Fexinidazole for the treatment of human African trypanosomiasis.

On November 15, 2018, Fexinidazole Winthrop received a positive opinion from the European Medicines Agency (EMA) (under Article 58) for treatment of first-stage (hemolymphatic) and second-stage (meningoencephalitic) human African trypanosomiasis caused by Trypanosoma gambiense (gHAT) in adults and children 6 years and older and weighing 20 or more kg. This is the first oral regimen for gHAT that is effective in treating both disease stages. Although fexinidazole has potential to simplify current therapies, it does not entirely eliminate the need for disease staging by lumbar puncture because patients with severe stage 2 disease (CSF WBC [cerebrospinal fluid white blood cells] greater than 100 cells/µL) should only be treated with fexinidazole if no other suitable treatment is available. Nausea and vomiting are a common side effect and the drug must be administered during or after the patient's main meal under direct observation by trained health personnel. Due to late relapses, the EMA recommends follow-up to 24 months after treatment.

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.

Peptoid inhibition of trypanothione reductase as a potential antitrypanosomal and antileishmanial drug lead.

One route to the design of lead compounds for rational drug design approaches to developing drugs against trypanosomiasis, Chagas' disease and leishmaniasis is to develop novel inhibitors of the parasite-specific enzyme trypanothione reductase. A lead inhibitor based on a peptoid structure was designed in the present study based on the known strong competitive inhibition of trypanothione reductase by N-benzoyl-Leu-Arg-Arg-beta-naphthylamide and N-benzyloxycarbonyl-Ala-Arg-Arg-4-methoxy- beta-naphthylamide. In the target peptoid the arginyl residues were replaced by alkylimidazolium units and the benzyloxycarbonyl group by the benzylaminocarbonyl function. The peptoid was synthesised using t-butoxycarbonyl protection chemistry and couplings were activated by 2-(1H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate. The resulting peptoid was shown to be a competitive inhibitor of recombinant trypanothione reductase from Trypanosoma cruzi with a K(i) value of 179 microM and with only weak inhibition of human erythrocyte glutathione reductase (the inhibition of glutathione reductase was at least 291-fold weaker than of trypanothione reductase).

Antiprotozoal and cytotoxicity evaluation of sulfonamide and urea analogues of quinacrine.

Sulfonamide and urea derivatives of quinacrine with varying methylene spacer lengths were synthesised and tested for inhibition of trypanothione reductase (TryR) and for activity in vitro against strains of the parasitic protozoa Trypanosoma, Leishmania, and Plasmodium. These derivatives are superior inhibitors of TryR relative to quinacrine with the best compound being 40 times more potent. Urea derivatives generally displayed good in vitro activity against all parasites.

Molecular characterisation of mitochondrial and cytosolic trypanothione-dependent tryparedoxin peroxidases in Trypanosoma brucei.

In trypanosomatids, removal of hydrogen peroxide and other aryl and alkyl peroxides is achieved by the NADPH-dependent trypanothione peroxidase system, whose components are trypanothione reductase (TRYR), trypanothione, tryparedoxin (TRYX) and tryparedoxin peroxidase (TRYP). Here, we report the cloning of a multi-copy tryparedoxin peroxidase gene (TRYP1) from Trypanosoma brucei brucei encoding a protein with two catalytic VCP motifs similar to the cytosolic TRYP from Crithidia fasciculata. In addition, we characterise a novel single copy gene encoding a second tryparedoxin peroxidase (TRYP2). TRYP2 shows 51% similarity to TRYP1, possesses a putative mitochondrial import sequence at its N-terminus and has a variant IPC motif replacing the second VCP motif implicated in catalysis in other 2-Cys peroxiredoxins. TRYP1 and TRYP2 were expressed in Escherichia coli, and the purified recombinant proteins shown to utilise hydrogen peroxide in the presence of NADPH, trypanothione, TRYR and TRYX from T. brucei, similar to the C. fasciculata cytoplasmic system. Western blots showed that TRYX, TRYP1 and TRYP2 are expressed in both bloodstream and procyclic forms of the life cycle. To determine the precise localisation of TRYX, TRYP1 and TRYP2 in the parasite, polyclonal antibodies to purified recombinant TRYX and TRYP1 and monoclonal antibody to TRYP2 were generated in mice. In-situ immunofluorescence and immunoelectron microscopy revealed a colocalisation of TRYX and TRYP1 in the cytosol, whereas TRYP2 was principally localised in the mitochondrion.

