Quantification of acetylation at proximal lysine residues using isotopic labeling and tandem mass spectrometry.

With the emergence of the histone code as a key determinant in the regulation of gene expression, it has been important to develop tools that can not only identify the types and locations of myriad modifications, but also determine how the levels of these modifications change as a result of various processes in a cell. Mass spectrometry has become a method of choice for the investigation of post-translational modifications in histone proteins. Described in this article is a mass spectrometric method that is useful for direct quantification of levels of acetylation at lysines residues in close proximity to one another, as is the case for the amino terminal tail of histone H4. This method involves fragmentation of peptides into b and y ions that contain one or more sites of modification and isotopic labeling which ensures equivalent ionization and fragmentation.