Little is known about the amino acid (AA) biosynthetic capacity and requirements of premature infants. This study assessed the synthesis of seven biochemically nonessential AA from a universal precursor, glucose, in stable, parenterally fed, premature neonates. Seven infants (six boys, one girl) were studied at a mean age of 6.3 +/- 0.6 (SEM) days; mean gestational age was 29.7 +/- 1.3 (SEM) weeks, and mean birth weight was 1.222.8 +/- 176.5 (SEM) grams. All infants were parenterally fed a mixture of 7.5% to 12.5% dextrose and 2.2% Trophamine, with or without lipid. Mean caloric intake was 93 +/- 8.4 (SEM) kcal/kg/d, and total AA intake was standardized at 2.86 g/kg/d AA, plus supplemental cysteine (30 mg/g AA/d). Each infant received a 4-hour continuous, unprimed intravenous infusion of a stable isotope tracer of D-[(UC)-C-13] glucose (200 mg/kg). Blood samples were obtained before and at the end of the infusion. Conversion of the glucose tracer into seven biochemically nonessential AA (cysteine [Cys], proline [Pro], aspartate [Asp], serine [Ser], glutamate [Glu], alanine [Ala], and glycine [Gly]) was assessed by measuring their isotopic enrichment in plasma, using gas chromatography/mass spectrometry (GC/MS), and expressed as mole percent excess (MPE) (mean +/- SEM). The isotopic enrichment of plasma glucose was also measured using GC/MS. Free plasma AA concentrations (mean +/- SD) were measured using an automated amino acid analyzer. Mean MPE for M + 1, M + 2 and M + 3 Cys, and for M + 1 and M + 3 Pro were not significantly different from 0; M + 2 Pro barely achieved statistical significance (P =.048). In contrast, there was significant (P <.05) enrichment of all isotopomers of all the other test AA except M + 2 Ala (P =.06). The mean free plasma Cys concentration (29.2 +/- 13.4 mu mol/L) was five-fold lower than the published value for term infants (153 +/- 25.5 mu mol/L), despite supplementation. Detectable enrichment of all glucose isotopomers confirmed precursor incorporation into the glycolytic/gluconeogenic pathways; the M + 6 isotopomer (precursor) enrichment was 6.87 +/- 0.53 MPE. These results provide in vivo evidence to suggest that Cys and Pro are essential amino acids in parenterally fed, premature neonates. Copyright (C) 1995 By W.B. Saunders Company.
