A method for obtaining 13C isotopomer populations in 13C-enriched glucose

Tracer-based assessments of hepatic anaplerotic and TCA cycle flux: practicality, stoichiometry, and hidden assumptions

Two groups recently used different tracer methods to quantify liver-specific flux rates. The studies had a similar goal, i.e., to characterize mitochondrial oxidative function. These efforts could have a direct impact on our ability to understand metabolic abnormalities that affect the pathophysiology of fatty liver and allow us to examine mechanisms surrounding potential therapeutic interventions. Briefly, one method couples the continuous infusion of [(13)C]acetate with direct real-time measurements of [(13)C]glutamate labeling in liver; the other method administers [(13)C]propionate, in combination with other tracers, and subsequently measures the (13)C labeling of plasma glucose and/or acetaminophen-glucuronide. It appears that a controversy has arisen, since the respective methods yielded different estimates of the anaplerotic/TCA flux ratio (VANA:VTCA) in "control" subjects, i.e., the [(13)C]acetate- and [(13)C]propionate-derived VANA:VTCA flux ratios appear to be ∼1.4 and ∼5, respectively. While the deep expertise in the respective groups makes it somewhat trivial for each to perform the tracer studies, the data interpretation is inherently difficult. The current perspective was undertaken to examine potential factors that could account for or contribute to the apparent differences. Attention was directed toward 1) matters of practicality, 2) issues surrounding stoichiometry, and 3) hidden assumptions. We believe that the [(13)C]acetate method has certain weaknesses that limit its utility; in contrast, the [(13)C]propionate method likely yields a more correct answer. We hope our discussion will help clarify the differences in the recent reports. Presumably this will be of interest to investigators who are considering tracer-based studies of liver metabolism.

Efficient synthesis of D-branched-chain amino acids and their labeled compounds with stable isotopes using D-amino acid dehydrogenase

D-Branched-chain amino acids (D-BCAAs) such as D-leucine, D-isoleucine, and D-valine are known to be peptide antibiotic intermediates and to exhibit a variety of bioactivities. Consequently, much effort is going into achieving simple stereospecific synthesis of D-BCAAs, especially analogs labeled with stable isotopes. Up to now, however, no effective method has been reported. Here, we report the establishment of an efficient system for enantioselective synthesis of D-BCAAs and production of D-BCAAs labeled with stable isotopes. This system is based on two thermostable enzymes: D-amino acid dehydrogenase, catalyzing NADPH-dependent enantioselective amination of 2-oxo acids to produce the corresponding D-amino acids, and glucose dehydrogenase, catalyzing NADPH regeneration from NADP(+) and D-glucose. After incubation with the enzymes for 2 h at 65°C and pH 10.5, 2-oxo-4-methylvaleric acid was converted to D-leucine with an excellent yield (>99 %) and optical purity (>99 %). Using this system, we produced five different D-BCAAs labeled with stable isotopes: D-[1-(13)C,(15)N]leucine, D-[1-(13)C]leucine, D-[(15)N]leucine, D-[(15)N]isoleucine, and D-[(15)N]valine. The structure of each labeled D-amino acid was confirmed using time-of-flight mass spectrometry and nuclear magnetic resonance analysis. These analyses confirmed that the developed system was highly useful for production of D-BCAAs labeled with stable isotopes, making this the first reported enzymatic production of D-BCAAs labeled with stable isotopes. Our findings facilitate tracer studies investigating D-BCAAs and their derivatives.

