Synthetic single and double aza-scorpiand macrocycles acting as inhibitors of the antioxidant enzymes iron superoxide dismutase and trypanothione reductase in Trypanosoma cruzi with promising results in a murine model

Synthetic scorpiand-like azamacrocycles selectively inhibit SOD and TR enzymes of Trypanosoma cruzi in mice causing death of the parasites and increasing the mouse survival rate after infection and treatment.

Bioinformatic and experimental fishing for artemisinin-interacting proteins from human nasopharyngeal cancer cells

Determining interacting cellular partners of drugs by chemical proteomic techniques is complex and tedious. Most approaches rely on activity-based probe profiling and compound-centric chemical proteomics. The anti-malarial artemisinin also exerts profound anti-cancer activity, but the mechanisms of action are incompletely understood. In the present investigation, we present a novel approach to identify artemisinin-interacting target proteins. Our approach overcomes usual problems in traditional fishing procedures, because the drug was attached to a surface without further chemical modification. The proteins identified effect among others, cell cycle arrest, apoptosis, inhibition of angiogenesis, disruption of cell migration, and modulation of nuclear receptor responsiveness. Furthermore, a bioinformatic approach confirmed experimentally identified proteins and suggested a large number of other interacting proteins. Theoretically predicted interaction partners may serve as a starting point to complete the whole set of proteins binding artemisinin.

2,3-diphenyl-1,4-naphthoquinone: a potential chemotherapeutic agent against Trypanosoma cruzi

Chagas disease, caused by Trypanosoma cruzi, is a widespread infection in Latin America. Currently, only 2 partially effective and highly toxic drugs, i.e., benznidazole and nifurtimox, are available for the treatment of this disease, and several efforts are underway in the search for better chemotherapeutic agents. Here, we have determined the trypanocidal activity of 2,3-diphenyl-1 ,4-naphthoquinone (DPNQ), a novel quinone derivative. In vitro, DPNQ was highly cytotoxic at a low, micromolar concentration (LD50 = 2.5 microM) against epimastigote, cell-derived trypomastigote, and intracellular amastigote forms of T. cruzi, but not against mammalian cells (LD50 = 130 microM). In vivo studies on the murine model of Chagas disease revealed that DPNQ-treated animals (3 doses of 10 mg/kg/day) showed a significant delay in parasitemia peak and higher (up to 60%) survival rate 70 days post-infection, when compared with the control group (infected, untreated). We also observed a 2-fold decrease in parasitemia between the control group (infected, untreated) and the treated group (infected, treated). No apparent drug toxicity effects were noticed in the control group (uninfected, treated). In addition, we determined that DPNQ is the first competitive inhibitor of T. cruzi lipoamide dehydrogenase (TcLipDH) thus far described. Our results indicate that DPNQ is a promising chemotherapeutic agent against T. cruzi.

Trypanothione-dependent synthesis of deoxyribonucleotides by Trypanosoma brucei ribonucleotide reductase

Trypanosoma brucei, the causative agent of African sleeping sickness, synthesizes deoxyribonucleotides via a classical eukaryotic class I ribonucleotide reductase. The unique thiol metabolism of trypanosomatids in which the nearly ubiquitous glutathione reductase is replaced by a trypanothione reductase prompted us to study the nature of thiols providing reducing equivalents for the parasite synthesis of DNA precursors. Here we show that the dithiol trypanothione (bis(glutathionyl)spermidine), in contrast to glutathione, is a direct reductant of T. brucei ribonucleotide reductase with a K(m) value of 2 mm. This is the first example of a natural low molecular mass thiol directly delivering reducing equivalents for ribonucleotide reduction. At submillimolar concentrations, the reaction is strongly accelerated by tryparedoxin, a 16-kDa parasite protein with a WCPPC active site motif. The K(m) value of T. brucei ribonucleotide reductase for T. brucei tryparedoxin is about 4 micrometer. The disulfide form of trypanothione is a powerful inhibitor of the tryparedoxin-mediated reaction that may represent a physiological regulation of deoxyribonucleotide synthesis by the redox state of the cell. The trypanothione/tryparedoxin system is a new system providing electrons for a class I ribonucleotide reductase, in addition to the well known thioredoxin and glutaredoxin systems described in other organisms.

2- and 3-substituted 1,4-naphthoquinone derivatives as subversive substrates of trypanothione reductase and lipoamide dehydrogenase from Trypanosoma cruzi: synthesis and correlation between redox cycling activities and in vitro cytotoxicity

Trypanothione reductase (TR) is both a valid and an attractive target for the design of new trypanocidal drugs. Starting from menadione, plumbagin, and juglone, three distinct series of 1,4-naphthoquinones (NQ) were synthesized as potential inhibitors of TR from Trypanosoma cruzi (TcTR). The three parent molecules were functionalized at carbons 2 and/or 3 by various polyamine chains. Optimization of TcTR inhibition and TcTR specificity versus human disulfide reductases was achieved with the 3,3'-[polyaminobis(carbonylalkyl)]bis(1,4-NQ) series 19-20, in which an optimum chain length was determined for inhibition of the trypanothione disulfide reduction. The most active derivatives against trypanosomes in cultures were also studied as subversive substrates of TcTR and lipoamide dehydrogenase (TcLipDH). The activities were measured by following NAD(P)H oxidation as well as coupling the reactions to the reduction of cytochrome c which permits the detection of one-electron transfer. For TcTR, 20(4-c) proved to be a potent subversive substrate and an effective uncompetitive inhibitor versus trypanothione disulfide and NADPH. Molecular modeling studies based on the known X-ray structures of TcTR and hGR were conducted in order to compare the structural features, dimensions, and accessibility of the cavity at the dimer interface of TcTR with that of hGR, as one of the putative NQ binding sites. TcLipDH reduced the plumbagin derivatives by an order of magnitude faster than the corresponding menadione derivatives. Such differences were not observed with the pig heart enzyme. The most efficient and specific subversive substrates of TcTR and TcLipDH exhibited potent antitrypanosomal activity in in vitro T. brucei and T. cruzi cultures. The results obtained here confirm that reduction of NQs by parasitic flavoenzymes is a promising strategy for the development of new trypanocidal drugs.

(2,2':6',2"-Terpyridine)platinum(II) complexes are irreversible inhibitors of Trypanosoma cruzi trypanothione reductase but not of human glutathione reductase

(2,2':6',2"-terpyridine)platinum(II) complexes possess pronounced cytostatic activities against trypanosomes and leishmania. As shown here, the complexes are irreversible inhibitors of trypanothione reductase (TR) from Trypanosoma cruzi, the causative agent of Chagas' disease. The most effective derivatives are the (4'-chloro-2, 2':6',2"-terpyridine)platinum(II) ammine and the (4-picoline)(4'-p-bromophenyl-2,2':6',2" -terpyridine)platinum(II) complexes which in the presence of NADPH inhibit TR with second-order rate constants of about 1.3 x 10(4) M(-1) s(-1). The modified enzyme species possess increased oxidase activities. The inhibition is not reversed upon dialysis or treatment with low-molecular-mass thiols. Kinetic and spectroscopic data suggest that Cys52 in the active site has been specifically altered. Inhibition of this key enzyme of parasite thiol metabolism probably contributes to the antitrypanosomal activity of the compounds. In contrast to the parasite enzyme, most (terpyridine)platinum complexes interact only reversibly with human glutathione reductase and an initial inhibition is completely abolished during the course of the assay.

