Quantifying endogenous glucose production and contributing source fluxes from a single 2 H NMR spectrum

Metabolic incorporation of H218 O into specific glucose-6-phosphate oxygens by red-blood-cell lysates as observed by 13 C isotope-shifted NMR signals

Water enriched with oxygen-18 (H₂¹⁸ O) is a potential tracer for evaluating the sources of glucose and glycogen synthesis since it is incorporated into specific sites of glucose-6-phosphate via specific enzyme-mediated exchange/addition mechanisms. Unlike ² H, ¹⁸ O does not experience significant isotope effects for any of these processes. Therefore, H₂¹⁸ O might provide more precise estimates of endogenous carbohydrate synthesis compared with deuterated water provided that positional ¹⁸ O enrichments of glucose can be measured. As a proof of concept, H₂¹⁸ O was incorporated into a well characterized hemolysate model of sugar phosphate metabolism and ¹³ C NMR was applied to quantify positional ¹⁸ O enrichment of glucose-6-phosphate oxygens. Human erythrocyte hemolysate preparations were incubated overnight at 37 °C with a buffer containing sugar phosphate precursors and 20% (n = 5) and 80% (n = 1) H₂¹⁸ O. Enrichment of glucose-6-phosphate was analyzed by ¹³ C NMR analysis of ¹⁸ O-shifted versus unshifted signals following derivatization to monoacetone glucose (MAG). ¹³ C NMR MAG spectra from hemolysate revealed resolved ¹⁸ O-shifted signals in Positions 1-5. Mean ¹⁸ O enrichments were 16.4 ± 1.6% (Position 1), 13.3 ± 1.3% (Position 2), 4.1 ± 1.1% (Position 3), 12.6 ± 0.8% (Position 4), 10.7 ± 1.4% (Position 5), and no detectable enrichment of Position 6. No ¹⁸ O-shifted glucose-6-phosphate signals were detected in preparations containing sugar phosphate precursors only. H₂¹⁸ O is incorporated into Positions 1-5 of glucose-6-phosphate in accordance with spontaneous aldose hydration and specific enzymatic reaction mechanisms. This provides a basis for its deployment as a tracer for glucose and glycogen biosynthesis.

Limitations to Starch Utilization in Barramundi (Lates calcarifer) as Revealed by NMR-Based Metabolomics

Practical diets for commercial barramundi production rarely contain greater than 10% starch, used mainly as a binding agent during extrusion. Alternative ingredients such as digestible starch have shown some capacity to spare dietary protein catabolism to generate glucose. In the present study, a carnivorous fish species, the Asian seabass (Lates calcarifer) was subjected to two diets with the same digestible energy: Protein (P) – with high protein content (no digestible starch); and Starch (S) – with high digestible (pregelatinized) starch content. The effects of a high starch content diet on hepatic glycogen synthesis as well as the muscle and liver metabolome were studied using a complementary approach of ¹H and ²H NMR. The hepatosomatic index was lower for fish fed high starch content diet while the concentration of hepatic glycogen was similar between groups. However, increased glycogen synthesis via the direct pathway was observed in the fish fed high starch content diet which is indicative of increased carbohydrate utilization. Multivariate analysis also showed differences between groups in the metabolome of both tissues. Univariate analysis revealed more variations in liver than in muscle of fish fed high starch content diet. Variations in metabolome were generally in agreement with the increase in the glycogen synthesis through direct pathway, however, this metabolic shift seemed to be insufficient to keep the growth rate as ensured by the diet with high protein content. Although liver glycogen does not make up a substantial quantity of total stored dietary energy in carnivorous fish, it is a key regulatory intermediate in dietary energy utilization.

Glycerol not lactate is the major net carbon source for gluconeogenesis in mice during both short and prolonged fasting

Objective

Fasting results in major metabolic changes including a switch from glycogenolysis to gluconeogenesis to maintain glucose homeostasis. However, the relationship between the length of fasting and the relative contribution of gluconeogenic substrates remains unclear. We investigated the relative contribution of glycogen, lactate, and glycerol in glucose production of male C57BL/6 J-albino mice after 6, 12, and 18 h of fasting.

Methods

We used non-perturbative infusions of ¹³C₃ lactate, ¹³C₃ glycerol, and ¹³C₆ glucose combined with liquid chromatography mass spectrometry and metabolic flux analysis to study the contribution of substrates in gluconeogenesis (GNG).

