Pentapeptide endorphins, found in many parts of the brain, that bind to specific receptor sites, some of which may be pain-related opiate receptors; hypothesized as endogenous neurotransmitters and nonaddicting analgesics. Metenkephalin is Tyr–Gly–Gly–Phe–Met; leuenkephalin has Leu in place of Met; proenkephalin has Pro in place of Met.
Medical dictionary. 2011.