MORPHINE ALLOSTERICALLY MODULATES THE BINDING OF [LEUCINE-H-3] ENKEPHALIN TO A PARTICULATE FRACTION OF RAT-BRAIN

  • 1 January 1982
    • journal article
    • research article
    • Vol. 21  (3) , 538-547
Abstract
Equilibrium binding studies demonstrated that [3H]leucine enkephalin labels a single class of binding sites in a particulate fraction of rat brain with a Kd of 3.2 .+-. 0.1 nM. Met enkephalin was a competitive inhibitor of [3H]leucine enkephalin binding, changing the Kd to 14.1 .+-. 1.5 nM. 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. Morphine of 500 nM caused a 45% decrease in the number of binding sites and an increase in the Kd. The Kd of morphine was 400 .+-. 17 nM. The noncompetitive inhibition by morphine of [3H]leucine enkephalin binding was 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 demonstrated that an allosteric model, not a 2-site model, best describes the observed data. 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.