Phenylalanine is an essential aromatic amino acid since our body isn’t able to synthesize its benzene ring. It is a precursor of melanin, dopamine, noradrenalin, and thyroxine. At the same time, tyrosine amino acid can be obtained from sufficient dietary intake of phenylalanine. This is explained by the fact that the main way of the conversion of phenylalanine begins with its oxidation (more precisely, hydroxylation) into tyrosine. The hydroxylation reaction is catalyzed by a specific enzyme phenylalanine-4-monooxygenase, which, like all other hydroxylases, contains tetrahydrobiopterin as a coenzyme. The blocking of this reaction, observed when the synthesis of phenylalanine-4-monooxygenase in the liver is impaired, leads to the development of a serious hereditary disease – phenylketonuria (phenylpyruvid oligophrenia). In the process of transamination, tyrosine is converted to n-hydroxyphenylpyruvic acid, which under the action of a specific oxidase is subjected to oxidation, decarboxylation, hydroxylation, and intramolecular displacement of the side chain to form homogentisic acid; this reaction requires the presence of ascorbic acid. Further conversion of homogentisic acid into maleic-acetoacetic acid is catalyzed by homogentisinic acid oxidase. Maleic-acetoacetic acid under the action of a specific isomerase in the presence of glutathione is converted to fumarylacetoacetic acid, which undergoes hydrolysis with the formation of fumaric and acetoacetic acids (5).
Catecholamines are stored in vesicles and released from the cells only upon stimulation. Small amount of catecholamines that remain in the cytoplasm are a major source of metabolites. The metabolism of the transient aldehyde intermediate 3,4 dihydroxyphenylacetalcehyde by a dehydrogenase is dependent upon the presence (in noradrenaline and adrenaline) or absence (in dopamine) of the ?-hydroxyl group. Its absence in dopamine and 3,4-dihydroxyphenylacetaldehyde favors oxidation by aldehyde dehydrogenase. Its presence on noradrenaline, adrenaline and 3,4-dihydroxyphenylglycolaldehyde favors reduction by aldehyde reductase or aldose reductase. Therefore, dopamine is preferentially converted to an acid metabolite, and noradrenaline and adrenaline are preferentially converted to an alcohol metabolite. Generally, about one half of the dopamine generated in the body is not converted to noradrenaline and is degraded to inactive metabolites. Catecholaminesa are degraded in the liver with by two enzymes: COMT and MAO. Epinephrine and norepinephrine are converted to metanephrine and normetanephrine by the action of COMT. Both products of this reaction are excreted from the body with the urine.
Phelynalanine’s metabolism results in production of the amino acid tyrosine that’s involved in the creation of melanin pigments. Also, phenylalanine can be hydroxylated to tyrosine by hepatic phenylalanine hydroxylase using tetrahydrobiopterinas an electorn-donor.