title: 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-(13)C(3)]glycerol (25-30 mg/kg) and paracetamol (10-12 mg/kg). (13)C Isotopomers of hepatic G6P were quantified by (13)C NMR spectroscopy of urinary glucuronide. [1,2,3-(13)C(3)]- and [4,5,6-(13)C(3)]glucuronide isotopomers, representing the conversion of [1,2,3-(13)C(3)]glycerol to G6P via dihydroxyacetone phosphate, were resolved from [1,2-(13)C(2)]- and [5,6-(13)C(2)]glucuronide (13)C-isotopomers, derived from metabolism of [1,2,3-(13)C(3)]glycerol via pyruvate and phosphoenolpyruvate. Enrichment of [1,2,3-(13)C(3)]glucuronide was significantly less than that of [4,5,6-(13)C(3)]glucuronide (1.30 +/- 0.57% versus 1.67 +/- 0.42%, P < 0.05). Also, [1,2-(13)C(2)]glucuronide enrichment was significantly less than that of [5,6-(13)C(2)]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 (13)C-isotopomer data to a model of hepatic sugar phosphate metabolism. Triose phosphate isomerase exchange was approximately 99% complete and did not contribute significantly to the unequal (13)C-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-(3)H]glucose and (2)H(2)O.
authors.affiliation: NMR Research Unit, Department of Biochemistry and Center for Neurosciences and Cell Biology, University of Coimbra, Portugal. jones@cnc.cj.uc.pt
authors.forename: John G
authors.initials: JG
authors.lastname: Jones
authors.forename: Paula
authors.initials: P
authors.lastname: Garcia
authors.forename: Cristina
authors.initials: C
authors.lastname: Barosa
authors.forename: Teresa C
authors.initials: TC
authors.lastname: Delgado
authors.forename: M Madalena
authors.initials: MM
authors.lastname: Caldeira
authors.forename: Luisa
authors.initials: L
authors.lastname: Diogo
mesh_terms: D000082:Acetaminophen / Q000008:administration & dosage / Q000652:urine; D000284:Administration, Oral; D000328:Adult; D018712:Analgesics, Non-Narcotic / Q000008:administration & dosage / Q000652:urine; D002247:Carbon Isotopes; D005260:Female; D005965:Glucuronates / Q000652:urine; D005990:Glycerol / Q000008:administration & dosage / Q000652:urine; D006801:Humans; D008099:Liver / Q000201:enzymology* / Q000378:metabolism; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008297:Male; D014153:Transaldolase / Q000378:metabolism*
chemical_list: D018712:Analgesics, Non-Narcotic; D002247:Carbon Isotopes; D005965:Glucuronates; D000082:Acetaminophen; D014153:Transaldolase; D005990:Glycerol