Purine metabolism (salvage)

The purine nucleotides are not only required as components of nucleic acids, but also as cofactors of metabolic processes and as a source of energy (ATP). Apicomplexans cannot synthesise purine rings de novo and salvage them from host. The putative transporters involved in the uptake of purine bases and nucleosides from parasitophorous vacuole are present in apicomplexans including Toxoplasma gondii, Plasmodium falciparum and Cryptosporidium parvum. There were four transporters identified in T. gondii of which three are present in the genome of ME49 strain (genome analysed), whereas all four are present in VEG and GT1 strain genomes. The work of De Koning et al in T. gondii demonstrated the presence high affinity purine nucleoside transporter (TgAT2) and purine base transporter (TgNT1) in addition to the already characterised low affinity nucleoside transporter (TgAT1) [1]. TgAT1 has substrate specificity to adenosine and inosine and TgAT2 possess broad substrate specificity. TgNT1 can transport the bases hypoxanthine, guanine and xanthine. The presence of high affinity transporters TgAT2 and TgNT1 suggests that these parasites are capable of utilising very low concentration of nucleobases and nucleosides available in the parasitophorous vacuole.

The two most important enzymes involved in salvage of purine nucleotides in apicomplexans are hypoxanthine-guanine phosphoribosyltransferase (HGXPRTase) and adenosine kinase leading to guanine nucleotide generation and adenine nucleotide generation respectively. HGXPRTase can catalyse the synthesis of IMP, XMP and GMP from salvaged hypoxanthine, xanthine and guanine respectively. IMP dehydrogenase and GMP synthase are the other enzymes which are involved in guanine nucleotide generation. Of the 4 purine nucleosides incorporated into nucleic acids (adenosine, guanosine, inosine and xanthosine), adenosine is incorporated more than 12-folds higher than others. Adenosine kinase is the the most active of all enzymes and is the main route of adenosine incorporation and AMP generation [2] although there are other routes available for both adenosine incorporation and AMP generation. Adenosine can also be utilised for IMP generation catalysed by adenosine deaminase [3] and these IMP can either be converted to guanine nucleotide (HGXPRTase route) or to AMP via the action of adenylosuccinate synthase and adenylosuccinate lyase. In addition, the activity of purine nucleoside phosphorylase was detected with guanosine and inosine. The enzyme AMP deaminase catalysing conversion of AMP to IMP is also present in T. gondii. This and 10-fold higher adenylate kinase activity suggests that adenine nucleotide serves as substrate for generation of guanine nucleotides [2]. The enzyme adenosine kinase is absent in P. falciparum (MPMP purine metabolism pathway), where AMP generation depends on adenylosuccinate synthase and lyase activities. Many of the enzymes of purine salvage pathway in T. gondii were biochemically characterised and their kinetic properties were studied. The reduction of ribonucleotides to deoxyribonucleotides is catalysed by ribonucleoside diphosphate reductase enzyme by oxidising the co-substrate thioredoxin.

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Sources and fates of metabolites

Nucleoside catabolism

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