The phosphorus/oxygen ratio of mitochondrial oxidative phosphorylation

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.

Combined changes in temperature and pH mimicking exercise result in decreased efficiency in muscle mitochondria

It is well known that exercise efficiency declines at intensities above the lactate threshold, yet the underlying mechanisms are poorly understood. Some have suggested it is due to a decline in mitochondrial efficiency, but this is difficult to examine in vivo. Therefore, the aim of the current study was to examine how changes in temperature and pH, mimicking those that occur during exercise, affect mitochondrial efficiency in skeletal muscle mitochondria. This study was performed on quadriceps muscle of 20 wild-type mice. Muscle tissue was dissected and either permeabilized (n = 10) or homogenized for isolation of mitochondria (n = 10), and oxidative phosphorylation capacity and P/O ratio were assessed using high-resolution respirometry. Samples from each muscle were analyzed in both normal physiological conditions (37°C, pH 7.4), decreased pH (6.8), increased temperature (40°C), and a combination of both. The combination of increased temperature and decreased pH resulted in a significantly lower P/O ratio, mirrored by an increase in leak respiration and a decrease in respiratory control ratio (RCR), in isolated mitochondria. In permeabilized fibers, RCR and leak were relatively unaffected, though a main effect of temperature was observed. Oxidative phosphorylation capacity was unaffected by changes in pH and temperature in both isolated mitochondria and permeabilized fibers. These results indicate that exercise-like changes in temperature and pH lead to impaired mitochondrial efficiency. These findings offer some degree of support to the concept of decreased mitochondrial efficiency during exercise, and may have implications for the assessment of mitochondrial function related to exercise.NEW & NOTEWORTHY To the best of our knowledge, this is the first study to examine the effects of combined changes in temperature and pH, mimicking intramuscular alterations during exercise. Our findings suggest that mitochondrial efficiency is impaired during exercise of moderate to high intensity, which could be a possible mechanism contributing to the decline in exercise efficiency at intensities above the lactate threshold.

ATP yield of plant respiration: potential, actual and unknown

BACKGROUND AND AIMS

The ATP yield of plant respiration (ATP/hexose unit respired) quantitatively links active heterotrophic processes with substrate consumption. Despite its importance, plant respiratory ATP yield is uncertain. The aim here was to integrate current knowledge of cellular mechanisms with inferences required to fill knowledge gaps to generate a contemporary estimate of respiratory ATP yield and identify important unknowns.

METHOD

A numerical balance sheet model combining respiratory carbon metabolism and electron transport pathways with uses of the resulting transmembrane electrochemical proton gradient was created and parameterized for healthy, non-photosynthesizing plant cells catabolizing sucrose or starch to produce cytosolic ATP.

KEY RESULTS

Mechanistically, the number of c subunits in the mitochondrial ATP synthase Fo sector c-ring, which is unquantified in plants, affects ATP yield. A value of 10 was (justifiably) used in the model, in which case respiration of sucrose potentially yields about 27.5 ATP/hexose (0.5 ATP/hexose more from starch). Actual ATP yield often will be smaller than its potential due to bypasses of energy-conserving reactions in the respiratory chain, even in unstressed plants. Notably, all else being optimal, if 25 % of respiratory O2 uptake is via the alternative oxidase - a typically observed fraction - ATP yield falls 15 % below its potential.

CONCLUSIONS

Plant respiratory ATP yield is smaller than often assumed (certainly less than older textbook values of 36-38 ATP/hexose) leading to underestimation of active-process substrate requirements. This hinders understanding of ecological/evolutionary trade-offs between competing active processes and assessments of crop growth gains possible through bioengineering of processes that consume ATP. Determining the plant mitochondrial ATP synthase c-ring size, the degree of any minimally required (useful) bypasses of energy-conserving reactions in the respiratory chain, and the magnitude of any 'leaks' in the inner mitochondrial membrane are key research needs.

Substrate-dependent differential regulation of mitochondrial bioenergetics in the heart and kidney cortex and outer medulla

The kinetics and efficiency of mitochondrial oxidative phosphorylation (OxPhos) can depend on the choice of respiratory substrates. Furthermore, potential differences in this substrate dependency among different tissues are not well-understood. Here, we determined the effects of different substrates on the kinetics and efficiency of OxPhos in isolated mitochondria from the heart and kidney cortex and outer medulla (OM) of Sprague-Dawley rats. The substrates were pyruvate+malate, glutamate+malate, palmitoyl-carnitine+malate, alpha-ketoglutarate+malate, and succinate±rotenone at saturating concentrations. The kinetics of OxPhos were interrogated by measuring mitochondrial bioenergetics under different ADP perturbations. Results show that the kinetics and efficiency of OxPhos are highly dependent on the substrates used, and this dependency is distinctly different between heart and kidney. Heart mitochondria showed higher respiratory rates and OxPhos efficiencies for all substrates in comparison to kidney mitochondria. Cortex mitochondria respiratory rates were higher than OM mitochondria, but OM mitochondria OxPhos efficiencies were higher than cortex mitochondria. State 3 respiration was low in heart mitochondria with succinate but increased significantly in the presence of rotenone, unlike kidney mitochondria. Similar differences were observed in mitochondrial membrane potential. Differences in H2O2 emission in the presence of succinate±rotenone were observed in heart mitochondria and to a lesser extent in OM mitochondria, but not in cortex mitochondria. Bioenergetics and H2O2 emission data with succinate±rotenone indicate that oxaloacetate accumulation and reverse electron transfer may play a more prominent regulatory role in heart mitochondria than kidney mitochondria. These studies provide novel quantitative data demonstrating that the choice of respiratory substrates affects mitochondrial responses in a tissue-specific manner.

Substrate- and Calcium-Dependent Differential Regulation of Mitochondrial Oxidative Phosphorylation and Energy Production in the Heart and Kidney

Mitochondrial dehydrogenases are differentially stimulated by Ca2+. Ca2+ has also diverse regulatory effects on mitochondrial transporters and other enzymes. However, the consequences of these regulatory effects on mitochondrial oxidative phosphorylation (OxPhos) and ATP production, and the dependencies of these consequences on respiratory substrates, have not been investigated between the kidney and heart despite the fact that kidney energy requirements are second only to those of the heart. Our objective was, therefore, to elucidate these relationships in isolated mitochondria from the kidney outer medulla (OM) and heart. ADP-induced mitochondrial respiration was measured at different CaCl2 concentrations in the presence of various respiratory substrates, including pyruvate + malate (PM), glutamate + malate (GM), alpha-ketoglutarate + malate (AM), palmitoyl-carnitine + malate (PCM), and succinate + rotenone (SUC + ROT). The results showed that, in both heart and OM mitochondria, and for most complex I substrates, Ca2+ effects are biphasic: small increases in Ca2+ concentration stimulated, while large increases inhibited mitochondrial respiration. Furthermore, significant differences in substrate- and Ca2+-dependent O2 utilization towards ATP production between heart and OM mitochondria were observed. With PM and PCM substrates, Ca2+ showed more prominent stimulatory effects in OM than in heart mitochondria, while with GM and AM substrates, Ca2+ had similar biphasic regulatory effects in both OM and heart mitochondria. In contrast, with complex II substrate SUC + ROT, only inhibitory effects on mitochondrial respiration was observed in both the heart and the OM. We conclude that the regulatory effects of Ca2+ on mitochondrial OxPhos and ATP synthesis are biphasic, substrate-dependent, and tissue-specific.

