Studies on the interaction of bleomycin A₂ with rat lung microsomes. II. Involvement of adventitious iron and reactive oxyggen in bleomycin-mediated DNA chain breakage

Bleomycin-dependent DNA chain breakage catalyzed by rat lung microsomes and NADPH was significantly inhibited by the reactive oxygen scavengers superoxide dismutase and dimethylurea and by the metal chelator EDTA. Cytochrome c and nitro blue tetrazolium, compounds which interfere with microsomal electron transport, also inhibited bleomycin-mediated DNA chain breakage. In contrast to these agents, ascorbic acid significantly enhanced this bleomycin-mediated reaction. In addition to ascorbic acid, the redox cycling compounds paraquat, nitrofurantoin and mitomycin C also significantly increased the DNA damage by bleomycin. The stimulatory action of these redox cycling compounds was significantly inhibited by superoxide dismutase, demonstrating that reactive oxygen generated by the redox cycling of these compounds was diverted to the bleomycin-DNA complex. These collective observations support the concept that oxidation/reduction of adventitious iron bound by bleomycin and accompanying reactive oxygen generation participate in the microsome-catalyzed DNA damage mediated by bleomycin.