The voltage-dependent anion channel (VDAC): function in intracellular signalling, cell life and cell death.

Research over the last decade has extended the prevailing view of mitochondria to include functions well beyond the critical bioenergetics role in supplying ATP. It is now recognized that mitochondria play a crucial role in cell signaling events, inter-organelle communication, aging, many diseases, cell proliferation and cell death. Apoptotic signal transmission to the mitochondria results in the efflux of a number of potential apoptotic regulators to the cytosol that trigger caspase activation and lead to cell destruction. Accumulating evidence indicates that the voltage-dependent anion channel (VDAC) is involved in this release of proteins via the outer mitochondrial membrane. VDAC in the outer mitochondrial membrane is in a crucial position in the cell, forming the main interface between the mitochondrial and the cellular metabolisms. VDAC has been recognized as a key protein in mitochondria-mediated apoptosis since it is the proposed target for the pro- and anti-apoptotic Bcl2-family of proteins and due to its function in the release of apoptotic proteins located in the inter-membranal space. The diameter of the VDAC pore is only about 2.6-3 nm, which is insufficient for passage of a folded protein like cytochrome c. New work suggests pore formation by homo-oligomers of VDAC or hetero-oligomers composed of VDAC and pro-apoptotic proteins such as Bax or Bak. This review provides insights into the central role of VDAC in cell life and death and emphasizes its function in the regulation of mitochondria-mediated apoptosis and, thereby, its potential as a rational target for new therapeutics.

Oligomeric C-terminal truncated Bax preferentially releases cytochrome c but not adenylate kinase from mitochondria, outer membrane vesicles and proteoliposomes.

UNLABELLED: The mechanism by which the proapoptotic protein Bax releases cytochrome c from mitochondria is not fully understood. The present work approaches this problem using C-terminal truncated oligomeric Bax (BaxDeltaC). Micromolar concentrations of BaxDeltaC released cytochrome c from isolated rat heart and liver mitochondria, while the release of adenylate kinase was not significantly affected. BaxDeltaC also released cytochrome c but not adenylate kinase from outer membrane vesicles filled with these proteins. However, BaxDeltaC was ineffective in releasing cytochrome c when outer membrane vesicles were obtained in the presence of glycerol, conditions under which the number of contact sites was drastically reduced. BaxDeltaC did not liberate encapsulated cytochrome c and adenylate kinase from pure phospholipid vesicles or vesicles reconstituted with porin. However, when the hexokinase-porin-adenine nucleotide translocase complex from brain mitochondria was reconstituted in vesicles, BaxDeltaC released internal cytochrome c but not adenylate kinase. In all these systems, only a small portion of total cytochrome c present in either mitochondria or vesicles could be liberated by BaxDeltaC. BaxDeltaC also increased the accessibility of external cytochrome c to either oxidation by complex IV or reduction by complex III in intact liver and heart mitochondria. CONCLUSIONS: (1) BaxDeltaC selectively releases cytochrome c and enables a bidirectional movement of cytochrome c across the outer mitochondrial membrane. (2) A multiprotein complex that resembles the mitochondrial contact sites is a prerequisite for BaxDeltaC action. (3) A limited pool of cytochrome c becomes the first target for BaxDeltaC.

Adenine nucleotide translocator isoforms 1 and 2 are differently distributed in the mitochondrial inner membrane and have distinct affinities to cyclophilin D.

Different isoforms of the adenine nucleotide translocase (ANT) are expressed in a tissue-specific manner. It was assumed that ANT-1 and ANT-2 co-exist in every single mitochondrion and might be differently distributed within the membrane structures that constitute the peripheral inner membrane or the crista membrane. To discriminate between ANT originating from peripheral or from cristal inner membranes we made use of the fact that complexes between porin, the outer-membrane pore protein, and the ANT can be generated. Such complexes between porin and the ANT in the peripheral inner membrane were induced in rat heart mitochondria and isolated from rat brain and kidney. Using ANT-isotype-specific antibodies and sequence analysis of the N-terminal end, it was discovered that the peripheral inner membrane contained ANT-1 and ANT-2, whereas the cristal membrane contained exclusively ANT-2. Cyclophilin was co-purified with the porin-ANT complexes, whereas it was absent in the crista-derived ANT. This suggested that ANT-1 might have a higher affinity for cyclophilin. This specific intra-mitochondrial distribution of the two ANT isotypes and cyclophilin D suggests specific functions of the peripheral and crista-forming parts of the inner membrane and the two ANT isotypes therein.