Metabolic implications of non-electrogenic ATP/ADP exchange in cancer cells: A mechanistic basis for the Warburg effect

In post-mitotic cells, mitochondrial ATP/ADP exchange occurs by the adenine nucleotide translocator (ANT). Driven by membrane potential (ΔΨ), ANT catalyzes electrogenic exchange of ATP4- for ADP3-, leading to higher ATP/ADP ratios in the cytosol than mitochondria. In cancer cells, ATP/ADP exchange occurs not by ANT but likely via the non-electrogenic ATP-Mg/phosphate carrier. Consequences of non-electrogenic exchange are: 1) Cytosolic ATP/ADP decreases to stimulate aerobic glycolysis. 2) Without proton utilization for exchange, ATP/O increases by 35% for complete glucose oxidation. 3) Decreased cytosolic ATP/ADPPi increases NAD(P)H/NAD(P)+. Increased NADH increases lactate/pyruvate, and increased NADPH promotes anabolic metabolism. Fourth, increased mitochondrial NADH/NAD+ magnifies the redox span across Complexes I and III, which increases ΔΨ, reactive oxygen species generation, and susceptibility to ferroptosis. 5) Increased mitochondrial NADPH/NADP+ favors a reverse isocitrate dehydrogenase-2 reaction with citrate accumulation and export for biomass formation. Consequently, 2-oxoglutarate formation occurs largely via oxidation of glutamine, the preferred respiratory substrate of cancer cells. Overall, non-electrogenic ATP/ADP exchange promotes aerobic glycolysis (Warburg effect) and confers specific growth advantages to cancer cells.

Adenylate kinase derived ATP shapes respiration and calcium storage of isolated mitochondria

The ratio of ADP and ATP is a natural indicator of cellular bioenergetic state and thus a prominent analyte in metabolism research. Beyond adenylate interconversion via oxidative phosphorylation and ATPase activities, ADP and ATP act as steric regulators of enzymes, e.g. cytochrome C oxidase, and are major factors in mitochondrial calcium storage potential. Consideration of all routes of adenylate conversion is critical to successfully predict their abundance in an experimental system and to correctly interpret many aspects of mitochondrial function. We showcase here how adenylate kinases elicit considerable impact on the outcome of a variety of mitochondrial assays through their drastic manipulation of the adenylate profile. Parameters affected include cytochrome c oxidase activity, P/O ratio, and mitochondrial calcium dynamics. Study of the latter revealed that the presence of ATP is required for mitochondrial calcium to be shaped into a particularly dense form of mitochondrial amorphous calcium phosphate.

Distinct differences in rates of oxygen consumption and ATP synthesis of regionally isolated non-synaptic mouse brain mitochondria

Brain mitochondrial dysfunction has been implicated in several neurodegenerative diseases. The distribution and efficiency of mitochondria display large heterogeneity throughout the regions of the brain. This may imply that the selective regional susceptibility of neurodegenerative diseases could be mediated through inherent differences in regional mitochondrial function. To investigate regional cerebral mitochondrial energetics, the rates of oxygen consumption and adenosine-5'-triphosphate (ATP) synthesis were assessed in isolated non-synaptic mitochondria of the cerebral cortex, hippocampus, and striatum of the male mouse brain. Oxygen consumption rates were assessed using a Seahorse XFe96 analyzer and ATP synthesis rates were determined by an online luciferin-luciferase coupled luminescence assay. Complex I- and complex II-driven respiration and ATP synthesis, were investigated by applying pyruvate in combination with malate, or succinate, as respiratory substrates, respectively. Hippocampal mitochondria exhibited the lowest basal and adenosine-5'-diphosphate (ADP)-stimulated rate of oxygen consumption when provided pyruvate and malate. However, hippocampal mitochondria also exhibited an increased proton leak and an elevated relative rate of oxygen consumption in response to the uncoupler carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP), showing a large capacity for uncoupled respiration in the presence of pyruvate. When the complex II-linked substrate succinate was provided, striatal mitochondria exhibited the highest respiration and ATP synthesis rate, whereas hippocampal mitochondria had the lowest. However, the mitochondrial efficiency, determined as ATP produced/O₂ consumed, was similar between the three regions. This study reveals inherent differences in regional mitochondrial energetics and may serve as a tool for further investigations of regional mitochondrial function in relation to neurodegenerative diseases.

Muscle Free Fatty-Acid Uptake Associates to Mechanical Efficiency During Exercise in Humans

Intrinsic factors related to muscle metabolism may explain the differences in mechanical efficiency (ME) during exercise. Therefore, this study aimed to investigate the relationship between muscle metabolism and ME. Totally 17 healthy recreationally active male participants were recruited and divided into efficient (EF; n = 8) and inefficient (IE; n = 9) groups, which were matched for age (mean ± SD 24 ± 2 vs. 23 ± 2 years), BMI (23 ± 1 vs. 23 ± 2 kg m-2), physical activity levels (3.4 ± 1.0 vs. 4.1 ± 1.0 sessions/week), and V ˙ O2peak (53 ± 3 vs. 52 ± 3 mL kg-1 min-1), respectively, but differed for ME at 45% of V ˙ O2peak intensity during submaximal bicycle ergometer test (EF 20.5 ± 3.5 vs. IE 15.4 ± 0.8%, P < 0.001). Using positron emission tomography, muscle blood flow (BF) and uptakes of oxygen (m V ˙ O2), fatty acids (FAU) and glucose (GU) were measured during dynamic submaximal knee-extension exercise. Workload-normalized BF (EF 35 ± 14 vs. IE 34 ± 11 mL 100 g-1 min-1, P = 0.896), m V ˙ O2 (EF 4.1 ± 1.2 vs. IE 3.9 ± 1.2 mL 100 g-1 min-1, P = 0.808), and GU (EF 3.1 ± 1.8 vs. IE 2.6 ± 2.3 μmol 100 g-1 min-1, P = 0.641) as well as the delivery of oxygen, glucose, and FAU, as well as respiratory quotient were not different between the groups. However, FAU was significantly higher in EF than IE (3.1 ± 1.7 vs. 1.7 ± 0.6 μmol 100 g-1 min-1, P = 0.047) and it also correlated with ME (r = 0.56, P = 0.024) in the entire study group. EF group also demonstrated higher use of plasma FAU than IE, but no differences in use of plasma glucose and intramuscular energy sources were observed between the groups. These findings suggest that the effective use of plasma FAU is an important determinant of ME during exercise.

Contribution of mitochondria to injury of hepatocytes and liver tissue by hyperthermia

OBJECTIVE

The aim of this study was to investigate functional changes of liver mitochondria within the experimentally modeled transition zone of radiofrequency ablation and to estimate possible contribution of these changes to the energy status of liver cells and the whole tissue.

MATERIALS AND METHODS

Experiments were carried out on mitochondria isolated from the perfused liver and isolated hepatocytes of male Wistar rats. Hyperthermia was induced by changing the temperature of perfusion medium in the range characteristic for the transition zone (38-52°C). After 15-min perfusion, mitochondria were isolated to investigate changes in the respiration rates and the membrane potential. Adenine nucleotides extracted from isolated hepatocytes and perfused liver subjected to hyperthermic treatment were analyzed by HPLC.

RESULTS

Hyperthermic liver perfusion at 42-52°C progressively impaired oxidative phosphorylation in isolated mitochondria. Significant inhibition of the respiratory chain components was observed after perfusion at 42°C, irreversible uncoupling became evident after liver perfusion at higher temperatures (46°C and above). After perfusion at 50-52°C energy supplying function of mitochondria was entirely compromised, and mitochondria turned to energy consumers. Hyperthermia-induced changes in mitochondrial function correlated well with changes in the energy status and viability of isolated hepatocytes, but not with the changes in the energy status of the whole liver tissue.

