O2 solubility in aqueous media determined by a kinetic method

Identification of a functioning mitochondrial uncoupling protein 1 in thymus

We present evidence that rat and mouse thymi contain mitochondrial uncoupling protein (UCP 1). Reverse transcriptase-PCR detected RNA transcripts for UCP 1 in whole thymus and in thymocytes. Furthermore, using antibodies to UCP 1 the protein was also detected in mitochondria isolated from whole thymus and thymocytes but not in thymus mitochondria from UCP 1 knock-out mice. Evidence for functional UCP 1 in thymus mitochondria was obtained by a comparative analysis with the kinetics of GDP binding in mitochondria from brown adipose tissue. Both tissues showed equivalent B(max) and K(D) values. In addition, a large component of the nonphosphorylating oxygen consumption by thymus mitochondria was inhibited by GDP and subsequently stimulated by addition of nanomolar concentrations of palmitate. UCP 1 was purified from thymus mitochondria by hydroxyapatite chromatography. The isolated protein was identified by peptide mass mapping and tandem mass spectrometry by using MALDI-TOF and LC-MS/MS, respectively. We conclude that the thymus contains a functioning UCP 1 that has the capacity to regulate metabolic flux and production of reactive oxygen-containing molecules in the thymus.

Ubiquinone is not required for proton conductance by uncoupling protein 1 in yeast mitochondria

Q (coenzyme Q or ubiquinone) is reported to be a cofactor obligatory for proton transport by UCPs (uncoupling proteins) in liposomes [Echtay, Winkler and Klingenberg (2000) Nature (London) 408, 609-613] and for increasing the binding of the activator retinoic acid to UCP1 [Tomás, Ledesma and Rial (2002) FEBS Lett. 526, 63-65]. In the present study, yeast ( Saccharomyces cerevisiae ) mutant strains lacking Q and expressing UCP1 were used to determine whether Q was required for UCP function in mitochondria. Wild-type yeast strain and two mutant strains (CENDeltaCOQ3 and CENDeltaCOQ2), both not capable of synthesizing Q, were transformed with the mouse UCP1 gene. UCP1 activity was measured as fatty acid-dependent, GDP-sensitive proton conductance in mitochondria isolated from the cells. The activity of UCP1 was similar in both Q-containing and -deficient yeast mitochondria. We conclude that Q is neither an obligatory cofactor nor an activator of proton transport by UCP1 when it is expressed in yeast mitochondria.

The effect of aging and caloric restriction on mitochondrial protein density and oxygen consumption

It has been proposed that part of the anti-aging mechanism of caloric restriction (CR) involves changes in mitochondrial function. To investigate this hypothesis, mitochondria from various tissues of male Brown Norway rats (fully fed and CR) were isolated and respiration rates determined. In mitochondria from liver, heart, brain and kidney, there were no significant effects of CR on state 4 mitochondrial respiration rate. Further experiments using liver mitochondria under a variety of incubation conditions confirmed that CR does not alter mitochondrial respiration rate in this tissue. However, the respiration rate of mitochondria from brown adipose tissue (BAT) of CR animals was approximately three-fold higher compared to mitochondria from fully fed controls. Mitochondrial protein density was significantly higher in liver tissue of CR animals; it was significantly lower in heart and unchanged in BAT. It is concluded that whilst CR results in tissue-specific changes in mitochondrial respiration rate, these effects do not explain the CR-induced changes in free radical production reported previously for these organelles.

The characterization of mitochondrial permeability transition in clonal pancreatic beta-cells. Multiple modes and regulation

Mitochondrial permeability transition (MPT), which contributes substantially to the regulation of normal mitochondrial metabolism, also plays a crucial role in the initiation of cell death. It is known that MPT is regulated in a tissue-specific manner. The importance of MPT in the pancreatic beta-cell is heightened by the fact that mitochondrial bioenergetics serve as the main glucose-sensing regulator and energy source for insulin secretion. In the present study, using MIN6 and INS-1 beta-cells, we revealed that both Ca(2+)-phosphate- and oxidant-induced MPT is remarkably different from other tissues. Ca(2+)-phosphate-induced transition is accompanied by a decline in mitochondrial reactive oxygen species production related to a significant potential dependence of reactive oxygen species formation in beta-cell mitochondria. Hydroperoxides, which are indirect MPT co-inducers active in liver and heart mitochondria, are inefficient in beta-cell mitochondria, due to the low mitochondrial ability to metabolize them. Direct cross-linking of mitochondrial thiols in pancreatic beta-cells induces the opening of a low conductance ion permeability of the mitochondrial membrane instead of the full scale MPT opening typical for liver mitochondria. Low conductance MPT is independent of both endogenous and exogenous Ca(2+), suggesting a novel type of nonclassical MPT in beta-cells. It results in the conversion of electrical transmembrane potential into DeltapH instead of a decrease in total protonmotive force, thus mitochondrial respiration remains in a controlled state. Both Ca(2+)- and oxidant-induced MPTs are phosphate-dependent and, through the "phosphate flush" (associated with stimulation of insulin secretion), are expected to participate in the regulation in beta-cell glucose-sensing and secretory activity.

