Tryptophan metabolism: utility of plasmatic assay in phenylketonuria, a study in 6 adult patients.
Phenylketonuria (PKU, OMIM 261600) is an autosomal recessive inborn error of metabolism caused by a deficiency of the hepatic enzyme phenylalanine hydroxylase (PAH; EC 1.14.16.1). If untreated, the disease leads to an important intellectual disability (IQ <50). Although many facts are common between phenylalanine (Phe) and tryptophan (Trp) metabolism, little is known about Trp metabolism modification in PKU. Our aim was to evaluate the modifications of Trp metabolism in a phenylketonuric population. A monocentric study was conducted between October 2016 and March 2017. Every phenylketonuric fasting adults were included during their annual follow up. For each patient, 9 analytes of Trp metabolism were quantified in peripheral blood using liquid chromatography coupled with tandem mass spectrometry. Mann and Whitney tests (p <0.05) were carried out in StatView 5.0 software. A total of 6 PKU patients were studied. Significant modification of Trp metabolism was shown. Indeed, three analytes, i.e. tryptophan, kynurenine and 3-hydroxykynurenic acid, were significantly lower in phenylketonuric than in healthy population (p-value <0.05), without known confounding factors. This study shows a significant modification of Trp metabolism in peripheral blood of phenylketonuric patients. Nevertheless, more investigations are necessary to confirm the modification of Trp metabolism in PKU and to determine how this metabolism is involved in neurological symptoms.