Inherited disorders: Phenylketonuria

Last updated: Monday, March 13, 2017

Patients with phenylketonuria (PKU) are unable to break down the amino acid phenylalanine because the enzyme responsible, phenylalanine hydroxylase, is reduced or absent. This allows phenylalanine to build up to potentially toxic levels. Nerve cells in the brain are particularly sensitive to this amino acid and they can become damaged when exposed to high levels.

Symptoms of PKU can include behavioural difficulties, epilepsy, tremors, jerky movements, eczema, vomiting, and reduced pigment of the skin, hair and eyes. The main treatment is a controlled, low-protein diet to reduce intake of phenylalanine, but patients also have to take amino acid supplements.

Chemical structure for aspartame (left) and phenylalanine (right)

People with PKU must avoid the sweetener aspartame because it is converted to phenylalanine by the body. Aspartame is found in foods such as drinks, chewing gums, and also in some medicines. As a pharmacist you may be asked to check whether a medicine contains aspartame, or to advise on an aspartame-free alternative.