A child with phenylketonuria had normal phenylalanine hydroxylase activity in vitro. In addition, all known components of the phenylalanine hydroxylating system were within the normal range. Despite early treatment with a phenylalanine-restricted diet, the patient developed severe neurological symptoms. Although the primary molecular defect in this child is not known, there are indications that the defect lead to disturbances in phenylalanine metabolism and in the biosynthesis of L-dopa and L-5-hydroxytryptophan. The administration of these two precursors of neurotransmitters brought about a notable improvement in the patient's neurological symptoms.
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