Reaction of DNA-bound Co(II)bleomycin with dioxygen

The aerobic oxidation of Co(II)bleomycin bound to calf thymus DNA has been investigated in relation to the mechanism of reaction in solution in the absence of DNA. Kinetics of dioxygenation of the Co(II) complex were followed by spectrophotometric and electron spin resonance spectroscopy as well as dioxygen analysis. The reaction is slower than when carried out in solution; its rate is inversely related to the ratio of DNA base pairs to Co(II)bleomycin. The subsequent oxidation reaction, observed spectrophotometrically and by dioxygen analysis, is second order in cobalt complex. The calculated second order rate constant is also inversely related to the base pair to metal complex ratio. Once this ratio exceeds three, the reaction rate slows significantly with each additional increment of DNA added to the starting reaction mixture. Taking advantage of the high stability of O(2)-Co(II)bleomycin bound to greater than a 3-fold excess of DNA base pairs, it could be demonstrated that the rate constant for oxidation of two O(2)-Co(II)bleomycin molecules is much slower than that for O(2)-Co(II)bleomycin plus Co(II)bleomycin. With the same technique it was observed that the metal centers of O(2)-Co(II)bleomycin and Fe(II)bleomycin also undergo oxidation. The binding to DNA of both solution products of the oxidation of Co(II)bleomycin by O2 was examined by 1H NMR spectroscopy. Peroxy-Co(III)bleomycin, Form I, binds with higher affinity than Co(III)bleomycin, Form II. At lower ionic strength, the size of the DNA binding site for each form is about 2 base pairs/molecule of drug.