ATP-dependent ligases in trypanothione biosynthesis--kinetics of catalysis and inhibition by phosphinic acid pseudopeptides

Glutathionylspermidine is an intermediate formed in the biosynthesis of trypanothione, an essential metabolite in defence against chemical and oxidative stress in the Kinetoplastida. The kinetic mechanism for glutathionylspermidine synthetase (EC 6.3.1.8) from Crithidia fasciculata (CfGspS) obeys a rapid equilibrium random ter-ter model with kinetic constants K_GSH = 609 μm, K_Spd = 157 μm and K_ATP = 215 μm. Phosphonate and phosphinate analogues of glutathionylspermidine, previously shown to be potent inhibitors of GspS from Escherichia coli, are equally potent against CfGspS. The tetrahedral phosphonate acts as a simple ground state analogue of glutathione (GSH) (K_i ∼ 156 μm), whereas the phosphinate behaves as a stable mimic of the postulated unstable tetrahedral intermediate. Kinetic studies showed that the phosphinate behaves as a slow-binding bisubstrate inhibitor [competitive with respect to GSH and spermidine (Spd)] with rate constants k₃ (on rate) = 6.98 × 10⁴m⁻¹·s⁻¹ and k₄ (off rate) = 1.3 × 10⁻³ s⁻¹, providing a dissociation constant K_i = 18.6 nm. The phosphinate analogue also inhibited recombinant trypanothione synthetase (EC 6.3.1.9) from C. fasciculata, Leishmania major, Trypanosoma cruzi and Trypanosoma brucei with K_i^app values 20–40-fold greater than that of CfGspS. This phosphinate analogue remains the most potent enzyme inhibitor identified to date, and represents a good starting point for drug discovery for trypanosomiasis and leishmaniasis.

A comparative study of methylglyoxal metabolism in trypanosomatids

The glyoxalase system, comprising the metalloenzymes glyoxalase I (GLO1) and glyoxalase II (GLO2), is an almost universal metabolic pathway involved in the detoxification of the glycolytic byproduct methylglyoxal to d-lactate. In contrast to the situation with the trypanosomatid parasites Leishmania major and Trypanosoma cruzi, this trypanothione-dependent pathway is less well understood in the African trypanosome, Trypanosoma brucei. Although this organism possesses a functional GLO2, no apparent GLO1 gene could be identified in the T. brucei genome. The absence of GLO1 in T. brucei was confirmed by the lack of GLO1 activity in whole cell extracts, failure to detect a GLO1-like protein on immunoblots of cell lysates, and lack of d-lactate formation from methylglyoxal as compared to L. major and T. cruzi. T. brucei procyclics were found to be 2.4-fold and 5.7-fold more sensitive to methylglyoxal toxicity than T. cruzi and L. major, respectively. T. brucei also proved to be the least adept of the ‘Tritryp’ parasites in metabolizing methylglyoxal, producing l-lactate rather than d-lactate. Restoration of a functional glyoxalase system by expression of T. cruzi GLO1 in T. brucei resulted in increased resistance to methylglyoxal and increased conversion of methylglyoxal to d-lactate, demonstrating that GLO2 is functional in vivo. Procyclic forms of T. brucei possess NADPH-dependent methylglyoxal reductase and NAD⁺-dependent l-lactaldehyde dehydrogenase activities sufficient to account for all of the methylglyoxal metabolized by these cells. We propose that the predominant mechanism for methylglyoxal detoxification in the African trypanosome is via the methylglyoxal reductase pathway to l-lactate.