CONCLUSIONS

In this study the pattern of the adverse changes in mitochondrial functions that are progressing with increase in liver perfusion temperature was established. Results of experiments on isolated mitochondria and isolated hepatocytes indicate that hyperthermic treatment significantly and irreversibly inhibits energy-supplying function of mitochondria under conditions similar to those existing in the radiofrequency ablation transition zone and these changes can lead to death of hepatocytes. However, it was not possible to estimate contribution of mitochondrial injury to liver tissue energy status by estimating only hyperthermia-induced changes in adenine nucleotide amounts on the whole tissue level.

Ca(2+)-dependent nonspecific permeability of the inner membrane of liver mitochondria in the guinea fowl (Numida meleagris)

This comparative study presents the results of the induction of Ca(2+)-dependent nonspecific permeability of the inner membrane (pore opening) of rat and guinea fowl liver mitochondria by mechanisms that are both sensitive and insensitive to cyclosporin A (CsA). It was established that energized rat and guinea fowl liver mitochondria incubated with 1 mM of inorganic phosphate (Pi) are capable of swelling upon addition of at least 125 and 875 nmol of CaCl2 per 1 mg protein, respectively. Under these conditions, the Ca(2+) release from the mitochondria of these animals and a drop in Δψ are observed. All of these processes are inhibited by 1 μM of CsA. FCCP, causing organelle de-energization, induces pore opening in rat and guinea fowl liver mitochondria upon addition of 45 и 625 nmol of CaCl2 per 1 mg protein, respectively. These results suggest the existence of a CsA-sensitive mechanism for the induction of Ca(2+)-dependent pores in guinea fowl liver mitochondria, which has been reported in rat liver mitochondria. However, guinea fowl liver mitochondria have a significantly greater resistance to Ca(2+) as a pore inducer compared to rat liver mitochondria. It was found that the addition of α,ω-hexadecanedioic acid (HDA) to rat and guinea fowl liver mitochondria incubated with CsA and loaded with Ca(2+) causes organelle swelling and Ca(2+) release from the matrix. It is assumed that in contrast to the CsA-sensitive pore, the CsA-insensitive pore induced by HDA in the inner membrane of guinea fowl liver mitochondria, as well as in rat liver mitochondria, is lipid in nature.

Quantitative Analysis of Cereulide, an Emetic Toxin ofBacillus cereus, by Using Rat Liver Mitochondria

An emetic toxin cereulide, produced by Bacillus cereus, causes emetic food poisonings, but a method for quantitative measurement of cereulide has not been well established. A current detection method is a bioassay method using the HEp-2 cell vacuolation test, but it was unable to measure an accurate concentration. We established a quantitative assay for cereulide based on its mitochondrial respiratory uncoupling activity. The oxygen consumption in a reaction medium containing rat liver mitochondria was rapid in the presence of cereulide. Thus uncoupling effect of cereulide on mitochondrial respiration was similar to those of uncouplers 2,4-dinitrophenol (DNP), carbonylcyanide m-chlorophenylhydrazone (CCCP), and valinomycin. This method gave constant results over a wide range of cereulide concentrations, ranging from 0.05 to 100 microg/ml. The minimum cereulide concentration to detect uncoupled oxygen consumption was 50 ng/ml and increased dose-dependently to the maximum level. Semi-log relationship between the oxygen consumption rate and the cereulide concentration enables this method to quantify cereulide. The results of this method were highly reproducible as compared with the HEp-2 cell vacuolation test and were in good agreement with those of the HEp-2 cell vacuolation test. The enterotoxin of B. cereus or Staphylococcus aureus did not show any effect on the oxygen consumption, indicating this method is specific for the identification of cereulide as a causative agent of emetic food poisonings.

Effects of salinity on O₂ consumption, ROS generation and oxidative stress status of gill mitochondria of the mud crab Scylla serrata

Mitochondrial respiration, activities of electron transport chain enzymes and formation of oxidative stress parameters were investigated in mitochondria isolated from gill tissue of mud crabs (Scylla serrata) as a function of salinity (10 ppt, 17 ppt and 35 ppt). Mitochondrial oxygen consumption rate was higher for succinate as substrate compared with those of glutamate, malate and pyruvate. Complex I and complex II mediated respirations were higher at low salinity (10 ppt) than high salinity (17 ppt and 35 ppt). Although activities of electron transport chain enzymes particularly complexes I (EC 1.6.5.3), II (EC 1.3.99.1) and II-III (EC 1.3.2.1) were elevated linearly in response to salinity treatment, activity of complex V (ATPase, EC 3.6.1.34) was decreased at 35 ppt salinity. However, ATPase activity was higher at 17 ppt salinity in comparison to 10 ppt and 17 ppt salinity. Results of the experiment suggest that high salinity (35 ppt) causes hypoxic state in mitochondria of mud crabs. Hypoxic condition induced by high salinity was accompanied with increased hydrogen peroxide production resulting oxidative stress in mitochondria of crabs. A possible mechanism of hypoxia-induced reactive oxygen species generation and OS due to salinity stress in the crabs is discussed.

Does the oxidative stress theory of aging explain longevity differences in birds? I. Mitochondrial ROS production

Mitochondrial reactive oxygen species (ROS) production rates are reported to be inversely related to maximum lifespan potential (MLSP) in mammals and also to be higher in short-living mammals compared to short-living birds. The mammal-bird comparison, however, is mainly based on studies of rats and pigeons. To date, there has been no systematic examination of ROS production in birds that differ in MLSP. Here we report a comparison of mitochondrial ROS production in two short-living (quails) and three long-living bird species (parrots) that exhibit, on average, a 5-fold longevity difference. Mitochondrial ROS production was determined both in isolated mitochondria (heart, skeletal muscle and liver) as traditionally done and also in intact erythrocytes. In all four tissues, mitochondrial ROS production was similar in quails and parrots and showed no correspondence with known longevity differences. The lack of a consistent difference between quails and parrots was not due to differences in mitochondrial content as ROS production in relation to oxygen consumption (determined as the free radical leak) showed a similar pattern. These findings cast doubt on the robustness of the oxidative stress theory of aging.

The long life of birds: the rat-pigeon comparison revisited

The most studied comparison of aging and maximum lifespan potential (MLSP) among endotherms involves the 7-fold longevity difference between rats (MLSP 5y) and pigeons (MLSP 35y). A widely accepted theory explaining MLSP differences between species is the oxidative stress theory, which purports that reactive oxygen species (ROS) produced during mitochondrial respiration damage bio-molecules and eventually lead to the breakdown of regulatory systems and consequent death. Previous rat-pigeon studies compared only aspects of the oxidative stress theory and most concluded that the lower mitochondrial superoxide production of pigeons compared to rats was responsible for their much greater longevity. This conclusion is based mainly on data from one tissue (the heart) using one mitochondrial substrate (succinate). Studies on heart mitochondria using pyruvate as a mitochondrial substrate gave contradictory results. We believe the conclusion that birds produce less mitochondrial superoxide than mammals is unwarranted. We have revisited the rat-pigeon comparison in the most comprehensive manner to date. We have measured superoxide production (by heart, skeletal muscle and liver mitochondria), five different antioxidants in plasma, three tissues and mitochondria, membrane fatty acid composition (in seven tissues and three mitochondria), and biomarkers of oxidative damage. The only substantial and consistent difference that we have observed between rats and pigeons is their membrane fatty acid composition, with rats having membranes that are more susceptible to damage. This suggests that, although there was no difference in superoxide production, there is likely a much greater production of lipid-based ROS in the rat. We conclude that the differences in superoxide production reported previously were due to the arbitrary selection of heart muscle to source mitochondria and the provision of succinate. Had mitochondria been harvested from other tissues or other relevant mitochondrial metabolic substrates been used, then very different conclusions regarding differences in oxidative stress would have been reached.

