Reconstitution of partially purified opioid receptors with a GTP-binding protein

Partially purified opioid receptors, obtained from rat brains using an affinity resin, AF-Amino Toyopearl with D-Ala2, Leu5]enkephalin, were reconstituted with an inhibitory GTP-binding protein (Gi). In the reconstituted system, the displacement curve for the binding of a delta-agonist, D-Ala2, D-Leu5]enkephalin, showed two states, high and low affinity binding ones, with different affinities for the agonist. The high affinity binding was eliminated by the addition of a guanine nucleotide analog to the system. These results directly showed that opioid receptors, at least the delta-type, could interact with Gi.