Mu-receptor specificity of the opioid peptide irreversible reagent, [ 3H] DALECK

A novel affinity reagent DALECK, i.e. D-Ala 2-Leu 5-enkephalin with a C-terminal chloromethyl ketone group, was previously synthesized in normal and in tritiated form and shown to react irreversibly at opioid receptors, with some evidence for selectivity for the μ subtype. DALECK tritiated in its phenolic group has been synthesized at 13-fold higher specific radioactivity than in the previous study. In the irreversible reaction of this product at pH 8.1 with rat brain membranes it was confirmed that only one polypeptide there is labelled, of apparent M r 58,000. Competition between this reaction and ligands highly selective for the μ, δ or κ binding sites yielded curves demonstrating the very high selectivity of the DALECK irreversible reaction for the μ site. The results provide evidence that the μ opioid receptor protein contains only one type of binding subunit, whose apparent M r is 58,000, this size being dependent upon the conditions used in the gel electrophoresis and being higher when stringent conditions which would reduce all internal disulphide bonds are applied.