Kinetic control of oxygen consumption during contractions in self-perfused skeletal muscle

Fast kinetics of muscle oxygen consumption (VO2) is characteristic of effective physiological systems integration. The mechanism of VO2 kinetic control in vivo is equivocal as measurements are complicated by the twin difficulties of making high-frequency direct measurements of VO2 and intramuscular metabolites, and in attaining high [ADP]; complexities that can be overcome utilising highly aerobic canine muscle for the investigation of the transition from rest to contractions at maximal VO2. Isometric tetanic contractions of the gastrocnemius complex of six anaesthetised, ventilated dogs were elicited via sciatic nerve stimulation (50 Hz; 200 ms duration; 1 contraction s(−1)). Muscle VO2 and lactate efflux were determined from direct Fick measurements. Muscle biopsies were taken at rest and every ∼10 s during the transient and analysed for [phosphates], [lactate] and pH. The temporal VO2 vs. [PCr] and [ADP] relationships were not well fitted by linear or classical hyperbolic models (respectively), due to the high sensitivity of VO2 to metabolic perturbations early in the transient. The time course of this apparent sensitisation was closely aligned to that of ATP turnover, which was lower in the first ∼25 s of contractions compared to the steady state. These findings provide the first direct measurements of skeletal muscle VO2 and [PCr] in the non-steady state, and suggest that simple phosphate feedback models (which are adequate for steady-state observations in vitro) are not sufficient to explain the dynamic control of VO2 in situ. Rather an allosteric or 'parallel activation' mechanism of energy consuming and producing processes is required to explain the kinetic control of VO2 in mammalian skeletal muscle.

Acetoacetate as regulator of palmitic acid-induced uncoupling involving liver mitochondrial ADP/ATP antiporter and aspartate/glutamate antiporter

The effect of acetoacetate on palmitate-induced uncoupling with the involvement of ADP/ATP antiporter and aspartate/glutamate antiporter has been studied in liver mitochondria. The incubation of mitochondria with acetoacetate during succinate oxidation in the presence of rotenone, oligomycin, and EGTA suppresses the accumulation of conjugated dienes. This is considered as a display of antioxidant effect of acetoacetate. Under these conditions, acetoacetate does not influence the respiration of mitochondria in the absence or presence of palmitate but eliminates the ability of carboxyatractylate or aspartate separately to suppress the uncoupling effect of this fatty acid. The action of acetoacetate is eliminated by beta-hydroxybutyrate or thiourea, but not by the antioxidant Trolox. In the absence of acetoacetate, the palmitate-induced uncoupling is limited by a stage sensitive to carboxyatractylate (ADP/ATP antiporter) or aspartate (aspartate/glutamate antiporter); in its presence, it is limited by a stage insensitive to the effect of these agents. In the presence of Trolox, ADP suppresses the uncoupling action of palmitate to the same degree as carboxyatractylate. Under these conditions, acetoacetate eliminates the recoupling effects of ADP and aspartate, including their joint action. This effect of acetoacetate is eliminated by beta-hydroxybutyrate or thiourea. It is supposed that the stimulating effect of acetoacetate is caused both by increase in the rate of transfer of fatty acid anion from the inner monolayer of the membrane to the outer one, which involves the ADP/ATP antiporter and aspartate/glutamate antiporter, and by elimination of the ability of ADP to inhibit this transport. Under conditions of excessive production of reactive oxygen species in mitochondria at a high membrane potential and in the presence of small amounts of fatty acids, such effect of acetoacetate can be considered as one of the mechanisms of antioxidant protection.

Neuromuscular blockade of slow twitch muscle fibres elevates muscle oxygen uptake and energy turnover during submaximal exercise in humans

We tested the hypothesis that a greater activation of fast-twitch (FT) fibres during dynamic exercise leads to a higher muscle oxygen uptake (VO2 ) and energy turnover as well as a slower muscle on-kinetics. Subjects performed one-legged knee-extensor exercise for 10 min at an intensity of 30 W without (CON) and with (CUR) arterial injections of the non-depolarizing neuromuscular blocking agent cisatracurium. In CUR, creatine phosphate (CP) was unaltered in slow twitch (ST) fibres and decreased (P < 0.05) by 28% in FT fibres, whereas in CON, CP decreased (P < 0.05) by 33% and 23% in ST and FT fibres, respectively. From 127 s of exercise, muscle VO2 was higher (P < 0.05) in CUR compared to CON (425 +/- 25 (+/- S.E.M.) versus 332 +/- 30 ml min(-1)) and remained higher (P < 0.05) throughout exercise. Using monoexponential fitting, the time constant of the exercise-induced muscle VO2 response was slower (P < 0.05) in CUR than in CON (55 +/- 6 versus 33 +/- 5 s). During CUR and CON, muscle homogenate CP was lowered (P < 0.05) by 32 and 35%, respectively, and also muscle lactate production was similar in CUR and CON (37.8 +/- 4.1 versus 35.2 +/- 6.2 mmol). Estimated total muscle ATP turnover was 19% higher (P < 0.05) in CUR than in CON (1196 +/- 90 versus 1011 +/- 59 mmol) and true mechanical efficiency was lower (P < 0.05) in CUR than in CON (26.2 +/- 2.0 versus 30.9 +/- 1.5%). In conclusion, the present findings provide evidence that FT fibres are less efficient than ST fibres in vivo at a contraction frequency of 1 Hz, and that the muscle VO2 kinetics is slowed by FT fibre activation.

Control of OXPHOS efficiency by complex I in brain mitochondria

In the present work we have analysed the efficiency (P/O ratio) of energy production by oxidative phosphorylation (OXPHOS) in rat brain, liver and heart mitochondria. This study has revealed tissue-specific differences in the mean values of P/O ratios and ATP production rates. A marked dependence of the P/O ratio on the respiration rates has been observed with complex I (NADH:ubiquinone oxidoreductase), but not with complex II (succinate dehydrogenase) respiratory substrates. The physiological impact of the P/O variations with complex I substrates has been further confirmed by extending the analysis to brain mitochondria from three independent groups of animals utilized to study the effects of dietary treatments on the age-related changes of OXPHOS. The general site-specificity of the rate-dependent P/O variability indicates that the decoupling, i.e. decreased coupling between electron transfer and proton pumping, is likely to be mostly due to slip of mitochondrial complex I. These findings suggest an additional mechanism for the pivotal role played by the energy-conserving respiratory complex I in the physiological and adaptive plasticity of mitochondrial OXPHOS.

