Dynamics of Ca2+ transport in rat liver mitochondria in anoxia]

Tetracycline was used as a fluorescent test-antibiotic for Ca2+ ions in rat liver mitochondria. Incubation of the isolated mitochondria under anaerobic conditions at 20 degrees C resulted in a rapid (in 30-min) loss by the mitochondria of the property to accumulate Ca2+. Disturbances of the mitochondrial Ca2+-accumulating property during the survival of the liver developed much more slowly (it took over 2 hours) and were not monotonous; the maximal values were recorded during the 5th-10th and the 60th minutes of survival.     

Structure and dynamics of the mitochondrial inner membrane cristae

Three-dimensional images of mitochondria provided by electron tomography reveal that the micro-compartments (cristae) defined by the inner membrane are connected to the periphery of this membrane by narrow tubular junctions, which likely restrict diffusion. The tomograms also strongly suggest that inner membrane topology represents a balance between membrane fusion and fission processes. The hypothesis being developed is that inner membrane topology is a regulated property of mitochondria. This review summarizes the evidence about how inner membrane shape influences mitochondrial function and, conversely, what is known about the factors that determine this membrane's topology.     

Understanding ion transport by quantification of mitochondrial swelling

Suspensions of mitochondria are turbid and scatter light. An increase in the matrix volume (swelling) due to the influx of permeable solutes results in a decrease in the amount of light scattered. This property can be used to study solute fluxes across the mitochondrial inner membrane. A rapid method for isolating mitochondria is presented along with three swelling experiments using energized and non-energized mitochondria to illustrate ion transport across energy transducing membranes.     

Isolation and partial characterization of the glutamate/aspartate transporter from pea leaf mitochondria using a specific monoclonal antibody

A library of monoclonal antibodies directed against the proteins of the inner mitochondrial membrane was screened for antibodies that could bind to the glutamate/aspartate transporter of pea mitochondria and thereby inhibit its activity. One antibody, 2C7, had the property of inhibiting glutamate and aspartate-dependent oxaloacetate metabolism by pea mitochondria without affecting the metabolism of other substrates. The antibody specifically recognized a 21,000 dalton protein, which was tentatively identified as the glutamate/aspartate transporter. The antibody was used to follow the extraction of this protein by Triton X-114 and cardiolipin and the partial purification of the protein by centrifugation and chromatography on hydroxylapatite. The partially purified preparation was reconstituted into azolectin vesicles and shown to catalyze glutamate/glutamate and glutamate/aspartate exchange in an apparently nonelectrogenic manner. The antibody was shown to specifically bind to the glutamate/aspartate exchanger by its ability to inhibit this reconstituted exchange reaction.     

Characterization and location of Src-dependent tyrosine phosphorylation in rat brain mitochondria

Analysis of protein phosphorylation in highly purified rat brain mitochondria revealed the presence of several alkali-stable phosphoproteins whose phosphorylation markedly increases upon treatment with peroxovanadate and Mn(2+), a property indicating tyrosine phosphorylation. These include three prominent bands, with apparent sizes of 50, 60, and 75 kDa, which are detectable by anti-phosphotyrosine. Tyrosine phosphorylation disappears when mitochondria are treated with PP2, an inhibitor of the Src kinase family, suggesting the presence of members of this family in rat brain mitochondria. Immunoblotting and immunoprecipitation assays of mitochondrial lysates confirmed the presence of Fyn, Src and Lyn kinases, as well as Csk, a protein kinase which negatively controls the activity of the Src kinase family. Results show that tyrosine-phosphorylated proteins are membrane-bound and that they are located on the inner surface of the outer membrane and/or the external surface of the inner membrane. Instead, Src tyrosine kinases are mainly located in the intermembrane space - in particular, as revealed by immunogold experiments for Lyn kinase, in the cristal lumen. Rat brain mitochondria were also found to possess a marked level of tyrosine phosphatase activity, strongly inhibited by peroxovanadate.     

Effects of N-tricyanovinylamines on oxidative phosphorylation and level of SH-groups in rat liver mitochondria.

