Determination of [amide-15N]glutamine in plasma with gas chromatography-mass spectrometry

A gas chromatographic-mass spectrometric method is described for determination of isotopic enrichment in [amide-15N]glutamine. The method involves the isolation of glutamine with an anionic exchange resin and the enzymatic hydrolysis of glutamine with glutaminase. The ammonia derived from the amide group is converted enzymatically to glutamic acid and isotopic enrichment in the latter compound is determined following formation of the N-TFA-glutaminate derivative. The method gave a linear result for [amide-15N]glutamine in the 0.5--21.5 atom % excess range with a mean coefficient of variance of 7.8%. Determination of plasma glutamine by isotope dilution showed a mean recovery of 94%. The appearance of plasma [amide-15N]glutamine was measured in a healthy adult following the intravenous administration of 15NH4Cl (350 mumol). A peak enrichment of 12 atom % excess was observed by 3.5 min.