Kinetic properties of aspartate transport in rat heart mitochondrial inner membranes

The mitochondrial glutamate-aspartate exchange carrier catalyzes the electrogenic exchange of intramitochondrial aspartate for extramitochondrial glutamate. Protons are cotransported with glutamate in a 1:1 ratio. In the present study, the effects of pH and glutamate concentration on glutamate entry into intact mitochondria were determined. Hydrogen ions were found to decrease the K_m for glutamate entry. In addition, using glutamate-loaded submitochondrial particles, aspartate transport into the particles was measured as a function of internal and external glutamate concentrations, pH, and electrical potential across the membrane. Glutamate, was a competitive inhibitor of aspartate transport when both amino acids were present on the same side of the membrane, while H⁺ was a noncompetitive inhibitor of aspartate entry into the particles. A decrease in glutamate concentration on the inside of the particles brought about a parallel decrease in V and K_m for aspartate outside of the particles, thus suggesting a ping-pong mechanism for the carrier. The uncoupling agent, carbonyl cyanide p-trifluoromethoxy-phenylhydrazone (FCCP), lowered both the K_m and V of aspartate transport, while the effect on V was somewhat larger. Data obtained in the presence of KSCN was similar to that obtained with FCCP, and therefore it is concluded that both K_m and V changes are dependent on a change of electrical potential across the membrane. A model for the carrier is proposed, which is consistent with the data presented. The model includes a single binding site specific for either glutamate or aspartate, and a separate binding site for the cotransported proton. The affinity of the binding site for protons is increased by simultaneous glutamate binding, but decreased by aspartate binding. The data suggest that an increase in the membrane potential increases the mobility of the charged carrier-aspartate complex, but also facilitates some additional step in the exchange cycle involving subsequent return of the carrier to the matrix side of the membrane. The additional membrane-potential-dependent step could be proton binding on the cytosolic side of the carrier.