Multi-site binding models for the delta opioid receptor were studied in vitro with [3H]DPDPE as the labeled ligand using analogues of deltorphin C (H-Tyr-D-Ala-Phe-Asp-Val-Val-Gly-NH2) altered at position 4. Modifications included a change in chirality (L- to D-Asp4), increased length of the anionic side-chain (Glu4), elimination of the charged group (Abu4), addition of an anionic group (Gla4), and change in backbone conformation (Pro4). All of the peptides had relatively high delta affinities (0.09 to 1.15 nM); the major variability in delta selectivity resided in changes in mu affinities (1.6 to 530 nM). Three analogues (Glu4, D-Asp4 and Pro4) revealed better fits to two-site binding models (Hill coefficients < 0.850 with narrow 95% confidence intervals and P < 0.0001). Deltorphin C and analogues containing Gla4 and Abu4 (which were weakly delta selective), as well as deltorphin A (H-Tyr-D-Met-Phe-His-Leu-Met-Asp-NH2), fitted one-site binding models. Molecular dynamics simulations performed on deltorphin C and Abu4 exhibited similarities in the tertiary structure of their low energy conformers, while differing from the three-dimensional structures of the analogues containing Glu4, D-Asp4 and Pro4 substitutions. The data provide support that the three-dimensional architecture of an opioid peptide is an important factor in the designation of delta opioid receptor subtypes.
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