Base Excision Repair of <i>N</i>⁶<i>-</i>Deoxyadenosine Adducts of 1,3-Butadiene

The important industrial and environmental carcinogen 1,3-butadiene (BD) forms a range of adenine adducts in DNA, including <i>N</i>⁶-(2-hydroxy-3-buten-1-yl)-2′-deoxyadenosine (<i>N</i>⁶-HB-dA), 1,<i>N</i>⁶-(2-hydroxy-3-hydroxymethylpropan-1,3-diyl)-2′-deoxyadenosine (1,<i>N</i>⁶-HMHP-dA), and <i>N</i>⁶,<i>N</i>⁶-(2,3-dihydroxybutan-1,4-diyl)-2′-deoxyadenosine (<i>N</i>⁶,<i>N</i>⁶-DHB-dA). If not removed prior to DNA replication, these lesions can contribute to A → T and A → G mutations commonly observed following exposure to BD and its metabolites. In this study, base excision repair of BD-induced 2′-deoxyadenosine (BD-dA) lesions was investigated. Synthetic DNA duplexes containing site-specific and stereospecific (<i>S</i>)-<i>N</i>⁶-HB-dA, (<i>R</i>,<i>S</i>)-1,<i>N</i>⁶-HMHP-dA, and (<i>R</i>,<i>R</i>)-<i>N</i>⁶,<i>N</i>⁶-DHB-dA adducts were prepared by a postoligomerization strategy. Incision assays with nuclear extracts from human fibrosarcoma (HT1080) cells have revealed that BD-dA adducts were recognized and cleaved by a BER mechanism, with the relative excision efficiency decreasing in the following order: (<i>S</i>)-<i>N</i>⁶-HB-dA > (<i>R</i>,<i>R</i>)-<i>N</i>⁶,<i>N</i>⁶-DHB-dA > (<i>R</i>,<i>S</i>)-1,<i>N</i>⁶-HMHP-dA. The extent of strand cleavage at the adduct site was decreased in the presence of BER inhibitor methoxyamine and by competitor duplexes containing known BER substrates. Similar strand cleavage assays conducted using several eukaryotic DNA glycosylases/lyases (AAG, Mutyh, hNEIL1, and hOGG1) have failed to observe correct incision products at the BD-dA lesion sites, suggesting that a different BER enzyme may be involved in the removal of BD-dA adducts in human cells

Gas-Phase Studies of Substrates for the DNA Mismatch Repair Enzyme MutY

The gas-phase thermochemical properties (tautomeric energies, acidity, and proton affinity) have been measured and calculated for adenine and six adenine analogues that were designed to test features of the catalytic mechanism used by the adenine glycosylase MutY. The gas-phase intrinsic properties are correlated to possible excision mechanisms and MutY excision rates to gain insight into the MutY mechanism. The data support a mechanism involving protonation at N7 and hydrogen bonding to N3 of adenine. We also explored the acid-catalyzed (non-enzymatic) depurination of these substrates, which appears to follow a different mechanism than that employed by MutY, which we elucidate using calculations