Hepatic gluconeogenesis and Krebs cycle fluxes in a CCl4 model of acute liver failure

Acute liver failure was induced in rats by CCl4 administration and its effects on the hepatic Krebs cycle and gluconeogenic fluxes were evaluated in situ by 13C NMR isotopomer analysis of hepatic glucose following infusion of [U-13C]propionate. In fed animals, CCl4 injury caused a significant increase in relative gluconeogenic flux from 0.80+/-0.10 to 1.34 +/-0.24 times the flux through citrate synthase (p<0.01). In 24-h fasted animals, CCl4-injury also significantly increased relative gluconeogenic flux from 1.36+/-0.16 to 1.80+/-0.22 times the flux through citrate synthase (p<0.01). Recycling of PEP via pyruvate and oxaloacetate was extensive under all conditions and was not significantly altered by CCl4 injury. CCl4 injury significantly reduced hepatic glucose output by 26% (42.8+/-7.3 vs 58.1+/-2.4 micromol/kg/min, p=0.005), which was attributed to a 26% decrease in absolute gluconeogenic flux from PEP (85.6+/-14.6 vs 116+/-4.8 micromol/kg/min, p<0.01). These changes were accompanied by a 47% reduction in absolute citrate synthase flux (90.6+/-8.0 to 47.6+/-8.0 micromol/kg/min, p<0.005), indicating that oxidative Krebs cycle flux was more susceptible to CCl4 injury. The reduction in absolute fluxes indicate a significant loss of hepatic metabolic capacity, while the significant increases in relative gluconeogenic fluxes suggest a reorganization of metabolic activity towards preserving hepatic glucose output.

Quantitation of gluconeogenesis by (2)H nuclear magnetic resonance analysis of plasma glucose following ingestion of (2)H(2)O

We present a simple (2)H NMR assay of the fractional contribution of gluconeogenesis to hepatic glucose output following ingestion of (2)H(2)O. The assay is based on the measurement of relative deuterium enrichment in hydrogens 2 and 3 of plasma glucose. Plasma glucose was enzymatically converted to gluconate, which displays fully resolved deuterium 2 and 3 resonances in its (2)H NMR spectrum at 14.1 T. The signal intensity of deuterium 3 relative to deuterium 2 in the gluconate derivative as quantitated by (2)H NMR was shown to provide a precise and accurate measurement of glucose enrichment in hydrogen 3 relative to hydrogen 2. This measurement was used to estimate the fractional contribution of gluconeogenesis to hepatic glucose output for two groups of rats; one group was fasted for 7 h and the other was fasted for 29 h. Rats were administered (2)H(2)O to enrich total body water to 5% over the last 4-5 h of each fasting period. For the 7-h fasted group, the hydrogen 3/hydrogen 2 enrichment ratio of plasma glucose was 0.32 +/- 0.09 (n = 7). This indicates that gluconeogenesis contributed 32 +/- 9% of total hepatic glucose output with glycogenolysis contributing the remainder. For the 29-h fasted group, the hydrogen 3/hydrogen 2 enrichment ratio of plasma glucose was 0.81 +/- 0.10 (n = 6), indicating that gluconeogenesis supplied the bulk of hepatic glucose output (81 +/- 10%).

Measurement of hepatic glucose output, krebs cycle, and gluconeogenic fluxes by NMR analysis of a single plasma glucose sample

13C and 1H NMR spectroscopy of plasma glucose was used to resolve the isotopomer contributions from tracer levels of [1,6-13C2]glucose, a novel tracer of glucose carbon skeleton turnover, and [U-13C]propionate, a tracer of hepatic citric acid cycle metabolism. This allowed simultaneous measurements of hepatic glucose production and citric acid cycle fluxes from the NMR analysis of a single plasma glucose sample in fasted animals. Glucose carbon skeleton turnover, as reported by the dilution of [1,6-13C2]glucose, was 56 +/- 2 micromol/kg/min in the presence of labeling from [U-13C]propionate and 53 +/- 4 micromol/kg/min in its absence. Therefore, as expected, the labeling contributions from [U-13C]propionate metabolism did not have a significant effect on the measurement of glucose turnover. For the group infused with both tracers, citric acid cycle flux estimates from the analysis of glucose C2 isotopomer ratios were consistent with those from our recent experiments where only [U-13C]propionate was infused, verifying that the presence of [1,6-13C2]glucose did not interfere with these measurements. This integrated analysis of hepatic glucose output and citric acid cycle fluxes from plasma glucose isotopomers yielded a noninvasive estimate of hepatic citrate synthase flux of 74 +/- 12 micromol/kg/min for 24-h fasted rats.

