Metabolism of alpha-D-[1,2-13C]glucose pentaacetate and alpha-D-glucose penta[2-13C]acetate in rat hepatocytes

Hepatocytes from fed rats were incubated for 120 min in the presence of alpha-D-1,2-13C]glucose pentaacetate (1.7 mM), both D-1,2-13C]glucose (1.7 mM) and acetate (8.5 mM), alpha-D-glucose penta2-13C]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 beta-hydroxybutyrate recovered in the incubation medium were comparable under the first two experimental conditions. The vast majority of D-glucose isotopomers consisted of alpha- and beta-D1,2-13C]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 alpha-D-glucose penta2-13C]acetate, the large majority of 13C atoms was recovered as 2-13C]acetate and, to a much lesser extent, beta-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 alpha-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 alpha-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.