Characterization of Protein Phosphorylation by Mass Spectrometry Using Immobilized Metal Ion Affinity Chromatography with On-Resin (beta)-Elimination and Michael Addition

A protocol combining immobilized metal ion affinity chromatography and (beta)-elimination with concurrent Michael addition has been developed for enhanced analysis of protein phosphorylation. Immobilized metal ion affinity chromatography was initially used to enrich for phosphorylated peptides. (beta)-Elimination, with or without concurrent Michael addition, was then subsequently used to simultaneously elute and derivatize phosphopeptides bound to the chromatography resin. Derivatization of the phosphate facilitated the precise determination of phosphorylation sites by MALDI-PSD/LIFT tandem mass spectrometry, avoiding complications due to ion suppression and phosphate lability in mass spectrometric analysis of phosphopeptides. Complementary use of immobilized metal ion affinity chromatography and (beta)-elimination with concurrent Michael addition in this manner circumvented several inherent disadvantages of the individual methods. In particular, (i) the protocol discriminated O-linked glycosylated peptides from phosphopeptides prior to (beta)-elimination/Michael addition and (ii) the elution of peptides from the chromatography resin as derivatized phosphopeptides distinguished them from unphosphorylated species that were also retained. The chemical derivatization of phosphopeptides greatly increased the information obtained during peptide sequencing by mass spectrometry. The combined protocol enabled the detection and sequencing of phosphopeptides from protein digests at low femtomole concentrations of initial sample and was employed to identify novel phosphorylation sites on the cell adhesion protein p120 catenin and the glycoprotein fetuin.