Enzymes of purine salvage in Trypanosoma cruzi

Synthesis and evaluation of a collection of purine-like C-nucleosides as antikinetoplastid agents

The kinetoplastid parasites Trypanosoma brucei, Trypanosoma cruzi and Leishmania spp. are the causative agents of neglected tropical diseases with a serious burden in several parts of the world. These parasites are incapable of synthesizing purines de novo, and therefore rely on ingenious purine salvage pathways to acquire and process purines from their host. Purine nucleoside analogs that may interfere with these pathways therefore constitute a privileged source of new antikinetoplastid agents. In this study, we synthetized a collection of C-nucleosides employing five different heterocyclic nucleobase surrogates. C-nucleosides are chemically and enzymatically stable and allow for extensive structural modification. Inspired by earlier 7-deazaadenosine nucleosides and known antileishmanial C-nucleosides, we introduced different modifications tailored towards antikinetoplastid activity. Both adenosine and inosine analogs were synthesized with the aim of discovering new antikinetoplastid hits and expanding knowledge of structure-activity relationships. Several promising hits with potent activity against Trypanosoma brucei, Trypanosoma cruzi and Leishmania infantum were discovered, and the nature of the nucleobase surrogate was found to have a profound influence on the selectivity profile of the compounds.

Mode of action of natural and synthetic drugs against Trypanosoma cruzi and their interaction with the mammalian host

Current knowledge of the biochemistry of Trypanosoma cruzi has led to the development of new drugs and the understanding of their mode of action. Some trypanocidal drugs such as nifurtimox and benznidazole act through free radical generation during their metabolism. T. cruzi is very susceptible to the cell damage induced by these metabolites because enzymes scavenging free radicals are absent or have very low activities in the parasite. Another potential target is the biosynthetic pathway of glutathione and trypanothione, the low molecular weight thiol found exclusively in trypanosomatids. These thiols scavenge free radicals and participate in the conjugation and detoxication of numerous drugs. Inhibition of this key pathway could render the parasite much more susceptible to the toxic action of drugs such as nifurtimox and benznidazole without affecting the host significantly. Other drugs such as allopurinol and purine analogs inhibit purine transport in T. cruzi, which cannot synthesize purines de novo. Nitroimidazole derivatives such as itraconazole inhibit sterol metabolism. The parasite's respiratory chain is another potential therapeutic target because of its many differences with the host enzyme complexes. The pharmacological modulation of the host's immune response against T. cruzi infection as a possible chemotherapeutic target is discussed. A large set of chemicals of plant origin and a few animal metabolites active against T. cruzi are enumerated and their likely modes of action are briefly discussed.

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.

Potential chemotherapeutic targets in the purine metabolism of parasites

Parasites are responsible for a wide variety of infectious diseases in human as well as in domestic and wild animals, causing an enormous health and economical blight. Current containment strategies are not entirely successful and parasitic infections are on the rise. In the absence of availability of antiparasitic vaccines, chemotherapy remains the mainstay for the treatment of most parasitic diseases. However, there is an urgent need for new drugs to prevent or combat some major parasitic infections because of lack of a single effective approach for controlling the parasites (e.g., trypanosomiasis) or because some serious parasitic infections developed resistance to presently available drugs (e.g., malaria). The rational design of a drug is usually based on biochemical and physiological differences between pathogens and host. Some of the most striking differences between parasites and their mammalian host are found in purine metabolism. Purine nucleotides can be synthesized by the de novo and/or the so-called "salvage" pathways. Unlike their mammalian host, most parasites studied lack the pathways for de novo purine biosynthesis and rely on the salvage pathways to meet their purine demands. Moreover, because of the great phylogenic separation between the host and the parasite, there are in some cases sufficient distinctions between corresponding enzymes of the purine salvage from the host and the parasite that can be exploited to design specific inhibitors or "subversive substrates" for the parasitic enzymes. Furthermore, the specificities of purine transport, the first step in purine salvage, diverge significantly between parasites and their mammalian host. This review highlights the unique transporters and enzymes responsible for the salvage of purines in parasites that could constitute excellent potential targets for the design of safe and effective antiparasitic drugs.

