What is PKU?

PKU stands for Phenylketonuria (PHE-NYL-KE-TON-URIA). Phenylketonuria is an inherited disorder. About one in every 15,000 infants born in the United States has PKU. People who are born with PKU are normal in every way except to stay healthy they must follow a strict diet which limits phenylalanine, a common part of most food. People with PKU have an inactive liver enzyme. (See figure 1)

Phenylalanine is only one of the many amino acids which are joined together to form proteins. Normally, when a person eats foods containing protein, their body uses the amino acids from that protein for growth and repair of body tissues. Often we eat amino acids in excess of the body's needs. These excess amino acids are chemically changed by enzymes into other compounds or used for energy. Since individuals with PKU are missing the enzyme for normal phenylalanine break down, the excess eaten in foods accumulates in the blood and begins to damage the brain.

If blood phenylalanine levels stay too high for a long time, the damage to the developing brain is severe and irreversible. The harmful effects of PKU can be wprevented if a diet low in phenylalanine is started in early infancy and maintained throughout life. The phenylalanine restricted diet is the only way to bring blood phenylalanine levels down to a safe level. At these safe levels, that is less than 10 mg/dL, the brain can function normally and the person with PKU can learn easily and have stable emotions.

phenylalanine is converted to tyrosine by an enzyme called phenylalanine hydroxylas
Figure 1

Normally, phenylalanine is converted to tyrosine by an enzyme called phenylalanine hydroxylase. However, in individuals with PKU, this enzyme is not present and results in a damaging build-up of phenylalanine in the body.


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