Definition:

Phenylketonuria (PKU) is one of the most common inborn errors of metabolism marked by a deficiency of the hepatic enzyme phenylalanine hydroxylase (PAH), leading to a toxic accumulation of phenylalanine in the blood and multiple tissues and potentially to intellectual disability, delayed speech, seizures, and behavior abnormalities. Deficiency of tetrahydrobiopterin (BH4), the cofactor of PAH, is caused by defects of the enzymes in pterin biosynthesis. Because BH4 is also a cofactor of tyrosine hydroxylase and tryptophan hydroxylase in neurotransmitter synthesis, BH4-deficient hyperphenylalaninemia is characterized by neurotransmitter deficiencies.