Reconstitution of the isolated phosphate-transport system of pig-heart mitochondria

The mitochondrial phosphate carrier: Role in oxidative metabolism, calcium handling and mitochondrial disease

The mitochondrial phosphate carrier (PiC) is a mitochondrial solute carrier protein, which is encoded by SLC25A3 in humans. PiC delivers phosphate, a key substrate of oxidative phosphorylation, across the inner mitochondrial membrane. This transport activity is also relevant for allowing effective mitochondrial calcium handling. Furthermore, PiC has also been described to affect cell survival mechanisms via interactions with cyclophilin D and the viral mitochondrial-localized inhibitor of apoptosis (vMIA). The significance of PiC has been supported by the recent discovery of a fatal human condition associated with PiC mutations. Here, we present first the early studies that lead to the discovery and molecular characterization of the PiC, then discuss the very recently developed mouse models for PiC and pathological mutations in the human SLC25A3 gene.

Toward understanding the mechanism of ion transport activity of neuronal uncoupling proteins UCP2, UCP4, and UCP5

Neuronal uncoupling proteins (UCP2, UCP4, and UCP5) have crucial roles in the function and protection of the central nervous system (CNS). Extensive biochemical studies of UCP2 have provided ample evidence of its participation in proton and anion transport. To date, functional studies of UCP4 and UCP5 are scarce. In this study, we show for the first time that, despite a low level of amino acid sequence identity with the previously characterized UCPs (UCP1-UCP3), UCP4 and UCP5 share their functional properties. Recombinantly expressed in Escherichia coli, UCP2, UCP4, and UCP5 were isolated and reconstituted into liposome systems, where their conformations and ion (proton and chloride) transport properties were examined. All three neuronal UCPs are able to transport protons across lipid membranes with characteristics similar to those of the archetypal protein UCP1, which is activated by fatty acids and inhibited by purine nucleotides. Neuronal UCPs also exhibit transmembrane chloride transport activity. Circular dichroism spectroscopy shows that these three transporters exist in different conformations. In addition, their structures and functions are differentially modulated by the mitochondrial lipid cardiolipin. In total, this study supports the existence of general conformational and ion transport features in neuronal UCPs. On the other hand, it also emphasizes the subtle structural and functional differences between UCPs that could distinguish their physiological roles. Differentiation between structure-function relationships of neuronal UCPs is essential for understanding their physiological functions in the CNS.

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.

Cardiolipin, a critical determinant of mitochondrial carrier protein assembly and function

The ability of phospholipids to act as determinants of membrane protein structure and function is probably best exemplified by cardiolipin (CL), the signature phospholipid of mitochondria. Early efforts to reconstitute individual respiratory complexes and members of the mitochondrial carrier family, most notably the ADP/ATP carrier (AAC), often demonstrated the importance of CL. Over the past decade, the significance of CL in the organization of components of the electron transport chain into higher order assemblies, termed respiratory supercomplexes, has been established. Another protein required for oxidative phosphorylation, AAC, has received comparatively little attention likely stemming from the fact that AACs were thought to function in isolation as either homodimers or monomers. Recently however, AACs have been demonstrated to interact with the respiratory supercomplex, other members of the mitochondrial carrier family, and the TIM23 translocon. Interestingly, many if not all of these interactions depend on CL. As the paradigm for the mitochondrial carrier family, these discoveries with AAC suggest that other members of this large group of important proteins may be more gregarious than anticipated. Moreover, it is proposed that AAC and perhaps additional members of the mitochondrial carrier family might represent downstream targets of pathological states involving alterations in CL.

Agmatine is transported into liver mitochondria by a specific electrophoretic mechanism

Agmatine, a divalent diamine with two positive charges at physiological pH, is transported into the matrix of liver mitochondria by an energy-dependent mechanism the driving force of which is DeltaPsi (electrical membrane potential). Although this process showed strict electrophoretic behaviour, qualitatively similar to that of polyamines, agmatine is most probably transported by a specific uniporter. Shared transport with polyamines by means of their transporter is excluded, as divalent putrescine and cadaverine are ineffective in inhibiting agmatine uptake. Indeed, the use of the electroneutral transporter of basic amino acids can also be discarded as ornithine, arginine and lysine are completely ineffective at inducing the inhibition of agmatine uptake. The involvement of the monoamine transporter or the existence of a leak pathway are also unlikely. Flux-voltage analysis and the determination of activation enthalpy, which is dependent upon the valence of agmatine, are consistent with the hypothesis that the mitochondrial agmatine transporter is a channel or a single-binding centre-gated pore. The transport of agmatine was non-competitively inhibited by propargylamines, in particular clorgilyne, that are known to be inhibitors of MAO (monoamine oxidase). However, agmatine is normally transported in mitoplasts, thus excluding the involvement of MAO in this process. The I2 imidazoline receptor, which binds agmatine to the mitochondrial membrane, can also be excluded as a possible transporter since its inhibitor, idazoxan, was ineffective at inducing the inhibition of agmatine uptake. Scatchard analysis of membrane binding revealed two types of binding site, S1 and S2, both with mono-co-ordination, and exhibiting high-capacity and low-affinity binding for agmatine compared with polyamines. Agmatine transport in liver mitochondria may be of physiological importance as an indirect regulatory system of cytochrome c oxidase activity and as an inducer mechanism of mitochondrial-mediated apoptosis.