Results

During infusion studies, both lactate and glycerol significantly label about 60% and 30–50% glucose carbon, respectively, but glucose labels much more lactate (∼90%) than glycerol carbon (∼10%). Our analyses indicate that lactate, but not glycerol is largely recycled during all fasting periods such that lactate is the largest direct contributor to GNG via the Cori cycle but a minor source of new glucose carbon (overall contribution). In contrast, glycerol is not only a significant direct contributor to GNG but also the largest overall contributor to GNG regardless of fasting length. Prolonged fasting decreases both the whole body turnover rate of glucose and lactate but increases that of glycerol, indicating that the usage of glycerol in GNG become more significant with longer fasting.

Conclusion

Collectively, these findings suggest that glycerol is the dominant overall contributor of net glucose carbon in GNG during both short and prolonged fasting.

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Prolonged fasting significantly decreases the turnover rate of glucose and lactate but increases the glycerol turnover rate in mice.

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In both short and prolonged fasting, lactate is the largest direct contributor to gluconeogenesis but a minor source of new carbon entry.

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Glycerol is the second largest direct contributor to gluconeogenesis and the dominant overall carbon contributor during both short and prolonged fasting.

Disposition of a Glucose Load into Hepatic Glycogen by Direct and Indirect Pathways in Juvenile Seabass and Seabream

In carnivorous fish, conversion of a glucose load to hepatic glycogen is widely used to assess their metabolic flexibility towards carbohydrate utilization, but the activities of direct and indirect pathways in this setting are unclear. We assessed the conversion of an intraperitoneal glucose load (2 g.kg-1) enriched with [U-13C6]glucose to hepatic glycogen in juvenile seabass and seabream. 13C-NMR analysis of glycogen was used to determine the contribution of the load to glycogen synthesis via direct and indirect pathways at 48-hr post-injection. For seabass, [U-13C6]glucose was accompanied by deuterated water and 2H-NMR analysis of glycogen 2H-enrichment, allowing endogenous substrate contributions to be assessed as well. For fasted seabass and seabream, 47 ± 5% and 64 ± 10% of glycogen was synthesized from the load, respectively. Direct and indirect pathways contributed equally (25 ± 3% direct, 21 ± 1% indirect for seabass; 35 ± 7% direct, 29 ± 4% indirect for seabream). In fasted seabass, integration of 2H- and 13C-NMR analysis indicated that endogenous glycerol and anaplerotic substrates contributed an additional 7 ± 2% and 7 ± 1%, respectively. In fed seabass, glucose load contributions were residual and endogenous contributions were negligible. Concluding, direct and indirect pathways contributed equally and substantially to fasting hepatic glycogen repletion from a glucose load in juvenile seabream and seabass.

Demonstration of glucose-6-phosphate hydrogen 5 enrichment from deuterated water by transaldolase-mediated exchange alone

PURPOSE

Enrichment of glucose position 5 (H5) from deuterated water ((2)H2O) is widely used for quantifying gluconeogenesis. Exchanges of hexose and triose phosphates mediated by transaldolase have been postulated to enrich H5 independently of gluconeogenesis, but to date this mechanism has not been proven. We determined the enrichment of glucose-6-phosphate (G6P), the immediate precursor of endogenously produced glucose, from (2)H2O in erythrocyte hemolysate preparations. Here, transaldolase exchange is active but gluconeogenesis is absent.

METHODS

Hemolysates were prepared from human erythrocytes and incubated with a buffer containing 5% [U-(13)C]G6P, unlabeled fructose 1,6-bisphosphate, and 10% (2)H2O. G6P (2)H-enrichment and (13)C-isotopomer distributions were analyzed by (2)H and (13)C NMR following derivatization to monoacetone glucose.

RESULTS

(2)H NMR analysis revealed high (2)H-enrichment of G6P hydrogens 2, 4, and 5; low enrichment of hydrogen 3, and residual enrichments of hydrogens 1, 6R, and 6S. (13)C NMR isotopomer analysis revealed that [U-(13)C]G6P was converted to [1,2,3-(13)C3]G6P, a predicted product of transaldolase-mediated exchange, as well as [1,2-(13)C2]G6P and [3-(13)C]G6P, predicted products of combined transaldolase and transketolase exchanges.