Measurement of gluconeogenesis and pyruvate recycling in the rat liver: a simple analysis of glucose and glutamate isotopomers during metabolism of [1,2,3-(13)C3]propionate

Simple equations that relate glucose and glutamate 13C-NMR multiplet areas to gluconeogenesis and pyruvate recycling during metabolism of [1,2,3-(13)C3]propionate are presented. In isolated rat livers, gluconeogenic flux was 1.2 times TCA cycle flux and about 40% of the oxaloacetate pool underwent recycling to pyruvate prior to formation of glucose. The 13C spectra of glucose collected from rats after gastric versus intravenous administration of [1,2,3-(13)C3]propionate indicated that pyruvate recycling was slightly higher in vivo (49%) while glucose production was unchanged. This indicates that a direct measure of gluconeogenesis and pyruvate recycling may be obtained from a single 13C-NMR spectrum of blood collected after oral administration of enriched propionate.

Relationship between gluconeogenesis and phosphoenergetics in rat liver assessed by in vivo 13C and 31P NMR spectroscopy

The relationship between the phosphoenergetic state and gluconeogenesis in the liver after ischemic damage was investigated using living rats. The ATP level was determined with in vivo 31P nuclear magnetic resonance spectroscopy, and gluconeogenesis was evaluated with in vivo 31C NMR spectroscopy using L-[3-13C]alanine as a tracer. These two measurements were alternated repeatedly. The rats were divided into three groups: without ischemia (group A); with 10 min ischemia (group B); and with 30 min ischemia (group C). ATP was depleted to 20% of the preischemic state after 10 min ischemia and this level was maintained during 30 min ischemia. After reperfusion, the ATP level was partially restored, but the recovery was smaller in group C. Infusion of [3-13C]alanine was started immediately after the reperfusion. In vivo 13C NMR disclosed changes in the alanine C3, glutamine/glutamate C2 and C3, glucose C1-6, and glycogen C1 signals in the liver. After 60 min infusion of [3-13C]alanine, the ATP level correlated negatively with the signal intensity of alanine (r = -0.664, p = 0.008) and positively with those of glucose and glyogen (r = 0.586, p = 0.023, and r = 0.643, p = 0.011, respectively). These results suggest that the ATP level participates in gluconeogenesis and glycogenesis in the liver. Such multinuclear in vivo NMR observations might uncover new aspects of the metabolic function of the liver in the in vivo state.

Orientation-conserved transfer of symmetric Krebs cycle intermediates in mammalian tissue

Metabolism of [2-13C]-, [3-13C]-, and [1,2,3-13C]propionate in perfused rat livers and [2-13C]-acetate in perfused rat hearts has been examined in tissue extracts by 13C NMR. Label from [2-13C]-propionate was preferentially incorporated into the C2 carbon of lactate, alanine, and aspartate in liver tissue while label from [3-13C]propionate appeared preferentially in the C3 carbon of those same molecules. These data suggest that 13C may not be completely randomized in the symmetric citric acid cycle intermediates succinate and fumarate as is normally assumed but that some fraction of those intermediates may be transferred between enzymes in this span of the cycle with conservation of spatial orientation, consistent with recent results obtained in yeast [Sumegi et al. (1990) Biochemistry 29, 9106-9110]. This was confirmed by performing similar experiments with [1,2,3-13C]propionate. Time-dependent asymmetry was also observed between the intensities of the glutamate C2 and C3 resonances and between the aspartate C2 and C3 resonances in 13C NMR spectra of intact hearts and heart extracts during early perfusion with [2-13C]-acetate. A model is presented which predicts that isotopic asymmetry is observed only during the first 2-3 turns of the cycle pools when isotope enters the cycle via acetyl-CoA even if all symmetric cycle intermediates retain a unique molecular orientation on each pass through the citric acid cycle.