Structure-activity relationships of the delta-opioid-selective agonists, deltorphins.

Deltorphins are naturally occurring peptides with high affinity and selectivity for delta-opioid receptors. They share with dermorphin, another mu-selective opioid agonist, the same N-terminal tripeptide Tyr-D-Xaa-Phe, where D-Xaa is a D-Ala or a D-Met residue. This common sequence appears to be essential for the best fitting of the peptides to both mu- or delta-opioid sites. We studied the changes in receptor affinity and selectivity and in biological potency of deltorphins due to shortening of the sequence, C-terminal deamidation or single amino acid substitutions. The results support the view that a code addressing the molecule towards delta-opioid sites is expressed in the C-terminal region of these peptides. This addressing domain confers high delta-selectivity to the ligand in the following two ways: (i) increased affinity for delta-sites; (ii) decreased affinity for mu-sites. The sequence of the C-terminal tripeptide appears to be responsible for the high delta-affinity of the molecules. Negatively charged side chains inhibit mu-binding and enhance delta-selectivity.