Human DNA Topoisomerase I: Quantitative Analysis of the Effects of Camptothecin Analogs and the Benzophenanthridine Alkaloids Nitidine and 6-Ethoxydihydronitidine on DNA Topoisomerase I-Induced DNA Strand Breakage

Human DNA topoisomerase I (topo I) has been purified from normal placenta and from a recombinant baculovirus expression system. A new radiolabeled plasmid DNA assay has been used to quantitate the activity of the purified enzymes and to compare the ability of several types of topo I-targeted drugs to induce topo I-mediated DNA strand breaks. The 100-kDa recombinant enzyme form isolated from the baculovirus expression system is able to relax 2564 ng of supercoiled M-13 mp19 plasmid per minute per nanogram of enzyme. The addition of camptothecin (1 μM) to the reaction lowers the rate to 1282 ng per minute per nanogram of enzyme. The 100-kDa topo I from human placenta is able to relax 1092 ng of supercoiled plasmid per minute per nanogram of enzyme and the 68-kDa topo I form from placenta is able to relax 2069 ng of supercoiled plasmid per minute per nanogram of enzyme. Camptothecin (1 μM) decreases the relaxation rate of the placental enzymes about 50%. In the presence of several different types of topo I-targeted drugs, both the recombinant and placental enzymes are induced to cleave plasmid DNA. Quantitative DNA cleavage assays with radioactive plasmid DNA and 9-aminocamptothecin, topotecan, SN-38, 10,11-methylenedioxycamptothecin, 7-ethyl-10,11-methylenedioxycamptothecin, 7-chloromethyl-10,11-methylenedioxycamptothecin, nitidine, and 6-ethoxy-5,6-dihydronitidine indicate that the order of potency in inducing topo I-mediated DNA breakage is methylenedioxycamptothecin analogs > SN-38 > 9-aminocamptothecin > topotecan and camptothecin > nitidine compounds. The order of potency correlates with the half-lives of the topo I-DNA drug complex determined with radiolabeled DNA in 0.45 M NaCl at 30°C. The half-life of the complex formed with 7-chloromethyl-10,11-methylenedioxycamptothecin is greater than 90 min whereas the half-life of the topo I–DNA complex with 6-ethoxy-5,6-dihydronitidine is less than 15 s. The other drugs tested were found to have drug complex half-lives which fall between these two extremes.