Purification by affinity chromatography of the dicarboxylate carrier from bovine heart mitochondria

Human mitochondrial carriers of the SLC25 family function as monomers exchanging substrates with a ping-pong kinetic mechanism

Members of the SLC25 mitochondrial carrier family link cytosolic and mitochondrial metabolism and support cellular maintenance and growth by transporting compounds across the mitochondrial inner membrane. Their monomeric or dimeric state and kinetic mechanism have been a matter of long-standing debate. It is believed by some that they exist as homodimers and transport substrates with a sequential kinetic mechanism, forming a ternary complex where both exchanged substrates are bound simultaneously. Some studies, in contrast, have provided evidence indicating that the mitochondrial ADP/ATP carrier (SLC25A4) functions as a monomer, has a single substrate binding site, and operates with a ping-pong kinetic mechanism, whereby ADP is imported before ATP is exported. Here we reanalyze the oligomeric state and kinetic properties of the human mitochondrial citrate carrier (SLC25A1), dicarboxylate carrier (SLC25A10), oxoglutarate carrier (SLC25A11), and aspartate/glutamate carrier (SLC25A13), all previously reported to be dimers with a sequential kinetic mechanism. We demonstrate that they are monomers, except for dimeric SLC25A13, and operate with a ping-pong kinetic mechanism in which the substrate import and export steps occur consecutively. These observations are consistent with a common transport mechanism, based on a functional monomer, in which a single central substrate-binding site is alternately accessible.

Modeling mitochondrial bioenergetics with integrated volume dynamics

Mathematical models of mitochondrial bioenergetics provide powerful analytical tools to help interpret experimental data and facilitate experimental design for elucidating the supporting biochemical and physical processes. As a next step towards constructing a complete physiologically faithful mitochondrial bioenergetics model, a mathematical model was developed targeting the cardiac mitochondrial bioenergetic based upon previous efforts, and corroborated using both transient and steady state data. The model consists of several modified rate functions of mitochondrial bioenergetics, integrated calcium dynamics and a detailed description of the K⁺-cycle and its effect on mitochondrial bioenergetics and matrix volume regulation. Model simulations were used to fit 42 adjustable parameters to four independent experimental data sets consisting of 32 data curves. During the model development, a certain network topology had to be in place and some assumptions about uncertain or unobserved experimental factors and conditions were explicitly constrained in order to faithfully reproduce all the data sets. These realizations are discussed, and their necessity helps contribute to the collective understanding of the mitochondrial bioenergetics.

Mathematically modeling biological systems challenges our current understanding of the physical and biochemical events contributing to the observed dynamics. It requires careful consideration of hypothesized mechanisms, model development assumptions and details regarding the experimental conditions. We have adopted a modeling approach to translate these factors that explicitly considers the thermodynamic constraints, biochemical states and reaction mechanisms during model development. Such models have numerous constant parameters that must be determined. Integrating thermodynamics and detailed mechanistic representation of the principal phenomena help constrain these parameter values; therefore, only a handful of the total number of model parameters (∼10%) must be adjusted during parameter estimation through model simulations. Additionally, all models must undergo some form of corroboration prior to application. In practice, this corroboration should challenge all possible dynamics of the model, but it is recognized that in this data rich world, we are surprisingly data poor. Eventually such developed and corroborated models are capable of supporting current hypotheses, guiding experimental designs and contributing to the overall knowledge base of biological processes.

Cardiolipin and mitochondrial carriers

Members of the mitochondrial carrier family interact with cardiolipin (CL) as evident from a variety of functional and structural effects. CL stabilises carrier proteins on isolation with detergents, with the P(i) carrier as the prime example. CL is required for transport in reconstituted vesicles, prime examples are the P(i)- and ADP/ATP carrier (AAC). CL binds to the AAC in a graded manner; 6 CL/AAC dimer bind tightly as measured on the (31)P NMR time scale. 2 additional CL/dimer bind reversibly and a fast exchanging envelope of phospholipids includes CL as measured on the ESR time scale. In the crystal structure of the CAT-AAC complex 3 CL bind to the periphery of the AAC in a three-fold pseudo-symmetry. The binding of CL is implicated to contribute lowering the high transition energy barriers in the AAC. Para-functions of the AAC, as in the mitochondrial pore transition (MPT) and in cell death are linked to the CL binding of the AAC. Ca(++) or oxidants can sequester or destroy AAC bound CL, rendering AAC labile, allowing pore formation and degradation. Thus AAC, by being vital for energy transfer, constitutes an Achilles heel in the eukaryotic cell. AAC together with CL is also engaged in respiratory supercomplexes. Different from AAC the similarly structured uncoupling protein (UCP1) has no tightly bound CL, but CL addition lowers affinity of the inhibitory nucleotide binding that may contribute to the physiological regulation of the uncoupling activity by ATP.

