Incorporation of N-ethylmaleimide into the membrane-bound ADP/ATP translocator. Isolation of the protein labeled with N-[3H]ethylmaleimide

A 20/20 view of ANT function in mitochondrial biology and necrotic cell death

The adenosine nucleotide translocase (ANT) family of proteins are inner mitochondrial membrane proteins involved in energy homeostasis and cell death. The primary function of ANT proteins is to exchange cytosolic ADP with matrix ATP, facilitating the export of newly synthesized ATP to the cell while providing new ADP substrate to the mitochondria. As such, the ANT proteins are central to maintaining energy homeostasis in all eukaryotic cells. Evidence also suggests that the ANTs constitute a pore-forming component of the mitochondrial permeability transition pore (MPTP), a structure that forms in the inner mitochondrial membrane that is thought to underlie regulated necrotic cell death. Additionally, emerging studies suggest that ANT proteins are also critical for mitochondrial uncoupling and for promoting mitophagy. Thus, the ANTs are multifunctional proteins that are poised to participate in several aspects of mitochondrial biology and the greater regulation of cell death, which will be discussed here.

The effect of N-ethylmaleimide on permeability transition as induced by carboxyatractyloside, agaric acid, and oleate

In this work, we studied the effect of N-ethylmaleimide on permeability transition. The findings indicate that the amine inhibited the effects of carboxyatractyloside and agaric acid. It is known that these reagents interact with the adenine nucleotide carrier through the cytosolic side. When oleate, which interacts through the matrix side, was used it was found that the amine amplified the effects of oleate on permeability transition. The results also show that N-ethylmaleimide strengthened the inhibition induced by carboxyatractyloside, agaric acid, and oleate on ADP exchange. Furthermore, it was also found that oleate improved the binding of eosin-5-maleimide on the adenine nucleotide translocase.

The ADP and ATP transport in mitochondria and its carrier

Different from some more specialised short reviews, here a general although not encyclopaedic survey of the function, metabolic role, structure and mechanism of the ADP/ATP transport in mitochondria is presented. The obvious need for an "old fashioned" review comes from the gateway role in metabolism of the ATP transfer to the cytosol from mitochondria. Amidst the labours, 40 or more years ago, of unravelling the role of mitochondrial compartments and of the two membranes, the sequence of steps of how ATP arrives in the cytosol became a major issue. When the dust settled, a picture emerged where ATP is exported across the inner membrane in a 1:1 exchange against ADP and where the selection of ATP versus ADP is controlled by the high membrane potential at the inner membrane, thus uplifting the free energy of ATP in the cytosol over the mitochondrial matrix. Thus the disparate energy and redox states of the two major compartments are bridged by two membrane potential responsive carriers to enable their symbiosis in the eukaryotic cell. The advance to the molecular level by studying the binding of nucleotides and inhibitors was facilitated by the high level of carrier (AAC) binding sites in the mitochondrial membrane. A striking flexibility of nucleotide binding uncovered the reorientation of carrier sites between outer and inner face, assisted by the side specific high affinity inhibitors. The evidence of a single carrier site versus separate sites for substrate and inhibitors was expounded. In an ideal setting principles of transport catalysis were elucidated. The isolation of intact AAC as a first for any transporter enabled the reconstitution of transport for unravelling, independently of mitochondrial complications, the factors controlling the ADP/ATP exchange. Electrical currents measured with the reconstituted AAC demonstrated electrogenic translocation and charge shift of reorienting carrier sites. Aberrant or vital para-functions of AAC in basal uncoupling and in the mitochondrial pore transition were demonstrated in mitochondria and by patch clamp with reconstituted AAC. The first amino acid sequence of AAC and of any eukaryotic carrier furnished a 6-transmembrane helix folding model, and was the basis for mapping the structure by access studies with various probes, and for demonstrating the strong conformation changes demanded by the reorientation mechanism. Mutations served to elucidate the function of residues, including the particular sensitivity of ATP versus ADP transport to deletion of critical positive charge in AAC. After resisting for decades, at last the atomic crystal structure of the stabilised CAT-AAC complex emerged supporting the predicted principle fold of the AAC but showing unexpected features relevant to mechanism. Being a snapshot of an extreme abortive "c-state" the actual mechanism still remains a conjecture.

