Synthesis and receptor binding analysis of dermorphin hepta-, hexa- and pentapeptide analogues

Sixteen dermorphin analogues were synthesized and characterized for μ- and δ-opioid receptor binding properties using [³H]DAGO and [³H]DPDPE, respectively. The analogues included the following: substitutions at position 4 and/or the C-terminal residue; deletions of Gly⁴ or Pro⁶-Ser⁷; inclusion of Z or an acetyl group on the β-amino group of Lys⁷; and the presence of either a C-terminal amide or free acid group. Two peptides, [Lys7-OH]- and [Lys⁷-NH₂]dermorphin, had μ-affinities (K_iμ= 0.15–0.13 nm ) and μ-selectivities (K_iδ/K_iμ= 1158–1482) higher than dermorphin (K_iμ= 0.28 nm ; K_iδ/K_iμ= 295) and best fitted a one-site binding model similar to dermorphin. Significantly better (P <0.0001) fits to a two-site binding model vs. a one-site model were observed with four dermorphin analogues: [Lys(Z)⁷-OH]heptapeptide, [des-Gly⁴(Tyr⁴,Pro⁵,Asn⁶-OH)]hexapeptide and two pentapeptides, [Tyr⁵-NH₂] and [Trp⁴,Asn⁵-OH]. Our data revealed a complex binding pattern for dermorphin analogues to brain μ-receptors in which Hill coefficients less than 0.85 generally suggest heterogeneity of μ-receptors; however, only detailed analyses of the data derived from the non-linear regression fits for one- or two-components gave evidence for the possible existence of two separate [³H]DAGO binding sites. Eight of our dermorphin analogues had significantly better fits for a two-site model (P <0.05), but only four seemed to have two distinct Kⁱ, values (P <0.0001).