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-hydroxybutyra te was studied in rat brain with in vivo and in vitro C-13 NMR spectro scopy, 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 a nd their metabolism to [1-C-13]bicarbonate could be detected with in v ivo C-13 NMR spectroscopy. At the end of the infusion experiment, meth anol/HCl/HClO4 extracts of the brain tissue were further analysed by h igh resolution C-13 NMR spectroscopy. The C-13 spin coupling patterns revealed entirely different isotopomer distributions for the closely r elated cerebral metabolites glutamate, glutamine and 4-aminobutyric ac id. A quantitative analysis of the C-13 Spectra demonstrated (i) the e xistence of two kinetically distinct pools of glutamate, (ii) a pronou nced CO2 fixation via pyruvate carboxylase in the glial cells accounti ng for as much as 38% of the oxaloacetate synthesis in the glial trica rboxylic acid cycle, (iii) a cerebral pyruvate recycling system contri buting maximally 17% of the pyruvate metabolism through the pyruvate d ehydrogenase in neurons, and (iv) a predominant production of 4-aminob utyric acid from glutamate synthesized in the neurons. In addition, th e labelling pattern of N-acetyl aspartate upon infusion of labelled gl ucose 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, th e aspartyl moiety is not in direct contact with the intermediates of g lycolysis or of the tricarboxylic acid cycles.