Mechanistic Insights into the Michael Addition of Deoxyguanosine to Catechol Estrogen-3,4-quinones

The reaction of catechol estrogen quinones with DNA to produce the depurinating adducts, 4-OHE 2(E 1)-1-N7Gua and 4-OHE 2(E 1)-1-N3Ade, has been linked to the initiation of breast and other human cancers. A better understanding into the mechanism of how these adducts are formed would be useful to studies aimed at correlating adduct formation to DNA damage. Possible reaction intermediates, produced as a result of Michael addition of deoxyguanosine (dG) to catechol estrogen-3,4-quninones, have been modeled using density functional theory to determine likely intermediates on the potential energy surface (PES) of this reaction. Specifically, the sequence of elimination events, glycosidic bond cleavage and rearomatization of the estrogen A ring, was explored. Consistent with known experimental procedures, B3LYP calculations indicate that a proton source is needed to effect the Michael addition. Calculations also indicate that a catalytic mechanism, where one catechol estrogen quinone could adduct multiple purine bases, is unlikely. Experimental investigation toward an observed cationic reaction intermediate was also consistent with a stoichiometric reaction between estrone-3,4-quinone (E 1-3,4-Q) and dG. HPLC-MS analysis indicates that the cationic reaction intermediate contains the 2'-deoxyribose moiety. Assay of 4-OHE 1-1-N7Gua adduct formation and 2'-deoxyribose formation at different times during the reaction of E 1-3,4-Q with dG indicates that equimolar amounts of each are produced, further supporting a stoichiometric process with respect the catechol estrogen quinone. Differences in the UV spectroscopy of cationic reaction intermediate and the 4-OHE 1-1-N7Gua adduct allowed for kinetic analysis of the glycosidic bond cleavage process. Kinetic scanning analysis indicates that the decomposition of the cationic reaction intermediate is a first-order process with a t 1/2 of 40 min at 30 degrees C. Measurement of the unimolecular rate constant k at different temperatures afforded an Arrhenius plot, which provided values for Delta H, Delta S, and Delta G of 24.7 kcal/mol, 7.2 eu, and 26.8 kcal/mol, respectively. The computational data in conjunction with experimental results are consistent with a mechanism that involves a proton-assisted Michael addition to form an alpha-ketoenol ring system, followed by slow loss of the proton at C1 to restore the aromatic A ring, then fast cleavage of the glycosidic bond to form the 4-OHE 1-1-N7Gua adduct.