Pyruvate transport

Figure21-4. The provision of acetyl-CoA and NADPH for lipogenesis. (PPP, pentose phosphate pathway T, tricarboxylate transporter K, a-ketoglutarate transporter P, pyruvate transporter.)...

A specialized pyruvate transporter is responsible for this step. 

Not all the transporters discussed above are present in aU types of mitochondria the set of activities present in mitochondria depends on the functional needs of the cells from which the mitochondria are isolated. The adenine nucleotide and phosphate transporters are present in all mitochondria thus far studied. This reflects the fact that the major function of mitochondria is the synthesis of ATP. Even in the rare instances (e.g., brown fat mitochondria [55] and mitochondria in anaerobically growing yeast [56]) where the major function is not ATP synthesis, mitochondria normally have active adenine nucleotide transport. The pyruvate transporter also appears to be ubiquitous. The carnitine transporter has been studied in liver [57], heart [35] and sperm [58] and is probably present in all mitochondria which use long-chain fatty acids. 

The pyruvate transporter [201] and the carnitine translocase [202] have both been isolated but not characterized in any detail. The pyruvate transporter and the carnitine translocase, like the phosphate transporter, are inhibited by maleimide derivatives and mercurials, although at higher concentrations of the sulfhydryl reagents. The pyruvate transporter has been isolated in inactive form covalently linked to phenyl maleimide. Identification was based on the correlation of labelling of the protein with inhibition of transport, and by the fact that mercurials prevented the labelling. The molecular weight of the isolated monomeric protein is surprisingly low, approximately 15000. 

The mitochondrial translocators which have been most carefully assessed with respect to their role in control of metabolism are (1) the adenine nucleotide translocator with respect to its role in the control of respiration (2) the liver pyruvate transporter and the control of gluconeogenesis and (3) kidney glutamate and glutamine transport and their control of ammoniagenesis. 

Halestrap initially concluded that the increases in mitochondrial pyruvate transport and carboxylation were due to an increase in A pH secondary to stimulation of the electron transport chain in the cytochrome fee, region [255]. The conclusion was based largely on spectral measurements of the redox state of these cytochromes in the control and stimulated states. The spectral measurements were later found to be artifactual due to low amplitude Ca swelling of the mitochondria. Halestrap then suggested that the stable changes in the mitochondria might reside in the lipid components of the membrane due to phospholipase A 2 activity [261,262], but he has been unable to confirm this with lysophospholipid measurements [263]. On the other hand, using an EPR spin label probe of the lipid environment of the isolated mitochondria, Hoek has found differences between control and treated mitochondria [264]. 

Lin, R.Y., Vera, J.C., Chaganti, R.S., and Golde, D.W (1998) Human monocarboxylate transporter 2 (MCT2) is a high affinity pyruvate transporter. The Journal of Biological Chemistry, 273, 28959-28965. 

By increasing substrate availability through stimulation of mitochondrial respiration. This decreases the intramitochondrial concentration of H and increases the rate of pyruvate transport. The [ATP]/[ADP] ratio also rises. 

Wiemer, E. A. C., Ter Kuile, B, H., Michels, P. A. M. and Opperdoes, F. R. (1992) Pyruvate transport across the plasma membrane of the bloodstream form of Trypanosoma brucei is mediated by a facilitated diffusion carrier. Biochem. Biophys. Res. Commun. 184 1028-1034. 

Carboxylic acids such as pyruvate, succinate, and citrate are transported into the matrix by the pyruvate transporter, the dicarboxylic acid transporter, and the tricarboxylic acid transporter, respectively. Pymvate transport operates as an antiporter with hydroxide ion. The other transporters are driven by concentration gradients for their substrates. For example, high concentrations of citrate in the matrix lead to export of citrate to the cytoplasm, where it can inhibit phospho-fmctokinase (see Chapter 9). 

An important question is whether mitochondrial pyruvate transport can regulate pyruvate metabolism. One way to approach this problem is to carry out careful titrations of pyruvate-dependent processes with the transport inhibitors. So far, such experiments have not been done. Inspection of the kinetic properties of the pyruvate translocator, however, shows that limitation of pyruvate metabolism by its transport into the mitochondria is possible. For liver the average reported is 70 nmol/min/mg mitochondrial protein, which is 900 jumol/g dry weight of liver tissue/h (Table 1). The maximum rate of glucose synthesis from lactate in hepato-cytes is about 430 jumol/g dry weight/h [86], so that flux through the pyruvate translocator under these conditions is 2 X 430 = 860 jumol/g dry weight/h, which is close to its In the presence of lactate plus ethanol mitochondrial pyruvate... 

The study of Oglno et al. (30) on the metabolic regulations and pyruvate transport in anaerobic coli cells is of special interest because the [1- C] glucose metabolites were detected by proton correlation spectroscopy and only signals from extracellular metabolites which had diffused through the cell membrane and accumulated in the medium were observed (acetate, lactate, ethanol, succinate, and pyruvate). This allowed the evaluation of perturbations to the cell on the influx and egress of pyruvate. 

Fig. 13.1.1. Schematic overview of mitochondrial oxidative phosphorylation. A part of the mitochondrion is represented, showing the outer mitochondrial membrane (OMM), inner mitochondrial membrane (IMM) and crista (an invagination of the inner membrane). Substrates for oxidation enter the mitochondrion through specific carrier proteins, e.g., the pyruvate transporter, (PyrT). Reducing equivalents from fatty acyl CoA dehydrogenases, pyruvate dehydrogenase and the TCA cycle are delivered to the electron transport chain through NADH, succinate ubiquinol oxidoreductase (SQO), electron transfer flavoprotein (ETF) and its ubiquinol-...

Halestrap, A.P. (1978), Pyruvate transport across mitochondrial and plasma membranes. In Regulatory Mechanisms of Carbohydrate Research (ed. V. Esmann), Pergamon Press, Oxford, pp. 61-70. 

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