beta-Lapachone, a novel DNA topoisomerase I inhibitor with a mode of action different from camptothecin

beta-Lapachone is a plant product that has been found to have many pharmacological effects. To date, very little is known about its biochemical target. In this study, we found that beta-lapachone inhibits the catalytic activity of Topoisomerase I from calf thymus and human cells. But, unlike camptothecin, beta-lapachone does not stabilize the cleavable complex, indicating a different mechanism of action. beta-Lapachone inhibits Topoisomerase I-mediated DNA cleavage induced by camptothecin. Incubation of Topoisomerase I with beta-lapachone before adding DNA substrate dramatically increases this inhibition. Incubation of Topoisomerase I with DNA prior to beta-lapachone makes the Enzyme refractory, and treatment of DNA with beta-lapachone before Topoisomerase has no effect. These results suggest a direct interaction of beta-lapachone with Topoisomerase I rather than DNA substrate. beta-Lapachone does not inhibit binding of Enzyme to DNA substrate. In cells, beta-lapachone itself does not induce a SDS-K(+)-precipitable complex, but it inhibits complex formation with camptothecin. We propose that the direct interaction of beta-lapachone with Topoisomerase I does not affect the assembly of the enzyme-DNA complex but does inhibit the formation of cleavable complex.