HPLC-MS/MS identification of positionally isomeric benzo[c]phenanthrene diol epoxide adducts in duplex DNA.

LC-MS and LC-MS/MS analyses were used to investigate the chemoselectivity of the carcinogenic diol epoxide metabolite, (-)-(1R,2S,3S,4R)-1,2-epoxy-3,4-dihydroxy-1,2,3, 4-tetrahydrobenzo[c]phenanthrene [(-)-(R,S,S,R)-BcPh DE-2], on reaction in vitro with an oligonucleotide dodecamer derived from the HPRT gene. The sequence of this dodecamer, 5'-T(1)A(2)G(3)T(4)C(5)A(6)A(7)G(8)G(9)G(10)C(11)A(12)-3', contains a base (corresponding to A(7)) which is a hot spot for mutagenesis in the hprt gene induced by the carcinogenic (R,S,S,R)-enantiomer of benzo[a]pyrene 7,8-diol 9,10-epoxide, and an adjacent base (corresponding to A(6)) which gave no mutations with this diol epoxide. Modified oligonucleotides were generated by reaction of (-)-BcPh DE-2 with both the single-stranded and duplex forms of the dodecamer. Multiple purine targets in both strands led to the formation of complex reaction mixtures of regioisomeric BcPh DE-modified oligonucleotides, which were partially separated by reverse phase HPLC on a polystyrene-divinylbenzene column. On-line LC-MS data allowed facile distinction between adducts on the two strands of the duplex, and MS/MS analysis permitted unambiguous assignment of the major sites of modification in the regioisomeric, adducted strands. In the duplex, these sites were at A(6), A(7), and G(8). Interestingly, the "hot spot" A(7)w as about 3 times more reactive with the BcPh DE than the "cold spot" A(6). Adduct formation from the single-stranded dodecamer was less selective, and resulted in more extensive alkylation of G residues.