Phe or Tyr-X1-X2-Phe were synthesized and their opioid activities were evaluated by the guinea pig ileum and radioreceptor assays. Among the peptides tested, the human /?-casein(51~54) amide, Tyr-Pro-Phe-Val-NH2 (valmuceptin) was the most active, more than the bovine counterpart, morphiceptin. The human /?-casein(41~44) amide, Tyr-Pro-Ser-Phe-NH2, and the a-lactalbumin
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