside with the exception of xanthosine. Doubling times in this medium varied between 7 and 14 hours depending on the purine source, with hypoxanthine, inosine and adenosine representing the purines utilized most efficiently for growth. The ability to grow on adenine or adenosine was studied using enzyme assays, revealing deamination of adenosine but not adenine by H. pylori 26695 cell lysates. Using mutant analysis we show that a strain lacking the gene encoding a NupC homolog (HP1180) was growth-retarded in a defined medium supplemented with certain purines. This strain was attenuated for uptake of radiolabeled adenosine, guanosine, and inosine, showing a role for this transporter in uptake of purine nucleosides. Deletion of the GMP biosynthesis gene guaA had no discernible effect on mouse stomach colonization, in contrast to findings in numerous bacterial pathogens. In this study we define a more comprehensive model for purine acquisition and salvage in H. pylori that includes purine uptake by a NupC homolog and catabolism of adenosine via adenosine deaminase.

New determinants in the catalytic mechanism of nucleoside hydrolases from the structures of two isozymes from Sulfolobus solfataricus

The purine- and pyrimidine-specific nucleoside hydrolases (NHs) from the archaeon Sulfolobus solfataricus participate in the fundamental pathway of nucleotide catabolism and function to maintain adequate levels of free nitrogenous bases for cellular function. The two highly homologous isozymes display distinct specificities toward nucleoside substrates, and both lack the amino acids employed for activation of the leaving group in the hydrolytic reaction by the NHs characterized thus far. We determined the high-resolution crystal structures of the purine- and pyrimidine-specific NHs from S. solfataricus to reveal that both enzymes belong to NH structural homology group I, despite the different substrate specificities. A Na(+) ion is bound at the active site of the pyrimidine-specific NH instead of the prototypical Ca(2+), delineating a role of the metals in the catalytic mechanism of NHs in the substrate binding rather than nucleophile activation. A conserved His residue, which regulates product release in other homologous NHs, provides crucial interactions for leaving group activation in the archaeal isozymes. Modeling of the enzyme-substrate interactions suggests that steric exclusion and catalytic selection underlie the orthogonal base specificity of the two isozymes.

Diverse viscerotropic isolates of Leishmania all express a highly conserved secretory nuclease during human infections

Previously, we characterized a gene encoding the unique nuclease (LdNuc(s)) from a Sudanese isolate of the human pathogen Leishmania donovani. This parasite secretory enzyme is involved in the salvage of host-derived purines and is constitutively expressed by both developmental forms of the parasite. Currently, we assessed whether an LdNuc(s)-like nuclease was conserved among other geographically disparate isolates of L. donovani and whether this enzyme was produced by intracellular amastigotes during human infections. Using RT-PCR and Southern blotting, we showed that LdNuc(s) gene homologs were present in each of the viscerotropic Leishmania tested (i.e., L. donovani isolates from the Sudan, Ethiopia and India as well as L. infantum). Further results of in situ enzyme activity gel analyses showed that each of these parasite isolates also expressed a released/secreted LdNuc(s)-like nuclease activity. In Western blots, our anti-LdNuc(s) (Sudan) peptide-specific antibody reacted with only a single ~35 kDa protein in each of the viscerotropic Leishmania isolates. Further, the ~35 kDa nuclease secreted by each of these isolates was specifically immunoprecipitated by the anti-LdNuc(s) antibody above. In situ gel analyses showed that each of these immunoprecipitates had LdNuc(s)-like nuclease activity. Moreover, sera from acute visceral leishmaniasis patients from India, Sudan and Brazil all immunoprecipitated an LdNuc(s)-HA expressed nuclease demonstrating, that these patients possessed antibodies against this parasite secretory enzyme. Cumulatively, these results showed that the LdNuc(s) homologs were functionally conserved among geographically disparate visceral Leishmania spp. and that amastigotes of these parasites must produce this nuclease enzyme during the course of human disease.

Purine stress in crithidia: adaptation of a parasite to environmental stress

How parasitic protozoa survive varying nutrient levels is a key issue in parasitology. Here, Annette Gero explains how the Trypanosomatid Crithidia luciliae responds to purine stress by increasing the rates of transport of nucleosides and bases from the environment and by increasing the activity of the ectoenzyme 3'-nucleotidase (3'NTase), which breaks down external nucleotides so that they can be salvaged as nucleosides. The increase in activity of the purine transporters, and the 3'NTase activity is simultaneous with a general increase in the purine metabolic pathway, hence ensuring that purines are readily available to the parasite during purine stress.

Molecular Cloning and Characterization of P4 Nuclease from Leishmania infantum

Parasite of the genus Leishmania is reliant on the salvage pathway for recycling of ribonucleotides. A class I nuclease enzyme also known as P4 nuclease is involved in salvage of purines in cutaneous Leishmania species but the relevant enzymes have not been characterized in Leishmania infantum (L. infantum). The aim of this study was to clone and characterize the gene encoding class I nuclease in L. infantum. DNA extracted from L. infantum was used for amplification of P4 nuclease gene (Li-P4) by PCR. The product was cloned, sequenced, and expressed in E. coli for further characterization. Analysis of the sequence of Li-P4 revealed that the gene consists of an ORF of 951 bp. Sequence similarity analysis indicated that Li-P4 has a high homology to relevant enzymes of other kintoplastids with the highest homology (88%) to p1/s1 class I nuclease from L. donovani. Western blotting of antirecombinant Li-P4 with promastigote and amastigote stages of L. infantum showed that this nuclease is present in both stages of parasite with higher expression in amastigote stage. The highly conserved nature of this essential enzyme in Leishmania parasites suggests it as a promising drug target for leishmaniasis.