Brave new world of post-genomics!

The Royal Society Discussion Meeting on "Utilizing the Genome Sequence of Parasitic Protozoa" was held at the Royal Society in London, UK, 21-22 March 2001.

[Inactivation of Trypanosoma cruzi trypanothione reductase by phenothiazine cationic free radicals]. / Inactivación de la tripanotiona reductasa de Trypanosoma cruzi por radicales libres catiónicos de fenotiazinas.

Peroxidase/H2O2/phenothiazine systems produced irreversible inhibition (inactivation) of Trypanosoma cruzi trypanothione reductase (TR). The enzyme inactivation depended on (a) the incubation time of TR with the peroxidase/H2O2/phenothiazine system; (b) the peroxidase nature and (c) the phenothiazine structure. With the more effective peroxidase/H2O2/phenothiazine systems, TR inactivation kinetics presented a relatively fast initial phase, lasting for about 10 min, in which most of the enzyme activity disappeared. This phase was followed by a slower one and, after 30 min incubation, TR was totally inactivated. Three peroxidases were assayed as catalysts of TR inactivation: the horseradish peroxidase (HRP), leukocyte myeloperoxidase (MPO) and modified myoglobin (Mb). Under comparable experimental conditions, the peroxidase system activity decreased in the given order. With HRP systems, 10 microM Thioridazine (TRDZ), Promazine (PZ), Trimeprazine (TMPZ), Prochlorperazine (PCZ), Propionylpromazine (PPZ), Chlorpromazine (CPZ) and Perphenazine (PFZ), produced 95-100% inactivation of TR. With the MPO/H2O2 systems, PZ. TRDZ and TMPZ were the most effective. Under similar experimental condition, the Mb/H2O2/PZ,/TMPZ, /TRDZ and CPZ systems effectively inactivated TR. The presence of alkylamino, piperazinyl, or piperidinyl groups in PTZ N atom (position 10) and -Cl, -CF3, -SCH3, COCH2CH3 and -CN in position C2 exerted significant influence on phenothiazine activity. Glutathione (GSH) prevented TR inactivation by the HRP/H2O2/PZ and MPO/H2O2/PZ systems. The HRP/H2O2 and MPO/H2O2/phenothiazines systems generated the corresponding cationic radicals (FTZ.+) the stability of which was limited by their conversion into phenothiazine-sulfoxides (PTZ-SO). The latter ones were inactive on TR. GSH rapidly reacted with PTZ+.; thus producing cation radical detoxication. These reactions fit in well with GSH protection of TR against the peroxidase/H2O2/phenothiazine system, as well as with the FTZ.+ role in phenothiazine cytotoxicity.

Ovothiol and trypanothione as antioxidants in trypanosomatids.

The relative amounts of ovothiol A (N(1)-methyl-4-mercaptohistidine) and trypanothione [N(1),N(8)-bis(glutathionyl)spermidine] have been determined in all life cycle stages of representative trypanosomatids (Leishmania spp, Crithidia fasciculata, Trypanosoma cruzi and T. brucei). Ovothiol A is present in all insect stages with intracellular concentrations of >1 mM for five species of Leishmania promastigotes and <0.25 mM for other trypanosomatids. In Leishmania promastigotes, ovothiol A can exceed trypanothione content particularly in late logarithmic and stationary phases of growth. In the other trypanosomatids, it represents less than 10% of the total thiol pool. Although amastigotes of L. major and L. donovani contain equivalent amounts of glutathione and trypanothione, ovothiol A is present in the former but absent in the latter. Ovothiol A is present in all developmental stages of T. cruzi but absent in bloodstream trypomastigotes of T. brucei. No ovothiol reductase activity could be detected in dialysed parasite extracts. Ovothiol disulphide is not a substrate for trypanothione reductase, although it can be reduced by the concerted action of trypanothione and trypanothione reductase. No ovothiol-dependent peroxidase activity was present in Leishmania extracts. Although ovothiol A can act as a non-enzymatic scavenger of hydrogen peroxide, it is less efficient than trypanothione. Second order rate constants were determined with trypanothione>glutathionylspermidine>ovothiol>glutathione. Given the presence of an active trypanothione peroxidase system in all these trypanosomatids, it is concluded that under physiological conditions, ovothiol is unlikely to play a major role in the metabolism of hydrogen peroxide in intact cells. Nonetheless, since ovothiol is absent in host macrophage, kidney and CHO cells, this metabolite may have other important functional roles in trypanosomatids that could be exploited as a chemotherapeutic target.

