PHENYLALANINE AND TYROSINE METABOLISM IN NEONATES RECEIVING PARENTERAL-NUTRITION DIFFERING IN PATTERN OF AMINO-ACIDS

Tyrosine is considered to be an indispensable dietary amino acid in th e neonate, yet achieving adequate parenteral tyrosine intake is diffic ult due to its poor solubility. Increasing the supply of phenylalanine is the most common means of compensating for low tyrosine levels. Unf ortunately, plasma phenylalanine concentrations are sometimes elevated in infants receiving high phenylalanine intake. This led us to study the phenylalanine and tyrosine metabolism in 16 neonates randomized to receive total parenteral nutrition with either a high or a moderate p henylalanine-containing amino acid solution. A primed, 24-h continuous stable isotope infusion of L-[1-C-13]phenylalanine and L-[3,3-H-2(2)] tyrosine was given to enable the measurement of phenylalanine and tyro sine kinetics. Results demonstrated that 1) phenylalanine hydroxylatio n was significantly greater in infants receiving high phenylalanine, 2 ) phenylalanine oxidation and percent dose oxidized was also significa ntly greater in infants receiving high phenylalanine, 3) apparent phen ylalanine retention was greater in neonates receiving high phenylalani ne, and 4) alternate catabolites of phenylalanine and tyrosine metabol ism were significantly greater in infants receiving high phenylalanine compared with moderate phenylalanine. We conclude that neonates respo nd to increased parenteral phenylalanine intake by increasing their hy droxylation and oxidation rates. The greater oxidation of phenylalanin e in infants receiving high phenylalanine in conjunction with the urin ary excretion of alternate catabolites of phenylalanine and tyrosine s uggests that the high phenylalanine intake may be in excess of needs. However, the lower apparent phenylalanine retention observed in infant s receiving moderate phenylalanine suggests that the total aromatic am ino acid level of moderate phenylalanine may be deficient for neonatal needs.