PHENYLALANINE-HYDROXYLASE ACTIVITY IN PRETERM INFANTS - IS TYROSINE ACONDITIONALLY ESSENTIAL AMINO-ACID

To assess the production of the nonessential amino acid tyrosine in pr eterm infants, we estimated the activity of phenylalanine hydroxylase (PAH) in three groups of infants by measuring the conversion of phenyl alanine to tyrosine, using a model based on a primed constant 200-min intravenous infusion of [H-2(5)] phenylalanine. We determined the isot opic enrichments of [H-2(5)] phenylalanine and [H-2(4)] tyrosine by se lected-ion-monitoring gas chromatography-mass spectrometry (GCMS). Gro up 1 (n = 7, mean gestational age 29.7 +/- 1.5 wk, mean birth weight 1 .4 +/- 0.4 kg) was studied during the first 4 d of life before initiat ion of amino acid nutrition. Group 2 (n = 7, mean gestational age 29.7 +/- 1.5 wk, mean birth weight 1.4 +/- 0.4 kg) was studied at 4-6 d of life after receiving amino acid nutrition. Group 3 (n = 4, mean gesta tional age 28.5 +/- 0.9 wk, mean birth weight 1.1 +/- 0.1 kg) was stud ied during the first 4 d of life after receiving amino acid nutrition. Calculated from the observed enrichments, phenylalanine conversion to tyrosine was 5.9 +/- 2.6, 19.4 +/- 8.8, and 11 +/- 1.8 mu mol . kg(-1 ). h(-1) in groups 1, 2, and 3, respectively. The rate of conversion o f phenylalanine to tyrosine increased significantly after initiation o f amino acid nutrition. We conclude that preterm infants are capable o f converting phenylalanine to tyrosine. Provision of phenylalanine in the context of parenteral amino acid nutrition solution accelerated PA H conversion of phenylalanine to tyrosine, suggesting that the enzyme system is capable of responding normally to provision of substrate. Fu rther, under the circumstances of parenteral phenylalanine administrat ion, PAH conversion of phenylalanine to tyrosine occurred at a rate eq uivalent to estimated tyrosine needs in preterm infants. Therefore, we conclude that tyrosine is not a conditionally essential amino acid fo r preterm infants under these conditions.