Tetrapeptide Derivatives of [D-Pen~2, D-Pen~5]-Enkephalin (DPDPE) Lacking an N-Terminal Tyrosine Residue Are Agonists at the #mu#-Opioid Receptor

摘要

The Phe~1 cyclic tetrapeptide Phe-c(D-Cys-Phe-D-Pen]NH2 (Et) (JH-54) has been shown previously to exhibit high affinity and selectivity for the JL-opioid receptor. To examine the role of the Phe 1 residue in the unexpected high affinity of this peptide, 11 analogs of JH-54 have been synthesized and evaluated for opioid ligand binding and for efficacy using the (35S]GTPyS assay. Alteration of the bridging groups between the D-Cys2 and D-Pen4 residues of JH-54 from dithioether to disulfide revealed the importance of the relative position of the aromatic rings of the first and third residues in determining JL- and D-affinities. The one carbon distance between the a carbon and phenyl ring in the N-terminal residue was critical. Additional steric bulk in the N-terminal Phe 1 residue was accommodated without large reductions in affinity in two naphthyl analogs, but not with 3,3-(diphenyl)alanine. Conformational restriction of the Ca-C/3 and/or C/3-Cy bonds had little effect on affinities in two peptides with 2-amino-2-carboxytetralin in position 1, but it abolished activity in an isoquinoline analog and differentially altered activity in four phenylproline 1-containing peptides. Most surprisingly, replacement of the Phe1 aromatic ring with cyclo- hexyl resulted in a peptide of moderate affinity (Kj = 32.5 nM) and potency (ECso = 58.8 nM). Thus, the tyrosyl para-hydroxyl substituent and even aromaticity in the N-terminal amino acid of these tetrapeptides are shown to be important, but not critical, features for IL-opioid receptor affinity, agonist potency, and efficacy.