Nutrient transport and pathogenesis in selected parasitic protozoa

Parasitic protozoa, such as malaria parasites, trypanosomes, and Leishmania, acquire a plethora of nutrients from their hosts, employing transport proteins located in the plasma membrane of the parasite. Application of molecular genetic approaches and the completion of genome projects have allowed the identification and functional characterization of a cohort of transporters and their genes in these parasites. This review focuses on a subset of these permeases that have been studied in some detail, that import critical nutrients, and that provide examples of approaches being undertaken broadly with these and other parasite transporters. Permeases reviewed include those for hexoses, purines, iron, polyamines, carboxylates, and amino acids. Topics of special emphasis include structure-function approaches, critical roles for transporters in parasite viability and physiology, regulation of transporter expression, and subcellular targeting. Investigations of parasite transporters impact a broad spectrum of basic biological problems in these protozoa.

Purine restriction induces pronounced translational upregulation of the NT1 adenosine/pyrimidine nucleoside transporter in Leishmania major

Leishmania and other parasitic protozoa are unable to synthesize purines de novo and are reliant upon purine nucleoside and nucleobase transporters to import preformed purines from their hosts. To study the roles of the four purine permeases NT1-NT4 in Leishmania major, null mutants in each transporter gene were prepared and the effect of each gene deletion on purine uptake was monitored. Deletion of the NT3 purine nucleobase transporter gene or both NT3 and the NT2 nucleoside transporter gene resulted in pronounced upregulation of adenosine and uridine uptake mediated by the NT1 permease and also induced up to a 200-fold enhancement in the level of the NT1 protein but not mRNA. A similar level of upregulation of NT1 was achieved in wild-type promastigotes that were transferred to medium deficient in purines. Pulse labelling and treatment of cells with the translation inhibitor cycloheximide revealed that control of NT1 expression occurs primarily at the level of translation and not protein turnover. These observations imply the existence of a translational control mechanism that enhances the ability of Leishmania parasites to import essential purines when they are present at limiting concentrations.

Protein preparation, crystallization and preliminary X-ray analysis of Trypanosoma cruzi nucleoside diphosphate kinase 1

The flagellated protozoan parasite Trypanosoma cruzi is the aetiological agent of Chagas disease. Nucleoside diphosphate kinases (NDPKs) are enzymes that are involved in energy management and nucleoside balance in the cell. T. cruzi TcNDPK1, a canonical isoform, was overexpressed in Escherichia coli as an N-terminally poly-His-tagged fusion protein and crystallized. Crystals grew after 72 h in 0.2 M MgCl(2), 20% PEG 3350. Data were collected to 3.5 A resolution using synchrotron X-ray radiation at the National Synchrotron Light Laboratory (Campinas, Brazil). The crystals belonged to the trigonal space group P3, with unit-cell parameters a = b = 127.84, c = 275.49 A. Structure determination is under way and will provide relevant information that may lead to the first step in rational drug design for the treatment of Chagas disease.

Active site plasticity revealed from the structure of the enterobacterial N-ribohydrolase RihA bound to a competitive inhibitor

Background

Pyrimidine-preferring N-ribohydrolases (CU-NHs) are a class of Ca²⁺-dependent enzymes that catalyze the hydrolytic cleavage of the N-glycosidic bond in pyrimidine nucleosides. With the exception of few selected organisms, their physiological relevance in prokaryotes and eukaryotes is yet under investigation.

Results

Here, we report the first crystal structure of a CU-NH bound to a competitive inhibitor, the complex between the Escherichia coli enzyme RihA bound to 3, 4-diaminophenyl-iminoribitol (DAPIR) to a resolution of 2.1 Å. The ligand can bind at the active site in two distinct orientations, and the stabilization of two flexible active site regions is pivotal to establish the interactions required for substrate discrimination and catalysis.

Conclusions

A comparison with the product-bound RihA structure allows a rationalization of the structural rearrangements required for an enzymatic catalytic cycle, highlighting a substrate-assisted cooperative motion, and suggesting a yet overlooked role of the conserved His82 residue in modulating product release. Differences in the structural features of the active sites in the two homologous CU-NHs RihA and RihB from E. coli provide a rationale for their fine differences in substrate specificity. These new findings hint at a possible role of CU-NHs in the breakdown of modified nucleosides derived from RNA molecules.

The crystal structure and activity of a putative trypanosomal nucleoside phosphorylase reveal it to be a homodimeric uridine phosphorylase

Purine nucleoside phosphorylases (PNPs) and uridine phosphorylases (UPs) are closely related enzymes involved in purine and pyrimidine salvage, respectively, which catalyze the removal of the ribosyl moiety from nucleosides so that the nucleotide base may be recycled. Parasitic protozoa generally are incapable of de novo purine biosynthesis; hence, the purine salvage pathway is of potential therapeutic interest. Information about pyrimidine biosynthesis in these organisms is much more limited. Though all seem to carry at least a subset of enzymes from each pathway, the dependency on de novo pyrimidine synthesis versus salvage varies from organism to organism and even from one growth stage to another. We have structurally and biochemically characterized a putative nucleoside phosphorylase (NP) from the pathogenic protozoan Trypanosoma brucei and find that it is a homodimeric UP. This is the first characterization of a UP from a trypanosomal source despite this activity being observed decades ago. Although this gene was broadly annotated as a putative NP, it was widely inferred to be a purine nucleoside phosphorylase. Our characterization of this trypanosomal enzyme shows that it is possible to distinguish between PNP and UP activity at the sequence level based on the absence or presence of a characteristic UP-specificity insert. We suggest that this recognizable feature may aid in proper annotation of the substrate specificity of enzymes in the NP family.

