Kinetics of Phenylalanine Disappearance after Intravenous Load in Phenylketonuria and Its Genetic Variants

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Intravenous phenylalanine loading tests were performed in 22 children affected by different types of hyperphenylalaninemia: six cases of classical phenylketonuria (PKU), six cases of atypical PKU, five cases of mild permanent hyperphenylalaninemia, three cases of “transient” PKU, and two cases of the “new variant with progressive neurological illness resistant to dietary treatment,” one of them suffering from DHP reductase deficiency and the other being affected by a defect in biopterin synthesis. A 0.06 M L-phenylalanine (500 ml/m2) solution in NaCl was perfused during 3 hr so that a plasma level of about 2-3 µmol·ml-1 was obtained. Results were compared to those obtained in five controls.


The demonstration of a zero-order kinetics of phenylalanine disappearance in transient PKU suggests the accumulation of an intermediate released during the hydroxylation process, which might occur from a phenylalanine hydroxylase-stimulating protein defect.

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