also called phenylpyruvic oligophrenia, a hereditary enzymopathy in which the absence of phenylalanine hydroxylase or a marked decrease in its activity result in defective amino acid metabolism. The disorder, described in 1934 by the Norwegian scientist A. Fölling, became known as Fölling’s disease.
PKU is transmitted as an autosomal recessive trait, and its incidence in the newborn is one per 10,000–15,000. In phenylketonuria the level of activity of phenylalanine hydroxylase is reduced to approximately 5 percent, which causes a disruption in the metabolism of phenylalanine and, consequently, of other amino acids as well, such as tyrosine and tryptophan. The result is an accumulation of intermediate metabolic products—for example, phenylethylamine and phenylpyruvic acid—and a deficiency of metabolites needed for the normal functioning of the organism. Specifically, a deficiency of such nervous system mediators as epinephrine, norepinephrine, and serotonin is the probable cause of mental disorders. PKU thus involves a set of interrelated metabolic disorders, the initial enzymopathy leading to other metabolic disturbances.
The most common manifestation of PKU is marked mental deficiency, as in idiocy or imbecility. The condition may be quickly identified in the first few days of life by microbiological or biochemical diagnostic methods, in which indicators are used to determine the level of pyruvic acid in the urine (Fölling’s test). Treatment generally consists of a special diet, with sharply restricted intake of foods containing phenylalanine.