O6-methylguanine methyltransferase in rat liver.

The protein which catalyzes the repair of O6-methylguanine in DNA has been purified 3800-fold from rat liver. This protein acts as a methyltransferase, with the methyl group transferred to a protein-associated cysteine residue. From kinetic and physical studies, we conclude that the methyl group is transferred to the protein responsible for the activity, resulting in inactivation of the enzyme. The enzyme is asymmetric, with a molecular weight of approximately 18 500. Following methylation, there is an apparent aggregation of methylated proteins which is independent of the concentration of NaCl or nonionic detergent. Upon denaturation and analysis by gel electrophoresis, the aggregated methylated protein migrates as a single peak with a molecular weight of 18 900. The activity does not require any cofactors or divalent cations but is inhibited by NaCl. The activity also shows a preference for double-stranded DNA in terms of kinetics and efficiency of repair.