Selective inhibition of the BPDE-I-induced modification of the replicating DNA of S-phase cells, by benzamide and 3-aminobenzamide.

Treatment of human skin fibroblasts in early S-phase with (+-)7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10- tetrahydrobenzo[a]pyrene (BPDE-I) results in more extensive modification of early replicating DNA than parental DNA. We have investigated the effects of benzamide (BZ) and 3-aminobenzamide (3-ABZ), inhibitors of transformation, on the modification of parental and replicating DNA of cells in early S-phase by BPDE-I. Synchronized cells were exposed to 5-bromodeoxyuridine at S-phase entry and treated 3 h later with 0.114 microM BPDE-I for 30 min. The cells at the time of treatment represent a radiolabeling index of 40 +/- 5% of the total number of cells. The replicated DNA was isolated from the non-replicated parental DNA on a CsCl gradient. A 32P-postlabeling procedure was used to quantitate the carcinogen-DNA adducts. The level of modification per nucleotide residue of the early replicated DNA was 1.6-2.2 times higher compared to the level of modification of the parental DNA. Addition of BZ inhibited the BPDE-I modification of the replicated DNA by 27-53%. There was no significant effect on the parental DNA modification. The major adduct that was quantitatively suppressed in the early replicated DNA was BPDE-I-trans-N2-dG. The addition of 3-ABZ also inhibited the modification of the dG by approximately 50% without significantly inhibiting the BPDE-I-dG adducts in the parental DNA. The data suggest that BZ and 3-ABZ inhibit the modification of specific sites in the replicating DNA leading to inhibition of transformation.