Metabolism of α-d-[1,2-13C]Glucose Pentaacetate and α-d-Glucose Penta[2-13C]acetate in Rat Hepatocytes

Hepatocytes from fed rats were incubated for 120 min in the presence of α-d-[1,2-13C]glucose pentaacetate (1.7 mM), both d-[1,2-13C]glucose (1.7 mM) and acetate (8.5 mM), α-d-glucose penta[2-13C]acetate (1.7 mM), or d-[1,2-13C]glucose (8.3 mM). The amounts of 13C-enriched l-lactate and d-glucose and those of acetate and β-hydroxybutyrate recovered in the incubation medium were comparable under the first two experimental conditions. The vast majority of d-glucose isotopomers consisted of α- and β-d[1,2-13C]glucose. The less abundant single-labeled isotopomers of d-glucose were equally labeled on each C atom. The output of 13C-labeled l-lactate, mainly l-[2-13C]lactate and l-[3-13C]lactate, was 1 order of magnitude lower than that found in hepatocytes exposed to 8.3 mM d-[1,2-13C]glucose, in which case the total production of the single-labeled species of d-glucose was also increased and that of the C3- or C4-labeled hexose was lower than that of the other 13C-labeled isotopomers. In cells exposed to α-d-glucose penta[2-13C]acetate, the large majority of 13C atoms was recovered as [2-13C]acetate and, to a much lesser extent, β-hydroxybutyrate labeled in position 2 and/or 4. Nevertheless, l-[2-13C]lactate, l-[3-13C]lactate, and single-labeled d-glucose isotopomers were also produced in amounts higher or comparable to those found in cells exposed to α-d-[1,2-13C]glucose pentaacetate. However, a modest preferential labelling of the C6-C5-C4 moiety of d-glucose, relative to its C1-C2-C3 moiety, and a lesser isotopic enrichment of the C3 (or C4), relative to that of C1 (or C6) and C2 (or C5), were now observed. These findings indicate that, despite extensive hydrolysis of α-d-glucose pentaacetate (1.7 mM) in the hepatocytes, the catabolism of its d-glucose moiety is not more efficient than that of unesterified d-glucose, tested at the same molar concentration (1.7 mM) in the presence of the same molar concentration of unesterified acetate (8.5 mM), and much lower than that found at a physiological concentration of the hexose (8.3 mM). The present results also argue against any significant back-and-forth interconversion of d-glucose 6-phosphate and triose phosphates, under conditions in which sizeable amounts of d-glucose are formed de novo from 13C-enriched Krebs cycle intermediates generated from either d-[1,2-13C]glucose or [2-13C]acetate.