Transaldolase exchange and its effects on measurements of gluconeogenesis in humans

Abstract

The deuterated water method is used extensively to measure gluconeogenesis in humans. This method assumes negligible exchange of the lower three carbons of fructose 6-phsophate via transaldolase exchange since this exchange will result in enrichment of carbon 5 of glucose in the absence of net gluconeogenesis. The present studies tested this assumption. ²H₂O and acetaminophen were ingested and [1-¹³C]acetate infused in 11 nondiabetic subjects after a 16-h fast. Plasma and urinary glucuronide enrichments were measured using nuclear magnetic resonance spectroscopy before and during a 0.35 mU·kg FFM⁻¹·min⁻¹ insulin infusion. Rates of endogenous glucose production measured with [3-³H]- and [6,6-²H₂]glucose did not differ either before (14.0 ± 0.7 vs. 13.8 ± 0.7 μmol·kg⁻¹·min⁻¹) or during the clamp (10.4 ± 0.9 vs. 10.9 ± 0.7 μmol·kg⁻¹·min⁻¹), consistent with equilibration and quantitative removal of tritium during triose isomerase exchange. Plasma [3-¹³C] glucose-to-[4-¹³C]glucose and urinary [3-¹³C] glucuronide-to-[4-¹³C]glucuronide ratios were <1.0 (P < 0.001) in all subjects both before (0.66 ± 0.04 and 0.60 ± 0.04) and during (059 ± 0.05 and 0.56 ± 0.06) the insulin infusion, respectively, indicating that ∼35-45% of the labeling of the 5th carbon of glucose by deuterium was due to transaldolase exchange rather than gluconeogenesis. When corrected for transaldolase exchange, rates of gluconeogenesis were lower (P < 0.001) and glycogenolysis higher (P < 0.001) than uncorrected rates both before and during the insulin infusion. In conclusion, assuming negligible dilution by glycerol and near-complete triose isomerase equilibration, these data provide strong experimental evidence that transaldolase exchange occurs in humans, resulting in an overestimate of gluconeogenesis and an underestimate of glycogenolysis when measured with the ²H₂O method. Use of appropriate ¹³C tracers provides a means of correcting for transaldolase exchange.

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).…”

Elevated NEFA levels impair glucose effectiveness by increasing net hepatic glycogenolysis

“…Plasma samples for GC-MS were derivatised after protein precipitation to the aldehyde penta-acetate with hydroxylamine hydrochloride acetic anhydride. Measurements of GNG were performed at the Mayo Clinic, using Landau's established method which measures deuterium enrichment at C-2 and C-5 on plasma glucose [18]. Rates of EGP and glucose uptake (Rd) were calculated as previously described [10,19].…”

Effects of transaldolase exchange on estimates of gluconeogenesis in type 2 diabetes

“…We have demonstrated that TA activity occurs in healthy humans based on a higher enrichment of glucose carbon 4 over carbon 3 (i.e., 13 C3/ 13 C4 Ͻ 1.0) from [1][2][3][4][5][6][7][8][9][10][11][12][13] C]acetate (3). In this study, the asymmetric labeling of glucose from this tracer could also be explained by incomplete exchange of the label between glyceraldehyde 3-phosphate and dihydroxyacetone phosphate mediated by incomplete triose phosphate isomerase (TPI; see Fig.…”

“…With a few exceptions (14), stable isotope tracer studies indicate an increased fractional gluconeogenesis in overnight-fasted T2DM subjects compared with healthy nondiabetic (ND) controls (6,10,15,16,28). Transaldolase (TA) exchange activity has been implicated in both ND and T2DM subjects from the depletion of position 5 relative to position 3 label following metabolism of [3, H 2 ]galactose to glucose (7). However, it is not known whether transaldolase exchange activity is different between ND and T2DM under conditions of fasting and hyperglycemia (frequently seen with meals).…”

“…Exchange of fructose 6-phosphate and triose phosphate mediated by transaldolase has been implicated by us (3,7,9) and others (12) in overestimation of rates of gluconeogenesis, utilizing the deuterated water method. In this method, glucose derived from glycogenolysis (GGL) via glucose 6-phosphate (G6P) source can become labeled in carbons 4, 5, and 6 from either a carbonlabeled gluconeogenic precursor or deuterated water independent of gluconeogenesis.…”