Large-scale phosphoproteome of human whole saliva using disulfide-thiol interchange covalent chromatography and mass spectrometry

To date, only a handful of phosphoproteins with important biological functions have been identified and characterized in oral fluids, and these include some of the abundant protein constituents of saliva. Whole saliva (WS) samples were trypsin digested, followed by chemical derivatization using dithiothreitol (DTT) of the phospho-serine/threonine-containing peptides. The DTT-phosphopeptides were enriched by covalent disulfide-thiol interchange chromatography and analysis by nanoflow liquid chromatography and electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS). The specificity of DTT chemical derivatization was evaluated separately under different base-catalyzed conditions with NaOH and Ba(OH)₂, blocking cysteine residues by iodoacetamide and enzymatic O-deglycosylation prior to DTT reaction. Further analysis of WS samples that were subjected to either of these conditions provided supporting evidence for phosphoprotein identifications. The combined chemical strategies and mass spectrometric analyses identified 65 phosphoproteins in WS; of these, 28 were based on two or more peptide identification criteria with high confidence and 37 were based on a single phosphopeptide identification. Most of the identified proteins (∼80%) were previously unknown phosphoprotein components. This study represents the first large-scale documentation of phosphoproteins of WS. The origins and identity of WS phosphoproteome suggest significant implications for both basic science and the development of novel biomarkers/diagnostic tools for systemic and oral disease states.