Distribution and repair of O6-methylguanine in different fractions of rat liver DNA after dimethylnitrosamine administration.

The removal of the promutagenic DNA alkylation product O6-methylguanine from different fractions of rat liver DNA has been examined using the technique of DNA-DNA reassociation. Male Wistar rats were given a low non-toxic dose of N,N-dimethylnitrosamine (DMN) (2 mg/kg) and killed 3 or 18 h later (a period corresponding to the removal of 50% of the O6-methylguanine from 'total' liver (DNA). DNA was extracted from liver, denatured in alkali and incubated at 60 degrees C for periods corresponding to the reassociation of highly repetitive (polycopy), middle repetitive and 'unique' sequences i.e. different 'kinetic' classes of DNA. Reassociated and single-stranded DNA were separated by hydroxyapatite chromatography and analyse for O6-methylguanine content. Three hours after administration of DMN the levels of O6-methylguanine in the reassociated and single-stranded DNA were the same after each period of reassociation indicating that O6-methylguanine was randomly distributed among the DNA classes. At 18 h the levels of O6-methylguanine were again the same in the reassociated and single-stranded DNA but approx. 50% lower than in the 3 h DNA samples. The rate of loss of O6-methylguanine from the three DNA classes was thus the same and there was therefore no indication of preferential removal of this base from any one kinetic class of DNA under the conditions used.