Covariance of tricarboxylate carrier activity and lipogenesis in liver of polyunsaturated fatty acid (n-6) fed rats

The mitochondrial tricarboxylate (citrate) carrier plays an important role in hepatic intermediary metabolism because, among other functions, it supplies the cytosol with acetyl units for fatty-acid synthesis. In this study, the effect of polyunsaturated fatty acids (PUFA, n-6) on the function of this mitochondrial transporter and on lipogenic enzyme activities was investigated by feeding rats for 4 weeks with a 15%-fat diet composed of high linoleic safflower oil. Citrate transport was strongly reduced in liver mitochondria isolated from PUFA-treated rats. A reduced transport activity was also observed when solubilized mitochondrial citrate carrier from PUFA-treated rats was reconstituted into liposomes. In the same animals, a decrease of cytosolic lipogenic enzyme activities was observed. These results indicate a coordinated modulation of citrate carrier and of lipogenic enzyme activities by PUFA feeding. Kinetic analysis of the carrier activity showed that only V(max) decreased, whereas K(m) was almost virtually unaffected. The PUFA-mediated effect is most likely due to the reduced mRNA level and lower content of the citrate carrier protein observed in the safflower oil-fed rats.

Temperature dependence of the mitochondrial inner membrane anion channel. The relationship between temperature and inhibition by protons

In this paper, we investigate the temperature and pH dependence of the mitochondrial inner membrane anion channel (IMAC) that is believed to be involved in mitochondrial volume homeostasis. At pH 7. 4, the flux of malonate is highly temperature-dependent with rates increasing from 1 nmol/min mg at 5 degrees C to 1900 nmol/min mg at 45 degrees C. The Arrhenius plot is nonlinear with the activation energy increasing from 21 kJ/mol (Q10 = 1.3) to 193 kJ/mol (Q10 = 13) as the temperature is decreased. This temperature dependence is unusual and not seen with solutes that are transported through the bilayer such as NH4OAc, malonamide, and KSCN (plus valinomycin) or even for cytochrome c oxidase-dependent uptake of potassium (plus valinomycin). The temperature dependence of IMAC is closely related to the inhibition of IMAC by protons. Thus, we find that the pIC50 for protons decreases from 9.3 (Hill coefficient = 1.0) at 5 degrees C to 7.1 (Hill coefficient = 2.5) at 45 degrees C. This behavior is explained on the basis of a new kinetic model for IMAC in which the net open probability is not only modulated by the binding of three protons but also by temperature via effects on the open probability of the unprotonated channel and the pK of one of the inhibitory protonation sites.

Unusual chromatographic behaviour and one-step purification of a novel membrane proteinase from Bacillus cereus

Cell envelopes of Bacillus cereus contain a casein-cleaving membrane proteinase (CCMP) and an insulin-cleaving membrane proteinase (ICMP), which differ in their substrate and inhibitor specificity from all Bacillus proteinases described previously. They remained localized in the cytoplasmic membrane after treatment with lysozyme and mutanolysin and they are strongly attached to the membrane compared with other known membrane proteinases. Only high a concentration of the Zwitterionic detergent sulfobetain SB-12 enabled an effective solubilization of both membrane proteinases. The usual conventional purification methods, such as chromatofocusing, ion-exchange chromatography and hydrophobic interaction chromatography in the presence of detergent concentrations beyond their critical micelle concentration, could not be applied to the purification, because the solubilized membrane proteinases bound strongly and irreversibly to the chromatographic matrix. In the search for other purification methods, we used a tentacle ion-exchanger (EMD trimethylaminoethyl-Fractogel) to reduce the hydrophobic interactions between the proteinases and the matrix. All contaminating proteins could be removed by a first gradient of sodium chloride without elution of CCMP; a second gradient with isopropanol and a decreasing salt concentration resulted in an efficiently purified CCMP. The ICMP was irreversibly denaturated. Purified CCMP is a member of the metalloproteinase family with a pH optimum in the neutral range and a temperature optimum of 40 degrees C, whose properties differ from the serine-type membrane proteinase of Bacillus subtilis described by Shimizu et al. [Agric. Biol. Chem., 47 (1983) 1775]. It consists of two subunits in sodiumdodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under reducing conditions (Mr 53,000 and 65,000); however, the molecular mass of the purified enzyme could not be determined by size exclusion or SDS-PAGE, because the purified enzyme aggregated at the top of the gel matrix. CCMP solubilized before the purification process, could be eluted in the presence of 0.1% octylphenol-poly(ethyleneglycol ether)9-10 (Triton X-100) in two peaks of Mr 56,000 and 128,000, respectively. We discuss this special chromatographic behaviour of the CCMP from Bacillus cereus, with regard to the strong hydrophobic interactions of the enzyme with the chromatographic matrix and additional self-aggregation, which could only be dissolved by solvents such as isopropanol.

