Mossbauer study of iron bleomycin and its activation intermediates

Using Mossbauer spectroscopy, we have examined iron-bleomycin in various oxidation states and in complexes with dioxygen or carbon monoxide. Ferrous bleomycin is a high spin ferrous complex. Addition of O₂ converts it into an EPR-silent oxygenated complex. Mossbauer studies in strong applied magnetic fields show that oxygenated bleomycin is diamagnetic. At 4.2 K, the quadrupole splitting ΔE(Q) = -2.96 mm/s and the isomer shift δ = 0.16 mm/s suggest that its electronic structure is best described as low spin ferric iron bound to superoxide anion. A single electron reduction yields activated bleomycin, an EPR-active form that still retains oxygen and which is kinetically competent to initiate DNA cleavage. We have produced this complex by exposing ferrous bleomycin to O₂ or by reacting ferric bleomycin with H₂O₂. The Mossbauer spectra give convincing evidence that the iron of activated bleomycin is low spin ferric. The decay of activated bleomycin yields low spin ferric bleomycin, a complex with Mossbauer parameters nearly identical with those reported for ferric cytochrome P-450. Although iron bleomycin does not have a polyaromatic structure like heme, many features of its electronic structure at the iron are very similar to those produced by the sulfur-coordinated heme iron of ferric cytochrome P-450, a protein that catalyzes a similar oxygen-dependent reaction.