The mitochondrial oxoglutarate carrier protein contains a disulfide bridge between intramembranous cysteines 221 and 224

Examination of the superoxide/hydrogen peroxide forming and quenching potential of mouse liver mitochondria

Pyruvate dehydrogenase (PDHC) and α-ketoglutarate dehydrogenase complex (KGDHC) are important sources of reactive oxygen species (ROS). In addition, it has been found that mitochondria can also serve as sinks for cellular hydrogen peroxide (H₂O₂). However, the ROS forming and quenching capacity of liver mitochondria has never been thoroughly examined. Here, we show that mouse liver mitochondria use catalase, glutathione (GSH), and peroxiredoxin (PRX) systems to quench ROS. Incubation of mitochondria with catalase inhibitor 3-amino-1,2,4-triazole (triazole) induced a significant increase in pyruvate or α-ketoglutarate driven O₂^-/H₂O₂ formation. 1-Choro-2,4-dinitrobenzene (CDNB), which depletes glutathione (GSH), elicited a similar effect. Auranofin (AF), a thioredoxin reductase-2 (TR2) inhibitor which disables the PRX system, did not significantly change O₂^-/H₂O₂ formation. By contrast catalase, GSH, and PRX were all required to scavenging extramitochondrial H₂O₂. In this study, the ROS forming potential of PDHC, KGDHC, Complex I, and Complex III was also profiled. Titration of mitochondria with 3-methyl-2-oxovaleric acid (KMV), a specific inhibitor for O₂^-/H₂O₂ production by KGDHC, induced a ~86% and ~84% decrease in ROS production during α-ketoglutarate and pyruvate oxidation. Titration of myxothiazol, a Complex III inhibitor, decreased O₂^-/H₂O₂ formation by ~45%. Rotenone also lowered ROS production in mitochondria metabolizing pyruvate or α-ketoglutarate indicating that Complex I does not contribute to ROS production during forward electron transfer from NADH. Taken together, our results indicate that KGDHC and Complex III are high capacity sites for O₂^-/H₂O₂ production in mouse liver mitochondria. We also confirm that catalase plays a role in quenching either exogenous or intramitochondrial H₂O₂.

Mitochondrial Glutathione in Diabetic Nephropathy

Although there are many etiologies for diabetic nephropathy (DN), one common characteristic of all cases involves mitochondrial oxidative stress and consequent bioenergetic dysfunction. As the predominant low-molecular-weight, intramitochondrial thiol reductant, the mitochondrial glutathione (mtGSH) pool plays important roles in how this organelle adapts to the chronic hyperglycemia and redox imbalances associated with DN. This review will summarize information about the processes by which this important GSH pool is regulated and how manipulation of these processes can affect mitochondrial and cellular function in the renal proximal tubule. Mitochondria in renal proximal tubular (PT) cells do not appear to synthesize GSH de novo but obtain it by transport from the cytoplasm. Two inner membrane organic anion carriers, the dicarboxylate carrier (DIC; Slc25a10) and 2-oxoglutarate carrier (OGC; Slc25a11) are responsible for this transport. Genetic modulation of DIC or OGC expression in vitro in PT cells from diabetic rats can alter mitochondrial function and susceptibility of renal PT cells to oxidants, with overexpression leading to reversion of bioenergetic conditions to a non-diabetic state and protection of cells from injury. These findings support the mtGSH carriers as potential therapeutic targets to correct the underlying metabolic disturbance in DN.

The mitochondrial oxoglutarate carrier: from identification to mechanism

The 2-oxoglutarate carrier (OGC) belongs to the mitochondrial carrier protein family whose members are responsible for the exchange of metabolites, cofactors and nucleotides between the cytoplasm and mitochondrial matrix. Initially, OGC was characterized by determining substrate specificity, kinetic parameters of transport, inhibitors and molecular probes that form covalent bonds with specific residues. It was shown that OGC specifically transports oxoglutarate and certain carboxylic acids. The substrate specificity combination of OGC is unique, although many of its substrates are also transported by other mitochondrial carriers. The abundant recombinant expression of bovine OGC in Escherichia coli and its ability to functionally reconstitute into proteoliposomes made it possible to deduce the individual contribution of each and every residue of OGC to the transport activity by a complete set of cys-scanning mutants. These studies give experimental support for a substrate binding site constituted by three major contact points on the even-numbered α-helices and identifies other residues as important for transport function through their crucial positions in the structure for conserved interactions and the conformational changes of the carrier during the transport cycle. The results of these investigations have led to utilize OGC as a model protein for understanding the transport mechanism of mitochondrial carriers.