Molecular structure of two C-aryl-iminocyclitols studied by X-ray andab initiocalculations

(1), C18H26N2O7,Mr= 382.41,P212121,a= 9.7215(9),b= 10.687(1),c= 18.399(2) Å,V= 1911.6(3) Å3,Z= 4,R1= 0.0395. (2), C12H18ClNO4,Mr= 275.72,P21,a= 10.431(1),b= 6.9223(8),c= 18.043(2) Å,β= 102.085(7)°,V= 1273.9(2) Å3,Z= 4,R1= 0.0578. The five membered ring is in a twist conformation in (1) and in the two independent molecules of (2) in an envelope conformation. In both compounds the hydroxyl moieties are involved in hydrogen bonds. The compounds were studied by HF/6-31G** computations.

Preclinical assessment of the treatment of second-stage African trypanosomiasis with cordycepin and deoxycoformycin

Background

There is an urgent need to substitute the highly toxic compounds still in use for treatment of the encephalitic stage of human African trypanosomiasis (HAT). We here assessed the treatment with the doublet cordycepin and the deaminase inhibitor deoxycoformycin for this stage of infection with Trypanosoma brucei (T.b.).

Methodology/Principal Findings

Cordycepin was selected as the most efficient drug from a direct parasite viability screening of a compound library of nucleoside analogues. The minimal number of doses and concentrations of the drugs effective for treatment of T.b. brucei infections in mice were determined. Oral, intraperitoneal or subcutaneous administrations of the compounds were successful for treatment. The doublet was effective for treatment of late stage experimental infections with human pathogenic T.b. rhodesiense and T.b. gambiense isolates. Late stage infection treatment diminished the levels of inflammatory cytokines in brains of infected mice. Incubation with cordycepin resulted in programmed cell death followed by secondary necrosis of the parasites. T.b. brucei strains developed resistance to cordycepin after culture with increasing concentrations of the compound. However, cordycepin-resistant parasites showed diminished virulence and were not cross-resistant to other drugs used for treatment of HAT, i.e. pentamidine, suramin and melarsoprol. Although resistant parasites were mutated in the gene coding for P2 nucleoside adenosine transporter, P2 knockout trypanosomes showed no altered resistance to cordycepin, indicating that absence of the P2 transporter is not sufficient to render the trypanosomes resistant to the drug.

Conclusions/Significance

Altogether, our data strongly support testing of treatment with a combination of cordycepin and deoxycoformycin as an alternative for treatment of second-stage and/or melarsoprol-resistant HAT.

There is an urgent need to substitute the highly toxic arsenic compounds still in use for treatment of the encephalitic stage of African trypanosomiasis, a disease caused by infection with Trypanosoma brucei. We exploited the inability of trypanosomes to engage in de novo purine synthesis as a therapeutic target. Cordycepin was selected from a trypanocidal screen of a 2200-compound library. When administered together with the adenosine deaminase inhibitor deoxycoformycin, cordycepin cured mice inoculated with the human pathogenic subspecies T. brucei rhodesiense or T. brucei gambiense even after parasites had penetrated into the brain. Successful treatment was achieved by intraperitoneal, oral or subcutaneous administration of the compounds. Treatment with the doublet also diminished infection-induced cerebral inflammation. Cordycepin induced programmed cell death of the parasites. Although parasites grown in vitro with low doses of cordycepin gradually developed resistance, the resistant parasites lost virulence and showed no cross-resistance to trypanocidal drugs in clinical use. Our data strongly support testing cordycepin and deoxycoformycin as an alternative for treatment of second-stage and/or melarsoprol-resistant HAT.

Comparative expression profiling of Leishmania: modulation in gene expression between species and in different host genetic backgrounds

Background

Genome sequencing of Leishmania species that give rise to a range of disease phenotypes in the host has revealed highly conserved gene content and synteny across the genus. Only a small number of genes are differentially distributed between the three species sequenced to date, L. major, L. infantum and L. braziliensis. It is not yet known how many of these genes are expressed in the disease-promoting intracellular amastigotes of these species or whether genes conserved between the species are differentially expressed in the host.

Methods/Principal Findings

We have used customised oligonucleotide microarrays to confirm that all of the differentially distributed genes identified by genome comparisons are expressed in intracellular amastigotes, with only a few of these subject to regulation at the RNA level. In the first large-scale study of gene expression in L. braziliensis, we show that only ∼9% of the genes analysed are regulated in their RNA expression during the L. braziliensis life cycle, a figure consistent with that observed in other Leishmania species. Comparing amastigote gene expression profiles between species confirms the proposal that Leishmania transcriptomes undergo little regulation but also identifies conserved genes that are regulated differently between species in the host. We have also investigated whether host immune competence influences parasite gene expression, by comparing RNA expression profiles in L. major amastigotes derived from either wild-type (BALB/c) or immunologically compromised (Rag2^−/− γ_c^−/−) mice. While parasite dissemination from the site of infection is enhanced in the Rag2^−/− γ_c^−/− genetic background, parasite RNA expression profiles are unperturbed.