Trypanosoma cruzi dihydrolipoamide dehydrogenase is inactivated by myeloperoxidase-generated "reactive species"

Dihydrolipoamide dehydrogenase (LADH) from Trypanosoma cruzi was inactivated by treatment with myeloperoxidase (MPO)-dependent systems. With MPO/H2O2/NaCl, LADH lipoamide reductase and diaphorase activities significantly decreased as a function of incubation time. Iodide, bromide, thiocyanide and chloride effectively supplemented the MPO/H2O2 system, KI and NaCl being the most and the least effective supplements, respectively. LADH inactivation by MPO/H2O2/NaCl and by NaOCl was similarly prevented by thiol compounds such as GSH, L-cysteine, N-acetylcysteine, penicillamine and N-(2-mercaptopropionyl-glycine) in agreement with the role of HOCI in LADH inactivation by MPO/H2O2/NaCl. LADH was also inactivated by MPO/NADH/halide, MPO/H2O2/NaNO2 and MPO/NADH/NaNO2 systems. Catalase prevented the action of the NADH-dependent systems, thus supporting H2O2 production by NADH-supplemented LADH. MPO inhibitors (4-aminobenzoic acid hydrazide, and isoniazid), GSH, L-cysteine, L-methionine and L-tryptophan prevented LADH inactivation by MPO/H2O2/NaNO2. Other MPO systems inactivating LADH were (a) MPO/H2O2/chlorpromazine; (b) MPO/H2O2/monophenolic systems, including L-tyrosine, serotonin and acetaminophen and (c) MPO/H2O2/di- and polyphenolic systems, including norepinephrine, catechol, nordihydroguaiaretic acid, caffeic acid, quercetin and catechin. Comparison of the above effects and those previously reported with pig myocardial LADH indicates that both enzymes were similarly affected by the MPO-dependent systems, allowance being made for T. cruzi LADH diaphorase inactivation and the greater sensitivity of its LADH lipoamide reductase activity towards the MPO/H2O2/NaCl system and NaOCl.

Inactivation of Trypanosoma cruzi dihydrolipoamide dehydrogenase by leukocyte myeloperoxidase systems: role of hypochloride and nitrite related radicals

Dihydrolipoamide dehydrogenase (LADH) from Trypanosoma cruzi, the causative agent of Chagas' disease, was inactivated by treatment with myeloperoxidase (MPO)-dependent systems. LADH lipoamide reductase and diaphorase activities decreased as a function of incubation time and composition of the MPO/H2O2/halide system, a transient increase preceding the loss of diaphorase activity. Iodide, bromide, thiocyanide and chloride were effective components of MPO/H2O2 or MPO/NADH systems. Catalase prevented LADH inactivation by the MPO/NADH/halide systems in agreement with H2O2 production by NADH-supplemented LADH. Thiol compounds (L-cysteine, N-acetylcysteine, penicillamine, N-(2-mercaptopropionylglycine) and Captopril prevented LADH inactivation by the MPO/H2O2/NaCl system and by NaOCl, thus supporting HOCl as agent of the MPO/H2O2/NaCl system. MPO/H2O2/NaNO2 and MPO/NADH/NaNO2 inactivated LADH, the reaction being prevented by MPO inhibitors and thiol compounds. T. cruzi LADH was affected by MPO-dependent systems like myocardial LADH, allowance being made for the variation of the diaphorase activity and the greater sensitivity of the T. cruzi enzyme to MPO/H2O2/halide systems.

Ribonucleotide reductase is regulated via the R2 subunit during the life cycle of Trypanosoma brucei

We have examined the occurrence of the R1 and R2 subunits of ribonucleotide reductase during the life cycle of Trypanosoma brucei. Whereas the R1 protein is present throughout the life cycle, the R2 protein is not found in cell cycle-arrested short stumpy trypanosomes. RT-PCR/hybridization analysis revealed almost equal amounts of the R1 and R2 mRNAs in all life cycle stages of the parasite. The data indicate that ribonucleotide reductase of African trypanosomes is developmentally controlled by post-transcriptional regulation of the R2 subunit.

Identification and functional characterization of thioredoxin from Trypanosoma brucei brucei

Trypanosomes and Leishmania, the causative agents of several tropical diseases, lack the glutathione/glutathione reductase system but have trypanothione/trypanothione reductase instead. The uniqueness of this thiol metabolism and the failure to detect thioredoxin reductases in these parasites have led to the suggestion that these protozoa lack a thioredoxin system. As presented here, this is not the case. A gene encoding thioredoxin has been cloned from Trypanosoma brucei, the causative agent of African sleeping sickness. The single copy gene, which encodes a protein of 107 amino acid residues, is expressed in all developmental stages of the parasite. The deduced protein sequence is 56% identical with a putative thioredoxin revealed by the genome project of Leishmania major. The relationship to other thioredoxins is low. T. brucei thioredoxin is unusual in having a calculated pI value of 8.5. The gene has been overexpressed in Escherichia coli. The recombinant protein is a substrate of human thioredoxin reductase with a K(m) value of 6 microM but is not reduced by trypanothione reductase. T. brucei thioredoxin catalyzes the reduction of insulin by dithioerythritol, and functions as an electron donor for T. brucei ribonucleotide reductase. The parasite protein is the first classical thioredoxin of the order Kinetoplastida characterized so far.

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.

Inhibition of Trypanosoma cruzi trypanothione reductase by acridines: kinetic studies and structure-activity relationships

Series of 9-amino and 9-thioacridines have been synthesized and studied as inhibitors of trypanothione reductase (TR) from Trypanosoma cruzi, the causative agent of Chagas' disease. The compounds are structural analogues of the acridine drug mepacrine (quinacrine), which is a competitive inhibitor of the parasite enzyme, but not of human glutathione reductase, the closest related host enzyme. The 9-aminoacridines yielded apparent K(i) values for competitive inhibition between 5 and 43 microM. The most effective inhibitors were those with the methoxy and chlorine substituents of mepacrine and NH(2) or NHCH(CH(3))(CH(2))(4)N(Et)(2) at C9. Detailed kinetic analyses revealed that in the case of 9-aminoacridines more than one inhibitor molecule can bind to the enzyme. In contrast, the 9-thioacridine derivatives inhibit TR with mixed-type kinetics. The kinetic data are discussed in light of the three-dimensional structure of the TR-mepacrine complex. The conclusion that structurally very similar acridine compounds can give rise to completely different inhibition patterns renders modelling studies and quantitative structure-activity relationships difficult.

