Aspartate transport

Figure 12-13. Malate shuttle for transfer of reducing equivalents from the cytosol into the mitochondrion. Ketoglutarate transporter , glutamate/aspartate transporter (note the proton symport with glutamate).

Storck, T., Schulte, S., Hofmann, K., and Stoffel, W. (1992) Structure, expression, and functional analysis of a Na+-dependent glutamate/aspartate transporter from rat brain. Proc. Natl. Acad. Sci. USA 89, 10955-10959. 

Tolner, B., Poolman, B., Wallace, B., and Konings, W. N. (1992) Revised nucleotide sequence of the gltP gene, which encodes the proton-glutamate-aspartate transport protein of Escherichia coli K-12. J. Bacteriol. 174, 2391-2393. 

Although all five glutamate transporter subtypes support uncoupled chloride conductances, there are a number of distinct differences between the transporter subtypes. The current generated by EAAT4 and EAAT5 is predominantly owing to chloride ions, and in the case of L-aspartate transport by EAAT4, it has been estimated that up to 95%... 

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. 

It is apparent that methodological problems limit the usefulness of the kinetic approach to studying mechanisms. However, data of an entirely different nature argue against a sequential model for the aspartate transporter. Studies of the metabolism of cysteine sulfinic acid show that it is transported by the glutamate/aspartate transporter and that it transaminates with a-ketoglutarate or oxalacetate to yield glutamate or aspartate and jS-sulfinyl pyruvate, which spontaneously hydrolyzes into sulfite and pyruvate [148,149]. 

Matsnda K, Ueda Y, Doi T, Tono T, Haruta A, Toyama K, Komnne S. Increase in glutamate-aspartate transporter (GLAST) mRNA during kanamycin-induced cochlear insnlt in rats. Hear Res 1999 133(1-2) 10-16. 

Lawton DM, Furness DN, Lindemann B, Hackney CM. 2000. Localization of the glutamate-aspartate transporter, GLAST, in rat taste buds. Eur J Neurosci 12 3163-3171. 

TTie actual meanings of the acronyms (GLAST, glutamate-aspartate transporter GLT. glutamate transporter EAAC. excitatory amino acid carrier EAAT. excitatory amino acid transporter) are not important, as they do not reflect functional differences among the transporters. 

Derouiche A, Rauen T (1995) Coincidence of L-glutamate/L-aspartate transporter (GLAST) and glutamine synthetase (GS) immunoreactions in retinal glia evidence for coupling of GLAST and GS in transmitter clearance. J Neurosci Res 42 131-143. 

Furness DN, Lehre KP (1997) Immunocytochemical localization of a high-affinity glutamate-aspartate transporter, GLAST, in the rat and guinea-pig cochlea. Eur J Neurosci 9 1961-1969. 

Lundy DF, McBean GJ (1995) Pre-incubation of synaptosomes with arachidonic acid potentiates inhibition of [ H]D-aspartate transport. Eur J Pharmacol 297 273-279. 

Glutamate-aspartate transporter of mitochondrial inner membrane... 

Transport of adenine nucleotides is presumably fully electrophoretic, and therefore driven by the mitochondrial membrane potential, because ADP exchanges with ATP [9], Aspartate transport is both electrophoretic and -coupled because undissociated glutamate exchanges with the aspartate anion [10]. Since in the intact cell the mitochondrial membrane potential is positive outside and the mitochondrial matrix slightly alkaline with respect to the cytosol, it follows that in vivo movement of ATP and of aspartate is unidirectional, out of the mitochondria. 

Obviously, both views are incompatible. One possibility is that the diffusion gradient of matrix aspartate, observed at 4°C, does not exist at 37°C. Also, the assumption that the for matrix aspartate can be measured in submitochondrial particles may be incorrect because of alterations in properties of the carrier during particle preparation. In fact, the of aspartate transport in these particles was... 

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