 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.Met-enkephalin is found mainly in the adrenal medulla and throughout the central nervous system (CNS), including in the striatum, cerebral cortex, olfactory tubercle, hippocampus, septum, thalamus, and periaqueductal gray, as well as the dorsal horn of the spinal cord.