Cerebral metabolism of [1,2-13C2]glucose and [U-13C4]3-hydroxybutyrate in rat brain as detected by 13C NMR spectroscopy

The metabolism of [1,2-13C2]glucose and [U-13C4]3-hydroxybutyrate was studied in rat brain with in vivo and in vitro 13C NMR spectroscopy, taking advantage, in particular, of homonuclear 13C-13C spin coupling patterns. After infusion of [1,2-13C2]glucose or [U-13C4]3-hydroxybutyrate into rats, the uptake of the substrates in brain and their metabolism to [1-13C]bicarbonate could be detected with in vivo 13C NMR spectroscopy. At the end of the infusion experiment, methanol/HCl/HClO4 extracts of the brain tissue were further analysed by high resolution 13C NMR spectroscopy. The 13C spin coupling patterns revealed entirely different isotopomer distributions for the closely related cerebral metabolites glutamate, glutamine and 4-aminobutyric acid. A quantitative analysis of the 13C spectra demonstrated (i) the existence of two kinetically distinct pools of glutamate, (ii) a pronounced CO2 fixation via pyruvate carboxylase in the glial cells accounting for as much as 38% of the oxaloacetate synthesis in the glial tricarboxylic acid cycle, (iii) a cerebral pyruvate recycling system contributing maximally 17% of the pyruvate metabolism through the pyruvate dehydrogenase in neurons, and (iv) a predominant production of 4-aminobutyric acid from glutamate synthesized in the neurons. In addition, the labelling pattern of N-acetyl aspartate upon infusion of labelled glucose or 3-hydroxybutyrate provided insight into the synthesis of this compound in mammalian brain. While the acetyl moiety originates from the metabolic equivalent of the C-1-C-2 part of cerebral glutamate, the aspartyl moiety is not in direct contact with the intermediates of glycolysis or of the tricarboxylic acid cycles.