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 50066. Comparison of the deduced amino acid sequence of LipDH from 7: crzizi 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 lpdl of 7: 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 AsnlO which represents the N-terminus of the mature protei-n. This system allowed the synthesis of 1000 U 7: cruzi LipDH/l bacterial cell culture. The recombinant protein was purified to homogeneity by (NH,),SO,-precipitation and affinity chromatography on 5' AMP-Sepharose. The K,,, 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 7: 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 7: 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 7: cruzi LipDH in drug activation and the crystallization of the protein.
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.
Although trypanothione [T(S)2] is the major thiol component in trypanosomatidae, significant amounts of glutathione are present in Trypanosoma cruzi. This could be explained by the existence of enzymes using glutathione or both glutathione and T(S)2 as cofactors. To assess these hypotheses, a cytosolic fraction of T. cruzi epimastigotes was subjected to affinity chromatography columns using as ligands either S-hexylglutathione or a non-reducible analogue of trypanothione disulphide. A similar protein of 52 kDa was eluted in both cases. Its partial amino acid sequence indicated that it was identical with the protein encoded by the TcAc2 cDNA previously described [Schoneck, Plumas-Marty, Taibi et al. (1994) Biol. Cell 80, 1-10]. This protein showed no significant glutathione transferase activity but surprisingly catalysed the thiol-disulphide exchange between dihydrotrypanothione and glutathione disulphide. The kinetic parameters were in the same range as those determined for trypanothione reductase toward its natural substrate. This trypanothione-glutathione thioltransferase provides a new target for a specific chemotherapy against Chagas' disease and may constitute a link between the glutathione-based metabolism of the host and the trypanothione-based metabolism of the parasite.
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