The ratio of acetate-to-glucose oxidation in astrocytes from a single ¹³C NMR spectrum of cerebral cortex

The ¹³C-labeling patterns in glutamate and glutamine from brain tissue are quite different after infusion of a mixture of ¹³C-enriched glucose and acetate. Two processes contribute to this observation, oxidation of acetate by astrocytes but not neurons, and preferential incorporation of α-ketoglutarate into glutamate in neurons, and incorporation of α-ketoglutarate into glutamine in astrocytes. The acetate:glucose ratio, introduced previously for analysis of a single ¹³C NMR spectrum, provides a useful index of acetate and glucose oxidation in the brain tissue. However, quantitation of relative substrate oxidation at the cell compartment level has not been reported. A simple mathematical method is presented to quantify the ratio of acetate-to-glucose oxidation in astrocytes, based on the standard assumption that neurons do not oxidize acetate. Mice were infused with [1,2-¹³C]acetate and [1,6-¹³C]glucose, and proton decoupled ¹³C NMR spectra of cortex extracts were acquired. A fit of those spectra to the model indicated that ¹³C-labeled acetate and glucose contributed approximately equally to acetyl-CoA (0.96) in astrocytes. As this method relies on a single ¹³C NMR spectrum, it can be readily applied to multiple physiologic and pathologic conditions.