Determination of amino- and amide-15N glutamine enrichment with tertiary butyldimethylsilyl derivatives.

We have developed a simple and rapid method for the selective synthesis of tetra-tertiarybutyldimethylsilyl (TBDMS) glutamine, which allows the simultaneous quantitation of glutamine (2-15N) and (5-15N) isotopic enrichment by selected ion monitoring (SIM) gas chromatographic/mass spectrometric analysis. The tetra-TBDMS glutamine (4S-gln) has an electron impact mass fragment at m/z 258 that contains only the amino-N and larger fragments (e.g., at m/z 545) that have both nitrogens. Derivatization with acetonitrile (ACN) and N-methyl-N-(tert-butyldimethylsilyl)trifluoroacetamide yields primarily tri-TBDMS glutamine (3S-gln) and quantities of 4S-gln too small to allow accurate SIM and tracer/tracee ratio determinations. However, when N,N-dimethylformamide, a more polar aprotic solvent, was substituted for ACN and the sample was heated for 30 min at 125 degrees C, greater than 80% of derivatized glutamine appeared as 4S-gln. Derivatized plasma samples that had been mixed with amide- and/or amino-15N glutamine and analyzed by SIM demonstrated strong agreement (r > or = 0.998, p = 0.0001) between theoretical and observed enrichment values for the 4S-gln fragments at m/z 258 and 545. Deamidation of glutamine to glutamate is negligible during sample processing and analysis. This procedure will facilitate the investigation of the specific sources and fates of glutamine amide and amino nitrogen as well as stable isotope studies involving amino acid transamination, ammonia clearance, urea production and other areas of nitrogen metabolism.