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Quantification of hepatic transaldolase exchange activity and its effects on tracer measurements of indirect pathway flux in humans
Author(s) -
Jones John G.,
Garcia Paula,
Barosa Cristina,
Delgado Teresa C.,
Caldeira M. Madalena,
Diogo Luisa
Publication year - 2008
Publication title -
magnetic resonance in medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.696
H-Index - 225
eISSN - 1522-2594
pISSN - 0740-3194
DOI - 10.1002/mrm.21451
Subject(s) - transaldolase , isotopomers , dihydroxyacetone phosphate , chemistry , triosephosphate isomerase , phosphoenolpyruvate carboxykinase , glyceraldehyde , metabolism , glycerol , biochemistry , glucuronidation , pentose phosphate pathway , phosphate , glycolysis , enzyme , dehydrogenase , organic chemistry , molecule , microsome
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 ≈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. Magn Reson Med, 2008. © 2008 Wiley‐Liss, Inc.