Enzymes trypanosomes

Melarsoprol, a trivalent organic melaminophenyl arsenic compound, kills intracerebral parasites of both T. brucei gambiense and T. brucei rhodesiense. Melarsoprol accumulates via an adenosine/adenine transporter in trypanosomes and is believed to inhibit glycolytic enzymes. Melarsoprol leads to a rapid lysis of trypanosomes. Melarsoprol is highly toxic to humans. 

Suramin, a symmetrical, polysulfonated naphthyl-amine, inhibits a number of trypanosomal enzymes however, the importance of these effects on parasite killing is not clear. 

S.2.3.3 Treatment of Trypanosomiasis The difluoromethylornithine (DFMO), eflomithine is a mechanism-based inhibitor of ornithine decarboxylase— a pyridoxal-dependent key enzyme of the polyamine s biosynthesis from ornithine. Fluorine atoms are essential for the inhibition process (cf. Chapter 7). Eflornithine was first clinically developed for cancer, but its development has been abandoned for this indication. The activity of eflornithine on trypanosomes was then discovered. Now, despite its very low bioavailability, eflornithine is the best therapy for sleeeping sickness (trypanosomiasis)—in particular, at the cerebral stage—due to Trypanosoma brucei gambiense parasite. Eflornithine is registered with orphan drug status and is distributed by the WHO. 

N-Ribohydrolases have been found to be involved in novel pathways of purine salvage in protozoan parasites as well as in nucleic acid repair, and exhibit other interesting biological activities [178]. In order to investigate the molecular electrostatic potential surface of the enzyme from the trypanosome Crithidia fasci-culata, several l,4-dideoxy-l,4-imino-D-ribitol derivatives were synthesized as nucleoside analogues and their inhibitory powers were tested [179,180]. In the course of this work, l,4-dideoxy-l,4-imino-l(S)-phenyl-D-ribitol (97) was found to inhibit this enzyme with K 30 nmol/1. 

Kooy and Royall (1994) have shown that cultured endothelial cells produce peroxynitrite when stimulated by bradykinen. They used an ultrasensitive chemiluminescent assay based on luminol developed by Radi et al. (1993). Peroxynitrite is a potent toxin to trypanosomes, attacking both sulfhydryl dependent enzymes and respiratory enzymes (Rubbo et al., 1994). Radi et al. (1994) have also shown that it is far more damaging to mitochondria than nitric oxide. 

Our goal is to discover and design selective inhibitors of trypanosomal glycolsysis. Thusfar, three enzymes have been targeted. Whereas little success was obtained with TIM, substantial progress is being made with GAPDH and PGK. 

In mammalian cells, ornithine decarboxylase undergoes rapid turnover—that is, a constant round of enzyme degradation and synthesis. In some trypanosomes, however, the enzyme—for reasons not well understood—is stable, not readily replaced by newly synthesized enzyme. An inhibitor of ornithine decarboxylase that binds permanently to the enzyme would thus have little effect on human cells, which could rapidly replace inactivated enzyme, but would adversely affect the parasite. 

Figure 5-16 (A) Electron micrograph of the network of catenated DNA circles in the mitochondrion of the trypanosome Crithidia fasciculata. (B) and (C) The same network after treatment with a topoisomerase from bacteriophage T4 that catalyzes a decatenation to form individual covalently closed circles (Chapter 27). Five times as much enzyme was added in (C) as in (B). Two sizes of circles are present. Most are "minicircles", each containing about 2300 bp but a smaller number of larger 35-kb "maxicircles" are also present. One of these is marked by the arrow. From Marini, Miller, and Englund.183...

Nucleoside hydrolase has been proposed to participate in purine salvage in the trypanosome Crithidia fasciculata. The enzyme hydrolyses the N-glycosidic linkage of the naturally occurring purine and pyrimidine nucleosides. A geometric model of the transition state for nucleoside hydrolase for the reaction (Horenstein et al., 1991)... 

The conversion of giycerol-3-phosphate (G-3-P) into dihydroxyacetone phosphate (DHAP) in glycosomes generates reducing equivalents, which enter the mitochondria and convert molecular oxygen into water. This respiratory reaction takes place in the presence of the glycerol-3-phosphate oxidase (glycerolphosphate oxidase, GPO) system. It is a complex enzyme system, which is unique to salivarian trypanosomes this is not found in mammalian cells [7]. Further, GPO is cytochrome free and is bound to the inner membrane of mitochondria (Chart 2). 

Although the exact nature of the various enzymes involved in pyrimidine biosynthesis is not fully worked out, it seems that leishmania and trypanosomes possess phosphoribosyl transferase, which is specific for uracil. This makes the protozoal phosphoribosyl transferase distinct from the mammalian orotate phosphoribosyl transferase and, therefore, may be explored in protozoal chemotherapy. 

The fivevalent antimonials per se do not exert toxic effects on leishmania and trypanosomes. After entering the human body, the fivevalent antimonials are reduced to the threevalent form [42]. This threevalent form reacts with the sulphhy-dryl groups of the enzymes/proteins essential for parasites and inactivate them [34,43]. 

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