title: Inhibition of human lymphoma DNA-dependent RNA polymerase activity by 6-mercaptopurine ribonucleoside triphosphate.
abstract: 6-Mercaptopurine was found to inhibit the growth of cultured human lymphoma P3HR-1 cells and the incorporation of [3H]-uridine into trichloroacetic acid-precipitable materials of the cells. One of the derivatives of 6-mercaptopurine, 6-mercaptopurine ribonucleoside triphosphate (6-thio-ITP), was found to inhibit in vitro RNA synthesis (both engaged and free enzyme activities) of the isolated nuclei from P3HR-1 cells. The alpha-amanitin-resistant RNA polymerase (polymerase I) and alpha-amanitin-sensitive RNA polymerase (polymerase II) of the cells were isolated and partially purified by either diethylaminoethyl cellulose or diethylaminoethyl Sephadex column chromatography, followed by DNA-cellulose affinity chromatography. It was found that these partially purified enzymes were also sensitive to 6-thio-ITP inhibition. Kinetic studies showed that the inhibition of RNA polymerase activities by 6-thio-ITP could be reversed by increasing concentrations of guanosine 5'-triphosphate in the reaction mixture, indicating that 6-thio-ITP may act as a competitive inhibitor of the enzymes by competing with guanosine 5'-triphosphate for its enzyme-binding site. These data suggest that inhibition of RNA transcription by 6-thio-ITP may be considered as one of the mechanisms of the cytotoxic action of 6-mercaptopurine in human tumor cells.
mesh_terms: D002460:Cell Line; D002470:Cell Survival / Q000187:drug effects; D012321:DNA-Directed RNA Polymerases / Q000037:antagonists & inhibitors*; D006801:Humans; D008223:Lymphoma / Q000201:enzymology* / Q000473:pathology; D015122:Mercaptopurine / Q000494:pharmacology; D012318:RNA Polymerase I / Q000037:antagonists & inhibitors; D012319:RNA Polymerase II / Q000037:antagonists & inhibitors; D013873:Thionucleotides / Q000494:pharmacology*
chemical_list: D013873:Thionucleotides; C001707:6-thioinosine-5'-triphosphate; D015122:Mercaptopurine; D012319:RNA Polymerase II; D012321:DNA-Directed RNA Polymerases; D012318:RNA Polymerase I