COMPARTMENTATION OF ENDOGENOUSLY SYNTHESIZED AMINO-ACIDS IN NEONATES

The conversion of D-[U-C-13]glucose to proline (Pro), aspartate (Asp), and cysteine (Cys) is limited in premature neonates, implying that th ese amino acids (AA) are conditionally essential. This study was perfo rmed to determine whether these findings resulted from an insufficient precursor dose or intracellular compartmentation of newly synthesized amino acids, rather than inadequate synthesis. In the first phase of this study, seven total parenteral nutrition-fed, premature neonates r eceived IV D-[(UC)-C-13]glucose at 5 mg/kg/min for 4 hr. In the second phase, a separate cohort of eight patients received an identical infu sion. Blood was obtained before and at the end of the infusion. Isotop ic enrichments of the free plasma AA and glucose were measured using g as chromatography/mass spectrometry in both studies. In phase 2, the i sotopic enrichments of the AA bound to the hepatically synthesized pro teins, fibrinogen and VLDL-apolipoprotein B-100 (apo B-100), were meas ured. In phase 1, despite a glucose precursor enrichment greater than 66%, Pro, Asp, and Cys remained the least enriched of all amino acids studied (P < 0.05). Asp, but not Pro, demonstrated very high enrichmen ts in apo B-100 (P < 0.001), reflecting distinct intracellular compart mentation. We conclude that the limited conversion of D-[U-C-13]glucos e to Pro, Asp, and Cys did not result from low precursor glucose enric hment and that there is evidence of Asp compartmentation (intracellula r) in premature neonates. However, the low Pro enrichment in the free plasma AA pool and the absence of intracellular Pro compartmentation s uggest that Pro may be a conditionally essential AA for premature neon ates. (C) 1996 Academic Press, Inc.