Interaction of S-activated enkephalin analogs with opiate receptors

Enkephalin analogs containing a thiol activated by a thiomethyl (SCH3)*** or 3-nitro-2-pyridinesulfenyl (Npys) group were synthesized. Incubation of such S-activated enkephalin analogs as [D-Ala2, Leu(CH2S)SCH(3)5]enkephalin or [D-Ala2,Leu(CH2S)Npys5]enkephalin with guinea pig ileum (GPI) resulted in the continuous stimulation of the mu opiate receptors. This sustained GPI-activity was completely reversed with the antagonist naloxone, while subsequent washings elicited again the full enkephalin activity. When GPI showing full enkephalin activity was incubated with 1 mM dithiothreitol, about 70% of the activity was eliminated. Examination of enkephalin analogs containing Cys(Npys) at position 1, 5, or 6 suggested that no other thiols occur near the enkephalin binding site of the mu receptor. From these results, it is considered that only one thiol group exists near the binding site of the mu receptor in GPI. Similar results were also obtained for the mu receptors in mouse vas deferens.