Evidence that Processes Other than Gluconeogenesis may influence the Ratio of Deuterium on the Fifth and Third Carbons of Glucose: Implications for the use of ²H₂0 to measure gluconeogenesis in humans

Abstract

Objective: The deuterated water method uses the ratio of deuterium on carbons five and two (C5/C2) or three and two (C3/C2) to estimate the fraction of glucose derived from gluconeogenesis. The current studies determined if C3 and C5 glucose enrichment is influenced by processes other than gluconeogenesis.

Research Design: Six non-diabetic subjects were infused with [3,5-²H₂] glucose and insulin while glucose was clamped at ∼5 mmol/L; the C5/C3 ratio was measured in the in UDP glucose pool using NMR and the acetaminophen glucuronide method.

Results: Whereas, the C5/C3 ratio of the infusate was 1.07, the ratio in UDP-glucose was less than 1.0 in all subjects both prior to (0.75±0.07) and during (0.67±0.05) the insulin infusion.

Conclusions: These data indicate that the deuterium on C5 of glucose is lost more rapidly relative to the deuterium on C3. The decrease in the C5/C3 ratio could result from exchange of the lower three carbons of fructose-6-phosphate with unlabeled three carbon precursors via the transaldolase reaction and/or selective retention of the C3 deuterium at the level of triosephosphate isomerase due to a kinetic isotope effect. Following ingestion of ²H₂O, these processes would increase the enrichment of C5 and decrease the enrichment of C3 respectively with the former causing an overestimation of gluconeogenesis using the C2/C5 ratio and the latter an underestimation using the C3/C2 ratio. Future studies will be required to determine whether the impact of these processes on the measurement of gluconeogenesis differs amongst the disease states being evaluated (e.g. diabetes or obesity).