The electrogenic carriers

Deviation from expected kinetic behavior for the gated pore model has also been observed in the case of the two electrogenic transporters, the glutamate/aspartate and the adenine nucleotide carriers. 

Wilhamson s experiments were carried out in the presence of transaminase inhibitors, and his kinetic parameters are inconsistent with rates of aspartate transport measured under more physiological conditions, using isolated mitochondria [134,140,142]. Although consistent with fluxes and metabolite levels measured in hepatocytes under a limited range of conditions [102,139], examination of a broader range of conditions reveals order of magnitude discrepancies between predicted and observed rates [143]. 

Studies performed by the authors of this review [142,144], suggest that the discrepancies are due to functional microcompartmentation between the aspartate aminotransferase and the aspartate transporter. The apparent for aspartate efflux can be dramatically decreased by generation of intramitochondrial aspartate by the aminotransferase reaction. Detailed isotopic studies using labelled matrix aspartate in liver mitochondrial [142] and labelled intramitochondrial glutamate in kidney mitochondria [144] confirmed the initial suggestion. 

In contrast to the result of Williamson [139], the present authors find that efflux of aspartate from the mitochondria is a first order process. Since no saturation of the carrier can be detected, determination of effects of Atp and external glutamate on the of matrix aspartate cannot be measured, even at the highest levels of aspartate loading possible. 

In order to circumvent the problem, kinetics of transport were measured in glutamate-loaded submitochondrial particles from liver mitochondria, where the matrix surface of the membrane is exposed to the external media. These data were 

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