Specific Nonopiate Receptors for β-Endorphin

Iodinated $\beta _{H}$-[2-D-alanine]endorphin exhibits specific binding to cultured human lymphocytes. The binding is inhibited by low concentrations of β-endorphin and its D-alanine$^{2}$ derivative, but is not affected by opiate agonists and antagonists, or by enkephalin analogs, β-lipotropin, adrenocorticotrophic hormone, or α -melanocyte-stimulating hormone; this suggests the existence of a specific, nonopiate binding site (receptor) for β-endorphin. The carboxy-terminal region of β-endorphin is essential for this binding activity, since α-endorphin is not active. β-Endorphin may be a circulating hormone with peripheral physiological effects that are not primarily mediated through interactions with opiate or enkephalin receptors.