Relationship of Serum Phenylalanine Levels and Ability of Phenylketonurics to Hydroxylate Tryptophan

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Abstract
Summary 1. Following a decrease in serum phenylalanine levels (induced by feeding of a low-phenylalanine diet) phenylketonurics excrete increased amounts of 5-hydroxyindoleacetic acid. 2. The ratio of urinary 5-hydroxyindoleacetic acid to creatinine is about 5-fold greater for young phenylketonuric children, fed a low-phenylalanine diet for 4–10 months, than for untreated phenylketonurics. 3. Oral administration of L-tryptophan to untreated phenylketonuric was followed by increase in 5-hydroxyindoleacetic acid excretion. 4. Following oral administration of 5-hydroxytryptophan, 10–15% was excreted as urinary 5-hydroxyindoleacetic acid by an otherwise untreated phenylketonuric.