Decoupling of oxidative phosphorylation and photophosphorylation

The bioenergetic landscape of cancer

BACKGROUND

Bioenergetic remodeling of core energy metabolism is essential to the initiation, survival, and progression of cancer cells through exergonic supply of adenosine triphosphate (ATP) and metabolic intermediates, as well as control of redox homeostasis. Mitochondria are evolutionarily conserved organelles that mediate cell survival by conferring energetic plasticity and adaptive potential. Mitochondrial ATP synthesis is coupled to the oxidation of a variety of substrates generated through diverse metabolic pathways. As such, inhibition of the mitochondrial bioenergetic system by restricting metabolite availability, direct inhibition of the respiratory Complexes, altering organelle structure, or coupling efficiency may restrict carcinogenic potential and cancer progression.

SCOPE OF REVIEW

Here, we review the role of bioenergetics as the principal conductor of energetic functions and carcinogenesis while highlighting the therapeutic potential of targeting mitochondrial functions.

MAJOR CONCLUSIONS

Mitochondrial bioenergetics significantly contribute to cancer initiation and survival. As a result, therapies designed to limit oxidative efficiency may reduce tumor burden and enhance the efficacy of currently available antineoplastic agents.

Theoretical and Experimental Study of the Interaction of Protonophore Uncouplers and Decoupling Agents with Functionally Active Mitochondria

The purpose of this work was to quantitatively characterize the effectiveness of oxidative phosphorylation uncouplers and decoupling agents in functionally active mitochondria, taking into account their content in the hydrophobic region of the inner membrane of these organelles. When conducting theoretical studies, it is accepted that uncouplers and decouplers occupy part of the volume of mitochondria to exhibit their activity, which is defined as the effective volume. The following quantities characterizing the action of these reagents are considered: (1) concentrations of reagents that cause double stimulation of mitochondrial respiration in state 4 (C 200 ); (2) effective distribution coefficient (E MW ) - the ratio of the amount of reagents in the effective volume of mitochondria and the water volume; (3) the relative amount of reagents associated with the effective volume of mitochondria (U M / U T ); (4) specific activity of reagents localized in the effective volume of mitochondria (A M ). We have developed methods for determining these values, based on an analysis of the dependence of the rate of mitochondrial respiration on the concentration of uncouplers and decoupling agents at two different concentrations of mitochondrial protein in the incubation medium. During experimental studies, we compared the effects of the classical protonophore uncouplers 2,4-dinitrophenol (DNP) and сarbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP), the natural uncouplers lauric and palmitic acids, and the natural decouplers α,ω-tetradecanedioic (TDA) and α,ω-hexadecanedioic (HDA) acids that differ both in the structure of the molecule and in the degree of solubility in lipids. Using the developed methods, we have clarified the dependence of the degree of activity of these uncouplers and decoupling agents on the distribution of their molecules between the effective volume of mitochondria and the water volume.

A model naphthenic acid decouples oxidative phosphorylation through selective inhibition of mitochondrial complex activity

The naphthenic acid fraction compound (NAFC), 3,5-dimethyladamantane-1-acetic acid, was tested for its ability to uncouple mitochondrial oxidative phosphorylation. Mitochondria isolated from rainbow trout (Oncorhynchus mykiss) liver were exposed to 3,5-dimethyladamantane-1-acetic acid in state 3 and 4 respiration, and mitochondrial membrane potential were quantified. Electron transport chain (ETC) protein complexes were isolated using pharmacological agents and inhibitors, and their activities measured. The NAFC compound completely inhibited states 3 and 4 respiration with an IC50 of 0.77 and 1.01 mM, respectively. The NAFC compound partially uncoupled mitochondrial membrane potential in state 3 and 4 respiration with an IC50 of 2.19 and 1.73 mM, respectively. The NAFC impaired the activities of ETC protein complexes with a 9.5-fold range in sensitivity. The relative inhibitory effect of the ETC protein complexes to NAFC was CIV≥CI>CIII>CII. The impairment of mitochondrial oxidative phosphorylation by adamantane 3,5-dimethyladamantane-1-acetic acid is mediated via its inhibition of ETC protein complexes.

Modulators reducing the efficiency of oxidative ATP synthesis in mitochondria: protonophore uncouplers, cyclic redox agents, and decouplers

This work considers the main indicators of the oxidative phosphorylation efficiency in mitochondria: the ADP/O and H+/O ratios. Three groups of modulators that reduce the efficiency of oxidative phosphorylation are compared: protonophore uncouplers, cyclic redox compounds, and decouplers. It is noted that some of them are considered effective therapeutic agents. The paper analyzes the authors' original data on the mechanism of action of natural decouplers, represented by long-chain α,ω-dioic acids, as antioxidants. In conclusion, we discuss the hypothesis of their participation in the rescue of hepatocytes in various disorders of carbohydrate and lipid metabolism.

Metabolic Reprogramming in Amyotrophic Lateral Sclerosis

Mitochondrial dysfunction in the spinal cord is a hallmark of amyotrophic lateral sclerosis (ALS), but the neurometabolic alterations during early stages of the disease remain unknown. Here, we investigated the bioenergetic and proteomic changes in ALS mouse motor neurons and patients' skin fibroblasts. We first observed that SODG93A mice presymptomatic motor neurons display alterations in the coupling efficiency of oxidative phosphorylation, along with fragmentation of the mitochondrial network. The proteome of presymptomatic ALS mice motor neurons also revealed a peculiar metabolic signature with upregulation of most energy-transducing enzymes, including the fatty acid oxidation (FAO) and the ketogenic components HADHA and ACAT2, respectively. Accordingly, FAO inhibition altered cell viability specifically in ALS mice motor neurons, while uncoupling protein 2 (UCP2) inhibition recovered cellular ATP levels and mitochondrial network morphology. These findings suggest a novel hypothesis of ALS bioenergetics linking FAO and UCP2. Lastly, we provide a unique set of data comparing the molecular alterations found in human ALS patients' skin fibroblasts and SODG93A mouse motor neurons, revealing conserved changes in protein translation, folding and assembly, tRNA aminoacylation and cell adhesion processes.

