MOLECULAR-DYNAMICS CONFORMATIONS OF DELTORPHIN ANALOGS ADVOCATE DELTA-OPIOID BINDING-SITE MODELS

Multi-site binding models for the delta opioid receptor were studied i n vitro with [H-3]DPDPE as the labeled ligand using anologues of delto rphin C (H-Tyr-D-Ala-Phe-Asp-Val-Val-Gly-NH2) altered at position 4. M odifications included a change in chirality (L- to D-Asp(4)), increase d length of the anionic side-chain (Glu(4)), elimination of the charge d group (Abu(4)) addition of an anionic group (Gla(4)), and change in backbone conformation (Pro(4)). All of the peptides had relatively hig h delta affinities (0.09 to 1.15 nM); the major variability in delta s electivity resided in changes in mu affinities (1.6 to 530 nM). Three analogues (Glu(4), D-Asp(4) and Pro(4)) revealed better fits to two-si te binding models (Hill coefficients < 0.850 with narrow 95% confidenc e intervals and P <0.0001). Deltorphin C and analogues containing Gla( 4) and Abu(4) (which were weakly delta selective), as well as deltorph in A (H-Tyr-D-Met-Phe- His-Leu-Met-Asp-NH2), fitted one-site binding m odels. Molecular dynamics simulations performed on deltorphin C and Ab u(4) exhibited similarities in the tertiary structure of their low ene rgy conformers while differing from the three-dimensional structures o f the analogues containing Glu(4), D-Asp(4) and Pro(4) substitutions. The data provide support that the three-dimensional architecture of an opioid peptide is an important factor in the designation of delta opi oid receptor subtypes.