Aromatic acids in urine of healthy infants, persistent hyperphenylalaninemia, and phenylketonuria, before and after phenylalanine load

Abstract

Aromatic acids in urine were studied by gas chromatography and mass spectrometry in 3 premature and 7 full term healthy infants, in 2 patients with persistent hyperphenylalaninemia, and in 11 patients with phenylketonuria. Eleven aromatic acids were determined quantitatively. On a free diet, patients with phenylketonuria excreted large amounts of phenylacetic, mandelic, phenyllactic, o-OH- phenylacetic, and phenylpyruvic acids (“phenylketonuria metabolites”), whereas the two patients with persistent hyperphenylalaninemia showed only a slightly abnormal excretion of these compounds. No or only very small amounts of phenylketonuria metabolites were found in healthy infants on normal diet, as well as in patients with persistent hyperphenylalaninemia or phenylketonuria on low phenylalanine diet. After an oral L-phenylalanine load (100 mg/kg) no or only a slight increase of phenylketonuria metabolites was observed in the urine during the subsequent 24 hr in healthy infants on normal diet, as well as in the two patients with persistent hyperphenylalaninemia on low phenylalanine diet; in contrast, the concentration of these metabolites increased markedly in patients with phenylketonuria on low phenylalanine diet. This divergent response of the aromatic acids in urine to an oral phenylalanine load administered during a low phenylalanine diet probably represents a useful criterion for the differential diagnosis of these two conditions. The quantitative determination of the aromatic acids in urine before and after phenylalanine load can be considered as an indirect measurement of phenylalanine hydroxylase activity. © 1974 International Pediatric Research Foundation, Inc.