NAD(P)H cytochrome b5 oxidoreductase deficiency in Leishmania major results in impaired linoleate synthesis followed by increased oxidative stress and cell death

NAD(P)H cytochrome b(5) oxidoreductase (Ncb5or), comprising cytochrome b(5) and cytochrome b(5) reductase domains, is widely distributed in eukaryotic organisms. Although Ncb5or plays a crucial role in lipid metabolism of mice, so far no Ncb5or gene has been reported in the unicellular parasitic protozoa Leishmania species. We have cloned, expressed, and characterized Ncb5or gene from Leishmania major. Steady state catalysis and spectral studies show that NADH can quickly reduce the ferric state of the enzyme to the ferrous state and is able to donate an electron(s) to external acceptors. To elucidate its exact physiological role in Leishmania, we attempted to create NAD(P)H cytochrome b(5) oxidoreductase from L. major (LmNcb5or) knock-out mutants by targeted gene replacement technique. A free fatty acid profile in knock-out (KO) cells reveals marked deficiency in linoleate and linolenate when compared with wild type (WT) or overexpressing cells. KO culture has a higher percentage of dead cells compared with both WT and overexpressing cells. Increased O(2) uptake, uncoupling and ATP synthesis, and loss of mitochondrial membrane potential are evident in KO cells. Flow cytometric analysis reveals the presence of a higher concentration of intracellular H(2)O(2), indicative of increased oxidative stress in parasites lacking LmNcb5or. Cell death is significantly reduced when the KO cells are pretreated with BSA bound linoleate. Real time PCR studies demonstrate a higher Δ12 desaturase, superoxide dismutase, and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) mRNA with a concomitant fall in Δ9 desaturase mRNA expression in LmNcb5or null cell line. Together these findings suggest that decreased linoleate synthesis, and increased oxidative stress and apoptosis are the major consequences of LmNcb5or deficiency in Leishmania.

The role of nitrite and free nitrous acid (FNA) in wastewater treatment plants

Nitrite is known to accumulate in wastewater treatment plants (WWTPs) under certain environmental conditions. The protonated form of nitrite, free nitrous acid (FNA), has been found to cause severe inhibition to numerous bioprocesses at WWTPs. However, this inhibitory effect of FNA may possibly be gainfully exploited, such as repressing nitrite oxidizing bacteria (NOB) growth to achieve N removal via the nitrite shortcut. However, the inhibition threshold of FNA to repress NOB (∼0.02 mg HNO2-N/L) may also inhibit other bioprocesses. This paper reviews the inhibitory effects of FNA on nitrifiers, denitrifiers, anammox bacteria, phosphorus accumulating organisms (PAO), methanogens, and other microorganisms in populations used in WWTPs. The possible inhibition mechanisms of FNA on microorganisms are discussed and compared. It is concluded that a single inhibition mechanism is not sufficient to explain the negative impacts of FNA on microbial metabolisms and that multiple inhibitory effects can be generated from FNA. The review would suggest further research is necessary before the FNA inhibition mechanisms can be more effectively used to optimize WWTP bioprocesses. Perspectives on research directions, how the outcomes may be used to manipulate bioprocesses and the overall implications of FNA on WWTPs are also discussed.

Synergism of ammonium and palmitic acid in uncoupling of electron transfer and ATP synthesis in chloroplasts

Uncoupling by ammonium of electron transfer and ATP synthesis during linear transfer of electrons from water to photosystem 1 acceptors was studied in pea chloroplasts. It was shown that 40 microM palmitic acid decreased several-fold the ammonium concentrations necessary for 50% inhibition of ATP synthesis. The protonophore carbonyl cyanide m-chlorophenylhydrazone has no such property. The enhancement by palmitate of ammonium-induced uncoupling is accompanied by acceleration of basal electron transfer and decrease in the photoinduced uptake of hydrogen ions (H+). In the absence of ammonium, palmitate has no effect on basal transport and stimulates uptake of hydrogen ions. This means that in the case of combined action of palmitate and ammonium an additional leakage of H+ takes place, resulting in dissipation of the pH gradient. Synergic action of two metabolites, free fatty acid and ammonium, is supposed to provide for functioning of a system of mild regulation of energy coupling processes in native plant cell chloroplasts. Possible mechanisms of synergism are discussed.

Acidic lipids, H(+)-ATPases, and mechanism of oxidative phosphorylation. Physico-chemical ideas 30 years after P. Mitchell's Nobel Prize award

Peter D. Mitchell, who was awarded the Nobel Prize in Chemistry 30 years ago, in 1978, formulated the chemiosmotic theory of oxidative phosphorylation. This review initially analyzes the major aspects of this theory, its unresolved problems, and its modifications. A new physico-chemical mechanism of energy transformation and coupling of oxidation and phosphorylation is then suggested based on recent concepts regarding proteins, including ATPases that work as molecular motors, and acidic lipids that act as hydrogen ion (H(+)) carriers. According to this proposed mechanism, the chemical energy of a redox substrate is transformed into nonequilibrium states of electron-transporting chain (ETC) coupling proteins. This leads to nonequilibrium pumping of H(+) into the membrane. An acidic lipid, cardiolipin, binds with this H(+) and carries it to the ATP-synthase along the membrane surface. This transport generates gradients of surface tension or electric field along the membrane surface. Hydrodynamic effects on a nanolevel lead to rotation of ATP-synthase and finally to the release of ATP into aqueous solution. This model also explains the generation of a transmembrane protonmotive force that is used for regulation of transmembrane transport, but is not necessary for the coupling of electron transport and ATP synthesis.

Profound effects of the general anesthetic etomidate on oxidative phosphorylation without effects on their yield

We investigated the effects of the general anesthetic Etomidate on oxidative phosphorylation in isolated rat liver mitochondria. The study of each electron transfer site shows that there is an inhibition: mainly at complex I but also, to a lesser extent, at complex III. Moreover, with succinate as substrate, the increase in non-phosphorylating respiration is accompanied by a decrease in DeltaPsi. However, this effect is not due to classical uncoupling of oxidative phosphorylation, since ADP addition at high Etomidate concentrations restores the transmembrane difference of electrical potential. Also, in the same range of Etomidate concentration, the ATP/O ratio is not significantly affected. In conclusion, the main effect of Etomidate is to decrease the oxidative phosphorylation rate without changing yield. The H(+) leak which appears under non-phosphorylating conditions becomes negligible in physiological conditions.

Alleviation of fatty acid and hypoxia-reoxygenation-induced proximal tubule deenergization by ADP/ATP carrier inhibition and glutamate

Kidney proximal tubules develop a severe but highly reversible energetic deficit due to nonesterified fatty acid (NEFA)-induced dissipation of mitochondrial membrane potential (DeltaPsi(m)) during reoxygenation after severe hypoxia. To assess the mechanism for this behavior, we have compared the efficacies of different NEFA for inducing mitochondrial deenergization in permeabilized tubules measured using safranin O uptake and studied the modification of NEFA-induced deenergization by inhibitors of the ADP/ATP carrier and glutamate using both normoxic tubules treated with exogenous NEFA and tubules deenergized during hypoxia-reoxygenation (H/R). Among the long-chain NEFA that accumulate during H/R of isolated tubules and ischemia-reperfusion of the kidney in vivo, oleate, linoleate, and arachidonate had strong effects to dissipate DeltaPsi(m) that were slightly greater than palmitate, while stearate was inactive at concentrations reached in the cells. This behavior correlates well with the protonophoric effects of each NEFA. Inhibition of the ADP/ATP carrier with either carboxyatractyloside or bongkrekic acid or addition of glutamate to compete for the aspartate/glutamate carrier improved DeltaPsi(m) in the presence of exogenous oleate and after H/R. Effects on the two carriers were additive and restored safranin O uptake to as much as 80% of normal under both conditions. The data strongly support NEFA cycling across the inner mitochondrial membrane using anion carriers as the main mechanism for NEFA-induced deenergization in this system and provide the first evidence for a contribution of this process to pathophysiological events that impact importantly on energetics of intact cells.

