C-NMR spectroscopic evaluation of the citric acid cycle flux in conditions of high aspartate transaminase activity in glucose-perfused rat hearts

A new mathematical model, based on the observation of ¹³C-NMR spectra of two principal metabolites (glutamate and aspartate), was constructed to determine the citric acid cycle flux in the case of high aspartate transaminase activity leading to the formation of large amounts of labeled aspartate and glutamate. In this model, the labeling of glutamate and aspartate carbons by chemical and isotopic exchange with the citric acid cycle are considered to be interdependent. With [U-¹³C]Glc or [1,2-¹³C]acetate as a substrate, all glutamate and aspartate carbons can be labeled. The isotopic transformations of 32 glutamate isotopomers into 16 aspartate isotopomers or vice versa were studied using matrix operations; the results were compiled in two matrices. We showed how the flux constants of the citric acid cycle and the ¹³C-enrichment of acetyl-CoA can be deduced from ¹³C-NMR spectra of glutamate and/or aspartate. The citric acid cycle flux in beating Wistar rat hearts, aerobically perfused with [U-¹³C]glucose in the absence of insulin, was investigated by ¹³C-NMR spectroscopy. Surprisingly, aspartate instead of glutamate was found to be the most abundantly-labeled metabolite, indicating that aspartate transaminase (which catalyses the reversible reaction: (glutamate + oxaloacetate ↔ 2-oxoglutarate + aspartate) is highly active in the absence of insulin. The amount of aspartate was about two times larger than glutamate. The quantities of glutamate (G_o) or aspartate (A_O) were approximately the same for all hearts and remained constant during perfusion: G₀ = (0.74 ±0.03) μmol/g; A₀ = (1.49±0.05) μmol/g. The flux constants, i.e., the fraction of glutamate and aspartate in exchange with the citric acid cycle, were about 1.45 min⁻¹ and 0.72 min⁻¹, respectively; the flux of this cycle is about (1.07±0.02) μmol min^-1 g^-1. Excellent agreement between the computed and experimental data was obtained, showing that: i) in the absence of insulin, only 41% of acetyl-CoA is formed from glucose while the rest is derived from endogenous substrates; and ii) the exchange between aspartate and oxaloacetate or between glutamate and 2-oxoglutarate is fast in comparison with the biological transformation of intermediate compounds by the citric acid cycle.