Isotopic discrimination between D-[1-13C]fructose and D-[2-13C]fructose in rat liver cells

When liver cells from either normal or hereditarily diabetic rats are exposed to C-13-enriched D-fructose (10 mM) and unlabelled D-glucose (also 10 mM) in the presence of DO, the output of C-13-enriched D-glucose generated from D-[1-C-13]fructose is significantly lower than that from D-[2-C-13] fructose. This coincides with a higher generation of C-13-enriched L-lactate and L-alanine from D-[1-C-13] fructose, as compared to D-[2-C-13] fructose. In absolute terms, the mean paired difference in the output of C-13-enriched D-glucose generated from D-[1-C-13] fructose versus D-[2-C-13]fructose is not significantly different from the mean paired difference in the production of C-13-enriched L-lactate and L-alanine from the same precursors, with an overall mean value of 7.01 +/- 1.59 mumol (n = 8; P < 0.005). It is proposed that these findings indicate isotopic discrimination at the phosphoglucoisomerase level between C-12 and C-13 for the carbon atom in position I (as compared to that in position 2) of D-fructose 6-phosphate. (C) 2003 Elsevier SAS. All rights reserved.