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.
There are numerous disorders of phenylalanine and tyrosine metabolism (see the table). See also Approach to the Patient With a Suspected Inherited Disorder of Metabolism and testing for suspected inherited disorders of metabolism.
Transient Tyrosinemia of the Newborn
Transient immaturity of metabolic enzymes, particularly 4-hydroxyphenylpyruvic acid dioxygenase, sometimes leads to elevated plasma tyrosine levels (usually in preterm infants, particularly those receiving high-protein diets); metabolites may show up on routine neonatal screening for phenylketonuria (PKU).
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
This disorder is an autosomal recessive condition caused by deficiency of fumarylacetoacetate hydroxylase, an enzyme important for tyrosine metabolism.
Disease may manifest as fulminant liver failure in the neonatal period or as indolent subclinical hepatitis, painful peripheral neuropathy, and renal tubular disorders (eg, normal anion gap metabolic acidosis, hypophosphatemia, ) in older infants and children. Children who do not die of associated liver failure in infancy have a significant risk of developing hepatocellular carcinoma.
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.
1).
A diet low in phenylalanine and tyrosine is also recommended. Liver transplantation is effective.
Tyrosinemia type I reference
1. Chinsky JM, Singh R, Ficicioglu C, et al. Diagnosis and treatment of tyrosinemia type I: a US and Canadian consensus group review and recommendations. Genet Med. 2017;19(12). doi:10.1038/gim.2017.101
Tyrosinemia Type II
This rare autosomal recessive disorder is caused by tyrosine transaminase deficiency.
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 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. It causes dark patches of skin pigmentation in people of all skin tones (ochronosis). Alkaptonuria also causes 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).
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.
More Information
The following English-language resource may be useful. Please note that THE MANUAL is not responsible for the content of this resource.
Online Mendelian Inheritance in Man (OMIM) database: Complete gene, molecular, and chromosomal location information
