Structures and Functions of Opioid Peptides Derived from Casein.

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Studies on the isolation, chemical syntheses and biological activities of opioid peptides derived from casein and whey proteins were performed to clarify possible physiological functions of milk proteins.Peptide sequences, Tyr-X-Phe and Tyr-X-Y-Phe, which are common to known opioid peptides, were found in <alpha> -lactalbumin and <beta> -lactoglobulin, respectively. The corresponding peptide amides were synthesized. Both peptides showed rather weak opioid agonist activity in the guinea pig ileum assay system.A peptide having affinity for rat brain opioid receptors was isolated from a peptic digest of bovine <kappa> -casein. The structure of the peptide was Ser-Arg-Tyr-Pro-Ser-Tyr- <OCH_3> , which corresponded to the 33-38th residues of <kappa> -casein. The terminal carboxyl group has been esterified during the extraction process. The peptide showed an opioid antagonist activity in the guinea pig ileum assay system. Peptides, Tyr-Pro-Ser-Tyr- <OCH_3> and Arg-Tyr-Pro-Ser-Tyr- <OCH_3> , were as active as Ser-Arg-Tyr-Pro-Ser-Tyr- <OCH_3> . These peptides were named casoxin 4, 5 and 6, respectively. The antagonist activity was retained after the reduction of the terminal Tyr- <OCH_3> to tyrosinol while the peptide with free carboxyl terminal tyrosine had only weak activity.A peptide Tyr-Pro-Tyr-Tyr, the homologous sequence of bovine casoxin 4 found in human <kappa> -casein was synthesized and the antagonist activity was evaluated. The peptide with a free terminal carboxyl group was active at 200 <micro> M. The methyl ester of the peptide, Tyr-Pro-Tyr-Tyr- <OCH_3> , was active at 2 <micro> M and is about 10 times more active than the bovine casoxin 4. The peptide antagonized an analgesic activity of morphiceptin when they are co-injected into mouse intracerebroventricularly.