Conclusion/Significance

These findings support the hypothesis that Leishmania amastigotes are pre-adapted for intracellular survival and undergo little dynamic modulation of gene expression at the RNA level. Species-specific parasite factors contributing to virulence and pathogenicity in the host may be limited to the products of a small number of differentially distributed genes or the differential regulation of conserved genes, either of which are subject to translational and/or post-translational controls.

The single-celled parasite Leishmania, transmitted by sand flies in more than 88 tropical and sub-tropical countries globally, infects man and other mammals, causing a spectrum of diseases called the leishmaniases. Over 12 million people are currently infected worldwide with 2 million new cases reported each year. The type of leishmaniasis that develops in the mammalian host is dependent on the species of infecting parasite and the immune response to infection (that can be influenced by host genetic variation). Our research is focused on identifying parasite factors that contribute to pathogenicity in the host and understanding how these might differ between parasite species that give rise to the different clinical forms of leishmaniasis. Molecules of this type might lead to new therapeutic tools in the longer term. In this paper, we report a comparative analysis of gene expression profiles in three Leishmania species that give rise to different types of disease, focusing on the intracellular stages that reside in mammalian macrophages. Our results show that there are only a small number of differences between these parasite species, with host genetics playing only a minor role in influencing the parasites' response to their intracellular habitat. These small changes may be significant, however, in determining the clinical outcome of infection.

Cytokine-dependent and-independent gene expression changes and cell cycle block revealed in Trypanosoma cruzi-infected host cells by comparative mRNA profiling

Background

The requirements for growth and survival of the intracellular pathogen Trypanosoma cruzi within mammalian host cells are poorly understood. Transcriptional profiling of the host cell response to infection serves as a rapid read-out for perturbation of host physiology that, in part, reflects adaptation to the infective process. Using Affymetrix oligonucleotide array analysis we identified common and disparate host cell responses triggered by T. cruzi infection of phenotypically diverse human cell types.

Results

We report significant changes in transcript abundance in T. cruzi-infected fibroblasts, endothelial cells and smooth muscle cells (2852, 2155 and 531 genes respectively; fold-change ≥ 2, p-value < 0.01) 24 hours post-invasion. A prominent type I interferon response was observed in each cell type, reflecting a secondary response to secreted cytokine in infected cultures. To identify a core cytokine-independent response in T. cruzi-infected fibroblasts and endothelial cells transwell plates were used to distinguish cytokine-dependent and -independent gene expression profiles. This approach revealed the induction of metabolic and signaling pathways involved in cell proliferation, amino acid catabolism and response to wounding as common themes in T. cruzi-infected cells. In addition, the downregulation of genes involved in mitotic cell cycle and cell division predicted that T. cruzi infection may impede host cell cycle progression. The observation of impaired cytokinesis in T. cruzi-infected cells, following nuclear replication, confirmed this prediction.

Conclusion

Metabolic pathways and cellular processes were identified as significantly altered at the transcriptional level in response to T. cruzi infection in a cytokine-independent manner. Several of these alterations are supported by previous studies of T. cruzi metabolic requirements or effects on the host. However, our methods also revealed a T. cruzi-dependent block in the host cell cycle, at the level of cytokinesis, previously unrecognized for this pathogen-host cell interaction.

Trypanosoma brucei brucei: effects of ferrous iron and heme on ecto-nucleoside triphosphate diphosphohydrolase activity

Trypanosoma brucei brucei is the causative agent of animal African trypanosomiasis, also called nagana. Procyclic vector form resides in the midgut of the tsetse fly, which feeds exclusively on blood. Hemoglobin digestion occurs in the midgut resulting in an intense release of free heme. In the present study we show that the magnesium-dependent ecto-nucleoside triphosphate diphosphohydrolase (E-NTPDase) activity of procyclic T. brucei brucei is inhibited by ferrous iron and heme. The inhibition of E-NTPDase activity by ferrous iron, but not by heme, was prevented by pre-incubation of cells with catalase. However, antioxidants that permeate cells, such as PEG-catalase and N-acetyl-cysteine prevented the inhibition of E-NTPDase by heme. Ferrous iron was able to induce an increase in lipid peroxidation, while heme did not. Therefore, both ferrous iron and heme can inhibit E-NTPDase activity of T. brucei brucei by means of formation of reactive oxygen species, but apparently acting through distinct mechanisms.

