EVIDENCE THAT PHENYLALANINE HYDROXYLATION RATES ARE OVERESTIMATED IN NEONATAL SUBJECTS RECEIVING TOTAL PARENTERAL-NUTRITION WITH A HIGH PHENYLALANINE CONTENT

Recent publications have indicated that the parenterally fed neonate h as a substantial ability to hydroxylate phenylalanine. Examination of these data suggests that, at high phenylalanine intakes, estimated rat es of hydroxylation exceed rates of intake. This implies significant n et tissue breakdown. However, the quantitative validity of the estimat es of phenylalanine hydroxylation cannot be assessed without nitrogen balance data. We have recently developed a parenterally fed neonatal p iglet model and have used this to study aromatic amino acid metabolism in piglets fed different amino acid solutions. Reappraisal of the dat a from these studies has allowed us to estimate both phenylalanine hyd roxylation and tissue protein accretion. Piglets were parenterally fed Vamin (292 mu mol of Phe.kg(-1).h(-1), 26 mu mol of Tyr.kg(-1).h(-1)) , Vaminolact + Phe (VLP, 277 mu mol of Phe.kg(-1).h(-1)), 26 mu mol Ty r.kg(-1).h(-1), or Vaminolact + glycyl-L-tyrosine (VLGT, 152 mu mol of Phe.kg(-1).h(-1), 159 mu mol of Tyr.kg(-1).h(-1)) for 8 d. Nitrogen b alance was measured over the last 5 study d, and aromatic amino acid k inetics were determined using a primed continuous infusion of L-[1-C-1 4]phenylalanine on d 8. Average body protein gain, derived from nitrog en balance, was 11 g.kg(-1).d(-1). For the Vamin and VLP groups, the r ates of phenylalanine hydroxylation were estimated to be 139 and 90% o f intake, respectively. However, phenylalanine hydroxylation was only 16% of intake for the VLGT group. In view of the tissue protein accret ion data, it appears that the rate of phenylalanine hydroxylation may be overestimated in neonates fed high phenylalanine parenteral nutriti on. The extent to which the parenterally fed neonate can adapt to a hi gh phenylalanine intake, by increasing the rate of phenylalanine hydro xylation, remains to be determined.