Tyrosine

Phenylketonuria (PKU)¹

PKU is an autosomal recessive inborn error of L-phenylalanine (Phe) metabolism, caused by deficiency in the enzyme phenylalanine hydroxylase (PAH), which converts Phe to tyrosine. PAH deficiency leads to accumulation of Phe in the blood and brain. Symptoms of untreated PKU include irreversible intellectual disability, microcephaly, motor deficits, seizures and developmental delay.

Dietary management of PKU involves a low Phe diet in combination with Phe-free protein substitutes (PS). As Phe is a precursor to L-tyrosine, L-tyrosine becomes an essential amino acid when Phe intake is restricted. L-tyrosine is important in the production of brain neurotransmitters, thyroxin and melanin skin pigments. As L-tyrosine is already added to Phe-free PS, additional L-tyrosine supplementation is not usually required in routine care.

Individuals who do not take the prescribed amount of PS may have an inadequate intake of L-tyrosine. It is recommended that supplemental L-tyrosine be provided if blood L-tyrosine concentrations are consistently below the normal range.