Simultaneous quantitation of 7-methyl- and O6-methylguanine adducts in DNA by liquid chromatography–positive electrospray tandem mass spectrometry

A methodology has been developed and validated for the simultaneous quantitation of O6-methyl- and 7-methylguanine in DNA isolated from in vitro exposure to the model alkylating agents: N-methyl-N-nitrosourea (MNU) and methyl methane sulfonate (MMS). After exposure, DNA was isolated and directly hydrolyzed under acid conditions to hydrolytes containing DNA bases (modified and unmodified). The hydrolytes were used for direct O6- and 7-methylguanine quantitation using a rapid and selective liquid chromatography–electrospray tandem mass spectrometry (LC/ESI-MS/MS). The lower limits of quantitation for O6-methyl- and 7-methylguanine were 75.8 and 151.5 fmol, respectively. Linearity of the calibration curve was greater than 0.999 from 75.8 to 151,600.0 fmol for O6-methylguanine and 0.999 from 151.5 to 303,200.0 fmol for 7-methylguanine. The intra-day assay precision relative standard deviation (R.S.D.) values for O6-methylguanine for quality control (QC) samples were ≤9.2% with accuracy values ranging from 90.8 to 118%, and for 7-methylguanine the R.S.D. values for QC samples were ≤11%, with accuracy values ranging from 92.9 to 119%. The inter-day assay precision (R.S.D.) values for O6-methylguanine QC samples were ≤7.9% with accuracy values ranging from 94.5 to 116%, and for 7-methylguanine QC samples were ≤7.1% with accuracy values ranging from 95.2 to 110.2%. This method was used for simultaneous determination of the levels of 7-methyl- and O6-methylguanine in DNA acidic hydrolytes present in a series of incubations from salmon testis DNA treated with either MNU or MMS.