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RESEARCH PRODUCT

Identification of Casein Phosphopeptides in β-casein and Commercial Hydrolysed Casein by Mass Spectrometry

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Casein phosphopeptides (CPPs) in commercial hydrolysed casein (CE90CPP) and in β-CN (β-CN) after simulated gastrointestinal digestion (gastric stage pepsin, pH =2, 37°C 2h) and intestinal stage (pancreatic-bile extract, pH =5.2, 37°C 2h) were sequenced by on-line reversed-phase high performance liquid chromatography coupled to electrospray ionisation tandem mass spectrometry (RP-HPLC-ESIMS/MS). In β-CN digest five peptides that contained four to five phosphate groups and the cluster sequence SpSpSpEE (residues 17-21) were identified. All CPPs with one exception β-CN(1-24)4P, had the protein fragment β-CN(1-25)4P, which is one of the main CPPs produced in vivo digestion of casein and the results of in vitro studies showed that this fragment enhanced calcium, iron and zinc absorption. In commercial hydrolysed casein CE90CPP 13 peptides were identified, only one of them, αs2-CN (1-13)3P, contained the cluster sequence SpSpSpEE but all the peptides have one or two phosphoserine residues with mineral binding capacity. These CPPs were shorter (527-2061 Da vs 2966-6512 Da) and less phosphorylated (1-3 P vs 4-5 P) than those released after simulated gastrointestinal digestion of β-CN. In both samples, the potential mineral chelating properties of these peptides in relation to their amino acid sequences and the presence of the phosphorylated cluster are discussed.