Cobalt-bleomycins and deoxyribonucleic acid: sequence-dependent interactions, action spectrum for nicking, and indifference to oxygen

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Abstract
Light-induced strand scission of DNA by Co-bleomycins [an antineoplastic drug] is more likely to occur at certain base sequences than others. By use of 32P-end-labeled [calf thymus, bacteriophage T-7, bacteriophage .vphi.X-174] DNA restriction fragments as the substrates for cleavage, products were analyzed on high-resolution polyacrylamide gels and compared to those produced by Fe bleomycin. The sites of damage to DNA are apparently similar in both cases; pyrimidine residues located at the 3'' side of a guanine are preferentially attacked. Consistent with the observed nicking specificity, interactions between Co-bleomycin and guanine residues in the trinucleotide sequence GGT are revealed in a dimethyl sulfate methylation experiment. The action spectrum for the light-induced DNA cleavage reaction correlates with the absorption spectrum of Co-bleomycin in the wavelength range between 330-450 nm. In contrast to Fe-bleomycin, the extent of DNA degradation by light-activated Co-bleomycins appears to be indifferent to the concentration of dissolved O2 in the reaction medium, little or no base propenal is produced. Bases (e.g., thymine) are released by both agents. Fluorescence quenching experiments show that apparent binding constants of Co-bleomycin complexes with DNA are in the 107 M-1 range in 25 mM tris(hydroxymethyl)aminomethane, pH 8, 1 mM NaCl and 1 mM ethylenediaminetetraacetic acid at 25.degree. C.