title: Toxoplasma gondii: mechanism of the parasitostatic action of 6-thioxanthine.
abstract: In contrast to the cytocidal effect of 6-thiopurines on mammalian cells, the action of 6-thioxanthine on Toxoplasma gondii was only parasitostatic. 6-Thioxanthine was a substrate of the parasite's hypoxanthine-guanine phosphoribosyltransferase. That enzyme converted 6-thioxanthine to 6-thioxanthosine 5'-phosphate which accumulated to near millimolar concentrations within parasites incubated intracellularly in medium containing the drug. 6-Thioxanthosine 5'-phosphate was the only detectable metabolite of 6-thioxanthine. The absence of 6-thioguanine nucleotides explains the lack of a parasitocidal effect because the incorporation of 6-thiodeoxyguanosine triphosphate into DNA is the mechanism of the lethal effect of 6-thiopurines on mammalian cells. Extracellular parasites that had accumulated a high concentration of 6-thioxanthosine 5'-phosphate incorporated more labeled hypoxanthine or xanthine into their nucleotide pools than did control parasites. The basis for this increased nucleobase salvage remains unexplained. It was not due to up-regulation of hypoxanthine-guanine phosphoribosyltransferase and could not be explained by reduced use of labeled nucleotides for nucleic acid synthesis. Extracellular parasites that had accumulated a high concentration of 6-thioxanthosine 5'-phosphate used labeled hypoxanthine almost entirely to make adenine nucleotides while control parasites made both adenine and guanine nucleotides. Both extracellular parasites that had accumulated a high concentration of 6-thioxanthosine 5'-phosphate and control parasites efficiently used labeled xanthine to make guanine nucleotides. These observations suggested that inosine 5'-phosphate-dehydrogenase was inhibited while guanosine 5'-phosphate synthase was not. Assay of inosine 5'-phosphate dehydrogenase in soluble extracts of T. gondii confirmed that 6-thioxanthosine 5'-phosphate was an inhibitor. We conclude that 6-thioxanthine blocks the growth of T. gondii by a depletion a guanine nucleotides.
authors.affiliation: Department of Microbiology and Immunology, Dartmouth Medical School, Hanover, New Hampshire, 03755-3842, USA. R.Pfefferkorn@Dartmouth.EDU
authors.forename: E R
authors.initials: ER
authors.lastname: Pfefferkorn
authors.forename: D J
authors.initials: DJ
authors.lastname: Bzik
authors.forename: C P
authors.initials: CP
authors.lastname: Honsinger
mesh_terms: D000818:Animals; D000963:Antimetabolites / Q000378:metabolism / Q000494:pharmacology*; D000981:Antiprotozoal Agents / Q000378:metabolism / Q000494:pharmacology*; D002851:Chromatography, High Pressure Liquid; D006150:Guanine Nucleotides / Q000037:antagonists & inhibitors / Q000096:biosynthesis; D006801:Humans; D019271:Hypoxanthine / Q000378:metabolism; D007041:Hypoxanthine Phosphoribosyltransferase / Q000378:metabolism; D007168:IMP Dehydrogenase / Q000037:antagonists & inhibitors; D013873:Thionucleotides / Q000378:metabolism; D014122:Toxoplasma / Q000187:drug effects* / Q000254:growth & development / Q000378:metabolism; D014970:Xanthines / Q000378:metabolism / Q000494:pharmacology*
chemical_list: D000963:Antimetabolites; D000981:Antiprotozoal Agents; D006150:Guanine Nucleotides; D013873:Thionucleotides; D014970:Xanthines; C054700:6-thioxanthine; D019271:Hypoxanthine; C074054:6-thioxanthine 5'-monophosphate; D007168:IMP Dehydrogenase; D007041:Hypoxanthine Phosphoribosyltransferase