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 bind 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.
* Met-enkephalin is Tyr-Gly-Gly-Phe-Met. 
* Leu-enkephalin is Tyr-Gly-Gly-Phe-Leu.