An active intermediate formed in the reaction of bleomycin-Fe(II) complex with oxygen

The base-release activity of oxygen adduct of bleomycin-Fe(II) complex [BLM-Fe(II)] from DNA decreased with a half-life of 5.2 minutes, when incubated at 0 degrees C in 0.05 M Tris-HCl buffer at pH 7.8 in the absence of DNA. Under the same condition, however, visible and ESR spectra showed that the adduct was immediately converted into the ferric complex. The ESR study further indicated the simultaneous formation of two kinds of the low-spin BLM-Fe(III) complex. One of them disappeared in parallel with the decrease of the base-release activity and transformed into the other. The latter Fe(III) complex was stable but inactive. However, by addition of hydrogen peroxide to the latter, the former was regenerated and the base-release activity appeared. Oxygen concentration measurements by oxygraph showed that one mole of BLM-Fe(II) consumed approximately 0.5 mole of molecular oxygen instantly, but did not any more thereafter in the absence of a reducing agent. While in the presence of 2-mercaptoethanol, the oxygen consumption proceeded biphasically, and equimolar oxygen was consumed by BLM-Fe(II) in the first rapid reaction. These results suggest that oxygen adduct of BLM-Fe(II) is reduced by one electron transfer from an external electron donor and the resulting BLM-Fe(III)-O2H- [or its deprotonated form: BLM-Fe(III)-O2(2)-] shows the activity to break DNA accompanying the base-release.