Photolabile opioid derivatives of D-Ala2-Leu5-enkephalin and their interactions with the opiate receptor

Photolabile derivatives of D-Ala2-Leu5-enkephalin were prepared by synthetic procedures in which a 2-nitro-4-azidophenyl group is linked to the terminal carboxyl group of the enkephalin by means of an ethylenediamine or ethylenediamine beta-alanine spacer. These peptides bind to opiate receptors with nanomolar affinities and inhibit electrically stimulated contractions of the mouse vas deferens and adenylate cyclase activity of NG108-15 neuroblastoma x glioma hybrid cell membranes. Both inhibitions are reversed by the opiate antagonist naloxone. Photolysis of the ligands bound to rat brain membranes results in the loss of approximately 50% of the receptor sites. This decrease in receptor number is blocked by naloxone and requires light. A photolabile 3H]enkephalin derivative labels an equivalent number of sites under similar irradiation conditions.