Trypanosoma cruzi trypanothione reductase is inactivated by peroxidase-generated phenothiazine cationic radicals.

Trypanosoma cruzi trypanothione reductase (TR) was irreversibly inhibited by peroxidase/H2O2 /phenothiazine (PTZ) systems. TR inactivation depended on (a) time of incubation with the phenothiazine system; (b) the peroxidase nature and (c) the PTZ structure and concentration. With the most effective systems, TR inactivation kinetics were biphasic, with a relatively fast initial phase during which about 75% of the enzyme activity was lost, followed by a slower phase leading to total enzyme inactivation. GSH prevented TR inactivation by the peroxidase/H2O2/PTZ+* systems. Production of PTZ+* cation radicals by PTZ peroxidation was essential for TR inactivation. Horseradish peroxidase, leukocyte myeloperoxidase (MPO) and the pseudo-peroxidase myoglobin (Mb) were effective catalysts of PTZ+* production. Promazine, thioridazine, chlorpromazine, propionylpromazine prochlorperazine, perphenazine and trimeprazine were effective constituents of the HRP/H2O2 /PTZ system. The presence of substituents at the PTZ nucleus position 2 exerted significant influence on PTZ activity, as shown by the different effects of 2-trifluoromethyl and 2-H or 2-chlorophenothiazines. The PTZ+* cation radicals disproportionation regenerated the non-radical PTZ molecule and produced the PTZ sulfoxide that was inactive on TR. Thiol compounds including GSH interacted with PTZ+* cation radicals transferring an electron from the sulfide anion to the PTZ+*, thus nullifying the PTZ+* biological and chemical activities.

Specific peptide inhibitors of trypanothione reductase with backbone structures unrelated to that of substrate: potential rational drug design lead frameworks.

By introducing cationic charge sites novel peptide lead inhibitor structures for trypanothione reductase have been designed using molecular modelling methods. The inhibitors showed reversible, linear competitive inhibition and the strongest peptide inhibitor to date was found to be N-benzyloxycarbonyl-Ala-Arg-Arg-4-methoxy-beta-naphthylamide with a Ki value of 2.4 microM and a selectivity for parasitic enzyme (trypanothione reductase) over the host enzyme (human glutathione reductase) of over 3 orders of magnitude.

Inactivación de la tripanotiona reductasa de Trypanosoma cruzi por radicales libres catiónicos de fenotiazinas / Inactivation of Trypanosoma cruzi trypanothione reductase by phenothiazine cationic free radicals

Peroxidase/H2O2/phenothiazine systems produced irreversible inhibition (inactivation) of Trypanosoma cruzi trypanothione reductase (TR). The enzyme inactivation depended on (a) the incubation time of TR with the peroxidase/H2O2/phenothiazine system; (b) the peroxidase nature and (c) the phenothiazine structure. With the more effective peroxidase/H2O2/phenothiazine systems, TR inactivation kinetics presented a relatively fast initial phase, lasting for about 10 min, in which most of the enzyme activity disappeared. This phase was followed by a slower one and, after 30 min incubation, TR was totally inactivated. Three peroxidases were assayed as catalysts of TR inactivation: the horseradish peroxidase (HRP), leukocyte myeloperoxidase (MPO) and modified myoglobin (Mb). Under comparable experimental conditions, the peroxidase system activity decreased in the given order. With HRP systems, 10 microM Thioridazine (TRDZ), Promazine (PZ), Trimeprazine (TMPZ), Prochlorperazine (PCZ), Propionylpromazine (PPZ), Chlorpromazine (CPZ) and Perphenazine (PFZ), produced 95-100 inactivation of TR. With the MPO/H2O2 systems, PZ. TRDZ and TMPZ were the most effective. Under similar experimental condition, the Mb/H2O2/PZ,/TMPZ, /TRDZ and CPZ systems effectively inactivated TR. The presence of alkylamino, piperazinyl, or piperidinyl groups in PTZ N atom (position 10) and -Cl, -CF3, -SCH3, COCH2CH3 and -CN in position C2 exerted significant influence on phenothiazine activity. Glutathione (GSH) prevented TR inactivation by the HRP/H2O2/PZ and MPO/H2O2/PZ systems. The HRP/H2O2 and MPO/H2O2/phenothiazines systems generated the corresponding cationic radicals (FTZ.+) the stability of which was limited by their conversion into phenothiazine-sulfoxides (PTZ-SO). The latter ones were inactive on TR. GSH rapidly reacted with PTZ+.; thus producing cation radical detoxication. These reactions fit in well with GSH protection of TR against the peroxidase/H2O2/phenothiazine system, as well as with the FTZ.+ role in phenothiazine cytotoxicity.