Efficient identification of inhibitors targeting the closed active site conformation of the HPRT from Trypanosoma cruzi

BACKGROUND

Currently, only two drugs are recommended for treatment of infection with Trypanosoma cruzi, the etiologic agent of Chagas' disease. These compounds kill the trypomastigote forms of the parasite circulating in the bloodstream, but are relatively ineffective against the intracellular stage of the parasite life cycle. Neither drug is approved by the FDA for use in the US. The hypoxanthine phosphoribosyltransferase (HPRT) from T. cruzi is a possible new target for antiparasite chemotherapy. The crystal structure of the HPRT in a conformation approximating the transition state reveals a closed active site that provides a well-defined target for computational structure-based drug discovery.

RESULTS

A flexible ligand docking program incorporating a desolvation correction was used to screen the Available Chemicals Directory for inhibitors targeted to the closed conformation of the trypanosomal HPRT. Of 22 potential inhibitors identified, acquired and tested, 16 yielded K(i)'s between 0.5 and 17 microM versus the substrate phosphoribosylpyrophosphate. Surprisingly, three of eight compounds tested were effective in inhibiting the growth of parasites in infected mammalian cells.

CONCLUSIONS

This structure-based docking method provided a remarkably efficient path for the identification of inhibitors targeting the closed conformation of the trypanosomal HPRT. The inhibition constants of the lead inhibitors identified are unusually favorable, and the trypanostatic activity of three of the compounds in cell culture suggests that they may provide useful starting points for drug design for the treatment of Chagas' disease.

Reduced purine accumulation is encoded on an amplified DNA in Leishmania mexicana amazonensis resistant to toxic nucleosides

Nucleoside analogs are potential anti-Leishmania agents. To better understand how these compounds might lose their effectiveness, Leishmania were independently selected for resistance to inosine dialdehyde or tubercidin. Each of the resistant cells exhibited resistance to inosine dialdehyde and tubercidin as well as to formycin B and allopurinol ribonucleoside. Resistant cells had a greatly reduced capability of accumulating exogenous adenosine, guanosine, thymidine and guanine. This decreased ability to accumulate nucleosides and at least one nucleobase appeared to be due to reduced activity of a number of distinct purine transporters, as the differences between purine metabolizing enzymes were not sufficiently different to account for the decreased accumulation capability. The resistance to toxic nucleosides and the decreased ability to accumulate purines were due to the presence in the resistant cells of an extrachromosomal DNA approximately 55 kb in size. The extrachromosomal DNA was not detected in wild-type cells or revertants which have lost resistance to toxic nucleosides. Except for a 1.2-kb difference, the extrachromosomal DNA from both independently selected resistant cells appeared to be identical. The resistant cells contained 2-4 times as much DNA homologous to the extrachromosomal DNA as compared to wild type cells. When cloned into an E. coli/Leishmania shuttle vector, a portion of the amplified DNA had the ability to confer upon wild-type cells resistance to the toxic purine nucleoside analogs tubercidin and inosine dialdehyde. These transformed cells also exhibited a decreased ability to accumulate non-toxic purine nucleosides.

Specific adenosine phosphorylase from hepatopancreas of gastropod Helix pomatia

1. Specific adenosine phosphorylase from Helix pomatia hepatopancreas was separated from inosine-guanosine phosphorylase and purified 100-165 times; molecular weights were found to be 71,000 and 90,000, respectively. 2. The enzyme is specific for deoxy- and adenosine; it is inactive for 5'-methylthioadenosine and 5-amino-4-imidazole-carboxyamide riboside. Its sensitivity to several inhibitors differs from that of eucaryotic and bacterial purine nucleoside phosphorylases, and is not identical with the sensitivity of the S. mansoni adenosine-splitting enzyme. 3. H. pomatia adenosine phosphorylase differs in its mol. wt and Km values for P(i), ribose 1P, Ado and Ade from adenosine phosphorylase of B. subtilis.

