Detection of different types of damage in alkylated DNA by means of human corrective endonuclease (correndonuclease).

Corrective endonuclease (correndunclease) activity of HeLa cells was assayed with alkylated DNA. Double-stranded, covalently closed DNA from phage PM II was treated with methyl methanesulfonate, N-methyl-N-nitrosourea, beta-propiolactone, or diepoxybutane to introduce alkylated bases and alkali-labile sites into the DNA. The damaged DNA was incubated with an extract of HeLa cells that catalyzes the formation of breaks at apurinic sites in double-stranded DNA. Methylated DNA was broken at every alkali-labile site by the HeLa correndonuclease, which indicated that these sites are similar to the apurinic sites produced by heating at acid pH. DNA alkylated with beta-propiolactone or diepoxybutane containing the same number of alkali-labile sites was broken to a far lesser extent. This indicates the presence of a second type of alkali-labile damage that is correndonuclease-insensitive.