Biological nitrogen removal with nitrification and denitrification via nitrite pathway

Presently, the wastewater treatment practices can be significantly improved through the introduction of new microbial treatment technologies. To meet increasingly stringent discharge standards, new applications and control strategies for the sustainable removal of ammonium from wastewater have to be implemented. Partial nitrification to nitrite was reported to be technically feasible and economically favorable, especially when wastewater with high ammonium concentrations or low C/N ratios is treated. For successful implementation of the technology, the critical point is how to maintain partial nitrification of ammonium to nitrite. Partial nitrification can be obtained by selectively inhibiting nitrite oxidizing bacteria through appropriate regulation of the system's DO concentration, microbial SRT, pH, temperature, substrate concentration and load, operational and aeration pattern, and inhibitor. The review addressed the microbiology, its consequences for their application, the current status regarding application, and the future developments.

Impairment of glucose metabolism and energy transfer in the rat heart

The metabolic pathways involved in ATP production in hypertriglyceridemic rat hearts were evaluated. Hearts from male Wistar rats with sugar-induced hypertriglyceridemia were perfused in an isolated organ system. Mechanical performance, oxygen uptake and beat rate were evaluated under perfusion with different oxidizable substrates. Age- and weight-matched animals were used as control. The hypertriglyceridemic (HTG) hearts showed a decrease in the mechanical work and slight diminution in the oxygen uptake when perfused with glucose, pyruvate or lactate. No differences were found when perfused with palmitate, octanoate or beta-hydroxybutyrate. The glycolytic flux in HTG hearts was 2.4 times lower than in control hearts. Phosphofructokinase-I (PFK-I) was 16% decreased in HTG hearts, whereas pyruvate kinase activity did not change. The increased levels of glucose-6-phosphate in HTG heart, suggested a flux limitation by the PFK-I. Pyruvate dehydrogenase in its active form (PDHa) diminished as well. The PDHa level in the HTG hearts was restored to control values by dichloroacetate; however, this addition did not significantly improve the mechanical performance. Levels of ATP and phosphocreatine as well as total creatine kinase activity and the MB fraction were significant lower in the HTG hearts perfused with glucose. The data suggested that supply of ATP by glucose oxidation did not suffice to support cardiac work in the HTG hearts; this impairment was exacerbated by the diminution of the creatine kinase system output.

Changed energy state and increased mitochondrial beta-oxidation rate in liver of rats associated with lowered proton electrochemical potential and stimulated uncoupling protein 2 (UCP-2) expression: evidence for peroxisome proliferator-activated receptor-alpha independent induction of UCP-2 expression

Lowering of plasma triglyceride levels by hypolipidemic agents is caused by a shift in the liver cellular metabolism, which become poised toward peroxisome proliferator-activated receptor (PPAR) alpha-regulated fatty acid catabolism in mitochondria. After dietary treatment of rats with the hypolipidemic, modified fatty acid, tetradecylthioacetic acid (TTA), the energy state parameters of the liver were altered at the tissue, cell, and mitochondrial levels. Thus, the hepatic phosphate potential, energy charge, and respiratory control coefficients were lowered, whereas rates of oxygen uptake, oxidation of pyridine nucleotide redox pairs, beta-oxidation, and ketogenesis were elevated. Moderate uncoupling of mitochondria from TTA-treated rats was confirmed, as the proton electrochemical potential (Delta(p)) was 15% lower than controls. The change affected the Delta(Psi) component only, leaving the (Delta)pH component unaltered, suggesting that TTA causes induction of electrogenic ion transport rather than electrophoretic fatty acid activity. TTA treatment induced expression of hepatic uncoupling protein 2 (UCP-2) in rats as well as in wild type and PPARalpha-deficient mice, accompanied by a decreased double bond index of the mitochondrial membrane lipids. However, changes of mitochondrial fatty acid composition did not seem to be related to the effects on mitochondrial energy conductance. As TTA activates PPARdelta, we discuss how this subtype might compensate for deficiency of PPARalpha. The overall changes recorded were moderate, making it likely that liver metabolism can maintain its function within the confines of its physiological regulatory framework where challenged by a hypolipemic agent such as TTA, as well as others.

Intrinsic and extrinsic uncoupling of oxidative phosphorylation

This article reviews parameters of extrinsic uncoupling of oxidative phosphorylation (OxPhos) in mitochondria, based on induction of a proton leak across the inner membrane. The effects of classical uncouplers, fatty acids, uncoupling proteins (UCP1-UCP5) and thyroid hormones on the efficiency of OxPhos are described. Furthermore, the present knowledge on intrinsic uncoupling of cytochrome c oxidase (decrease of H(+)/e(-) stoichiometry=slip) is reviewed. Among the three proton pumps of the respiratory chain of mitochondria and bacteria, only cytochrome c oxidase is known to exhibit a slip of proton pumping. Intrinsic uncoupling was shown after chemical modification, by site-directed mutagenesis of the bacterial enzyme, at high membrane potential DeltaPsi, and in a tissue-specific manner to increase thermogenesis in heart and skeletal muscle by high ATP/ADP ratios, and in non-skeletal muscle tissues by palmitate. In addition, two mechanisms of respiratory control are described. The first occurs through the membrane potential DeltaPsi and maintains high DeltaPsi values (150-200 mV). The second occurs only in mitochondria, is suggested to keep DeltaPsi at low levels (100-150 mV) through the potential dependence of the ATP synthase and the allosteric ATP inhibition of cytochrome c oxidase at high ATP/ADP ratios, and is reversibly switched on by cAMP-dependent phosphorylation. Finally, the regulation of DeltaPsi and the production of reactive oxygen species (ROS) in mitochondria at high DeltaPsi values (150-200 mV) are discussed.

