Detection of two minor phosphorylation sites for bovine κ-casein macropeptide by reversed-phase liquid chromatography–tandem mass spectrometry

Abstract

This work addresses the characterization of phosphopeptides in bovine κ-casein macropeptide by reversed-phase liquid chromatography-electrospray ionization-tandem mass spectrometry (RPLC-ESI-MS2). Two different mass spectrometers, equipped with an ion trap (IT) or a quadrupole time-of-flight (Q-TOF) analyzer, were used to perform an accurate phosphorylation site assignment. A total of 8 phosphopeptides from 26 identified peptides were characterized. MS2 spectra of phosphopeptides were dominated by the neutral loss of a phosphoric acid molecule (H 3PO4) and sufficient informative fragment ions resulting from peptide backbone cleavages enabling the elucidation of the phosphopeptide sequence. A higher number of sequence informative b and y ions were detected using a Q-TOF instead of an IT analyzer. In addition to the well-established phosphorylation sites at Ser149 and Ser127, this study also revealed the presence of two minor phosphorylation sites at Thr 145 and Ser166. These findings indicate that RPLC-ESI-MS2 on a Q-TOF analyzer is a useful technique for identifying low-abundance phosphorylation sites in caseins. © 2011 American Chemical Society.