Endomorphin 2 analogues containing Dmp residue as an aromatic amino acid surrogate with high μ-opioid receptor affinity and selectivity Original Research Article

To investigate the effectiveness of a 2′,6′-dimethylphenylalanine (Dmp) residue as an aromatic amino acid surrogate, endomorphin 2 (EM2: Tyr-Pro-Phe-Phe-NH2) analogues were prepared, in which the constitutive aromatic amino acids (Tyr1, Phe3, or Phe4) were replaced by Dmp or its isomer, d-Dmp. Replacement of Phe3 by Dmp increased the affinity over 10-fold for both μ- and δ-opioid receptors, without affecting receptor selectivity. In contrast, replacement of Phe4 considerably reduced the μ-receptor affinity and selectivity. These data indicated that the Dmp-substitution of Phe3, but not Phe4, in EM2 is favorable for improving μ-receptor specificity. Inversion of the chirality of the substituted Dmp residue resulted in marked decrease in the μ-receptor affinity. Replacement of Tyr1 by Dmp yielded an analogue that exhibited only a limited decrease in μ-receptor affinity and GPI potency, despite the lack of a phenolic hydroxyl group at the N-terminal residue. In contrast, d-Dmp1- or Phe1-substitution of Tyr1 resulted in a significant decrease in μ-receptor affinity and GPI potency. These results suggested that the Dmp residue can mimic Tyr1, which is one of the critical structural elements of opioid peptides.