Toxoplasma Gondii: Mechanism of the Parasitostatic Action of 6-thioxanthine

Exp Parasitol. 2001 Dec;99(4):235-43. doi: 10.1006/expr.2001.4673.


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.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • Antimetabolites / metabolism
  • Antimetabolites / pharmacology*
  • Antiprotozoal Agents / metabolism
  • Antiprotozoal Agents / pharmacology*
  • Chromatography, High Pressure Liquid
  • Guanine Nucleotides / antagonists & inhibitors
  • Guanine Nucleotides / biosynthesis
  • Humans
  • Hypoxanthine / metabolism
  • Hypoxanthine Phosphoribosyltransferase / metabolism
  • IMP Dehydrogenase / antagonists & inhibitors
  • Thionucleotides / metabolism
  • Toxoplasma / drug effects*
  • Toxoplasma / growth & development
  • Toxoplasma / metabolism
  • Xanthines / metabolism
  • Xanthines / pharmacology*


  • Antimetabolites
  • Antiprotozoal Agents
  • Guanine Nucleotides
  • Thionucleotides
  • Xanthines
  • 6-thioxanthine
  • Hypoxanthine
  • 6-thioxanthine 5'-monophosphate
  • IMP Dehydrogenase
  • Hypoxanthine Phosphoribosyltransferase