Antiparasitic chemotherapy: tinkering with the purine salvage pathway

Distinguishable differences between infectine organisms and their respective hosts with respect to metabolism and macromolecular structure provide scopes for detailed characterization of target proteins and/or macromolecules as the focus for the development of selective inhibitors. In order to develop a rational approach to antiparasitic chemotherapy, finding differences in the biochemical pathways of the parasite with respect to the host it infects is therefore of primary importance. Like most parasitic protozoan, the genus Leishmania is an obligate auxotroph of purines and hence for requirement of purine bases depends on its own purine salvage pathways. Among various purine acquisition routes used by the parasite, the pathway involved in assimilation of adenosine nucleotide is unique and differs significantly in the extracellular form of the parasite (promastigotes) from its corresponding intracellular form (amastigotes). Adenosine kinase (AdK) is the gateway enzyme of this pathway and displays stage-specific activity pattern. Therefore, understanding the catalytic mechanism of the enzyme, its structural complexities and mode of its regulation have emerged as one of the major areas of investigation. This review, in general, discusses possible strategies to validate several purine salvage enzymes as targets for chemotherapeutic manipulation with special reference to adenosine kinase of Leishmania donovani. Systemic endotheliosis, commonly known as Kala-azar in India, is caused by the parasitic protozoon Leishmania donovani. The spread of leishmaniases follows the distribution of these vectors in the temperate, tropical and subtropical regions of the world leading to loss of thousands of human lives.' WHO has declared leishmaniasis among one of the six major diseases namely leishmaniasis, malaria, amoebiasis, filariasis, Chagas disease and schistosomiasis in its Special Programme for Research and Training in Tropical Diseases. Strategies for better prophylaxis and urgent therapies must be therefore devised to control this menace among poor and under privileged population. However, the possible availability of antiparasitic vaccines appears remote in near future. Therefore, chemotherapy remains the mainstay for the treatment of most parasitic diseases. Selectivity of an antiparasitic compound must depend upon its mode of specific inhibition of parasite replication leaving host processes unaffected. In principle, these agents are expected to exert their selective actions against growth of the invading organisms by having one or both of the following properties: (i) Selective activation of compounds in question by enzyme (s) from the invading organisms, which are not present in the uninfected cells. (ii) Selective inhibition of vital enzyme(s), which are essential for replication of the parasites. In order to design specific compounds with the above characteristics, it is essential to have a thorough knowledge of the properties of the enzyme(s) and/or macromolecules which are unique to the parasite. Phylogenetic studies suggested that trypanosomatid parasites are relatively early-branching eukaryotic cells and indeed their cellular organization differs considerably from their mammalian hosts counterpart. Various enzymes, metabolites or proteins identified in parasites and known to be absent from or strikingly different in the mammalian hosts were considered as ideal drug targets. Among the various metabolic pathways that are presently being studied for their prospects to be exploited as the target for chemotherapeutic manipulation, the most important are (i) purine salvage (ii) polyamine and thiol metabolism (iii) folate biosynthesis (iv) DNA replication (v) glycolytic and (vi) fatty acid biosynthetic pathways etc. A number of excellent reviews, describing the prospects and efficacies of these pathways, already exist in the literature. Our laboratory is engaged in studying the pathways responsible for synthesis and assimilation ofpurine nucleotides in the parasitic protozoon Leishmania donovani. Therefore, we shall, for the constraint of space, try to restrict the discussion mostly with the purine salvage pathways of various Leishmania parasites with particular reference to the unique features of one of the enzymes of the purine salvage pathway viz AdK and its prospects as the chemotherapeutic target. However, contributions of other workers will also be discussed whenever essential and analogy will be drawn in order to make the reading coherent. The Leishmania genus goes through a dimorphic life cycle. It exists as a promastigote (extracellular form) in the sand fly vector but is converted to an amastigote (intracellular form) upon entry into mammalian macrophages. During this transformation process, the activities of a large number of proteins and/or enzymes have been reported to be stage-specifically altered and hence they could be prospective targets for development of chemotherapeutic regimen based on the exploitable differences of the parasitic proteins from their respective host counterpart.

Efficacy of the tubercidin antileishmania action associated with an inhibitor of the nucleoside transport

Tubercidin (TUB) is an adenosine analog with potent antiparasite action, unfortunately associated with severe host toxicity. Prevention of TUB toxicity can be reached associating nitrobenzylthioinosine (NBMPR), an inhibitor of the purine nucleoside transport, specifically target to the mammal cells. It was demonstrated that this nucleoside transport inhibitor has no significant effect in the in vitro uptake of TUB by Schistosoma mansoni and Trypanosoma gambiense. Seeking to evaluate if the association of these compounds is also effective against leishmania, we analyzed the TUB-NBMPR combined treatment in in vitro cultures of promastigote forms of Leishmania (L.) amazonensis, Leishmania (L.) chagasi, Leishmania (L.) major, and Leishmania (V.) braziliensis as well as in cultures of amastigote forms of L. (L.) amazonensis, mice macrophages infected with L. (L.) amazonensis, and in vivo tests in BALB/c mice infected with L. (L.) amazonensis. We demonstrated that TUB-NBMPR combined treatment can be effective against leishmania cells protecting mammalian cells from TUB toxicity.

Trypanosoma cruzi: multiple nucleoside diphosphate kinase isoforms in a single cell

Nucleoside diphosphate kinases (NDPKs) are multifunctional enzymes involved mainly in the conservation of nucleotides and deoxynucleotides at intracellular levels. Here we report the characterization of two NDPKs from the protozoan parasite Trypanosoma cruzi, the etiological agent of Chagas disease. TcNDPK1 and TcNDPK2 were biochemically characterized presenting different kinetic parameters and regulation mechanisms. NDPK activity was mainly detected in soluble fractions according to the digitonin extraction technique; however 20% of the activity remains insoluble at digitonin concentrations up to 5 mg ml(-1). TcNDPK1 is a short enzyme isoform, whereas TcNDPK2 is a long one containing a DM10 motif. In addition, two other putative NDPK genes (TcNPDK3 and TcNDPK4) were detected by data mining at the T. cruzi genome database. The large number and diversity of NDPK isoforms are in agreement with those previously observed for other T. cruzi phosphotransferases, such as adenylate kinases.

