Reaction of DNA-bound ferrous bleomycin with dioxygen: activation versus stabilization of dioxygen.

The properties of binding of dioxygen to ferrous bleomycin [Fe(II)Blm] in the presence of DNA have been examined. Dioxygen reacts rapidly with Fe(II)Blm.DNA, forming an adduct that is increasingly stable to oxidation-reduction as the ratio of base pairs to drug becomes larger. This species has a spectrum similar to that reported for a proposed dioxygenated intermediate in the solution reaction of Fe(II)Blm with O2. It contains Fe(II) as measured with bathophenanthroline disulfonate (BPS) and O2 according to direct observation of stoichiometric release of O2 upon chelation of Fe(II) by BPS. The dioxygenated form O2-Fe(II)Blm.DNA, is highly stable; bound O2 dissociates upon purging with N2 with a rate constant of 0.16 min-1. Although Fe(II)Blm or Fe(II)Blm.DNA reacts almost completely with BPS within the time of mixing, O2-Fe(II) Blm.DNA reacts much slower in a process that is first order in Fe(II) and in BPS at constant DNA concentration. The rate is inversely related to DNA concentration reaching a limiting value at 50-100 base pairs per drug molecule. The kinetics of oxidation of Fe(II)Blm bound to DNA by O2 are biphasic and depend on the base pair to drug ratio. After formation of O2-Fe(II)Blm.DNA, another reaction occurs in which O2 is released, which is second order in dioxygen donor, and in which the second-order rate constant declines with increasing base pairs to FeBlm ratio. The rate of this second-order process accelerates at higher ionic strength.(ABSTRACT TRUNCATED AT 250 WORDS)