Pyruvate translocator

After the discovery of the pyruvate translocator in 1971 by Papa et al. [73] and the introduction of the a-cyanocinnamates as specific inhibitors of this transport system... 

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... 

In 1975 Garrison and Haynes [96] showed that mitochondria isolated from glucagon-treated hepatocytes metabolised added pyruvate at higher rates than normal. Later studies demonstrated that transport of pyruvate into the mitochondria was accelerated [97,98]. This is not due to a direct stimulation of the pyruvate translocator but rather due to an increased matrix pH [98]. This, in turn, is caused by activation of mitochondrial electron transport, which results in the generation of a higher proton-motive force [98]. 

For the phosphotransferase system there is no problem in identifying the energy source for the establishment of the thermodynamic potential gradient. It is apparent that a high-energy phosphate bond in phospho-enol-pyruvate is the immediate source of the energy responsible for the vectorial translocation of the sugar molecule across the membrane (17). 

A single holocarboxylase synthetase (biotin protein ligase, EC 6.3.4.10) acts on the apoenzymes of acetyl CoA, pyruvate, propionyl CoA, and methylcrotonyl CoA carboxylases. Acetyl CoA carboxylase is a cytosolic enzyme, whereas the other three enzymes are mitochondrial. Although holocarboxylase synthetase is found in both the cytosol and mitochondria, it is not clear whether biotin is incorporated into the mitochondrial enzymes before or after they are translocated into the mitochondria. 

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. 

This is the first of the three C4 metabolic routes. PEP is carboxylated to oxaloacetate, reduced to malate and shunted into the bundle sheath. The malate is then decarboxylated to pyruvate by NADP-linked malic enzyme in the bundle-sheath chloroplasts. This generates NADPH, so the chloroplasts lack PSll, and make no oxygen m situ either, which is an unexpected bonus. Pyruvate is then translocated back to the mesophyll, and phosphorylated back to PEP, using 2 ATP (Fig. 13.17). 

Translocation systems of the inner mitochondrial membrane are listed in Table 14-5. Anion translocators are responsible for electroneutral movement of dicarboxylates, tricarboxylates, a-ketoglutarate, glutamate, pyruvate, and inorganic phosphate. Specific electrogenic translocator systems exchange ATP for ADP, and glutamate for aspartate, across the membrane. The metabolic function of the translocators is to provide appropriate substrates (e.g., pyruvate and fatty acids) for mitochondrial oxidation that is coupled to ATP synthesis from ADP and Pj. 

The major site for fatty acid synthesis is the cytosol. However, acetyl-CoA, the substrate for fatty acid synthesis, is formed from pyruvate in the mitochondria and, since the inner membrane is impermeable to acetyl-CoA, is translocated to the cytoplasm indirectly as citrate (Fig. 8). When the rate of production of acetyl-CoA from pyruvate is high, the rate of formation of citrate, catalyzed by citrate synthase in the citric acid cycle, is also elevated, and citrate accumulates in mitochondria. Citrate is then translocated into the... 

Fig. 1 Oxidative metabolism and energy production by mitochondria. The oxidation of pyruvate and free fatty acids (FFA) inside mitochondria produces NADH and FADH2, which transfer their electrons to the mitochondrial respiratory chain. The flow of electrons in mitochondrial complexes I, III, and IV is coupled with the extrusion of protons from the mitochondrial matrix into the intermembrane space. When energy is needed, these protons reenter the matrix through ATP synthase, to generate ATP from ADP. The adenine nucleotide translocator (ANT) then exchanges the formed ATP for cytosolic ADP. G-6-P Glucose 6-phosphate, PDH pyruvate dehydrogenase, LCFA-CoA long-chain fatty acyl-CoA, CPTI carnitine palmitoyltransferase I, TCA cycle tricarboxylic acid cycle, c cytochrome c...

Fig. 17 Mitochondrial effects of valproic acid. Valproic acid freely enters mitochondria, and thus translocates protons into the mitochondrial matrix. This protonophoric effect can slightly imcouple mitochondrial respiration, and can help trigger mitochondrial permeability transition (MPT). Inside the matrix, valproate is extensively transformed into valproyl-CoA, thus sequestering intramitochondrial CoA. The lack of CoA impairs both mitochondrial fatty acid 6-oxidation and pyruvate oxidation. Valproate is also dehydrogenated by cytochrome P450 (CYP) into 4-ene-valproate, which then forms 4-ene-valproyl-CoA and 2,4-diene-valproyl-CoA within mitochondria. The latter is an electrophilic metabolite, which may inactivate 6-oxidation enzymes...

Transport of phosphate, pyruvate and glutamate via their respective translocators is electroneutral and H -coupled. Although in Fig. 1 their transport has been written as cotransport of the anion with H (symport), it is also possible that the actual transport mechanism involves an exchange of the anion with OH (antiport). Experimentally, these two mechanisms cannot be distinguished. The translocators for a-oxoglutarate and malate (the latter is also called the dicarboxylate translocator) also mediate electroneutral exchanges. 

At equilibrium of the citrate-malate exchange system log[(citrate/ malate), /(citrate/malate), ] = ApH, since one H" " accompanies citrate in its exchange for malate. This value must be equal to the logarithm of the pyruvate and glutamate concentration gradients if both the pyruvate and glutamate translocators are also at equilibrium. 

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