Opioid Activities of Morphiceptin Analogs Derived from Human β-Casein

Abstract

Abstract Four tetrapeptide amides with the N-terminal Tyr–Pro sequence were synthesized as possible opioid agonists that can be produced by degradation of human β-casein. When these peptides were tested for their ability to bind to the μ and δ opioid receptors in rat brain, only H–Tyr–Pro–Phe–Val-NH2 was active, showing the 60% increased affinity for the μ receptors as compared with morphiceptin (H–Tyr–Pro–Phe–Pro–NH2) derived from bovine β-casein. It was highly μ-selective, as well as morphiceptin, with the μ/δ-selectivity ratio of 285. Other three analogs, H–Tyr–Pro–Ser–Phe–NH2, H–Tyr–Pro–Val–Arg–NH2 and H–Tyr–Pro–Val–Pro–NH2, were almost completely inactive. These results suggested that, for the morphiceptin-like tetrapeptide amides, the presence of Phe at position 3 is essential to elicit an activity to bind to the μ opioid receptors. Conformational considerations by measuring the CD spectra indicated that the sequence of tetrapeptide amide Tyr–Pro–Xxx–Pro(or Val)–NH2 is an important structural requirement to interact with the μ opioid receptors.