The Achilles' heel of trypanosomatids: trypanothione-mediated hydroperoxide metabolism

Unique thiol metabolism in trypanosomatids: Redox homeostasis and drug resistance

Trypanosomatids are mainly responsible for heterogeneous parasitic diseases: Leishmaniasis, Sleeping sickness, and Chagas disease and control of these diseases implicates serious challenges due to the emergence of drug resistance. Redox-active biomolecules are the endogenous substances in organisms, which play important role in the regulation of redox homeostasis. The redox-active substances like glutathione, trypanothione, cysteine, cysteine persulfides, etc., and other inorganic intermediates (hydrogen peroxide, nitric oxide) are very useful as defence mechanism. In the present review, the suitability of trypanothione and other essential thiol molecules of trypanosomatids as drug targets are described in Leishmania and Trypanosoma. We have explored the role of tryparedoxin, tryparedoxin peroxidase, ascorbate peroxidase, superoxide dismutase, and glutaredoxins in the anti-oxidant mechanism and drug resistance. Up-regulation of some proteins in trypanothione metabolism helps the parasites in survival against drug pressure (sodium stibogluconate, Amphotericin B, etc.) and oxidative stress. These molecules accept electrons from the reduced trypanothione and donate their electrons to other proteins, and these proteins reduce toxic molecules, neutralize reactive oxygen, or nitrogen species; and help parasites to cope with oxidative stress. Thus, a better understanding of the role of these molecules in drug resistance and redox homeostasis will help to target metabolic pathway proteins to combat Leishmaniasis and trypanosomiases.

The trypanothione system and the opportunities it offers to create drugs for the neglected kinetoplast diseases

Parasitic trypanosomatids (Kinetoplastida) are the causative agents of devastating and hard-to-treat diseases such as African sleeping sickness, Chagas disease and various forms of Leishmaniasis. Altogether they affect > 30 Million patients, account for half a million fatalities p.a. and cause substantial economical problems in the Third World due to human morbidity and life stock losses. The design of efficacious and safe drugs is expected from inhibition of metabolic pathways that are unique and essential to the parasite and absent in the host. In this respect, the trypanothione system first detected in the insect-pathogenic trypanosomatid Crithidia fasciculata qualified as an attractive drug target area. The existence of the system in pathogenic relatives was established by homology cloning and PCR. The vital importance of the system was verified in Trypanosoma brucei by dsRNA technology or knock-out in other trypanosomatids, respectively, and is explained by its pivotal role in the parasite's antioxidant defense and DNA synthesis. The key system component is the bis-glutathionyl derivative of spermidine, trypanothione. It is the proximal reductant of tryparedoxin which substitutes for thioredoxin-, glutaredoxin- and glutathione-dependent reactions. Heterologous expression, functional characterization and crystallization of recombinant system components finally enable structure-based rational inhibitor design.

Depletion of the thioredoxin homologue tryparedoxin impairs antioxidative defence in African trypanosomes

In trypanosomes, the thioredoxin-type protein TXN (tryparedoxin) is a multi-purpose oxidoreductase that is involved in the detoxification of hydroperoxides, the synthesis of DNA precursors and the replication of the kinetoplastid DNA. African trypanosomes possess two isoforms that are localized in the cytosol and in the mitochondrion of the parasites respectively. Here we report on the biological significance of the cTXN (cytosolic TXN) of Trypanosoma brucei for hydroperoxide detoxification. Depending on the growth phase, the concentration of the protein is 3-7-fold higher in the parasite form infecting mammals (50-100 microM) than in the form hosted by the tsetse fly (7-34 microM). Depletion of the mRNA in bloodstream trypanosomes by RNA interference revealed the indispensability of the protein. Proliferation and viability of cultured trypanosomes were impaired when TXN was lowered to 1 muM for more than 48 h. Although the levels of glutathione, glutathionylspermidine and trypanothione were increased 2-3.5-fold, the sensitivity against exogenously generated H2O2 was significantly enhanced. The results prove the essential role of the cTXN and its pivotal function in the parasite defence against oxidative stress.

