These papers present studies concerning the enzymatic defect in phenylpyruvic oligophrenia. Patients with this disorder have a markedly reduced ability to convert phenylalanine to tyrosine. The enzyme system which catalyzes this conversion has been demonstrated in the liver but has been found lacking in livers from patients with phenylpyruvic oligophrenia. This enzyme has been separated into two protein fractions: Fraction I is present only in the liver and Fraction II is found in almost all tissues, including brain. The present studies indicate that it is Fraction I which is lacking in the liver of patients with phenylpyruvic oligophrenia. The amount of Fraction II appears to be the same in the liver from patients with phenylpyruvic oligophrenia as in normals. The conversion of phenylalanine to tyrosine takes place only in the liver and it is suggested by these studies that the absence of Fraction I in the liver is the primary defect in phenylpyruvic oligophrenia. The disturbances in the brain in these patients is considered to result from the accumulation of phenylalanine and other intermediary metabolites, due to lack of Fraction I in the liver, rather than an enzymatic defect in the brain or other tissues.
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