Phenylketonuria (PKU) is a rare metabolic disorder caused by a deficiency in the production of the hepatic (liver) enzyme phenylalanine hydroxylase (PAH).
PKU is the most serious form of a class of diseases referred to as hyperphenylalaninemia, all of which involve above normal (elevated) levels of phenylalanine in the blood. The primary symptom of untreated PKU, mental retardation, is the result of consuming foods that contain the amino acid phenylalanine, which is toxic to brain tissue.
PKU is an inherited, autosomal recessive disorder. It is the most common genetic disease involving amino acid metabolism. As of 2004, PKU was incurable, but early, effective treatment can prevent the development of serious mental incapacity.
PKU is caused by the liver's inability to produce a particular type of PAH enzyme. This enzyme converts (metabolizes) the amino acid called phenylalanine into another amino acid, tyrosine. This is the only role of PAH in the body. A lack of PAH results in the buildup of abnormally high phenylalanine concentrations (or levels) in the blood and brain. Above normal levels of phenylalanine are toxic to the cells that make up the nervous system and cause irreversible abnormalities in brain structure and function in PKU patients. Phenylalanine is a type of teratogen (any substance or organism that can cause birth defects in a developing fetus).
The liver is the body's chief protein-processing center. Proteins are one of the major food nutrients. They are generally very large molecules composed of strings of smaller building blocks or molecules called amino acids. About twenty amino acids exist in nature. The body breaks down proteins from food into individual amino acids and then reassembles them into human proteins. Proteins are needed for growth and repair of cells and tissues and are the key components of enzymes, antibodies, and other essential substances.