Identification of a novel gene encoding the yeast mitochondrial dicarboxylate transport protein via overexpression, purification, and characterization of its protein product

A gene encoding the mitochondrial dicarboxylate transport protein (DTP) has been identified for the first time from any organism. Our strategy involved overexpression of putative mitochondrial transporter genes, selected based on analysis of the yeast genome, followed by purification and functional reconstitution of the resulting protein products. The DTP gene from the yeast Saccharomyces cerevisiae encodes a 298-residue basic protein which, in common with other mitochondrial anion transporters of known sequence and function, displays the mitochondrial transporter signature motif, three homologous 100-amino acid sequence domains, and six predicted membrane-spanning regions. The product of this gene has been abundantly expressed in Escherichia coli where it accumulates in inclusion bodies. Upon solubilization of the overexpressed DTP from isolated inclusion bodies with Sarkosyl, 28 mg of DTP was obtained per liter of E. coli culture at a purity of 75%. The purified, overexpressed DTP was then reconstituted in phospholipid vesicles where both its kinetic properties (i.e. Km = 1. 55 mM and Vmax = 3.0 micro;mol/min/mg protein) and its substrate specificity were determined. The intraliposomal substrates malonate, malate, succinate, and phosphate effectively supported [14C]malonate uptake, whereas other anions tested did not. External substrate competition studies revealed a similar specificity profile. Inhibitor studies indicated that the reconstituted transporter was sensitive to inhibition by n-butylmalonate, p-chloromercuribenzoate, mersalyl, and to a lesser extent pyridoxal 5'-phosphate but was insensitive to N-ethylmaleimide and selective inhibitors of other mitochondrial anion transporters. In combination, the above findings indicate that the identified gene encodes a mitochondrial transport protein which upon overexpression and reconstitution displays functional properties that are virtually identical to those of the native mitochondrial dicarboxylate transport system. In conclusion, the present investigation has resulted in identification of a gene encoding the mitochondrial DTP and thus eliminates a major impediment to molecular studies with this metabolically important transporter. Based on both structural and functional considerations, the yeast DTP is assignable to the mitochondrial carrier family. Additionally, the development of a procedure that enables the expression and isolation of large quantities of functional DTP provides the foundation for comprehensive investigations into the structure/function relationships within this transporter via site-directed mutagenesis, as well as for the initiation of crystallization trials.

The mitochondrial sulfonylurea receptor: identification and characterization

Biochemical identification of mitochondrial sulfonylurea receptors has been carried out through binding studies performed with [3H]glibenclamide. The presence of a single class of low affinity binding sites for glibenclamide in the inner mitochondrial membrane has been found, with a KD of 360 +/- 48 nM and BMAX of 48 +/- 7 pmoles/mg in beef heart mitochondria. Glibenclamide binding was affected by other sulfonylureas (glipizide, glisoxepide) but not by potassium channel openers (diazoxide, pinacidil, RP66471). In both rat liver and beef heart mitochondria adenine nucleotides (ATP, ADP, AMP) and nucleotide analogs (triazine dyes) produced large inhibition (from 60 to 80%) of [3H]glibenclamide binding. Photoaffinity labeling of submitochondrial particles with [125I]-glibenclamide revealed a single specifically labeled polypeptide band of 28 kDa by SDS-PAGE that is postulated to be (or to form a part of) the mitochondrial sulfonylurea receptor.

Characterization, purification and properties of the yeast mitochondrial dicarboxylate carrier (Saccharomyces cerevisiae)

