CEREBRAL METABOLISM OF [1,2-C-13(2)]GLUCOSE AND [U-C-13(4)]3-HYDROXYBUTYRATE IN RAT-BRAIN AS DETECTED BY C-13 NMR-SPECTROSCOPY

The metabolism of [1,2-C-13(2)]glucose and [(UC4)-C-13]3-hydroxybutyrate was studied in rat brain with in vivo and in vitro C-13 NMR spectroscopy, taking advantage, in particular, of homonuclear C-13-C-13 spin coupling patterns. After infusion of [1,2-C-13(2)]glucose or [U-C-13(4)]3-hydroxybutyrate into rats, the uptake of the substrates in brain and their metabolism to [1-C-13]bicarbonate could be detected with in vivo C-13 NMR spectroscopy. At the end of the infusion experiment, methanol/HCl/HClO4 extracts of the brain tissue were further analysed by high resolution C-13 NMR spectroscopy. The C-13 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 C-13 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.