Properties and Products of the Degradation of DNA by Bleomycin and Iron(II)

Bleomycin and Fe(II) together act to cause the highly efficient degradation of adenovirus-2 DNA to acid-soluble products when the drug and metal ion are present in a one to twofold excess of DNA. Where the bleomycin DNA ratio is less than 1, breakage of DNA occurs to a greater extent than is seen with Fe(II) in the absence of bleomycin. Breakage of DNA occurs from pH 4.5 to greater than 10 and is not greatly dependent on temperature. The appearance of activity between pH 4 and 6 is related to the appearance of a 1:1 Fe(lI)-bleomycin complex with ʎmax 476 nm, ϵm (apparent) = 3.8 X 102, and pK = 5.2. The reaction proceeds to completion in less than 1 min at 37 °C and requires the presence of molecular oxygen. Increased ionic strength inhibits the reaction. Degradation of DNA by Fe(II)-bleomycin is not inhibited by superoxide dismutase or catalase at high bleomycin concentrations; however, at low concentrations of bleomycin inhibition by superoxide dismutase, but not by catalase, is observed. All four bases in DNA are released by Fe(II)-bleomycin (T > C > A > G). Discrete nucleaseresistant oligonucleotides of average length 7-10 residues are formed in the limit degradation of DNA by Fe(II)-bleomycin. Mononucleotides and inorganic phosphate are not produced. A product which resembles malondialdehyde is found and its formation is maximal at a 1:1 Fe(II) to bleomycin ratio in reactions containing excess DNA. The products of the degradation of DNA by Fe(II)-bleomycin are in accord with the proposal that Fe(II) is the ultimate cofactor of bleomycin in degrading DNA.