Differentiation and quantification of C1 and C2 (13)C-labeled glucose by tandem mass spectrometry.

The fragmentation patterns of various (13)C-labeled glucose molecules were analyzed by electrospray ionization tandem mass spectrometry. Derivatization of glucose to yield methylglucosamine makes the C-C bond between C1 and C2 a favored cleavage site. This is in contrast to underivatized glucose, which favorably undergoes loss of a fragment containing both C1 and C2. Based on the fragmentation pattern of methylglucoasmine, we developed a method to distinguish and quantify C1 and C2 (13)C-labeled glucose by derivatization with methylamine followed by multiple reaction monitoring scans in a Q-trap mass spectrometer. Fragment ion ratios in the tandem mass spectra showed an isotope effect with (13)C or deuterium labeling, so a "correction factor" was introduced to make the quantification more accurate. The current approach can be applied to individually monitor the metabolic origin and fate of C1 and C2 atoms in (13)C-labeled glucose. This method provides a new means of quantifying glucose isotopomers in metabolic studies.