Both phenylalanine and tyrosine are aromatic amino acids that are used in protein synthesis. In humans, phenylalanine is an essential amino acid that must be obtained from the diet. In contrast, tyrosine can be synthesized from phenylalanine and is considered a non-essential amino acid. Both compounds can be altered into chemicals that affect mood and brain functioning. Because of these effects, phenylalanine and tyrosine are frequently taken as nutritional supplements.
All amino acids have a basic core structure, but vary in their side chains. There is a group of three amino acids that are collectively known as aromatic amino acids because they are attached to a phenyl ring. Phenylalanine and tyrosine have such a ring in their structure. Phenylalanine has just the phenyl ring, a six carbon ring with three double bonds. Tyrosine is synthesized by the enzyme phenylalanine hydroxylase, which adds a hydroxyl, or OH, group to the ring on phenylalanine.
Both phenylalanine and tyrosine are obtained from eating high protein foods such as meat, dairy, avocados, soy products, and certain nuts. Some people need to be very careful and eat a low phenylalanine diet because they suffer from a genetic disorder called phenylketuria (PKU), in which phenylalanine is not metabolized. It can build up to toxic levels and cause retardation and death. In the United States, newborn babies are tested for this genetic disease within the first several days of their life.
The artificial sweetener aspartame is a common source of phenylalanine that people with PKU must avoid. This compound is a combination of aspartic acid and phenylalanine and is a common component in diet sodas. It is starting to be replaced by alternative sweeteners, however.
Amino acids can be found in two forms: D and L. These are mirror images of each other and are known as stereoisomers. The common form for amino acids in protein synthesis is the L-form. Humans obtain L-phenylalanine from their diet. D-phenylalanine is synthesized chemically. It is not involved in protein synthesis and is not metabolized to tyrosine. Supplements of phenylalanine are sold in D-, L-, and DL-phenylalanine (DLPA) combinations.
A number of exaggerated claims are made for the health benefits of taking phenylalanine as a nutrient supplement, and it should not be taken by everyone. It is clear from biochemical studies that phenylalanine and tyrosine are starting blocks for the neurotransmitter dopamine as well as the hormones adrenaline and noradrenaline. Changes in dopamine levels can have great effects on mood disorders such as depression and diseases such as Parkinson’s. People with PKU or who take antidepressants as well as women who are pregnant or breastfeeding should not take supplements of phenylalanine.