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 H2O to Measure Gluconeogenesis in Humans

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-to-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. After ingestion of H2O, 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-to-C5 ratio and the latter an underestimation using the C3-to-C2 ratio. Future studies will be required to determine whether the impact of these processes on the measurement of gluconeogenesis differs among the disease states being evaluated (e.g., diabetes or obesity). Diabetes 57:50–55, 2008 Plasma glucose concentration is determined by the balance between the rate of glucose production and glucose uptake. Glucose released from the liver is derived from either the degradation of glycogen, referred to as glycogenolysis, or the synthesis of new glucose from three-carbon precursors, referred to as gluconeogenesis. Multiple methods have been used to estimate rates of glycogenolysis and gluconeogenesis in humans with the deuterated water method perhaps being the most widely accepted (1–7). This method is based on the assumption that after ingestion of H2O, the fifth carbon of glucose (C5) is labeled with deuterium at the level of the triosephosphate isomerase reaction during gluconeogenesis, whereas the second carbon of glucose (C2) is labeled with deuterium during both glycogenolysis and gluconeogenesis (1). Therefore the plasma C5-to-C2 glucose ratio is used to estimate the fraction of glucose derived from gluconeogenesis. Because the third carbon of glucose (C3) and C5 are concurrently labeled during the triose isomerase reaction, the C3-to-C2 ratio also has been used to assess gluconeogenesis (8). The absolute rate of gluconeogenesis is calculated by multiplying the fraction of glucose derived from gluconeogenesis times the tracerdetermined rate of endogenous glucose production (9– 11). The rate of glycogenolysis is then determined by subtracting the rate of gluconeogenesis from glucose production. In general, estimates derived with the deuterated water method are concordant with those derived using other methods of estimating gluconeogenesis (2,3). In addition, gluconeogenesis measured with the deuterated water method changes in a biologically plausible manner during perturbations such as fasting or diabetes (1,11–14). However, unfortunately there is no “gold standard” against which results derived with this method can be compared. An alternative approach is to test the assumptions of the method. As noted above, a key assumption is that labeling of the fifth carbon of glucose with deuterium after ingestion of H2O only occurs during gluconeogenesis (1). The specific biochemical step in which the C5 hydrogen is exchanged for body water is during the equilibration between glyceraldhyde-3 phosphate (G3P) and dihydroxyacetone phosphate (DHAP) at the triosephosphate isomerase reaction (1). The C3 hydrogen also is lost during this exchange. Therefore, whenever a triose molecule is conFrom the Division of Endocrinology, Diabetes, Metabolism, and Nutrition, Department of Medicine, Mayo Clinic College of Medicine, Rochester, Minnesota; The Advanced Imaging Research Center, Case Western Reserve University School of Medicine, Cleveland, Ohio; and the University of Texas Southwestern Medical Center, Dallas, Texas. Address correspondence and reprint requests to Robert A. Rizza, MD, Mayo Clinic, 200 1st St. SW, Rm. 5-194 Joseph, Rochester, MN 55905. E-mail: [email protected]. Received for publication 22 May 2007 and accepted in revised form 1 October 2007. Published ahead of print at http://diabetes.diabetesjournals.org on 12 October 2007. DOI: 10.2337/db07-0694. DHAP, dihydroxyacetone phosphate; G3P, glyceraldhyde-3 phosphate; MAG, monoacetoneglucose; NMR, nuclear magnetic resonance; THF, tetrahydrofuran. †B.R.L. is deceased. © 2008 by the American Diabetes Association. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. ORIGINAL ARTICLE