Effect of temperature on skeletal muscle energy turnover during dynamic knee-extensor exercise in humans

The present study examined the effect of elevated temperature on muscle energy turnover during dynamic exercise. Nine male subjects performed 10 min of dynamic knee-extensor exercise at an intensity of 43 W (SD 10) and a frequency of 60 contractions per minute. Exercise was performed under normal (C) and elevated muscle temperature (HT) through passive heating. Thigh oxygen uptake (V(O2)) was determined from measurements of thigh blood flow and femoral arterial-venous differences for oxygen content. Anaerobic energy turnover was estimated from measurements of lactate release as well as muscle lactate accumulation and phosphocreatine utilization based on analysis of muscle biopsies obtained before and after each exercise. At the start of exercise, muscle temperature was 34.5 degrees C (SD 1.7) in C compared with 37.2 degrees C (SD 0.5) during HT (P < 0.05). Thigh V(O2) after 3 min was 0.52 l/min (SD 0.11) in C and 0.63 l/min (SD 0.13) in HT, and at the end of exercise it was 0.60 l/min (SD 0.14) and 0.61 l/min (SD 0.10) in C and HT, respectively (not significant). Total lactate release was the same between the two temperature conditions, as was muscle lactate accumulation and PCr utilization. Total ATP production (aerobic + anaerobic) was the same between each temperature condition [505.0 mmol/kg (SD 107.2) vs. 527.1 mmol/kg (SD 117.6); C and HT, respectively]. In conclusion, within the range of temperatures studied, passively increasing muscle temperature before exercise has no effect on muscle energy turnover during dynamic exercise.

P/O ratios of mitochondrial oxidative phosphorylation

Mitochondrial mechanistic P/O ratios are still in question. The major studies since 1937 are summarized and various systematic errors are discussed. Values of about 2.5 with NADH-linked substrates and 1.5 with succinate are consistent with most reports after apparent contradictions are explained. Variability of coupling may occur under some conditions but is generally not significant. The fractional values result from the coupling ratios of proton transport. An additional revision of P/O ratios may be required because of a report of the structure of ATP synthase (D. Stock, A.G.W. Leslie, J.E. Walker, Science 286 (1999) 1700-1705) which suggests that the H+/ATP ratio is 10/3, rather than 3, consistent with P/O ratios of 2.3 with NADH and 1.4 with succinate, values that are also possible.

Protons may leak through pure lipid bilayers via a concerted mechanism

Protons are known to permeate pure lipid bilayers at a rate that is anomalous compared to those of other small monovalent cations. The prevailing mechanism via which they cross the membrane is still unclear, and it is unknown how to probe the mechanism directly by experiment. One of the more popular theories assumes the formation of membrane-spanning single-file water wires providing a matrix along which the protons can "hop" over the barrier. However, free energy calculations on such structures (without the presence of an excess proton) suggest that this mechanism alone cannot account for the observed permeation rates. We use the multistate empirical valence bond method to directly study water structures surrounding a (delocalized) excess proton on its way through the membrane. We find that membrane-spanning networks, rather than single-file chains, are formed around the proton. We also find that such structures are considerably stabilized in the presence of the proton, with lifetimes of several hundreds of picoseconds. The observed structures are suggestive of a new, concerted, mechanism and provide some direction for further investigation.

Oxygen solubilities of media used in electrochemical respiration measurements

Solubility data are presented as equations from which the oxygen concentration of arbitrary media may be calculated with an accuracy of about 1%. These equations, covering the range 5-40 degrees C, are based on measurements with a modification of the physical method of St. Helen and Fatt (I. Fatt, Polarographic Oxygen Sensors, CRC Press, Cleveland (1976)). Solutions of the following compounds were measured: EDTA (ethylenediaminetetraacetic acid), EGTA (ethyleneglycol-bis(2-aminoethylether)tetraacetic acid), glucose, Hepes (N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid), lactobionic acid (galactosylgluconic acid), magnesium chloride, mannitol, Mes (2-(N-morpholino)ethanesulfonic acid), potassium chloride, potassium phosphate, sodium chloride, sucrose, taurine, and Tris (tris(hydroxymethyl)aminomethane). The present data are compared with solubility data obtained with the method involving oxidation of calibrated amounts of reduced nicotinamide adenine dinucleotide (e.g., R.W. Estabrook, Methods Enzymol. 10 (1967) 41-47) and with the kinetic method of Reynafarje et al. (Anal. Biochem. 145 (1985) 406-418). It is concluded that the former method produces reliable results provided that some simple precautions are taken. The kinetic method, however, appears to have produced calibration errors up to about 5%.

ATP and heat production in human skeletal muscle during dynamic exercise: higher efficiency of anaerobic than aerobic ATP resynthesis

The aim of the present study was to simultaneously examine skeletal muscle heat production and ATP turnover in humans during dynamic exercise with marked differences in aerobic metabolism. This was done to test the hypothesis that efficiency is higher in anaerobic than aerobic ATP resynthesis. Six healthy male subjects performed 90 s of low intensity knee-extensor exercise with (OCC) and without thigh occlusion (CON-LI) as well as 90 s of high intensity exercise (CON-HI) that continued from the CON-LI bout. Muscle heat production was determined by continuous measurements of muscle heat accumulation and heat release to the blood. Muscle ATP production was quantified by repeated measurements of thigh oxygen uptake as well as blood and muscle metabolite changes. All temperatures of the thigh were equalized to approximately 37 degrees C prior to exercise by a water-perfused heating cuff. Oxygen uptake accounted for 80 +/- 2 and 59 +/- 4 %, respectively, of the total ATP resynthesis in CON-LI and CON-HI, whereas it was negligible in OCC. The rise in muscle temperature was lower (P < 0.05) in OCC than CON-LI (0.32 +/- 0.04 vs. 0.37 +/- 0.03 degrees C). The mean rate of heat production was also lower (P < 0.05) in OCC than CON-LI (36 +/- 4 vs. 57 +/- 4 J s-1). Mechanical efficiency was 52 +/- 4 % after 15 s of OCC and remained constant, whereas it decreased (P < 0.05) from 56 +/- 5 to 32 +/- 3 % during CON-LI. During CON-HI, mechanical efficiency transiently increased (P < 0.05) to 47 +/- 4 %, after which it decreased (P < 0.05) to 36 +/- 3 % at the end of CON-HI. Assuming a fully coupled mitochondrial respiration, the ATP turnover per unit of work was calculated to be unaltered during OCC (approximately 20 mmol ATP kJ-1), whereas it increased (P < 0.05) from 21 +/- 4 to 29 +/- 3 mmol ATP kJ-1 during CON-LI and further (P < 0.05) to 37 +/- 3 mmol ATP kJ-1 during CON-HI. The present data confirm the hypothesis that heat loss is lower in anaerobic ATP resynthesis than in oxidative phosphorylation and can in part explain the finding that efficiency declines markedly during dynamic exercise. In addition, the rate of ATP turnover apparently increases during constant load low intensity exercise. Alternatively, mitochondrial efficiency is lowered as exercise progresses, since ATP turnover was unaltered during the ischaemic exercise bout.

