The lead enzymes of polyamine biosynthesis, i.e. ornithine decarboxylase (ODC) and arginine decarboxylase (ADC), were not detected in Toxoplasma gondii [the limit of detection for ODC and ADC was 5 pmol min "1 (mg protein) "1 ], indicating that T. gondii lacks a forward-directed polyamine biosynthetic pathway, and is therefore a polyamine auxotroph. The biochemical results were supported by results obtained from data-mining the T. gondii genome. However, it was possible to demonstrate the presence of a highly active backconversion pathway that formed spermidine from spermine, and putrescine from spermidine, via the combined action of spermidine/ spermine N 1 -acetyltransferase (SSAT) or spermidine N 1 -acetyltransferase (SAT) and polyamine oxidase (PAO). With spermine as the substrate, T. gondii SSAT had a specific activity of 1.84 nmol min "1 (mg protein) "1 , and an apparent K m for spermine of 180 mM; with spermidine as the substrate, the SAT had a specific activity of 3.95 nmol min "1 (mg protein) "1 , and a K m for spermidine of 240 mM. T. gondii PAO had a specific activity of 10.6 nmol min "1 (mg protein) "1 , and a K m for acetylspermine of 36 mM. Furthermore, the results demonstrated that T. gondii SSAT was 50 % inhibited by 30 mM di(ethyl)norspermine. The parasite actively transported arginine and ornithine, which were converted via the arginine dihydrolase pathway to citrulline and carbamoyl phosphate, resulting in the formation of ATP via carbamate kinase. The lack of polyamine biosynthesis by T. gondii is contrasted with polyamine metabolism by other apicomplexans.

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