Kinetic characterization of the reconstituted ornithine carrier from rat liver mitochondria

The ornithine carrier was purified from rat liver mitochondria and reconstituted into liposomes by removing the detergent from mixed micelles by hydrophobic chromatography on Amberlite XAD-2. The efficiency of reconstitution was optimized with respect to the concentration of protein and phospholipid, the Triton X-100/phospholipid ratio, the Amberlite/detergent ratio and the number of passages through a single Amberlite column. The activity of the carrier was influenced by the phospholipid composition of the liposomes, increasing in the presence of acidic phospholipids and decreasing in the presence of dioleoylphosphatidylcholine. In the reconstituted system the incorporated ornithine carrier catalyzed a first-order reaction of ornithine/ornithine or ornithine/citrulline exchange. The maximum transport rate of external [14C]ornithine was 3.2 mmol/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 (ornithine, citrulline and lysine). The half-saturation constant (Km) was 0.16 mM for ornithine, 1.2 mM for lysine and 3.6 mM for citrulline. The activation energy of the ornithine/ornithine exchange reaction was 89 kJ/mol. The rate of exchange had a pH optimum at 8 and was inhibited by cations.

Functional properties of purified and reconstituted mitochondrial metabolite carriers

Eight mitochondrial carrier proteins were solubilized and purified in the authors' laboratories using variations of a general procedure based on hydroxyapatite and Celite chromatography. The molecular mass of all the carriers ranges between 28 and 34 kDa on SDS-PAGE. The purified carrier proteins were reconstituted into liposomes mainly by using a method of detergent removal by hydrophobic chromatography on polystyrene beads. The various carriers were identified in the reconstituted state by their kinetic properties . A complete set of basic kinetic data including substrate specificity, affinity, interaction with inhibitors, and activation energy was obtained. These data closely resemble those of intact mitochondria, as far as they are available from the intact organelle. Mainly on the basis of kinetic data, the asymmetric orientation of most of the reconstituted carrier proteins were established. Several of their functional properties are significantly affected by the type of phospholipids used for reconstitution. All carriers which have been investigated in proteoliposomes function according to a simultaneous (sequential) mechanism of transport; i.e., a ternary complex, made up of two substrates and the carrier protein, is involved in the catalytic cycle. The only exception was the carnitine carrier, where a ping-pong mechanism of transport was found. By reaction of particular cysteine residues with mercurial reagents, several carriers could be reversibly converted to a functional state different from the various physiological transport modes. This "unphysiological" transport mode is characterized by a combination of channel-type and carrier-type properties.

Functional properties of the reconstituted phosphate carrier from bovine heart mitochondria: evidence for asymmetric orientation and characterization of three different transport modes

The phosphate carrier from bovine heart mitochondria was reconstituted into liposomes by the removal of detergent using hydrophobic ion-exchange columns. Reversible blocking of the carrier function during chromatographic steps was possible by the application of the inhibitor p-(chloromercuri)benzenesulfonate at low temperature. Thus, both forward and backward exchange experiments for kinetic characterization of Pi/Pi-antiport as well as the Pi/H(+)-symport could be performed. The maximum rate of Pi/Pi-antiport was 90 mumol min-1 (mg protein)-1. Only one single half-saturation constant (Km) for phosphate was observed at each side of the membrane under antiport conditions, 1.8 mM at the external and 9.4 mM at the internal side. By comparing the Km values at both sides of the membrane with the values found in intact mitochondria, a right-side-out orientation of the reconstituted phosphate carrier was concluded. Furthermore, the influence of various sulfhydryl reagents on the carrier was investigated. After modification with HgCl2, the phosphate carrier reveals a third (nonphysiological) unidirectional transport mode. This was characterized by a significantly reduced substrate specificity. In view of similar observations with several other mitochondrial carriers, these results again indicate that the phosphate carrier is a member of the postulated functional family of mitochondrial carrier proteins.