The role of the mitochondrial glycine cleavage complex in the metabolism and virulence of the protozoan parasite Leishmania major

For the human pathogen Leishmania major, a key metabolic function is the synthesis of thymidylate, which requires 5,10-methylenetetrahydrofolate (5,10-CH(2)-THF). 5,10-CH(2)-THF can be synthesized from glycine by the mitochondrial glycine cleavage complex (GCC). Bioinformatic analysis revealed the four subunits of the GCC in the L. major genome, and the role of the GCC in parasite metabolism and virulence was assessed through studies of the P subunit (glycine decarboxylase (GCVP)). First, a tagged GCVP protein was expressed and localized to the parasite mitochondrion. Second, a gcvP(-) mutant was generated and shown to lack significant GCC activity using an indirect in vivo assay after incorporation of label from [2-(14)C]glycine into DNA. The gcvP(-) mutant grew poorly in the presence of excess glycine or minimal serine; these studies also established that L. major promastigotes require serine for optimal growth. Although gcvP(-) promastigotes and amastigotes showed normal virulence in macrophage infections in vitro, both forms of the parasite showed substantially delayed replication and lesion pathology in infections of both genetically susceptible or resistant mice. These data suggest that, as the physiology of the infection site changes during the course of infection, so do the metabolic constraints on parasite replication. This conclusion has great significance to the interpretation of metabolic requirements for virulence. Last, these studies call attention in trypanosomatid protozoa to the key metabolic intermediate 5,10-CH(2)-THF, situated at the junction of serine, glycine, and thymidylate metabolism. Notably, genome-based predictions suggest the related parasite Trypanosoma brucei is totally dependent on the GCC for 5,10-CH(2)-THF synthesis.

Kinetic and mutational analysis of the Trypanosoma brucei NBT1 nucleobase transporter expressed in Saccharomyces cerevisiae reveals structural similarities between ENT and MFS transporters

Parasitic protozoa are unable to synthesise purines de novo and thus depend on the uptake of nucleosides and nucleobases across their plasma membrane through specific transporters. A number of nucleoside and nucleobase transporters from Trypanosoma brucei brucei and Leishmania major have recently been characterised and shown to belong to the equilibrative nucleoside transporter (ENT) family. A number of studies have demonstrated the functional importance of particular transmembrane segments (TMS) in nucleoside-specific ENT proteins. TbNBT1, one of only three bona fide nucleobase-selective members of the ENT family, has previously been shown to be a high-affinity transporter for purine nucleobases and guanosine. In this study, we use the Saccharomyces cerevisiae expression system to build a biochemical model of how TbNBT1 recognises nucleobases. We next performed random in vitro and site-directed mutagenesis to identify residues critical for TbNBT1 function. The identification of residues likely to contribute to permeant binding, when combined with a structural model of TbNBT1 obtained by homology threading, yield a tentative three-dimensional model of the transporter binding site that is consistent with the binding model emerging from the biochemical data. The model strongly suggests the involvement of TMS5, TMS7 and TMS8 in TbNBT1 function. This situation is very similar to that concerning transporters of the major facilitator superfamily (MFS), one of which was used as a template for the threading. This point raises the possibility that ENT and MFS carriers, despite being considered evolutionarily distinct, might in fact share similar topologies and substrate translocations pathways.

CRE recombinase-based positive-negative selection systems for genetic manipulation in Trypanosoma brucei

The limited repertoire of drug-resistance markers imposes a serious obstacle to genetic manipulation of Trypanosoma brucei. Here we describe experiments with a fusion protein that allows positive selection for genome integration followed by CRE recombinase-mediated excision of the marker cassette that can be selected by ganciclovir, although the excision event is so efficient that selection is not strictly necessary. We describe two variants of the tetracycline-inducible pLEW100-based CRE-expression vector that reduced its toxicity when stably integrated into the genome, and we demonstrate that transient transfection of circular pLEW100-CRE is highly efficient at catalyzing marker excision. We used this approach to delete the last two enzymes of the pyrimidine synthesis pathway, creating a cell line that is resistant to fluoroorotic acid, which would allow the same enzymes (PYR6-5) to be used as an alternative negative selectable marker.

Episomally driven antisense mRNA abrogates the hyperinducible expression and function of a unique cell surface class I nuclease in the primitive trypanosomatid parasite, Crithidia luciliae

Here, we show that Crithidia luciliae, a primitive trypanosomatid, purine auxotroph, up-expressed its unique, bi-functional, surface membrane 3'-nucleotidase/nuclease (Cl 3'NT/NU) activity by approximately 1000-fold in response to purine starvation. A second surface membrane phospho-monoesterase, i.e. a tartrate-resistant acid phosphatase (Cl MAcP) was also found to be up-expressed in such purine-starved cells. Here, we used homologous episomal-expression of an antisense construct of the Cl3'NT/NU to dissect the functional expression of these two surface membrane enzymes. In antisense transfected cells, a large excess of the antisense transcript was produced and no trace of any endogenous Cl3'NT/NU sense message was detected. Further, the purine-starvation hyper-induced levels of 3'NT/NU enzyme activity were completely abrogated in these transfected cells versus controls. Moreover, such antisense transcription completely abolished the ability of these transfectants to grow in poly(A)-containing medium demonstrating the essential nature of the 3'NT/NU for the growth/survival of this parasite. In contrast, antisense transcription had no apparent deleterious effects on either endogenous or purine-starvation-induced levels of MAcP enzyme activity, its steady-state mRNA levels, or the constitutive expression of house-keeping genes (e.g. Cl alpha-tubulin) in these transfectants. Cumulatively, results of our antisense experiments demonstrated that the functional nuclease activity of the surface membrane Cl 3'NT/NU was, in fact, critical/essential for the growth and development of these primitive parasites.

