title: A comparison of topoisomerase I activity in normal and transformed cells.
abstract: Many viral oncogenes encode protein-tyrosine kinase activities. However, important in vivo substrates of these enzymes have yet to be identified. Recently, type I topoisomerases were shown to be in vitro substrates for two tyrosine kinases. Following tyrosine phosphorylation, topoisomerase I activity was reduced 10-fold (Tse-Dinh et al. Nature 312: 785-786, 1984). To determine whether topoisomerase I activity was modulated by tyrosine phosphorylation in vivo, we have measured topoisomerase I activity in nuclear lysates prepared from both normal fibroblasts and cells transformed by two different viral oncogenes (v-abl, v-src). Under a variety of experimental conditions, we have found no evidence to support the notion that type I topoisomerase activity is modulated by tyrosine phosphorylation in vivo.
mesh_terms: D000818:Animals; D002467:Cell Nucleus / Q000201:enzymology*; D002471:Cell Transformation, Neoplastic; D002478:Cells, Cultured; D004264:DNA Topoisomerases, Type I / Q000378:metabolism*; D005347:Fibroblasts / Q000201:enzymology; D007700:Kinetics; D051379:Mice; D008815:Mice, Inbred Strains; D009857:Oncogenes
chemical_list: D004264:DNA Topoisomerases, Type I