Functional binding of cardiolipin to cytochrome c oxidase

Bovine cytochrome c oxidase usually contains 3-4 mol of tightly bound cardiolipin per cytochrome aa3 complex. At least two of these cardiolipins are required for full electron transport activity. Without the tightly bound cardiolipin, cytochrome c oxidase has only 40-50% of its original activity when assayed in detergents that support activity, e.g., dodecyl maltoside. By measuring the restoration of electron transport activity, functional binding constants for cardiolipin and a number of cardiolipin analogues have been evaluated (Kd,app = 1 microM for cardiolipin). These binding constants agree reasonably well with direct measurement of the binding using [14C]-acetyl-cardiolipin (Kd < 0.1 microM) when the enzyme is solubilized with Triton X-100. These data are discussed in relationship to the wealth of data that is known about the association of cardiolipin with cytochrome c oxidase and the other mitochondrial electron transport complexes and transporters.

Decreased activity of the phosphate carrier and modification of lipids in cardiac mitochondria from senescent rats

1. A comparative study of the effects of aging on the transport of phosphate and on the lipid composition in cardiac mitochondria isolated from young and aged rats was carried out. 2. Mitochondria from aged rats (26 month old) translocate phosphate much more slowly than do mitochondria from young control rats (4 month old). 3. Kinetic analysis of the phosphate transport show that only the Vmax of this process is decreased while there is no change in the Km value. 4. There is no appreciable difference in either the respiratory control ratios or in the ADP/O ratios between mitochondria from young and aged rats. 5. The heart mitochondrial lipid composition is altered in aged rats; in particular, the cholesterol/phospholipid molar ratio increases and the content of cardiolipin decreases with aging.

The effect of aging and acetyl-L-carnitine on the activity of the phosphate carrier and on the phospholipid composition in rat heart mitochondria

The effect of aging and treatment with acetyl-L-carnitine on the activity of the phosphate carrier and on the phospholipid composition in rat heart mitochondria was studied. It was found that the activity of the phosphate carrier was reduced by aging. Treatment of aged rats with acetyl-L-carnitine reversed this effect. The mitochondrial level of cardiolipin was decreased with aging. Treatment of aged rats with acetyl-L-carnitine restored the level of cardiolipin to that of young rats. It is proposed that acetyl-L-carnitine may restore the correct phospholipid composition (cardiolipin level) of the mitochondrial membrane, altered by aging, thereby restoring the activity of the phosphate carrier.

The influence of hypothyroidism on the transport of phosphate and on the lipid composition in rat-liver mitochondria

The influence of hypothyroidism on the transport of phosphate and on the lipid composition in rat-liver mitochondria was examined. It was found that the rate of phosphate transport is reduced (around 40%) in mitochondria from hypothyroid rats compared to that obtained in mitochondria from normal rats. Treatment of hypothyroid rats with thyroid hormone reverses this effect completely. Kinetic analysis of the phosphate transport indicates that only the Vmax of this process is affected, while there is no change in the Km values. The lower rate of phosphate transport in mitochondria from hypothyroid rats is also demonstrated by swelling experiments. There is no significant difference either in the respiratory control ratios or in the ADP/O ratios between these two types of mitochondria. The hepatic mitochondrial lipid composition is altered significantly in hypothyroid rats. The total cholesterol increases, the phospholipids decrease and the cholesterol/phospholipid molar ratio increases (around 40%). Among the phospholipids, cardiolipin shows the greatest alteration (30% decrease in the hypothyroid rats). The phosphatidylethanolamine/phosphatidylcholine ratio also decreases. Alterations were also found in the pattern of fatty acids. These changes in lipid composition may be responsible, at least in part, for the depression of the phosphate carrier activity in mitochondria from hypothyroid rats.

Effect of thyroxine on the activity of mitochondrial cardiolipin synthase in rat liver

Cardiolipin is a major mitochondrial membrane lipid and plays a vital role in mitochondrial function. The effect of thyroxine on the activity of liver mitochondrial cardiolipin synthase was examined in this study. Treatment with thyroxine (250 micrograms/100 g) for 5 days increased cardiolipin synthase activity by 52%. Mitochondrial levels of cardiolipin appear to be regulated in part by the effect of thyroid hormone on the activity of cardiolipin synthase.

