Kinetics of DNA alkylation, depurination and hydrolysis of anti diol epoxide of benzo(a)pyrene and the effect of cadmium on DNA alkylation

Anti benzo[a]pyrene diol epoxide (BPDE) alkylates guanines of DNA at N7 in the major groove and at the exocyclic amino group in the minor groove. In t his report we investigated the rates of BPDE hydrolysis, DNA alkylation and subsequent depurination of BPDE-adducted pBR322 DNA fragment using polyacr ylamide gel electrophoresis. Preincubation studies showed that it hydrolyze d completely in triethanolamine buffer in <2 min. The depurination kinetics showed that a fraction of the N7 alkylated guanine depurinated rapidly; ho wever a significant amount of N7 guanine alkylation remained stable to spon taneous depurination over a 4-h period. Similar results were obtained for t he hydrolysis and alkylation rates of syn isomer but it required nearly 500 times more concentration to induce similar levels of N7 guanine alkylation . Cadmium ion strongly inhibited the N7 guanine alkylation of both isomers. But the minor groove alkylation was not affected as demonstrated by postla beling assay which confirmed the presence of heat-and cadmium-stable minor groove adducts in BPDE-treated calf thymus DNA. Based on these and our earl ier findings, we propose a mechanism for the synergistic effect of cadmium in chemically induced carcinogenesis. (C) 2000 Elsevier Science Ireland Ltd . All rights reserved.