Long-chain fatty acids promote opening of the reconstituted mitochondrial permeability transition pore.

Adenine nucleotide translocase-porin-hexokinase complex isolated from rat brain, when reconstituted into phospholipid-cholesterol vesicles, exhibits all properties of the mitochondrial permeability transition pore [Beutner, G., Rück, A., Riede, B., Welte, W. and Brdiczka, D. (1996) FEBS Lett. 396, 189-195]. In the present work, the effect of long-chain fatty acids on such reconstituted pore was examined. Opening of the pore was measured by leakage of either malate or fluorescein sulphonate entrapped inside the vesicles. It was found that myristate and oleate in the presence of 50 or 100 microM Ca(2+) produced a partial release of the probes in a dose-dependent way. A dicarboxylic fatty acid analogue, that appeared inactive as protonophore in intact mitochondria, exerted no effect on pore opening in the reconstituted system. 100 microM Ca(2+) alone was without effect. Pore opening by fatty acids in the reconstituted system was partly prevented by cyclosporin A. The pore opening also occurred when the vesicles were incubated in the presence of pancreatic phospholipase A(2). In this case, the opening was decreased by cyclosporin A or serum albumin. These results indicate that long-chain fatty acids elicit opening of the permeability transition pore reconstituted in phospholipid vesicles in a similar way as in intact mitochondria [Wi&ecedil;ckowski, M.R. and Wojtczak, L. (1998) FEBS Lett. 423, 339-342].

Inverse dose-rate effects at the level of proteins observed in the presence of lipids.

PURPOSE: Radical-chain mechanisms such as lipid peroxidation are known to show an inverse dose-rate effect, i.e. the radiation effect increases with decreasing dose rate at identical doses applied. The present study was intended to investigate whether an inverse dose-rate effect can be transferred from the level of lipids to the level of proteins. METHOD: Functional inactivation or structural modification by 80kV X-rays of two classes of proteins was investigated: membrane proteins with a natural environment of lipids like the Ca-ATPase of the sarcoplasmic reticulum, succinate dehydrogenase and F0F1-ATPase from the inner mitochondrial membrane. The second class comprises the water-soluble proteins cytosolic creatine kinase (MM-CK) and bovine serum albumin (BSA). Their modification by free radicals of water radiolysis was investigated in the absence and presence of lipid vesicles. RESULTS: For all proteins investigated, an inverse dose-rate effect was observed in the presence of lipids. This also holds for the water-soluble proteins MM-CK and BSA. In the latter two cases, the dose-rate effect disappeared either in the absence of (unsaturated) lipids or in the presence of alpha-tocopherol. CONCLUSION: The largely identical results obtained for a variety of different proteins indicate that inverse dose-rate effects are a normal consequence of radiation induced protein damage in the presence of lipids. In view of the high amount of cellular lipids, this should also hold for the situation in vivo, although due to the comparatively high concentration of intracellular antioxidants the dose-rate dependence might be strongly reduced or even virtually abolished.

Free radical-induced inactivation of creatine kinase: influence on the octameric and dimeric states of the mitochondrial enzyme (Mib-CK).

Free radicals of X-ray-induced water radiolysis, either directly or indirectly via their reaction products, reduce the activity of both dimeric cytoplasmic muscle-type creatine kinase (MM-CK) and octameric mitochondrial creatine kinase (Mi-CK) to virtually zero. Similarly values of the characteristic D(37)-dose of enzyme inactivation (dose required to reduce enzyme activity to 37%) were found for the two isoenzymes of CK under identical conditions. Octamer stability was not significantly affected within the dose range considered. However, both the dissociation of octamers into dimers by a transition-state analogue complex (TSAC), and the reassociation of the dimers into octamers, showed dose-dependent reduction. Binding of the TSAC to the active centre was found to protect the enzyme against inactivation by free radicals. No protection was observed for the radiation-induced decrease of the endogenous tryptophan fluorescence. The experimental results are in line with the following interpretation: (i) the reduction of Mi(b)-CK dimer association is due to free radical-induced modification of Trp-264, situated at the dimer/dimer interface; (ii) the active-site Trp-223 is not a prime target for free radicals and is not involved in the inactivation of the enzyme; (iii) the inhibition of TSAC-induced dissociation of Mi(b)-CK, like enzyme inactivation, is primarily due to a modification of the active-site Cys-278.

Octamer-dimer transitions of mitochondrial creatine kinase in heart disease.

