Synthesis and receptor binding characteristics of [d‐Ala2, cysteamine5]enkephalin, a thiol‐containing probe for structural elements of opiate receptors

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Abstract
For the elucidation of structural elements in the opiate receptors, a thiol-containing enkephalin analog, [D-Ala2, cysteamine5]enkephalin, and its dimeric analog were synthesized and evaluated in the radio-ligand receptor binding assays using rat brain membranes. The dimeric analog was very potent in both delta and mu assays. Comparison of receptor affinities of the thiol-containing enkephalin with those of standard mu or delta receptor specific ligands suggested that the mu receptor contains an essential thiol group which may interact with the thiol group at the C-terminus of the enkephalin analog. It also appears that no metal-ion site, postulated for the delta receptors, is present in the delta binding site.