The effects of N-substituted tricyanovinylamines on oxidative phosphorylation as well as on glutathione and total SH group concentrations in rat liver mitochondria was studied. The N-TCVA derivatives studied (N-cyclohexyl; N-isobutyl; N-benzyl; N-phenyl; N-4-Br-phenyl; N-3-nitrophenyl) had an uncoupling effection on the oxidative phosphorylation. They stimulated the respiration of mitochondria and influenced their membrane potential. In their property as SH agents, the N-TCVA derivatives reduced the level of TSH groups of the mitochondria present in concentrations of 2 mumol/mg protein. The activity of succinate dehydrogenase was decreased by N-TCVA by 13%. N-TCVA derivatives changed the redox state of glutathione in mitochondria. This effect was observed at the concentration 0.3 mumol/mg protein. The results obtained in the present study support the view that the glutathione status is more sensitive than the total level of SH groups to incubation of mitochondria with SH agents such as N-TCVA derivatives.     

The human mitochondrial import receptor, hTom20p, prevents a cryptic matrix targeting sequence from gaining access to the protein translocation machinery

Yeast Mas70p and NADH cytochrome b5 reductase are bitopic integral proteins of the mitochondrial outer membrane and are inserted into the lipid-bilayer in an Nin-Ccyto orientation via an NH2-terminal signal- anchor sequence. The signal anchor of both proteins is comprised of a short, positively charged domain followed by the predicted transmembrane segment. The positively charged domain is capable of functioning independently as a matrix-targeting signal in yeast mitochondria in vitro but does not support import into mammalian mitochondria (rat or human). Rather, this domain represents a cryptic signal that can direct import into mammalian mitochondria only if proximal components of the outer membrane import machinery are removed. This can be accomplished either by treating the surface of the intact mitochondria with trypsin or by generating mitoplasts. The import receptor Tom20p (Mas20p/MOM19) is responsible for excluding the cryptic matrix-targeting signal from mammalian mitochondria since replacement of yeast Tom20p with the human receptor confers this property to the yeast organelle while at the same time maintaining import of other proteins. In addition to contributing to positive recognition of precursor proteins, therefore, the results suggest that hTom20p may also have the ability to screen potential matrix-targeting sequences and exclude certain proteins that would otherwise be recognized and imported by distal components of the outer and inner membrane protein- translocation machinery. These findings also indicate, however, that cryptic signals, if they exist within otherwise native precursor proteins, may remain topogenically silent until the precursor successfully clears hTom20p, at which time the activity of the cryptic signal is manifested and can contribute to subsequent translocation and sorting of the polypeptide.     

Structural features within the nascent chain regulate alternative targeting of secretory proteins to mitochondria

Protein targeting to specified cellular compartments is essential to maintain cell function and homeostasis. In eukaryotic cells, two major pathways rely on N‐terminal signal peptides to target proteins to either the endoplasmic reticulum (ER) or mitochondria. In this study, we show that the ER signal peptides of the prion protein‐like protein shadoo, the neuropeptide hormone somatostatin and the amyloid precursor protein have the property to mediate alternative targeting to mitochondria. Remarkably, the targeting direction of these signal peptides is determined by structural elements within the nascent chain. Each of the identified signal peptides promotes efficient ER import of nascent chains containing α‐helical domains, but targets unstructured polypeptides to mitochondria. Moreover, we observed that mitochondrial targeting by the ER signal peptides correlates inversely with ER import efficiency. When ER import is compromised, targeting to mitochondria is enhanced, whereas improving ER import efficiency decreases mitochondrial targeting. In conclusion, our study reveals a novel mechanism of dual targeting to either the ER or mitochondria that is mediated by structural features within the nascent chain.     

Reduction in cardiac mitochondrial calcium loading capacity is observable during alpha-naphthylisothiocyanate-induced acute cholestasis: a clue for hepatic-derived cardiomyopathies?

Cardiovascular changes of still obscure origin are sometimes correlated with co-existing liver diseases, as cholestasis.The aim of this work was to examine and compare cardiac mitochondrial bioenergetics and calcium loading capacity from rats injected with a single dose of alpha-naphthylisothiocyanate (ANIT), a cholestasis-inducing compound. Forty-eight hours after ANIT administration, blood samples were collected and markers for hepatic disease were determined. Heart mitochondria from both control and ANIT-injected rats were isolated and subjected to biochemical characterization, including the susceptibility to the calcium-dependent permeability transition. The results showed that cardiac mitochondria from cholestatic animals did not have significant changes in respiratory parameters or in the basal levels of adenine nucleotide. The most impressive result from this work was that cardiac mitochondria from ANIT-injected animals had a lower calcium loading capacity. The prevention of this property by cyclosporin-A, a specific inhibitor of the mitochondrial permeability transition, showed that this phenomenon was reason for the reduced calcium loading capacity in ANIT-injected animals. The results suggest that, during the development of ANIT-induced cholestasis, heart mitochondria loose their default ability to buffer calcium. Our results may contribute to explain the occurrence of cardiomyopathies sometimes associated with cholestatic disease.     