Inactivación de la tripanotiona reductasa de Trypanosoma cruzi por radicales libres catiónicos de fenotiazinas / Inactivation of Trypanosoma cruzi trypanothione reductase by phenothiazine cationic free radicals

Peroxidase/H2O2/phenothiazine systems produced irreversible inhibition (inactivation) of Trypanosoma cruzi trypanothione reductase (TR). The enzyme inactivation depended on (a) the incubation time of TR with the peroxidase/H2O2/phenothiazine system; (b) the peroxidase nature and (c) the phenothiazine structure. With the more effective peroxidase/H2O2/phenothiazine systems, TR inactivation kinetics presented a relatively fast initial phase, lasting for about 10 min, in which most of the enzyme activity disappeared. This phase was followed by a slower one and, after 30 min incubation, TR was totally inactivated. Three peroxidases were assayed as catalysts of TR inactivation: the horseradish peroxidase (HRP), leukocyte myeloperoxidase (MPO) and modified myoglobin (Mb). Under comparable experimental conditions, the peroxidase system activity decreased in the given order. With HRP systems, 10 microM Thioridazine (TRDZ), Promazine (PZ), Trimeprazine (TMPZ), Prochlorperazine (PCZ), Propionylpromazine (PPZ), Chlorpromazine (CPZ) and Perphenazine (PFZ), produced 95-100 inactivation of TR. With the MPO/H2O2 systems, PZ. TRDZ and TMPZ were the most effective. Under similar experimental condition, the Mb/H2O2/PZ,/TMPZ, /TRDZ and CPZ systems effectively inactivated TR. The presence of alkylamino, piperazinyl, or piperidinyl groups in PTZ N atom (position 10) and -Cl, -CF3, -SCH3, COCH2CH3 and -CN in position C2 exerted significant influence on phenothiazine activity. Glutathione (GSH) prevented TR inactivation by the HRP/H2O2/PZ and MPO/H2O2/PZ systems. The HRP/H2O2 and MPO/H2O2/phenothiazines systems generated the corresponding cationic radicals (FTZ.+) the stability of which was limited by their conversion into phenothiazine-sulfoxides (PTZ-SO). The latter ones were inactive on TR. GSH rapidly reacted with PTZ+.; thus producing cation radical detoxication. These reactions fit in well with GSH protection of TR against the peroxidase/H2O2/phenothiazine system, as well as with the FTZ.+ role in phenothiazine cytotoxicity.(AU)

Cloning of a pyruvate phosphate dikinase from Trypanosoma cruzi.

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

Inactivación de la tripanotiona reductasa de Trypanosoma cruzi por radicales libres catiónicos de fenotiazinas. / [Inactivation of Trypanosoma cruzi trypanothione reductase by phenothiazine cationic free radicals]

Peroxidase/H2O2/phenothiazine systems produced irreversible inhibition (inactivation) of Trypanosoma cruzi trypanothione reductase (TR). The enzyme inactivation depended on (a) the incubation time of TR with the peroxidase/H2O2/phenothiazine system; (b) the peroxidase nature and (c) the phenothiazine structure. With the more effective peroxidase/H2O2/phenothiazine systems, TR inactivation kinetics presented a relatively fast initial phase, lasting for about 10 min, in which most of the enzyme activity disappeared. This phase was followed by a slower one and, after 30 min incubation, TR was totally inactivated. Three peroxidases were assayed as catalysts of TR inactivation: the horseradish peroxidase (HRP), leukocyte myeloperoxidase (MPO) and modified myoglobin (Mb). Under comparable experimental conditions, the peroxidase system activity decreased in the given order. With HRP systems, 10 microM Thioridazine (TRDZ), Promazine (PZ), Trimeprazine (TMPZ), Prochlorperazine (PCZ), Propionylpromazine (PPZ), Chlorpromazine (CPZ) and Perphenazine (PFZ), produced 95-100