Trypanosoma cruzi adenine nucleoside phosphorylase. Purification and substrate specificity

An adenine nucleoside phosphorylase has been partially purified from extracts of epimastigotes of the Peru strain of Trypanosoma cruzi, the causative agent of Chagas' disease. The purification procedure separated this enzyme from the three other nucleoside-cleaving enzymes found in extracts. The adenine nucleoside phosphorylase, which efficiently cleaved 5'-deoxy-5'-methylthioadenosine (MTA), had a particle weight of 68,000 and exhibited a broad pH optimum between pH 6 and 8. In addition to MTA, the purified enzyme cleaved and synthesized adenosine and 2'-deoxyadenosine with high efficiency. This contrasts to the enzyme from S-180 cells which has been reported to cleave adenosine poorly and not to cleave 2'-deoxyadenosine. Several observations suggested that the three substrates, MTA, adenosine and 2'-deoxyadenosine, use a common catalytic site: (a) all served as alternate-substrate inhibitors exhibiting mutually competitive inhibition with Ki values equivalent to their respective Km values, (b) 5'-chloroformycin A exhibited a competitive Ki value of 4 microM with each nucleoside substrate, and (c) the Km value of phosphate derived from initial velocity studies (180 +/- 20 microM) was independent of the nucleoside substrate. Substrate specificity studies in both the synthesis and cleavage direction indicated that the enzyme had a broad specificity for bases and nucleosides. For the synthesis of nucleosides, the enzyme demonstrated a preference for an amino group in the position equivalent to the 6 position of purine. Compounds containing a hydroxyl group in this position were not substrates. Although a hydrogen or methyl group could substitute for a 6-amino group, a marked decrease in substrate efficiency was observed with these compounds. Alterations in the purine ring led to decreases in the maximal velocity values as evidenced by the substrate or nonsubstrate properties of 1-, 3-, and 7-deazaadenine and 4-aminopyrazolo[3,4-d]pyrimidine. The Km values for 5-methylthioribose 1-phosphate, ribose 1-phosphate and 2'-deoxyribose 1-phosphate with adenine serving as acceptor were 21, 150 and 370 microM. For nucleoside cleavage, the T. cruzi enzyme catalyzed the phosphorolysis of a variety of 5'-substituted adenine-containing nucleosides including those possessing 5'-hydrogen-, hydroxyl-, halogeno-, alkylthio-, amino- and azido-moieties. Inclusion of an ionized group in the 5'-position, such as 5'-carboxy-5'-deoxyadenosine or AMP, precluded substrate activity. 3'-Deoxyadenosine, arabinosyladenine and alpha-adenosine did not serve as substrates.(ABSTRACT TRUNCATED AT 400 WORDS)

Defective transport of pyrazolopyrimidine ribosides in insensitive Trypanosoma cruzi wild strains is a parasite-stage specific and reversible characteristic

1. By using freshly isolated blood trypomastigotes of twelve T. cruzi wild type strains we have found eight strains sensitive to FoB and FoA, while four and one were FoA- and FoB-insensitive respectively to the drug-mediated growth inhibition. 2. This was not so for APPR, to which most strains were transitory insensitive except two which were clearly sensitive. 3. All these pyrazolopyrimidines blocked trypomastigote-amastigote transformation. 4. Incubation of pyrazolopyrimidine-insensitive wild strains with [3H]FoA, [3H]FoB and [14C]APPR respectively indicates that insensitive cells can only accumulate low concentrations of phosphorylated metabolites. 5. This is due to a defective or impaired pyrazolopyrimidine riboside transport system in the wild type insensitive cells, as we did not detect significant variations in the levels of the various nucleoside and nucleobase metabolism enzymes studied. 6. Additional experiments suggested that FoA and FoB are incorporated by different nucleoside transport systems, as Y and ES strains were FoA-insensitive but FoB-sensitive. 7. Epimastigotes of the same T. cruzi strains were highly sensitive to low concentrations of the three pyrazolopyrimidine ribosides studied. However, when this parasitic form was allowed to transform into trypomastigotes, these cells showed the same pyrazolopyrimidine sensitivity found before, suggesting that in T. cruzi pyrazolopyrimidine riboside-insensitivity is a parasite-stage specific and reversible biochemical characteristic.