Roles of adjoining Asp and Cys residues of first matrix-facing loop in transport activity of yeast and bovine mitochondrial ADP/ATP carriers

The mitochondrial ADP/ATP carrier (AAC) transports substrate by interconversion of its conformation between m- and c-states. The 1st loop facing the matrix (LM1) is extruded into the matrix in the m-state and is suggested to intrude into the mitochondrial membrane on conversion to the c-state conformation [Hashimoto, M., Majima, E., Goto, S., Shinohara, Y., and Terada, H. (1999) Biochemistry 38, 1050-1056]. To elucidate the mechanism of the translocation of LM1, we examined the effects of site-directed mutagenesis of two adjoining residues, Cys56 and Asp55 in the bovine type 1 AAC and Cys73 and Asp72 in the yeast type 2 AAC, on the substrate transport activity. We found that (i) replacement of the Cys by bulky and hydrophilic residues was unfavorable for efficient transport activity, (ii) the carboxyl groups of the Asp residues of the bovine and yeast AACs were essential and strictly position-specific, and (iii) hence, the mutation to Glu showed transport activity comparable to that of the native AACs. Based on these results, we discussed the functional role of LM1 in the transport activity of AAC.

Role of critical thiol groups on the matrix surface of the adenine nucleotide translocase in the mechanism of the mitochondrial permeability transition pore

Opening of the mitochondrial permeability transition pore (MPTP) is sensitized to [Ca(2+)] by oxidative stress (diamide) and phenylarsine oxide (PAO). We have proposed that both agents cross-link two thiol groups on the adenine nucleotide translocase (ANT) involved in ADP and cyclophilin-D (CyP-D) binding. Here, we demonstrate that blocking Cys(160) with 80 microM eosin 5-maleimide (EMA) or 500 microM N-ethylmaleimide (NEM) greatly decreased ADP inhibition of the MPTP. The ability of diamide, but not PAO, to block ADP inhibition of the MPTP was antagonized by treatment of mitochondria with 50 microM NEM to alkylate matrix glutathione. Binding of detergent-solubilized ANT to a PAO-affinity matrix was prevented by pre-treatment of mitochondria with diamide, EMA or PAO, but not NEM. EMA binding to the ANT in submitochondrial particles (SMPs) was prevented by pre-treatment of mitochondria with either PAO or diamide, implying that both agents modify Cys(160). Diamide and PAO pre-treatments also inhibited binding of solubilized ANT to a glutathione S-transferase-CyP-D affinity column, both effects being blocked by 100 microM EMA. Intermolecular cross-linking of adjacent ANT molecules via Cys(57) by copper phenanthroline treatment of SMPs was abolished by pre-treatment of mitochondria with diamide and PAO, but not with EMA. Our data suggest that PAO and diamide cause intramolecular cross-linking between Cys(160) and Cys(257) directly (not antagonized by 50 microM NEM) or using glutathione (antagonized by 50 microM NEM) respectively. This cross-linking stabilizes the "c" conformation of the ANT, reducing the reactivity of Cys(57), while enhancing CyP-D binding to the ANT and antagonizing ADP binding. The two effects together greatly sensitize the MPTP to [Ca(2+)].

Carboxyatractyloside increases the effect of oleate on mitochondrial permeability transition

Addition of a low concentration of carboxyatractyloside (0.075 microM) renders mitochondria susceptible to the opening of the non-specific pore by 5 microM oleate, in a cyclosporin A-sensitive fashion. Matrix Ca2+ efflux as well as collapse of the transmembrane potential reveal permeability transition. The effect of oleate is reached after the titration, by carboxyatractyloside, of 38 pmol of adenine nucleotide translocase per mg mitochondrial protein. We propose that permeability transition may result from an additive action of carboxyatractyloside plus oleate on the ADP/ATP carrier.

Phosphate transport in mitochondria: past accomplishments, present problems, and future challenges

The requirement of inorganic phosphate (Pi) for oxidative phosphorylation in eukaryotic cells is fulfilled through specific Pi transport systems. The mitochondrial proton/phosphate symporter (Pic) is a membrane-embedded protein which translocates Pi from the cytosol into the mitochondrial matrix. Pic is responsible for the very rapid transport of most of the Pi used in ATP synthesis. During the past five years there have been advances on several fronts. Genomic and cDNA clones for yeast, bovine, rat, and human Pic have been isolated and sequenced. Functional expression of yeast Pic in yeast strains deficient in Pi transport and expression in Escherichia coli of a chimera protein involving Pic and ATP synthase alpha subunit have been accomplished. Pic, in contrast to other members of the family of transporters involved in energy metabolism, was demonstrated to have a presequence, which optimizes the import of the precursor protein into mitochondria. Six transmembrane segments appear to be a structural feature shared between Pic and other mitochondrial anion carriers, and recent-site directed mutagenesis studies implicate structure-functional relationships to bacteriorhodopsin. These recent advances on Pic will be assessed in light of a more global interpretation of transport mechanism across the inner mitochondrial membrane.

