Specific enrichment and direct detection of phosphopeptides on insoluble transition metal oxide particles in matrix-assisted laser desorption/ionization mass spectrometry applications.

Several transition metal oxides, such as iron (III), nickel (II) and zirconium (IV) oxides, were examined in detail for the specific enrichment and the purification of phosphopeptides from a digested peptide mixture solution. Phosphopeptide enrichment was performed on the metal oxide particles using a peptide mixture obtained by tryptic digestion of β-casein. The mixture of protein digests containing bovine serum albumin (BSA):β-casein digests (100: 1 mole ratio) was also used for the phosphopeptide enrichment. Furthermore, non-fat milk digest was examined as a complex biological sample. In each phosphopeptide enrichment process, phosphopeptides were specifically enriched and separated from the non-phosphopeptides. The phosphopeptides were adsorbed onto the metal oxide surface at acidic pH values between 1.0 and 2.0 and, for desorption of phosphopeptides from metal oxide particles, pH values were examined and optimized in the enrichment studies. The analysis of phosphopeptides were carried out by matrix-a...