Interaction between SV40 DNA and Camptothecin, an Antitumor Alkaloid¹

Camptothecin specifically interacted with closed superhelical circular SV40 DNA during incubation in 1.0 m NaCl at 37°C and induced an alkali-labile linkage in the E strand. No interaction occurred in the reaction mixture containing 0.1 m NaCl, or at 4°C. As camptothecin did not affect linear SV40 DNA, the superhelical structure of DNA appeared to be essential. The site of the alkali-labile linkage induced in SV40 DNA I through interaction with camptothecin was near the origin of replication on the basis of the results of experiments with restriction enzymes. Neither sulfhydryl reagents nor EDTA affected the interaction between camptothecin and SV40 DNA I, so the action of camptothecin is different from those of antitumor antibiotics, bleomycin or neocarzinostatin. Analysis of the $s20,w0$ value of SV40 DNA I after the interaction with camptothecin and the sedimentation profiles of DNA after heating in the reaction mixture indicated that the interaction between camptothecin and SV40 DNA I was different from those of intercalating or alkylating agents such as ethidium bromide and methyl-methanesulfonate. Replacement of the OH group at C-20 in the E ring of camptothecin by H⁻, CH₃⁻, and Cl⁻ resulted in the reduction, in this order, of the potency for interaction with SV40 DNA I to induce an alkali-labile linkage.