Met-enkephalin, also known as metenkefalin (INN), sometimes referred to as opioid growth factor (OGF), is a naturally occurring, endogenous opioid peptide that has opioid effects of a relatively short duration. It is one of the two forms of enkephalin, the other being leu-enkephalin. The enkephalins are considered to be the primary endogenous ligands of the δ-opioid receptor, due to their high potency and selectivity for the site over the other endogenous opioids. History Met-enkephalin was discovered and characterized by John Hughes, Hans Kosterlitz, et al. in 1975 after a search for endogenous ligands of the opioid receptors. Chemistry Met-enkephalin is a pentapeptide with the amino acid sequence tyr-gly-gly-phe-met. The tyrosine residue at position 1 is thought to be analogous to the 3-hydroxyl group on morphine. Biochemistry Distribution Met- enkephalin is found mainly in the adrenal medulla and throughout the central nervous system (CNS), including in the striatum, cerebral...