The dicarboxylate carrier has been characterized and purified from mitochondria of wild strain Saccharomyces cerevisiae. The mitochondria were solubilized with Triton X-100 and the detergent extract was chromatographed on hydroxylapatite. SDS-PAGE of the hydroxylapatite pass-through showed five protein bands with M(r)s ranging from 28,000 to 35,000, by silver nitrate staining. The n-butylmalonate-sensitive succinate(out)/malate(in) exchange activity of the hydroxylapatite pass-through reconstituted into liposomes, was increased nine-fold with respect to the activity of the Triton X-100 extract. The exchange activity was inhibited by p-chloromercuriphenylsulfonate (PMPS), 4.4'diisothiocyanostilbene-2.2'-disulfonate (DIDS) and pyridoxal-phosphate, suggesting that one or more thiol groups and basic residues are implicated in the binding mechanism. The purification of the carrier was achieved by affinity chromatography on Sepharose-immobilized malate dehydrogenase. The purified protein presented the same properties as the dicarboxylate carrier in native mitochondria and displayed a single protein band with an M(r) of 28,000 as determined by SDS-PAGE. The specific activity of the purified carrier showed a 53-fold increase compared to that of the initial material. The Km for the reconstituted exchange was 2 mM for succinate with a V of 1.5 mumol min-1 mg-1 protein at 22 degrees C. The high purification state achieved for the yeast dicarboxylate carrier should allow the study of its molecular properties.

Purification of the rat-liver mitochondrial dicarboxylate carrier by affinity chromatography on immobilized malate dehydrogenase

The dicarboxylate carrier of rat-liver mitochondria, extracted by Triton X-100 and partially purified by hydroxylapatite chromatography, was retained by malate dehydrogenase immobilized on Sepharose gel, and eluted with 0.4 M NaCl. SDS-polyacrylamide gel electrophoresis of the eluate showed a predominant peptide band with an M(r) of 28,000. The purified protein, incorporated into liposomes, mediated a butylmalonate sensitive malonate(out)/malate(in) exchange that was inhibited by p-chloromercuriphenylsulfonate. Sulfate, malate and phosphate decreased the rate of exchange. The highly purified protein displayed all the properties of the dicarboxylate carrier. Moreover, the results suggest a possible functional interaction between mitochondrial carrier protein and malate dehydrogenase.

The activity of pyruvate carrier in a reconstituted system: substrate specificity and inhibitor sensitivity

The pyruvate carrier, of molecular mass 34 kDa, was purified from mitochondria isolated from rat liver, rat brain, and bovine heart, by affinity chromatography on immobilized 2-cyano-4-hydroxycinnamate. Its activity after reconstitution in phosphatidylcholine vesicles was measured either as uptake of [1-14C]pyruvate or as exchange with different 2-oxoacids. All preparations exhibited similar apparent Km values for pyruvate, but somewhat different V(max) values. The ability to exchange different anions of physiological significance, including branched-chain 2-oxoacids, confirmed the known substrate specificity described for the pyruvate carrier in mitochondria. The sensitivity of pyruvate transport toward phenylglyoxal suggested an important role of arginyl residues in the transport activity, while a role of lysyl and histidyl residues was not confirmed.

The effect of insulin supplementation on diabetes-induced alterations in the extractable levels of functional mitochondrial anion transport proteins

The effect of insulin supplementation on diabetes-induced alterations in the levels of functional mitochondrial anion transport proteins has been determined. The experimental approach consisted of the extraction of the pyruvate, dicarboxylate, and citrate transport proteins from the mitochondrial inner membrane with Triton X-114 using rat liver mitoplasts (prepared from control, diabetic, or insulin-supplemented diabetic animals) as the starting material, followed by the reconstitution of the function of each transporter in a proteoliposomal system. This experimental strategy permitted the quantification of the functional levels of these three transporters in the absence of the complications that arise when such measurements are carried out with intact mitochondria (or mitoplasts). We found that treatment of diabetic rats (i.e., animals that were injected with streptozotocin 3 weeks earlier) on a daily basis with insulin for 3 weeks resulted in a reversal of the diabetes-induced (a) increase in the extractable and reconstitutable total (and specific) transport activities of the pyruvate and dicarboxylate transporters and (b) decrease in the activity of the citrate transporter. These findings indicate that diabetes-induced alterations in the functional levels of mitochondrial anion transport proteins are a direct consequence of the insulin insufficiency that characterizes this disease. Furthermore, this study provides the first demonstration that insulin participates in the regulation of the functional levels of liver mitochondrial anion transport proteins.

Azido derivative of tricarboxylic acid for photoaffinity labeling

A new photoaffinity probe, 5-(1-hydroxy-4-azidophenylazo)-1,2,3-benzenetricarboxylic acid, was synthesized and characterized. This reagent can be potentially used in photoaffinity labeling of the mitochondrial tricarboxylate carrier, as well as of enzymes interacting with tricarboxylic acids. Inhibition and labeling of the mitochondrial tricarboxylate carrier is presented.