Muscle heat production and anaerobic energy turnover during repeated intense dynamic exercise in humans

1. The aim of the present study was to examine muscle heat production, oxygen uptake and anaerobic energy turnover throughout repeated intense exercise to test the hypotheses that (i) energy turnover is reduced when intense exercise is repeated and (ii) anaerobic energy production is diminished throughout repeated intense exercise. 2. Five subjects performed three 3 min intense one-legged knee-extensor exercise bouts (EX1, EX2 and EX3) at a power output of 65 +/- 5 W (mean +/- S.E.M.), separated by 6 min rest periods. Muscle, femoral arterial and venous temperatures were measured continuously during exercise for the determination of muscle heat production. In addition, thigh blood flow was measured and femoral arterial and venous blood were sampled frequently during exercise for the determination of muscle oxygen uptake. Anaerobic energy turnover was estimated as the difference between total energy turnover and aerobic energy turnover. 3. Prior to exercise, the temperature of the quadriceps muscle was passively elevated to 37.02 +/- 0.12 degrees C and it increased 0.97 +/- 0.08 degrees C during EX1, which was higher (P < 0.05) than during EX2 (0.79 +/- 0.05 degrees C) and EX3 (0.77 +/- 0.06 degrees C). In EX1 the rate of muscle heat accumulation was higher (P < 0.05) during the first 120 s compared to EX2 and EX3, whereas the rate of heat release to the blood was greater (P < 0.05) throughout EX2 and EX3 compared to EX1. The rate of heat production, determined as the sum of heat accumulation and release, was the same in EX1, EX2 and EX3, and it increased (P < 0.05) from 86 +/- 8 during the first 15 s to 157 +/- 7 J s(-1) during the last 15 s of EX1. 4. Oxygen extraction was higher during the first 60 s of EX2 and EX3 than in EX 1 and thigh oxygen uptake was elevated (P < 0.05) during the first 120 s of EX2 and throughout EX3 compared to EX1. The anaerobic energy production during the first 105 s of EX2 and 150 s of EX3 was lower (P < 0.05) than in EX1. 5. The present study demonstrates that when intense exercise is repeated muscle heat production is not changed, but muscle aerobic energy turnover is elevated and anaerobic energy production is reduced during the first minutes of exercise.

Do sterols reduce proton and sodium leaks through lipid bilayers?

Proton and/or sodium electrochemical gradients are critical to energy handling at the plasma membranes of all living cells. Sodium gradients are used for animal plasma membranes, all other living organisms use proton gradients. These chemical and electrical gradients are either created by a cation pumping ATPase or are created by photons or redox, used to make ATP. It has been established that both hydrogen and sodium ions leak through lipid bilayers at approximately the same rate at the concentration they occur in living organisms. Although the gradients are achieved by pumping the cations out of the cell, the plasma membrane potential enhances the leakage rate of these cations into the cell because of the orientation of the potential. This review proposes that cells use certain lipids to inhibit cation leakage through the membrane bilayers. It assumes that Na(+) leaks through the bilayer by a defect mechanism. For Na(+) leakage in animal plasma membranes, the evidence suggests that cholesterol is a key inhibitor of Na(+) leakage. Here I put forth a novel mechanism for proton leakage through lipid bilayers. The mechanism assumes water forms protonated and deprotonated clusters in the lipid bilayer. The model suggests how two features of lipid structures may inhibit H(+) leakage. One feature is the fused ring structure of sterols, hopanoids and tetrahymenol which extrude water and therefore clusters from the bilayer. The second feature is lipid structures that crowd the center of the bilayer with hydrocarbon. This can be accomplished either by separating the two monolayers with hydrocarbons such as isoprenes or isopranes in the bilayer's cleavage plane or by branching the lipid chains in the center of the bilayers with hydrocarbon. The natural distribution of lipids that contain these features are examined. Data in the literature shows that plasma membranes exposed to extreme concentrations of cations are particularly rich in the lipids containing the predicted qualities. Prokaryote plasma membranes that reside in extreme acids (acidophiles) contain both hopanoids and iso/anteiso- terminal lipid branching. Plasma membranes that reside in extreme base (alkaliphiles) contain both squalene and iso/anteiso- lipids. The mole fraction of squalene in alkaliphile bilayers increases, as they are cultured at higher pH. In eukaryotes, cation leak inhibition is here attributed to sterols and certain isoprenes, dolichol for lysosomes and peroxysomes, ubiquinone for these in addition to mitochondrion, and plastoquinone for the chloroplast. Phytosterols differ from cholesterol because they contain methyl and ethyl branches on the side chain. The proposal provides a structure-function rationale for distinguishing the structures of the phytosterols as inhibitors of proton leaks from that of cholesterol which is proposed to inhibit leaks of Na(+). The most extensively studied of sterols, cholesterol, occurs only in animal cells where there is a sodium gradient across the plasma membrane. In mammals, nearly 100 proteins participate in cholesterol's biosynthetic and degradation pathway, its regulatory mechanisms and cell-delivery system. Although a fat, cholesterol yields no energy on degradation. Experiments have shown that it reduces Na(+) and K(+) leakage through lipid bilayers to approximately one third of bilayers that lack the sterol. If sterols significantly inhibit cation leakage through the lipids of the plasma membrane, then the general role of all sterols is to save metabolic ATP energy, which is the penalty for cation leaks into the cytosol. The regulation of cholesterol's appearance in the plasma membrane and the evolution of sterols is discussed in light of this proposed role.

Oxyconformity in the intertidal worm Sipunculus nudus: the mitochondrial background and energetic consequences

The energetic consequences of strict oxyconformity in the intertidal worm S. nudus were studied by characterizing the Po2 dependence of respiration in mitochondria isolated from the body wall tissue. Mitochondrial respiration rose in a Po2 range between 2.8 and 31.3 kPa from a mean of 56.5 to 223.9 nmol O mg protein(-1) h(-1). Respiration was sensitive to both salicylhydroxamic acid (SHAM) and KCN. Po2 dependence remained unchanged with saturating and non-saturating substrate levels (malate, glutamate and ADP). A concomitant decrease of the ATP/O ratio revealed a lower ATP yield of aerobic metabolism at elevated Po2. Obviously, oxyconforming respiration implies progressive uncoupling of mitochondria. The decrease in ATP/O ratios at higher Po2 was completely reversible. Addition of 90.9 micromol H2O2 l(-1) did not inhibit ATP synthesis. Both observations suggest that oxidative injury did not contribute to oxyconformity. The contribution of the rates of mitochondrial ROS production and proton leakiness to mitochondrial oxygen consumption and uncoupling was investigated by using oligomycin as a specific inhibitor of the ATP synthase. The maximum contribution of oligomycin independent respiration to state 3 respiration remained below 6% and showed a minor, insignificant increase at elevated Po2, at a slope significantly lower than the increment of state 3 respiration. Therefore, Po2 dependent mitochondrial proton leakage or ROS production cannot explain oxyconformity. In conclusion Po2 dependent state 3 respiration likely relates to the progressive contribution of an alternative oxidase (cytochrome o), which is characterized by a low affinity to oxygen and an ATP/O ratio similar to the branched respiratory system of bacteria. The molecular nature of the alternative oxidase in lower invertebrates is still obscure.

Quinone-annonaceous acetogenins: synthesis and complex I inhibition studies of a new class of natural product hybrids

The natural product hybrids quinone-mucocin and quinone- squamocin D were synthesized. In these hybrids, the butenolide unit of the annonaceous acetogenins mucocin and squamocin D is exchanged for the quinone moiety of the natural complex I substrate ubiquinone. For both syntheses, a modular, highly convergent approach was applied. Quinone-mucocin was constructed out of a tetrahydropyran (THP) component 1, a tetrahydrofuran (THF) unit 2, and a quinone precursor 3. A stereoselective, organometallic coupling reaction was chosen for the addition of the THP unit to the rest of the molecule. In the final step, the oxidation to the free quinone was achieved by using cerium(IV) ammonium nitrate (CAN) as the oxidizing agent. Quinone-squamocin D was assembled in a similar manner, from the chiral side chain bromide 16, the central bis-THF core 17, and the quinone precursor 18. Inhibition of complex I (isolated from bovine heart mitochondria) by the quinone acetogenins and several smaller building blocks was examined; quinone mucocin and quinone-squamocin D act as strong inhibitors of complex I. These results and the data from the smaller substructures indicate that other substructures of the acetogenins besides the butenolide group, such as the polyether component and the lipophilic left-hand side chain, are necessary for the strong binding of the acetogenins to complex I.

