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.

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 k,,,/K, ratio had the highest values for both natural substrates, glutathionylspermidine disulfide (2.65 x lo6 M-' s-' 1 and trypanothione disulfide (4.63 x lo6 M-' s f l 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)-l-nitrosourea (BCNU). Nifurtimox, the drug used in the treatment of Chagas' disease, is a stronger inhibitor of glutathione reductase (Ki = 40 pM) than of trypanothione reductase (ICso = 200 pM).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, 5374-53781 a nitrofuran derivative, 2-(5 -nitro -2-furanylmethy1idene)-N,W-[1,4-piperazinediylbis (1,3 -propanediyl)]bishydrazinecarboximidamide tetrahydrobromide, was found to be a better inhibitor for trypanothione reductase (Ki = 0.5 pM) than for glutathione reductase (ICso = 10 pM). A naphthoquinone derivative, 2,3-bis[3-(2-amidinohydrazono)-butyl]-l,4-naphthoquinone dihydrochloride, turned out to be both an inhibitor = 1 pM) and an NADPHoxidation-inducing substrate (K, = 14 pM). 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. Enzymes. Trypanothione reductase (EC 1.6.4.-); glutathione reductase (EC 1.6.4.2). donovani) and oriental sore (Leishmania tropica) are caused by these protozoa.In contrast to all other known eukaryotes [l, 21 Trypanosomatidae do not possess the flavoenzyme glutathione reductase that is responsible for a high intracellular glutathione/ glutathione disulfide ratio. The thiol metabolism of trypanosomes and leishmanias is based on conjugates between glutathione and spermidine, namely trypanothione [3] and glutathionylspermidine [4]. These thiols are kept in the reduced state by trypanothione reductase which catalyzes the following reactions: trypanothione disulfideTrypanothione reductase is an FAD-cystine oxidoreductase which has been purified from the insect parasite Crithidia fasciculata [5] and, in crystalline form, from T . cruzi [6]. The DNA sequence is known for the enzyme from T. congolense [7].In vitro studies on trypanothione reductase are hampered by the fact that the natural dis...

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.