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13 C Nuclear Magnetic Resonance Evidence for γ‐Aminobutyric Acid Formation via Pyruvate Carboxylase in Rat Brain: A Metabolic Basis for Compartmentation0
Author(s) -
Brainard James R.,
Kyner Eric,
Rosenberg Gary A.
Publication year - 1989
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1111/j.1471-4159.1989.tb07426.x
Subject(s) - glutamate receptor , pyruvate carboxylase , citric acid cycle , biochemistry , tricarboxylic acid , pyruvate dehydrogenase complex , glycolysis , metabolism , glutamic acid , amino acid , glutamate dehydrogenase , biology , chemistry , enzyme , receptor
The compartmentation of amino acid metabolism is an active and important area of brain research. I3 C labeling and 13 C nuclear magnetic resonance (NMR) are powerful tools for studying metabolic pathways, because information about the metabolic histories of metabolites can be determined from the appearance and position of the label in products. We have used 13 C labeling and 13 C NMR in order to investigate the metabolic history of γ‐aminobutyric acid (GABA) and glutamate in rat brain. [l‐ l3 C]Glucose was infused into anesthetized rats and the 13 C labeling patterns in GABA and glutamate examined in brain tissue extracts obtained at various times after infusion of the label. Five minutes after infusion, most of the l3 C label in glutamate appeared at the C4 position; at later times, label was also present at C2 and C3. This l3 C labeling pattern occurs when [1‐ l3 Cjglucose is metabolized to pyruvate by glycolysis and enters the pool of tricarboxylic acid (TCA) intermediates via pyruvate dehydrogenase. The label exchanges into glutamate from the TCA cycle pool through glutamate transaminases or dehydrogenase. After 30 min of infusion, approximately 10% of the total 13 C in brain extracts appeared in GABA, primarily (>80%) at the amino carbon (C4), indicating that the GABA detected is labeled through pyruvate carboxylase. The different labeling patterns observed for glutamate and GABA show that the large detectable glutamate pool does not serve as the precursor to GABA. Our NMR data support previous experiments suggesting compartmentation of metabolism in brain, and further demonstrate that GABA is formed from a pool of TCA cycle intermediates derived from an anaplerotic pathway involving pyruvate carboxylase.