Leucine enkephalin noncompetitively inhibits the binding of [3H]naloxone to the opiate mu-recognition site: evidence for delta----mu binding site interactions in vitro.

Using quantitative methods, this study examined the hypothesis that delta-ligands are noncompetitive inhibitors at a population of mu-binding sites. Evidence is presented that with the defined set of in vitro assay conditions utilized, [3H]naloxone labels two binding sites: the mu binding site and a second site tentatively identified as a kappa binding site, and that leucine enkephalin is a noncompetitive inhibitor at the mu recognition site.