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Gas liquid chromatography–mass spectrometry of trifluoroacetyl n-butyl ester derivatives of plasma amino acids has been used to determine directly the 15N-enrichment of plasma glycine in rabbits, following the intravenous administration of a single dose of 15N-glycine. The isotope enrichment time decay curves of plasma glycine were linear over the course of the measurements (5–45 min). Glycine pool sizes and turnover rate constants were obtained from the intercepts and slopes of non weighted least-squares lines, describing the isotope enrichment time decay curve. In fed animals, glycine pool sizes showed only modest variations over a 24 h period, while glycine turnover rate constants showed a statistically significant daily variation with a minimum at 10 PM. In fasted animals, the glycine pool sizes decreased and turnover rate constants increased with the duration of the fast. The turnover rate constants appeared to represent the utilization of glycine, as an energy or nitrogen source in the liver, rather than uptake of glycine into the free amino acid pool of muscle. These data indicate that physiologically significant variations in amino acid dynamics can be obtained from single dose tracer experiments, when GC-MS is used to determine directly the isotope enrichments of plasma amino acids. |
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