From the Manufacturer:
DLPA is a 50/50 blend of L-phenylalanine and D-phenylalanine (DPA). Researchers believe DPA inhibits the action of an enzyme that breaks down endorphins and enkephalins - proteins that bind to opiate receptor sites in the brain, thereby influencing the perception of discomfort. By obstructing the action of this enzyme, DPA may extend the life of endorphins and enkephalins, and their positive influence on comfort levels