1. Toxoplasma gondii: mechanism of the parasitostatic action of 6-thioxanthine.
- Author
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Pfefferkorn ER, Bzik DJ, and Honsinger CP
- Subjects
- Animals, Antimetabolites metabolism, Antiprotozoal Agents metabolism, Chromatography, High Pressure Liquid, Guanine Nucleotides antagonists & inhibitors, Guanine Nucleotides biosynthesis, Humans, Hypoxanthine metabolism, Hypoxanthine Phosphoribosyltransferase metabolism, IMP Dehydrogenase antagonists & inhibitors, Thionucleotides metabolism, Toxoplasma growth & development, Toxoplasma metabolism, Xanthines metabolism, Antimetabolites pharmacology, Antiprotozoal Agents pharmacology, Toxoplasma drug effects, Xanthines pharmacology
- 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., (Copyright 2001 Elsevier Science (USA).)
- Published
- 2001
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