A new stable isotope tracer technique to assess human neonatal amino acid synthesis

The amino acid (AA) synthetic ability and requirements of human infants are undefined. A stable isotope tracer technique was employed in neonates to assess conversion of uniformly labeled 13C glucose into biochemically nonessential AA (NEAA). Ten neonates (5 males, 5 females) were studied at a mean age of 7 +/- 2.0 (SEM) days. The mean gestational age was 35.5 +/- 1.1 weeks, and the mean weight at time of study was 2,191 +/- 181 g. Six infants were fed enterally, and four received only intravenous 10% dextrose (D10W). Blood samples were obtained before, and 30, 60, and 120 minutes after an orogastric bolus of D-[U-13C]glucose (100 mg/kg). The conversion of glucose carbon into seven NEAA was assessed by measuring their isotopic enrichments in plasma, using gas chromatography/mass spectrometry (GC/MS), and was expressed as mole percent excess (MPE), with detectable MPE defined as > or = 0.2. The isotopic enrichment of plasma glucose also was measured using GC/MS. Free plasma AA concentrations were assayed using an automated AA analyzer and expressed in micromoles per liter. The mean glucose enrichment was 9.33 +/- 1.8 MPE (range, 5.82 to 13.48). Detectable 13C-labeling of the NEAA was observed as follows: Glu in 100% of infants; Gly, 100%; Ala, 90%; Ser, 80%; Asp, 70%; Cys, 60%; and Pro, 60%. Detectable Pro enrichment was observed in none of three premature infants on D10W. Free plasma Cys concentration was markedly lower than normal (19.8 v 86 mumol/L).(ABSTRACT TRUNCATED AT 250 WORDS)