Phosphopeptides interacting with colloidal calcium phosphate isolated by tryptic hydrolysis of bovine casein micelles

After extended tryptic hydrolysis of large bovine casein micelles, a mineral-rich peptide fraction was recovered by ultracentrifugation. Its mineral part contained 72% of the colloidal Ca and 49% of the colloidal P-i originally present in the native micelle. Colloidal nitrogenous components were also present, amounting to 27% of the original N content. They contained most of the phosphopeptides and 82% of the micellar phosphoseryl residues. These tryptic peptides were characterized by reversed-phase HPLC on-line electrospray ion source-mass spectrometry analysis. Among the peptides produced 14 phosphopeptides were identified: alpha(s2)-CN(1-24), alpha(s2)-CN(1-21), alpha(s1)-CN(43-79), alpha(s1)-CN(35-79)7P, alpha(s1)-CN(35-79)8P, alpha(s1)-CN(37-79), alpha(s1)-CN(104-119), alpha(s1)-CN(104-124), beta-CN(1-25), beta-CN(1-28), beta-CN(1-29), beta-CN(30-97), beta-CN(33-97) and beta-CN(29-97). The proportion of the phosphopeptides interacting with colloidal calcium phosphate was correlated with their relative content of phosphoserine residues, since phosphopeptides containing more than four phosphoserine residues were consistently present within this fraction. It also appeared that other types of peptides, some of them hydrophobic in nature, were also partly or completely present within the colloidal fraction, including alpha(s1)-CN(91-100), alpha(s1)-CN(152-193), alpha(s1)-CN(23-34), alpha(s1)-CN(125-193), alpha(s1)-CN(125-199), beta-CN(177-209), beta-CN(184-209), beta-CN(114-169) and beta-CN(108-169). Their possible involvement in the micellar backbone is discussed.