Chemical and genetic validation of dihydrofolate reductase-thymidylate synthase as a drug target in African trypanosomes

The phenotypes of single- (SKO) and double-knockout (DKO) lines of dihydrofolate reductase–thymidylate synthase (DHFR–TS) of bloodstream Trypanosoma brucei were evaluated in vitro and in vivo. Growth of SKO in vitro is identical to wild-type (WT) cells, whereas DKO has an absolute requirement for thymidine. Removal of thymidine from the medium triggers growth arrest in S phase, associated with gross morphological changes, followed by cell death after 60 h. DKO is unable to infect mice, whereas the virulence of SKO is similar to WT. Normal growth and virulence could be restored by transfection of DKO with T. brucei DHFR–TS, but not with Escherichia coli TS. As pteridine reductase (PTR1) levels are unchanged in SKO and DKO cells, PTR1 is not able to compensate for loss of DHFR activity. Drugs such as raltitrexed or methotrexate with structural similarity to folic acid are up to 300-fold more potent inhibitors of WT cultured in a novel low-folate medium, unlike hydrophobic antifols such as trimetrexate or pyrimethamine. DKO trypanosomes show reduced sensitivity to these inhibitors ranging from twofold for trimetrexate to >10 000-fold for raltitrexed. These data demonstrate that DHFR–TS is essential for parasite survival and represents a promising target for drug discovery.

Leishmania trypanothione synthetase-amidase structure reveals a basis for regulation of conflicting synthetic and hydrolytic activities

The bifunctional trypanothione synthetase-amidase catalyzes biosynthesis and hydrolysis of the glutathione-spermidine adduct trypanothione, the principal intracellular thiol-redox metabolite in parasitic trypanosomatids. These parasites are unique with regard to their reliance on trypanothione to determine intracellular thiol-redox balance in defense against oxidative and chemical stress and to regulate polyamine levels. Enzymes involved in trypanothione biosynthesis provide essential biological activities, and those absent from humans or for which orthologues are sufficiently distinct are attractive targets to underpin anti-parasitic drug discovery. The structure of Leishmania major trypanothione synthetase-amidase, determined in three crystal forms, reveals two catalytic domains. The N-terminal domain, a cysteine, histidine-dependent amidohydrolase/peptidase amidase, is a papain-like cysteine protease, and the C-terminal synthetase domain displays an ATP-grasp family fold common to C:N ligases. Modeling of substrates into each active site provides insight into the specificity and reactivity of this unusual enzyme, which is able to catalyze four reactions. The domain orientation is distinct from that observed in a related bacterial glutathionylspermidine synthetase. In trypanothione synthetase-amidase, the interactions formed by the C terminus, binding in and restricting access to the amidase active site, suggest that the balance of ligation and hydrolytic activity is directly influenced by the alignment of the domains with respect to each other and implicate conformational changes with amidase activity. The potential inhibitory role of the C terminus provides a mechanism to control relative levels of the critical metabolites, trypanothione, glutathionylspermidine, and spermidine in Leishmania.

Target assessment for antiparasitic drug discovery

Drug discovery is a high-risk, expensive and lengthy process taking at least 12 years and costing upwards of US$500 million per drug to reach the clinic. For neglected diseases, the drug discovery process is driven by medical need and guided by pre-defined target product profiles. Assessment and prioritisation of the most promising targets for entry into screening programmes is crucial for maximising the chances of success. Here, we describe criteria used in our drug discovery unit for target assessment and introduce the 'traffic-light' system as a prioritisation and management tool. We hope this brief review will stimulate basic scientists to acquire additional information necessary for drug discovery.

Increased levels of thiols protect antimony unresponsive Leishmania donovani field isolates against reactive oxygen species generated by trivalent antimony

The current trend of antimony (Sb) unresponsiveness in the Indian subcontinent is a major impediment to effective chemotherapy of visceral leishmaniasis (VL). Although contributory mechanisms studied in laboratory-raised Sb-R parasites include an up-regulation of drug efflux pumps and increased thiols, their role in clinical isolates is not yet substantiated. Accordingly, our objectives were to study the contributory role of thiols in the generation of Sb unresponsiveness in clinical isolates. Promastigotes were isolated from VL patients who were either Sb responsive (n=2) or unresponsive (n=3). Levels of thiols as measured by HPLC and flow cytometry showed higher basal levels of thiols and a faster rate of thiol regeneration in Sb unresponsive strains as compared with sensitive strains. The effects of antimony on generation of reactive oxygen species (ROS) in normal and thiol-depleted conditions as also their H2O2 scavenging activity indicated that in unresponsive parasites, Sb-mediated ROS generation was curtailed, which could be reversed by depletion of thiols and was accompanied by a higher H2O2 scavenging activity. Higher levels of thiols in Sb-unresponsive field isolates from patients with VL protect parasites from Sb-mediated oxidative stress, thereby contributing to the antimony resistance phenotype.

