Pyrimidine metabolism (biosynthesis and salvage)

The apicomplexans Toxoplasma gondii, Neospora caninum and Plasmodium falciparum can synthesise pyrimidine de novo from aspartate and glutamine. The Cryptosporidium species cannot synthesise pyrimidines de novo and salvages it from host. The Coccidians, T. gondii and N. caninum also possess the salvage mechanisms in addition to de novo synthesis, whereas P. falciparum does not possess salvaging ability. The measurement of activities of five of six enzymes catalysing pyrimidine biosynthesis by Hill et al in wide range of protozoan parasites demonstrated the presence of this pathway in P. berghei and T. gondii [1]. The work by same group in P. berghei also demonstrated the detection of all enzymes of pyrimidine biosynthesis pathway and it is identified that carbamoyl-phosphate synthase accepts glutamine rather than ammonia as amine group donor [2]. All the six enzymes leading to UMP generation were biochemically characterised in T. gondii [3]. The carbamoyl-phosphate synthase enzyme of T. gondii have bifunctional N-terminal glutamine amidotransferase domain fused with C-terminal carbamoyl-phosphate synthase domains. It is a unique feature of apicomplexans and it is suggested that it does not possess the allosteric regulation by PRPP as in mammalian enzyme. In addition, acivicin, a glutamine antagonist was demonstrated to inhibit T. gondii growth in vitro suggesting it to be a possible drug target [4, 5]. Pyrimidine biosynthesis can also be indirectly targeted with atovaquone, a ubiquinone analog as blocking mitochondrial electron flow will lead to inhibition of dihydroorotate dehydrogenase, the enzyme catalysing fourth step of UMP biosynthesis (refer to Electron transport chain pathway). The inhibition of thymidylate synthase enzyme (refer to folate biosynthesis pathway) also indirectly targets pyrimidine biosynthesis.

The enzyme CTP synthase is present in all apicomplexans and is the only enzyme involved in the conversion of uridine to cytidine nucleotides. In addition to these, the enzyme cytidine/dCTP deaminase, an enzyme involved in deamination of dCTP to dUTP is present in P. falciparum, N. caninum and T. gondii. Other salvage enzymes present in T.gondii and N. caninum and absent in P. falciparum includes dCMP deaminase, uracil phosphoribosyltransferase (UPRT) and pyrimidine (uridine) phosphorylase. There are also biochemical evidence available for the presence of salvage pathway enzymes mainly UPRT in T. gondii. These also suggest that disruption of either biosynthesis or salvage pathway has no effect on growth in vitro although biosynthesis pathway is required for virulence in mammals [5].

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Sources and fates of metabolites