Origin of malondialdehyde from DNA degraded by Fe(II)-bleomycin

Ferrous bleomycin is known to break DNA efficiently in vitro in the presence of O₂, giving rise to oligonucleotides, bases, and compounds resembling malondialdehyde in their chromogenic reaction with 2-thiobarbituric acid. Chromatography of radiolabeled DNA reaction mixtures resolves three kinds of malondialdehyde-like products, related by sequential conversions. The first chromogenic product is linked to DNA, and its formation does not entail the release of a base. It decomposes readily to the second product, a compound containing the base and deoxyribose carbons 1'-3'. Hydrolysis of either product yields the third, which is indistinguishable from authentic malondialdehyde. These findings suggest that the oxygen-dependent cleavage of DNA of Fe(II)·bleomycin can begin with the rupture of the deoxyribose 3'-4'-carbon bond. The initiation of these events is concurrent with the initiation of another mode of DNA degradation, involving the release of free base alone, in a yield similar to that of chromogen.