Design and Improvement of Food Proteins by the Introduction of New Function

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Biologically active peptides are screened for in enzymatic digests of food proteins. Derivatives of theses peptides were synthesized and their structure-activity relationships were studied to design orally effective biologically active peptides, which could be introduced into food proteins by protein engineering. Casoxin C, Tyr- Ile-Pro-Ile-Gln-Tyr-Val-Leu-Ser-Arg, which is released from bovine kappa-casein by the action of trypsin is a multifunctional peptide. Homology is found between the penta- peptide sequence at the carboxyl terminus of casoxin C and the laminin penta-peptide, Tyr-Ile-Gly-Ser-Arg. Unlike laminin penta-peptide, casoxin C or pepta-peptide at its carboxyl terminus did not inhibit tumor metastasis. The [Ile^7,Gly^8]-casoxin C, which has laminin penta-peptide sequence at the carboxyl terminus inhibited tumor metastasis in mice.Casoxin D, Tyr-Val-Pro-Phe-Pro-Pro-Phe, is a vaso-relaxing peptide derived from human alpha_<51>-casein. Casoxin D binds to bradykinin BK1-receptors of endothelium. The endothelium-derived relaxing factor released by casoxin D stimulusis is prostacyclin. Among casoxin D derivatives synthesized, Tyr-Pro-Phe-Pro-Pro-Phe, Val-Val-Phe-Pro- Pro-Phe and Tyr-Val-Phe-Pro-Pro-Phe showed vaso-relaxing activity at the concentrations 1/4, 1/7 and 1/10 that of casoxin D. Tyr-Pro-Phe-Pro-Pro-Phe lowered blood pressure of spontaneously hypertensive rats after oral administration while other peptides were active only after intravenous administration. Another derivative Tyr- Pro-Phe-Pro-Pro-Leu showed vaso-relaxing activity at the concentration 1/10,000 that of casoxin D. Mode of action of this peptide was different from that of casoxin D.Opioid peptides were isolated from enzymatic digests of wheat gluten and potent derivatives were obtained by substitution of single amino acid residues.