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Tyrosine Metabolism Disorders

By

Matt Demczko

, MD, Sidney Kimmel Medical College of Thomas Jefferson University

Last full review/revision Apr 2020| Content last modified Apr 2020
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Tyrosine is an amino acid that is a precursor of several neurotransmitters (eg, dopamine, norepinephrine, epinephrine), hormones (eg, thyroxine), and melanin; deficiencies of enzymes involved in its metabolism lead to a variety of syndromes.

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Transient tyrosinemia of the newborn

Most infants are asymptomatic, but some have lethargy and poor feeding.

Tyrosinemia is distinguished from PKU by elevated plasma tyrosine levels.

Most cases resolve spontaneously. Symptomatic patients should have dietary tyrosine restriction (2 g/kg/day) and be given vitamin C 200 to 400 mg orally once a day.

Tyrosinemia type I

Diagnosis of tyrosinemia type I is suggested by elevated plasma levels of tyrosine; it is confirmed by genetic testing or a high level of succinylacetone in plasma or urine and by low fumarylacetoacetate hydroxylase activity in blood cells or liver biopsy specimens. Treatment with nitisinone (NTBC) is effective in acute episodes and slows progression.

A diet low in phenylalanine and tyrosine is recommended. Liver transplantation is effective.

Tyrosinemia type II

Accumulation of tyrosine causes cutaneous and corneal ulcers. Secondary elevation of phenylalanine, though mild, may cause neuropsychiatric abnormalities if not treated.

Diagnosis of tyrosinemia type II is by elevation of tyrosine in plasma, absence of succinylacetone in plasma or urine, and genetic testing; measurement of decreased enzyme activity in liver biopsy is usually not needed.

This disorder is easily treated with mild to moderate restriction of dietary phenylalanine and tyrosine.

Alkaptonuria

This rare autosomal recessive Autosomal Recessive Genetic disorders determined by a single gene (Mendelian disorders) are easiest to analyze and the most well understood. If expression of a trait requires only one copy of a gene (one allele)... read more disorder is caused by homogentisic acid oxidase deficiency; homogentisic acid oxidation products accumulate in and darken skin, and crystals precipitate in joints.

The condition is usually diagnosed in adults and causes dark skin pigmentation (ochronosis) and arthritis. Urine turns dark when exposed to air because of oxidation products of homogentisic acid. Diagnosis of alkaptonuria is by finding elevated urinary levels of homogentisic acid (> 4 to 8 g/24 hours).

There is no effective treatment for alkaptonuria, but ascorbic acid 1 g orally once a day may diminish pigment deposition by increasing renal excretion of homogentisic acid.

Oculocutaneous albinism Albinism Oculocutaneous albinism is an inherited defect in melanin formation that causes diffuse hypopigmentation of the skin, hair, and eyes. Ocular albinism affects the eyes and usually not the skin... read more Albinism

Tyrosinase deficiency results in absence of skin and retinal pigmentation, causing a much increased risk of skin cancer and considerable vision loss. Nystagmus is often present, and photophobia is common.

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