Heart metabolic disturbances in cardiovascular diseases

Myocardial function depends on adenosine triphosphate (ATP) supplied by oxidation of several substrates. In the adult heart, this energy is obtained primarily from fatty acid oxidation through oxidative phosphorylation. However, the energy source may change depending on several factors such as substrate availability, energy demands, oxygen supply, and metabolic condition of the individual. Surprisingly, the role of energy metabolism in development of cardiac diseases has not been extensively studied. For instance, alterations in glucose oxidation and transport developed in diabetic heart may compromise myocardial performance under conditions in which ATP provided by glycolysis is relevant, such as in ischemia and reperfusion. In some cardiac diseases such as ischemic cardiomyopathy, heart failure, hypertrophy, and dilated cardiomyopathy, ATP generation is diminished by derangement of fatty acid delivery to mitochondria and by alteration of certain key enzymes of energy metabolism. Shortage of some co-factors such as L-carnitine and creatine also leads to energy depletion. Creatine kinase system and other mitochondrial enzymes are also affected. Initial attempts to modulate cardiac energy metabolism by use of drugs or supplements as a therapeutic approach to heart disease are described.

Metabolic changes induced by cold stress in rat liver mitochondria

The mechanisms involved in the metabolic changes induced by cold stress in isolated rat liver mitochondria were studied. Respiration, ATP synthesis, and membrane potential as well as the contents of several metabolites were determined in liver mitochondria from cold-exposed rats. At different times of cold exposure, the force-flux relationships showed net variation in flux (enhanced respiration, diminished ATP synthesis) with no associated variation in force (H+ gradient); this suggested that decoupling rather than classical uncoupling was involved in the effects of cold stress. The flux control coefficient of the H+ leak on basal respiration was slightly increased by 380 h of cold exposure. Cold stress also induced a diminution in total membrane fatty acids, Zn2+, Fe3+, ATP, and ADP/O ratios; the content of cytochromes c + c1 and b oscillated. The contents of Ca2+, Na+, Pi, and cytochromes a + a3 were not affected, whereas matrix ADP, AMP, K+, and Mg2+ were markedly increased. Basal and oleic acid-stimulated respiration of mitochondria from cold-stressed rats was inhibited by GDP, carboxyatractyloside, or albumin. These agents did not affect basal respiration in control mitochondria. Western blot analysis showed enhanced expression of a protein of about 35 kDa, presumably the uncoupling protein 2, induced by long-term cold exposure. The overall data suggest that cold stress promoted decoupling of oxidative phosphorylation, and hence, changes in several matrix metabolites, by increasing free fatty acids and the UCP2 content.

Proton re-uptake partitioning between uncoupling protein and ATP synthase during benzohydroxamic acid-resistant state 3 respiration in tomato fruit mitochondria

The yield of oxidative phosphorylation in isolated tomato fruit mitochondria depleted of free fatty acids remains constant when respiratory rates are decreased by a factor of 3 by the addition of n-butyl malonate. This constancy makes the determination of the contribution of the linoleic acid-induced energy-dissipating pathway by the ADP/O method possible. No decrease in membrane potential is observed in state 3 respiration with increasing concentration of n-butyl malonate, indicating that the rate of ATP synthesis is steeply dependent on membrane potential. Linoleic acid decreases the yield of oxidative phosphorylation in a concentration-dependent manner by a pure protonophoric process like that in the presence of FCCP. ADP/O measurements allow calculation of the part of respiration leading to ATP synthesis and the part of respiration sustained by the dissipative H(+) re-uptake induced by linoleic acid. Respiration sustained by this energy-dissipating process remains constant at a given LA concentration until more than 50% inhibition of state 3 respiration by n-butyl malonate is achieved. The energy dissipative contribution to oxygen consumption is proposed to be equal to the protonophoric activity of plant uncoupling protein divided by the intrinsic H(+)/O of the cytochrome pathway. It increases with linoleic acid concentration, taking place at the expense of ADP phosphorylation without an increase in the respiration.

The mechanism of channel formation by alamethicin as viewed by molecular dynamics simulations

Alamethicin is a 20-residue channel-forming peptide that forms a stable amphipathic alpha-helix in membrane and membrane-mimetic environments. This helix contains a kink induced by a central Gly-X-X-Pro sequence motif. Alamethicin channels are activated by a cis positive transbilayer voltage. Channel activation is suggested to correspond to voltage-induced insertion of alamethicin helices in the bilayer. Alamethicin forms multi-conductance channels in lipid bilayers. These channels are formed by parallel bundles of transmembrane helices surrounding a central pore. A change in the number of helices per bundle switches the single channel conductance level. Molecular dynamics simulations of alamethicin in a number of different environments have been used to explore its channel-forming properties. These simulations include: (i) alamethicin in solution in water and in methanol; (ii) a single alamethicin helix at the surface of a phosphatidylcholine bilayer; (iii) single alamethicin helices spanning a phosphatidylcholine bilayer; and (iv) channels formed by bundles of 5, 6, 7 or 8 alamethicin helices spanning a phosphatidylcholine bilayer. The total simulation time is c. 30 ns. Thus, these simulations provide a set of dynamic snapshots of a possible mechanism of channel formation by this peptide.

Diphenylamine as an important structure of nonsteroidal anti-inflammatory drugs to uncouple mitochondrial oxidative phosphorylation

A marked difference has been observed in the inhibitory effects of nonsteroidal anti-inflammatory drugs (NSAIDs) on oxidative phosphorylation of rat liver mitochondria. It should be noted that some of the potent inhibitors, N-phenylanthranilic acids and diclofenac, have a similar "skeleton" structure, diphenylamine. Diphenylamine itself was found to inhibit oxidative phosphorylation significantly, although its inhibition potency was weaker than that of NSAIDs with a diphenylamine structure. In addition to decreases in the respiration control index (ratio of state 3 to state 4 respiration), these compounds released oligomycin-inhibited state 3 respiration. These results demonstrated that diphenylamine, as well as N-phenylanthranilic acids and diclofenac, was an uncoupler of oxidative phosphorylation of rat liver mitochondria. Thus, diphenylamine was suggested to play an important role in the uncoupling effects of NSAIDs with a diphenylamine skeleton.

