Identification of phosphorylated peptides from complex mixtures using negative-ion orifice-potential stepping and capillary liquid chromatography/electrospray ionization mass spectrometry.

A rapid method for identifying and characterizing sites of phosphorylation of peptides and proteins is described. High-performance capillary liquid chromatography (HPLC) coupled with electrospray ionization mass spectrometry (ESI-MS) is used to distinguish non-phosphorylated and phosphorylated peptides originating from mixtures as complex as enzyme digests. The method relies on the ability to produce a fragment ion characteristic and unique to phosphopeptides (m/z 79, PO3) by stepping the orifice potential of the mass spectrometer as a function of mass. At low m/z values, a high orifice potential is applied to induce extensive fragmentation of the peptide, leading to the formation of the m/z 79 phosphate-derived ion. This method is analogous to that described by Carr et al. for the identification of glycopeptides from enzymatic digestion of glycoproteins (S.A. Carr, M.J. Huddleston, M.F. Bean, Protein Science 2, 183 (1993)). The method was first evaluated and validated for a mixture of non-, mono- and di-phosphorylated synthetic peptides. Both mono- and di-phosphorylated peptides were found to generate fragment ions characteristic of PO3 whereas the non-phosphorylated peptide did not. Application of the method was extended to identifying phosphopeptides generated from an endoprotease Lys-C digestion of beta-casein. Both the expected mono- and tetra-phosphorylated Lys-C peptides were observed and identified rapidly in the LC/SEI-MS analysis. The procedure was used additionally to identify the site(s) of phosphorylation of the cytosolic non-receptor tyrosine kinase, pp60(c-src).