Properties of the O6-alkylguanine-DNA repair system of mammalian cells.

O6-Alkylguanine is an important lesion produced in DNA after exposure to N-nitrosodimethylamine or N-nitrosodiethylamine and may lead to mutagenesis or carcinogenesis if unrepaired. Repair of this product is accomplished by a unique DNA-repair activity which resides in a single protein. This protein catalyses the transfer of the alkyl group from the guanine O6-position to a cysteine residue. The cysteine acceptor site appears to be present on the same protein and is not regenerated. There is, therefore, a stoichiometric relationship between the amount of this protein and the number of O6-alkylguanine residues that can be repaired. The protein has been partially purified from rat liver and from a number of human tissues. The rodent and human O6-alkylguanine-DNA alkyltransferases have similar properties and a molecular weight of about 23 000. The protein is specific for O6-alkylguanine in DNA, prefers double-stranded DNA as substrate and binds tightly to double-stranded DNA, whether alkylated or not. Both methyl and ethyl groups are removed, although the rate of removal of the ethyl groups is four times slower. Liver has the highest content of this protein among all tissues tested from both rats and humans, but all human tissues have substantially higher levels than the equivalent rodent tissue. O6-Alkylguanine-DNA alkyltransferase was found in all primary human tumours and tissues tested (including brain, which in rodents has very little activity). It is concluded that human tissues are likely to be able to repair O6-alkylguanine but that the capacity of repair is tissue-specific and linked to the level of this protein.