Peptide E is a μ-selective opioid peptide derived from proenkephalin A which contains [Met5]-enkephalin at the amino end and [Leu5]-enkephalin at the carboxyl end. Peptide E is further processed both centrally and peripherally to a [Leu5]-enkephalincontaining fragment which was investigated to determine if processing leads to alterations in receptor selectivity. Peptide E-(15-25) inhibited electrically stimulated contractions in both the mouse vas deferens, longitudinal muscle, myenteric (IC50 = 459 nmol/ L), and guinea pig ileum (IC50 = 2630 nmol/L), indicating a sixfold δ-receptor selectivity. When administered intracerebroventricularly to mice, peptide E-(15-25) also produced potent analgesia which was completely antagonized by naloxone pretreatment, but the peptide had no effect on intestinal transit as measured by the radiochromium geometric center method. This is consistent with earlier findings that intracerebroventricular δ-opioid-selective agents are analgesic but do not inhibit intestinal transit. In vitro radioligand binding assays were performed using male Sprague-Dawley rat whole brain homogenates. The IC50 for peptide E against [3H]naloxone was 1.8 nmol/L compared with the δ-opioid ligand, [3H] [d-Pen2, d-Pen5]-enkephalin of 38.8 nmol/L. The IC50 for peptide E-(15-25) against [3H]naloxone was 497 nmol/L, but for [3H] [d-Pen2, d-Pen5]-enkephalin it was 50.6 nmol/L. Therefore, peptide E loses μ-opioid receptor affinity (1.8-497 nmol/L) after proteolytic processing and the loss of the amino terminal tyrosine but maintains a high δ-opioid affinity (38.8-50.6 nmol/L). These studies demonstrate that enzymatic peptide processing of peptide E to peptide E-(15-25) leads to a shift from μ to δ-receptor selectivity and a different spectrum of biological effects on gut motility.
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