Abstract
We have studied the receptor binding activities of C -terminal free and amidated enkephalins with and without the dehydrophenylalanine 4 residue. For the selective labeling of so-called δ and μ opiate receptors, specific tracers were used at low concentrations in rat brain membranes and neuroblastoma cells containing pure δ receptors. C -Terminal free enkephalins are five to eight times more potent in the assay for δ receptors than in μ assay, while the amides are almost equipotent in both assays. The presence of a C-terminal carboxyl group is a determining factor for selective activity. [D-Ala 2, ΔPhe 4, Met 5]-enkephalin amide is very potent in all of the binding assays examined, and, in particular, twice as active as the saturated amide and the C -terminal free enkephalin in the δ assay. We suggest that the steric arrangement of the dehydrophenylalanine residue in position 4 is very important to the enhanced interaction with the δ receptors.
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