Photoinactivation of the μ opioid receptor using a novel synthetic morphiceptin analog

Abstract

The benzophenone chromophore has been incorporated into a synthetic amino acid (p-benzoyl-L-phenylalanine; L-Bpa) to produce a chemically stable photoaffinity probe. L-Bpa was found to retain the photochemical reactivity of benzophenone. To test the utility of this synthetic amino acid as a photo-reactive probe for receptors, a tetrapeptide analog of morphiceptin was made as a model peptide in which the C-terminal prolinamide was replaced by L-Bpa amide. The affinity of the μ opioid receptor for this peptide is comparable to that for the parent compound, morphiceptin. Irradiation of the peptide-receptor complex reduced the subsequent binding of [ 3H]naloxone and virtually eliminated that of [ 3H]Tyr-D-Ala-Gly-NMe-Phe-Gly-ol (DAGO). Binding studies with [ 3H]naloxone indicated that both the affinity and the capacity were reduced. Competition studies with [ 3H]D-Ala 2-D-Leu 5-enkephalin (DADLE) and naloxone indicated selective inactivation of a μ type opioid receptor.