Adenosine accumulation in Saccharomyces cerevisiae cultured in medium containing low levels of adenine

By monitoring the in vivo incorporation of low concentrations of radiolabeled adenine into acid-soluble compounds, we observed the unusual accumulation of two nucleosides in Saccharomyces cerevisiae that were previously considered products of nucleotide degradation. Under the culture conditions used in the present study, radiolabeled adenosine was the major acid-soluble intracellular derivative, and radiolabeled inosine was initially detected as the second most prevalent derivative in a mutant lacking adenine aminohydrolase. The use of yeast mutants defective in the conversion of adenine to hypoxanthine or to AMP renders very unlikely the possibility that the presence of adenosine and inosine is attributable to nucleotide degradation. These data can be explained by postulating the existence of two enzyme activities not previously reported in S. cerevisiae. The first of these activities transfers ribose to the purine ring and may be attributable to purine nucleoside phosphorylase (EC 2.4.2.1) or adenosine phosphorylase (EC 2.4.2.-). The second enzyme converts adenosine to inosine and in all likelihood is adenosine aminohydrolase (EC 3.5.4.4).

Purine-metabolising enzymes in Entamoeba histolytica

The enzymes that catalyse the salvage of purines in Entamoeba histolytica trophozoites have been surveyed. Adenine deaminase (EC 3.5.4.2), adenosine deaminase (EC 3.5.4.4), guanine deaminase (EC 3.5.4.3), adenine phosphoribosyltransferase (PRTase) (EC 2.4.2.7), xanthine PRTase (EC 2.4.2.22) and hypoxanthine PRTase (EC 2.4.2.8) were all detected in cell homogenates but only at low activities, whereas AMP deaminase (EC 3.5.4.6) and guanine PRTase (EC 2.4.2.8) were not found. Phosphorylases (EC 2.4.2.1) active in both anabolic and catabolic directions were present and all nucleosides tested were phosphorylated by kinases (EC 2.7.1.15, EC 2.7.1.20, EC 2.7.1.73). 3'-Nucleotidase (EC 3.1.3.6) and 5'-nucleotidase (EC 3.1.3.5) were found, the former being mainly particulate. Nucleotide interconversion enzymes (adenylosuccinate lyase, EC 4.3.2.2; adenylosuccinate synthetase, EC 6.3.4.4; IMP dehydrogenase, EC 1.2.1.14; GMP synthetase, EC 6.3.5.2 and GMP reductase, EC 1.6.6.8) were not detected. The results suggest that in E. histolytica the main route of nucleotide synthesis is from the individual bases through the actions of phosphorylases and kinases.

Adenosine cycle in African trypanosomes

African trypanosomes can convert adenosine to adenosine monophosphate. However, in Trypanosoma brucei, as in T. vivax and T. congolense, most of the adenosine is broken down to adenine before conversion to the nucleotide by adenine phosphoribosyltransferase. Trypanosoma brucei and T. vivax use the purine nucleoside hydrolase for adenosine cleavage while T. congolense uses purine nucleoside phosphorylase for the nucleoside cleavage. Trypanosoma vivax also deaminates adenine to hypoxanthine before its conversion to adenosine monophosphate by way of inosine monophosphate. All African trypanosomes lack adenosine deaminase. This finding particularly demonstrates that the effectiveness of the therapy of African trypanosomiasis with adenosine analogue drugs will depend upon the strain of trypanosome which causes the infection.

Purine and pyrimidine metabolism in the Trypanosomatidae

The pathways leading to purine and pyrimidine nucleotide production in members of the family Trypanosomatidae are discussed with special emphasis on data relating to pathogenic species published from 1974 to 1983 inclusive. Trypanosomes and leishmania in general lack a de novo purine biosynthetic pathway, but have a multiplicity of possible routes for purine salvage. In contrast, pyrimidine nucleotides can be produced by either de novo or salvage pathways. The properties of these pathways in trypanosomatids are compared and contrasted with those of their hosts.

