Reaction of acid-activated mitomycin C with calf thymus DNA and model guanines: elucidation of the base-catalyzed degradation of N7-alkylguanine nucleosides

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Abstract
Mitomycin C (MC, 1) forms covalent adducts under acidic activating conditions (pH .apprx. 4) with deoxyguanosine, d(GpC), and guanine residues of calf thymus DNA. In the case of deoxyguanosine, five adducts arise from a common precursor, N7-(2".beta.,7"-diaminomitosen-1"-yl)-2''-deoxyguanosine (10a; not isolated), which hydrolyzes spontaneously via two pathways: (i) scission of the glycosidic bond to form N7-(2".beta.,7"-diaminomitosen-1" .alpha.-yl)guanine (5) and its 1".beta.-isomer (6) and (ii) imidazolium ring opening to generate three 2,6-diamino-4-hydroxy-5-(N-formyl-2".beta.,7"-diaminomitosen-1".beta.-yl)pyrmidine (FAPyr) derivatives that are substituted at N6 by isomeric 2''-deoxyribose units [i.e., 1''.beta.-furanose (7), 1''.alpha.-furanose (8), and 1''.beta.-pyranose (9)]. The structures of 5-9 were determined by spectroscopic methods. The same five adducts were obtained from d(GpC), but only the guanine adducts 5 and 6 were formed in DNA. Adducts 7-9 interconvert during high-performance liquid chromatography (HPLC). The unexpected isomerization of the deoxyribose moiety of the initially formed 1''.beta.-furanose adduct 7 to those of 8 and 9 occurs upon imidazolium ring opening, as discerned by the course of imidazolium cleavage of the simple models N7-ethyl- and N7-methylguanosine and N7-methyl-2''-deoxyguanosine. All ring-opened N7-alkylguanosine derivatives studied here exist as a mixture of distinct N-formyl rotamers, manifested by multiple interconverting peaks on HPLC and in the 1H NMR spectra. In the UV spectra of such derivatives, a new and diagnostic maximum at 218 nm (at pH 7) is observed. Acid-activated MC is found to alkylate preferentially the Gua-N7 position in deoxyguanosine or d(GpC), in contrast to reductively activated MC, which preferentially alkylates the Gua-N2 position. This finding is explained by the different electronic structures of acid- and reduction-activated MC. In DNA, the N7 specificity of acid-activated MC is partially offset by steric factors.