CONCLUSION

Hydrogen 5 of G6P was enriched from (2)H2O through exchanges mediated by transaldolase. These studies prove that G6P can be enriched in hydrogen 5 by (2)H2O independently of gluconeogenesis.

Effects of food-deprivation and refeeding on the regulation and sources of blood glucose appearance in European seabass (Dicentrarchus labrax L.)

Sources of blood glucose in European seabass (initial weight 218.0±43.0g; mean±S.D., n=18) were quantified by supplementing seawater with deuterated water (5%-(2)H2O) for 72h and analyzing blood glucose (2)H-enrichments by (2)H NMR. Three different nutritional states were studied: continuously fed, 21-day of fast and 21-day fast followed by 3days of refeeding. Plasma glucose levels (mM) were 10.7±6.3 (fed), 4.8±1.2 (fasted), and 9.3±1.4 (refed) (means±S.D., n=6), showing poor glycemic control. For all conditions, (2)H-enrichment of glucose position 5 was equivalent to that of position 2 indicating that blood glucose appearance from endogenous glucose 6-phosphate (G6P) was derived by gluconeogenesis. G6P-derived glucose accounted for 65±7% and 44±10% of blood glucose appearance in fed and refed fish, respectively, with the unlabeled fraction assumed to be derived from dietary carbohydrate (35±7% and 56±10%, respectively). For 21-day fasted fish, blood glucose appearance also had significant contributions from unlabeled glucose (52±16%) despite the unavailability of dietary carbohydrates. To assess the role of hepatic enzymes in glycemic control, activity and mRNA levels of hepatic glucokinase (GK) and glucose 6-phosphatase (G6Pase) were assessed. Both G6Pase activity and expression declined with fasting indicating the absence of a classical counter-regulatory stimulation of hepatic glucose production in response to declining glucose levels. GK activities were basal during fed and fasted conditions, but were strongly stimulated by refeeding. Overall, hepatic G6Pase and GK showed limited capacity in regulating glucose levels between feeding and fasting states.

Disposition of [U-2H7]glucose into hepatic glycogen in rat and in seabass

The stimulation of hepatic glycogenesis is a ubiquitous response to a glucose challenge and quantifying its contribution to glucose uptake informs its role in restoring euglycemia. Glycogenesis can be quantified with labeled water provided that exchange of glucose-6-phosphate hydrogen 2 (G6P-H2) and body water via glucose-6-phosphate isomerase, and exchange of positions 4, 5 and 6 hydrogens (G6P-H456) via transaldolase, are known. These exchanges were quantified in 24-h fasted rats (Rattus norvegicus; n=6) and 21-day fasted seabass (Dicentrarchus labrax; n=8) by administration of a glucose load (2000mg·kg(-1)) enriched with [U-(2)H7]glucose and by quantifying hepatic glycogen (2)H-enrichments after 2h (rats) and 48h (seabass). Direct pathway contributions of the glucose load to glycogenesis were also estimated. G6P-H2 and body water exchange was 61±1% for rat and 47±3% for seabass. Transaldolase-mediated exchange of G6P-H456 was 5±1% for rat and 10±1% for seabass. Conversion of the glucose load to hepatic glycogen was significant in seabass (249±54mg·kg(-1)) but negligible in rats (12±1mg·kg(-1)). Preload plasma glucose levels were similar for seabass and rats (3.3±0.7 and 4.4±0.1mmol·L(-1), respectively) but post-load plasma glucose was significantly higher in seabass compared to rats (14.6±1.8 versus 5.8±0.3mmol·L(-1), p<0.01). In conclusion, G6P-H2 and body water exchange is incomplete for both species and has to be accounted for in estimating hepatic glycogen synthesis and direct pathway activities with labeled water tracers. Transaldolase-mediated exchange is insignificant. Hepatic direct pathway glycogenesis plays a prominent role in seabass glucose load disposal, but a negligible role in the rat.

