A photoaffinity reagent to label the opiate receptors of guinea pig ileum and mouse vas deferens

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Abstract
An enkephalin derivative, [D-Ala2, Leu5]enkephalin N-[(2-nitro-4-azidophenyl)amino]ethylamide, was synthesized as a photoaffinity label for the opiate receptor. This compound retained the full biological activity of [D-Ala2, Leu5]enkephalin in guinea pig ileum and mouse vas deferens tests with IC50 [median inhibitory concentration] values of 4.4 and 2.6 nM, respectively, and inhibited the binding of [3H]naloxone to rat brain membrane preparations with an IC50 value of 2.5 nM. Photolysis of a muscle strip of the guinea pig ileum or of the mouse vas deferens in the presence of the peptide derivative caused irreversible inhibition of electrically stimulated contractions with high efficiencies (80 and 66%, respectively), while the inhibitory effect in the dark was fully reversed by washing. The irreversible inhibition during photolysis was completely prevented by the presence of [D-Ala2, Leu5]enkephalin. [D-Ala2, Leu5]-enkephalin N-[(2-nitro-4-azidophenyl)amino]ethylamide is evidently a prominent candidate as a photoaffinity label for the opiate receptor.