Functional levels of mitochondrial anion transport proteins in non-insulin-dependent diabetes mellitus

The effect of non-insulin-dependent diabetes mellitus (i.e., NIDDM; type 2 diabetes) on the levels of functional mitochondrial anion transport proteins has been determined utilizing a chemically-induced neonatal model of NIDDM. We hypothesized that moderate insulin deficiency exacerbated by the insulin resistance, which is characteristic of NIDDM, would cause changes in mitochondrial anion transporter function that were similar to those we have previously shown to occur in insulin-dependent diabetes mellitus (i.e., IDDM; type 1 diabetes) (Arch. Biochem. Biophys. 280: 181-191, 1990). Our 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 diabetic and control animals) as the starting material, followed by the functional reconstitution of each transporter in a proteoliposomal system. This 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 experimental NIDDM did not cause significant changes in the extractable and reconstitutable specific (and total) transport activities of the pyruvate, dicarboxylate, and citrate transporters. These results are in marked contrast to our previous findings obtained using rats with IDDM and negated our hypothesis. The present results, in combination with our earlier findings, allow us to conclude that insulin plays an important role in the regulation of mitochondrial anion transporter function.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetic characterization of the reconstituted carnitine carrier from rat liver mitochondria

The carnitine carrier was purified from rat liver mitochondria and reconstituted into liposomes by removing the detergent from mixed micelles by Amberlite. Optimal transport activity was obtained with 1 microgram/ml and 12.5 mg/ml of protein and phospholipid concentration, respectively, with a Triton X-100/phospholipid ratio of 1.8 and with 16 passages through the same Amberlite column. The activity of the carrier was influenced by the phospholipid composition of the liposomes, being increased in the presence of cardiolipin and decreased in the presence of phosphatidylinositol. In the reconstituted system the incorporated carnitine carrier catalyzed a carnitine/carnitine exchange which followed a first-order reaction. The maximum transport rate of external [3H]carnitine was 1.7 mmol/min per g protein at 25 degrees C and was independent of the type of countersubstrate. The half-saturation constant (Km) for carnitine was 0.51 mM. The affinity of the carrier for acylcarnitines was in the microM range and depended on the carbon chain length. The activation energy of the carnitine/carnitine exchange was 133 kJ/mol. The carrier function was independent of the pH in the range between 6 and 8 and was inhibited at pH below 6.

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.

Effect of aging on the activity of the phosphate carrier and on the lipid composition in rat liver mitochondria

The effect of aging on the activity of the phosphate carrier and on the lipid composition in rat liver mitochondria has been investigated. It was found that the rate of phosphate transport in mitochondria from aged rats (28 months old) is significantly reduced (around 40%) compared to that obtained in mitochondria from young control rats (5 months old). Kinetic analysis of the phosphate transport indicates that only the Vmax of this process is affected, while there is no change in the Km values. The lower activity of the phosphate carrier in mitochondria from aged rats is also documented by swelling experiments. The age-related decrement in the activity of the phosphate carrier was found not to be due neither to a change in the endogenous content of phosphate nor to a change in the transmembrane delta pH value. Inhibitor titrations with mersalyl provide no evidence for a lower content of functional phosphate translocase in mitochondria from aged rats. There is no difference either in the respiratory control ratios or in the ADP/O ratios between mitochondria from young and aged animals. The hepatic mitochondrial lipid composition is altered significantly in aged rats: the total cholesterol increases (31%), the phospholipids decrease (12%), and the cholesterol/phospholipid molar ratio increases (44%). Among the phospholipids cardiolipin shows the greatest alteration (30% decrease with age). Alterations were also found in the pattern of fatty acids. The age-related decrement in the activity of the phosphate carrier appears to be dependent on changes in the lipid domain surrounding the carrier protein molecule in the mitochondrial membrane.

Kinetic characterization of the reconstituted tricarboxylate carrier from rat liver mitochondria

The tricarboxylate carrier from rat liver mitochondria was purified by chromatography on hydroxyapatite/celite and reconstituted in phospholipid vesicles by removing the detergent using hydrophobic chromatography on Amberlite. Optimal transport activity was obtained by using a Triton X-114/phospholipid ratio of 0.8, 6% cardiolipin and 24 passages through a single Amberlite column. In the reconstituted system the incorporated tricarboxylate carrier catalyzed a first-order reaction of citrate/citrate or citrate/malate exchange. The activation energy of the exchange reaction was 70.1 kJ/mol. The rate of the exchange had a pH optimum between 7 and 8. The half-saturation constant was 0.13 mM for citrate and 0.76 mM for malate. All these properties were similar to those described for the tricarboxylate transport system in intact mitochondria. In proteoliposomes the maximum exchange rate at 25 degrees C reached 2000 mumols/min per g protein. This value was independent of the type of substrate present at the external or internal space of the liposomes (citrate or malate).

