¹³C isotopomer model for estimation of anaplerotic substrate oxidation via acetyl-CoA

A previous model using ¹³C nuclear magnetic resonance isotopomer analysis provided for direct measurement of the oxidation of ¹³C-enriched substrates in the tricarboxylic acid cycle and/or their entry via anaplerotic pathways. This model did not allow for recycling of labeled metabolites from tricarboxylic acid cycle intermediates into the acetyl-CoA pool. An extension of this model is now presented that incorporates carbon flow from oxaloacetate or malate to acetyl-CoA. This model was examined using propionate metabolism in the heart, in which previous observations indicated that all of the propionate consumed was oxidized to CO₂ and water. Application of the new isotopomer model shows that 2 mM [3-¹³C]propionate entered the tricarboxylic acid cycle as succinyl-CoA (an anaplerotic pathway) at a rate equal to 52% of tricarboxylic acid cycle turnover and that all of this carbon entered the acetyl-CoA pool and was oxidized. This was verified using standard biochemical analysis; from the rate (μmol · min^-1 · g dry wt^-1) of propionate uptake (4.0 ± 0.7), the estimated oxygen consumption (24.8 ± 5) matched that experimentally determined (24.4 ± 3).