Caseins of various origins and biologically active casein peptides and oligosaccharides: structural and physiological aspects

Mapping and sequence-specific identification of phosphopeptides in unfractionated protein digest mixtures by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

We have demonstrated a procedure for the rapid (minutes), sensitive (less than pmol), and sequence-specific identification of phosphopeptides in unfractionated digests of phosphoproteins using matrix-assisted UV laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometry. The mass-dependent identification of one specific 13-residue phosphopeptide (S105-K117), observed among the 153 possible trypsin digest fragments of human beta-casein (211 residues), was confirmed by amino acid sequence analysis of the 33P-labeled peptide after isolation by reverse-phase HPLC. MALDI-TOF was also used to monitor the rate and extent to which an 18-residue N-terminal beta-casein peptide (R1-K18) was phosphorylated in vitro. These results demonstrate that MALDI-TOF may be used (i) to facilitate the identification of sequence-specific sites of protein phosphorylation and dephosphorylation, (ii) to monitor protein and peptide phosphorylation and dephosphorylation reaction rates, even in complex unfractionated mixtures, (iii) to determine the minimum primary structure necessary for the phosphorylation of specific protein surface domains, and (iv) to evaluate the effects of intact protein phosphorylation and dephosphorylation on susceptibility to subsequent proteolytic events.

Identification of C-terminal peptides of bovine beta-casein that enhance proliferation of rat lymphocytes

A casein polypeptidic fraction, obtained from a pepsin-chymosin digestion of caseins, showed a mitogenic effect on primed lymph node (LN) cells and unprimed spleen cells of rats. A biologically active C-terminal sequence of bovine beta-casein (residues 192-209) was characterized. The corresponding synthetic peptide had a stimulatory effect on primed LN cells but failed to enhance proliferation of spleen cells. We prepared two chymosin digests (PA and PB) of bovine beta-casein which contained, respectively, 80% and 95% of the sequence including residues 193-209. They induced a significant proliferative response in both LN and spleen cells. It is therefore possible that other active peptides in the PA preparation may be involved in mitogenic activity.

Isolation and characterization of monoclonal antibody directed against bovine alpha s2-casein

A monoclonal antibody 62-1A of isotype IgM, directed against bovine alpha s2-casein, was isolated and characterized. Monoclonal antibody 62-1A recognized bovine alpha s2-11P- and alpha s2-9P-caseins by indirect solid phase radioimmunoassay and Western blot analysis. Crossreactivity toward native genetic variants (A, B, and C) of alpha s1-casein was similar but lower (approximately 60 to 70%) than that for alpha s2-11P-casein). Little or no crossreactivity was observed for other bovine milk or serum proteins. Antibody affinity for alpha s2-11P-casein was 1.3 x 10(9)/M. As little as 25 ng/ml (.5 ng/well) of alpha s2-11P-casein was detected by solid phase radioimmunoassay.

Biologically active polypeptides in milk

Many biologically active polypeptides have been described in the milk of several species. Various functions for these polypeptides in addition to nutrition have been proposed in the maternal body and in the breast-fed infant. These polypeptides are derived from several sources and multiple factors control their secretion into milk as well as their fate in the mother and infant. An increasing body of evidence supports the concept that they may function physiologically.