Stimulation of phosphate transport in rat-liver mitochondria by thyroid hormones

The effect of hyperthyroidism on the transport of phosphate in rat-liver mitochondria has been examined. Thyroid hormones administered in vivo increased carrier mediated (mersalyl-sensitive) phosphate transport. Kinetic analysis of the phosphate transport showed that the thyroid hormone affects the Vmax of this process, while having no effect on the Km values. The higher activity of the phosphate carrier was found not to be due to a change in the endogenous content of phosphate nor to a change in the transmembrane delta pH value. Inhibitor titrations with mersalyl showed that mitochondria from both control and hyperthyroid rats required the same concentrations of inhibitor to produce total inhibition of phosphate transport, thus suggesting that the amount of functional translocase present is unaffected. The level of cardiolipin was significantly higher in mitochondrial membranes from hyperthyroid rats as compared to the control rats. The thyroid hormone induced change in the activity of the phosphate carrier appears to be due to a more favorable lipid microenvironment (cardiolipin content) surrounding the carrier molecule in the mitochondrial membrane.

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.

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.

Purification of the mitochondrial carnitine carrier by chromatography on hydroxyapatite and celite

The carnitine carrier from rat liver mitochondria has been extracted with Triton X-100 ad partially purified by chromatography on hydroxyapatite and celite. During purification the activity of the carrier was monitored by functional reconstitution into liposomes. The purified fraction is 250-fold enriched with respect to the N-ethylmaleimide-sensitive carnitine/carnitine transport activity. The substrate specificity and the inhibitor sensitivity of carnitine transport in liposomes resemble closely those described for the transport of carnitine in mitochondria.

Kinetics of the reconstituted 2-oxoglutarate carrier from bovine heart mitochondria

The 2-oxoglutarate carrier from the inner membrane of bovine heart mitochondria was purified by chromatography on hydroxyapatite/celite and reconstituted with egg yolk phospholipid vesicles by the freeze-thaw-sonication technique. In the reconstituted system the incorporated 2-oxoglutarate carrier catalyzed a first-order reaction of 2-oxoglutarate/2-oxoglutarate exchange. The substrate affinity for 2-oxoglutarate was determined to be 65 +/- 18 microM (15 determinations) and the maximum exchange rate at 25 degrees C reaches 4000-22,000 mumol/min per g protein, in dependence of the particular reconstitution conditions. The activation energy of the exchange reaction is 54.3 kJ/mol. The transport is independent of pH in the range between 6 and 8. When the first fraction of the hydroxyapatite/celite column eluate was used for reconstitution, besides the 2-oxoglutarate/2-oxoglutarate exchange, a significant activity of unidirectional uptake was observed. This activity may be due to a population of the carrier protein which is in a different state.

Solute carriers involved in energy transfer of mitochondria form a homologous protein family

The sequences of three mitochondrial carriers involved in energy transfer, the ADP/ATP carrier, phosphate carrier and uncoupling carrier, are analyzed. Similarly to what has been previously reported for the ADP/ATP carrier and the uncoupling protein, now also the phosphate carrier is found to have a tripartite structure comprising three similar repeats of approx. 100 residues each. The three sequences show a fair overall homology with each other. More significant homologies are found by comparing the repeats within and between the carriers in a scheme where the sequences are spliced into repeats, which are arranged for maximum homology by allowing possible insertions or deletions. A striking conservation of critical residues, glycine, proline, of charged and of aromatic residues is found throughout all nine repeats. This is indicative of a similar structural principle in the repeats. Hydropathy profiles of the three proteins and a search for amphipathic alpha-spans reveal six membrane-spanning segments for each carrier, providing further support for the basic structural identity of the repeats. The proposed folding pattern of the carriers in the membrane is exemplified with the phosphate carrier. A possible tertiary arrangement of the repeats and the membrane-spanning helices is shown. The emergence of a mitochondrial carrier family by triplication and by divergent evolution from a common gene of about 100 residues is discussed.

Inhibition of the mitochondrial tricarboxylate carrier by arginine-specific reagents

The effect of arginine-specific reagents on the activity of the partially purified and reconstituted tricarboxylate carrier of the inner mitochondrial membrane has been studied. It has been found that 1,2-cyclohexanedione, 2,3-butanedione, phenylglyoxal and phenylglyoxal derivatives inhibit the reconstituted citrate/citrate exchange activity. The inhibitory potency of the phenylglyoxal derivatives increases with increasing hydrophilic character of the molecule. Citrate protects the tricarboxylate carrier against inactivation caused by the arginine-specific reagents. Other tricarboxylates, which are not substrates of the carrier, have no protective effect. The results indicate that at least one essential arginine residue is located at the substrate-binding site of the tricarboxylate carrier and that the vicinity of the essential arginine(s) has a hydrophilic character.

