Quantitative in vitro kinase reaction as a guide for phosphoprotein analysis by mass spectrometry

A simple calculation using the radioactive decay of (32)P incorporated into a protein during in vitro kinase reactions is described that allows the overall stoichiometry of phosphorylation for the substrate protein or peptide to be calculated. Prior to using techniques such as diagnostic ion scanning to identify the molecular weight of an unknown phosphopeptide in a complex mixture followed by tandem mass spectrometry (MS/MS) to locate the phosphorylated residue within the phosphopeptide, such calculations are predictive of the chances for successful characterization by these methods. An example of estimating the stoichiometry of peptide phosphorylation will be presented along with calculations that predict when adequate phosphopeptide is present in any given spot on the thin-layer chromatography (TLC) plates used for two-dimensional phosphopeptide (2DPP) mapping to allow extraction and complete characterization by MS/MS.