[Effect of some nonintercalative antitumor drugs on the activity of calf thymus DNA topoisomerase I].

DNA topoisomerase I has been isolated from the nuclei of calf thymus by PEG fractionation and chromatography on P11 and on Bio-Rex 70. Either a positive or negative supercoiled pBR322 DNA can be relaxed by the enzyme. The activity of Topo I is Mg2+ and ATP independent. Some of nonintercalative antitumor drugs such as camptothecine, hydroxycamptothecine, cyclophosphamide, methotrexate and mitomycin C were found to inhibit the activity of Topo I. The results suggest that DNA Topo I can be used to screen new nonintercalative antitumor drugs as a target protein.