Chemical modification of nucleic acids. Methylation of calf thymus DNA investigated by mass spectrometry and liquid chromatography

Mass spectrometry provides an extremely sensitive method for the identification and quantification of modified nucleosides and hence for determining chemical modifications of nucleic acids. When mass spectrometry is used in conjunction with a new high‐performance liquid chromatographic system capable of separating 15 methylated and naturally occurring nucleosides, this allows the quantification of products of in vitro DNA methylation. With synthetic (2H3)methyl‐labeled methylnucleosides as internal references, the distribution of methylated products formed when calf thymus DNA was reacted with N‐methyl‐N‐nitrosourea(MeNU) was determined. Five modified products, 1‐methyldeoxyadenosine(m1dA), 3‐methyldeoxycytidine(m3dC), 7‐methyldeoxyguanosine(m7dG), 3‐methylthymidine(m3T) and O4‐methylthymidine(m4T) were detected and the relative distributions were measured. The ability of mass spectrometry/mass spectrometry (tandem mass spectrometry) to increase specificity and sensitivity in this determination is demonstrated and its application to in vivo studies is suggested.