Mass Spectral Analysis of Unstable N7-Aralkyl DNA Adducts Resulting from Reaction of 7-Sulfooxymethyl-12-methylbenz[ a ]anthracene (SMBA) with DNA and Deoxynucleotides

The unified hypothesis for PAH activation predicts that SMBA plays a role in the metabolic activation and carcinogenicity of 7,12-dimethylbenz a ]anthracene (DMBA). SMBA and closely related aralkylating agents are derived from 7-hydroxymethyl-12-methylbenz a ]anthracene (HMBA), a direct metabolite of DMBA, and react with bases in nucleic acids. This occurs by generation of a benzylic carbonium ion owing to the fact that sulfate is a good leaving group. Previous characterization of reaction products with deoxynucleotide-3'-monophosphates detected stable adducts as primarily resulting from reaction with adenine at the N6- and guanine at the N2-amino groups, respectively. Pyrimidine adducts were also found; however, examination of SMBA-reacted DNA confirmed that the purine bases were the major targets for reaction. We now report evidence for the formation of unstable DNA adducts, most likely by reaction with purine N-7 positions. Treatment of SMBA-reacted DNA with formic acid at 70°C for 60 min and examination of acidified reaction products with electrospray-positive mode mass spectrometry (ESI + MS) disclosed m/z 390 and 406 products corresponding to 12-methylbenz a ]anthracene-7-methylene-N7-adenine or -guanine, respectively. Release of these adducts is expected to be accompanied by generation of apurinic sites in DNA structure. Examination of alternative leaving groups on the aralkylating agent suggest the following relative reactivities with deoxyguanosine or deoxyadenosine: chloro > sulfooxy > acetoxy > iodo. It is our expectation that development of these analytical methodologies will enable us to assign a role for the participation of aralkylating agents in PAH carcinogenicity.