beta-Endorphin: surface binding and internalization in thymoma cells.

The opioid peptide beta-endorphin binds to specific nonopioid binding sites (Mr 72,000) that are present on the surface of thymoma cells. beta-Endorphin is then internalized, apparently via the Mr 72,000 species, and is subsequently found within intracellular, vesicular structures. This process is accompanied by the down-regulation of the Mr 72,000 binding sites. Our findings suggest that beta-endorphin may modulate cellular functions, such as T-lymphocyte proliferation, at intracellular rather than cell surface sites.