The 35 kDa DCCD-binding protein from pig heart mitochondria is the mitochondrial porin

The protein which can be labelled by low concentrations of dicyclohexylcarbodiimide in the Mr region of 30 000-35 000 has been purified from pig heart mitochondria with a high yield and as a single band of apparent Mr 35 000 in dodecyl sulphate-containing gels. The protein is not identical with the phosphate carrier as suggested before, since the two proteins behave differently during isolation. Incorporation of the isolated 35 kDa dicyclohexylcarbodiimide-binding protein into lipid bilayer membranes causes an increase of the membrane conductance in definite steps, due to the formation of pores. The specific pore-forming activity increases during the purification procedure. The single pore conductance is about 4.0 nS, suggesting a diameter of 1.7 nm of the open pore. The pore conductance is dependent on the voltage across the membrane. Anion permeability of the pore is higher than cation permeability. These properties are similar to those described for isolated mitochondrial and bacterial porins. It is concluded that the 35 kDa dicyclohexylcarbodiimide-binding protein from pig heart mitochondria is identical with porin from outer mitochondrial membrane.

Specific elution from hydroxylapatite of the mitochondrial phosphate carrier by cardiolipin

The role of cardiolipin in the purification of the mitochondrial phosphate carrier by hydroxylapatite has been investigated. Without added cardiolipin, the reconstituted phosphate-transport activity in the hydroxylapatite eluate is small and only confined to the first fraction. With cardiolipin added to the extract, the eluted activity is much higher and present until fraction 6. The activity retained by hydroxylapatite in the absence of cardiolipin is eluted after addition of this phospholipid to the column. The requirement of added cardiolipin diminishes on increasing the concentration of solubilized mitochondria. The hydroxylapatite eluate contains five protein bands in the Mr-region of 30 000-35 000, which are differently distributed in the various fractions. Among these, only the presence and the relative amount of band 3 of Mr 33 000 corresponds to the phosphate transport activity. Cardiolipin is the only phospholipid tested which causes elution of band 3 from hydroxylapatite; on the other hand, it prevents the elution of band 2 and retards that of band 5 (the ADP/ATP carrier). Band 1 starts to appear in the second fraction even without cardiolipin. On increasing the concentration of cardiolipin, in the first fraction of the hydroxylapatite eluate band 3 increases and the contamination of band 4 decreases. Under optimal conditions a preparation of band 3 about 90% pure and with high reconstituted phosphate transport activity is obtained. It is concluded that the elution of the phosphate carrier from hydroxylapatite requires cardiolipin and that the phosphate carrier is identical with (or with part of) band 3 of the hydroxylapatite eluate.

Reactivity of the -SH groups of the mitochondrial phosphate carrier under native, solubilized and reconstituted conditions

The isolated and liposome-reconstituted mitochondrial phosphate carrier exhibits a sigmoidal inhibition curve by mersalyl, similar to that found with intact mitochondria. In contrast a hyperbolic inhibition curve is found (a) by titration of the soluble carrier with mersalyl before reconstitution in liposomes and (b) by titration of the reconstituted carrier with mersalyl after successively pretreatment of the mitochondria with low, non-inhibitory concentrations of mersalyl, excess N-ethylmaleimide and dithiothreitol. The inhibition of the reconstituted, but not of the soluble, phosphate carrier by mersalyl can be reversed by dithiothreitol. Cupric di(1,10-phenanthroline) inhibits the soluble but not the reconstituted phosphate carrier. The inhibited phosphate carrier can be reactivated by dithiothreitol in the soluble state but not after reconstitution in liposomes. The data support the previously suggested model of the phosphate carrier, assuming a dimer of two identical subunits for the active unit.

Doxorubicin inhibits the phosphate-transport protein reconstituted in liposomes. A study on the mechanism of the inhibition

The phosphate transport protein (PTP) has been isolated from beef heart mitochondria in the presence of cardiolipin and reconstituted in asolectin and phosphatidylcholine vesicles. As expected, the activity of the reconstituted PTP is inhibited by N-ethylmaleimide and mersalyl. It is also inhibited by adriamycin and Br-daunomycin. Storage of isolated mitochondria at -80 degrees C prior to the isolation of the PTP decreases the maximal activity of the unidirectional transport of phosphate (Pi) in reconstituted vesicles. The sensitivity of the system to the four inhibitors remains the same. The inhibition of the PTP by adriamycin is reversed by KCl and prevented by MgSO4. Since the interaction between adriamycin and cardiolipin is of ionic type, KCl apparently replaces adriamycin on the negative charges of cardiolipin. By contrast, MgSO4 complexes adriamycin directly, eliminating its inhibitory effect on the PTP. No PTP activity is found after reconstitution in phosphatidylcholine vesicles in the absence of cardiolipin. Addition of buffer-dispersed cardiolipin to the vesicles restores the Pi-transport activity. The addition of adriamycin to the vesicles together with cardiolipin removes the reactivation. Succinylation of the PTP at pH 8.0 eliminates the inhibitory effect of adriamycin in the reconstituted system. The effects of N-ethylmaleimide and mersalyl are not modified. The succinylated preparation reconstituted in phosphatidylcholine vesicles is reactivated by cardiolipin, but in this case the reactivation is not counteracted by adriamycin. Succinylation of the PTP at pH 9.0 results in its complete inhibition. The results indicate, (a) that the sites of interaction of MalNEt /mersalyl and adriamycin with the PTP X cardiolipin complex are different, and (b) that the adriamycin-binding site in the complex is probably on cardiolipin molecules essential to the PTP activity. It is postulated that succinylation of the PTP prevents the interaction of adriamycin with these molecules.

