Repair of O6-methyl-guanine residues in DNA takes place by a similar mechanism in extracts from HeLa cells, human liver, and rat liver.

Extracts from HeLa S3 cells, human liver, and rat liver were found to contain an activity that transfers the methyl group from O6-methyl-guanine residues in DNA to a cysteine residue of an acceptor protein. The molecular weights of the acceptor proteins in HeLA cells and human liver are 24,000 +/- 1,000 and 23,000 +/- 1,000, respectively. Assuming that each acceptor molecule is used only once, the average number of acceptor molecules in HeLa cells was calculated to be about 50,000. The extracts also contained 3-methyl-adenine-DNA glycosylase activity and 7-methyl-guanine-DNA glycosylase activity, although the latter activity was not detected in extracts from human liver in our assay system. Thus, the three major alkylation products resulting from the effect of methylating agents, such as N-methyl-N-nitroso urea, can all be repaired in animal cells. Pretreatment of HeLa cells with N-methyl-N'-nitro-N-nitrosoguanidine (0.1 micrograms/ml) strongly reduced the capacity of HeLa cell extracts to repair O6-methyl-guanine residues, while the activity of three DNA-N-glycosylases was essentially unaltered. This inactivation was not caused by a direct methylation of the enzyme by the carcinogen. The results demonstrate that the mechanism of repair of O6-methyl-guanine residues in DNA is strikingly similar in E coli and animal cells, including humans.