Streptozotocin-induced alterations in the levels of functional mitochondrial anion transport proteins

The effect of streptozotocin-induced diabetes on the levels of functional mitochondrial anion transport proteins has been determined. The experimental approach utilized for these studies consisted of the extraction of each of four mitochondrial anion transport proteins from rat liver mitoplasts (isolated from diabetic and control animals) with the nonionic detergent Triton X-114, followed by the functional reconstitution of each transporter in a liposomal system via the freeze-thaw-sonication technique. This approach permitted the quantification of transporter function without the complications that occur when such measurements are carried out with intact mitochondria (or mitoplasts). We found that experimental diabetes caused an increase in the extractable and reconstitutable specific (and total) transport activities of the pyruvate and dicarboxylate transporters, a decrease in the activity of the citrate transporter, and no significant change in the activity of the phosphate transporter relative to control values. An examination of the time course of the appearance of changes in the reconstitutable activities of the pyruvate and citrate transporters following the injection of streptozotocin revealed differences. Thus, whereas the activity of the pyruvate transporter displayed the most pronounced increase (193%) 1 week following streptozotocin injection and then subsequently declined from this peak and plateaued at later times (99% and 96% increases at 3 and 8 weeks, respectively), the activity of the citrate transporter progressively decreased with time (31-51% decreases at 1-8 weeks). We suggest that the observed diabetes-induced changes in mitochondrial anion transporter function are predictable on the basis of diabetes-induced alterations in the activities of enzymes that constitute metabolic pathways to which these transporters either supply substrate or remove product. Furthermore, we speculate that mitochondrial anion transport proteins may be regulated in coordination with the enzymes of such associated metabolic pathways.

Significance and redox state of SH groups in pyruvate carrier isolated from bovine heart mitochondria

The role and properties of -SH groups of purified pyruvate (monocarboxylate) carrier were investigated. After isolation, this protein has all -SH groups in the oxidized state. Upon reduction, the carrier can be labelled with eosin-5-maleimide. The shift in apparent Mr after the labelling points to the presence of at least two cysteine residues. Pyruvate uptake in the reconstituted system is inhibited by both permeable (eosin-5-maleimide at 1 mM concentration) and impermeable (mersalyl, p-chloromercuribenzoate) -SH group reagents. Phenylarsine oxide inhibits pyruvate transport only slightly (20%), but the inhibition is enhanced after preincubation with the substrate.

Kinetics of the reconstituted dicarboxylate carrier from rat liver mitochondria

The dicarboxylate carrier from rat liver mitochondria was purified by the Amberlite/hydroxyapatite procedure and reconstituted in egg yolk phospholipid vesicles by removing the detergent with Amberlite. The efficiency of reconstitution was optimized with respect to the ratio of detergent/phospholipid, the concentration of phospholipid and the number of Amberlite column passages. In the reconstituted system the incorporated dicarboxylate carrier catalyzed a first-order reaction of malate/phosphate exchange. V of the reconstituted malate/phosphate exchange was determined to be 6000 mumol/min per g protein at 25 degrees C. This value was independent of the type of substrate present at the external or internal space of the liposomes (malate, phosphate or malonate). The half-saturation constant was 0.49 mM for malate, 0.54 mM for malonate and 1.41 mM for phosphate. The activation energy of the exchange reaction was determined to be 95.8 kJ/mol. The transport was independent of the external pH in the range between pH 6 and 8.

Azido derivatives of dicarboxylic acids for photoaffinity labeling of mitochondrial carriers

New photoaffinity probes, N-(4-azidosalicylic)-aminosuccinic acid, 3-(4-azidophenylazo)-4-hydroxyphenylmalonic acid, (4-azido-2-nitroanilino)-N-succinic acid, 4-azidophenacylthiosuccinic acid and 4-azidophenylsuccinic acid, were synthesized and characterized chemically. They differ in the distance between dicarboxylic and azido groups, hydrophobicity and acidic moiety. These between dicarboxylic and azido groups, hydrophobicity and acidic moiety. These reagents can be applied for photoaffinity labeling of mitochondrial anion carriers and enzymes interacting with dicarboxylic acids. Inhibition and labeling of the dicarboxylate carrier is presented.

Novel reconstitution and enzymatic assay of the mitochondrial tricarboxylate carrier

The tricarboxylate carrier from beef liver mitochondria was reconstituted into liposomes using a protocol based on the absorption of Triton X-100 to hydrophobic Amberlite XAD-2 beads. The activity of the reconstituted carrier was determined spectroscopically by measuring the citrate/isocitrate exchange with an enzymatic assay. The Km for citrate obtained with this method was 35 microM and the Ki of 1,2,3-benzenetricarboxylate was 27 microM.