What is Doctor’s Best D-Phenylalanine? Doctor’s Best D-phenylalanine is a non-protein amino acid related to the essential amino acid L-phenylalanine. D- and L-phenylalanine are structural mirror images of each other. D-phenylalanine is reported to inhibit an enzyme (enkephalinase) that degrades enkephalins, which are naturally occurring substances in the body that belong to the class of compounds known as “endorphins.” D-phenylalanine slows the action of the enzymes carboxypeptidase A, endorphinase and enkephalinase that degrade endorphins. Endorphins are neurotransmitters your body releases that bind to specialized receptors that trigger two actions. This process blocks the neurotransmitters that cause you to experience pain. It also makes your brain release more of the neurotransmitter dopamine. D-phenylalanine has twice the endorphinase inhibitory effect of D, L-phenylalanine. Slowing down this endorphin-reducing mechanism may help diminish pain.