Schumann, William C.

To determine the source(s) of blood and very low density lipoprotein (VLDL)-triglyceride glycerol during fasting, four men ingested2H2O from 14 to 20 h into a 60-h fast to achieve ∼0.5% body water enrichment. At 60 h of fasting, glycerol flux was measured using [2-14C]glycerol. Blood was taken for measurement of2H enrichment at carbon 6 of glucose and at carbon 3 of free glycerol and VLDL-triglyceride glycerol.2H enrichment of the 2 hydrogens bound to carbon 3 of VLDL-triglyceride glycerol was 105 ± 2% of the2H enrichment of the 2 hydrogens bound to carbon 6 of glucose, indicating isotopic equilibrium between hepatic glyceraldehyde 3- P and glycerol 3- P. The2H enrichment of the 2 hydrogens bound to carbon 3 of free glycerol was 17 ± 3% of VLDL-triglyceride glycerol, indicating that a significant percentage of free glycerol in blood originated from the hydrolysis of circulating VLDL-triglyceride or a pool of glycerol with similar2H enrichment. Glycerol flux was 6.3 ± 1.1 μmol · kg−1· min−1. Glycerol appearing from nonadipose tissue sources was then ∼1.1 μmol · kg−1· min−1. Seven other subjects were fasted for 12, 42, and 60 h. A small percentage of glycerol in the circulation after 12 h of fasting was enriched with2H. The enrichment of the 2 hydrogens bound to carbon 3 of free glycerol in the longer periods of fasting was ∼16% of the enrichment of the 2 hydrogens bound to carbon 6 of glucose. Therefore, as much as 15–20% of systemic glycerol turnover during fasting is not from lipolysis of adipose tissue triglyceride.

OBJECTIVE To determine whether deuterium enrichment on carbons 5 and 3 (C5/C3) in plasma glucose is influenced by processes other than gluconeogenesis and, if so, whether these processes are altered by type 2 diabetes. RESEARCH DESIGN AND METHODS In this study, 10 obese diabetic and 10 obese nondiabetic subjects were infused intravenously with [3,5-2H2] galactose enriched at a C5-to-C3 ratio of 1.0 as well as the enrichment of deuterium on C5 and C3 of plasma glucose, measured with nuclear magnetic resonance using the acetaminophen glucuronide method. RESULTS The ratio of deuterium enrichment on C5 and C3 of glucose was <1 (P < 0.001) in all of the diabetic and nondiabetic subjects, resulting in a means ± SE C5-to-C3 ratio that did not differ between groups (0.81 ± 0.01 vs. 0.79 ± 0.01, respectively). CONCLUSIONS That the C5-to-C3 glucose ratio is <1 indicates that transaldolase exchange, selective retention of deuterium at the level of the triose-isomerase reaction, or both occur in humans. This also indicates that the net effect of these processes on the C5-to-C3 ratio is the same in people with and without type 2 diabetes. The possible effects of transaldolase exchange or selective retention of deuterium (or tritium) at the level of the triose-isomerase reaction on tracee labeling and tracer metabolism should be considered when the deuterated water method is used to measure gluconeogenesis or [3-3H] glucose is used to measure glucose turnover in humans.

OBJECTIVE—The deuterated water method uses the ratio of deuterium on carbons 5 and 2 (C5/C2) or 3 and 2 (C3/C2) to estimate the fraction of glucose derived from gluconeogenesis. The current studies determined whether C3 and C5 glucose enrichment is influenced by processes other than gluconeogenesis. RESEARCH DESIGN AND METHODS—Six nondiabetic subjects were infused with [3,5-2H2]glucose and insulin while glucose was clamped at ∼5 mmol/l; the C5-to-C3 ratio was measured in the in UDP-glucose pool using nuclear magnetic resonance and the acetaminophen glucuronide method. RESULTS—Whereas the C5-to-C3 ratio of the infusate was 1.07, the ratio in UDP-glucose was <1.0 in all subjects both before (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-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 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-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).