Endomorphin 2 analogues containing Dmp residue as an aromatic amino acid surrogate with high mu-opioid receptor affinity and selectivity.

To investigate the effectiveness of a 2',6'-dimethylphenylalanine (Dmp) residue as an aromatic amino acid surrogate, endomorphin 2 (EM(2): Tyr-Pro-Phe-Phe-NH(2)) analogues were prepared, in which the constitutive aromatic amino acids (Tyr(1), Phe(3), or Phe(4)) were replaced by Dmp or its isomer, D-Dmp. Replacement of Phe(3) by Dmp increased the affinity over 10-fold for both mu- and delta-opioid receptors, without affecting receptor selectivity. In contrast, replacement of Phe(4) considerably reduced the mu-receptor affinity and selectivity. These data indicated that the Dmp-substitution of Phe(3), but not Phe(4), in EM(2) is favorable for improving mu-receptor specificity. Inversion of the chirality of the substituted Dmp residue resulted in marked decrease in the mu-receptor affinity. Replacement of Tyr(1) by Dmp yielded an analogue that exhibited only a limited decrease in mu-receptor affinity and GPI potency, despite the lack of a phenolic hydroxyl group at the N-terminal residue. In contrast, D-Dmp(1)- or Phe(1)-substitution of Tyr(1) resulted in a significant decrease in mu-receptor affinity and GPI potency. These results suggested that the Dmp residue can mimic Tyr(1), which is one of the critical structural elements of opioid peptides.