An enkephalin is a pentapeptide involved in regulating nociception (pain sensation) in the body. The enkephalins are termed endogenous ligands, as they are internally derived (and therefore endogenous) and bind as ligands to the body's opioid receptors. Discovered in 1975, two forms of enkephalin have been found, one containing leucine ("leu"), and the other containing methionine ("met"). Both are products of the proenkephalin gene:[2]
There are three well-characterized families of opioid peptides produced by the body: enkephalins, β-endorphin, and dynorphins. The met-enkephalin peptide sequence is coded for by the enkephalin gene; the leu-enkephalin peptide sequence is coded for by both the enkephalin gene and the dynorphin gene.[3] The proopiomelanocortin gene (POMC) also contains the met-enkephalin sequence on the N-terminus of β-endorphin, but the endorphin peptide is not processed into enkephalin.[citation needed]
