Glutamate metabolism in brain and its relationship to the operation of the tricarboxylic acid (TCA) cycle. CO2 indicates reactions in which CO2 can be fixed to pyruvate to yield malate (MAL) or oxaloacetate (OAA) for replenishing a-ketoglutarate exported for de novo synthesis of glutamate (GLU) (anaplerotic reactions). AcCoA, acetyl CoA; ASP, aspartate; CIT, citrate; FUM, fumarate; GDH, glutamate dehydrogenase; GLN, glutamine; SSA, succinic semialdehyde; SUC, succinate.