Quantification of hepatic transaldolase exchange activity and its effects on tracer measurements of indirect pathway flux in humans

Abstract

Exchange of hepatic glucose-6-phosphate (G6P) and glyceraldehyde-3-phosphate via transaldolase modifies hepatic G6P enrichment from glucose or gluconeogenic tracers. Transaldolase exchange was quantified in five healthy, fed subjects following an oral bolus of [1,2,3-13C3]glycerol (25-30 mg/kg) and paracetamol (10-12 mg/kg). 13C Isotopomers of hepatic G6P were quantified by 13C NMR spectroscopy of urinary glucuronide. [1,2,3-13C3]- and [4,5,6-13C3]glucuronide isotopomers, representing the conversion of [1,2,3-13C3]glycerol to G6P via dihydroxyacetone phosphate, were resolved from [1,2-13C2]- and [5,6-13C2]glucuronide 13C-isotopomers, derived from metabolism of [1,2,3-13C3]glycerol via pyruvate and phosphoenolpyruvate. Enrichment of [1,2,3-13C3]glucuronide was significantly less than that of [4,5,6-13C3]glucuronide (1.30 ± 0.57% versus 1.67 ± 0.42%, P < 0.05). Also, [1,2-13C2]glucuronide enrichment was significantly less than that of [5,6-13C2]glucuronide (0.28 ± 0.08% versus 0.36 ± 0.03%, P < 0.05). Transaldolase and triose phosphate isomerase exchange activities were estimated by applying the 13C-isotopomer data to a model of hepatic sugar phosphate metabolism. Triose phosphate isomerase exchange was ~99% complete and did not contribute significantly to the unequal 13C-isotopomer distributions of the glucuronide triose halves. Instead, this was attributable to 25 ± 23% of hepatic G6P flux undergoing transaldolase exchange. This results in substantial overestimates of indirect pathway contributions to hepatic glycogen synthesis with tracers such as [5-3H]glucose and 2H2O. Magn Reson Med, 2008. © 2008 Wiley-Liss, Inc.