Tyrosine Kinetics and Requirements during Total Parenteral Nutrition in the Neonatal Piglet: The Effect of Glycyl-L-Tyrosine Supplementation1

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Abstract
Tyrosine may be a conditionally indispensable amino acid in the neonate; however, the provision of aromatic amino acids to neonates receiving total parenteral nutrition (TPN) is complicated by the poor solubility of crystalline tyrosine. In the present study, we investigated tyrosine kinetics and requirements during TPN, when tyrosine was supplied as the soluble dipeptide, glycyl-L-tyrosine in a neonatal piglet model. Fifteen 3-d-old male Yorkshire piglets were fitted with extermal jugular and femoral catheters and randomized to one of five tyrosine intakes: 0.11, 0.31, 0.41, 0.51 and 0.71 g·kg-1·d-1. Total parenteral amino acid and energy intakes were 15.0 g·kg-1·d-1 and 1.1 MJ·kg-1·d-1, respectively. Piglets were maintained on TPN for 6 d, with nitrogen balance measured over the final 3 d of the study. On the final study day, tyrosine kinetics were measured during a 4-h primed-constant infusion of L-[114C]tyrosine. Nitrogen retention was 67% at the lowest tyrosine intake and increased significantly (p< 0.05) at intakes of 0.31 g·kg-1·d-1 and above(84, 86, 87, and 88% for intakes of 0.31, 0.41, 0.51, and 0.71 g·kg-1·d-1, respectively). Plasma tyrosine concentrations and tyrosine oxidation (expressed as either a percentage of the dose oxidized or when corrected for flux) were low and similar at the two lowest intakes, but increased significantly at the higher intakes. Two-phase regression analysis of the data (plasma tyrosine, tyrosine oxidation) yielded estimates of a mean tyrosine requirement of 0.31 and 0.35, respectively, with estimated safe intakes (upper 95% confidence limit) of 0.44 and 0.42 g·kg-1·d-1. The present work also indicates that oxidation techniques may be suitable for the estimation of amino acid requirements during TPN in the neonate.