Excision of N-acetoxy-2-acetylaminofluorene-induced DNA adducts from chromatin fractions of human fibroblasts.

The excision and persistence of covalent DNA adducts formed by N-acetoxy-2-acetylaminofluorene (AAAF) were studied in human skin fibroblasts. The changes in adduct concentration as a function of posttreatment incubation were measured in purified nucleosomal core DNA and in total nuclear DNA, and from these data the changes in adduct concentration in nucleosomal linker DNA were calculated. Immediately after N-acetoxy-2-acetylaminofluorene treatment which introduced 20 to 38 mumol adducts per mol DNA-P, the adduct concentration was 4 to 5 times higher in linker DNA than in core DNA. Adduct removal was rapid during the first 8 hr of incubation and occurred 3.5 to 4 times more efficiently from linker DNA than from core DNA. After 24 hr incubation, adduct removal continued only at a very low rate, leaving a substantial fraction of adducts unexcised. This fraction of persistent adducts had a value of 0.5 and was independent of the initial adduct concentration in the range of 12 to 115 mumol adducts per mol DNA-P. Approximately 65% of the persisting adducts were located in nucleosomal cores. The initial differences in adduct concentration between linker DNA and core DNA diminished during posttreatment incubation. This was entirely due to preferential early adduct excision from linker DNA. No evidence was obtained for repair-induced long-range nucleosomal movement in normal fibroblasts or constitutive movement in the absence of excision repair in xeroderma pigmentosum fibroblasts. Nucleosomal movement would tend to diminish the concentration differences between linker DNA and core DNA.