Specific BPDE I modification of replicating and parental DNA from early S phase human foreskin fibroblasts

Replicating DNA was modified by BPDE I to a greater extent than parental DNA when human fibroblast cells were treated with the carcinogen for 30 min in early S phase. Synchronized cells were exposed to 5-bromodeoxyuridine and treated with non-radioactive BPDE I and [methyl-3H]thymidine in early S phase. The density- and tritium-labeled, replicated DNA was separated from parental DNA in a CsCl gradient. The individual carcinogen-DNA adduct levels in both samples were quantitated by using the 32P-postlabeling method. The total modification of replicated DNA was 1.4-2.4 times greater than parental DNA. This difference was mainly reflected by differences in the main adducts, identified as the 3', [5'-32P]bisphosphates of 7R and 7S-BPDE I-dG. Confirmation of the identity of these two specific carcinogen-DNA adducts was accomplished by co-chromatography on t.l.c. with 3H-labeled 3',5'-bisphosphate adducts. The two 3H- and 32P-labeled adducts were isolated and dephosphorylated. The resultant 3H-labeled deoxyribonucleoside adducts were analyzed on h.p.l.c. and identified by co-chromatography with authentic standards. These results suggest that preferential modification of replicating DNA occurs when human cells are treated with BPDE I in early S phase. The ultimate result of this specific modification is the expression of a transformed phenotype.