The formation and removal of covalent aflatoxin B1 (AFB1): DNA adducts were studied under nontoxic conditions in metabolically active epithelioid human lung cells (A549). Following 24 hr exposure to 0.82 µm AFB1, the adduct level was in the range of 0.16 to 1.34 µmol adduct per mol DNA-phosphate. Approximately 60% of the total initial AFB1 adducts were removed rapidly during the first 24 hr of posttreatment incubation of A549 cells, while only 15% of the total adducts were released from free AFB1:DNA isolated from A549 cells within the same time period under physiological conditions in vitro. The remaining 40% of the adducts were removed only very slowly in A549 cells, leaving a sizable fraction of residual lesions in the DNA over several generation times.
Of three products which can be distinguished by Sephadex LH 20 and high-pressure liquid chromatography in formic acid hydrolysates of DNA from A549 cells immediately following AFB1 treatment, 42 to 64% was 2,3-dihydro-2-(N7-guanyl)-3-hydroxyaflatoxin B1. The disappearance of lesions assayed as 2,3-dihydro-2-(N7-guanyl)-3-hydroxyaflatoxin B1 in acid hydrolysates of DNA was rapid and occurred with similar kinetics upon incubation of intact cells in culture or of free DNA in vitro under physiological conditions. For free AFB1:DNA, the disappearance of these lesions is in part due to their transformation to more stable secondary derivatives which remain attached to the DNA backbone. In analogy to the in vitro situation, it is likely that a portion of the primary lesions in A549 cells are also first modified either spontaneously or enzymatically to secondary lesions which are then processed by the cellular repair pathways.
This work was supported by United States Public Health Service Grant GM 18617 and by Grant 3′305.78 of the Swiss National Science Foundation.