Alterations in Membrane Potential in Mitochondria Isolated from Brain Subregions During Focal Cerebral Ischemia and Early Reperfusion: Evaluation Using Flow Cytometry

Mitochondria isolated from brain tissue following middle cerebral artery occlusion or during early reperfusion were tested for their ability to generate a membrane potential under standard conditions in vitro. Membrane potential was evaluated based on rhodamine 123 fluorescence in the mitochondria as detected using flow cytometry. Compared with equivalent samples from the contralateral hemisphere, the geometric mean fluorescence was significantly lower in mitochondria prepared from the striatum and perifocal tissue in the cortex at 3 h ischemia. During reperfusion, this property was decreased in mitochondria from tissue in the striatum and cortex that had been part of severely ischemic core tissue during the arterial occlusion. These findings provide additional evidence that mitochondria develop changes during ischemia and reperfusion that are likely to limit their ability to respond to changing energy requirements and contribute to cell dysfunction and cell death. It also demonstrates the ability to gain a sensitive measure of these mitochondrial changes using flow cytometry.     

Inhibitory effects of some long-chain unsaturated fatty acids on mitochondrial beta-oxidation. Effects of streptozotocin-induced diabetes on mitochondrial beta-oxidation of polyunsaturated fatty acids.

Evidence showing that some unsaturated fatty acids, and in particular docosahexaenoic acid, can be powerful inhibitors of mitochondrial beta-oxidation is presented. This inhibitory property is, however, also observed with the cis- and trans-isomers of the C18:1(16) acid. Hence it is probably the position of the double bond(s), and not the degree of unsaturation, which confers the inhibitory property. It is suggested that the inhibitory effect is caused by accumulation of 2,4-di- or 2,4,7-tri-enoyl-CoA esters in the mitochondrial matrix. This has previously been shown to occur with these fatty acids, in particular when the supply of NADPH was limiting 2,4-dienoyl-CoA reductase (EC 1.3.1.-) activity [Hiltunen, Osmundsen & Bremer (1983) Biochim. Biophys. Acta 752, 223-232]. Liver mitochondria from streptozotocin-diabetic rats showed an increased ability to beta-oxidize 2,4-dienoyl-CoA-requiring acylcarnitines. Docosahexaenoylcarnitine was also found to be less inhibitory at lower concentrations with incubation under coupled conditions. With uncoupling conditions there was little difference between mitochondria from normal and diabetic rats in these respects. This correlates with a 5-fold stimulation of 2,4-dienoyl-CoA reductase activity found in mitochondria from streptozotocin-diabetic rats.     

Glutathione binding to the Bcl-2 homology-3 domain groove: a molecular basis for Bcl-2 antioxidant function at mitochondria

Bcl-2 protects cells against mitochondrial oxidative stress and subsequent apoptosis. However, the mechanism underlying the antioxidant function of Bcl-2 is currently unknown. Recently, Bax and several Bcl-2 homology-3 domain (BH3)-only proteins (Bid, Puma, and Noxa) have been shown to induce a pro-oxidant state at mitochondria (1-4). Given the opposing effects of Bcl-2 and Bax/BH3-only proteins on the redox state of mitochondria, we hypothesized that the antioxidant function of Bcl-2 is antagonized by its interaction with the BH3 domains of pro-apoptotic family members. Here, we show that BH3 mimetics that bind to a hydrophobic surface (the BH3 groove) of Bcl-2 induce GSH-sensitive mitochondrial dysfunction and apoptosis in cerebellar granule neurons. BH3 mimetics displace a discrete mitochondrial GSH pool in neurons and suppress GSH transport into isolated rat brain mitochondria. Moreover, BH3 mimetics and the BH3-only protein, Bim, inhibit a novel interaction between Bcl-2 and GSH in vitro. These results suggest that Bcl-2 regulates an essential pool of mitochondrial GSH and that this regulation may depend upon Bcl-2 directly interacting with GSH via the BH3 groove. We conclude that this novel GSH binding property of Bcl-2 likely plays a central role in its antioxidant function at mitochondria.     

