The majority of the Phe we consume in our diets is metabolized (converted) in the liver by the PAH enzyme. The liver uses the PAH enzyme, along with a helper (or cofactor) called BH4, to convert some of the Phe we eat into another amino acid known as tyrosine.
Tyrosine is a building block for many proteins and compounds including several important brain chemicals (called neurotransmitters) such as dopamine. Neurotransmitters help brain cells communicate and they have a role in influencing our mood, as well as the way we think and act.
In people with PKU, the PAH enzyme is either missing or not working properly. As a result, not enough Phe gets converted to tyrosine, causing a buildup of Phe in the blood.
When someone with PKU eats high-protein foods, such as chicken, meat, eggs, dairy, nuts, grains, and legumes, his or her body cannot break down the Phe, and Phe levels build up in the blood, which can affect thinking, mood, and behavior.