Nitrofuran drugs as common subversive substrates of Trypanosoma cruzi lipoamide dehydrogenase and trypanothione reductase

Lipoamide dehydrogenase (LipDH), trypanothione reductase (TR), and glutathione reductase (GR) catalyze the NAD(P)H-dependent reduction of disulfide substrates. TR occurs exclusively in trypanosomatids which lack a GR. Besides their physiological reactions, the flavoenzymes catalyze the single-electron reduction of nitrofurans with the concomitant generation of superoxide anions. Here, we report on the interaction of clinically used antimicrobial nitrofurans with LipDH and TR from Trypanosoma cruzi, the causative agent of Chagas' disease (South American trypanosomiasis), in comparison to mammalian LipDH and GR. The compounds were studied as inhibitors and as subversive substrates of the enzymes. None of the nitrofurans inhibited LipDH, although they did interfere with the disulfide reduction of TR and GR. When the compounds were studied as substrates, T. cruzi LipDH showed a high rate of nitrofuran reduction and was even more efficient than its mammalian counterpart. Several derivatives were also effective subversive substrates of TR, but the respective reaction with human GR was negligible. Nifuroxazide, nifuroxime, and nifurprazine proved to be the most promising derivatives since they were redox-cycled by both T. cruzi LipDH and TR and had pronounced antiparasitic effects in cultures of T. cruzi and Trypanosoma brucei. The results suggest that those nitrofuran derivatives which interact with both parasite flavoenzymes should be revisited as trypanocidal drugs.

Enzymes of parasite thiol metabolism as drug targets

The potential for chemotherapeutic exploitation of thiol metabolism in parasitic protozoa is reviewed here by Luise Krauth-Siegel and Graham Coombs. The review is based largely on discussions held at a meeting of the COST B9 Action entitled 'Chemotherapy of Protozoal Infections'*. The major questions posed were: which enzymes are the best to target; what further information is required to allow their use for rational drug development; and how can this be achieved most efficiently? Not surprisingly, only partial answers could be obtained in many cases, but the interactive discussion between the multidisciplinary group of participants provided thought-provoking ideas and will help direct future research.

Enzymatic reduction studies of nitroheterocycles

The nitroimidazole derivative Megazol is a highly active compound used against several strains of Trypanosoma cruzi, the causative agent of Chagas' disease (American trypanomiasis). With the aim of gaining an insight into the probable mode of action, the interaction of Megazol with different redox enzymes was studied in comparison to that of Nifurtimox and Metronidazole. The three nitroaromatic compounds are reduced by L-lactate cytochrome c-reductase, adrenodoxin reductase, and NADPH:cytochrome P-450 reductase (EC 1.6.2.4), the efficiencies of the enzymatic reductions being roughly related to the reduction potentials of these pseudo-substrates. As the enzyme responsible for the reduction of Megazol within the parasite has not yet been identified, the nitroimidazole was assayed with T. cruzi lipoamide dehydrogenase and trypanothione reductase. Megazol did not inhibit the physiological reactions but proved to be a weak substrate of both flavoenzymes. The single electron reduction of the compound by NADPH:cytochrome P-450 reductase, by rat liver as well as by trypanosome microsomes was confirmed by ESR experiments. As shown here, Megazol interferes with the oxygen metabolism of the parasite, but its extra activity when compared to Nifurtimox may be related to other features not yet identified.

Ajoene is an inhibitor and subversive substrate of human glutathione reductase and Trypanosoma cruzi trypanothione reductase: crystallographic, kinetic, and spectroscopic studies

Ajoene ((E,Z)-4,5,9-trithiadodeca-1,6,11-triene 9-oxide), a garlic-derived natural compound, is a covalent inhibitor as well as a substrate of human glutathione reductase (GR) and Trypanosoma cruzi trypanothione reductase (TR). The 2.1-A resolution crystal structure of GR inhibited by (E)-ajoene revealed a mixed disulfide between the active site Cys58 and the CH2=CH-CH2-SO-CH2-CH=CH-S moiety of ajoene. The modified enzyme has a markedly increased oxidase activity when compared to free GR. GR reduces (Z)-ajoene with a kcat/Km of 6.8 x 10(3) M-1 s-1 yielding 4,5,9-trithiadodeca-1, 6,11-triene (deoxyajoene) and 4,8,9,13-tetrathiahexadeca-1,6,10, 15-tetraene as stable reaction products. The reaction leads also to the formation of single-electron reduced products and concomitantly superoxide anion radicals as shown by coupling the reaction to the reduction of cytochrome c. The interactions between the flavoenzymes and ajoene are expected to increase the oxidative stress of the respective cell. The antiparasitic and cytostatic actions of ajoene may at least in part be due to the multiple effects on key enzymes of the antioxidant thiol metabolism.

Glutamate dehydrogenase, the marker protein of Plasmodium falciparum--cloning, expression and characterization of the malarial enzyme

The gene of an NADP+-specific glutamate dehydrogenase was cloned from Plasmodium falciparum, the causative agent of tropical malaria. Southern-blot analysis indicates a single-copy gene. The gene encodes a protein with 470 residues which has 50% of all residues identical with those of the glutamate dehydrogenases from other low eukaryotes and eubacteria. In contrast, the sequence identity with the human enzyme is marginal, which underlines the long evolutionary distance between parasite and host. The gene was overexpressed in Escherichia coli. The kinetic properties of the recombinant enzyme are in good agreement with those of the authentic enzyme. The parasite enzyme is inhibited by D-glutamate and glutarate, but not by chloroquine. Like other coenzyme-specific glutamate dehydrogenases, but in contrast to the dual-specific mammalian enzymes, the P. falciparum enzyme is not affected by GTP and ADP. The physical and chemical properties of the protein are in accordance with the cytosol being the major localization. The gene does not encode a cleavable mitochondrial presequence and the Mr of the recombinant protein and the protein isolated from the parasite are indistinguishable on SDS/PAGE. Western-blot analysis of stage-specific parasites shows that glutamate dehydrogenase is present in all intraerythrocytic stages. The signal increased continuously from rings, early trophozoites to late trophozoites and decreased slightly in the segmenter stage. Glutamate dehydrogenase, suggested to be the major source of NADPH in the parasite, is an attractive target molecule for the rational development of new antimalarial drugs.

Role of active site tyrosine residues in catalysis by human glutathione reductase

Tyr114 and Tyr197 are highly conserved residues in the active site of human glutathione reductase, Tyr114 in the glutathione disulfide (GSSG) binding site and Tyr197 in the NADPH site. Mutation of either residue has profound effects on catalysis. Y197S and Y114L have 17% and 14% the activity of the wild-type enzyme, respectively. Mutation of Tyr197, in the NADPH site, leads to a decrease in Km for GSSG, and mutation of Tyr114, in the GSSG site, leads to a decrease in Km for NADPH. This behavior is predicted for enzymes operating by a ping-pong mechanism where both half-reactions partially limit turnover. Titration of the wild-type enzyme or Y114L with NADPH proceeds in two phases, Eox to EH2 and EH2 to EH2-NADPH. In contrast, Y197S reacts monophasically, showing that excess NADPH fails to enhance the absorbance of the thiolate-FAD charge-transfer complex, the predominant EH2 form of glutathione reductase. The reductive half-reactions of the wild-type enzyme and of Y114L are similar; FAD reduction is fast (approximately 500 s-1 at 4 degreesC) and thiolate-FAD charge-transfer complex formation has a rate of 100 s-1. In Y197S, these rates are only 78 and 5 s-1, respectively. The oxidative half-reaction, the rate of reoxidation of EH2 by GSSG, of the wild-type enzyme is approximately 4-fold faster than that of Y114L. These results are consistent with Tyr197 serving as a gate in the binding of NADPH, and they indicate that Tyr114 assists the acid catalyst His467'.

