Protein Phosphorylation Degree: Determination by Capillary Liquid Chromatography and Inductively Coupled Plasma Mass Spectrometry

Abstract
Capillary liquid chromatography (μLC) interfaced to inductively coupled plasma mass spectrometry (ICPMS) is introduced as a new micromethod to determine the phosphorylation degree in phosphoproteins and phosphopeptides containing cysteine and/or methionine residues. The stoichiometric phosphorus to sulfur (31P to 32S) ratio is experimentally determined by μLC−ICPMS and converted into the degree of phosphorylation using protein/peptide sequence information. The method is applied to the phosphoproteins α-casein, β-casein, and recombinant protein kinase A catalytic subunit and to synthetic phosphopeptides. The accurate data obtained by μLC−ICPMS allow quantitative assessment of the compound-specific discrimination of the electrospray ionization process between nonphosphorylated and phosphorylated proteins and peptides.

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