This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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n. Either of two closely related pentapeptides having opiate qualities and occurring in the brain, spinal cord, and other parts of the body.

n. Any of a group of pentapeptide endorphins that have opiate-like effects

n. an endorphin having opiate qualities that occurs in the brain and spinal cord and elsewhere

"The principal objection came from Hughes and Kosterlitz, who preferred the term enkephalin."

endorphin   

cell-death    cellsurface    signal-transduction    gastrin    sulfoxide