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phenylalanine (fĕnˌəlălˈənēnˌ), organic compound, one of the 22 α-amino acids commonly found in animal proteins. Only the L-stereoisomer appears in mammalian protein. It is one of several essential amino acids needed in the diet; human beings cannot synthesize it from simpler metabolites. Young adults need about 31 mg of this amino acid per day per kg (14 mg per lb) of body weight. Phenylalanine can be degraded into simpler compounds by the enzymes of the body and is readily converted to the amino acid tyrosine. Phenylketonuria (PKU), an inherited disease that, if left untreated, results in retarded mental development in children, has been shown to be associated with the lack of activity of the enzyme that converts phenylalanine to tyrosine. This results in the buildup of phenylalanine in the blood, an event leading to several pathological consequences. The incidence of this disease, about one in every 10,000 births, is high enough to have prompted several states to institute regular screening procedures for the detection of the disease in newborns. If diagnosed early the disease can be controlled to a great extent by administering a diet very low in phenylalanine. Phenylalanine contributes to the structure of proteins into which it has been incorporated by the tendency of its side chain to participate in hydrophobic interactions (see isoleucine). This amino acid was first isolated from a natural source (lupine sprouts) in 1879; it was first chemically synthesized in 1882.
(also alpha-amino-beta-phenylpropionic acid), C6H5CH2CH(NH2)COOH, an aromatic amino acid. Phenylalanine exists in dextrorotatory (D), levorotatory (L), and racemic (DL) forms. In its L form, the amino acid is present (3–8 percent) in all natural proteins except protamines, and it occurs in the free state in animals, plants, and microorganisms. L-phenylalanine is an essential amino acid, the daily requirement of which is 4.3 mg/kg for adult males, 3.1 mg/kg for adult females, and 90 mg/kg for children. It is continuously formed in the body from the breakdown of proteins derived from food and tissue proteins. The need for phenylalanine increases in the absence of the amino acid tyrosine in food; tyrosine is normally formed in the liver by the hydroxylation of phenylalanine with the participation of the enzyme phenylalanine hydroxylase. The disruption of this process through a genetically caused defect leads to the accumulation of phenylalanine in the body cells and fluids. Disruption of the normal pathway for the conversion of phenylalanine produces secondary biochemical reactions (Figure 1) that lead to the formation of phenylpyruvic, phenyllactic, and phenylacetic acids in the body and to the development of a disease known as phenylketonuria.
In normal metabolism, phenylalanine is converted through tyrosine to dihydroxyphenylalanine, melanins, norepinephrine, and adrenaline; to a slight extent, it undergoes transamination. The direct precursor in the biosynthesis of phenylalanine in plants and microorganisms is phenylpyruvic acid, the aromatic ring of which is synthesized in a complex sequence of enzyme-catalyzed reactions from phosphophenylpyruvic acid and D-erythrose-4-phosphate by way of shikimic acid. In the breakdown of L-phenylalanine in the body, eight of the nine carbon atoms are introduced into the tricarboxylic acid cycle in the form of acetyl coenzyme A and fumarate; one carbon atom is converted into CO2. In the decomposition of proteins, especially in the intestines of animals and humans, phenylalanine is converted into the biogenic amine phenethylamine.
REFERENCESMeister, A. Biokhimiia aminokislot. Moscow, 1961. (Translated from English.)
Harris, H. Osnovy biokhimicheskoi genetiki cheloveka. Moscow, 1973. (Translated from English.)
E. N. SAFONOVA