Formation and removal of B[a]P diol epoxide--DNA adducts in human fibroblasts

Using xeroderma pigmentosum fibroblasts, deficient in excision repair, as controls to measure the initial rate of (+/-)7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (B[a]PDE)--DNA adducts removal in normal human fibroblasts, it was found that the maximum amount of carcinogen DNA adducts occurred 1 h after the addition of B[a]PDE, and that during the first hour approximately 12% of the DNA--carcinogen adducts had already been removed. Thus the formation and removal of DNA--carcinogen adducts occurred simultaneously within the first hour after B[a]PDE addition to confluent fibroblasts. Examination of excision repair over an extended period showed that during a further 6 h, DNA adducts were removed at a rate four times slower than that observed during the first hour. Since the maximum level of B[a]PDE--DNA adducts was observed 1 h after the addition of B[a]PDE to the cells in culture, this suggested that the rate of breakdown of B[a]PDE was much slower than that observed in vitro. Further experiments indeed indicated that the rate of hydrolysis of B[a]PDE within the cell was significantly decreased. Thus, the stability of B[a]PDE inside the cell is governed by very different parameters than those observed in vitro.