Molecular genetic analysis of purine nucleobase transport in Leishmania major

Leishmania major and all other parasitic protozoa are unable to synthesize purines de novo and are therefore reliant upon uptake of preformed purines from their hosts via nucleobase and nucleoside transporters. L. major expresses two nucleobase permeases, NT3 that is a high affinity transporter for purine nucleobases and NT4 that is a low affinity transporter for adenine. nt3((-/-)) null mutant promastigotes were unable to replicate in medium containing 10 microM hypoxanthine, guanine, or xanthine and replicated slowly in 10 microM adenine due to residual low affinity uptake of that purine. The NT3 transporter mediated the uptake of the anti-leishmanial drug allopurinol, and the nt3((-/-)) mutants were resistant to killing by this drug. Expression of the NT3 permease was profoundly downregulated at the protein but not the mRNA level in stationary phase compared with logarithmic phase promastigotes. The nt4((-/-)) null mutant was quantitatively impaired in survival within murine bone marrow-derived macrophages. Extensive efforts to generate an nt3((-/-))/nt4((-/-)) dual null mutant were not successful, suggesting that one of the two nucleobase permeases must be retained for robust growth of the parasite. The phenotypes of these null mutants underscore the importance of purine nucleobase transporters in the Leishmania life cycle and pharmacology.

Molecular and functional analyses of a novel class I secretory nuclease from the human pathogen, Leishmania donovani

The primitive protozoan pathogen of humans, Leishmania donovani, resides and multiplies in highly restricted micro-environments within their hosts (i.e. as promastigotes in the gut lumen of their sandfly vectors and as amastigotes in the phagolysosomal compartments of infected mammalian macrophages). Like other trypanosomatid parasites, they are purine auxotrophs (i.e. lack the ability to synthesize purines de novo) and therefore are totally dependent upon salvaging these essential nutrients from their hosts. In that context, in this study we identified a unique 35-kDa, dithiothreitol-sensitive nuclease and showed that it was constitutively released/secreted by both promastigote and amastigote developmental forms of this parasite. By using several different molecular approaches, we identified and characterized the structure of LdNuc(s), a gene that encodes this new 35-kDa class I nuclease family member in these organisms. Homologous episomal expression of an epitope-tagged LdNuc(s) chimeric construct was used in conjunction with an anti-LdNuc(s) peptide antibody to delineate the functional and biochemical properties of this unique 35-kDa parasite released/secreted enzyme. Results of coupled immunoprecipitation-enzyme activity analyses demonstrated that this "secretory" enzyme could hydrolyze a variety of synthetic polynucleotides as well as several natural nucleic acid substrates, including RNA and single- and double-stranded DNA. Based on these cumulative observations, we hypothesize that within the micro-environments of its host, this leishmanial "secretory" nuclease could function at a distance away from the parasite to harness (i.e. hydrolyze/access) host-derived nucleic acids to satisfy the essential purine requirements of these organisms. Thus, this enzyme might play an important role(s) in facilitating the survival, growth, and development of this important human pathogen.

Molecular cloning and biochemical characterization of Leishmania donovani serine hydroxymethyltransferase

Serine hydroxymethyltransferase (SHMT) catalyzes the inter conversion of serine and tetrahydrofolate (H(4)-folate) to form glycine and 5,10-methylene H(4)-folate and generates one-carbon fragments for the synthesis of nucleotides, methionine, thymidylate, choline, etc. In spite of being an indispensable enzyme of the thymidylate cycle, SHMT in Leishmania donovani remains uncharacterized. The study of L. donovani SHMT (ldSHMT) becomes important as this gene is preferentially expressed in the amastigote stage of parasite, which resides in human macrophages. Here we report cloning, expression and purification of a catalytically active ldSHMT. The homogeneity of recombinant protein was analyzed by denaturing gel electrophoresis and protein was found to be 95% pure having yield of 1mg/l. The recombinant protein is a tetramer of 216kDa as evidenced by gel filtration chromatography and uses serine and tetrahydrofolate as substrates with Km of 1.6 and 2.4mM, respectively. Further biochemical studies revealed that pH optimum of ldSHMT is 7.8 and enzyme is thermally stable up to 45 degrees C. ldSHMT was found sensitive towards denaturants as manifested by loss of enzyme activity at the concentration of 1M urea or 0.25M guanidine hydrochloride. This is the first report of purification and characterization of recombinant SHMT from any protozoan source. Studies on recombinant ldSHMT will help in evaluating this enzyme as potential drug target.

Comparative proteomics of glycosomes from bloodstream form and procyclic culture form Trypanosoma brucei brucei

Peroxisomes are present in nearly every eukaryotic cell and compartmentalize a wide range of important metabolic processes. Glycosomes of Kinetoplastid parasites are peroxisome-like organelles, characterized by the presence of the glycolytic pathway. The two replicating stages of Trypanosoma brucei brucei, the mammalian bloodstream form (BSF) and the insect (procyclic) form (PCF), undergo considerable adaptations in metabolism when switching between the two different hosts. These adaptations involve also substantial changes in the proteome of the glycosome. Comparative (non-quantitative) analysis of BSF and PCF glycosomes by nano LC-ESI-Q-TOF-MS resulted in the validation of known functional aspects of glycosomes and the identification of novel glycosomal constituents.

