title: Repair of O6-methylguanine in DNA by demethylation is lacking in Mer- human tumor cell strains.
abstract: The ability of extracts of human tumor cells to demethylate O6-methylguanine (O6-MeG) in DNA was assayed using the synthetic DNA polymer poly(dC,dG,m6dG). Cell strains proficient in repair of O6-MeG in vivo (Mer+ phenotype) contained a methyltransferase activity while repair deficient cells (Mer- phenotype) had little or no activity. Mixing extracts of different Mer- strains did not result in the appearance of the activity. Extracts of Mer- cells did not inhibit the activity in extracts of Mer+ cells. Both Mer+ and Mer- strains contained methylnitrosourea-damage-specific endonuclease activity. The data suggest that the Mer- strains are deficient in methyltransferase and that this is the fundamental reason for their hypersensitivity to the cytotoxic effects of DNA alkylation. The activity was partially purified from a Mer+ colon carcinoma cell strain. Its kinetics parallel the repair of O6-MeG in DNA in vivo and suggest that the activity is inactivated during repair of DNA.
mesh_terms: D004260:DNA Repair; D004273:DNA, Neoplasm / Q000378:metabolism*; D005260:Female; D006147:Guanine / Q000031:analogs & derivatives* / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D008769:Methylnitronitrosoguanidine / Q000633:toxicity*; D008780:Methyltransferases / Q000378:metabolism*; D009369:Neoplasms / Q000378:metabolism*; D019853:O(6)-Methylguanine-DNA Methyltransferase; D010641:Phenotype
chemical_list: D004273:DNA, Neoplasm; D008769:Methylnitronitrosoguanidine; D006147:Guanine; C008449:O-(6)-methylguanine; D008780:Methyltransferases; D019853:O(6)-Methylguanine-DNA Methyltransferase