Measurement of relative amounts of phospho‐ and dephospho‐B‐50(GAP‐43) peptides by fast atom bombardment‐mass spectrometry

Abstract

The biological role of phosphoproteins depends upon their degree of phosphorylation in vivo. Methods currently available to measure the degree of phosphorylation of a protein involve indirect procedures to detect the ³²P-phosphate incorporation. We report here a direct method to measure relative amounts of phospho- and dephospho-forms of peptides based upon a mass spectrometric technique. The intensities of the molecular ions corresponding to the two forms of the peptides are proportional to their relative amounts. This is demonstrated for a peptide fragment of the protein B-5O(GAP-43) and for kemptide, respectively substrates for protein kinases C and A, and demonstrates the applicability of fast atom bombardment-mass spectrometry to quantitate peptides bearing post-translational modifications.