Stereospecificity of amino acid side chains in deltorphin defines binding to opioid receptors

A series of individual D-amino acid replacement analogues of deltorphin A, several of which were in combination with a His4 deletion, were used to probe alterations of side-chain orientation on peptide binding parameters with rat brain opioid receptors. Peptides with D-amino acids in residues 1, 3, and 5 exhibited diminished affinities primarily for delta receptors (88-1200-fold) with selectivity decreasing by factors of 13-64-fold relative to deltorphin A (Ki delta = 0.45 nM; Ki mu/Ki delta = 764): the aromatic side chains Tyr1 and Phe3, which lie in the N-terminal "message" domain and the aryl side chain of Leu5 in the C-terminal "address" domain, appear to play essential roles in conferring high delta affinity and selectivity. Although D-His4 only decreased delta affinity by 6-fold and selectivity by a factor of 4, His appears to be involved as an integral component of both domains: [des-His4]deltorphin A and [des-His4] analogues containing consecutive D-amino acid replacements in the remaining residues exhibited weak binding to delta receptors and poor delta selectivity. Substitution of D-Met2 in deltorphin A by D-Ala or D-Nle decreased delta selectivities 3-6-fold through an elevation in mu affinities; however, the converse replacement, D-Met for D-Ala2 in deltorphin B, diminished beta selectivity by an order of magnitude only through the loss in delta affinity. The data show that the high delta affinity and selectivity of deltorphins correlate with and require a strict stereospecificity of the amino acid residue side chains.