The efficacy of inhibitors involved in spermidine metabolism in Plasmodium falciparum, Anopheles stephensi and Trypanosoma evansi

In the present study, we have tested the effect of different polyamine inhibitors of the spermidine metabolizing enzymes deoxyhypusine synthase and homospermidine synthase in different chloroquine resistant Plasmodium falciparum strains, in the mosquito Anopheles stephensi (Diptera: Culicidae) and in a Trypanosoma evansi clone I from strain STIB 806 K China. Recent experiments have shown that agmatine is a growth inhibitor of the malaria parasite P. falciparum (Kaiser et al. 2001) in vitro. A comparison of agmatine efficacy with the new antimalarials artemisinin, triclosan and conventional chloroquine showed similar or even better results on the basis of growth inhibition and the reduction of developmental forms. However, no effect of triclosan or agmatine was observed at the ribonucleic acid level. In a second set of experiments, we tested the effect of 1,7-diaminoheptane and agmatine on oocyst formation in A. stephensi after infection with Plasmodium yoelii. Agmatine had an antisporozoite effect since 1,000 microM led to a 59.5% inhibition of oocysts. A much weaker inhibitor of oocyst formation was 1,7-diaminoheptane. The most effective in in vitro inhibition of T. evansi was dicyclohexylamine, an inhibitor of spermidine biosynthesis with an IC(50 ) value of 47.44 microM and the deoxyhypusine inhibitor 1,7-diaminoheptane with an IC(50) value of 47.80 microM. However, both drugs were ineffective in in vivo experiments in a Trypanosoma mouse model. Two different spermidine analogues, 1,8-diaminooctane and 1,3-diaminopropane with IC(50) values of 171 microM and 181.37 microM, respectively, were moderate inhibitors in vitro and ineffective in vivo.

Targeting enzymes involved in spermidine metabolism of parasitic protozoa--a possible new strategy for anti-parasitic treatment

Sequencing data obtained from the Plasmodium, Anopheles gambiae and human genome projects provide a new basis for drug and vaccine development. One of the most characteristic features in the process of drug development against parasitic protozoa is target identification in a biological pathway. The next step must be a structure-based rational drug design if the target is not only present in the parasite. In mouse models of malaria, such drugs should be tested for efficacy of the new therapies. Here, we present data that pinpoint the existence of two enzymes of the polyamine pathway involved in spermidine metabolism in P. falciparum, i.e. deoxyhypusine synthase (DHS; EC 1.1.1.249) and homospermidine synthase (HSS; EC 2.5.1.45). Recent data obtained from the malaria genome databases showed that at least a putative gene encoding DHS is present in the parasite. Sequencing data from the P. falciparum genome project prove that the eukaryotic initiation factor eIF5A (the substrate for DHS) exists in P. falciparum. Here, we present the amino acid sequence of eIF5A from P. vivax, which causes tertiary malaria. EIF5A from P. vivax shows 82% nucleic acid and 97% amino acid identity to its homologue from P. falciparum. GC/MS data and inhibitor studies with agmatine prove that the triamine homospermidine occurs in the parasite. These data suggest a separate locus encoding HSS in P. falciparum. The hss gene recruits from the dhs gene in eukaryotes. Here, we present genomic DNA fragments obtained by amplification with primers of a conserved region (amino acid positions 550-1,043) between the putative P. falciparum DHS gene ( dhs) and the HSS gene ( hss) from the plant Senecio vulgaris (Asteraceae). The amplification product from different P. falciparum strains reveals differences in sequence identity, compared with the putative dhs gene from P. falciparum strain 3D7. Expression of the full-length clone and determination of HSS-specific activity will finally prove whether a separate region encoding HSS exists.

Peroxiredoxins

Present knowledge on peroxiredoxins is reviewed with special emphasis on catalytic principles, specificities and biological function. Peroxiredoxins are low efficiency peroxidases using thiols as reductants. They appear to be fairly promiscuous with respect to the hydroperoxide substrate; the specificities for the donor substrate vary considerably between the subfamilies, comprising GSH, thioredoxin, tryparedoxin and the analogous CXXC motifs in bacterial AhpF proteins. Peroxiredoxins are definitely responsible for antioxidant defense in bacteria (AhpC), yeast (thioredoxin peroxidase) and trypanosomatids (tryparedoxin peroxidase). They are considered to determine virulence of mycobacteria and trypanosomatids. In higher plants they are involved in balancing hydroperoxide production during photosynthesis. In higher animals peroxiredoxins appear to be involved in the redox-regulation of cellular signaling and differentiation, displaying in part opposite effects.