Inhibition and uncoupling of oxidative phosphorylation by nonsteroidal anti-inflammatory drugs: study in mitochondria, submitochondrial particles, cells, and whole heart

The effects of the anti-inflammatory drugs diclofenac, piroxicam, indomethacin, naproxen, nabumetone, nimesulide, and meloxicam on mitochondrial respiration, ATP synthesis, and membrane potential were determined. Except for nabumetone and naproxen, the other drugs stimulated basal and uncoupled respiration, inhibited ATP synthesis, and collapsed membrane potential in mitochondria incubated in the presence of either glutamate + malate or succinate. Plots of membrane potential versus ATP synthesis (or respiration) showed proportional variations in both parameters, induced by different concentrations of nimesulide, meloxicam, piroxicam, or indomethacin, but not by diclofenac. The activity of the adenine nucleotide translocase was blocked by diclofenac and nimesulide; diclofenac also slightly inhibited mitochondrial ATPase activity. Naproxen did not affect any of the mitochondrial parameters measured. Nabumetone inhibited respiration, ATP synthesis, and membrane potential in the presence of glutamate + malate, but not with succinate. NADH oxidation in submitochondrial particles also was inhibited by nabumetone. Nabumetone inhibited O2 uptake in intact cells and in whole heart, whereas the other five drugs stimulated respiration. These observations revealed that in situ mitochondria are an accessible target. Except for diclofenac, a negative inotropic effect on cardiac contractility was induced by the drugs. The data indicated that nimesulide, meloxicam, piroxicam, and indomethacin behaved as mitochondrial uncouplers, whereas nabumetone exerted a specific inhibition of site 1 of the respiratory chain. Diclofenac was an uncoupler too, but it also affected the adenine nucleotide translocase and the H+-ATPase.

Quantitative assay by flow cytometry of the mitochondrial membrane potential in intact cells

Mitochondrial membrane potential, in situ, is an important indicator of mitochondrial function and dysfunction. Because of recent interest in the role of mitochondria in signaling, cell injury and cell death, there is a need for a convenient, sensitive and accurate method for the measurement of the mitochondrial membrane potential, Deltapsim, in situ, in a heterogeneous cell population. We have adapted a flow cytometry method for the quantitative measurement of DeltaPsim which utilizes the lipophilic, cationic, fluorescent probe 3,3'-dihexyloxacarbocyanine iodide (DiOC6(3)). We developed a new protocol in which cells are equilibrated with very low dye concentrations (<1 nM). Only under these condition, the cell fluorescence appears to be correlated with the magnitude of DeltaPsim, as evident from the sensitivity of the fluorescence to low concentrations of uncouplers, ionophores and inhibitors of the mitochondrial proton pumps. The magnitude of the plasma membrane potential, DeltaPsip, also affects cell fluorescence, and a procedure that corrects for this effect is outlined. This method offers a distinct advantage over existing methods for estimation of Deltapsim by flow cytometry.

Mitochondria uncoupling by a long chain fatty acyl analogue

Mitochondria uncoupling by fatty acids in vivo is still questionable, being confounded by their dual role as substrates for oxidation and as putative genuine uncouplers of oxidative phosphorylation. To dissociate between substrate and the uncoupling activity of fatty acids in oxidative phosphorylation, the uncoupling effect was studied here using a nonmetabolizable long chain fatty acyl analogue. beta,beta'-Methyl-substituted hexadecane alpha,omega-dioic acid (MEDICA 16) is reported here to induce in freshly isolated liver cells a saturable oligomycin-insensitive decrease in mitochondrial proton motive force with a concomitant increase in cellular respiration. Similarly, MEDICA 16 induced a saturable decrease in membrane potential, proton gradient, and proton motive force in isolated liver and heart mitochondria accompanied by an increase in mitochondrial respiration. Uncoupling by MEDICA 16 in isolated mitochondria was partially suppressed by added atractyloside. Hence, fatty acids may act as genuine uncouplers of cellular oxidative phosphorylation by interacting with specific mitochondrial proteins, including the adenine nucleotide translocase.

Potassium collapses the deltaP in yeast mitochondria while the rate of ATP synthesis is inhibited only partially: modulation by phosphate

Addition of increasing concentrations of K+ to yeast mitochondria in the presence of 0 to 400 microM phosphate and 200 microM Mg2+ led to uncoupled respiration and decreased protonmotive force (deltaP):at 0 K+ deltaP = 213 mV, negative inside, where deltapsi = 180 mV and deltapH = 33 mV, while at 20 mM K+ deltaP = 28 mV, where deltapsi = 16 mV and deltapH = 12 mV. In contrast, the synthesis of ATP resulted in smaller values for the Km and the Vmax in 400 microM Pi and increasing ADP: in 0 K+, Km = 18.6 microM and Vmax = 75.4 nmol (min x mg protein)-1, while in 20 mM K+, Km = 5.2 microM and Vmax = 46.0 nmol (min x mg protein)-1, i.e., when K+ depleted most of the deltaP, and at ADP concentrations below the Km, the rate of ATP synthesis was essentially the same as in the absence of K+. At saturating ADP, the rate of ATP synthesis in the presence of K+ was about 60% of the rate observed without K+. The synthesis of ATP by yeast mitochondria was inhibited by oligomycin or uncouplers. K+ had no effects on rat liver mitochondria. Adenylate kinase activity was much smaller in yeast mitochondria than in rat liver mitochondria and thus did not account for the synthesis of ATP observed in the presence of K+. The effects of K+ on the deltaP of yeast mitochondria were prevented by increasing concentrations of phosphate (1 to 4 mM). At 4 mM phosphate, the deltaP was always above 200 mV and the kinetics of ATP synthesis were as follows: 0 K+ Km = 10.0 microM and Vmax = 88.3 nmol (min x mg protein)-1. At 20 mM K+, Km = 7.4 microM and Vmax = 133 nmol (min x mg protein)-1.

The toxicity of substituted phenolic compounds to a detoxifying and an acetic acid bacterium

In the detoxifying bacterium Acinetobacter calcoaceticus 69-V and in the acetic acid bacterium Acetobacter methanolicus MB 58, glucose and xylose are oxidized, respectively, via PQQ-dependent membrane-bound dehydrogenases, which are linked to the respiratory chain in a manner enabling energy conservation via electron transport phosphorylation (ETP) in the cytoplasmic membrane. Neither the glucose and gluconic acid nor the xylose and xylonic acid are metabolized. Therefore, measurements of sugar oxidation-driven ATP syntheses ought not to be disturbed by ATP drainage caused by anabolic processes. Studying the effect of substituted phenolic compounds on these energization processes reveals that their toxicity increases with an increasing degree of chlorination and that A. calcoaceticus 69-V is more stable than A. methanolicus MB 58 against chlorinated phenols. On the other hand, A. methanolicus MB 58 is more stable against 2,4-dinitrophenol (2,4-DNP) and 2,4-dichlorophenoxyacetic acid (2,4-D), especially in the acidic pH range, in which the sensitivity of ATP synthesis to the uncouplers is higher than that of respiration. The toxicity caused by protonophoric activities ought to be barely detectable by respiratory and dehydrogenase tests. The luminescence system of Photobacterium phosphoreum tested in the luminescent bacteria test was much more sensitive. This test system should be used as a screening tool and the effects measured must be confirmed by toxicity tests evaluating the stability of bacteria themselves involved in processes of detoxification as well as the production of toxic metabolites, monitored with respect to their velocity and efficiency.