Mitochondrial creatine kinase (Mi-CK) occurs in dimeric and octameric forms, both in vitro and in vivo. The Mi-CK octamer, however, is the predominant form in vivo and is important for various functions of the protein. In the present study we show for the first time a significant decrease of the octamer/dimer ratio in vivo, related to ischemia-induced damage, and a similar decrease of octamer stability in vitro, induced by peroxynitrite (PN) radicals. We used animal models to induce ischemia in two different ways: acute ischemia in intact heart (Langendorff perfusion) and chronic ischemia in vivo (LAD-infarction). In both models, impairment of heart function and mitochondrial energy metabolism was associated with a significant decrease of Mi-CK octamer/dimer ratios and of Mi-CK activities. These findings, together with recent data showing that the formation of PN is induced in ischemia and that Mi-CK is a prime target of peroxynitrite (PN)-induced damage, suggest that oxygen radicals generated during ischemia and reoxygenation could be an important factor for the decreased octamer stability. To test this hypothesis, we studied the effect of PN on pure Mi-CK in vitro, both on dissociation of octamers and reassociation of dimers. At 1 m m PN 66% of Mi-CK octamers dissociated into dimers, whereas octamerization of PN-modified dimers was already completely inhibited at 100 microm PN. Our data indicate that PN-induced damage could be responsible for the octamer-dimer transition of Mi-CK in ischemia. A loss of Mi-CK octamers would impair the channeling of high energy phosphate out of mitochondria and hence heart function in general.

Mass spectrometric mapping of ion channel proteins (porins) and identification of their supramolecular membrane assembly.

Mass spectrometric peptide mapping, particularly by matrix-assisted laser desorption-ionization (MALDI-MS), has recently been shown to be an efficient tool for the primary structure characterization of proteins. In combination with in situ proteolytic digestion of proteins separated by one- and two-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), mass spectrometric peptide mapping permits identification of proteins from complex mixtures such as cell lysates. In this study we have investigated several ion channel membrane proteins (porins) and their supramolecular assembly in mitochondrial membranes by peptide mapping in solution and upon digestion in the gel matrix. Porins are integral membrane proteins serving as nonspecific diffusion pores or as specific systems for the transport of substrates through bacterial and mitochondrial membranes. The well-characterized porin from Rhodobacter capsulatus (R.c.-porin) has been found to be a native trimeric complex by the crystal structure and was used as a model system in this study. R.c.-porin was characterized by MALDI-MS peptide mapping in solution, and by direct in situ-gel digestion of the trimer. Furthermore, in this study we demonstrate the direct identification of the noncovalent complex between a mitochondrial porin and the adenine nucleotide translocator from rat liver, by MALDI-MS determination of the specific peptides due to both protein sequences in the SDS-PAGE gel band. The combination of native gel electrophoresis and mass spectrometric peptide mapping of the specific gel bands should be developed as a powerful tool for the molecular identification of protein interactions.

Direct demonstration of a specific interaction between cyclophilin-D and the adenine nucleotide translocase confirms their role in the mitochondrial permeability transition.

A fusion protein between cyclophilin-D (CyP-D) and glutathione S-transferase (GST) was shown to bind to purified liver inner mitochondrial membranes (IMMs) in a cyclosporin A (CsA)-sensitive manner. Binding was enhanced by diamide treatment of the IMMs. Immobilized GST-CyP-D avidly bound a single 30 kDa protein present in Triton X-100-solubilized IMMs; immunoblotting showed this to be the adenine nucleotide translocase (ANT). Binding was prevented by pretreatment of the CyP-D with CsA, but not with cyclosporin H. Purified ANT also bound specifically to GST-CyP-D, but porin did not, even in the presence of ANT.

Reconstituted adenine nucleotide translocase forms a channel for small molecules comparable to the mitochondrial permeability transition pore.

Highly purified adenylate translocase (ANT) from rat heart mitochondria was functionally reconstituted as ATP/ADP exchange carrier in asolectin/cardiolipin vesicles. The ANT preparations used were free of porin, cyclophilin D, and Bax as analysed immunologically and by activity measurements. After pre-loading the ANT-containing proteoliposomes with ATP, malate or AMP, a gradual release of the trapped compounds by increasing the external Ca2+ concentrations could be demonstrated. N-Methyl-Val-4-cyclosporin did not inhibit the Ca2+ dependent release of internal substances from ANT liposomes. This inhibitor was found to be specific for the mitochondrial permeability transition pore (MTP) in intact mitochondria or reconstituted MTP-like protein complexes (e.g. hexokinase, porin, ANT complex). However, ADP in concentrations > 20 microM inhibited the liberation of internal compounds, while in contrast, atractyloside (30 microM) and HgCl2 (5 microM) both induced permeability of the ANT-containing liposomes resulting in a release of trapped substances. These results strongly suggest that ANT itself is capable to adopt a pore-like structure under conditions known to induce the permeability transition in mitochondria.

