Binding of 3H-beta-endorphin in rat brain.

The binding of 3H-beta-endorphin to rat brain homogenates, reported by several other laboratories, has suggested unique selective beta-endorphin binding sites. We now present additional evidence supporting the concept of distinct beta-endorphin binding (epsilon) sites in rat brain. In competitive displacement studies, 3H-beta-endorphin was inhibited far better by unlabeled beta-endorphin than a variety of opiates and enkephalins. Conversely, beta-endorphin inhibited the binding of a series of 3H-labeled ligands, including dihydromorphine, ethylketocyclazocine, SKF 10,047, naloxone and D-ala2-D-leu5-enkephalin, far less potently than their corresponding unlabeled drug. Other differences were also found. Compared to 3H-dihydromorphine and 3H-D-ala2-D-leu5-enkephalin binding, 3H-beta-endorphin binding was far less sensitive to the reagent N-ethylmaleimide and more sensitive to the proteolytic enzyme trypsin. The regional distribution for 3H-beta-endorphin binding was also distinct from other 3H-ligands tested. This evidence supports the concept of a distinct binding site for beta-endorphin which does not correspond to the previously defined opioid binding sites.