Biphasic removal of DNA adducts in a repetitive DNA sequence after dietary administration of 2-acetylaminofluorene.

Dietary administration of the hepatocarcinogen 2-acetylaminofluorene (2-AAF) to rats results in the formation of a major hepatic DNA adduct, N-(deoxyguanosin-8-yl)-2-aminofluorene (dG-C8-AF). In liver DNA, dG-C8-AF reaches steady-state conditions after approximately 2 weeks of feeding and is removed in a biphasic manner. In these experiments, we have quantified adduct concentrations in a 370 base-pair repetitive DNA fragment to determine if the adduct levels and kinetics of adduct removal were similar to those found in total genomic DNA. Male F344 rats were fed 0.02% 2-AAF for 28 days and were sacrificed at intermittent times up to 56 days after being returned to the control diet. Hepatic DNA adduct levels were measured by 32P-postlabeling or radioimmunoassay (RIA) in total genomic DNA and in a 370 base-pair fragment obtained by digesting genomic DNA with Hind III. Biphasic removal of dG-C8-AF, which composed about 90% of the total adducts measured, was observed in total genomic DNA, with comparable rate constants being detected by both 32P-postlabeling and RIA. 32P-Postlabeling also showed analogous biphasic removal of dG-C8-AF in the 370 base-pair fragment. A second adduct, 3-(deoxyguanosin-N2-yl)-2-AAF (dG-N2-AAF), which accounted for about 10% of the total adducts measured, showed similar biphasic removal kinetics in the total genomic DNA and the 370 base-pair fragment; however, as compared to dG-C8-AF, little removal of dG-N2-AAF was observed during the slow phase.