Determination of low 13C-glutamine enrichments using gas chromatography-combustion-isotope ratio mass spectrometry.

Glutamine is an essential fuel for tissues with high rates of cell replication, such as enterocytes and lymphocytes. Infusion of 13C-labeled glutamine tracers allows for measurement of the rates of production, utilization and oxidation of glutamine's carbon skeleton in vivo. The use of this tracer, however, has been limited by its high cost and/or the difficulty is measuring low enrichments in biological fluids using conventional gas chromatography-mass spectrometry (GC/MS) techniques. We have developed a method using gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) that allows for the determination of low 13C enrichments (down to 0.06 mol.% excess) with a precision of 2% or better, and a within-day and between-day variability better than 5%, in plasma free glutamine. The method was applied to measuring the incorporation of 13C in plasma glutamine over the course of infusion of 13C-labeled acetate in a human subject.