Human fetal amino acid metabolism at term gestation.
BACKGROUND
Knowledge on human fetal amino acid (AA) metabolism, largely lacking thus far, is pivotal in improving nutritional strategies for prematurely born infants. Phenylalanine kinetics is of special interest as is debate as to whether neonates will adequately hydroxylate phenylalanine to the semiessential AA tyrosine.


OBJECTIVE
Our aim was to quantify human fetal phenylalanine and tyrosine metabolism.


METHODS
Eight fasted, healthy, pregnant women undergoing elective cesarean delivery at term received primed continuous stable-isotope infusions of [1-(13)C]phenylalanine and [ring-D(4)]tyrosine starting before surgery. Umbilical blood flow was measured by ultrasound. Maternal and umbilical cord blood was collected and analyzed by gas chromatography-mass spectrometry for phenylalanine and tyrosine enrichments and concentrations. Data are expressed as medians (25th-75th percentile).


RESULTS
Women were in a catabolic state for which net fetal AA uptake was responsible for > or = 25%. Maternal and fetal hydroxylation rates were 2.6 (2.2-2.9) and 7.5 (6.2-15.5) micromol phenylalanine/(kg . h), respectively. Fetal protein synthesis rates were higher than breakdown rates: 92 (84-116) and 73 (68-87) micromol phenylalanine/(kg . h), respectively, which indicated an anabolic state. The median metabolized fraction of available phenylalanine and tyrosine in the fetus was <20% for both AAs.


CONCLUSIONS
At term gestation, fetuses still show considerable net AA uptake and AA accretion [converted to tissue approximately 12 g/(kg . d)]. The low metabolic uptake (AA usage) implies a very large nutritional reserve capacity of nutrients delivered through the umbilical cord. Fetuses at term are quite capable of hydroxylating phenylalanine to tyrosine.
van den Akker
Chris H P
CH
Department of Pediatrics, Division of Neonatology, Erasmus MC-Sophia Children's Hospital, Rotterdam, Netherlands.
Schierbeek
Henk
H
Dorst
Kristien Y
KY
Schoonderwaldt
Ernst M
EM
Vermes
Andras
A
Duvekot
Johannes J
JJ
Steegers
Eric A P
EA
van Goudoever
Johannes B
JB
D000328:Adult; D001783:Blood Flow Velocity; D002247:Carbon Isotopes; D002585:Cesarean Section; D005260:Female; D005312:Fetal Blood / Q000000981:diagnostic imaging / Q000378:metabolism*; D005326:Fetal Proteins / Q000096:biosynthesis*; D005333:Fetus / Q000378:metabolism*; D008401:Gas Chromatography-Mass Spectrometry / Q000379:methods; D005865:Gestational Age; D006801:Humans; D006900:Hydroxylation; D007223:Infant; D007231:Infant, Newborn / Q000378:metabolism; D008297:Male; D009751:Nutritional Requirements; D010649:Phenylalanine / Q000378:metabolism* / Q000493:pharmacokinetics; D011247:Pregnancy; D039401:Prenatal Nutritional Physiological Phenomena / Q000502:physiology; D012039:Regional Blood Flow; D014443:Tyrosine / Q000378:metabolism* / Q000493:pharmacokinetics; D014463:Ultrasonography
D002247:Carbon Isotopes; D005326:Fetal Proteins; D014443:Tyrosine; D010649:Phenylalanine