Trypanosoma brucei tryparedoxin, a thioredoxin-like protein in African trypanosomes

A gene has been cloned from Trypanosoma brucei which encodes a protein of 144 amino acid residues containing the thioredoxin-like motif WCPPCR. Overexpression of the gene in E. coli resulted in 4 mg pure protein from 100 ml bacterial cell culture. Recombinant T. brucei tryparedoxin acts as a thiol-disulfide oxidoreductase. It is spontaneously reduced by trypanothione. This dithiol, exclusively found in parasitic protozoa, also reduces E. coli glutaredoxin but not thioredoxin. The trypanothione/tryparedoxin couple is an effective reductant of T. brucei ribonucleotide reductase. Like thioredoxins it has a poor GSH:disulfide transhydrogenase activity. The catalytic properties of tryparedoxin are intermediate between those of classical thioredoxins and glutaredoxins which indicates that these parasite proteins may form a new class of thiol-disulfide oxidoreductases.

Cloning, sequencing and expression of ribonucleotide reductase R2 from Trypanosoma brucei

Ribonucleotide reductase (RR) is an attractive drug target molecule. The gene of the R2 protein of Trypanosoma brucei RR (nrd B) has been cloned. It encodes a protein of 337 residues which shows about 60% identity with other eukaryotic R2 proteins. All residues which bind the iron center, the tyrosyl radical or are supposed to participate in the radical transfer are conserved in the trypanosomal protein sequence. Overexpression of the gene in E. coli resulted in 2-5 mg pure R2 protein from 100 ml bacterial cell culture. Northern blot analysis revealed a transcript of 1.85 kb in bloodstream and procyclic forms of the parasite.

Glutathione reductase turned into trypanothione reductase: structural analysis of an engineered change in substrate specificity

Trypanosoma and Leishmania, pathogens responsible for diseases such as African sleeping sickness, Chagas' heart disease, or Oriental sore, are two of the very few genera that do not use the ubiquitous glutathione/glutathione reductase system to keep a stable cellular redox balance. Instead, they rely on trypanothione and trypanothione reductase to protect them from oxidative stress. Trypanothione reductase (TR) and the corresponding host enzyme, human red blood cell glutathione reductase (GR), belong to the same flavoprotein family. Despite their closely related three-dimensional structures and although their natural substrates share the common structural glutathione core, the two enzymes are mutually exclusive with respect to their disulfide substrates. This makes the parasite enzyme a potential target for antitrypanosomal drug design. While a large body of structural data on GR complexes is available, information on TR-ligand interactions is very limited. When the two amino acid changes Ala34Glu and Arg37Trp are introduced into human GR, the resulting mutant enzyme (GRTR) prefers trypanothione 700-fold over its original substrate, effectively converting a GR into a TR [Bradley, M., Bücheler, U. S., & Walsh, C. T. (1991) Biochemistry 30, 6124-6127]. The crystal structure of GRTR has been determined at 2.3 A resolution and refined to a crystallographic R factor of 20.9%. We have taken advantage of the ease with which ligand complexes can be produced in GR crystals, a property that extends to the isomorphous GRTR crystals, and have produced and analyzed crystals of GRTR complexes with glutathione, trypanothione, glutathionylspermidine and of a true catalytic intermediate, the mixed disulfide between trypanothione and the enzyme. The corresponding molecular structures have been characterized at resolutions between 2.3 and 2.8 A with R factors ranging from 17.1 to 19.7%. The results indicate that the Ala34Glu mutation causes steric hindrance leading to a large displacement of the side chain of Arg347. This movement combined with the change in charge introduced by the mutations modifies the binding cavity, forcing glutathione to adopt a nonproductive binding mode and permitting trypanothione and to a certain degree also the weak substrate glutathionylspermidine to assume a productive mode.

Cloning, sequencing and functional expression of dihydrolipoamide dehydrogenase from the human pathogen Trypanosoma cruzi

This work presents the complete sequences of a cDNA and the two allelic genes of dihydrolipoamide dehydrogenase (LipDH) from Trypanosoma cruzi, the causative agent of Chagas' disease (American trypanosomiasis). The full-length cDNA has an ORF of 1431 bp and encodes a protein of 477 amino acid residues. LipDH is a homodimeric protein with FAD as prosthetic group. The calculated molecular mass of the subunit of the mature protein with bound FAD is 50,066. Comparison of the deduced amino acid sequence of LipDH from T. cruzi with that of Trypanosoma brucei and man shows identities of 81% and 50%, respectively. An N-terminal nonapeptide, not present in the mature enzyme, represents a mitochondrial targeting sequence so far found only in trypanosomatids. The gene lpd1 of T. cruzi LipDH was expressed without the targeting sequence in Escherichia coli JRG1342 cells which are deficient for LipDH. For this purpose an ATG codon was introduced directly upstream the codon for Asn10 which represents the N-terminus of the mature protein. This system allowed the synthesis of 1000 U T. cruzi LipDH/1 bacterial cell culture. The recombinant protein was purified to homogeneity by (NH4)2SO4-precipitation and affinity chromatography on 5' AMP-Sepharose. The K(m) values for NAD+, NADH, lipoamide and dihydrolipoamide are identical with those of the enzyme isolated from the parasite. LipDH is present in all major developmental stages of T. cruzi as shown by northern and western blot analyses. This finding is in agreement with the citric acid cycle being active throughout the whole life cycle of the parasite. In vitro studies on a mammalian LipDH revealed the ability of the flavoenzyme to catalyze the redoxcycling and superoxide anion production of nitrofuran derivatives including the antitrypanosomal drug Nifurtimox. For that reason T. cruzi LipDH is regarded as a promising target for the structure-based development of new antiparasitic drugs. The bacterial expression system for the parasite enzyme will now allow the study of the role of T. cruzi LipDH in drug activation and the crystallization of the protein.

Charge is the major discriminating factor for glutathione reductase versus trypanothione reductase inhibitors

Benson et al. (Biochem. J. 1992, 286, 9) reported three novel competitive inhibitors of trypanothione reductase (TR), which were selected to complement a hydrophobic region identified on the TR structure which was not present on human glutathione reductase (hGR). Benson et al. also noted that chlorpromazine, a tricyclic antidepressant known to have trypanocidal activity, was an inhibitor of TR. Here we show that chlorpromazine is a competitive inhibitor of TRs from Crithidia fasciculata (Ki = 14 microM) and Trypanosoma cruzi (Ki = 10 microM), but the drug binds > 50-fold more weakly (Ki = 762 microM) to hGR. Analogues of chlorpromazine differing in the length of the side chain carrying the positively charged R-group are also selective TR inhibitors whereas, a tricyclic structure carrying a negatively charged side chain is a competitive inhibitor with selectivity for hGR (K(hGR)i = 165 microM vs. K(TR)i = 1400 microM). This finding suggests that simple charge characteristics, rather than differences in hydrophobicity, may account for a significant portion of the selectivity of this series of inhibitors for these two enzymes. Electrostatic analysis of the structures of TR and hGR thus provides a rationale for these results, and offers a new principle for inhibitor design. The principle gains further support from the observation that all known tricyclic competitive inhibitors of TR are positively charged. In order to investigate the in vivo relevance of our findings we have examined the effect of chlorpromazine and its negatively charged analogue on the growth of C. fasciculata parasites. Consistent with our kinetics, chlorpromazine (50 microM) inhibited the growth of parasites by 50%, while no measurable decrease in parasite growth rate was noted in the presence of the negatively charged inhibitor (400 microM). Furthermore, the highly similar inhibitory profiles of C. fasciculata TR and T. cruzi TR suggest that drug-design studies using the structurally better-studied C. fasciculata TR are also relevant to the human pathogen T. cruzi.