The effect of chloroform on mitochondrial energy transduction

The effect of chloroform on mitochondrial respiration with succinate was investigated by applying the method of Brand, Chien and Diolez [(1994) Biochem. J. 297, 27-29] to examine whether chloroform causes redox slip (fewer protons pumped per electron transferred) during mitochondrial electron transport. N,N,N',N'-Tetramethyl-p-phenylenediamine (TMPD), which lowers H+/O (the number of protons pumped to the external medium by the electron transport complexes per oxygen atom consumed) by altering the electron flow pathway, was investigated for comparison. Non-phosphorylating mitochondria that had been treated with 350 microM TMPD or 30 mM chloroform were titrated with malonate in the presence of submaximal concentrations of the uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP). Linear relations between CCCP-induced extra respiration and protonmotive force were obtained. These results showed that there was no measurable protonmotive force-dependent or rate-dependent slip in mitochondria treated with either TMPD or chloroform. However, both TMPD and chloroform seemed to decrease H+/O in a manner independent of protonmotive force and rate. The relationship between non-phosphorylating respiration and protonmotive force was simulated in mitochondria of which 25% of the total population were assumed to have been broken. The simulation showed that the apparent decrease in H+/O on the addition of TMPD or chloroform to mitochondria could be in principle accounted for by breakage. Assays of mitochondrial breakage (ATP hydrolysis in the presence of atractyloside and oxidation of exogenous NADH) showed that chloroform broke mitochondria but TMPD did not. We conclude that chloroform changes the measured H+/O as an artifact by causing mitochondrial breakage and does not cause measurable redox slip, whereas TMPD genuinely lowers H+/O.

Fatty acids as modulators of cytochrome c oxidase in proteoliposomes

The control of cytochrome c oxidase turnover in proteoliposomes by membrane potential (delta psi) and by pH gradient (delta pH) is probably kinetic in nature, and inhibition by valinomycin and stimulation by nigericin indicate that delta pH exerts a greater influence than does an equivalent delta psi. Oleic acid at 100 microM removes all delta psi and delta pH control, whereas a similar concentration of palmitic acid increases turnover but does not completely abolish control. Valinomycin acts synergistically with both fatty acids, indicating that the latter can act as H+/K+ exchangers, but neither fatty acid alone markedly affects delta pH, showing that they cannot fully mimic nigericin. Oleate, but not palmitate, diminishes delta psi, and can move electrophoretically as oleate anion. Submicromolar palmitic acid concentrations partly stimulate turnover in delta psi- and delta pH-controlled proteoliposomes, as reported by Labonia, Muller and Azzi [(1988) Biochem. J. 254, 130-145], which might represent a direct effect on cytochrome c oxidase. The ubiquity of fatty acids in biological membranes suggests that these substances might be responsible for limiting respiratory control and enzyme activity in vivo.

Mechanisms of inhibition and uncoupling of respiration in isolated rat liver mitochondria by the general anesthetic 2,6-diisopropylphenol

We investigated the effects of 2,6-diisopropylphenol on oxidative phosphorylation of isolated rat liver mitochondria. Diisopropylphenol strongly inhibits state-3 and uncoupled respiratory rates, when glutamate and malate are the substrates, as a direct consequence of the limitation of electron transfer at the level of complex I. In addition, diisopropylphenol acts as an uncoupler in non-phosphorylating mitochondria, which leads to an increase in respiratory rate and a large decrease in proton-motive force. However, such effects cannot be due to the classical protonophoric property of this drug, since addition of ADP plus oligomycin before diisopropylphenol avoids this increase in proton permeability, and in phosphorylating mitochondria, the ATP/O ratio is not significantly affected by diisopropylphenol addition. In the absence of added ADP, diisopropylphenol modifies some mitochondrial ATPases in such a way that they become insensitive to oligomycin and unable to couple proton movement to ATP synthesis or hydrolysis. However, these modified enzymes can catalyse passive proton permeability, which leads to uncoupling. Addition of ADP before diisopropylphenol prevents these changes. We propose that ADP induces a change in conformation of ATPase, which leads to insensitivity of this complex towards diisopropylphenol. In conclusion, we show that diisopropylphenol has two main effects on rat liver mitochondria: inhibition of the respiratory chain at the level of complex I level and modification of ATPase such that, in the absence of phosphorylation, it catalyses a H+ leak, which becomes negligible when oxidative phosphorylation is functional.

The coupling of the relative movement of the a and c subunits of the F0 to the conformational changes in the F1-ATPase

F0F1-ATPase structural information gained from X-ray crystallography and electron microscopy has activated interest in a rotational mechanism for the F0F1-ATPase. Because of the subunit stoichiometry and the involvement of both a- and c-subunits in the mechanism of proton movement, it is argued that relative movement must occur between the subunits. Various options for the arrangement and structure of the subunits involved are discussed and a mechanism proposed.

Photomodification of mitochondrial proteins by azido fatty acids and its effect on mitochondrial energetics. Further evidence for the role of the ADP/ATP carrier in fatty-acid-mediated uncoupling

Azido derivatives of long-chain fatty acids, 12-(4-azido-2-nitrophenylamino)dodecanoic acid (N3-NpNH-Lau) and 16-(4-azido-2-nitrophenylamino)hexadecanoic acid (N3-NpNH-Pam), were used to study the mechanism of the protonophoric function of long-chain fatty acids in mitochondrial membranes. N3-NpNH-Lau was found to increase resting-state respiration and decrease the membrane potential in a dose-dependent way in a manner similar to that of the natural fatty acid, myristate. Both effects of N3-NpNH-Lau as well as of the myristate were reversed or prevented by the inhibitor of the mitochondrial ADP/ATP carrier (AAC), carboxyatractyloside. This protective effect of carboxyatractyloside was well expressed in rat heart mitochondria and less so in mitochondria within digitonin-permeabilized Ehrlich ascites tumour cells. Photomodification of Ehrlich ascites tumour mitochondria by ultraviolet irradiation in the presence of N3-NpNH-Lau made them more resistant to the uncoupling effect of myristate, and photomodification of rat heart mitochondria resulted in a strong inhibition of AAC which could not be reversed by serum albumin. Photolabelling of rat heart mitochondria with tritiated N3-NpNH-Pam revealed around 10 labelled bands on SDS/polyacrylamide gel electrophoresis. Based on immunodetection with a specific antibody, one of them, corresponding to 30 kDa, was identified as AAC. Specific interaction of AAC with azido fatty acids was confirmed by a high radiolabelling of this band. The role of fatty acids in fine control of the efficiency of oxidative phosphorylation is discussed.

