Mapping sites of protein phosphorylation by mass spectrometry utilizing a chemical-enzymatic approach: Characterization of products from α-S1casein phosphopeptides

A novel chemical-enzymatic approach was developed to facilitate identification of phosphorylation sites in isolated phosphoproteins. ESI-TOF mass spectrometry was used to characterize products from the chemical-enzymatic cleavage of specific phosphorylation sites in bovine α-S1 casein and synthetic phosphopeptides containing substitutions at a single phosphorylation site. Further refinements to this approach for identification of protein phosphorylation sites and its utility for the quantification of phosphopeptides by isotope-dilution mass spectrometry are presented.