The permeability transition pore complex: a target for apoptosis regulation by caspases and bcl-2-related proteins.

Early in programmed cell death (apoptosis), mitochondrial membrane permeability increases. This is at least in part due to opening of the permeability transition (PT) pore, a multiprotein complex built up at the contact site between the inner and the outer mitochondrial membranes. The PT pore has been previously implicated in clinically relevant massive cell death induced by toxins, anoxia, reactive oxygen species, and calcium overload. Here we show that PT pore complexes reconstituted in liposomes exhibit a functional behavior comparable with that of the natural PT pore present in intact mitochondria. The PT pore complex is regulated by thiol-reactive agents, calcium, cyclophilin D ligands (cyclosporin A and a nonimmunosuppressive cyclosporin A derivative), ligands of the adenine nucleotide translocator, apoptosis-related endoproteases (caspases), and Bcl-2-like proteins. Although calcium, prooxidants, and several recombinant caspases (caspases 1, 2, 3, 4, and 6) enhance the permeability of PT pore-containing liposomes, recombinant Bcl-2 or Bcl-XL augment the resistance of the reconstituted PT pore complex to pore opening. Mutated Bcl-2 proteins that have lost their cytoprotective potential also lose their PT modulatory capacity. In conclusion, the PT pore complex may constitute a crossroad of apoptosis regulation by caspases and members of the Bcl-2 family.

Complexes between porin, hexokinase, mitochondrial creatine kinase and adenylate translocator display properties of the permeability transition pore. Implication for regulation of permeability transition by the kinases.

Complexes between hexokinase, outer membrane porin, and the adenylate translocator (ANT) were recently found to establish properties of the mitochondrial permeability transition pore in a reconstituted system. The complex was extracted by 0.5% Triton X-100 from rat brain membranes and separated by anion exchanger chromatography. The molecular weight was approximately 400 kDa suggesting tetramers of hexokinase (monomer 100kDa). By the same method a porin, creatine kinase octamer, ANT complex was isolated and reconstituted in liposomes. Vesicles containing the reconstituted complexes both retained ATP that could be used by either kinase to phosphorylate external creatine or glucose. Atractyloside inhibited this activity indicating that the ANT was involved in this process and was functionally reconstituted. Exclusively from the hexokinase complex containing liposome internal malate or ATP was released by addition of Ca2+ in a N-methylVal-4-cyclosporin sensitive way, suggesting that the hexokinase porin ANT complex might include the permeability transition pore (PTP). The Ca2+ dependent opening of the PTP-like structure was inhibited by ADP (apparent I(50), 8 microM) and ATP (apparent I(50), 84 microM). Also glucose inhibited the PTP-like activity, while glucose-6-phosphate abolished this effect. Although porin and ANT were functionally active in vesicles containing the creatine kinase octamer complex, Ca2+ did not induce a release of internal substrates. However, after dissociation of the creatine kinase octamer, the complex exhibited PTP-like properties and the vesicles liberated internal metabolites upon addition of Ca2+. The latter process was also inhibited by N-methylVal-4-cyclosporin. The activity of peptidyl-prolyl-cis-trans-isomerase (representing cyclophilin) was followed during complex isolation. Cyp D was co-purified with the hexokinase complex, while it was absent in the creatine kinase complex. The inhibitory effect of N-methylVal-4-cyclosporin on the creatine kinase complex may be explained by direct interaction with the creatine kinase dimer that appeared to support octamer formation.

Some new aspects of creatine kinase (CK): compartmentation, structure, function and regulation for cellular and mitochondrial bioenergetics and physiology.

