Synthesis and Identification of Benzo[a]pyrene-Guanine Nucleoside Adducts Formed by Electrochemical Oxidation and by Horseradish Peroxidase Catalyzed Reaction of Benzo[a]pyrene with DNA

One-electron oxidation plays an important role in the metabolism of many substrates and their covalent binding to macromolecules. This mechanism of activation can be demonstrated by elucidation of the structure of DNA adducts. In this paper, we report the synthesis of adducts by anodic oxidation of benzo[a]pyrene (BP) in the presence of deoxyguanosine (dG) or guanosine (G). By using ¹H and two-dimensional NMR spectroscopy as well as fast atom bombardment and collisionally activated decomposition (CAD) mass spectrometry, adducts were identified as BP bound at C-6-C-8 of guanine (Gua), dG, and G and to N-7 of Gua. Loss of deoxyribose from the N-7 adduct was anticipated, but it was unexpectedly found that about 30% of the C-8 adduct with dG lost the deoxyribose moiety. The C-8 adduct of G almost entirely retained the ribose moiety. These compounds were used as markers for high-pressure liquid chromatography (HPLC) to identify adducts formed in the horseradish peroxidase catalyzed binding of BP to DNA. By use of HPLC in two solvent systems, adducts were identified in the supernatant fraction obtained after ethanol precipitation of the DNA and in an enzymatic digest of the DNA. The supernatant, containing adducts lost by depurination, afforded 95% of the N-7 adduct and about half of the C-8 adduct. The major adduct identified in the DNA digest was the C-8 of dG. The structure of the N-7 adduct in the supernatant was confirmed by CAD mass spectrometry. These results demonstrate that horseradish peroxidase catalyzes binding of BP to DNA by one-electron oxidation.