Mitochondrial polymorphism. IV. Phospholipid composition of mitochondria of a wheat hybrid and its parents

Mitochondria of young seedlings of wheat genotypes 28, 31MS, and 31MS |MX 28 differed in total lipid and phospholipid. Hybrid mitochondria had more lipid and phospholipid than did the parents, and the three genotypes differed in fatty acid composition of the phospholipid fraction. Hybrid mitochondria exhibited heterosis in cytochrome oxidase activity. Although depletion of phospholipid greatly reduced cytochrome oxidase activity in hybrid mitochondria, the heterotic property was preserved. Regulatory aspects of lipid and phospholipid in mitochondria of the hybrid and complementing parental mixture are considered.This research was supported by DeKalb Ag-Research Inc. and in part by grant A-338 from Robert A. Welch Foundation of Houston, Texas.The preceding paper in this series is Sarkissian, I. V., and Srivastava, H. K. (1971). Biochem. Genet. 5:57.     

Mechanism of action of an old antibiotic revisited: Role of calcium ions in protonophoric activity of usnic acid

Usnic acid (UA), an old antibiotic and one of the first described mitochondrial uncouplers, has demonstrated many beneficial activities, such as antimicrobial, antiviral, antitumour and anti-inflammatory properties. Here, we performed a thorough investigation of effects of usnic acid and its analogues on artificial planar bilayer lipid membrane (BLM), rat liver mitochondria and bacteria. Surprisingly enough, all of the three hydroxyl groups of UA appeared to be involved in its proton-shuttling activity on BLM. We ascribed this fact to an ability of UA to form complexes with calcium ions, aiding it in cycling protons across the membrane. Actually, the addition of calcium ions markedly stimulated the UA-induced electrical current across BLM. By using the calcium ionophore A23187, we proved the involvement of calcium ions in the UA uncoupling action on isolated rat liver mitochondria. The calcium-chelating property of UA was demonstrated here by the method of extracting metal ions into a hydrophobic phase. Modification of any of the hydroxyl groups in UA dramatically reduced not only the UA-induced current across BLM and the UA-mediated calcium extraction, but also the uncoupling activity of UA in mitochondria and the inhibiting effect of UA on the growth of Bacillus subtilis. The ability of UA to cause dissipation of membrane potential in isolated liver mitochondria and bacterial cells was shown here for the first time. In view of the data obtained, the protonophoric activity of UA is considered to make a significant contribution to its antibacterial action.     

The anti-cancer agent nemorosone is a new potent protonophoric mitochondrial uncoupler

Nemorosone, a natural-occurring polycyclic polyprenylated acylphloroglucinol, has received increasing attention due to its strong in vitro anti-cancer action. Here, we have demonstrated the toxic effect of nemorosone (1-25 μM) on HepG2 cells by means of the MTT assay, as well as early mitochondrial membrane potential dissipation and ATP depletion in this cancer cell line. In mitochondria isolated from rat liver, nemorosone (50-500 nM) displayed a protonophoric uncoupling activity, showing potency comparable to the classic protonophore, carbonyl cyanide m-chlorophenyl hydrazone (CCCP). Nemorosone enhanced the succinate-supported state 4 respiration rate, dissipated mitochondrial membrane potential, released Ca(2+) from Ca(2+)-loaded mitochondria, decreased Ca(2+) uptake and depleted ATP. The protonophoric property of nemorosone was attested by the induction of mitochondrial swelling in hyposmotic K(+)-acetate medium in the presence of valinomycin. In addition, uncoupling concentrations of nemorosone in the presence of Ca(2+) plus ruthenium red induced the mitochondrial permeability transition process. Therefore, nemorosone is a new potent protonophoric mitochondrial uncoupler and this property is potentially involved in its toxicity on cancer cells.     

