Sources of hepatic glycogen synthesis during an oral glucose tolerance test: Effect of transaldolase exchange on flux estimates

Abstract

Sources of hepatic glycogen synthesis during an oral glucose tolerance test were evaluated in six healthy subjects by enrichment of a 75-g glucose load with 6.67% [U-(13)C]glucose and 3.33% [U-(2)H(7)]glucose and analysis of plasma glucose and hepatic uridine diphosphate-glucose enrichments (sampled as urinary menthol glucuronide) by (2)H and (13)C nuclear magnetic resonance. The direct pathway contribution, as estimated from the dilution of [U-(13)C]glucose between plasma glucose and glucuronide, was unexpectedly low (36 +/- 5%). With [U-(2)H(7)]glucose, direct pathway estimates based on the dilution of position 3 (2)H-enrichment between plasma glucose and glucuronide were significantly higher (51 +/- 6%, P = 0.05). These differences reflect the exchange of the carbon 4, 5, and 6 moiety of fructose-6-phosphate and glyceraldehyde-3-phosphate catalyzed by transaldolase. As further evidence of this exchange, (2)H-enrichments in glucuronide positions 4 and 5 were inferior to those of position 3. From the difference in glucuronide positions 5 and 3 enrichments, the fraction of direct pathway carbons that experienced transaldolase exchange was estimated at 21 +/- 4%. In conclusion, the direct pathway contributes only half of hepatic glycogen synthesis during an oral glucose tolerance test. Glucose tracers labeled in positions 4, 5, or 6 will give significant underestimates of direct pathway activity because of transaldolase exchange.

An update on the use of benzoate, phenylacetate and phenylbutyrate ammonia scavengers for interrogating and modifying liver nitrogen metabolism and its implications in urea cycle disorders and liver disease

“…Metabolic flux analysis is useful firstly to improve our understanding of the disease, and secondly to monitor pharmaceutical and lifestyle interventions. In recent years, liver intermediary carbohydrate metabolism fluxes have been non-invasively evaluated using chemical agents that are excreted in urine, such as acetaminophen [86, 87], menthol [88], and also PA/PB and their excretable urinary by-product, PAGN [89, 90]. For example, Burgess and colleagues assessed the 13 C-labeling pattern of PAGN to evaluate hepatic TCA cycle fluxes assuming that the labeling pattern of PAGN glutamine moiety reflects that of the liver TCA cycle intermediate, alpha-ketoglutarate.…”

2H enrichment distribution of hepatic glycogen from2H2O reveals the contribution of dietary fructose to glycogen synthesis

“…Transaldolase exchange activity is another possible confounder of glycogen position 5 enrichment from indirect pathway activity. By exchanging an unlabeled hexose C456 moiety with enriched glyceraldehyde-3-phosphate, transaldolase exchange can increase the enrichment of positions 4, 5, and 6 independently of net synthesis from indirect pathway sources (1,11), thereby systematically increasing the apparent contributions of all indirect pathway sources and decreasing that of the direct pathway. Although transaldolase exchange may account for ϳ15-20% of indirect pathway contributions in humans (11), its effects appear to be less significant in rodents (Jones JG, unpublished observations).…”

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

“…Transaldolase exchange activity has been indirectly inferred from the unequal distribution of 13 C-enrichment from gluconeogenic precursors in the triose units of glucose and UDP-glucose (6,(8)(9)(10) and from the selective depletion of 5-2 H relative to 3-2 H following the conversion of [3,5-2 H 2 ]glucose (7) and [U-2 H 7 ]glucose (11) to UDP-glucose. For gluconeogenic tissues such as the liver, gluconeogenesis and transaldolase exchange both contribute to the 2 Henrichment distribution of glucose from 2 H 2 O.…”

“…The resulting G6P isotopomers generated by the various carbon transfer and exchange reactions of this metabolic network were then analyzed by 13 C NMR spectroscopy and fitted to a metabolic flux model. Using this approach, they found that [U-13 C]G6P was rapidly converted to [1,2,[3][4][5][6][7][8][9][10][11][12][13] C 3 ]G6P, consistent with exchange of G6P and fructose-6-P (F6P) by means of G6P-isomerase coupled with exchange of F6P carbons 456 and unlabeled G3P, mediated by transaldolase (12). We hypothesized that this process would also result in the 2 Henrichment of G6P H5 from 2 H 2 O (see Figure 1), thereby proving that this process could occur independently of gluconeogenesis.…”