Nonenzymatic methylation of DNA by the intracellular methyl group donor S-adenosyl-L-methionine is a potentially mutagenic reaction.
Incubation of DNA with S-adenosyl-L-methionine (SAM) in neutral aqueous solution leads to base modification, with formation of small amounts of 7-methylguanine and 3-methyladenine. The products have been identified by high performance liquid chromatography of DNA hydrolysates and by the selective release of free 3-methyladenine from SAM-treated DNA by a specific DNA glycosylase. We conclude that SAM acts as a weak DNA-alkylating agent. Several control experiments including extensive purification of [3H-methyl]SAM preparations and elimination of the alkylating activity by pretreatment of SAM with a phage T3-induced SAM cleaving enzyme, have been performed to determine that the activity observed was due to SAM itself and not to a contaminating substance. We estimate that SAM, at an intracellular concentration of 4 X 10(-5) M, causes DNA alkylation at a level similar to that expected from continuous exposure of cells to 2 X 10(-8) M methyl methane-sulphonate. This ability of SAM to act as a methyl donor in a nonenzymatic reaction could result in a background of mutagenesis and carcinogenesis. The data provide an explanation for the apparently universal occurrence of multiple DNA repair enzymes specific for methylation damage.
D002851:Chromatography, High Pressure Liquid; D045647:DNA Glycosylases; D004269:DNA, Bacterial / Q000378:metabolism*; D006147:Guanine / Q000031:analogs & derivatives / Q000378:metabolism; D008745:Methylation; D008837:Micrococcus; D009699:N-Glycosyl Hydrolases / Q000378:metabolism; D012436:S-Adenosylmethionine / Q000378:metabolism*
D004269:DNA, Bacterial; D006147:Guanine; C008450:7-methylguanine; D012436:S-Adenosylmethionine; C021565:3-methyladenine-DNA glycosylase; D045647:DNA Glycosylases; D009699:N-Glycosyl Hydrolases