Further insights into the mechanism of the reaction of activated bleomycin with DNA.

Abstract

Bleomycin (BLM) is a glycopeptide anticancer drug that effectively carries out single- and double-stranded DNA cleavage. Activated BLM (ABLM), a low-spin ferric-hydroperoxide, BLM-Fe(III)-OOH, is the last intermediate detected before DNA cleavage. We have previously shown through experiments and DFT calculations that both ABLM decay and reaction with H atom donors proceed via direct H atom abstraction. However, the rate of ABLM decay had been previously found, based on indirect methods, to be independent of the presence of DNA. In this study, we use a circular dichroism (CD) feature unique to ABLM to directly monitor the kinetics of ABLM reaction with a DNA oligonucleotide. Our results show that the ABLM + DNA reaction is appreciably faster, has a different kinetic isotope effect, and has a lower Arrhenius activation energy than does ABLM decay. In the ABLM reaction with DNA, the small normal k(H)/k(D) ratio is attributed to a secondary solvent effect through DFT vibrational analysis of reactant and transition state (TS) frequencies, and the lower E(a) is attributed to the weaker bond involved in the abstraction reaction (C-H for DNA and N-H for the decay in the absence of DNA). The DNA dependence of the ABLM reaction indicates that DNA is involved in the TS for ABLM decay and thus reacts directly with BLM-Fe(III)-OOH instead of its decay product.