Iron(II)-bleomycin. Biochemical and spectral properties in the presence of radical scavengers

Consistent with a recent literature report (Repine, J. E. etal. (1981) Proc.Nat.Acad.Sci.USA7̄8̄, 1001-1003), the release of [³H]-thymine from PM-2 DNA by Fe(II)-H₂O₂-generated ·OH was suppressed by dimethyl sulfoxide. In contrast, DMSO did not affect [³H]-thymine release mediated by Fe(II)-bleomycin. Under aerobic conditions in the presence of t-butyl phenylnitrone, Fe(II)-BLM produces an epr signal that has been presumed to arise by transfer of ·OH or O₂^• from the "active complex" of bleomycin to the spin trap. Remarkably, high concentrations (80 mM) of PBN had no effect on the ability of Fe(II)-BLM to solubilize [³H]-thymine, although the ability of authentic ·OH to degrade DNA was completely suppressed under these condition. The suproxide dismutase catalyst tetrakis(4-N-methylpyridyl)porphineiron(III) also failed to suppress BLM-mediated DNA degradation. Moreover, the epr signal observed with 1.6 mM Fe(II)-BLM in the presence of 80 mM PBN was found to be much less intense than that produced by 1.6 mM Fe(II) and 290 mM H₂O₂, but equivalent in intensity to that obtained with 45 mM Fe(II) and exoess H₂O₂. We conclude that the fragmentation of DNA produced by Fe(II)-BLM can be due neither to free ·OH nor to O₂^•. We suggest that DNA degradation is initiated by an "active complex" consisting of BLM, metal and oxygen that functions by abstracting H· from susceptible sites on DNA.