Abnormalities of ADP/ATP carrier protein in J-2-N cardiomyopathic hamsters

ADP/ATP carrier protein (AAC) is located in the mitochondrial inner membrane and has an important function in mitochondrial energy supply. This protein transports ATP to the cytoplasm and counter transports ADP into the mitochondria. J-2-N cardiomyopathic hamsters were investigated to determine the AAC content in cardiac mitochondria. After recording an electrocardiogram and collecting blood, the cardiac mitochondria were isolated. The mitochondrial membranes were labelled with eosin-5-maleimide (EMA) and separated on SDS polyacrylamide gels. The position of the AAC component was identified by exposing the gel under UV light, and the AAC content was determined by densitometry after staining with Coomassie blue. The AAC content ratio was significantly decreased in both 10-week-old and 1-year survived J-2-N hamsters when compared to control Golden hamster. Among 10-week-old J-2-N hamsters, the decrease in the AAC content ratio was more marked for the animals with more severe myocardial damage. The H(+)-ATPase activities of mitochondrial membrane were higher in 10-week-old J-2-N hamsters than in control hamsters. These results suggest that the decrease of AAC in J-2-N hamster plays an important role in the pathogenesis of cardiomyopathy in J-2-N hamsters.

Peroxidative damage to cardiac mitochondria: identification and purification of modified adenine nucleotide translocase

Rat myocardial membranes exposed to free radical-generating systems exhibit both lipid peroxidation and protein alterations. The most sensitive protein, a 28-kDa polypeptide, was previously shown to increase slightly in apparent molecular weight before disappearing completely from the protein profile [N. L. Parinandi, C. W. Zwizinski, and H. H. O. Schmid (1991) Arch. Biochem. Biophys. 289, 118-123]. We now report that isolated cardiac mitochondria contain a 28-kDa protein which responds in the same manner to treatment with Cu2+/t-butylhydroperoxide. The protein exhibits several characteristic properties of the mitochondrial adenine nucleotide translocase. This assignment is supported by the finding that carboxyatractyloside, a specific inhibitor of the adenine nucleotide translocase, can prevent the oxidant-induced changes in the 28-kDa protein. Efficient purification schemes for the isolation of milligram quantities of unmodified and oxidatively altered adenine nucleotide translocase from rat heart mitochondria are described.

N-ethylmaleimide and mercurials modulate inhibition of the mitochondrial inner membrane anion channel by H+, Mg2+ and cationic amphiphiles

Previously it has been shown that the mitochondrial inner membrane anion channel is reversibly inhibited by matrix Mg2+, matrix H+ and cationic amphiphiles such as propranolol. Furthermore, the IC50 values for both Mg2+ and cationic amphiphiles are dependent on matrix pH. It is now shown that pretreatment of mitochondria with N-ethylmaleimide, mersalyl and p-chloromercuribenzenesulfonate increases the IC50 values of these inhibitors. The effect of the mercurials is most evident when cysteine or thioglycolate is added to the assay medium to reverse their previously reported inhibitory effect (Beavis, A.D. (1989) Eur. J. Biochem. 185, 511-519). Although the IC50 values for Mg2+ and propranolol are shifted they remain pH dependent. Mersalyl is shown to inhibit transport even in N-ethylmaleimide-treated mitochondria indicating that N-ethylmaleimide does not react at the inhibitory mercurial site. However, the effects of N-ethylmaleimide and mersalyl on the IC50 for H+ are not additive which suggests that mercurials and N-ethylmaleimide react at the same 'regulatory' site. It is suggested that modification of this latter site exerts an effect on the binding of Mg2+, H+ and propranolol by inducing a conformational change. It is also suggested that a physiological regulator may exist which has a similar effect in vivo.

