This simply means that glucose and tracer must be separately measurable. Usually it means that glucose is isotopically labeled. The most frequently used labels are 14C or 3H (radioactive) and 12C or 2H (stable). Labels are not all equivalent and the method of measurement
may affect the results. For example, the use of [U-13C]glucose and the measurement of the M+6 isotopomer (all six carbons of each glucose molecule are labeled) yields results equivalent to the use of [3-3H]glucose. Both will lose their (measured) label in the equilibration in the triose phosphate pool. [U-14C]glucose, which appears to be similarly labeled, only has a uniformly distributed label; not all carbons in each molecule are labeled. Any measurement of [U-14C]glucose will therefore include label that has been recycled via glycolysis and gluconeogenesis, which is not the case for [U-13C]glucose. If both these carbon labels undergo combustion to CO2, however, an analogous measurement of their enrichment will be obtained. Label choices and methods of measurement must therefore always be made with the metabolic application in mind.
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