Patterns of resistance to exonuclease digestion of oligonucleotides containing polycyclic aromatic hydrocarbon diol epoxide adducts at N-6 of deoxyadenosine

The effect of adduct stereochemistry on the susceptibility to hydrolysis by snake venom (VPD) and bovine spleen (SPD) phosphodiesterases was investiga ted with short deoxyoligonucleotides containing defined adducts derived fro m alkylation of the exocyclic 6-amino group of dA by polycyclic aromatic hy drocarbon diol epoxides (DEs). In accordance with several earlier reports, we have found that adducts with R configuration at the site of attachment o f dA to the DE moiety derived from either benzo[alpha ]pyrene (BaP) or benz o[c]phenanthrene (BcPh) are generally more resistant to hydrolysis by VPD t han are their (S)-diastereomers. The reaction with VPD initially yields a f ragment containing the adducted dA residue at its 3 ' -end, which slowly hy drolyzes to a dimer (pXpA*) with an intact 5 ' -phosphodiester bond to the adducted dA. With several of the adducts studied, this dimer underwent clea vage to release eventually the monomeric adduct p(dA*). Adducts derived fro m cis opening of the epoxide ring of both BaP and BcPh DEs were considerabl y more resistant to VPD than the corresponding trans-opened adducts. Althou gh several previous investigations had suggested that oligonucleotides cont aining adducts which have S configuration at the site of attachment of the hydrocarbon to adenine are more resistant to cleavage by SPD than are their (R)-diastereomers, the present results with a more extensive set of oligon ucleotides indicate that SPD, in contrast to VPD, exhibits little discrimin ation between adducts with R and S configuration at the site of attachment to the base. Notably, for both enzymes, the most resistant internucleotide linkage (the bond 3 ' -sugar to phosphate for VPD and 5 ' -sugar to phospha te for SPD) is between the modified base and the base immediately 5 ' to it , regardless of the configuration of the adduct.