Crystal structure of the Trypanosoma cruzi trypanothione reductase.mepacrine complex

The three-dimensional structure of the complex between Trypanosoma cruzi trypanothione reductase (TR) (EC 1.6.4.8) and the antiparasitic drug mepacrine (quinacrine) has been solved at 2.9 angstoms resolution. Mepacrine is a competitive inhibitor of TR but does not affect human glutathione reductase (GR), a closely related host enzyme. Of particular importance for inhibitor binding are four amino acid residues in the disulfide substrate-binding site of TR that are not conserved in human GR, namely, Glu-18 (Ala-34 in GR), Trp-21 (Arg-37), Ser-109 (Ile-113), and Met-113 (Asn-117). The acridine ring of mepacrine is fixed at the active site close to the hydrophobic wall formed by Trp-21 and Met-113. Specific pairwise interactions between functional groups of the drug and amino acid side chains include the ring nitrogen and Met-113, the chlorine atom and Trp-21, and the oxymethyl group and Ser-109. The alkylamino chain of mepacrine points into the inner region of the active site and is held in position by a solvent-mediated hydrogen bond to Glu-18. The structure of the complex shows for the first time the atomic interactions between TR and an inhibitory ligand. This is a crucial step towards the rational design of inhibitors that might be suited as drugs against Chagas' disease.

Inhibition of human glutathione reductase by 10-arylisoalloxazines: crystalline, kinetic, and electrochemical studies

A series of newly synthesized N(10)-arylisoalloxazines--some of which are known to be antimalarial agents--were studied as inhibitors of human glutathione reductase (GR;NADPH + GSSG + H(+) <==> NADP(+) + 2GSH). The flavoenzyme was inhibited with IC(50) values between </- 1 and 100 microM in the presence of 100 microM NADPH. The isoalloxazines and N(3)-methylisoalloxazines with a 4'-chlorophenyl or a 3',5'-dichlorophenyl group at N10 were found to be the most promising inhibitors of GR, although even the bulkier 10-naphthyl and -anthryl derivatives were also effective inhibitors. In contrast, at position N3 of the isoalloxazine ring, the size of the substituent was found to strongly influence the inhibitory effect. Introduction of a carboxymethyl group at N3--which markedly increased the solubility of the derivative in aqueous solutions-- caused a rise in the IC(50) values by 1 order of magnitude. 8-Fluoro- and 8-azido-10-arylisoalloxazines were potent inhibitors of GR; consequently position C8 of the benzenoid subnucleus instead of N3 should be considered for introducing substituents. No correlation was observed between the inhibitory strength of several isoalloxazines and their redox potential as measured by cyclovoltammetry. The crystallographic analysis of GR complexed with 10-(4'-chlorophenyl)-3-(carboxymethyl)isoalloxazine and 10-(3',5'-dichlorophenyl)-3-(carboxymethyl)isoalloxazine, respectively, revealed the presence of one inhibitor molecule bound at the 2-fold axis of the homodimeric protein. This location is consistent with fluorescence titration measurements and enzyme kinetic studies in solution which gave no indication for binding at the substrate sites.

Glutathione reductase and glutamate dehydrogenase of Plasmodium falciparum, the causative agent of tropical malaria

The use of glutathione reductase inhibitors in chemotherapy is the raison d'être for this study. Two enzymes were purified to homogeneity from the intraerythrocytic malarial parasite Plasmodium falciparum: glutathione disulfide reductase, an antioxidative enzyme, which appears to play an essential role for parasite growth and differentiation, and glutamate dehydrogenase, an enzyme not occurring in the host erythrocyte. The two proteins were copurified and separated by gel electrophoresis with yields of approximately 20%. Malarial glutathione reductase, a homodimer of 110 kDa with a pH optimum of 6.8 and a high preference for NADPH over NADH, was shown to contain FAD as its prosthetic group. The N-terminal sequence, VYDLIVIGGGSGGMA, which can be aligned with residues 20-34 of human glutathione reductase, represents the first beta strand and the diphosphate-fixing helix of the FAD domain. Glutamate dehydrogenase was confirmed as a hexamer with blocked N-termini; it is an enzyme that is highly specific for NADP and NADPH. The copurification of the proteins and the potential of P.falciparum glutathione reductase as a drug target are discussed.

Crystal structure of the Trypanosoma cruzi trypanothione reductase.mepacrine complex

The three-dimensional structure of the complex between Trypanosoma cruzi trypanothione reductase (TR) (EC 1.6.4.8) and the antiparasitic drug mepacrine (quinacrine) has been solved at 2.9 angstoms resolution. Mepacrine is a competitive inhibitor of TR but does not affect human glutathione reductase (GR), a closely related host enzyme. Of particular importance for inhibitor binding are four amino acid residues in the disulfide substrate-binding site of TR that are not conserved in human GR, namely, Glu-18 (Ala-34 in GR), Trp-21 (Arg-37), Ser-109 (Ile-113), and Met-113 (Asn-117). The acridine ring of mepacrine is fixed at the active site close to the hydrophobic wall formed by Trp-21 and Met-113. Specific pairwise interactions between functional groups of the drug and amino acid side chains include the ring nitrogen and Met-113, the chlorine atom and Trp-21, and the oxymethyl group and Ser-109. The alkylamino chain of mepacrine points into the inner region of the active site and is held in position by a solvent-mediated hydrogen bond to Glu-18. The structure of the complex shows for the first time the atomic interactions between TR and an inhibitory ligand. This is a crucial step towards the rational design of inhibitors that might be suited as drugs against Chagas' disease.

Flavoprotein structure and mechanism. 5. Trypanothione reductase and lipoamide dehydrogenase as targets for a structure-based drug design

Trypanothione reductase (TR) is a flavoenzyme that has been found only in parasitic protozoa of the order Kinetoplastida. The enzyme catalyzes the NADPH-dependent reduction of glutathionylspermidine conjugates and is a key enzyme of the parasite's thiol metabolism. Consequently, TR is an attractive target molecule for a structure-based drug development against Chagas' disease, African sleeping sickness, and other diseases caused by trypanosomes and leishmanias. The three-dimensional structures of TR and of three enzyme substrate complexes have been solved. Several classes of compounds are discussed as guide structures for the design of specific inhibitors. Among them are tricyclic compounds such as acridines and phenothiazines, which competitively inhibit TR but not the related host enzyme glutathione reductase, as well as oxidase activity-inducing quinones and nitrofurans. Lipoamide dehydrogenase (LipDH) is another flavoprotein discussed as a target molecule for an antitrypanosomal therapy. In Trypanosoma cruzi, an organism that is highly susceptible to oxidative stress, LipDH participates in the redox cycling of nifurtimox, one of the most effective anti-Chagas agents. In conclusion, the structurally related enzymes TR and LipDH exhibit an unusually high one-electron-reducing capacity. Consequently, turncoat inhibitors and other compounds inducing an oxidase activity in both enzymes are promising drug candidates against Chagas' disease.

