Enkephalin

Enkephalins are endogenous opioid pentapeptides that are produced mainly in the central nervous system, adrenal medulla, and other peripheral tissues. There are two structurally different enkephalin peptides: Met-enkephalin (YGGFM) and Leu-enkephalin (YGGFL). These are generated from a precursor protein called proenkephalin by posttranslational proteolytic cleavage. In mammals, proenkephalin contains seven enkephalin sequences: six copies of Met-enkephalin and one copy of Leu-enkephalin. Thus, the activity of the proenkephalin gene is amplified via the production of multiple enkephalin peptides. Of the three classical opioid receptors (δ-opioid receptor, κ-opioid receptor, and μ-opioid receptor), enkephalin peptides preferentially bind to the δ-opioid receptor.