Tyrosine
Description
Tyrosine is a nonessential amino acid, ie. it is manufactured from other amino acids in the liver and does not have to be obtained through diet.
Tyrosine is only generated from one essential amino acid--phenylalanine. Individuals who have a genetic impairment in the ability to convert phenylalanine to tyrosine (called phenylala-nine hydroxylase deficiency--phenylketonuria) may then require tyrosine as an essential amino acid, based upon their unique biochemistry. For instance, infants who suffer from phenyl-ketonuria can be maintained by withholding phenylalanine from their diets or by supplementing them with tyrosine. It has been suggested part of the mental retardation occuring with phenylketonuria may be a consequence of tyrosine deficiency as much as phenylalanine excess. This represents an example of where tyrosine becomes an essential amino acid due to a genetic uniqueness in metabolism.
Tyrosine is also the immediate precursor to the thyroid hormone thyroxin and melanin, which is the pigment in the skin that allows for tanning. Deficiencies of phenylalanine or tyrosine may result in interruption of the synthesis of melanin or thyroxin. Adverse side effects of tyrosine supplementation can be aggravation of depression or the production of sleep disturbances. Therapeutic doses of tyrosine may have an impact on thyroid hormone production and basal metabolism.
Tyrosine supplementation has been used in animals to treat elevated and low blood pressure, which illustrates its blood pressure normalizing effects. Tyrosine has also been suggested to be useful for the management of certain nervous system disorders which are associated with low dopamine output such as depression. In those depressive patients who do not respond to phenylalanine or tryptophan supplementation, a trial with tyrosine might prove beneficial. Doses are generally in the range of 250 to 1,000mg per day. Supplementation should be discontinued if headaches or anxiety attacks occur or the depression is aggravated. Therapeutic doses of tyrosine may have an impact upon thyroid hormone product and basal metabolism.
Recommended Dietary Allowances
Tyrosine is a nonessential amino acid, therefore an RDA has not been established.
Food Sources
Tyrosine is a nonessential amino acid, which means it is manufactured from other amino acids in the liver; it does not have to be obtained directly through the diet.
Method of Action
Tyrosine is a precursor of the dopamine family of hormones. Dopamines are synthesized in the adrenal medulla and the central nervous system, and regulate central and peripheral nervous system activity. These hormones include adrenaline and noradrenaline.
The major route of degradation of tyrosine is the conversion to parahydroxyphenylpyruvate. This compound is then further degraded by an enzyme called dioxygenase to homogentinsic acid. One of the first inborn errors of metabolism to be recognized was called alkaptonuria, which was a lack of dioxygenase resulting in increased spill in the urine of parahydroxyphenylpyruvate. This condition is easily identified by blackening of the urine upon standing.
Tyrosine is also metabolized to the thyroid hormones thyroxin and triiodothyronine. The thyroid gland is rich in iodide. This reacts under the influence of a peroxidase enzyme to iodinate tyrosine to form the active thyroid hormones. Tyrosine is metabolized to catechol derivatives, which may play important roles as neurotransmitters. They are also the precursors of the tanning pigment in skin and the black pigment in hair. Route of formation of the catecholamines is through change of tyrosine into tyramine, and subsequent conversion to dopa, which is effective in treating Parkinson's disease. Debilitating symptoms are thought to be a result of a lack of dopamine in certain areas of the brain. Conversion of dopamine by way of vitamin C and a copper-containing enzyme produces noradrenaline. Methylation of this compound produces adrenaline, an important hormone and neurotransmitter.
The metabolic breakdown of adrenaline and noradrenaline occur by way of an enzyme called monoamine oxidase (MAO), with the ultimate excretory product being vanillic acid which is excreted in the urine. The second breakdown route for these neurotransmitters is by way of catecholamine O-methyl transferase (COMT), a very active enzyme in neural tissues. Noradrenaline and adrenaline are related to the proper metabolism of tyrosine and phenylalanine.
References
Anderson, G. & V. Ashley. Correlation of Plasma Tyrosine to Phenylalanine Ratio with Energy Intake in Self-Selecting Rats, Life Sci. 21:1227, 1977.
Blackburn, G., J. Grant, V. Young, ed. Amino Acids Metabolism and Medical Applications.
Gibson, C., Gelenberg, A. Tyrosine for depression. Adv. Biol Psychiat 10 (1983) 148-159.
Morgan, M., J. Milson & S. Sherlock. Plasma Ratio of Valine, Leucine and Isoleucine to Phenylalanine & Tyrosine in Liver Disease, Gut, v.19, 1978.
Munro, H. & M. Crim. The Proteins and Amino Acids. Modern Nutri. in Health & Disease, eds: R. Goodhart & M. Shils, Phila. Lea & Febiger, 1980.
Rudman, D., M. Kutner, J. Ansley, R. Jansen, J. Chipponi, & R. Bain. Hypotyrosinemia, Hypocystinemia, and Failure to Retain Nitrogen during Total Parenteral Nutrition of Cirrhotic Patients, Gastroenterology, 81:1025-35, 1981.
Young, V., M. Meguid, D. Meredith, & D. Bier. Recent Developments in Knowledge of Human Amino Acid Requirements, in Nitrogen Metabolism in Man, eds: J. Waterlow & J. Stephen, London: Applied Science Publishers, 1981.
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