Hyperthermia impairs liver mitochondrial function in vitro

The effects of temperature on the relationships among the rates of pyruvate carboxylation, O(2) uptake (J(o)), oxidative phosphorylation (J(p)), and the free energy of ATP hydrolysis (G(p)) were studied in liver mitochondria isolated from 250-g female rats. Pyruvate carboxylation was evaluated at 37, 40, and 43 degrees C. In disrupted mitochondria, pyruvate carboxylase maximal reaction velocity increased from 37 to 43 degrees C with an apparent Q(10) of 2.25. A reduction in ATP/ADP ratio decreased enzyme activity at all three temperatures. In contrast, in intact mitochondria, increasing temperature failed to increase pyruvate carboxylation (malate + citrate accumulation) but did result in increased J(o) and decreased extramitochondrial G(p). J(p) was studied in respiring mitochondria at 37 and 43 degrees C at various fractions of state 3 respiration, elicited with a glucose + hexokinase ADP-regenerating system. The relationship between J(o) and G(p) was similar at both temperatures. However, hyperthermia (43 degrees C) reduced the J(p)/J(o) ratio, resulting in lower G(p) for a given J(p). Fluorescent measurements of membrane phospholipid polarization revealed a transition in membrane order between 40 and 43 degrees C, a finding consistent with increased membrane proton conductance. It is concluded that hyperthermia augments nonspecific proton leaking across the inner mitochondrial membrane, and the resultant degraded energy state offsets temperature stimulation of pyruvate carboxylase. As a consequence, at high temperatures approaching 43 degrees C, the pyruvate carboxylation rate of intact liver mitochondria may fail to exhibit a Q(10) effect.

Heat production in human skeletal muscle at the onset of intense dynamic exercise

1. We hypothesised that heat production of human skeletal muscle at a given high power output would gradually increase as heat liberation per mole of ATP produced rises when energy is derived from oxidation compared to phosphocreatine (PCr) breakdown and glycogenolysis. 2. Five young volunteers performed 180 s of intense dynamic knee-extensor exercise ( approximately 80 W) while estimates of muscle heat production, power output, oxygen uptake, lactate release, lactate accumulation and ATP and PCr hydrolysis were made. Heat production was determined continuously by (i) measuring heat storage in the contracting muscles, (ii) measuring heat removal to the body core by the circulation, and (iii) estimating heat transfer to the skin by convection and conductance as well as to the body core by lymph drainage. 3. The rate of heat storage in knee-extensor muscles was highest during the first 45 s of exercise (70-80 J s-1) and declined gradually to 14 +/- 10 J s-1 at 180 s. 4. The rate of heat removal by blood was negligible during the first 10 s of exercise, rising gradually to 112 +/- 14 J s-1 at 180 s. The estimated rate of heat release to skin and heat removal via lymph flow was < 2 J s-1 during the first 5 s and increased progressively to 24 +/- 1 J s-1 at 180 s. The rate of heat production increased significantly throughout exercise, being 107 % higher at 180 s compared to the initial 5 s, with half of the increase occurring during the first 38 s, while power output remained essentially constant. 5. The contribution of muscle oxygen uptake and net lactate release to total energy turnover increased curvilinearly from 32 % and 2 %, respectively, during the first 30 s to 86 % and 8 %, respectively, during the last 30 s of exercise. The combined energy contribution from net ATP hydrolysis, net PCr hydrolysis and muscle lactate accumulation is estimated to decline from 37 % to 3 % comparing the same time intervals. 6. The magnitude and rate of elevation in heat production by human skeletal muscle during exercise in vivo could be the result of the enhanced heat liberation during ATP production when aerobic metabolism gradually becomes dominant after PCr and glycogenolysis have initially provided most of the energy.

Ca(2+) activation of heart mitochondrial oxidative phosphorylation: role of the F(0)/F(1)-ATPase

Ca(2+) has been postulated as a cytosolic second messenger in the regulation of cardiac oxidative phosphorylation. This hypothesis draws support from the well-known effects of Ca(2+) on muscle activity, which is stimulated in parallel with the Ca(2+)-sensitive dehydrogenases (CaDH). The effects of Ca(2+) on oxidative phosphorylation were further investigated in isolated porcine heart mitochondria at the level of metabolic driving force (NADH or Deltapsi) and ATP production rates (flow). The resulting force-flow (F-F) relationships permitted the analysis of Ca(2+) effects on several putative control points within oxidative phosphorylation, simultaneously. The F-F relationships resulting from additions of carbon substrates alone provided a model of pure CaDH activation. Comparing this curve with variable Ca(2+) concentration ([Ca(2+)]) effects revealed an approximate twofold higher ATP production rate than could be explained by a simple increase in NADH or Deltapsi via CaDH activation. The half-maximal effect of Ca(2+ )at state 3 was 157 nM and was completely inhibited by ruthenium red (1 microM), indicating matrix dependence of the Ca(2+) effect. Arsenate was used as a probe to differentiate between F(0)/F(1)-ATPase and adenylate translocase activity by a futile recycling of ADP-arsenate within the matrix, catalyzed by the F(0)/F(1)-ATPase. Ca(2+) increased the ADP arsenylation rate more than twofold, suggesting a direct effect on the F(0)/F(1)-ATPase. These results suggest that Ca(2+) activates cardiac aerobic respiration at the level of both the CaDH and F(0)/F(1)-ATPase. This type of parallel control of both intermediary metabolism and ATP synthesis may provide a mechanism of altering ATP production rates with minimal changes in the high-energy intermediates as observed in vivo.

Simulation of state 4 → state 3 transition in isolated mitochondria

The mathematical dynamic model of oxidative phosphorylation developed previously and in the accompanying paper was modified to involve isolated mitochondria conditions; it was also used to simulate state 4 --> state 3 transition in rat liver mitochondria incubated with succinate as respiratory substrate and glucose-hexokinase as an ADP-regenerating system. Changes in the respiration rate, protonmotive force and reduction level of ubiquinone and cytochrome c as well as the internal (i) and external (e) ATP/ADP ratio between state 4 and state 3 were calculated and compared with the experimental data. Flux control coefficients with respect to oxygen consumption flux for different reactions and processes of oxidative phosphorylation were simulated for different values of the respiration rate (state 4, state 3 and intermediate states). Flux control coefficients for the oxidation, phosphorylation and proton leak subsystems with respect to the oxidation, phosphorylation and proton leak fluxes for different values of the respiration rate were also calculated. These theoretical data were compared with the experimental results obtained in the frame of metabolic control analysis and the 'top-down' approach to this analysis. A good agreement was obtained. Simulated time courses of the respiration rate, the protonmotive force (Deltap) and other parameters after addition of a small amount of ADP to mitochondria in state 4 mimicked at least semiquantitatively the experimentally measured time courses of these parameters. It was concluded, therefore, that in the present stage, the model is able to reflect different properties of the oxidative phosphorylation system in a broad range of conditions fairly well, allows deeper insight into the mechanisms responsible for control and regulation of this process, and can be used for simulation of new experiments, thus inspiring experimental verification of the theoretical predictions.

