Elsevier

Metabolism

Volume 47, Issue 5, May 1998, Pages 549-554
Metabolism

Role of glucose and glutamine synthesis in the differential recovery of 13CO2 from infused [2-13C] versus [1-13C] acetate

https://doi.org/10.1016/S0026-0495(98)90238-4Get rights and content

Abstract

Carbon exchange in the Krebs cycle may result in underestimation of substrate oxidation measured with 13C-labeled substrates, since carbon labeled in position 2 of acetyl-coenzyme A (CoA) could be incorporated into glucose (via gluconeogenesis) and glutamine. Five healthy volunteers were therefore infused with [1-13C] and [2-13C] acetate at a rate of 0.5 μmol · kg−1 · min−1 for 165 minutes on two different occasions in randomized order. Whole body acetate turnover did not differ between the two tracers: 7.9 ± 0.3 and 7.5 ± 0.6 μmol · kg−1 · min−1 (nonsignificant [NS]) for [1-13C] and [2-13C] acetate, respectively. Isotopic 13C enrichment was higher in expired CO2 (0.177 ± 0.021 v 0.089 ± 0.009 atom percent excess [APE], P < .01) and lower in glucose (0.074 ± 0.017 v 0.291 ± 0.061 mole percent excess [MPE], P < .01) for [1-13C] acetate compared with [2-13C] acetate, respectively, at the end of the infusions. Glutamine isotopic enrichment was slightly but not significantly higher when infusing [1-13C] acetate versus [2-13C] acetate (0.348 ± 0.038 v 0.495 ± 0.069 MPE, NS, respectively). At the end of the experiment, the recovery of 13CO2 from [1-13C] acetate was 44.8% ± 2.7%, and from [2-13C] acetate, 22.6% ± 1.3%. A significant correlation was observed between the differences in 13C enrichment of CO2 for the two tracers and glucose (ΔCO2 = 0.424 · Δglucose + 0.001, R2 = .9856, P = .0007) or glutamine (ΔCO2 = 0.621 · Δglutamine + 0.004, R2 = .9573, P = .0038) during the infusion. These results suggest that (1) although gluconeogenesis appears to be more responsible than glutamine for the differential recovery of [2-13C] versus [1-13C] acetate, other secondary pathways are probably also implicated; and (2) different recovery correction factors should be applied when measuring substrate oxidation with a stable isotope tracer depending on the expected position of 13C in acetyl-CoA.

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Supported in part by grants from the Ministère de la Recherche et de l'Enseignement Supérieur (programme Aliment 2000), the Ministère de l'Agriculture, de la Pêche et de l'Alimentation, la Direction de la recherche clinique, Centre Hopital Universitaire de Nantes, and Royal Canin.

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