Creatine kinase (CK) isoenzymes, specifically located at places of energy demand and energy production, are linked by a phosphocreatine/creatine (PCr/Cr) circuit, found in cells with intermittently high energy demands. Cytosolic CKs, in close conjunction with Ca(2+)-pumps, play a crucial role for the energetics of Ca(2+)-homeostasis. Mitochondrial Mi-CK, a cuboidal-shaped octamer with a central channel, binds and crosslinks mitochondrial membranes and forms a functionally coupled microcompartment with porin and adenine nucleotide translocase for vectorial export of PCr into the cytosol. The CK system is regulated by AMP-activated protein kinase via PCr/Cr and ATP/AMP ratios. Mi-CK stabilizes and cross-links cristae- or inner/outer membranes to form parallel membrane stacks and, if overexpressed due to creatine depletion or cellular energy stress, forms those crystalline intramitochondrial inclusions seen in some mitochondrial cytopathy patients. Mi-CK is a prime target for free radical damage by peroxynitrite. Mi-CK octamers, together with CK substrates have a marked stabilizing and protective effect against mitochondrial permeability transition pore opening, thus providing a rationale for creatine supplementation of patients with neuromuscular and neurodegenerative diseases.

The molecular structure of mitochondrial contact sites. Their role in regulation of energy metabolism and permeability transition.

Contact sites between the outer and peripheral inner membrane of mitochondria are involved in protein precursor uptake and energy transfer. Hexokinase and mitochondrial creatine kinase could be attributed by different techniques to the energy transfer contacts. Kinetic analyses suggested a functional interaction between the kinases, outer membrane pore protein, and inner membrane adenylate translocator (ANT). This suggestion was strongly supported by isolation of hexokinase and creatine kinase complexes that were constituted of kinase oligomers, porin and ANT. Phospholipid vesicles carrying reconstituted kinase-porin-ANT complexes enclosed internal ATP in contrast to vesicles containing free porin only. This indicated that unspecific transport through porin was regulated by its interaction with a specific antiporter, ANT. A direct interaction between porin and ANT in the hexokinase complex conferred the reconstituted system with permeability properties reminiscent of the mitochondrial permeability transition (PT) pore. In the creatine kinase complex this interaction between porin and ANT was replaced by contact of both with the kinase octamer. Thus PT-pore-like functions were not observed unless the creatine kinase octamer was dissociated, suggesting that the ANT was locked in the antiporter state by interaction with the octamer. Indeed, reconstituted pure ANT showed PT-pore-like properties concerning Ca2+ sensitivity. However, as cyclophilin was missing, sensitivity against cyclosporin was not observed.

Mitochondrial intermembrane inclusion bodies: the common denominator between human mitochondrial myopathies and creatine depletion, due to impairment of cellular energetics.

Mitochondrial inclusion bodies are often described in skeletal muscle of patients suffering diseases termed mitochondrial myopathies. A major component of these structures was discovered as being mitochondrial creatine kinase. Similar creatine kinase enriched inclusion bodies in the mitochondria of creatine depleted adult rat cardiomyocytes have been demonstrated. Structurally similar inclusion bodies are observed in mitochondria of ischemic and creatine depleted rat skeletal muscle. This paper describes the various methods for inducing mitochondrial inclusion bodies in rodent skeletal muscle, and compares their effects on muscle metabolism to the metabolic defects of mitochondrial myopathy muscle. We fed rats with a creatine analogue guanidino propionic acid and checked their solei for mitochondrial inclusion bodies, with the electron microscope. The activity of creatine kinase was analysed by measuring creatine stimulated oxidative phosphorylation in soleus skinned fibres using an oxygen electrode. The guanidino propionic acid-rat soleus mitochondria displayed no creatine stimulation, whereas control soleus did, even though the GPA solei had a five fold increase in creatine kinase protein per mitochondrial protein. The significance of these results in light of their relevance to human mitochondrial myopathies and the importance of altered cell energetics and metabolism in the formation of these crystalline structures are discussed.

The role of creatine kinase in inhibition of mitochondrial permeability transition.

Cyclosporin A sensitive swelling of mitochondria isolated from control mouse livers and from the livers of transgenic mice expressing human ubiquitous mitochondrial creatine kinase occurred in the presence of both 40 microM calcium and 5 microM atractyloside which was accompanied by a 2.5-fold increase over state 4 respiration rates. Creatine and cyclocreatine inhibited the latter only in transgenic liver mitochondria. Protein complexes isolated from detergent solubilised rat brain extracts, containing octameric mitochondrial creatine kinase, porin and the adenine nucleotide translocator, were reconstituted into malate loaded lipid vesicles. Dimerisation of creatine kinase in the complexes and exposure of the reconstituted complexes to >200 microM calcium induced a cyclosporin A sensitive malate release. No malate release occurred with complexes containing octameric creatine kinase under the same conditions.

