Regulation of pool sizes and turnover rates of amino acids in humans: 15N-glycine and 15N-alanine single-dose experiments using gas chromatography-mass spectrometry analysis.

Gas chromatography--mass spectrometry (GCMS) of plasma amino acid derivatives has been used to determine directly the 15N-enrichment of plasma glycine and alanine in ten volunteers at various metabolic states. Isotope-enrichment time-decay curves of plasma glycine and alanine, following a single intravenous dose of 15N-glycine or 15N-L alanine were obtained and provide an estimate of the extracellular compartment. Relatively narrow ranges were obtained for the glycine pool (7.7--11.8 micromole/100 g body wt), rate constants of transport (3.7--4.2 hr-1) and flux (28--43 micromole hr-1/100 g body wt) in the postabsorptive statcine pool (7.7--11.8 mumole/100 g body wt), rate constants of transport (3.7--4.2 hr-1) and flux (28--43 mumole hr-1/100 g body wt) in the postabsorptive state. In postprandial humans, pool sizes showed only a modest variation whereas the rate constants of transport of glycine and alanine were significantly lower. The plasma 15N-glycine and 15N-alanine isotope-enrichment time-decay curves over the first hour following a single i.v. dose of 15-amino acid represent mostly the hepatic uptake of glycine and alanine from the extracellular pool. The results presented in this study establish the stable isotope GCMS method as a more accurate, more convenient, and safe alternative to the use of radioactive labeled amino acids in studies of amino acid metabolism in human subjects.