Tryparedoxin peroxidase of Leishmania donovani: molecular cloning, heterologous expression, specificity, and catalytic mechanism

Tryparedoxin peroxidase (TXNPx) of Trypanosomatidae is the terminal peroxidase of a complex redox cascade that detoxifies hydroperoxides by NADPH (Nogoceke et al., Biol. Chem. 378, 827-836, 1997). A gene putatively coding for a peroxiredoxin-type TXNPx was identified in L. donovani and expressed in Escherichia coli to yield an N-terminally His-tagged protein (LdH6TXNPx). LdH6TXNPx proved to be an active peroxidase with tryparedoxin (TXN) 1 and 2 of Crithidia fasciculata as cosubstrates. LdH6TXNPx efficiently reduces H2O2, is moderately active with t-butyl and cumene hydroperoxide, but only marginally with linoleic acid hydroperoxide and phosphatidyl choline hydroperoxide. The enzyme displays ping-pong kinetics with a k(cat) of 11.2 s(-1) and limiting K(m) values for t-butyl hydroperoxide and CfTXN1 of 50 and 3.6 microM, respectively. Site-directed mutagenesis confirmed that C52 and C173, as in related peroxiredoxins, are involved in catalysis. Exchanges of R128 against D and T49 against S and V, supported by molecular modelling, further disclose that the SH group of C52 builds the center of a novel catalytic triad. By hydrogen bonding with the OH of T49 and by the positive charge of R128 the solvent-exposed thiol of C52 becomes deprotonated to react with ROOH. Molecular models of oxidized TXNPx show C52 disulfide-bridged with C173' that can be attacked by C41 of TXN2. By homology, the deduced mechanism may apply to most peroxiredoxins and complements current views of peroxiredoxin catalysis.

New drugs for the treatment of human African trypanosomiasis: research and development

Chemotherapy of human African trypanosomiasis is problematic because of the high frequency of severe adverse events, the long duration and high cost of treatment, and an increasing number of treatment-refractory cases. New cost-efficient, easy-to-use drugs are urgently needed. Whereas basic research on potential drug targets is anchored in academia, the complex, highly regulated and very expensive process of preclinical and clinical drug development is almost exclusively in the hands of pharmaceutical companies. Jennifer Keiser, August Stich and Christian Burri here review, from the angle of industrial drug research and development, the past ten years of research activities at different stages of the development of trypanocidal drugs, and assess future prospects. The absence of compounds in clinical development Phases I-III indicates no new drugs will become available in the next few years.

Glutathione and trypanothione in parasitic hydroperoxide metabolism

Thiol-dependent hydroperoxide metabolism in parasites is reviewed in respect to potential therapeutic strategies. The hydroperoxide metabolism of Crithidia fasciculata has been characterized to comprise a cascade of three enzymes, trypanothione reductase, tryparedoxin, and tryparedoxin peroxidase, plus two supportive enzymes to synthesize the redox mediator trypanothione from glutathione and spermidine. The essentiality of the system in respect to parasite vitality and virulence has been verified by genetic approaches. The system appears to be common to all genera of the Kinetoplastida. The terminal peroxidase of the system belongs to the protein family of peroxiredoxins which is also represented in Entamoeba and a variety of metazoan parasites. Plasmodial hydroperoxide metabolism displays similarities to the mammalian system in comprising glutathione biosynthesis, glutathione reductase, and at least one glutathione peroxidase homolog having the active site selenocysteine replaced by cysteine. Nothing precise is known about the antioxidant defence systems of Giardia, Toxoplasma, and Trichomonas species. Also, the role of ovothiols and mycothiols reportedly present in several parasites remains to be established. Scrutinizing known enzymes of parasitic antioxidant defence for suitability as drug targets leaves only those of the trypanosomatid system as directly or indirectly validated. By generally accepted criteria of target selection and feasibility considerations tryparedoxin and tryparedoxin peroxidase can at present be rated as the most appealing target structures for the development of antiparasitic drugs.

Crystal structure of Trypanosoma cruzi tyrosine aminotransferase: substrate specificity is influenced by cofactor binding mode

