New analogues of deltorphin I (DT I), in which the phenylalanine residue in position 3 is substituted with amphiphilic α,α-disubstituted amino acid enantiomers, (R) and (S)-α-hydroxymethylnaphtylalanine, were synthesized and tested for μ and δ opioid receptor affinity and selectivity. Although both analogues have lower affinity to δ receptors than DT I, they both expressed specificity to δreceptors.
To evaluate the role of aromatic amino-acids residues, four analogues of the μ-selec-tive opioid peptide agonist DALDA (H-Tyr-D-Arg-Phe-Lys-NH2) containing the amphiphilic, α,α-disubstituted amino acid (R)- or (S)-α-hydroxymethyltyrosine (HmTyr) in position 1 and (R)- or (S)-α-hydroxymethylphenylalanine (HmPhe) in position 3 of the peptide sequence were synthesized. Only the [(R)-HmPhe3)]DALDA analogue displayed full agonistic activity in both the guinea pig ileum and the mouse vas deferens assays and turned out to be a δ receptor-selective opioid agonist.
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