A comparative study of type I and type II tryparedoxin peroxidases in Leishmania major

The genome of Leishmania major, the causative agent of cutaneous leishmaniasis, contains three almost identical genes encoding putative glutathione peroxidases, which differ only at their N- and C-termini. Because the gene homologues are essential in trypanosomes, they may also represent potential drug targets in Leishmania. Recombinant protein for the shortest of these showed negligible peroxidase activity with glutathione as the electron donor indicating that it is not a bone fide glutathione peroxidase. By contrast, high peroxidase activity was obtained with tryparedoxin, indicating that these proteins belong to a new class of monomeric tryparedoxin-dependent peroxidases (TDPX) distinct from the classical decameric 2-Cys peroxiredoxins (TryP). Mass spectrometry studies revealed that oxidation of TDPX1 with peroxides results in the formation of an intramolecular disulfide bridge between Cys35 and Cys83. Site-directed mutagenesis and kinetic studies showed that Cys35 is essential for peroxidase activity, whereas Cys83 is essential for reduction by tryparedoxin. Detailed kinetic studies comparing TDPX1 and TryP1 showed that both enzymes obey saturation ping-pong kinetics with respect to tryparedoxin and peroxide. Both enzymes show high affinity for tryparedoxin and broad substrate specificity for hydroperoxides. TDPX1 shows higher affinity towards hydrogen peroxide and cumene hydroperoxide than towards t-butyl hydroperoxide, whereas no specific substrate preference could be detected for TryP1. TDPX1 exhibits rate constants up to 8 x 10(4) m(-1).s(-1), whereas TryP1 exhibits higher rate constants approximately 10(6) m(-1).s(-1). All three TDPX proteins together constitute approximately 0.05% of the L. major promastigote protein content, whereas the TryPs are approximately 40 times more abundant. Possible specific functions of TDPXs are discussed.

Bis-acridines as lead antiparasitic agents: structure-activity analysis of a discrete compound library in vitro

Parasitic diseases are of enormous public health significance in developing countries-a situation compounded by the toxicity of and resistance to many current chemotherapeutics. We investigated a focused library of 18 structurally diverse bis-acridine compounds for in vitro bioactivity against seven protozoan and one helminth parasite species and compared the bioactivities and the cytotoxicities of these compounds toward various mammalian cell lines. Structure-activity relationships demonstrated the influence of both the bis-acridine linker structure and the terminal acridine heterocycle on potency and cytotoxicity. The bioactivity of polyamine-linked acridines required a minimum linker length of approximately 10 A. Increasing linker length resulted in bioactivity against most parasites but also cytotoxicity toward mammalian cells. N alkylation, but less so N acylation, of the polyamine linker ameliorated cytotoxicity while retaining bioactivity with 50% effective concentration (EC(50)) values similar to or better than those measured for standard drugs. Substitution of the polyamine for either an alkyl or a polyether linker maintained bioactivity and further alleviated cytotoxicity. Polyamine-linked compounds in which the terminal acridine heterocycle had been replaced with an aza-acridine also maintained acceptable therapeutic indices. The most potent compounds recorded low- to mid-nanomolar EC(50) values against Plasmodium falciparum and Trypanosoma brucei; otherwise, low-micromolar potencies were measured. Importantly, the bioactivity of the library was independent of P. falciparum resistance to chloroquine. Compound bioactivity was a function of neither the potential to bis-intercalate DNA nor the inhibition of trypanothione reductase, an important drug target in trypanosomatid parasites. Our approach illustrates the usefulness of screening focused compound libraries against multiple parasite targets. Some of the bis-acridines identified here may represent useful starting points for further lead optimization.

