Human African trypanosomiasis is caused by two kinetoplastid flagellates, Trypanosoma brucei var. rhodesiense and Trypanosoma brucei var. gambiense, which are subspecies of T. brucei. The third subspecies, T. brucei var. brucei, is generally considered to be solely an animal pathogen. The T. brucei complex belongs to the order Kinetoplastida, family Trypanosomatidae, genus Trypanosoma, section salivaria and subgenus Trypanozoon. The three members of the T. brucei complex are phenotypically very similar, being
Edited by Stephen Gillespie and Richard D. Pearson © 2001 John Wiley & Sons Ltd morphologically identical and sharing major biochemical features. However, electrophoretic analysis of isoenzymes (zymodeme typing) and restriction fragment length polymorphism (RFLP) analysis has revealed considerable variation, not only within species but also within subspecies (Enyaru et al., 1993).
Phylogenetic analysis of 18S rRNA sequences indicates that T. brucei and T. cruzi have very different origins and divergent evolutionary patterns (Stevens et al, 1999a). The period during which this divergence occurred remains uncertain; palaeogeographical evidence dates the divergence to around 100 million years ago, when Africa became isolated from other continents, but estimates based on host-parasite associations place it at 260-500 million years ago (Haag et al., 1998). It is possible that T. brucei, in a background of continuous tsetse fly contact, co-evolved with primates in Africa for about 15 million years, with eventual emergence of the genus Homo about 3 million years ago. T. cruzi, on the other hand, could not have developed an evolutionary relationship with humans until human migration to the Americas, which is presumed to have occurred around 30 00040000 years ago (Stevens et al., 1999b).
These single-celled flagellated protozoa are characterised by the possession of an organelle unique to the Kinetoplastida, called a 'kineto-plast'. This DNA-containing organelle is located in the organism's single, complex mitochondrion, and resembles a nucleus on Giemsa staining. The kinetoplast-mitochondrion complex differs both morphologically and functionally among the different forms of the trypanosome that exist at the different stages of the life cycle. Studies by transmission electron microscopy have identified several other organelles within the trypanosome, including a Golgi apparatus, a nucleus with nucleolus and peripheral chromatin, an endo-plasmic reticulum, glycosomes and a basal body and flagellar pocket, from which extends a single flagellum. The trypanosomes also possess a cell membrane, attached to the inside of which is a complex network of microfilaments and micro-tubules. Surrounding the outside of the cell membrane is a surface coat, which contains the variant surface glycoprotein (VSG), the subject of antigenic variation. The flagellar pocket is devoid of both cytoskeletal attachments and VSG—this area has numerous receptors and provides a site for receptor-mediated endocyto-sis. In the procyclic forms of the trypanosome, VSG is replaced by procyclin.
In bloodstream trypanosomes, glucose cata-bolism is carried out by the Embden-Meyerhof pathway in a specialised organelle called a glycosome. ATP is generated by the substrate phosphorylation stages in this catabolic pathway. These stages lack lactate dehydrogenase and pyruvate decarboxylase, and pyruvate is hence excreted directly or transaminated into alanine. NADH generated during glycolysis is reoxidised by a dihydroxyacetone phosphate-glycerol-3-phosphate oxidase, which uses molecular oxygen as a terminal electron acceptor, does not require a respiratory chain and does not generate ATP (Fairlamb, 1989). In contrast, the procyclic forms possess the Krebs cycle and respiratory chain enzymes, and generate ATP primarily by oxida-tive phosphorylation.
Trypanosomes appear to have little capacity for the synthesis of amino acids, most of which are acquired directly from the host. Alanine, aspartate and glutamate are also acquired by transamination of pyruvate, oxaloacetate and a-ketoglutarate, respectively (Gutteridge et al., 1977). Trypanosomes do, however, synthesise polyamines (e.g. putrescine and spermidine), compounds which are essential for proliferation and differentiation of the bloodstream stages. A key step in polyamine biosynthesis is the decarboxylation of ornithine to putrescine via ornithine decarboxylase. In contrast to nearly all other eukaryotes, which have a thiol metabolism based on the glutathione/glutathione reduc-tase system, trypanosomatids lack glutathione
reductase (Fairlamb and Cerami, 1985). The main thiol compound is a conjugate between spermidine and glutathione called bis(glutathio-nyl)spermidine (trypanothione). Trypanothione metabolism plays several key roles in trypanoso-mal survival (see Figure 14a.1).
Trypanothione and trypanothione reductase contribute significantly to the maintenance of the correct intracellular thiol redox potential. Trypanothione reductase is an FAD-cystine-oxidoreductase unique to trypanosomes; it utilises NADPH to maintain trypanothione disulphide (T[S]2) as the dithiol, dihydro-trypanothione (T[SH]2) (Smith et al., 1991).
• Trypanothione and trypanothione peroxidase play an important role in defence against oxidant and radical damage.
• Thiol metabolism, particularly trypanothione, is crucial in defence against heavy metal toxicity.
Whereas trypanosomes synthesise pyrimidines by a pathway similar to that in mammals, there is no evidence of de novo purine biosynthesis. Salvage pathways are employed for this purpose (Hammond et al., 1984).
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