Allosteric modulation by leucine-enkephalin of [3H]naloxone binding in rat brain.

The interaction of morphine and leucine-enkephalin with the binding site labeled by [3H]naloxone in the presence of sodium was compared. The effect of fixed concentrations of morphine and leucine enkephalin on the saturation binding of [3H]naloxone demonstrated that whereas morphine was a competitive inhibitor, leucine enkephalin caused a dose-dependent masking of binding sites. From these data we conclude that the enkephalin receptor is allosterically coupled to the morphine receptor.