EFFECTS OF KETONE-BODIES ON ASTROCYTE AMINO-ACID-METABOLISM

The effects of acetoacetate and 3-hydroxybutyrate on glial amino acid metabolism were studied in primary cultures of astrocytes. The exchang e of nitrogen among amino acids was measured with N-15 as a metabolic probe and gas chromatography-mass spectrometry as a tool with which to quantify isotope abundance. Addition of either acetoacetate or 3-hydr oxybutyrate (5 mM) to the incubation medium did not alter the initial rate of appearance of [N-15]glutamate in the glia, but it did inhibit transamination of glutamate to [N-15]aspartate. Addition of acetoaceta te also inhibited formation of [2-N-15]glutamine, but 3-hydroxybutyrat e had a stimulatory effect. The presence in the medium of sodium aceta te (5 mM) was also associated with diminished production of [N-15]aspa rtate and [2-N-15]glutamine with [N-15]glutamate as precursor. Studies with [2-N-15]glutamine as precursor indicated that treatment of the a strocytes with ketone bodies did not alter flux through the glutaminas e pathway. Nor did the presence of the ketone bodies reduce significan tly the flux of nitrogen from [N-15]GABA to [2-N-15]glutamine when the former species served as a metabolic tracer. The concentration of int ernal citrate increased in the presence of acetoacetate, 3-hydroxybuty rate, and acetate. Studies with purified sheep brain glutamine synthet ase showed that citrate inhibited this enzyme. These findings are cons idered in terms of the known anticonvulsant effect of a ketogenic diet .