Opioid activities of morphiceptin-like peptides latent in various natural proteins

The Tyr-Pro-Phe peptide sequence, an N-terminal tripeptide fragment of opioid peptide morphiceptin, was searched on the database SEQDB (Peptide Institute, Osaka). More than 30 proteins were drawn in a list with 15 amino acid varieties at the position adjacent to Phe. Seven morphiceptin-like peptides with the H-Tyr-Pro-Phe-Xxx-NH2 sequence, where Xxx denotes the selected amino acids (Ala, Asp, Gly, Gln, Lys, Thr and Tyr), have been synthesized. Together with the side-chain protected analogs [Asp(OBzl), Lys(Z), and Thr(Bzl)], they have been evaluated for opioid activities. For the mu opioid receptors to which morphiceptin binds specifically and selectively, analogs with Xxx = Asp, Lys, Ala, Thr, Gln, Tyr and Gly showed a considerably weaker binding affinity than morphiceptin. In contrast, the Thr(Bzl) derivative demonstrated an affinity ten times greater than morphiceptin. Because of an extremely weak affinity for the delta receptors, its mu-selectivity became very high (260-fold). The present results and the increased activity of [Val4]morphiceptin (Sakaguchi et al., 36) indicate that the mu receptors tolerate the hydrophobic or aromatic residue at the site corresponding to position 4 of morphiceptin. When the circular dichroism (CD) spectra are compared between active and inactive analogs, no significant difference is found in both water and methanol. This suggests that the activity of morphiceptin analogs with the Tyr-Pro-Phe sequence is mainly determined by the structural characters of the amino acid residue in position 4 rather than by the molecular conformation. The results suggested a possible formation of morphiceptin-like peptides in various protein digests.