inactivation of TR. With the MPO/H2O2 systems, PZ. TRDZ and TMPZ were the most effective. Under similar experimental condition, the Mb/H2O2/PZ,/TMPZ, /TRDZ and CPZ systems effectively inactivated TR. The presence of alkylamino, piperazinyl, or piperidinyl groups in PTZ N atom (position 10) and -Cl, -CF3, -SCH3, COCH2CH3 and -CN in position C2 exerted significant influence on phenothiazine activity. Glutathione (GSH) prevented TR inactivation by the HRP/H2O2/PZ and MPO/H2O2/PZ systems. The HRP/H2O2 and MPO/H2O2/phenothiazines systems generated the corresponding cationic radicals (FTZ.+) the stability of which was limited by their conversion into phenothiazine-sulfoxides (PTZ-SO). The latter ones were inactive on TR. GSH rapidly reacted with PTZ+.; thus producing cation radical detoxication. These reactions fit in well with GSH protection of TR against the peroxidase/H2O2/phenothiazine system, as well as with the FTZ.+ role in phenothiazine cytotoxicity.

The structure of reduced tryparedoxin peroxidase reveals a decamer and insight into reactivity of 2Cys-peroxiredoxins.

Tryparedoxin peroxidase (TryP) is a recently discovered 2Cys-peroxiredoxin involved in defence against oxidative stress in parasitic trypanosomatids. The crystal structure of recombinant Crithidia fasciculata TryP, in the reduced state, has been determined using multi-wavelength anomalous dispersion methods applied to a selenomethionyl derivative. The model comprises a decamer with 52 symmetry, ten chloride ions with 23 water molecules and has been refined, using data to 3.2 A resolution (1 A=0.1 nm), to an R-factor and R(free) of 27.3 and 28.6 %, respectively. Secondary structure topology places TryP along with tryparedoxin and glutathione peroxidase in a distinct subgroup of the thioredoxin super-family. The molecular details at the active site support ideas about the enzyme mechanism and comparisons with an oxidised 2Cys-peroxiredoxin reveal structural alterations induced by the change in oxidation state. These include a difference in quaternary structure from dimer (oxidised form) to decamer (reduced form). The 2Cys-peroxiredoxin assembly may prevent indiscriminate oligomerisation, localise ten peroxidase active sites and contribute to both the specificity of reduction by the redox partner tryparedoxin and attraction of peroxides into the active site.

Synthesis and evaluation of 9,9-dimethylxanthene tricyclics against trypanothione reductase, Trypanosoma brucei, Trypanosoma cruzi and Leishmania donovani.

Derivatives of 9,9-dimethylxanthene were synthesised and evaluated against trypanothione reductase (TR) and in vitro against parasitic trypanosomes and leishmania. High in vitro antiparasitic activity was observed for some derivatives with one compound showing high activity against all three parasites (ED50 values of 0.02, 0.48 and 0.32 microM, for Trypanosoma brucei, Trypanosoma cruzi, and Leishmania donovani, respectively). The lack of correlation between inhibitory activity against TR and ED50 values suggests that TR is not the target.

The three-dimensional structure of a Plasmodium falciparum cyclophilin in complex with the potent anti-malarial cyclosporin A.

Cyclosporin A (CsA) is a potent anti-malarial compound in vitro and in vivo in mice though better known for its immunosuppressive properties in humans. Crystal structures of wild-type and a double mutant Plasmodium falciparum cyclophilin (PfCyP19 and mPfCyP19) complexed with CsA have been determined using diffraction terms to a resolution of 2.1 A (1 A=0.1 nm). The wild-type has a single PfCyP19/CsA complex per asymmetric unit in space group P1 and refined to an R-work of 0.15 and R-free of 0.19. An altered cyclophilin, with two accidental mutations, Phe120 to Leu in the CsA binding pocket and Leu171 to Trp at the C terminus, presents two complexes per asymmetric unit in the orthorhombic space group P2(1)2(1)2. This refined to an R-work of 0.18 and R-free 0.21. The mutations were identified from the crystallographic analysis and the C-terminal alteration helps to explain the different crystal forms obtained. PfCyP19 shares approximately 61 % sequence identity with human cyclophilin A (hCyPA) and the structures are similar, consisting of an eight-stranded antiparallel beta-barrel core capped by two alpha-helices. The fold creates a hydrophobic active-site, the floor of which is formed by side-chains of residues from four antiparallel beta-strands and the walls from loops and turns. We identified C-H.O hydrogen bonds between the drug and protein that may be an important feature of cyclophilins and suggest a general mode of interaction between hydrophobic molecules. Comparisons with cyclophilin-dipeptide complexes suggests that a specific C-H.O hydrogen bonding interaction may contribute to ligand binding. Residues Ser106, His99 and Asp130, located close to the active site and conserved in most cyclophilins, are arranged in a manner reminiscent of a serine protease catalytic triad. A Ser106Ala mutant was engineered to test the hypothesis that this triad contributes to CyP function. Mutant and wild-type enzymes were found to have similar catalytic properties.

