DNA adduct formation, removal and persistance in rat liver during one month of feeding 2-acetylaminofluorene

Male Wistar-Furth rats were fed 0.02% 2-acetylaminofluorene(AAF) for 3 days or 0.02% AAF for 25 days followed by 0.02%ring-³H]AAF for an additional 3 days. The concentration ofhepatic DNA adducts was then monitored by both radioimmunoassayand radiolabeling during 28 days of control diet. This approachallowed comparisons to be made of adduct accumulation, removaland persistence at both the beginning and end of a four weekcarcinogen feeding period. DNA adduct formation remained constantduring the month of AAF administration with an accumulationrate of 157 fmol adduct/µg DNA during days 1–3 anddays 25–28 of the experiment. Furthermore, the rate ofremoval of adducts formed during these three day periods wassimilar when both groups were fed control diets for 28 additionaldays. Continued AAF administration resulted in a slow accumulationof persistent adducts; thus, 91±6% of the adducts detectedafter 3 days of AAF feeding were removed during a subsequentmonth of control diet, while only 65±11% of the adductsdetected after 28 days of AAF diet were removed when rats werefed control diet for an additional 28 days. In a second experiment,the removal of adducts was compared in animals fed control orAAF diet after previously being fed 0.02% AAF for 17 days. Similarremoval curves were observed in both groups; therefore, continuedingestion of AAF did not affect the rate of adduct removal.In both experiments, biphasic repair curves were observed. Thesedata were used to develop a pharmacokinetic model. Two genomicregions were postulated, an area susceptible to fast repairand a region more resistant to the removal of AAF adducts. Atequilibrium, which was reached after 2–3 weeks of AAFfeeding, the concentration of adducts in each region was similarwith {small tilde}150 fmol adduct/µg DNA. Although thetotal number of adducts formed in the fast repair region duringone month of AAF administration was five times greater thanin the resistant region, the model predicted that the adductslocalized in regions resistant to repair were the persistentadducts detected after one month of control diet. Overall, theremoval of adducts formed during chronic AAF feeding was veryefficient since >93% of the adducts were removed by the endof a subsequent month of control diet.