A convenient LC-MS method for assessment of glucose kinetics in vivo with D-[13C6]glucose as a tracer

The isotope-labeled intravenous glucose tolerance test (IVGTT) combined with computer modeling is widely used to derive parameters related to glucose metabolism in vivo. Most of these methods involve use of either (2)H(2)-labeled or (13)C(1)-labeled D-glucose as a tracer with GC-MS to measure the isotope enrichment. These methods are challenging, both technologically and economically. We have developed a novel approach that is suitable for labeled-IVGTT studies involving a large cohort of individuals.

The tracer, D-[(13)C(6)]glucose, is a low-cost alternative with the significant advantage that the sixth isotope of natural glucose has virtually zero natural abundance, which facilitates isotopomer analysis with <1% labeled glucose in the infusate. After deproteinization of plasma samples collected at various times, glucose is converted to a stable derivative, purified by solid-phase extraction (SPE), and analyzed by HPLC-electrospray ionization mass spectrometry to accumulate the isotope-abundance data for the A+2, A+3, and A+6 ions of the glucose derivative. A 2-pool modeling program was used to derive standard kinetic parameters.

With labeled-IVGTT data from 10 healthy male individuals, the values for insulin sensitivity, glucose effectiveness, and the plasma clearance rate estimated with the 2-pool minimal model compared well with values obtained via traditional methods.

The relative simplicity and robustness of the new method permit the preparation and analysis of up to 48 samples/day, a throughput equivalent to 2 complete IVGTT experiments, and this method is readily adaptable to existing 96 well-format purification and analytical systems.