Trypanosoma lewisi infection in the rat: effect of adenine

Trypanosoma brucei adenine-phosphoribosyltransferases mediate adenine salvage and aminopurinol susceptibility but not adenine toxicity

•

African trypanosomes possess two distinct adenine phosphoribosyltransferases.

•

Trypanosoma brucei TbAPRT1 is cytosolic, TbAPRT2 localizes to the glycosome.

•

Aprt1,2 null mutants are viable but do not incorporate adenine into nucleotides.

•

Aprt1,2 null mutants are resistant to aminopurinol but still sensitive to adenine.

•

Aminopurinol is a trypanocide with submicromolar activity against T. brucei.

African trypanosomes, like all obligate parasitic protozoa, cannot synthesize purines de novo and import purines from their hosts to build nucleic acids. The purine salvage pathways of Trypanosoma brucei being redundant, none of the involved enzymes is likely to be essential. Nevertheless they can be of pharmacological interest due to their role in activation of purine nucleobase or nucleoside analogues, which only become toxic when converted to nucleotides. Aminopurine antimetabolites, in particular, are potent trypanocides and even adenine itself is toxic to trypanosomes at elevated concentrations. Here we report on the T. brucei adenine phosphoribosyltransferases TbAPRT1 and TbAPRT2, encoded by the two genes Tb927.7.1780 and Tb927.7.1790, located in tandem on chromosome seven. The duplication is syntenic in all available Trypanosoma genomes but not in Leishmania. While TbAPRT1 is cytosolic, TbAPRT2 possesses a glycosomal targeting signal and co-localizes with the glycosomal marker aldolase. Interestingly, the distribution of glycosomal targeting signals among trypanosomatid adenine phosphoribosyltransferases is not consistent with their phylogeny, indicating that the acquisition of adenine salvage to the glycosome happened after the radiation of Trypanosoma. Double null mutant T. brucei Δtbaprt1,2 exhibited no growth phenotype but no longer incorporated exogenous adenine into the nucleotide pool. This, however, did not reduce their sensitivity to adenine. The Δtbaprt1,2 trypanosomes were resistant to the adenine isomer aminopurinol, indicating that it is activated by phosphoribosyl transfer. Aminopurinol was about 1000-fold more toxic to bloodstream-form T. brucei than the corresponding hypoxanthine isomer allopurinol. Aminopurinol uptake was not dependent on the aminopurine permease P2 that has been implicated in drug resistance.

The availability of purines influences both the number of parasites and the splenocyte levels of purine-metabolizing enzymes in trypanosome-infected mice

Growth on Trypanosoma musculi in the murine host was limited by the availability of host purines. A portion of the spleen cells of infected mice (many of them granulocytes) displayed high levels of adenosine deaminase (ADA) and purine nucleoside phosphorylase, probably as a compensatory response to extracellular purine deficiency. Injections of adenosine or 2-deoxycoformycin stimulated significant increases in the growth of parasites. 2-Deoxycoformycin treatment also diminished parasite-induced splenomegaly. Treatment of mice with polyethylene glycol-modified ADA, a slowly catabolized form of ADA, had no effect on the course of T. musculi infection, indicating that the parasites can utilize purines other than adenosine. The apparent competition between parasites and host cells for available purines suggests that depletion of extracellular purines should be considered as an approach to treating extracellular trypanosome infections.

The complex Arom locus of Aspergillus nidulans. Evidence for multiple gene fusions and convergent evolution

The physical positions of the DNA sequences encoding the five consecutive enzyme activities required to metabolise 3-deoxy-D-arabino-heptulosonic acid-7-phosphate to 5-enolpyruvyl-shikimate-3phosphate, which are encoded by the A. nidulans Arom polypeptide have been determined. Subfragments of the Arom locus encoding EPSP synthase and 3-dehydroquinase have been expressed in appropriate E. coli aro mutants. The DNA sequence of the A. nidulans Arom locus has been shown to have homology with the corresponding unlinked E. coli aro loci strongly suggesting (I) divergent evolution from common ancestral sequences and (II) that the complex A. nidulans Arom locus arose by multiple gene fusion. The DNA and protein sequence of the two 3-dehydroquinase isoenzymes of A. nidulans share no homology, strongly indicating separate phylogenetic origins and their convergent evolution. The 5' and 3' non-translated DNA sequence of the A. nidulans Arom locus is presented along with the presumed sites for transcription initiation and polyadenylation determined by S1 nuclease protection experiments.