Abstract
Hydroxylation of phenylalanine to tyrosine is the first and rate-limiting step in phenylalanine catabolism. Currently, there are data on the rate of phenylalanine hydroxylation in infants and adults but not in healthy children. Thus, the aim of the study reported here was to measure the rate of phenylalanine hydroxylation and oxidation in healthy school-aged children both when receiving diets with and without tyrosine. In addition, hydroxylation rates calculated from the isotopic enrichments of amino acids in plasma and in very LDL apoB-100 were compared. Eight healthy 6- to 10-y-old children were studied while receiving a control and again while receiving a tyrosine-free diet. Phenylalanine flux, hydroxylation, and oxidation were determined by a standard tracer protocol using oral administration of 13C-phenylalanine and 2H2-tyrosine for 6 h. Phenylalanine hydroxylation rate of children fed a diet devoid of tyrosine was greater than that of children fed a diet containing tyrosine (40.25 ± 5.48 versus 29.55 ± 5.35 μmol · kg−1 · h−1; p < 0.01). Phenylalanine oxidation was not different from phenylalanine hydroxylation regardless of dietary tyrosine intake, suggesting that phenylalanine converted to tyrosine was mainly oxidized. In conclusion, healthy children are capable of converting phenylalanine to tyrosine, but the need for tyrosine cannot be met by providing extra phenylalanine.
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Abbreviations
- apoB:
-
apolipoprotein B
- EAR:
-
estimated average requirement
- Phe:
-
phenylalanine
- RMR:
-
resting metabolic rate
- Tyr:
-
tyrosine
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Acknowledgements
We thank Cynthia Boutte for recruiting subjects and coordinating the study, Ann McMeans for designing the diet, the research nurses and kitchen staff at the MRU of the CNRC for their dedicated subject care, the staff at the Calorimeter Core Laboratory for assisting with the room respiratory calorimetry, and Rebecca Newsom for administration work. We also thank Margaret Frazer, Melanie Delrosario, Shaji Chacko, and Dan Donaldson for technical assistance. Special thanks to the subjects who participated in this study and their parents.
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Supported by a USDA/ARS Cooperative Agreement.This work is a publication of the US Department of Agriculture/Agricultural Research Service Children's Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston, Texas.The contents of this publication do not necessarily reflect the view or policies of the US Department of Agriculture, nor does mention of trade names, commercial products, or organizations imply endorsement by the US Government.
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Hsu, J., Jahoor, F., Butte, N. et al. Rate of Phenylalanine Hydroxylation in Healthy School-Aged Children. Pediatr Res 69, 341–346 (2011). https://doi.org/10.1203/PDR.0b013e31820bbdcd
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DOI: https://doi.org/10.1203/PDR.0b013e31820bbdcd