Purification of the active mitochondrial tricarboxylate carrier by hydroxylapatite chromatography

The mitochondrial tricarboxylate carrier has been extracted from rat liver mitochondria or SMP with Triton X-100, in the presence of 1,2,3-BTA and DPG, and partially purified by chromatography on HTP. The purified fraction, which also contains the ADP/ATP carrier and the phosphate carrier, after incorporation into liposomes catalyzes a 1,2,3-BTA-sensitive [14C]citrate/citrate exchange. The tissue and substrate specificity, the inhibitor sensitivity and the kinetic properties of citrate transport in liposomes are similar to those described for the citrate transport in mitochondria. The maximal rate of citrate exchange in the reconstituted system is 338 mumol X min-1 X g protein-1, at 30 degrees C and pH 7.0.

The mitochondrial phosphate carrier reconstituted in liposomes is inhibited by doxorubicin

The phosphate carrier has been isolated from beef heart mitochondria in the presence of cardiolipin and reconstituted in asolectin vesicles. It has been found that 100 microM doxorubicin and 100 microM Br-daunomycin inhibit the unidirectional phosphate uptake in the reconstituted liposomes to the same extent as N-ethylmaleimide. The inhibition by Br-daunomycin is not due to covalent interaction with the carrier. The specific interaction between doxorubicin and cardiolipin is responsible for the inhibition of the phosphate carrier. Br-daunomycin interacts with 3 mitochondrial proteins of apparent Mr approximately 45 000, approximately 35 000 and approximately 30 000.

Specific and reversible activation and inactivation of the mitochondrial phosphate carrier by cardiolipin and nonionic detergents, respectively

The phosphate carrier of pig heart mitochondria was solubilized with Triton X-100 and purified by chromatography on hydroxylapatite. Incubation of the phosphate carrier fraction with cardiolipin stimulated the reconstituted [32P]phosphate exchange activity in liposomes, whereas increased Triton X-100 concentrations inhibited it. The effects of cardiolipin and Triton X-100 are reversible. The activation by cardiolipin is highly specific and could not be obtained with any other applied phospholipid.

Effect of tribenzylphosphate on the active phosphate transport and ATP synthesis in yeast mitochondria

Tribenzylphosphate (TBP), a specific inhibitor of the high affinity system for Pi transport in yeast mitochondria, inhibits the active Pi transport measured by the energy-linked swelling. The dependence of the rate of oligomycin sensitive ATP synthesis as a function of the external Pi concentration shows two kinetic systems. The high affinity system, corresponds to the range of the external Pi concentration which stimulates the respiratory rate. TBP inhibits both this system and the state 4 leads to state 3 transition.

The protein component(s) of the isolated phosphate-transport system of mitochondria

The ethylmaleimide-sensitive phosphate-transport system of heart mitochondria was isolated and the activity reconstituted in liposomes. The identification of the phosphate-carrier protein was complicated by several factors. 1. The phosphate-carrier fraction, isolated through different procedures in different laboratories, contains 4-5 protein components of very similar apparent molecular weight, as shown by gradient dodecylsulfate gel electrophoresis. 2. The amino acid composition of protein 1, 2 and 3(apparent Mr = 34 500, 34 000 and 33 000, respectively) is quite similar although not identical. 3. Including several protease inhibitors during isolation of the phosphate-carrier fraction does not influence the protein pattern. 4. Labeled N-ethylmaleimide binds only protein 1 and 3, labeled N,N'-dicyclohexylcarbodiimide bonds only to protein 1 and 2. 5. N,N'-Dicyclohexylcarbodiimide had no effect on the reconstituted phosphate-exchange activity after incubation with intact mitochondria or proteoliposomes. 6. In the presence of protecting mersalyl concentrations N-ethylmaleimide inhibits the phosphate transport in intact mitochondria. Under these conditions no label ethylmaleimide was bound to the phosphate-carrier fraction. 7. Comparison of the reconstituted transport activity with the protein pattern of fractions, isolated with or without cardiolipin, shows correlation of transport with the amount of protein 2 but not with that of protein 1, which binds ethylmaleimide. A model of the mitochondrial phosphate carrier is presented which suggests a proteolytic degradation of the phosphate-carrier protein during its isolation. The model explains the contradictory results of this investigation.