The effect of the lipid peroxidation product 4-hydroxynonenal and of its metabolite 4-hydroxynonenoic acid on respiration of rat kidney cortex mitochondria

In rat kidney cortex mitochondria, 4-hydroxynonenal inhibits state 3 respiration as well as uncoupled respiration at micromolar concentrations. The inhibition is more distinct for NAD-linked than for FAD-linked respiration. 4-Hydroxynonenal increases the state 4 respiration. It is assumed that 4-hydroxynonenal behaves like a decoupling agent. 4-Hydroxynonenal augments the inhibitory effect of 2,4-dinitrophenol observed at superoptimal concentrations. 4-Hydroxynonenal is metabolised by renal mitochondria, and 4-hydroxynonenoic acid is one of the metabolites generated. This metabolite is without effect on respiration at concentrations up to 50 microM. Therefore, the effect of 4-hydroxynonenal on respiration is not mediated by this fatty acid derivative formed during respiratory measurements.

The nature of uncoupling by n-hexane, 1-hexanethiol and 1-hexanol in rat liver mitochondria

We have analyzed the effects of n-hexane, 1-hexanethiol, and 1-hexanol on the coupled respiration of rat liver mitochondria. Incubation of mitochondria with n-hexane, 1-hexanethiol and 1-hexanol resulted in a stimulation, at low concentrations, and an inhibition, at high concentrations, of the state 4 mitochondrial respiration. Three criteria, all based on the comparison with the effect of DNP, have been used to establish whether the stimulation of respiration, at low concentrations of n-hexane, 1-hexanethiol, and 1-hexanol, depends on protonophoric mechanisms. First, the quantitative relationship between the extents of respiratory stimulation and membrane potential depression: a strong decrease of membrane potential was induced by increasing concentrations of DNP and a negligible depression by increasing concentrations of n-hexane or 1-hexanethiol. Only a slight decrease was induced by 1-hexanol. Second, the quantitative relationship between the extents of respiratory stimulation and of proton conductance increase: at equivalent rates of respiration, the enhancement of the proton conductance induced by DNP was very marked, by n-hexane and 1-hexanethiol practically negligible, and by 1-hexanol much smaller than that induced by DNP. Third, in titrations with redox inhibitors of the proton pumps, the pattern of the relationship between proton pump conductance and membrane potential was markedly different from protonophoric and non-protonophoric uncouplers: almost linear in the case of DNP, highly non-linear in the case of n-hexane, 1-hexanethiol and 1-hexanol. These three criteria support the view that n-hexane, 1-hexanethiol, and partially 1-hexanol, uncouple mitochondrial respiration by a non-protonophoric mechanism.

Photoaffinity labelling of the mitochondrial uncoupling protein by [3H]azido fatty acid affects the anion channel

Brown adipose tissue (BAT) mitochondria were incubated with the azido derivative of fatty acid (hexadecanoic) containing four tritium atoms, [3H]AzHA, and among all mitochondrial proteins only a few proteins were photolabelled after irradiation with UV. It suggests the existence of specific fatty acid binding sites on mitochondrial proteins. It was also possible to label with [3H]AzHA the isolated uncoupling protein (UcP) of BAT mitochondria with a low stoichiometry--lower than one AzHA per dimeric UcP. These results together with the observed competition (i.e. prevention of photolabelling) of various UcP anionic substrates with [3H]AzHA and its dodecanoic acid analogue, suggest the existence of the specific fatty acid binding site on UcP identical with the anion channel or anion translocating site.

The essential arginine residue at position 210 in the alpha subunit of the Escherichia coli ATP synthase can be transferred to position 252 with partial retention of activity

The substitution of arginine at position 210 in the alpha subunit of Escherichia coli F0F1-ATPase by either lysine or alanine causes dominance in complementation tests with a chromosomal c subunit mutation. Reversal of dominance was achieved for the alpha R210K mutation but not for the alpha R210A mutation by the presence of an aspartic acid residue at position 50 or at position 252 in the alpha subunit. It was concluded that position 210 in putative helix 4 of a previously proposed model of the alpha subunit is close to position 252 in putative helix 5 and to position 50 in putative helix 1. The juxtaposition of residues 252 and 210 was also indicated by the observation that the double mutant alpha R210Q/Q252R was partially functional. A revertant of the partially functional double mutant, isolated on succinate medium, was found to contain a third mutation resulting in Pro-204 in the alpha subunit being replaced by threonine. That the revertant phenotype was due to the alpha P204T change was confirmed by site-directed mutagenesis. ATP synthesis in the revertant strain was at near normal levels as judged by growth yield experiments, but the revertant strain was unable to pump protons in response to ATP hydrolysis.

Regulation of energy metabolism in liver

Energy metabolism in liver has to cope with the special tasks of this organ in intermediary metabolism. Main ATP-generating processes in the liver cell are the respiratory chain and glycolysis, whereas main ATP-consuming processes are gluconeogenesis, urea synthesis, protein synthesis, ATPases and mitochondrial proton leak. Mitochondrial respiratory chain in the intact liver cell is subject to control mainly by substrate (hydrogen donors, ADP, oxygen) transport and supply and proton leak/slip. Whereas hormonal control is mainly on substrate supply to mitochondria, proton leak/slip is supposed to play an important role in the modulation of the efficiency of oxidative phosphorylation.

The nature of mitochondrial respiration and discrimination between membrane and pump properties

A new criterion is utilized for the interpretation of flow-force relationships in rat liver mitochondria. The criterion is based on the view that the nature of the relationship between the H+/O ratio and the membrane potential can be inferred from the relationship between ohmic-uncoupler-induced extra respiration and the membrane potential. Thus a linear relationship between extra respiration and membrane potential indicates unequivocally the independence of the H+/O ratio from the membrane potential and the leak nature of the resting respiration [Brand, Chien, and Diolez (1994) Biochem. J. 297, 27-29]. On the other hand, a non-linear relationship indicates that the H+/O ratio is dependent on the membrane potential. The experimental assessment of this relationship in the presence of an additional ohmic leak, however, is rendered difficult by both the uncoupler-induced depression of membrane potential and the limited range of dependence of the H+/O ratio on the membrane potential. We have selected conditions, i.e. incubation of mitochondria at low temperatures, where the extent of non-linearity is markedly increased. It appears that the nature of the resting respiration of mitochondria in vitro is markedly dependent on the temperature: at low temperatures the percentage of resting respiration due to membrane leak decreases and that due to intrinsic uncoupling of the proton pumps increases.