High-throughput screening affords novel and selective trypanothione reductase inhibitors with anti-trypanosomal activity

Trypanothione reductase (TR), an enzyme that buffers oxidative stress in trypanosomatid parasites, was screened against commercial libraries containing approximately 134,500 compounds. After secondary screening, four chemotypes were identified as screening positives with selectivity for TR over human glutathione reductase. Thirteen compounds from these four chemotypes were purchased, and their in vitro activity against TR and Trypanosoma brucei is described.

Discovery of 2-iminobenzimidazoles as a new class of trypanothione reductase inhibitor by high-throughput screening

A high-throughput screening campaign of a library of 100,000 lead-like compounds identified 2-iminobenzimidazoles as a novel class of trypanothione reductase inhibitors. These 2-iminobenzimidazoles display potent trypanocidal activity against Trypanosoma brucei rhodesiense, do not inhibit closely related human glutathione reductase and have low cytotoxicity against mammalian cells.

Structure and reactivity of Trypanosoma brucei pteridine reductase: inhibition by the archetypal antifolate methotrexate

The protozoan Trypanosoma brucei has a functional pteridine reductase (TbPTR1), an NADPH-dependent short-chain reductase that participates in the salvage of pterins, which are essential for parasite growth. PTR1 displays broad-spectrum activity with pterins and folates, provides a metabolic bypass for inhibition of the trypanosomatid dihydrofolate reductase and therefore compromises the use of antifolates for treatment of trypanosomiasis. Catalytic properties of recombinant TbPTR1 and inhibition by the archetypal antifolate methotrexate have been characterized and the crystal structure of the ternary complex with cofactor NADP+ and the inhibitor determined at 2.2 A resolution. This enzyme shares 50% amino acid sequence identity with Leishmania major PTR1 (LmPTR1) and comparisons show that the architecture of the cofactor binding site, and the catalytic centre are highly conserved, as are most interactions with the inhibitor. However, specific amino acid differences, in particular the placement of Trp221 at the side of the active site, and adjustment of the beta6-alpha6 loop and alpha6 helix at one side of the substrate-binding cleft significantly reduce the size of the substrate binding site of TbPTR1 and alter the chemical properties compared with LmPTR1. A reactive Cys168, within the active site cleft, in conjunction with the C-terminus carboxyl group and His267 of a partner subunit forms a triad similar to the catalytic component of cysteine proteases. TbPTR1 therefore offers novel structural features to exploit in the search for inhibitors of therapeutic value against African trypanosomiasis.

Drug resistance in leishmaniasis

Leishmaniasis is a complex disease, with visceral and cutaneous manifestations, and is caused by over 15 different species of the protozoan parasite genus Leishmania. There are significant differences in the sensitivity of these species both to the standard drugs, for example, pentavalent antimonials and miltefosine, and those on clinical trial, for example, paromomycin. Over 60% of patients with visceral leishmaniasis in Bihar State, India, do not respond to treatment with pentavalent antimonials. This is now considered to be due to acquired resistance. Although this class of drugs has been used for over 60 years for leishmaniasis treatment, it is only in the past 2 years that the mechanisms of action and resistance have been identified, related to drug metabolism, thiol metabolism, and drug efflux. With the introduction of new therapies, including miltefosine in 2002 and paromomycin in 2005-2006, it is essential that there be a strategy to prevent the emergence of resistance to new drugs; combination therapy, monitoring of therapy, and improved diagnostics could play an essential role in this strategy.