Lineage-specific gene loss following mitochondrial endosymbiosis and its potential for function prediction in eukaryotes

MOTIVATION: The endosymbiotic origin of mitochondria has resulted in a massive horizontal transfer of genetic material from an alpha-proteobacterium to the early eukaryotes. Using large-scale phylogenetic analysis we have previously identified 630 orthologous groups of proteins derived from this event. Here we show that this proto-mitochondrial protein set has undergone extensive lineage-specific gene loss in the eukaryotes, with an average of three losses per orthologous group in a phylogeny of nine species. This gene loss has resulted in a high variability of the alphaproteobacterial-derived gene content of present-day eukaryotic genomes that might reflect functional adaptation to different environments. Proteins functioning in the same biochemical pathway tend to have a similar history of gene loss events, and we use this property to predict functional interactions among proteins in our set.     

Ability of energy metabolism of cells to activate repair of its genetic material after radiation damage

A new hypothesis on a mechanism of action of the radioprotective and medical preparations involving the activation of oxidative phosphorylation and biogenesis of mitochondria which provides the increase of dimension and rapidity of the cellular reparation has been suggested. A previously unknown property of mitochondrions to determine a cellular homeostasis has been found. An activation of oxidative phosphorylation of mitochondria is a way of cellular protection against extreme conditions (namely, against radiation exposure and tumor formation).     

Does fatty acid-binding protein play a role in fatty acid transport?

Summary The possible property of fatty acid-binding proteins (FABPs) to transport fatty acid was investigated in various model systems with FABP preparations from liver and heart. An effect of FABP, however, was not detectable with a combination of oleic acid-loaded mitochondria and vesicles or liposomes due to the rapid spontaneous transfer. Therefore, the mitochondria were separated from the vesicles in an equilibrium dialysis cell. The spontaneous fatty acid transfer was much lower and addition of FABP resulted in an increase of fatty acid transport. Oleic acid was withdrawn from different types of monolayers by FABP with rates up to 10%/min. When two separate monolayers were used, FABP increased fatty acid transfer between these monolayers and an equilibrium was reached.Abbreviations FABP(s) fatty acid-binding protein(s) - PC phosphatidylcholine - PS phosphatidylserine - PE phosphatidylethanolamine     

Ionophorous Properties of SY-1 (20-Deoxysalinomycin) in Rat Liver Mitochondri

SY-1 (20-deoxysalinomycin), a monocarboxylic polyether antibiotic closely related to salinomycin, caused a rapid release of previously accumulated alkali metal cations by valinomycin or monazomycin in rat liver mitochondria, and simultaneously reversed swelling of mitochondria.

With a strict specificity for substrate and cation, SY-1 exhibited a property of inhibiting mitochondrial functions such as substrate oxidation, oxidative phosphorylation and ATP hydrolysis induced by valinomycin or monazomycin, In comparative study with salinomycin, SY-1 was found to be more effective on the mitochondrial functions than salinomycin.

On the basis of the results so far obtained, the inhibitory effect of SY-1 on mitochondria is interpreted as a result of interaction with essential cations, especially with K⁺, in mitochondria.     

Accumulation of lipophilic dications by mitochondria and cells

Lipophilic monocations can pass through phospholipid bilayers and accumulate in negatively-charged compartments such as the mitochondrial matrix, driven by the membrane potential. This property is used to visualize mitochondria, to deliver therapeutic molecules to mitochondria and to measure the membrane potential. In theory, lipophilic dications have a number of advantages over monocations for these tasks, as the double charge should lead to a far greater and more selective uptake by mitochondria, increasing their therapeutic potential. However, the double charge might also limit the movement of lipophilic dications through phospholipid bilayers and little is known about their interaction with mitochondria. To see whether lipophilic dications could be taken up by mitochondria and cells, we made a series of bistriphenylphosphonium cations comprising two triphenylphosphonium moieties linked by a 2-, 4-, 5-, 6- or 10-carbon methylene bridge. The 5-, 6- and 10-carbon dications were taken up by energized mitochondria, whereas the 2- and 4-carbon dications were not. The accumulation of the dication was greater than that of the monocation methyltriphenylphosphonium. However, the uptake of dications was only described by the Nernst equation at low levels of accumulation, and beyond a threshold membrane potential of 90-100 mV there was negligible increase in dication uptake. Interestingly, the 5- and 6-carbon dications were not accumulated by cells, due to lack of permeation through the plasma membrane. These findings indicate that conjugating compounds to dications offers only a minor increase over monocations in delivery to mitochondria. Instead, this suggests that it may be possible to form dications within mitochondria that then remain within the cell.