The structure of Trypanosoma cruzi trypanothione reductase in the oxidized and NADPH reduced state

The three-dimensional structure of trypanothione reductase (TR) (EC 1.6.4.8) from Trypanosoma cruzi has been solved at 0.33 nm resolution by molecular replacement using the structure of C. fasciculata TR as a starting model. Elucidation of the T. cruzi TR structure represents the first step in the rational design of a drug against Chagas' disease. The structure of T. cruzi TR is compared with those of C. fasciculata TR as well as human and E. coli glutathione reductase (GR). In the FAD-binding domain, TR has two insertions, each about 10 residues long, which do not occur in GR. The first one is a rigid loop stabilizing the position of helix 91-117 which is responsible for the wider active site of TR as compared to GR. The second insertion does not occur where it is predicted by sequence alignment; rather the residues extend three strands of the 4-stranded beta-sheet by one or two residues each. This increases the number of hydrogen bonds within the sheet structure. The structure of the NADPH.TR complex has been solved at 0.33 nm resolution. The nicotinamide ring is sandwiched between the flavin ring and the side chain of Phe-198 which undergoes the same conformational change upon coenzyme binding as Tyr-197 in GR. In addition to Arg-222 and Arg-228, which are conserved in TR and GR, Tyr-221--the last residue of the second beta-sheet strand of the beta alpha beta dinucleotide binding fold--is in hydrogen bonding distance to the 2' phosphate group of NADPH.

Glutathione reductase in human and murine lung tumors: high levels of mRNA and enzymatic activity

The enzyme glutathione reductase (GR) (GSSG+NADPH+H+-->2 GSH+NADP+) plays a key role in the cellular defense against oxidative stress. High levels of GR activity are often associated with tumor growth and/or resistance mechanisms against drug and radiation therapy. In order to investigate the molecular basis of elevated glutathione reductase activities we studied the enzyme at the DNA, mRNA and protein levels in murine experimental tumor cell lines and in human lung tumors. A modified ultracentrifugation procedure was developed which allowed the simultaneous isolation of DNA and total cellular RNA. Out of 11 human bronchial carcinomas obtained from patients without prior chemotherapy, five tumors showed a GR activity which was 2.4 to 3.8 times higher than in the respective control tissues. In each case the elevated enzyme activity was accompanied by an elevated GRmRNA levels. For none of the tumors, GR gene rearrangement or amplification was observed by Southern blot analyses. The mouse tumor cell lines ASB XIV, Lewis lung carcinoma and EAT cells, also showed high levels of GRmRNA whereas this mRNA was hardly detectable in normal mouse lung tissue.

Crystallization and preliminary crystallographic analysis of trypanothione reductase from Trypanosoma cruzi, the causative agent of Chagas' disease

Trypanothione reductase from Trypanosoma cruzi is the most promising target molecule for the rational design of a specific drug against Chagas' disease. The recombinant protein was purified in a single chromatographic step and crystallized. Two crystal forms suitable for X-ray diffraction analysis were obtained. Tetragonal crystals (a = b = 87.4 A, c = 152.3 A) were grown from 30% polyethylene glycol (average M(r) = 8,000) in the presence of 0.2% beta-n-octylglucoside (space group either P4(2) with one dimer or P4(2)22 with one monomer in the asymmetric unit). Monoclinic crystals (space group P2, a = 136.3 A, b = 91.1 A, c = 126.0 A, beta = 94 degrees) were grown from 1.2 M sodium citrate in the presence of 2% octanoyl-N-methyl-glucamide. They contain two dimers of the enzyme in the asymmetric unit; both crystal forms diffract to 3 A resolution.

Purification and characterization of lipoamide dehydrogenase from Trypanosoma cruzi

From Trypanosoma cruzi, the causative agent of Chagas' disease, a lipoamide dehydrogenase was isolated. The enzyme, an FAD-cystine oxidoreductase, shares many physical and chemical properties with T. cruzi trypanothione reductase, the key enzyme of the parasite's thiol metabolism. 1. From 60 g epimastigotic T. cruzi cells, 2.7 mg lipoamide dehydrogenase was extracted. The flavoenzyme was purified 3000-fold to homogeneity with an overall yield of 26%. 2. The enzyme is a dimer with a subunit Mr of 55,000. With 1 mM lipoamide (Km approximately 5 mM) and 100 microM NADH (Km = 23 microM), the specific activity at pH 7.0 is 297 U/mg. 3. With excess NADH, the enzyme is reduced to the EH2.NADH complex and, by addition of lipoamide, it is reoxidized, indicating that it can cycle between the oxidized state E and the two-electron-reduced state, EH2. 4. As shown by N-terminal sequencing of the enzyme, 21 out of 30 positions are identical with those of pig heart and human liver lipoamide dehydrogenase. The sequenced section comprises the GGGPGG stretch, which represents the binding site for the pyrophosphate moiety of FAD. 5. After reduction of Eox to the two-electron-reduced state, the enzyme is specifically inhibited by the nitrosourea drug 1,3-bis(2-chloroethyl)-1-nitrosourea (Carmustine), presumably by carbamoylation at one of the nascent active-site thiols. 6. Polyclonal rabbit antibodies raised against T. cruzi lipoamide dehydrogenase and trypanothione reductase are specific for the respective enzyme, as shown by immunoblots of the pure proteins and of cell extracts.

Trypanothione reductase from Trypanosoma cruzi. Catalytic properties of the enzyme and inhibition studies with trypanocidal compounds

Trypanothione reductase of Trypanosoma cruzi is a key enzyme in the antioxidant metabolism of the parasite. Here we report on the enzymic and pharmacological properties of trypanothione reductase using glutathionylspermidine disulfide as a substrate. 1. Both pH optimum (7.5) and the ionic strength optimum (at 30 mM) are unusually narrow for this enzyme. 40 mM Hepes, 1 mM EDTA, pH 7.5 was chosen as a standard assay buffer because in this system the kcat/Km ratio had the highest values for both natural substrates, glutathionylspermidine disulfide (2.65 x 10(6) M-1 s-1) and trypanothione disulfide (4.63 x 10(6) M-1 s-1). 2. Using the standardized assay, trypanothione reductase and the phylogenetically related host enzyme, human glutathione reductase, were studied as targets of inhibitors. Both enzymes, in their NADPH-reduced forms, were irreversibly modified by the cytostatic agent, 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). Nifurtimox, the drug used in the treatment of Chagas' disease, is a stronger inhibitor of glutathione reductase (Ki = 40 microM) than of trypanothione reductase (IC50 = 200 microM). 3. Of the newly synthesized trypanocidal compounds [Henderson, G. B., Ulrich, P., Fairlamb, A. H., Rosenberg, I., Pereira, M., Sela, M. & Cerami, A. (1988) Proc. Natl Acad. Sci., 85, 5374-5378] a nitrofuran derivative, 2-(5-nitro-2-furanylmethylidene)-N,N'-[1,4-piperazinediylbis (1,3-propanediyl)]bishydrazinecarboximidamide tetrahydrobromide, was found to be a better inhibitor for trypanothione reductase (Ki = 0.5 microM) than for glutathione reductase (IC50 = 10 microM). A naphthoquinone derivative, 2,3-bis[3-(2-amidinohydrazono)-butyl]-1,4-naphthoquinone dihydrochloride, turned out to be both an inhibitor (IC50 = 1 microM) and an NADPH-oxidation-inducing substrate (Km = 14 microM). This effect was not observed with human glutathione reductase. Such compounds which lead to oxidative stress by more than one mechanism in the parasite are promising starting points for drug design based on the three-dimensional structures of glutathione and trypanothione reductases.

