A study of o₂- versus h₂o₂-supported activation of fe-bleomycin

The rates of formation of product resulting from the cleavage of d(CGCT₃A₃GCG) by Fe(II)-BLM + O₂ as well as Fe(III)-BLM + H₂O₂ have been measured under a variety of experimental conditions in order to permit comparison of drug activation with the appearance of DNA degradation products. The activation of bleomycin with Fe(II) under aerobic conditions was found to be fast at neutral pH but was inhibited by DNA. In the presence of reducing agents such as sodium ascorbate, however, activation was rapid even in the presence of DNA. Presumably this reflects the ability of external reductants to bypass an otherwise obligatory bimolecular collision of two Fe(II)-BLM’s in the presence of O₂ to achieve activation. The decay of the activated Fe-BLM obtained by admixture of Fe(II)-BLM + O₂ was also found to occur quickly, with t_1/2 ~2 min at 0 °C. The rates of product release, in contrast, were found to be much slower, although both free bases and base propenals were released on the same time scale. The activation of Fe(III)-BLM with H₂O₂ was found to be slow at pH 7.2 but rapid at both acidic and basic pH. These observations can be rationalized by a mechanism that posits a slow bimolecular reaction between Fe(IIl)-BLM and H₂O₂ at neutral pH. When activated at pH 5.8, the rates of product release from DNA obtained with activated Fe(HI)-BLM were found to be comparable to the rates observed for aerobically activated Fe(II)-BLM at the same pH. The activation of Fe(III)-BLM by H₂O₂ was not inhibited by DNA. Both methods of Fe-BLM activation produced active species that behaved in the same fashion with regard to their reactivity toward DNA.