Parkinsonism in phenylketonuria: a consequence of dopamine depletion?

Phenylketonuria (PKU) is caused by a deficiency or inactivity of the enzyme phenylalanine hydroxylase that converts phenylalanine (Phe) to tyrosine (Tyr). It has been proposed that a reduction of brain Tyr levels, as well as reduced activity of the key regulatory enzyme of dopamine (DA) synthesis tyrosine hydroxylase, leads to a depletion in DA activity in patients with PKU. We report a case of a 56-year-old woman with an intellectual disability due to late diagnosis of PKU and parkinsonism, with a modest clinical response to levodopa therapy.We hypothesize that the signs of parkinsonism might be caused by the depletion of DA activity in the brain. Clinicians should be alert on parkinsonian symptoms in patients with PKU, particularly in those treated with agents that negatively influence DA transmission.