Refinement of techniques for the propagation of Leishmania donovani in hamsters

Improved animal models are urgently required for drug and vaccine development against visceral leishmaniasis. Here we report refinements to the hamster model of infection that reduce the severity of the disease as well as the number of animals required to maintain infection while improving parasite yields. A comparison between infection via the intracardiac and intraperitoneal routes showed that the less commonly used intraperitoneal route is the simpler and preferred method. The KAtex latex agglutination test for visceral leishmaniasis accurately detected Leishmania donovani antigen in hamster urine as early as 6 weeks post-inoculation. With modification, this assay could be an important tool in the evaluation of experimental drugs and vaccines.

MYST family histone acetyltransferases in the protozoan parasite Toxoplasma gondii

The restructuring of chromatin precedes tightly regulated events such as DNA transcription, replication, and repair. One type of chromatin remodeling involves the covalent modification of nucleosomes by histone acetyltransferase (HAT) complexes. The observation that apicidin exerts antiprotozoal activity by targeting a histone deacetyltransferase has prompted our search for more components of the histone modifying machinery in parasitic protozoa. We have previously identified GNAT family HATs in the opportunistic pathogen Toxoplasma gondii and now describe the first MYST (named for members MOZ, Ybf2/Sas3, Sas2, and Tip60) family HATs in apicomplexa (TgMYST-A and -B). The TgMYST-A genomic locus is singular and generates a approximately 3.5-kb transcript that can encode two proteins of 411 or 471 amino acids. TgMYST-B mRNA is approximately 7.0 kb and encodes a second MYST homologue. In addition to the canonical MYST HAT catalytic domain, both TgMYST-A and -B possess an atypical C2HC zinc finger and a chromodomain. Recombinant TgMYST-A exhibits a predilection to acetylate histone H4 in vitro at lysines 5, 8, 12, and 16. Antibody generated to TgMYST-A reveals that both the long and short (predominant) versions are present in the nucleus and are also plentiful in the cytoplasm. Moreover, both TgMYST-A forms are far more abundant in rapidly replicating parasites (tachyzoites) than encysted parasites (bradyzoites). A bioinformatics survey of the Toxoplasma genome reveals numerous homologues known to operate in native MYST complexes. The characterization of TgMYST HATs represents another important step toward understanding the regulation of gene expression in pathogenic protozoa and provides evolutionary insight into how these processes operate in eukaryotic cells in general.

Specificity of the trypanothione-dependent Leishmania major glyoxalase I: structure and biochemical comparison with the human enzyme

Trypanothione replaces glutathione in defence against cellular damage caused by oxidants, xenobiotics and methylglyoxal in the trypanosomatid parasites, which cause trypanosomiasis and leishmaniasis. In Leishmania major, the first step in methylglyoxal detoxification is performed by a trypanothione-dependent glyoxalase I (GLO1) containing a nickel cofactor; all other characterized eukaryotic glyoxalases use zinc. In kinetic studies L. major and human enzymes were active with methylglyoxal derivatives of several thiols, but showed opposite substrate selectivities: N1-glutathionylspermidine hemithioacetal is 40-fold better with L. major GLO1, whereas glutathione hemithioacetal is 300-fold better with human GLO1. Similarly, S-4-bromobenzylglutathionylspermidine is a 24-fold more potent linear competitive inhibitor of L. major than human GLO1 (Kis of 0.54 microM and 12.6 microM, respectively), whereas S-4-bromobenzylglutathione is >4000-fold more active against human than L. major GLO1 (Kis of 0.13 microM and >500 microM respectively). The crystal structure of L. major GLO1 reveals differences in active site architecture to both human GLO1 and the nickel-dependent Escherichia coli GLO1, including increased negative charge and hydrophobic character and truncation of a loop that may regulate catalysis in the human enzyme. These differences correlate with the differential binding of glutathione and trypanothione-based substrates, and thus offer a route to the rational design of L. major-specific GLO1 inhibitors.

