Phenylalanine is an essential amino acid and the precursor for the amino acid tyrosine.
Phenylalanine is an essential amino acid, meaning that it can’t be synthesized in the human body and must be ingested through the diet. It can be viewed as a benzyl group substituted for the methyl group of alanine, or a phenyl group in place of a terminal hydrogen of alanine.
This essential amino acid is classified as neutral, and nonpolar because of the inert and hydrophobic nature of the benzyl side chain. The L-isomer is used to biochemically form proteins, coded for by DNA. Phenylalanine is a precursor for the amino acid tyrosine; the monoamine neurotransmitters dopamine, norepinephrine (noradrenaline), and epinephrine (adrenaline); and the skin pigment melanin.
Phenylalanine is also a direct precursor to the neuromodulator phenethylamine, a commonly used dietary supplement. As an essential amino acid, phenylalanine is not synthesized de novo in humans and other animals, who must ingest phenylalanine or phenylalanine-containing proteins
The L-form of Phenylalanine is incorporated into proteins, while the D-form acts as a painkiller.
Phenylalanine is an essential amino acid and the precursor for the amino acid tyrosine. Like tyrosine, it is the precursor of catecholamine in the body (tyramine, dopamine, epinephrine and norepinephrine). The L-form of Phenylalanine is incorporated into proteins, while the D-form acts as a painkiller. Absorption of ultraviolet radiation by Phenylalanine is used to quantify protein amounts.
DL-Phenylalanine (DLPA) is purported to have analgesic and antidepressant activities. DL-Phenylalanine is a mixture of D-phenylalanine and L-phenylalanine. The reputed analgesic activity of DL-phenylalanine may be explained by the possible blockage by D-phenylalanine of enkephalin degradation by the enzyme carboxypeptidase A. The mechanism of DL-phenylalanine's supposed antidepressant activity may be accounted for by the precursor role of L-phenylalanine in the synthesis of the neurotransmitters norepinephrine and dopamine. Elevated brain levels of norepinephrine and dopamine are thought to have an antidepressant effect.
Phenylalanine is highly concentrated in the human brain and plasma, normal metabolism of phenylalanine requires iron, niacin, vitamin B6, copper and vitamin C.
Phenylalanine can be an effective pain reliever. Its use in premenstrual syndrome and Parkinson's may enhance the effects of acupuncture and electric transcutaneous nerve stimulation.
Wheat germ is a good source of phenylalanine.
Phenylalanine is found naturally in the breast milk of mammals. It is used in the manufacture of food and drink products and sold as a nutritional supplement for its reputed analgesic and antidepressant effects.
Phenylalanine is also highly concentrated in high protein foods, such as meat, cottage cheese and wheat germ.
Phenylalanine should be avoided by phenylketonurics and pregnant women.
Phenylalanine uses the same active transport channel as tryptophan to cross the blood–brain barrier. In excessive quantities, supplementation can interfere with the production of serotonin and other aromatic amino acids as well as nitric oxide due to the overuse (eventually, limited availability) of the associated cofactors, iron or tetrahydrobiopterin. The corresponding enzymes in for those compounds are the aromatic amino acid hydroxylase family and nitric oxide synthase.
Phenylalanine should be avoided by phenylketonurics and pregnant women. Phenylketonurics, are people who have a genetic error of phenylalanine metabolism, and tend to have elevated serum plasma levels of phenylalanine up to 400 times normal. This is due to lack of the enzyme phenylalanine hydroxylase. Phenylketonurics often use blood tests to monitor the amount of phenylalanine in their blood.
Mild phenylketonuria, can also be an unsuspected cause of hyperactivity, learning problems, and other developmental problems in children.
A non-food source of phenylalanine is the artificial sweetener aspartame. This compound, is metabolized by the body into several chemical byproducts including phenylalanine. The breakdown problems phenylketonurics have with the buildup of phenylalanine in the body also occurs with the ingestion of aspartame, although to a lesser degree.
Low phenylalanine diets have been prescribed for certain cancers with mixed results. Some tumors use more phenylalanine (particularly melatonin-producing tumors called melanoma). One strategy is to exclude this amino acid from the diet e.g. a Phenylketonuria (PKU) diet (compliance is a difficult issue; it is hard to quantify and is under-researched). The other strategy is just to increase phenylalanine's competing amino acids e.g. tryptophan, valine, isoleucine and leucine, but not tyrosine.
An average adult ingests 5 g of phenylalanine per day and may optimally need up to 8 g daily.
The Food and Nutrition Board (FNB) of the U.S. Institute of Medicine set Recommended Dietary Allowances (RDAs) for essential amino acids in 2002. For phenylalanine plus tyrosine, for adults 19 years and older, 33 mg/kg body weight/day.