The mitochondrial aspartate/glutamate and ADP/ATP carrier switch from obligate counterexchange to unidirectional transport after modification by SH-reagents

The influence of various SH-reagents on the aspartate/glutamate carrier was investigated in the reconstituted system. When liposomes carrying partially purified carrier protein were treated with 5,5'-dithiobis(2-nitrobenzoic acid) or N-ethylmaleimide, antiport activity was strongly reduced. Several mercury compounds exerted a dual effect. They completely blocked the antiport and, in addition, induced an efflux pathway for internal aspartate. The maximum rate of this unidirectional flux was comparable to the original antiport activity. Induction of efflux always was coupled to inhibition of antiport. Efflux was neither due to unspecific leakage of proteoliposomes nor to a possible contamination by porin, but depended on active carrier protein, as elucidated by the sensitivity to proteinases and protein-modifying reagents. Besides efflux of aspartate, HgCl2 and mersalyl also induced a slow efflux of ATP from liposomes carrying coreconstituted aspartate/glutamate and ADP/ATP carrier. The two efflux activities could be discriminated taking advantage of the differential effectiveness of several inhibitors and proteinases. Although basic carrier properties were changed by the applied mercurials (Dierks, T., Salentin, A. and Krämer, R. (1990) Biochim. Biophys. Acta 1028, 281), aspartate and ATP efflux could clearly be correlated with the aspartate/glutamate and the ADP/ATP carrier, respectively. When purifying these two translocators the respective efflux activity copurified with the antiporter, thus elucidating that the two different transport functions are mediated by the same protein. These results argue for a participation of the aspartate/glutamate and the ADP/ATP carrier in the generally observed increase of mitochondrial permeability after treatment with SH-reagents.

The mitochondrial inner-membrane anion channel possesses two mercurial-reactive regulatory sites

The mitochondrial inner membrane anion channel catalyzes the electrophoretic transport of a wide variety of anions and is inhibited by matrix divalent cations and protons. In this paper, evidence is provided that mersalyl and p-chloromercuribenzene-sulfonate each interact with this uniporter at two distinct sites. Binding to site 1 causes a shift in the pH dependence of transport, characterized by a decrease in the pIC50 for protons from about 7.8 to about 7.3, and leads to substantial stimulation of transport in the physiological pH range. This effect is not reversed by addition of thiols such as thioglycolate. Binding of mersalyl and p-chloromercuribenzenesulfonate to site 2 inhibits the transport of most anions including Pi, citrate, malonate, sulfate and ferrocyanide. The transport of Cl- is inhibited about 60% by mersalyl, but is not inhibited by p-chloromercuribenzenesulfonate. These data suggest that inhibition is a steric effect dependent on the size of the anion and the size of the R group of the mercurial. This inhibition is reversed by thioglycolate. Dose/response curves indicate that mersalyl binds to site 1 as the dose increased from 7 to 13 nmol/mg, whereas it binds to site 2 as the dose is increased from 10 to 18 nmol/mg. Thus, at certain pH values both stimulatory and inhibitory phases can be seen in the same dose/response curve. It is suggested that these sites may contain thiol groups and that physiological regulators may exist which can effect changes in activity of the inner membrane anion uniporter similar to those exerted by mercurials.

Relationships between the NAD(P) redox state, fatty acid oxidation, and inner membrane permeability in rat liver mitochondria

Dysfunction of mitochondria after oxidation of endogenous NAD(P)H, especially after calcium accumulation, has been abundantly reported, but the causes of membrane perturbations did not receive a full explanation. In light of several additional observations reported in this study, we propose a general scheme which shows the sequential processes that are likely involved in the appearance of calcium-induced membrane leakiness. Addition of acetoacetate, oxaloacetate, or ketomalonate to rotenone-treated mitochondria led to a massive oxidation of both NADH and NADPH. Under these conditions, stimulation of fatty acid oxidation could be observed. This process was shown to be accompanied by a reduction of intramitochondrial NADP+. The reduction of NADP+ was inhibited by uncouplers, electron transfer inhibitors and N,N'-dicyclohexylcarbodiimide. It was thus probably catalyzed by the mitochondrial transhydrogenase. Oxidation of pyridine nucleotides in the presence of acetoacetate induced (i) a slight decrease in the number of sulfhydryl groups reactive with N-ethylmaleimide (but no change in the amount of intramitochondrial reduced glutathione) and (ii) modifications of the kinetics and the orientation of the ADP/ATP carrier. In the presence of calcium ions, acetoacetate-stimulated fatty acid oxidation promoted an extensive swelling of mitochondria. Uptake of calcium ions into the matrix was a critical factor for triggering the swelling. Thiols, if they were added at a sufficiently high concentration, suppressed the swelling. Also ligands of the ADP/ATP carrier which stabilized the m-state conformation of the protein, exerted an efficient protective action. Three essential interacting factors emerge from this study: (i) The crucial role of the ADP/ATP carrier orientation in promoting the calcium-induced membrane destabilization. More precisely, it has been shown that the ADP/ATP carrier adopts the c-state conformation (i.e., nucleotide binding site facing the cytoplasm) during fatty acid oxidation. (ii) The modification of a very small number of sulfhydryl groups of mitochondrial protein. These groups are probably in an oxidized state when the level of reduced pyridine nucleotides is low. (iii) The prevailing role of the transhydrogenase, the function of which is also intimately associated with fatty acid oxidation. After energization, transhydrogenase can hinder thiol oxidation and therefore partially protect the membrane structure.

