D. Matthews, J. Conway, V. Young
Sep 1, 1981
Citations
1
Influential Citations
71
Citations
Quality indicators
Journal
Metabolism: clinical and experimental
Abstract
Abstract [15N]Glycine is a commonly used tracer for measuring human whole body protein metabolism, even though the metabolism of the glycine N is not well defined for man. In this work, glycine N metabolism was studied by measuring the transfer of 15N from [15N]glycine to other plasma amino acids and urea. [15N]Glycine was given orally, every 4 hr for 60 hr to 6 young adult men, maintained on a 1.5 g kg−1d−1 egg protein diet. The 15N enrichments of plasma glycine, serine, and urea were measured during the entire 60 hr period. Although both glycine and serine 15N enrichments rose throughout the period (95% of plateau being reached by 33 hr), serine 15N enrichment was about half that of glycine from the first blood sample (4 hr) on. Plasma urea 15N reached only 88% of its predicted plateau value by 50 hr. The 15N enrichments in other plasma amino acids were determined in the plasma samples taken after isotopic steady state had been reached for glycine and serine (40–60 hr). The 15N enrichments relative to plasma [15N]glycine were serine 54%, urea 20%, glutamine/glutamate amino-N 15%, alanine 7%, leucine, isoleucine, valine, ornithine, proline, and methionine 3%–8%. Insignificant 15N was found in the remaining plasma amino acids. When the size of the various free amino acid pools were considered, 97% of the administered 15N in free amino acids was estimated to be in glycine, serine, glutamine/glutamate, and alanine amino-N. These data illustrate the rapid interconversion of glycine and serine in man and show that 15N from these amino acids can enter the major transaminating pool of amino acids via glutamate and alanine.