Electrogenic characteristics of the mitochondrial glutamate aspartate antiporter

Rat liver mitochondria were loaded with aspartate by treating glutamate loaded mitochondria with oxalacetate. Aspartate efflux was initiated by addition of extramitochondrial glutamate and was accompanied by the uptake of an equivalent amount of glutamate and protons. Proton uptake, however, did not occur in the presence of an uncoupling agent, and net transport of aspartate was facilitated by the presence of a source of energy from the oxidation of ascorbate and tetramethylphenylenediamine. In the absence of uncoupling agents or a source of energy, exchange of aspartate for glutamate resulted in acidification of the mitochondrial matrix and efflux of an equivalent amount of potassium. The data indicate that exchange of aspartate and glutamate across the mitochondrial membrane is an electrogenic process. The most plausible model is one in which a neutral glutamate carrier complex transports glutamate from outside the mitochondria to inside. The aspartate carrier complex formed on the inner surface of the mitochondrial membrane should then be negatively charged and transport of the anionic carrier aspartate complex would be facilitated by an electrical potential across the mitochondrial membrane.