Renal membrane transport of glutathione in toxicology and disease

Membrane transport processes, at both the plasma membranes and intracellular membranes, play critical roles in renal function and are a determining factor in the susceptibility of renal epithelial cells to blood-borne drugs and toxic chemicals. Proximal tubular epithelial cells possess a large array of transport proteins for organic anions, organic cations, and peptides on both basolateral and brush-border plasma membranes. Although these transporters function in excretion of waste products and reabsorption of nutrients, they also play a role in the susceptibility of the kidneys to drugs and other toxicants in the blood. The proximal tubules are typically the primary target cells because they are the first epithelial cell population exposed to such chemicals in either the renal plasma or glomerular filtrate and because of their large array of membrane transporters. Besides transport across the basolateral and brush-border plasma membranes, transport across intracellular membranes such as the mitochondrial inner membrane is a critical determinant of metabolite distribution. To illustrate the function of these transporters, carrier-mediated processes for transport of the tripeptide and antioxidant glutathione across the basolateral, brush-border, and mitochondrial inner membranes of the renal proximal tubule are reviewed. Studies are summarized that have identified the involvement of specific carrier proteins and characterized the role of these transporters in glutathione metabolism and turnover, susceptibility of the proximal tubules to oxidative and other stresses, and modulation in disease and other pathological processes.

Mitochondrial glutathione transport: physiological, pathological and toxicological implications

Although most cellular glutathione (GSH) is in the cytoplasm, a distinctly regulated pool is present in mitochondria. Inasmuch as GSH synthesis is primarily restricted to the cytoplasm, the mitochondrial pool must derive from transport of cytoplasmic GSH across the mitochondrial inner membrane. Early studies in liver mitochondria primarily focused on the relationship between GSH status and membrane permeability and energetics. Because GSH is an anion at physiological pH, this suggested that some of the organic anion carriers present in the inner membrane could function in GSH transport. Indeed, studies by Lash and colleagues in isolated mitochondria from rat kidney showed that most of the transport (>80%) in that tissue could be accounted for by function of the dicarboxylate carrier (DIC, Slc25a10) and the oxoglutarate carrier (OGC, Slc25a11), which mediate electroneutral exchange of dicarboxylates for inorganic phosphate and 2-oxoglutarate for other dicarboxylates, respectively. The identity and function of specific carrier proteins in other tissues is less certain, although the OGC is expressed in heart, liver, and brain and the DIC is expressed in liver and kidney. An additional carrier that transports 2-oxoglutarate, the oxodicarboxylate or oxoadipate carrier (ODC; Slc25a21), has been described in rat and human liver and its expression has a wide tissue distribution, although its potential function in GSH transport has not been investigated. Overexpression of the cDNA for the DIC and OGC in a renal proximal tubule-derived cell line, NRK-52E cells, showed that enhanced carrier expression and activity protects against oxidative stress and chemically induced apoptosis. This has implications for development of novel therapeutic approaches for treatment of human diseases and pathological states. Several conditions, such as alcoholic liver disease, cirrhosis or other chronic biliary obstructive diseases, and diabetic nephropathy, are associated with depletion or oxidation of the mitochondrial GSH pool in liver or kidney.

Purification and folding of recombinant bovine oxoglutarate/malate carrier by immobilized metal-ion affinity chromatography

A major obstacle to investigating the structure of membrane proteins is the difficulty in obtaining sufficient amounts of functional protein. The oxoglutarate carrier, an intrinsic membrane-transport protein of the inner membranes of bovine-heart mitochondria, has been cloned as a fusion protein containing a C-terminal hexa-histidine tag. This fusion protein has been expressed at an abundant level in a mutant strain of Escherichia coli BL21 (DE3) called C41 (DE3). The protein accumulated as inclusion bodies and none was detected in the bacterial inner membrane. The denatured protein was refolded to reconstitute functional properties similar to the native protein. Solubilized inclusion body protein was immobilized using nickel-chelating affinity chromatography, and purified and refolded in a single step. The protein eluted as a monomer which was stable in mild detergent, at a yield equivalent to 15 mg active protein/liter bacterial culture. The reconstituted fusion protein displayed the same transport characteristics as the wild-type, demonstrating that the tag does not perturb the structure of the protein. The oxoglutarate carrier is one member of an extensive family of mitochondrial transport proteins. These proteins transport many different metabolites across the inner mitochondrial membrane and share a common mechanism of action. Therefore, it is likely that this folding protocol can be applied successfully to other mitochondrial transport proteins, thus providing sufficient protein for extensive crystallization trials with a wide range of family members.