Acetaminophen glucuronide and plasma glucose report identical estimates of gluconeogenesis and glycogenolysis for healthy and prediabetic subjects using the deuterated water method

Plasma glucose (2) H-enrichment in positions 5 ((2) H5) and 2 ((2) H2) from deuterated water ((2) H2 O) provides a measure of the gluconeogenic contribution to endogenous glucose production. Urinary glucuronide analysis can circumvent blood sampling but it is not known if glucuronide and glucose enrichments are equal. Thirteen subjects with impaired fasting glucose/impaired glucose tolerance and 11 subjects with normal fasting glucose and normal glucose tolerance ingested (2) H2 O to ∼0.5% body water and acetaminophen. Glucose and glucuronide (2) H5 and (2) H2 were measured by (2) H NMR spectroscopy of monoacetone glucose. For normal fasting glucose/normal glucose tolerance, (2) H5 was 0.23 ± 0.02% and 0.25 ± 0.02% for glucose and glucuronide, respectively, whereas (2) H2 was 0.47 ± 0.01% and 0.49 ± 0.02%, respectively. For impaired fasting glucose/impaired glucose tolerance, (2) H5 was 0.22 ± 0.01% and 0.26 ± 0.02% for glucose and glucuronide, respectively, whereas (2) H2 was 0.46 ± 0.01% and 0.49 ± 0.02%, respectively. The gluconeogenic contribution to endogenous glucose production measured from glucose and glucuronide were identical for both normal fasting glucose/normal glucose tolerance (48 ± 4 vs. 51 ± 3%) and impaired fasting glucose/impaired glucose tolerance (48 ± 2 vs. 53 ± 3%).

Hepatic glycogen synthesis in farmed European seabass (Dicentrarchus labrax L.) is dominated by indirect pathway fluxes

Hepatic glycogen synthesis fluxes from direct and indirect pathways were quantified in seabass by postmortem (2)H NMR analysis of plasma water (PW) and glycogen glucosyl (2)H enrichments from (2)H-enriched seawater. Eighteen fish (28.0 ± 1.7 cm and 218.0 ± 43.0 g) were divided into three groups of 6 and studied over 24 days with transfer to 5% (2)H-seawater after day 21. Over this period, one group was fed daily with fishmeal, a second group was fasted, and a third group was fasted for 21 days followed by 3 days refeeding. Glycogen turnover and sources were determined from the ratio of glucosyl position 5 enrichment to that of plasma water (H5/PW). Glycogen levels of fed fish were significantly higher than fasted (665.4 ± 345.2 μmol.g(-1) liver versus 77.2 ± 59.5 μmol.g(-1) liver, P<0.05) while refed fish had comparable levels to fed (584.6 ± 140.4 μmol.g(-1) liver). Glycogen enrichment of fed fish was undetectable indicating negligible turnover over 3 days. For fasted fish, H5/PW was ~50% indicating that half of the glycogen had turned over via indirect pathway flux. For refed fish, H5/PW was ~100% indicating that the indirect pathway accounted for all net glycogen synthesis. Direct pathway conversion of dietary carbohydrate to glycogen was not detected in any of the groups.

Analysis of glucose metabolism in farmed European sea bass (Dicentrarchus labrax L.) using deuterated water

Glucose metabolism in free-swimming fasted and fed seabass was studied using deuterated water ((2)H(2)O). After transfer to seawater enriched with 4.9% (2)H(2)O for 6-h or for 72-h, positional and mole percent enrichment (MPE) of plasma glucose and water were quantified by (2)H NMR and ESI-MS/MS. Plasma water (2)H-enrichment reached that of seawater within 6h. In both fasted and fed fish, plasma glucose MPE increased asymptotically attaining ~55% of plasma water enrichment by 72 h. The distribution of (2)H-enrichment between the different glucose positions was relatively uniform. The gluconeogenic contribution to glucose that was synthesized during (2)H(2)O administration was estimated from the ratio of position 5 and 2 glucose enrichments. For both fed and fasted fish, gluconeogenesis accounted for 98±1% of the glucose that was produced during the 72-h (2)H(2)O administration period. For fasted fish, gluconeogenic contributions measured after 6h were identical to 72-h values (94±3%). For fed fish, the apparent gluconeogenic contribution at 6-h was significantly lower compared to 72-h (79±5% versus 98±1%, p<0.05). This may reflect a brief augmentation of gluconeogenic flux by glycogenolysis after feeding and/or selective enrichment of plasma glucose position 2 via futile glucose-glucose-6-phosphate cycling.