Characterization of Opioid Receptor Subtypes in Solution

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Abstract
Stable opioid receptor binding activity that retains distinct subtype specificities (μ, 6, and K) has been obtained in high yields in digitonin extracts of rat brain membranes that had been preincubated with Mg2+ prior to solubilization. The dependence on Mg2+ ions for receptor activity is also expressed in the soluble state, where the presence of Mg2+ leads to high‐affinity and high‐capacity opioid peptide binding to the δ, μ, and K sites (the latter subtype measured by the binding of [3H]dynorphin1–8). Binding of opiate alkaloids to soluble receptor sites is less dependent on Mg2+ than is opioid peptide binding. Soluble opioid binding activity shows the same sensitivity to Na+ ions and guanine nucleotides as the membrane‐bound receptor. The ligand‐receptor interactions give evidence of strong positive cooperativity, which is interpreted in terms of association‐dissociation of receptor subunits on ligand binding in solution. Binding of enkephalin peptides is associated with the large macromolecules present (apparent Stokes radii > 60 Å), whereas both those and several small species present (< 60 Å) bind opiate alkaloids and dynorphin1–8.