Morphine allosterically modulates the binding of [3H]leucine enkephalin to a particulate fraction of rat brain.

Equilibrium binding studies have demonstrated that [3H]leucine enkephalin labels a single class of binding sites in a particulate fraction of rat brain with a dissociation constant (KD) of 3.2 +/- 0.1 nM. Methionine enkephalin was a competitive inhibitor of [3H]leucine enkephalin binding, changing the KD to 14.1 +/- 1.5 nM. In contrast, Scatchard analysis of the binding of [3H]leucine enkephalin in the absence and presence of 10, 50, and 100 nM morphine demonstrated that these concentrations of morphine decreased the number of binding sites by 23%, 32%, and 42%, respectively, with no change in the KD. In contrast, morphine at 500 nM caused a 45% decrease in the number of binding sites and an increase in the KD. On the basis of these data, the inhibitory dissociation constant (KI) of morphine was calculated to 400 +/- 17 nM. The noncompetitive inhibition by morphine of [3H]leucine enkephalin binding was shown to be rapidly reversible, ruling out pseudoirreversible binding of morphine to the enkephalin binding site as the underlying mechanism. Computer analysis of the displacement [3H]leucine enkephalin binding by various concentrations of morphine has demonstrated that an allosteric model, not a two-site model, best describes the observed data. We conclude that at low concentration morphine binds to a receptor not labeled by [3H]leucine enkephalin and by doing so allosterically induces an apparent loss of enkephalin receptors.