Differential toxicity of antimonial compounds and their effects on glutathione homeostasis in a human leukaemia monocyte cell line

Trivalent antimonial compounds (Sb(III)), originally used in the treatment of leishmaniasis, are now being proposed as a novel therapy for acute promyelocytic leukaemia (APL). Here, we examine the effects of Sb(III) and pentavalent antimonial drugs (Sb(V)) on glutathione homeostasis, oxidative stress and apoptosis in the human leukaemia monocyte cell line, THP-1. Although growth of THP-1 macrophages is unaffected by Sb(V), macrophages are extremely sensitive to Sb(III). On exposure to Sb(III), intracellular free glutathione (GSH) levels in macrophages decrease linearly by 50% over 4h, associated with efflux of both GSH and accumulation of intracellular glutathione disulphide (GSSG). Together these effects increase the redox potential of the GSSG/GSH couple from -282 to -225mV. Sb(III)-induced GSH efflux from THP-1 macrophages is accompanied by the concomitant efflux of Sb(III) at a constant molar ratio of 3 (GSH) to 1 (Sb(III)), respectively. Sb(III) directly inhibits glutathione reductase activity in macrophages, significantly retarding the regeneration of GSH from GSSG, following diamide oxidation. Sb(III)-treated THP-1 macrophages go on to exhibit elevated levels of reactive oxygen species and show the early signs of apoptosis. The absence of these effects in Sb(V)-treated THP-1 cells suggests that macrophages do not efficiently reduce Sb(V) to Sb(III). Collectively, these findings suggest that Sb(III) seriously compromises thiol homeostasis in THP-1 macrophages and that this may be an early defining event in the mode of action of antimonials against leukaemia cells.

Time-dependent inhibitors of trypanothione reductase: analogues of the spermidine alkaloid lunarine and related natural products

The macrocyclic spermidine alkaloid lunarine 1 from Lunaria biennis is a competitive, time-dependent inhibitor of the protozoan oxidoreductase trypanothione reductase (TryR), a promising target in drug design against tropical parasitic diseases. Various molecules related to 1 and the alkaloid itself have been synthesized in racemic form and evaluated against TryR in order to determine the key features of 1 that are associated with time-dependent inhibition. Kinetic data are consistent with an inactivation mechanism involving a conjugate addition of an active site cysteine residue onto the C-24-C-25 double bond of the tricyclic nucleus of 1. Comparison of data for synthetic (+/-)-1, the natural product, and other derivatives 7-10 from L. biennis confirms the importance of the unique structure of the tricyclic core as a motif for inhibitor design and reveals that the non-natural enantiomer may be a more suitable scaffold upon which thiophilic groups may be presented.

Crystallization and preliminary X-ray analysis of Leishmania major glyoxalase I

Glyoxalase I (GLO1) is a putative drug target for trypanosomatids, which are pathogenic protozoa that include the causative agents of leishmaniasis. Significant sequence and functional differences between Leishmania major and human GLO1 suggest that it may make a suitable template for rational inhibitor design. L. major GLO1 was crystallized in two forms: the first is extremely disordered and does not diffract, while the second, an orthorhombic form, produces diffraction to 2.0 A. Molecular-replacement calculations indicate that there are three GLO1 dimers in the asymmetric unit, which take up a helical arrangement with their molecular dyads arranged approximately perpendicular to the c axis. Further analysis of these data are under way.

The genome of the African trypanosome Trypanosoma brucei

African trypanosomes cause human sleeping sickness and livestock trypanosomiasis in sub-Saharan Africa. We present the sequence and analysis of the 11 megabase-sized chromosomes of Trypanosoma brucei. The 26-megabase genome contains 9068 predicted genes, including approximately 900 pseudogenes and approximately 1700 T. brucei-specific genes. Large subtelomeric arrays contain an archive of 806 variant surface glycoprotein (VSG) genes used by the parasite to evade the mammalian immune system. Most VSG genes are pseudogenes, which may be used to generate expressed mosaic genes by ectopic recombination. Comparisons of the cytoskeleton and endocytic trafficking systems with those of humans and other eukaryotic organisms reveal major differences. A comparison of metabolic pathways encoded by the genomes of T. brucei, T. cruzi, and Leishmania major reveals the least overall metabolic capability in T. brucei and the greatest in L. major. Horizontal transfer of genes of bacterial origin has contributed to some of the metabolic differences in these parasites, and a number of novel potential drug targets have been identified.

