Opiorphin is an endogenous inhibitor of neprilysin and aminopeptidase N. Inhibits the breakdown of substance P, Mca-BK2 and Met-enkephalin by neprilysin in vitro with IC(50) values of 29 uM, 33 uM and 33 uM respectively. Inhibits the breakdown of Ala-pNA by aminopeptidase N in vitro with an IC(50) of 65 uM. Has a potent analgesic effect when administered to rats by intravenous injection. Abundantly expressed in lacrimal gland where it found in the secretory endpieces. Also expressed at modest levels in the submandibular gland. Belongs to the PROL1/PROL3 family.