Uptake of the nitroimidazole drug megazol by African trypanosomes.

Megazol, CL 64,855 (2-amino-5-[1-methyl-5-nitro-2-imidazolyl]-1,3, 4-thiazole) has been shown to be extremely effective in clearing experimental infections of African trypanosomes. An unusual amino-purine transporter termed P2, implicated in the transport of both the diamidine and melaminophenyl arsenical classes of drug in Trypanosoma brucei, recognised chemical groups on compounds which are also present on megazol. Megazol interacted with this carrier protein, as judged by its ability to inhibit P2 adenosine transport and to abrogate in vitro arsenical-induced lysis in a dose-dependent manner. However, parasites resistant to melaminophenyl arsenical and diamidine drugs due to lack of the P2 transporter showed no resistance to megazol. This is because passive diffusion represented the major route of entry. Initial rates of uptake were not saturable within the limit of megazol's solubility and did not conform to thermodynamic precepts compatible with carrier-mediated uptake. Adenosine and other P2 transporter substrates, even at high concentration, had little impact on megazol uptake. Uptake was biphasic, with a very rapid equilibration across the membrane followed by a slower accumulation over time. The equilibration phase represented a simple passive diffusion, with the subsequent uptake probably being due to metabolism of the drug.

Trypanosomes lacking trypanothione reductase are avirulent and show increased sensitivity to oxidative stress.

In Kinetoplastida, trypanothione and trypanothione reductase (TRYR) provide an intracellular reducing environment, substituting for the glutathione-glutathione reductase system found in most other organisms. To investigate the physiological role of TRYR in Trypanosoma brucei, we generated cells containing just one trypanothione reductase gene, TRYR, which was under the control of a tetracycline-inducible promoter. This enabled us to regulate TRYR activity in the cells from less than 1% to 400% of wild-type levels by adjusting the concentration of added tetracycline. In normal growth medium (which contains reducing agents), trypanosomes containing less than 10% of wild-type enzyme activity were unable to grow, although the levels of reduced trypanothione and total thiols remained constant. In media lacking reducing agents, hypersensitivity towards hydrogen peroxide (EC50 = 3.5 microM) was observed compared with the wild type (EC50 = 223 microM). The depletion of TRYR had no effect on susceptibility to melarsen oxide. The infectivity and virulence of the parasites in mice was dependent upon tetracycline-regulated TRYR activity: if the trypanosomes were injected into mice in the absence of tetracycline, no infection was detectable; and when tetracycline was withdrawn from previously infected animals, the parasitaemia was suppressed.

Synthesis and enzymology of modified N-benzyloxycarbonyl-L-cysteinylglycyl-3,3-dimethylaminopropylamide++ + disulphides as alternative substrates for trypanothione reductase from Trypanosoma cruzi: Part 3.

Kinetic data for alternative substrates of recombinant trypanothione reductase from Trypanosoma cruzi were measured for a series of N-substituted-L-cysteinylglycyl-3-dimethylaminopropylamides, in which the cysteine N-substituent was either a variant of the benzyloxycarbonyl group or was L-phenylalanine or L-tryptophan. Replacing the benzylic ether oxygen atom by CH2 or NH had relatively minor effects on kcat, but raised the value of K(m) 4.5- and 10-fold, respectively. Similarly, relative to the carbobenzoxy group, an N-L-phenylalanyl or N-L-tryptophanyl replacement on the cysteine hardly altered kcat, but increased K(m) values by 16.6 and 7.4 fold, respectively. These observations were consistent with the K(m) values referring primarily to binding for this series of nonspecific substrates.