Mitochondrial sulfhydryl groups under oligomycin-inhibited, aging, and uncoupling conditions: beneficial influence of cardioprotective drugs

Uncoupling, oligomycin-inhibited, and aging/swelling conditions comprise three models for mitochondrial dysfunction. In these models, the effects of cardioprotective agents on rat heart mitochondrial membrane -SH reactivity have been studied. For -SH detection two different chromophores were used: dithionitrobenzoate (NbS2) and monobromobimane (MB). The objective of this study is to reveal the influence of three cardioprotective substances against the loss of membrane -SH reactivity: (i) The thiol reagent 2-mercaptopropionylglycine (MPG) prevents the decrease of thiols caused by carbonylcyanide-p-trifluoromethoxyphenylhydrazone (FCCP), aging, and oligomycin measured with MB and NbS2, and the diminution by oleate detected with MB. The small amount of MPG (6 nmol/mg protein), necessary for the protection, agrees with oligomycin sensitivity of the -SH groups concerned. (ii) The active metabolite of molsidomine, 3-morpholinosydnonimine (SIN-1), protects against the decrease of thiols by FCCP, oleate, and aging monitored with MB. In the case of oligomycin -SH groups accessible to NbS2 are protected. (iii) Another antianginal drug, isosorbidedinitrate (ISDN) does not protect membrane thiol groups. In contrast to SIN-1, ISDN probably requires enzymatic activation. It is suggested that MPG as well as SIN-1 may help to restitute the original -SH status of the mitochondrial membrane.

Isolation and identification of proteins from the peptide-transport carrier in the scutellum of germinating barley (Hordeum vulgare L.) embryos

Peptide-transport proteins, intrinsic to the epithelial plasmalemmae of the scutella of germinating barley (Hordeum vulgare L.) embryos, have been selectively labelled with p-chloro-[(203)Hg]mercuribenzenesulphonate using both a substrate-screening technique and a procedure developed to label exclusively vicinal dithiol groups, which were shown previously (Walker-Smith and Payne, 1983, FEBS Lett. 160, 25-30) to be essential components of the peptide-transport system. After radioactive labelling, proteins from the scutellar membranes have been solubilised with lithium diiodosalicylate plus sodium dodecyl sulphate and separated by using polyacrylamide gel electrophoresis. Fluorography and silver staining of these gels has for the first time allowed identification of two presumptive components of the peptide-transport system. These components only become detectable in an extract of the scutellar epithelia after 15 h imbibition, concomitant with a dramatic increase in peptide-transport activity, and they remain present at least 3 d after the onset of germination. [(35)] Methionine was shown to be incorporated into these proteins between 15-20 h after imbibition, but its incorporation during a similar 5 h period into scutella isolated after 3 d was undetectable, implying a slow turnover of these proteins during the later stages of germination.

The reactivity of -SH groups in the ADP/ATP carrier isolated from beef heart mitochondria

The emergence of the reactivity of -SH groups associated with conformation changes has been studied on the ADP/ATP carrier, is isolated in three different inhibitor-protein complexes. 1. The bongkrekate-protein complex incorporates approximately one molecular more of N-ethylmaleimide than the carboxyatractylate-protein complex. After extensive denaturation by dodecylsulfate in urea, both inhibitor complexes exhibit four reactive -SH groups per subunit. Thus one of four -SH groups per subunit has been unmasked in the bongkrekate-protein complex. 2. The interconversion from the bongkrekate-protein complex to the carboxyatractylate-protein complex is inhibited after the -SH groups have been blocked. 3. The protein complex isolated with the more easily dissociable atractylate, is used to demonstrate, by the emergence of the -SH groups, the transition into the m-state. This transition is specifically catalyzed by ADP and ATP. 4. Using 2,2'-dinitro-5,5'-dithiodibenzoate, the appearance of the -SH groups on transition from the c-state to the m-state can be followed spectrophotometrically. The specificity for the catalyzing nucleotides is identical with that for the transport. The Km for ADP and ATP is in the range of 1 microM. In conclusion, the thiol groups of the isolated ADP/ATP carrier behave as in the mitochondrial membrane. The unmasking of -SH groups is in full accordance with the concept of two conformational states (c and m).