Enkephalin and enkephalin-like peptides have been detected in many animal species. In mammals these opioid pentapeptides are cleaved from a common precursor molecule which is expressed in a number of cell types, including neurons and endocrine cells associated with the sympathetic nervous system where they are often co-stored with catecholamines. They are known to be endogenous ligands for Sopioid receptors and are known to mediate analgesia at the spinal level. 
Met-enkephalin (tyr-gly-gly-phe-met) and leu-enkephalin (tyr-gly-gly-phe-leu) are pentapeptides which compete with and mimic the effects of opiate drugs. Although interest in enkephalins stems largely from their possible role in the brain, the richest source of these peptides is the adrenal gland. The amino acid sequence shows that the precursor is 267 amino acids long and contains 6 interspersed Met-enkephalin sequences and 1 Leu-enkephalin sequence. The precursor does not contain the sequences of dynorphin, alpha-neo-endorphin or beta-endorphin. (Because of structural similarities it had been postulated that beta-endorphin is precursor of Met-enkephalin, and that dynorphin or alpha-neo-endorphin is precursor of Leu-enkephalin.)