Deltakephalin, Tyr-D-Thr-Gly-Phe-Leu-Thr: a new highly potent and fully specific agonist for opiate delta-receptors

Deltakephalin, Tyr-D-Thr-Gly-Phe-Leu-Thr (DTLET) was rationally designed as pure delta-probe from proposed models of mu and delta opiate receptors. On peripheral organs, deltakephalin displays a 3000 times higher inhibitory potency on the electrically stimulated mouse vas deferens (IC50 = 0.15 nM) as on the guinea pig ileum (IC50 = 460 nM). As expected [3H]deltakephalin interacts at 35 degrees C in rat brain tissue to a single class of binding sites (delta) (Bmax = 0.115 pmole/mg protein) with a high affinity: KD = 1.35 nM from equilibrium measurements and KD = 0.43 nM from kinetic determinations. Deltakephalin occurs as the most specific ligand for delta-binding sites as shown by the following discrimination ratios KI(mu)/KI(delta): 0.31 for D-Ala2-D-Leu5-enkephalin; 0.15 for D-Ser2-Thr6-Leu-enkephalin and 0.05 for deltakephalin.