Pyrimidine metabolism in Trichomonas vaginalis

Pyrimidine metabolism in Trichomonas vaginalis was investigated using washed cell suspensions of the organism with radiolabelled pyrimidine ring precursors and preformed pyrimidines. The precursors [14C]orotate, [14C]bicarbonate and [14C]aspartate were not incorporated into the pyrimidine bases of trichomonal nucleic acids, indicating that the protozoan is unable to synthesise the pyrimidine ring and is dependent on the salvage of exogenous pyrimidines. [3H]uracil, [3H]uridine, [3H]cytidine, deoxy[3H]cytidine and [3H]thymidine were all efficiently salvaged, and interconversion between cytosine and uracil nucleotides was detected. Thymidylate synthase activity was not detected, suggesting that T. vaginalis is dependent upon an exogenous supply of thymidine for TMP synthesis.

Comparative aspects of purine metabolism in some African trypanosomes

Some enzymes of purine salvage were detected in the cell-free preparations from bloodstream forms of African trypanosomes: Trypanosoma vivax; T. brucei and T. congolense. Extracts of trypanosomes cleave adenosine and inosine hydrolytically except in T. congolense where adenosine cleavage was mediated by a phosphorylase. All the trypanosomes apparently lacked adenosine deaminase. Adenine aminohydrolase was found only in T. vivax while adenosine monophosphate deaminase was detected in T. brucei and T. congolense. There was no detectable adenosine kinase activity in T. brucei. A pathway is proposed for the metabolism of purines in these trypanosomes.

The enzymes of purine salvage in Trypanosoma cruzi, Trypanosoma brucei and Leishmania mexicana

We have previously shown the presence of various purine salvage enzymes in Trypanosoma cruzi, including phosphoribosyltransferase, aminohydrolase, kinase, phosphorylase and hydrolase activities. We now report that a similar situation occurs in Leishmania mexicana amazonensis and Trypanosoma brucei brucei. In all three organisms we found higher levels of activity for the phosphoribosyltransferase enzymes than for the nucleoside kinases, suggesting a preference for the salvage of purine bases rather than nucleosides. Similarly, absence of inosine phosphorylase activity suggests that only one route for the salvage of hypoxanthine is available to the three organisms. The most striking difference was that whereas T. cruzi and T. brucei possessed adenosine aminohydrolase activity, this was not detected in L. mexicana; instead adenine aminohydrolase activity was found. The overall similarity, as judged by the distribution of enzyme activities, of purine salvage in these three members of the kinetoplastida suggest a broad spectrum of activity for any inhibitor acting in this area; the plethora of alternative salvage pathways, however, suggests that in no case would such inhibition be cidal.

Purine metabolism in the bloodstream forms of Trypanosoma gambiense and Trypanosoma rhodesiense

Bloodstream forms of Trypanosoma brucei gambiense and Trypanosoma brucei rhodesiense are incapable of de novo purine synthesis. Purine bases are converted directly to ribonucleotides and with the exception of guanine, are stable. Guanine is incorporated directly into ribonucleotides and also deaminated to xanthine. Purine ribonucleosides are hydrolyzed rapidly; these reactions may limit their incorporation since purine bases label the nucleotide pools more efficiently than do ribonucleosides. The apparent order of salvage efficiency for ribonucleosides is adenosine greater than inosine greater than guanosine greater than xanthosine for both organisms. T. b. gambiense salvages purine bases in the same order, while T. b. rhodesiense salvages purine bases in the order hypoxanthine greater than adenine greater than guanine greater than xanthine.

Effect of allopurinol on Trypanosoma cruzi: metabolism and biological activity in intracellular and bloodstream forms

Allopurinol (4-hydroxypyrazolo [3,4-d]pyrimidine) is an effective agent in vitro against Trypanosoma cruzi. The important forms of this parasite, with respect to the pathogenesis of Chagas' disease in man, are the bloodstream (trypomastigote) and the intracellular forms. Experiments with radiolabeled allopurinol and analysis of the metabolic products of this compound by high-performance liquid chromatography showed that both the bloodstream and the intracellular forms of T. cruzi metabolize allopurinol in the same manner as has been shown for the epimastigotes in vitro. The metabolic pathways for pyrazolopyrimidines in the pathogenic forms were demonstrated with organisms isolated from infected animals and a tissue culture system infected with T. cruzi. Treatment of infected tissue culture with allopurinol eradicated the infection. This investigation implies that allopurinol may be useful in chemotherapy of T. cruzi infections, a supposition which has been borne out in one animal study.