Inhibitory effect of Mg2+ on the protonophoric activity of palmitic acid

To discriminate whether fatty acids are uncouplers that cause acceleration of State-4 respiration, associated with a decrease in the protonmotive force, or decouplers that increase respiration without associated decrease in the protonmotive force, we examined the effect of palmitate on functions of rat-liver mitochondria under various conditions. We found that palmitate itself increases State-4 respiration, releases oligomycin-inhibited State-3 respiration, inhibits ATP synthesis and ATP<->Pi exchange reaction, and increases H+ permeability in mitochondrial and model bilayer phospholipid membranes. Thus, palmitate is a classical uncoupler of oxidative phosphorylation. However, these effects were inhibited by Mg2+, due to rapid formation of a stable complex between palmitate and Mg2+.

Alkaliphiles: 'basic' molecular problems of pH tolerance and bioenergetics

Alkaliphilic Bacillus species provide experimental opportunities for examination of physiological processes under conditions in which the stress of the extreme environment brings issues of general biological importance into special focus. The alkaliphile, like many other cells, uses Na+/H+ antiporters in pH regulation, but its array of these porters, and other ion-flux pathways that energize and support their activity, result in an extraordinary capacity for pH homeostasis; this process nonetheless becomes the factor that limits growth at the upper edge of the pH range. Above pH 9.5, aerobic alkaliphiles maintain a cytoplasmic pH that is two or more units below the external pH. This chemiosmotically adverse delta pH is bypassed by use of an electrochemical gradient of Na+ rather than of protons to energize solute uptake and motility. By contrast, ATP synthesis occurs via completely proton-coupled oxidative phosphorylation that proceeds just as well, or better, at pH 10 and above as it does in the same bacteria growing at lower pH, without the adverse pH gradient. Various mechanisms that might explain this conundrum are described, and the current state of the evidence supporting them is summarized.

Bacterial resistance to uncouplers

Uncoupler resistance presents a potential challenge to the conventional chemiosmotic coupling mechanism. In E. coli, an adaptive response to uncouplers was found in cell growing under conditions requiring oxidative phosphorylation. It is suggested that uncoupler-resistant mutants described in the earlier literature might represent a constitutive state of expression of this "low energy shock" adaptive response. In the environment, bacteria are confronted by nonclassical uncoupling factors such as organic solvents, heat, and extremes of pH. It is suggested that the low energy shock response will aid the cell in coping with the effects of natural uncoupling factors. The genetic analysis of uncoupler resistance has only recently began, and is yielding interesting and largely unexpected results. In Bacillus subtilis, a mutation in fatty acid desaturase causes an increased content of saturated fatty acids in the membrane and increased uncoupler resistance. The protonophoric efficiency of uncouplers remains unchanged in the mutants, inviting nonorthodox interpretations of the mechanism of resistance. In E. coli, two loci conferring resistance to CCCP and TSA were cloned and were found to encode multidrug resistance pumps. Resistance to one of the uncouplers, TTFB, remained unchanged in strains mutated for the MDRs, suggesting a resistance mechanism different from uncoupler extrusion.

Nitric oxide reductase from Pseudomonas stutzeri, a novel cytochrome bc complex. Phospholipid requirement, electron paramagnetic resonance and redox properties

The nitric oxide reductase (NOR) from Pseudomonas stutzeri is a cytochrome bc complex which shows on SDS/PAGE two subunits with apparent molecular masses of 17 kDa and 38 kDa. Two other species of approximately 45 kDa and 74-78 kDa represent the undissociated enzyme complex and an aggregate of the cytochrome b subunit, respectively. The cytochrome b subunit is highly hydrophobic and results in aberrant electrophoretic mobility. The stability of the enzyme in various detergents and at different pH was investigated. The highest specific activity of 60 mumol NO min-1 mg-1 protein was obtained after electrophoresis in the presence of laurylpropanediol-3-phosphorylcholine ether. Purified NOR contained cardiolipin, phosphatidylglycerol, and phosphatidylethanolamine, the latter as the major component. A phospholipid was required for high catalytic activity with either cardiolipin or phosphatidylglycerol increasing the activity of the enzyme as isolated by a factor of up to 5. Free fatty acids inhibited NOR, with cis-9-octadecenoic acid (oleic acid) showing the most pronounced effect. Certain detergents substituted for the phospholipid requirement of NOR. The enzyme, as isolated, in 0.1% Triton X-100, 20 mM Tris/HCl pH 8.5, exhibited a complex set of EPR resonances at low magnetic field, with a prominent peak at g 6.34 resulting from Fe(III) high-spin cytochrome b. The second prominent feature arose from a low-spin Fe(III) heme center with strong lines at apparent g values of 3.02 and 2.29, and a broad resonance at g approximately 1.5 which we assigned to the cytochrome c component of the enzyme. From spin quantitation and computer simulations of the various EPR signals a ratio close to 1:1 for the low-spin/high-spin heme centers in NOR was estimated. Shifting the pH from 8.5 to 5.0, replacing Triton X-100 by other detergents, or adding soybean phospholipids to the protein, led to pronounced changes of the EPR signals in the g = 6 region. In contrast, the strong inhibitor oleic acid did not cause significant spectral changes. NOR which had been reduced by L-ascorbate/phenazine methosulfate prior to incubation with its substrate NO gave the characteristic Fe(II) nitrosyl triplet centered at g approximately 2.01, with a hyperfine splitting of 1.70 mT. In the absence of dioxygen, NOR was quantitatively reduced by either sodium dithionite, or photochemically with deazaflavin and oxalate; the enzyme was reoxidizable by ferricyanide in a fully reversible reaction. Spectroelectrochemical oxidoreductive titrations gave E'o (versus standard hydrogen electrode) = +322 mV for the cytochrome b and +280 mV for the cytochrome c component.

Effect of metallothionein I on mitochondrial oxygen consumption

The effect of the stress-induced, cysteine-rich protein, metallothionein I (MT), on oxygen consumption by rat liver mitochondria was studied. Using a Clark-type oxygen electrode we found that electron transport from succinate to oxygen was enhanced by MT whereas ADP-initiated oxygen consumption was inhibited by MT. The MT effect was concentration dependent. No evidence was found for the provision by MT of an alternate pathway to oxygen.

A threshold membrane potential accounts for controversial effects of fatty acids on mitochondrial oxidative phosphorylation

The uncoupling effect of free fatty acids on oxidative phosphorylation in mitochondria has been known for more than 35 years. The mechanism of action, however, remains controversial. In this report the physicochemical basis of uncoupling was elucidated by studying the effect of free fatty acids on the proton permeability and membrane potential of proteoliposomes containing reconstituted cytochrome c oxidase (COX). A threshold membrane potential of about 125 mV was identified for fatty acid-induced proton permeability. Only above this potential do free fatty acids translocate protons across the biological membrane. The data explain the controversial effects of long-chain fatty acids on oxidative phosphorylation as well as their role on non-shivering thermogenesis in larger mammals.