Enhanced catalytic activity of hexokinase by work-induced mitochondrial binding in fast-twitch muscle of rat.

Using a teased muscle fiber preparation, we determined the activity of mitochondrially bound hexokinase in rat fast-twitch muscle under control conditions and after low-frequency stimulation periods for up to 2 h. As compared to soluble hexokinase, mitochondrial binding led to stimulation of glucose 6-phosphate production. Low-frequency stimulation greatly enhanced glucose 6-phosphate formation which was 100% and 250% elevated after 1 and 2 h, respectively. These observations point to a mechanism which rapidly increases the catalytic activity of hexokinase through binding to the mitochondrial surface.

Complexes between kinases, mitochondrial porin and adenylate translocator in rat brain resemble the permeability transition pore.

In vitro incubation of isolated hexokinase isozyme I or isolated dimer of mitochondrial creatine kinase with the outer mitochondrial membrane pore led to high molecular weight complexes of enzyme oligomers. Similar complexes of hexokinase and mitochondrial creatine kinase could be extracted by 0.5% Triton X-100 from homogenates of rat brain. Hexokinase and creatine kinase complexes could be separated by subsequent chromatography on DEAE anion exchanger. The molecular weight, as determined by gel-permeation chromatography, was approximately 400 kDa for both complexes. The Mr suggested tetramers of hexokinase (monomer 100 kDa) and creatine kinase (active enzyme is a dimer of 80 kDa). The composition of the complexes was further characterised by specific antibodies. Besides either hexokinase or creatine kinase molecules the complexes contained porin and adenylate translocator. It was possible to incorporate the complexes into artificial bilayer membranes and to measure conductance in 1 M KCI. The incorporating channels had a high conductance of 6 nS that was asymmetrically voltage dependent. The complexes were also reconstituted in phospholipid vesicles that were loaded with ATP. Complex containing vesicles retained ATP while vesicles reconstituted with pure porin were leaky. The internal ATP could be used by creatine kinase and hexokinase in the complex to phosphorylate external creatine or glucose. This process was inhibited by atractyloside. The hexokinase complex containing vesicles were furthermore loaded with malate or ATP that was gradually released by addition of Ca2+ between 100 and 600 microM. The liberation of malate or ATP by Ca2+ could be inhibited by N-methylVal-4-cyclosporin, suggesting that the porin translocator complex constitutes the permeability transition pore. The results show the physiological existence of kinase porin translocator complexes at the mitochondrial surface. It is assumed that such complexes between inner and outer membrane components are the molecular basis of contact sites observed by electron microscopy. Kinase complex formation may serve three regulatory functions, firstly regulation of the kinase activity, secondly stimulation of oxidative phosphorylation and thirdly regulation of the permeability transition pore.

Differential effects of creatine depletion on the regulation of enzyme activities and on creatine-stimulated mitochondrial respiration in skeletal muscle, heart, and brain.

Guanidinopropionic acid (GPA), an analogue of creatine (Cr), is known to inhibit Cr uptake by cells. The metabolic effects of chronic Cr depletion on brain, heart and soleus muscle of rats were studied. In GPA hearts and soleus muscle, total specific creatine kinase (CK) activity was decreased by approx. 40% compared to controls, whereas in brain this same activity was elevated by a factor of two. Immunoblot analysis of soleus mitochondria from GPA rats showed an approximate 4-fold increase in Mi-CK protein and a concomitant 3-fold increase in adenine nucleotide translocator (ANT) protein, when compared to control. In GPA-fed rats, the specific activities of adenylate kinase (ADK) and succinate dehydrogenase were significantly higher in brain and soleus (2-fold), but heart remained the same. However, hexokinase (HK) decreased by approx. 50% both in heart and soleus, indicating that muscle and brain follow different strategies to compensate the energy deficit caused by creatine depletion. Skinned muscle fibres from Cr-depleted soleus attained approx. only 70% maximum state 3 respiration with 0.1 M ADP in the presence of 10 mM Cr compared to 100% in control fibres. This defect in Cr stimulated respiration was also seen in isolated heart mitochondria, but was normal in those from brain. The observed deficit of Cr-stimulated respiration, the significant accumulation of Mib-CK and ANT, concomitant with the formation of Mib-CK rich intra-mitochondrial inclusions shown by electron microscopy, indicate that Mib-CK function and coupling to oxidative phosphorylation (OXPHOS), is impaired in these abnormal mitochondria. In addition, our results show tissue-specific metabolic compensations to Cr depletion.