Trypanothione metabolism: a chemotherapeutic target in trypanosomatids

Chemotherapy of trypanosomotid infections continues to present challenges. Treatment of Trypanosoma cruzi infections is virtually impossible, while treatment of Leishmaniasis and African tryponosomiasis is, at best, difficult - often involving toxic drugs based on heavy metals such as antimony and arsenic. As with several other parasites, much recent research has focused on defining metabolic differences between parasite and host that could represent good targets for chemotherapy. As Graeme Henderson and Alan Fairlamb explain, there is something strange about glutothione in trypanosomotids - which seems to offer a very promising chemotherapeutic target.

Trypanothione biosynthesis in Leishmania major

Trypanothione plays a crucial role in regulation of intracellular thiol redox balance and in defence against chemical and oxidant stress. Crithidia fasciculata requires two enzymes for the formation of trypanothione, namely glutathionylspermidine synthetase (GspS; EC 6.3.1.8) and a glutathionylspermidine-dependent trypanothione synthetase (TryS; EC 6.3.1.9), whereas Trypanosoma cruzi and Trypanosoma brucei use a broad-specificity trypanothione synthetase to make trypanothione from glutathione (GSH) and spermidine. Here, we report the identification of two genes in Leishmania major with similarity to previously identified GSPS and TRYS. GSPS is an apparent pseudogene containing two frame shift mutations and two stop codons, whereas TRYS is in a single open-reading frame. The enzyme encoded by TRYS was expressed and found to catalyse formation of trypanothione with GSH and either spermidine or glutathionylspermidine. When GSH is varied as substrate the enzyme displays substrate inhibition (apparent Km=89 microM, Ki(s)=1mM, k(cat)=2s-1). At a fixed GSH concentration, the enzyme obeys simple hyperbolic kinetics with the other substrates with apparent Km values for spermidine, glutathionylspermidine and MgATP of 940, 40 and 63 microM, respectively. Immunofluorescence and sub-cellular fractionation studies indicate that TryS localises to the cytosol of L. major promastigotes. Phylogenetic analysis of the GspS and TryS amino acid sequences suggest that in the trypanosomatids, TryS has evolved to replace the GspS/TryS complex in C. fasciculata. It also appears that the L. major still harbours a redundant GSPS pseudogene that may be currently in the process of being lost from its genome.

A trypanothione-dependent glyoxalase I with a prokaryotic ancestry in Leishmania major

Glyoxalase I forms part of the glyoxalase pathway that detoxifies reactive aldehydes such as methylglyoxal, using the spontaneously formed glutathione hemithioacetal as substrate. All known eukaryotic enzymes contain zinc as their metal cofactor, whereas the Escherichia coli glyoxalase I contains nickel. Database mining and sequence analysis identified putative glyoxalase I genes in the eukaryotic human parasites Leishmania major, Leishmania infantum, and Trypanosoma cruzi, with highest similarity to the cyanobacterial enzymes. Characterization of recombinant L. major glyoxalase I showed it to be unique among the eukaryotic enzymes in sharing the dependence of the E. coli enzyme on nickel. The parasite enzyme showed little activity with glutathione hemithioacetal substrates but was 200-fold more active with hemithioacetals formed from the unique trypanosomatid thiol trypanothione. L. major glyoxalase I also was insensitive to glutathione derivatives that are potent inhibitors of all other characterized glyoxalase I enzymes. This substrate specificity is distinct from that of the human enzyme and is reflected in the modification in the L. major sequence of a region of the human protein that interacts with the glycyl-carboxyl moiety of glutathione, a group that is conjugated to spermidine in trypanothione. This trypanothione-dependent glyoxalase I is therefore an attractive focus for additional biochemical and genetic investigation as a possible target for rational drug design.