L-Phenylalanine is an essential, nine-carbon amino acid and is a precursor to the amino acid tyrosine. Because tyrosine is necessary for the synthesis of proteins and the production of the neurotransmitters dopamine and norepinephrine, phenylalanine is an extremely important nutrient that must be obtained through the diet or supplementation.
Phenylalanine occurs in two chemical forms: L-phenylalanine, a natural amino acid found in proteins; and its mirror image, D-phenylalanine, a form synthesized in a laboratory. Some research has involved the L-form, others the D-form, and still others a combination of the two known as DL-phenylalanine.
In the body, L-phenylalanine is converted into another amino acid called Tyrosine. Tyrosine in turn is converted into L-dopa, norepinephrine, and epinephrine, three key neurotransmitters (chemicals that transmit signals between nerve cells). Because some antidepressants work by raising levels of norepinephrine, various forms of phenylalanine have been tried as a possible treatment for depression.
D-phenylalanine (but not L-phenylalanine) has been proposed to treat chronic pain. It blocks enkephalinase, an enzyme that may act to increase pain levels in the body.