Dietary Supplement DL-Phenylalanine Supports an Alert State DL-Phenylalanine or DLPA, is a 50/50 mixture of the D and L forms of the amino acid phenylalanine (phenylalanine is one of the few amino acids that can be utilized in its D form). Researchers believe that D-phenylalanine (DPA) inhibits the action of an enzyme that breaks down endorphins and enkephalins. These are proteins that bind to opioid 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. L-Phenylalanine is the precursor to two neurotransmitters that help support an alert state.