allopurinol-riboside and formycin

The TOR gene (TOxic nucleoside Resistance gene) was mapped to a 2.3 kb fragment on the amplified DNA from tubercidin resistant Leishmania (TUB). This DNA fragment conferred upon wild type cells resistance to tubercidin, inosine dialdehyde, formycin A and B and allopurinol riboside and a reduced ability to accumulate purine nucleobases and nucleosides. These properties were characteristic of the parental TUB cells which carried the intact amplified DNA and have been hypothesized to be caused by a reduction in the activity of the multiple purine transporters within this organism. The TOR gene was found to be partially homologous to the rodent and human Oct-6/SCIP/Tst-1 gene. It lacked, however, the POU specific domain of this class of transcription factors and contained only the first two helices of the POU homeodomain. This truncated homeodomain was not required to confer resistance upon wild type cells to toxic nucleosides, suggesting that TOR was not a repressor with independent DNA binding capability.

Topics: Allopurinol; Amino Acid Sequence; Animals; Antiprotozoal Agents; Biological Transport; Cloning, Molecular; Drug Resistance; Formycins; Genes, Protozoan; Humans; Inosine; Leishmania mexicana; Molecular Sequence Data; Octamer Transcription Factor-6; Peptides; Purines; Restriction Mapping; Ribonucleosides; Transcription Factors; Transformation, Genetic; Tubercidin

Formycin B is cytotoxic toward Leishmania and is a potential chemotherapeutic agent for leishmaniasis. In order to determine the mechanism of action of formycin B, we have isolated and characterized clonal populations of formycin B-resistant Leishmania donovani. These formycin B-resistant clones are also cross-resistant to formycin A and allopurinol riboside-mediated growth inhibition. Incubation of the formycin B-resistant cells with [3H]formycin B indicates that, unlike wild type cells, the resistant populations cannot accumulate phosphorylated metabolites of exogenous [3H]formycin B. This is due to a defective transport system for formycin B in the resistant cells. However, wild type and mutant cells incorporate [3H]formycin A equally efficiently into [3H]formycin A-containing nucleotides and into RNA. These data suggest that formycin B cytotoxicity in Leishmania is not mediated by its incorporation as the adenosine analog into RNA. A plausible alternative hypothesis is proposed for the mechanism of action of the pyrazolo (4,3-d)pyrimidine C-nucleosides based upon depletion of an essential intracellular metabolite.

Topics: Allopurinol; Animals; Antibiotics, Antineoplastic; Cell Division; Chromatography, High Pressure Liquid; Drug Resistance; Formycins; Leishmania; Purines; Ribonucleosides; RNA