MOLECULE

L-Phenylalanine

Phenylalanine, abbreviated Phe or F, is a neutral, nonpolar, hydrophobic, essential amino acid. It is the precursor of the amino acid tyrosine, a precursor of the skin pigment melanin and a precursor of catecholamines that include tyramine, dopamine, epinephrine, and norepinephrine. Catecholamines are neurotransmitters that act as adrenalin-like substances. Interestingly, several psychotropic drugs (mescaline, morphine, codeine, and papaverine) also have phenylalanine as a constituent. Phenylalanine is highly concentrated in the human brain and plasma and is found naturally in mammalian breast milk. Normal metabolism of phenylalanine requires biopterin, iron, niacin, vitamin B6, copper, and vitamin C. An average adult ingests 5 g of phenylalanine per day and may optimally need up to 8 g daily. Phenylalanine is highly concentrated in several high protein foods, such as meat, cottage cheese, and wheat germ. Phenylalanine is also found in Cannabis plants (PMID: 6991645). An additional dietary source of phenylalanine is artificial sweeteners containing aspartame. Generally, aspartame should be avoided by phenylketonurics and pregnant women. When present in sufficiently high levels, phenylalanine can act as a neurotoxin and a metabotoxin. A neurotoxin is a compound that disrupts or attacks neural cells and neural tissue. A metabotoxin is an endogenously produced metabolite that causes adverse health effects at chronically high levels. Chronically high levels of phenylalanine are associated with at least five inborn errors of metabolism, including Hartnup disorder, hyperphenylalaninemia due to guanosine triphosphate cyclohydrolase deficiency, phenylketonuria (PKU), tyrosinemia type 2 (or Richner-Hanhart syndrome), and tyrosinemia type III (TYRO3). Untreated PKU can lead to intellectual disability, seizures, behavioural problems, and mental disorders. Classic PKU dramatically affects myelination and white matter tracts in untreated infants; this may be one major cause of neurological disorders associated with PKU. PKU patients have elevated serum plasma levels of phenylalanine, up to 400 times normal. High plasma concentrations of phenylalanine influence the blood-brain barrier transport of large neutral amino acids. The high plasma phenylalanine concentrations increase phenylalanine entry into the brain and restrict the entry of other large neutral amino acids (PMID: 19191004). Phenylalanine has been found to interfere with different cerebral enzyme systems. PKU individuals may have a musty smell and lighter skin, likely related to altered conversion of phenylalanine to the skin pigment melanin. Mild phenylketonuria may be an unsuspected cause of hyperactivity, learning problems, and other developmental problems in children. It has been recently suggested that PKU may resemble amyloid diseases, such as Alzheimer's disease and Parkinson's disease, due to the formation of toxic amyloid-like assemblies of phenylalanine (PMID: 22706200). Phenylalanine also has some potential benefits. Phenylalanine can act as an analgesic, providing relief for premenstrual syndrome and may enhance the effects of acupuncture and electric transcutaneous nerve stimulation (TENS) for Parkinson's patients. Phenylalanine and tyrosine, like L-DOPA, produce a catecholamine-like effect. Phenylalanine is better absorbed than tyrosine and may cause fewer headaches. Low phenylalanine diets have been prescribed for certain cancers with mixed results. For some tumours that use more phenylalanine than others (such as melatonin-producing tumours called melanomas) may benefit from low phenylalanine diets.