Transaldolase exchange and its effects on measurements of gluconeogenesis

terated 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]ac-etate infused in 11 nondiabetic subjects after a 16-h fast. Plasma and urinary glucuronide enrichments were measured using nuclear mag-netic resonance spectroscopy before and during a 0.35 mU·kg FFM1·min1 insulin infusion. Rates of endogenous glucose produc-tion measured with [3-3H]- and [6,6-2H2]glucose did not differ either before (14.0 0.7 vs. 13.8 0.7 mol·kg1·min1) or during the clamp (10.4 0.9 vs. 10.9 0.7 mol·kg1·min1), consistent with equilibration and quantitative removal of tritium during triose isomer-ase exchange. Plasma [3-13C] glucose-to-[4-13C]glucose and urinar