Glutamate and aspartate transport in rat brain mitochondria

1. Rat brain mitochondria did not swell in iso-osmotic solutions of ammonium or potassium (plus valinomycin) glutamate or aspartate, with or without addition of uncouplers. 2. Glutamate was able to reduce intramitochondrial NAD(P)⁺; aspartate was able to cause partial re-oxidation. 3. These effects were inhibited by threo-hydroxy-aspartate in whole but not in lysed mitochondria. 4. The existence of a ‘malate–aspartate shuttle’ for the oxidation of extramitochondrial NADH was demonstrated. This shuttle requires the net exchange of glutamate for aspartate across the mitochondrial membrane. 5. Extramitochondrial glutamate did not inhibit intramitochondrial glutaminase under conditions in which the inhibition in lysed mitochondria was virtually complete. 6. The glutaminase activity of these mitochondria was not energy-dependent. 7. We conclude that these mitochondria do not possess a glutamate–hydroxyl antiporter similar to that of liver mitochondria nor a glutamate–glutamine antiporter similar to that of pig kidney mitochondria, but that they do possess a glutamate–aspartate antiporter.