Topoisomerase-II activity in human leukemic and lymphoblastoid cells.

Topoisomerase-II activity was analyzed in various human leukemic and lymphoblastoid cell-lines with comparison to normal human peripheral blood lymphocytes. All of the examined tumor cells contained this enzyme in both the nuclear and cytoplasmic fractions, whereas no appreciable activity of the enzyme was detected in either fraction of the resting normal lymphocytes. Using pBR322 plasmid as a substrate, undialyzed extracts of the tumor cells exhibited the typical ATP-dependent relaxation of supercoiled circles and formation of linear and catenated structures, as well as the ATP-independent knotting activity. On the other hand, dialyzed extracts exerted only the ATP-dependent supercoil relaxation. Novobiocin inhibited the linearization and catenation but not the supercoil relaxing or knotting activities. This study provides indications for an excessive level of a structurally abnormal topoisomerase-II in these tumor cell-lines.