Inhibition of human glutathione reductase by the nitrosourea drugs 1,3-bis(2-chloroethyl)-1-nitrosourea and 1-(2-chloroethyl)-3-(2-hydroxyethyl)-1-nitrosourea. A crystallographic analysis

Glutathione reductase from human erythrocytes was inhibited by incubation with the drugs 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) and 1-(2-chloroethyl)-3-(2-hydroxyethyl)-1-nitrosourea (HeCNU) under quasi-physiological conditions. For reference purposes, iodoacetamide was used for inactivating alkylation of the enzyme. In each case the modified glutathione reductase was crystallized and its structure determined. These analyses showed that in all experiments the enzyme had reacted at the distal sulfur, that is at the thiol of Cys-58, and virtually nowhere else in the visible structure. The electron density of the HeCNU derivative at 0.3 nm resolution is consistent with a 2-hydroxyethyl group. This alkyl moiety has recently been identified by chemical analysis [Schirmer, R. H., Schöllhammer, T., Eisenbrand, G. and Krauth-Siegel, R. L. (1987) Free Radical Res. Commun. 3, 3-12]. The 0.2 nm resolution electron-density map of the BCNU-derivatized enzyme cannot be explained by a 2-hydroxyethyl group. Instead the modification appears as a carbamoyl moiety containing at least five non-hydrogen atoms. In this derivative the distal cysteine is forced into an unusual conformation.

Trypanothione reductase from Trypanosoma cruzi. Purification and characterization of the crystalline enzyme

The structural differences between trypanothione reductase of Trypanosoma cruzi and human glutathione reductase, an enzyme of known three-dimensional structure, offer an opportunity for rational drug design against Chagas' disease. As a first step in the analysis of the parasite enzyme we report its purification and characterization. 2.2 mg trypanothione reductase was extracted from 33 g wet weight of cultured epimastigotes or from 4 g lyophilized cells. The flavoenzyme was purified 2400-fold to homogeneity in three steps with an overall yield of 45%. The enzyme is a dimer with a subunit Mr of 50,000. Using NADPH (Km = 5 microM) and trypanothione disulfide (Km = 45 microM) as substrates, a turnover number of 14,200 min-1 was estimated. Trypanothione reductase, the parasite enzyme, and glutathione reductase, the host enzyme, exhibit mutually exclusive specificities for their respective disulfide substrates. When screening cell cultures or column eluates for the presence of trypanothione reductase, a microassay based on Ellman's reagent as indicator was used. A mixture of regioisomeric glutathionylspermidine disulfides isolated from Escherichia coli served as substrate in this microassay. Experimentally, the catalytic cycle of the enzyme can be subdivided into the half-reactions Eox + NADPH + H+----EH2 + NADP+, and EH2 + trypanothione disulfide----Eox + dihydrotrypanothione. This is also true for the crystallized enzyme in the presence of 2 M (NH4)2SO4. The spectral properties of trypanothione reductase both in the oxidized form (Eox) and in the two-electron-reduced form (EH2) closely resemble those of human glutathione reductase. Both proteins contain a flavin and a redox-active disulfide at the catalytic site. After reduction of Eox to EH2, trypanothione reductase can be inactivated by specifically alkylating one of the nascent active-site thiols.

Oxidative stress as a defense mechanism against parasitic infections

Many parasites--including the causative agents of malaria, Chagas' disease and schistosomiasis--are more susceptible to reactive oxygen species (ROS) than their hosts are. This is manifested by one or more of the following criteria: 1. Susceptibility of the parasite to ROS in vitro; 2. macrophage-based defense mechanisms against the parasite in vivo; 3. successful therapy using agents which lead to oxidative stress; 4. selection advantage (with respect to parasite infections) of human populations whose antioxidant capacity is impaired by a gene defect or by strong oxidants in their staple food. Our laboratory is involved in developing inhibitors against antioxidant enzymes thus mimicking natural experiments. Since glutathione reductase is a protein of known atomic structure the methods of drug design by receptor fit (DDRF) can be applied for this enzyme. Another promising target enzyme is trypanothione reductase which was found so far only in trypanosomatids, and specifically, not in their hosts. Consequently the trypanothione pathway may be a general target in the design of drugs against diseases caused by trypanosomes and leishmanias.

Mitochondrial adenylate kinase (AK2) from bovine heart. The complete primary structure

The sequence analysis of adenylate kinase isoenzyme 2 (AK2) was completed using a gas-phase sequencer constructed in our laboratory. The enzyme contains 238 amino acid residues in the following order: (sequence; see text) The four cysteine residues of AK2 were reinvestigated. Cys-41 and Cys-233 contain free thiols, which can be carboxymethylated in the intact protein without loss of enzymic activity. Chemical and model-building studies suggest that the pair Cys-43/Cys-93 forms a disulfide in native AK2. The relative molecular mass of AK2, as deduced from the sequence, is 26104. Other methods, including titration of -SH groups, sedimentation equilibrium ultracentrifugation and gel filtration yielded Mr values in the range from 26 000 to 31 500, each value depending on the respective method of determination. Bovine heart AK2 contains 44 residues more than the homologous isoenzyme AK1 (myokinase). As all but one single insertions and deletions cancel, the higher Mr of AK2 is due to 9 residues preceding the N terminus of AK1, a stretch of 30 residues in the middle of the molecule and 6 residues at the end. AK2 and AK1 are similar in their active-site geometry. In contrast, AK2 does not possess any of the three antigenic sites of AK1, which is consistent with the lack of immunological cross-reactivity between AK1 and AK2.

