Essential' amino acids can only be obtained from the food we eat as our body does not normally produce them. In 'classic PKU', the enzyme that breaks down phenylalanine phenylalanine hydroxylase, is completely or nearly completely deficient. This enzyme normally converts phenylalanine to another amino acid, tyrosine. Without this enzyme, phenylalanine and its' breakdown chemicals from other enzyme routes, accumulate in the blood and body tissues. These other disorders may be caused by a partial deficiency of the phenylalanine breakdown enzyme or the lack of another enzyme important to the processing of this amino acid.
Levels are somewhat less in the other disorders of hyperphenylalaninemia.
Chronically high levels of phenylalanine and some of its breakdown products can cause significant brain problems. Classic PKU is the most common cause of high levels of phenylalanine in the blood and will be the primary focus of this topic sheet. Classic PKU and the other causes of hyperphenylalaninemia affect about one of every 10,000 to 20,000 Caucasian or Oriental births. The incidence in African Americans is far less. These disorders are equally frequent in males and females.