Metabolism of [1,3-13C]glycerol-1,2,3-tris(methylsuccinate) and glycerol-1,2,3-tris(methyl[2,3-13C]succinate) in rat hepatocytes

Hepatocytes prepared from overnight-fasted rats were incubated for 120 minutes in the presence of 2.5 mmol/L [1,3-13C]glycerol-1,2,3-tris(methylsuccinate) or glycerol-1,2,3-tris(methyl[2,3-13C]succinate). The identification and quantification of 13C-enriched metabolites by a recently developed method for the deconvolution of nuclear magnetic resonance (NMR) spectra with multiplet structures and constraints documented a virtually complete recovery of [1,3-13C]glycerol-1,2,3-tris(methylsuccinate) in 13C-labeled glycerol, lactic acid, and glucose. In hepatocytes exposed to [1,3-13C]glycerol-1,2,3-tris(methylsuccinate), glucose was symmetrically labeled, with the vast majority of hexose molecules being enriched with 13C on both C1 and C3 and/or C6 and C4. The respective abundance of glucose isotopomers labeled either on both C3 and C4 or on only 1 of these 2 C atoms indicated that the triose phosphates generated from [1,3-13C]glycerol represented 44% +/- 1% of the total amount of triose phosphates incorporated into the hexose. In hepatocytes exposed to glycerol-1,2,3-tris(methyl[2,3-13C]succinate), the recovery of [2,3-13C]succinate, [2,3-13C]fumarate, and either double- or single-labeled malate, lactate, alanine, and glucose accounted for about half the initial 13C content of the ester. The majority of the glucose molecules were now labeled in both C, and C2 or C6 and C5, with a preferential labeling of C6-C5 relative to C1-C2, the paired C6/C1 and C5/C2 ratios averaging 1.33 +/-0.04. These findings show that glycerol-1,2,3-tris(methylsuccinate) is efficiently and extensively metabolized in hepatocytes. They reinforce the concept that the asymmetry of glucose 13C-labeling by triose phosphates generated from Krebs cycle intermediates is modulated by the availability of glycerol-derived triose phosphates. Lastly, the present study indicates that the latter triose esters, under the present experimental conditions which do not aim at duplicating the physiological in vivo situation, are largely directly channelled in the gluconeogenic pathway, with only a limited intrahepatic contribution of the "indirect" pathway involving their back-and-forth interconversion to and from pyruvate.