Bleomycin-induced strand-scission of DNA. Mechanism of deoxyribose cleavage

The bleomycins, a family of glycopeptides with antineoplastic activity, bind to DNA and cause strand-scission in a reaction requiring Fe(II) and oxygen. Strand-scission is accompanied by the simultaneous release of free bases and four low molecular weight compounds that react with 2-thiobarbituric acid to form chromophores absorbing maximally at 532 nm. The structure of these four products was elucidated as follows: formic acid hydrolysis released thymine, cytosine, adenine, and guanine, respectively; treatment of each compound with phenylhydrazine yielded 1-phenylpyrazole. The adenine-containing compound was reduced with sodium borohydride and trimethylsilyl derivative was prepared. High resolution mass spectrometry of this derivative revealed silylated 3-(adenin-9'yl)-2-propen-1-ol, consistent with formulation of the original compound as 3-(adenin-9'-yl)-2-propenal. The reaction product containing thymine was similarly shown to be 3-(thymin-1'-yl)-2-propenal. The 3-substituted propenal structures proposed were confirmed by chemical synthesis. Oligonucleotides, produced by bleomycin-induced strand-scission, were found to contain glycolic acid residues esterified through the hydroxyl group to 3'-phosphate termini. We conclude that the primary reaction products resulting from bleomycin-induced strand-scission of DNA have a common structure: Base-CH=CH-CHO. These observations are consistent with a reaction mechanism initiated by free radical abstraction of the C-4'proton of deoxyribose and leading to oxidative cleavage of the (C-3')-(C-4') bond.