Transition-state Complex of the Purine-specific Nucleoside Hydrolase of T.vivax: Enzyme Conformational Changes and Implications for Catalysis

Nucleoside hydrolases cleave the N-glycosidic bond of ribonucleosides. Crystal structures of the purine-specific nucleoside hydrolase from Trypanosoma vivax have previously been solved in complex with inhibitors or a substrate. All these structures show the dimeric T. vivax nucleoside hydrolase with an "open" active site with a highly flexible loop (loop 2) in its vicinity. Here, we present the crystal structures of the T. vivax nucleoside hydrolase with both soaked (TvNH-ImmH(soak)) and co-crystallised (TvNH-ImmH(co)) transition-state inhibitor immucillin H (ImmH or (1S)-1-(9-deazahypoxanthin-9-yl)-1,4-dideoxy-1,4-imino-D-ribitol) to 2.1 A and 2.2 A resolution, respectively. In the co-crystallised structure, loop 2 is ordered and folds over the active site, establishing previously unobserved enzyme-inhibitor interactions. As such this structure presents the first complete picture of a purine-specific NH active site, including leaving group interactions. In the closed active site, a water channel of highly ordered water molecules leads out from the N7 of the nucleoside toward bulk solvent, while Trp260 approaches the nucleobase in a tight parallel stacking interaction. Together with mutagenesis results, this structure rules out a mechanism of leaving group activation by general acid catalysis, as proposed for base-aspecific nucleoside hydrolases. Instead, the structure is consistent with the previously proposed mechanism of leaving group protonation in the T. vivax nucleoside hydrolase where aromatic stacking with Trp260 and an intramolecular O5'-H8C hydrogen bond increase the pKa of the N7 sufficiently to allow protonation by solvent. A mechanism that couples loop closure to the positioning of active site residues is proposed based on a comparison of the soaked structure with the co-crystallized structure. Interestingly, the dimer interface area increases by 40% upon closure of loop 2, with loop 1 of one subunit interacting with loop 2 of the other subunit, suggesting a relationship between the dimeric form of the enzyme and its catalytic activity.

Purine and pyrimidine transport in pathogenic protozoa: From biology to therapy

Purine salvage is an essential function for all obligate parasitic protozoa studied to date and most are also capable of efficient uptake of preformed pyrimidines. Much progress has been made in the identification and characterisation of protozoan purine and pyrimidine transporters. While the genes encoding protozoan or metazoan pyrimidine transporters have yet to be identified, numerous purine transporters have now been cloned. All protozoan purine transporter-encoding genes characterised to date have been of the Equilibrative Nucleoside Transporter family conserved in a great variety of eukaryote organisms. However, these protozoan transporters have been shown to be sufficiently different from mammalian transporters to mediate selective uptake of therapeutic agents. Recent studies are increasingly addressing the structure and substrate recognition mechanisms of these vital transport proteins.

The acquisition of purines by trypanosomatids

Parasites of the family Trypanosomatidae have an absolute requirement for purines, yet lack the intracellular machinery to synthesize their own purine ring de novo. As a result, the enzymes devoted to the transport and metabolism of purines are extremely important to the parasite. Here, Claudia Cohn and Michael Gottlieb emphasize the value of understanding purine salvage for the development of trypanocidal drugs, and discuss the putative transporters devoted to purine uptake.

Purification and characterization of RihC, a xanthosine-inosine-uridine-adenosine-preferring hydrolase from Salmonella enterica serovar Typhimurium

Salmonella enterica serovar Typhimurium normally salvage nucleobases and nucleosides by the action of nucleoside phosphorylases and phosphoribosyltransferases. In contrast to Escherichia coli, which catabolizes xanthosine by xanthosine phosphorylase (xapA), Salmonella cannot grow on xanthosine as the sole carbon and energy source. By functional complementation, we have isolated a nucleoside hydrolase (rihC) that can complement a xapA deletion in E. coli and we have overexpressed, purified and characterized this hydrolase. RihC is a heat stable homotetrameric enzyme with a molecular weight of 135 kDa that can hydrolyze xanthosine, inosine, adenosine and uridine with similar catalytic efficiency (k(cat)/Km=1 to 4 x 10(4) M(-1)s(-1)). Cytidine and guanosine is hydrolyzed with approximately 10-fold lower efficiency (k(cat)/Km=0.7 to 1.2 x 10(3) M(-1)s(-1)) while RihC is unable to hydrolyze the deoxyribonucleosides thymidine and deoxyinosine. The Km for all nucleosides except adenosine is in the mM range. The pH optimum is different for inosine and xanthosine and the hydrolytic capacity (k(cat)/Km) is 5-fold higher for xanthosine than for inosine at pH 6.0 while they are similar at pH 7.2, indicating that RihC most likely prefers the neutral form of xanthosine.

Trypanosoma brucei: expression of multiple purine transporters prevents the development of allopurinol resistance

Allopurinol is a hypoxanthine analogue used to treat Leishmania infections that also displays activity against the related parasite Trypanosoma brucei. We have investigated the ease by which resistance to this drug is established in Trypanosoma brucei brucei and correlated this to the mechanisms by which it is accumulated by the parasite. Long-term exposure of procyclic T. b. brucei to 3mM allopurinol did not induce resistance. This appears to be related to the fact that allopurinol was taken up through two distinct nucleobase transporters, H1 and H4, both with high affinity for the drug. The apparent Km for [3H]allopurinol transport by H4 (2.1+/-0.4 microM) was determined by expressing the encoding gene in Saccharomyces cerevisiae. Long-term allopurinol exposure did not change Km (hypoxanthine), Ki (allopurinol), or Vmax values of either H1 or H4 transporters and the cells retained their ability to proliferate with hypoxanthine as sole purine source. This study shows that transport-related resistance to purine antimetabolites is not easily induced in Trypanosoma spp. as long as uptake is mediated by multiple transporters.