title: Reaction of acid-activated mitomycin C with calf thymus DNA and model guanines: elucidation of the base-catalyzed degradation of N7-alkylguanine nucleosides.
abstract: Mitomycin C (MC, 1) forms covalent adducts under acidic activating conditions (pH approximately 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: scission of the glycosidic bond to form N7-(2'' beta, 7''-diaminomitosen-1'' alpha-yl)guanine (5) and its 1'' beta-isomer (6) and imidazolium ring opening to generate three 2,6-diamino-4-hydroxy-5-(N-formyl-2'' beta, 7''-diaminomitosen-1'' beta-yl)pyrimidine (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.
mesh_terms: D055598:Chemical Phenomena; D002621:Chemistry; D004247:DNA; D003845:Deoxycytosine Nucleotides; D003849:Deoxyguanosine / Q000031:analogs & derivatives; D006147:Guanine / Q000031:analogs & derivatives* / Q000138:chemical synthesis; D006863:Hydrogen-Ion Concentration; D007202:Indicators and Reagents; D009682:Magnetic Resonance Spectroscopy; D016685:Mitomycin; D008937:Mitomycins; D013053:Spectrophotometry; D013329:Structure-Activity Relationship
chemical_list: D003845:Deoxycytosine Nucleotides; D007202:Indicators and Reagents; D008937:Mitomycins; C037781:deoxycytidylyl-(3'-5')-deoxyguanosine; D016685:Mitomycin; D006147:Guanine; D004247:DNA; D003849:Deoxyguanosine