RESUMO
The effect of adduct stereochemistry on the susceptibility to hydrolysis by snake venom (VPD) and bovine spleen (SPD) phosphodiesterases was investigated with short deoxyoligonucleotides containing defined adducts derived from alkylation of the exocyclic 6-amino group of dA by polycyclic aromatic hydrocarbon diol epoxides (DEs). In accordance with several earlier reports, we have found that adducts with R configuration at the site of attachment of dA to the DE moiety derived from either benzo[a]pyrene (BaP) or benzo[c]phenanthrene (BcPh) are generally more resistant to hydrolysis by VPD than are their (S)-diastereomers. The reaction with VPD initially yields a fragment containing the adducted dA residue at its 3'-end, which slowly hydrolyzes to a dimer (pXpA*) with an intact 5'-phosphodiester bond to the adducted dA. With several of the adducts studied, this dimer underwent cleavage to release eventually the monomeric adduct p(dA*). Adducts derived from cis opening of the epoxide ring of both BaP and BcPh DEs were considerably more resistant to VPD than the corresponding trans-opened adducts. Although several previous investigations had suggested that oligonucleotides containing 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 oligonucleotides indicate that SPD, in contrast to VPD, exhibits little discrimination 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 phosphate for SPD) is between the modified base and the base immediately 5' to it, regardless of the configuration of the adduct.
