Enzymatic removal of O6-methylguanine from DNA by mammalian cell extracts

Extracts from various rat tissues were incubated with ³H]methylated DNA or chromatin in order to compare their abilities to catalyze the removal of labeled $\text{O}{}^6$-methylguanine from acid precipitable DNA. Liver extracts had the greatest activity. Kidney extracts had about 35% of the activity in liver and extracts from lung, colon, small intestine and brain were much less active. The enzyme responsible for this reaction does not appear to be an $\text{N}$-glycosidase because no labeled $\text{O}{}^6$-methylguanine could be detected in the supernatant fraction even though more than 50% of this base was lost from the DNA. The released radioactivity was present as methanol which is consistent with the possibility that the reaction may involve a demethylase action on either the DNA substrate or an oligonucleotide derived from it.