Measurement of unidirectional P(i)-->ATP flux in lamb myocardium in vivo

Unidirectional myocardial ATP synthesis, P(i)-->ATP flux, was studied in vivo using 31P magnetization transfer techniques in intact sheep hearts (n = 5) which were functioning aerobically. Myocardial oxygen consumption (MVO) expressed as mu moles of oxygen atoms/gm/min was estimated using linear regression analysis of data derived from sheep (n = 23), which had undergone continuous MVO measurement during graded stepups in epinephrine induced work loads. During the saturation transfer experiment, epinephrine, beginning at 1 microgram/kg per min was infused to achieve a higher steady-state work load and level of MVO. The unidirectional P(i)-->ATP flux was found to increase significantly (P < 0.05) during increases in rate pressure product and MVO. These data show that the unidirectional P(i)-->ATP flux is at least 3-times higher than the peak ATP synthesis rate, achieved through oxidative phosphorylation in these experiments, and more than a magnitude higher than the peak ATP synthesis rate through glycolysis. Therefore, forward P(i)-->ATP flux through glycolysis is the major contributor to the measured P(i)-->ATP flux and these ATP producing bidirectional glycolytic reactions are in a near equilibrium state. Furthermore, delta P(i)-->ATP/delta MVO, 2.70 +/- 0.29 (S.E.) elicited during epinephrine infusion is similar to classically derived P:O values, indicating that most of the change in unidirectional flux is due to oxidative phosphorylation and that minimal disturbance in the glycolytic near equilibrium occurs under these conditions.

Respiration in non-phosphorylating yeast mitochondria. Roles of non-ohmic proton conductance and intrinsic uncoupling

Respiratory rate, protonmotive force and charge/O ratio were measured under two different kinds of steady state in non-phosphorylating yeast mitochondria: (i) when the electron flux was modulated by a variable limitation in electron supply or (ii) when oxygen consumption was decreased by respiratory chain inhibitor titration. We showed that the relationships between either delta p or charge/O ratio and respiratory rate are different under the two kinds of steady state, indicating different degrees of intrinsic uncoupling in respiratory chain. Moreover, we observed a non-ohmic dependence between H(+)-conductance and delta p. We concluded that the high rate of static-head respiration in yeast mitochondria was determined both by the non-ohmic proton conductance of the inner membrane and the saturation of the redox proton pump slipping.

An assessment of the role of proton leaks in the mechanistic stoichiometry of oxidative phosphorylation

Rat liver mitochondria were incubated in the presence of varying concentrations of ATP, followed by ADP to initiate phosphorylation. Analysis of phosphorylation to oxygen ratios (P/O) was carried out with varied initial phosphorylation potentials (or ATP/ADP ratios). Rates of phosphorylation and respiration and magnitude of membrane potential (delta psi) were measured. The results are discussed in the framework of P/total O and P/"extra" O ratios in determination of the mechanistic P/O ratio. It is concluded that the former underestimates, and the latter overestimates the mechanistic P/O ratio.

Mechanistic stoichiometry of mitochondrial oxidative phosphorylation

P/O ratios of rat liver mitochondria were measured with particular attention to systematic errors. Corrections for energy loss during oxidative phosphorylation were made by measurement of respiration as a function of mitochondrial membrane potential. The corrected values were close to 1, 0.5, and 1 at the three coupling sites, respectively. These values are consistent with recent measurements of mitochondrial proton transport.

Differentiation between leaks and slips in oxidative phosphorylation

We have measured the thermodynamic efficiency of oxidative phosphorylation of isolated rat-liver mitochondria during oxidation of succinate. Furthermore, we have calculated what the effect of proton leak or slip in the redox pumps should be on the efficiency of energy transduction in oxidative phosphorylation. These calculations were compared with experiments in which the efficiency was determined in the presence of induced proton leak or redox slip. The results of these experiments are in agreement with the predictions. It is concluded that it is possible to distinguish experimentally between effects of proton leak and redox slip on energy transduction.

Respiratory control and ADP:O coupling ratios of isolated chick heart mitochondria

Heart mitochondria isolated from 14- to 21-day-old chicks are highly coupled and often have respiratory control ratio (RCR) values exceeding 100. This paper presents data from a study of some of the properties of these mitochondria. The studies show that: (a) The ADP:O ratios and the state 4 rates of respiration are highly dependent upon the concentration of mitochondria at which these parameters are measured. (b) The mitochondrial isolate is contaminated with at least two divalent cation-stimulated ATPase, of which one is the F1F0-ATPase of broken mitochondria. (c) The oligomycin-sensitive component of state 4 respiration is completely inhibited by ethylene glycol bis(beta-amino-ethylether) N,N'-tetraacetic acid (EGTA). This inhibition is biphasic and attributable to the differential affinity of EGTA for Ca(II) and Mg(II). (d) Ca(II) and Mg(II) stimulate state 4 respiration, thereby depressing RCR values. These cations also decrease ADP:O ratios from greater than or equal to 3.25 to 3.0 for some NAD-linked substrates. (e) Uncoupled (i.e., oligomycin-insensitive) state 4 respiration can be abolished by treating the mitochondria with Nagarse and by preincubating mitochondria with exogenous substrate. (f) The ADP:O ratios obtained when these heart mitochondria oxidize pyruvate/malate, alpha-ketoglutarate, and beta-hydroxybutyrate are fractional and significantly greater than 3.0.

Resting state respiration of mitochondria: reappraisal of the role of passive ion fluxes

Rat liver mitochondria respiring under resting state conditions in the presence of oligomycin were rapidly blocked with cyanide and the dissipation of the membrane potential, measured with a tetraphenylphosphonium-sensitive electrode, was followed over time. The plot of the rate of membrane potential dissipation versus the actual value of the membrane potential was nonlinear and identical to the plot of resting state respiration (titrated with small amounts of a respiratory inhibitor) versus the membrane potential. The relationship between the respiratory chain activity and the proton-motive force in mitochondria oxidizing succinate with either oxygen or ferricyanide as electron acceptors was also found to be identical. These results are interpreted as an indication that the passive permeability of the inner mitochondrial membrane toward ions is far more significant in maintaining resting state respiration than is the molecular slippage of the pumps in the respiratory chain. These results also confirm the non-ohmic characteristics of the inner mitochondrial membrane.

Slip and leak in mitochondrial oxidative phosphorylation

During oxidative phosphorylation by mammalian mitochondria part of the free energy stored in reduced substrates is dissipated and energy is released as heat. Here I review the mechanisms and the physiological significance of this phenomenon.

Enzymatic method for measuring the absolute value of oxygen concentration

An enzymatic method for measuring the absolute concentration of oxygen in aqueous solutions, using 4-hydroxybenzoate 3-monooxygenase and glucose oxidase, is described. The monooxygenase is used for quantitative oxidation of 4-hydroxybenzoate and NADPH with oxygen into 3,4-dihydroxybenzoate and NADP+; the amount of oxygen can be measured as the amount of NADPH decreased by the reaction. The monooxygenase reaction is performed in a syringe, a closed system. After the completion of the monooxygenase reaction, glucose oxidase is added to the assay solution to consume the oxygen from the atmosphere; this enables us to measure the NADPH concentration in the solution spectrophotometrically in an open system and to check the anaerobicity of closed systems. The oxygen concentrations at 25 degrees C of oxygen-saturated and air-saturated water were 1.10 and 0.23 mM, respectively. The value for argon-bubbled water was zero within the experimental error; this justifies the assay system. Thus, it is shown that a sample containing 8 microM-1.1 mM oxygen can be measured by this method.