Enrichment and functional reconstitution of glutathione transport activity from rabbit kidney mitochondria: further evidence for the role of the dicarboxylate and 2-oxoglutarate carriers in mitochondrial glutathione transport

In previous studies, we provided evidence for uptake of glutathione (GSH) by the dicarboxylate and the 2-oxoglutarate carriers in rat kidney mitochondria. To investigate further the role of these two carriers, GSH transport activity was enriched from rabbit kidney mitochondria and functionally reconstituted into phospholipid vesicles. Starting with 200 mg of mitoplast protein, 2 mg of partially enriched proteins were obtained after Triton X-114 solubilization and hydroxyapatite chromatography. The reconstituted proteoliposomes catalyzed butylmalonate-sensitive uptake of [(14)C]malonate, phenylsuccinate-sensitive uptake of [(14)C]2-oxoglutarate, and transport activity with [(3)H]GSH. The initial rate of uptake of 5 mM GSH was approximately 170 nmol/min per mg protein, with a first-order rate constant of 0.3 min(-1), which is very close to that previously determined in freshly isolated rat kidney mitochondria. The enrichment procedure resulted in an approximately 60-fold increase in the specific activity of GSH transport. Substrates and inhibitors for the dicarboxylate and the 2-oxoglutarate carriers (i.e., malate, malonate, 2-oxoglutarate, butylmalonate, phenylsuccinate) significantly inhibited the uptake of [(3)H]GSH, whereas most substrates for the tricarboxylate and monocarboxylate carriers had no effect. GSH uptake exhibited an apparent K(m) of 2.8 mM and a V(max) of 260 nmol/min per mg protein. Analysis of mutual inhibition between GSH and the dicarboxylates suggested that the dicarboxylate carrier contributes a somewhat higher proportion to overall GSH uptake and that both carriers account for 70 to 80% of total GSH uptake. These results provide further evidence for the function of the dicarboxylate and 2-oxoglutarate carriers in the mitochondrial transport of GSH.

Inactivation of the reconstituted oxoglutarate carrier from bovine heart mitochondria by pyridoxal 5'-phosphate

The effect of pyridoxal 5'-phosphate and some other lysine reagents on the purified, reconstituted mitochondrial oxoglutarate transport protein has been investigated. The inhibition of oxoglutarate/oxoglutarate exchange by pyridoxal 5'-phosphate can be reversed by passing the proteoliposomes through a Sephadex column but the reduction of the Schiff's base by sodium borohydride yielded an irreversible inactivation of the oxoglutarate carrier protein. Pyridoxal 5'-phosphate, which caused a time- and concentration-dependent inactivation of oxoglutarate transport with an IC50 of 0.5 mM, competed with the substrate for binding to the oxoglutarate carrier (Ki = 0.4 mM). Kinetic analysis of oxoglutarate transport inhibition by pyridoxal 5'-phosphate indicated that modification of a single amino acid residue/carrier molecule was sufficient for complete inhibition of oxoglutarate transport. After reduction with sodium borohydride [3H]pyridoxal 5'-phosphate bound covalently to the oxoglutarate carrier. Incubation of the proteoliposomes with oxoglutarate or L-malate protected the carrier against inactivation and no radioactivity was found associated with the carrier protein. In contrast, glutarate and substrates of other mitochondrial carrier proteins were unable to protect the carrier. Mersalyl, which is a known sulfhydryl reagent, also failed to protect the oxoglutarate carrier against inhibition by pyridoxal 5'-phosphate. These results indicate that pyridoxal 5'-phosphate interacts with the oxoglutarate carrier at a site(s) (i.e., a lysine residue(s) and/or the amino-terminal glycine residue) which is essential for substrate translocation and may be localized at or near the substrate-binding site.