The crystal structure of tyrosine aminotransferase (TAT) from the parasitic protozoan Trypanosoma cruzi, which belongs to the aminotransferase subfamily Igamma, has been determined at 2.5 A resolution with the R-value R = 15.1%. T. cruzi TAT shares less than 15% sequence identity with aminotransferases of subfamily Ialpha but shows only two larger topological differences to the aspartate aminotransferases (AspATs). First, TAT contains a loop protruding from the enzyme surface in the larger cofactor-binding domain, where the AspATs have a kinked alpha-helix. Second, in the smaller substrate-binding domain, TAT has a four-stranded antiparallel beta-sheet instead of the two-stranded beta-sheet in the AspATs. The position of the aromatic ring of the pyridoxal-5'-phosphate cofactor is very similar to the AspATs but the phosphate group, in contrast, is closer to the substrate-binding site with one of its oxygen atoms pointing toward the substrate. Differences in substrate specificities of T. cruzi TAT and subfamily Ialpha aminotransferases can be attributed by modeling of substrate complexes mainly to this different position of the cofactor-phosphate group. Absence of the arginine, which in the AspATs fixes the substrate side-chain carboxylate group by a salt bridge, contributes to the inability of T. cruzi TAT to transaminate acidic amino acids. The preference of TAT for tyrosine is probably related to the ability of Asn17 in TAT to form a hydrogen bond to the tyrosine side-chain hydroxyl group.

Activation of active-site cysteine residues in the peroxiredoxin-type tryparedoxin peroxidase of Crithidia fasciculata

Tryparedoxin peroxidase (TXNPx), recently identified as the hydroperoxide-detoxifying enzyme of trypanosomatidae [Nogoceke, E., Gommel, D. U., Kiess, M., Kalisz, H. M. & Flohé, L. (1997) Biol. Chem. 378, 827-836], is a member of the peroxiredoxin family and is characterized by two VCP motifs. Based on a consensus sequence of TXNPx and peroxiredoxin-type peroxidases, eight TXNPx variants were designed, heterologously expressed in Escherichia coli, checked for alpha-helix content by CD and kinetically analysed. The variant Q164E was fully active, C52S, W87D and R128E were inactive and C173S, W87H, W177E and W177H showed reduced activity. Wild-type TXNPx and Q164E exhibit ping-pong kinetics with infinite maximum velocities, whereas saturation kinetics were observed with C173S and W177E. The data comply with a mechanism in which C52, primarily activated by R128 and possibly by W87, is first oxidized by hydroperoxide to a sulfenic acid derivative. C173, supported by W177, then forms an intersubunit disulfide bridge with C52. If C173 is exchanged with a redox-inactive residue (Ser) or is insufficiently activated, the redox shuttle remains restricted to C52. The shift in the kinetic pattern and decrease in specific activity of C173S and W177E may result from a limited accessibility of the oxidized C52 to tryparedoxin, which in the oxidized wild-type TXNPx presumably attacks the C173 sulfur of the disulfide bridge. The proposed mechanism of action of TXNPx is consistent with that deduced for the homologous thioredoxin peroxidase of yeast [Chae, H. Z., Uhm, T. B. & Rhee, S. G. (1994) Proc. Natl Acad. Sci. USA 91, 7022-7026] and is supported by molecular modelling based on the structure of the human peroxiredoxin 'hORF6' [Choi, H.-J., Kang, S. W. Yang, C.-H., Rhee, S. G. & Ryu, S.-E. (1998) Nat. Struct. Biol. 5, 400-406].

Sequence, heterologous expression and functional characterization of tryparedoxin1 from Crithidia fasciculata

Tryparedoxin (TXN) has recently been discovered as a constituent of the complex peroxidase system in the trypanosomatid Crithidia fasciculata [Nogoceke et al. (1997) Biol. Chem. 378, 827-836] where it catalyzes the reduction of a peroxiredoxin-type peroxidase by trypanothione. Here we report on the full-length DNA sequence of the TXN previously isolated from C. fasciculata (TXN1). The deduced amino acid sequence comprises 147 residues and matches with all the peptide sequences of fragments obtained from TXN1. It shares a characteristic sequence motif YFSAxWCPPCR with some thioredoxin-related proteins of unknown function. This motif is homologous with the CXXC motif, which characterizes the thioredoxin superfamily of proteins and is known to catalyze disulfide reductions. Sequence conservations between TXNs and the typical thioredoxins are restricted to the intimate environment of the CXXC motif and three more remote residues presumed to contribute to the folding pattern of the thioredoxin-type proteins. The TXNs thus form a distinct molecular clade within the thioredoxin superfamily. TXN1 was expressed in Escherichia coli BL21 (DE3)pLysS as a C-terminally extended and His-tagged protein, isolated by chelate chromatography and characterized functionally. The recombinant product exhibited a kinetic pattern identical with, and kinetic parameters similar to those of the authentic enzyme in the trypanothione/peroxiredoxin oxidoreductase assay. The recombinant TXN1 can therefore be considered a valuable tool for the screening of specific inhibitors as potential trypanocidal agents.