FAD analogues as prosthetic groups of human glutathione reductase. Properties of the modified enzyme species and comparisons with the active site structure

Human glutathione reductase (NADPH + GSSG + H+ in equilibrium with NADP+ + 2 GSH) is a suitable enzyme for correlating spectroscopic properties and chemical reactivities of protein-bound FAD analogues with structural data. FAD, the prosthetic group of the enzyme, was replaced by FAD analogues, which were modified at the positions 8, 1, 2, 4, 5 and 6, respectively, of the isoalloxazine ring. When compared with a value of 100% for native glutathione reductase, the specific activities of most enzyme species ranged from 40% to 17%, in the order of the prosthetic groups 8-mercapto-FAD greater than 8-azido-FAD = 8-F-FAD = 8-C1-FAD greater than 4-thio-FAD = 1-deaza-FAD greater than 2-thio-FAD. The enzymic activities indicate a correct orientation of the bound analogues. The enzyme species containing 5-deaza-FAD and 6-OH-FAD, respectively, had no more glutathione reductase activity than the FAD-free apoenzyme. 5-Deaza-FAD X glutathione reductase was crystallized for X-ray diffraction analysis. Detailed studies were focussed on position 8 of the flavin. 8-Cl-FAD X glutathione reductase and 8-F-FAD X glutathione reductase reacted only poorly with HS- to give 8-mercapto-FAD X glutathione reductase, which suggests that the region around Val61 hinders the halogen anion from leaving the tetrahedral intermediate. Other experiments showed that position 8 is accessible to certain solvent-borne reagents. 8-Mercapto-FAD X glutathione reductase, for instance, reacted readily and stoichiometrically with the thiol reagent methylmethanethiosulfonate. 8-Mercapto-FAD X glutathione reductase does not exhibit a long wavelength charge transfer absorption band upon reduction, as it is the case for the 2-electron-reduced FAD-containing enzyme. This behaviour indicates that the charge transfer interaction between flavin and the thiolate of Cys63 in the native enzyme is not per se essential for catalysis. The absorption spectrum of the blue anionic 8-mercapto-FAD bound to glutathione reductase suggests that the protein concurs to the stabilization of a negative charge in the pyrimidine subnucleus. In light of the protein structure this effect is attributed to the dipole moment of alpha-helix 338-354 which starts out close to the N(1)/C(2)/O(2 alpha) region of the flavin. 1-Deaza-FAD binds as tightly as FAD to the apoenzyme. The resulting holoenzyme was found to be enzymically active but structurally unstable. In this respect 1-deaza-FAD . glutathione reductase mimics the properties of the enzyme species found in inborn glutathione reductase deficiency.(ABSTRACT TRUNCATED AT 400 WORDS)

Interaction of a glutathione S-conjugate with glutathione reductase. Kinetic and X-ray crystallographic studies

S-Conjugates of glutathione influence the glutathione/glutathione disulfide (GSH/GSSG) status of hepatocytes in at least two ways, namely by inhibition of GSSG transport into the bile [Akerboom et al. (1982) FEBS Lett. 140, 73-76] and by inhibition of the enzyme GSSG reductase (EC 1.6.4.2). The interaction of GSSG reductase with a well-studied conjugate, namely S-(2,4-dinitrophenyl)-glutathione and its electrophilic precursor 1-chloro-2,4-dinitrobenzene are described. For short exposures both compounds are reversible inhibitors of the enzyme, the Ki values being 30 microM and 22 microM respectively. After prolonged incubation, 1-chloro-2,4-dinitrobenzene blocks GSSG reductase irreversibly, which emphasizes the need for rapid conjugate formation in situ. As shown by X-ray crystallography the major binding site of S-(2,4-dinitrophenyl)-glutathione in GSSG reductase overlaps the binding site of the substrate, glutathione disulfide. However, the glutathione moiety of the conjugate does not bind in the same manner as either of the glutathiones in the disulfide.

Glutathione reductase from human erythrocytes. The sequences of the NADPH domain and of the interface domain

1. Sequence analysis of the NADPH domain (residues 158--293) and of the interface domain (365--478) was based on 12 CNBr fragments, which were isolated using ion-exchange chromatography and paper methods. Fragments with more than 15 residues were digested further with trypsin and chymotrypsin. The isolated peptides were sequenced by automated solid-phase Edman degradation. All sequenced peptides were ordered and overlapped by computerized comparisons with a complete sequence guessed from the electron density map of the protein. In the case of short CNBr fragments, this alignment was confirmed by the sequence analysis of protein fragments resulting from incomplete CNBr cleavage. 2. In the NADPH domain, residue 197, which is involved in an induced-fit mechanism, was identified as a tyrosine. The structure of the NADPH domain is probably homologous with the NAD domain of lipoamide dehydrogenase and with the FAD domain of several proteins, but not with NADPH domains of known chain-fold in other proteins. 3. The paper completes the sequence analysis of glutathione reductase so that the enzyme is now known in atomic detail. The numbering scheme of the chemically determined sequence will be used henceforth in crystallographic studies also. As inferred from the sequence data each of the two identical chains contains 478 amino acid residues, the composition being Cys10, Asp21, Asn17, Thr31, Ser31, Glu29, Gln11, Pro24, Gly43, Ala42, Val44, Met15, Ile29, Leu34, Tyr13, Phe14, Lys34, His16. Arg17, and Trp3. From these data an Mr of 2 x 51 600 was calculated for the FAD-free apoenzyme and an Mr of 2 x 42 400 for the holoenzyme.

Glutathione reductase from human erythrocytes: amino-acid sequence of the structurally known FAD-binding domain

Glutathione reductase (Mr 2 x 52 500), a flavoenzyme of known three-dimensional structure, catalyses the reduction of glutathione disulfide by NADPH. This paper describes the primary structure of the FAD-binding domain which ranges from AcAla-1 to Gly-157. The three CNBr-produced fragments (69, 10 and 80 residues) of the domain were fractionated further by enzymatic and chemical methods; isolated peptides were sequenced mainly by automatic solid-phase Edman degradation. The tryptic peptides were overlapped by chymotryptic peptides. A fragment which results from cleavage at the acid-labile bond between Asp-135 and Pro-136 supplied peptides for overlapping the CNBr-produced fragments. In addition, many peptides were ordered and overlapped by computerized comparison with a complete sequence guessed from the electron density map. With one exception the computer method and the chemical alignment gave the same results. The sequence data are discussed in the light of the secondary and tertiary structure (Schulz et al. (1978) Nature (Lond.) 273, 120--124]. The 17 N-terminal residues are not visible in the electron density map. Consequently our numbering scheme differs from that of Schulz et al. by approximately 20 residues. Acetylation of the N terminus and an unusual composition of the following residues may serve to protect the loose N-terminal section of the protein against proteolysis in situ. The four cysteinyl residues of the FAD domain are of special interest. Cys-2 at the tip of the N-terminal extension is likely to be involved in the aggregation behaviour of glutathione reductase. Cys-58 and Cys-63 (formerly Cys-41 and Cys-46) represent the enzyme's redox-active dithiol. Cys-90 with its location at the twofold axis forms a disulfide bridge with Cys-90 of the other peptide chain of the enzyme. This might be related to the fact that both peptide chains contribute to each of the two active centers. In view of the interchain disulfide bridge glutathione reductase should be regarded as a monomeric protein. The sequence of the FAD-binding domain was compared with the sequence of the NADPH-binding domain of glutathione reductase using a computer program. As discussed, the scarcity of sequence similarities does not argue against the assumption that the two nucleotide-binding domains of glutathione reductase originated by gene duplication. The pyrophosphate moiety of FAD binds to a part of the polypeptide chain which in geometric structure, in topology and in sequence resembles the phosphate loops of other nucleotide-binding proteins and of flavodoxin. Using the phosphate loop as a reference, the N-terminal sequence of five flavoproteins can be aligned. The results of Williams et al. on the sequence of lipoamide dehydrogenase (EC 1.6.4.3) and our data on glutathione reductase (EC 1.6.4.2) show clearly that these two mechanistically similar enzymes possess homologous structures.