We studied the components of the hepatic phenylalanine hydroxylating system in a child with phenylketonuria who showed substantial neurologic impairment despite early dietary control of elevated blood phenylalanine levels. Phenylalanine hydroxylase, dihydropteridine reductase and dihydrofolate reductase activities were normal. In contrast the level of hydroxylation cofactor, tetrahydrobiopterin, in liver was only 10 per cent of normal. In addition to this hepatic deficiency, serum and urinary levels of biopterin-like compounds were low, and the serum biopterin did not increase in response to a phenylalanine load as it does in normal and phenylketonuric subjects. The phenylalanine hydroxylase activity in this child, as determined by an in vivo tritium-release assay, was 2.3 per cent of the normal value. These results indicate that the child suffers from a variant form of phenylketonuria--a deficiency of a functional phenylalanine hydroxylating system secondary to a defect in biosynthesis of biopterin.
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