Pyrimidine metabolism (salvage)

The apicomplexans Toxoplasma gondii 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. T. gondii also possess the salvage mechanisms in addition to de novo synthesis, whereas P. falciparum does not possess salvaging ability. All the six enzymes leading to de novo biosynthesis of UMP present in P. falciparum and T. gondii are absent in Cryptosporidium species.

There are three important pyrimidine salvage enzymes in Cryptosporidia. They are thymidine kinase (TK), a monofunctional uracil phosphoribosyltransferase (UPRT) enzyme and a bifunctional UPRT-uridine kinase (UK) enzyme. Of these the bifunctional UPRT-UK and thymidine kinase enzymes are absent in T. gondii and Neospora caninum, whereas all three salvage enzymes are absent in P. falciparum. The enzyme CTP synthase is present in all apicomplexans and is the only enzyme involved in the conversion of uridine to cytidine nucleotides. The enzyme cytidine deaminase present in both P. falciparum and T. gondii is absent in Cryptosporidia. Another salvage enzyme dCMP deaminase present in T.gondii and absent in P. falciparum is also present in Cryptosporidia. Pyrimidine (uridine) phosphorylase present in Toxoplasma is absent in both Plasmodia and Cryptosporidia. There are biochemical evidences for the expression of active thymidine kinase and uridine kinase enzymes. The presence of active TK was confirmed in Cryptosporidium parvum with bromodeoxyuridine incorporation [1]. Pronounced effects in parasite development seen with nucleoside analogs such as cytosine-arabinoside suggests the activity of uridine kinase [2].

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Sources and fates of metabolites