Abstract
ABSTRACT: The cause of hyperphenylalaninemia in sick preterm infants has yet to be determined; one reason may be reduced tolerance to phenylalanine as a consequence of immaturity of phenylalanine hydroxylase. Phenylalanine metabolism was studied in vivo in 23 ventilated preterm infants of gestational age 23 to 36 wk within the first 6 d of life using a continuous i.v. infusion of the stable isotopelabeled amino acids [2H5]phenylalanine, [2H4] tyrosine, and [2H2]tyrosine. Phenylalanine hydroxylation was calculated from two different methods. In the first method, used in all 23 infants receiving glucose and in seven of these infants who subsequently received parenteral nutrition, phenylalanine hydroxylation was calculated from the plasma enrichments of [2H5]phenylalanine and [2H4]tyrosine and from the molar ratio of tyrosine to phenylalanine in mammalian tissue protein. In this instance, the mean hydroxylation was 16.0 (SD 10.9) and 48.4 (SD 14.9) µmol/kg/h, which was 17.3% (SD 8.4%) and 33.2% (SD 9.8%) of the total phenylalanine flux for infants receiving glucose and parenteral nutrition, respectively. Additionally, in six infants receiving glucose, hydroxylation was calculated from the measured phenylalanine (2H5), independent tyrosine (2H2) fluxes, and the plasma enrichments of (2H5) phenylalanine and its hydroxylation product [2H4]tyrosine. In this case, hydroxylation was 20.5 (SD 13.0) (µmol/kg/h, which represented 22.3% (SD 9.8%) of the phenylalanine flux. In the same six infants, phenylalanine hydroxylation derived using the first method was 22.2 (SD 13.1) µmol/kg/h, 23.6% (SD 9.9%) of the total phenylalanine flux. The close agreement between phenylalanine hydroxylation calculated from the enrichment of plasma with [2H2]tyrosine and estimated from the proportion of phenylalanine to tyrosine in body protein confirms that the independent measurement of tyrosine flux by a constant infusion of [2H2]tyrosine is not routinely required in the measurement of phenylalanine hydroxylation in preterm infants. These results do not support the hypothesis that phenylalanine hydroxylase activity is low in preterm infants.
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Shortland, G., Walter, J., Fleming, P. et al. Phenylalanine Kinetics in Sick Preterm Neonates with Respiratory Distress Syndrome. Pediatr Res 36, 713–718 (1994). https://doi.org/10.1203/00006450-199412000-00005
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DOI: https://doi.org/10.1203/00006450-199412000-00005