Uncoupling protein and ATP/ADP carrier increase mitochondrial proton conductance after cold adaptation of king penguins

Juvenile king penguins develop adaptive thermogenesis after repeated immersion in cold water. However, the mechanisms of such metabolic adaptation in birds are unknown, as they lack brown adipose tissue and uncoupling protein-1 (UCP1), which mediate adaptive non-shivering thermogenesis in mammals. We used three different groups of juvenile king penguins to investigate the mitochondrial basis of avian adaptive thermogenesis in vitro. Skeletal muscle mitochondria isolated from penguins that had never been immersed in cold water showed no superoxide-stimulated proton conductance, indicating no functional avian UCP. Skeletal muscle mitochondria from penguins that had been either experimentally immersed or naturally adapted to cold water did possess functional avian UCP, demonstrated by a superoxide-stimulated, GDP-inhibitable proton conductance across their inner membrane. This was associated with a markedly greater abundance of avian UCP mRNA. In the presence (but not the absence) of fatty acids, these mitochondria also showed a greater adenine nucleotide translocase-catalysed proton conductance than those from never-immersed penguins. This was due to an increase in the amount of adenine nucleotide translocase. Therefore, adaptive thermogenesis in juvenile king penguins is linked to two separate mechanisms of uncoupling of oxidative phosphorylation in skeletal muscle mitochondria: increased proton transport activity of avian UCP (dependent on superoxide and inhibited by GDP) and increased proton transport activity of the adenine nucleotide translocase (dependent on fatty acids and inhibited by carboxyatractylate).

Uncoupled and surviving: individual mice with high metabolism have greater mitochondrial uncoupling and live longer

Two theories of how energy metabolism should be associated with longevity, both mediated via free-radical production, make completely contrary predictions. The 'rate of living-free-radical theory' (Pearl, 1928; Harman, 1956; Sohal, 2002) suggests a negative association, the 'uncoupling to survive' hypothesis (Brand, 2000) suggests the correlation should be positive. Existing empirical data on this issue is contradictory and extremely confused (Rubner, 1908; Yan & Sohal, 2000; Ragland & Sohal, 1975; Daan et al., 1996; Wolf & Schmid-Hempel, 1989]. We sought associations between longevity and individual variations in energy metabolism in a cohort of outbred mice. We found a positive association between metabolic intensity (kJ daily food assimilation expressed as g/body mass) and lifespan, but no relationships of lifespan to body mass, fat mass or lean body mass. Mice in the upper quartile of metabolic intensities had greater resting oxygen consumption by 17% and lived 36% longer than mice in the lowest intensity quartile. Mitochondria isolated from the skeletal muscle of mice in the upper quartile had higher proton conductance than mitochondria from mice from the lowest quartile. The higher conductance was caused by higher levels of endogenous activators of proton leak through the adenine nucleotide translocase and uncoupling protein-3. Individuals with high metabolism were therefore more uncoupled, had greater resting and total daily energy expenditures and survived longest - supporting the 'uncoupling to survive' hypothesis.

Superoxide activates a GDP-sensitive proton conductance in skeletal muscle mitochondria from king penguin (Aptenodytes patagonicus)

We present the partial nucleotide sequence of the avian uncoupling protein (avUCP) gene from king penguin (Aptenodytes patagonicus), showing that the protein is 88-92% identical to chicken (Gallus gallus), turkey (Meleagris gallopavo), and hummingbird (Eupetomena macroura). We show that superoxide activates the proton conductance of mitochondria isolated from king penguin skeletal muscle. GDP abolishes the superoxide-activated proton conductance, indicating that it is mediated via avUCP. In the absence of superoxide there is no GDP-sensitive component of the proton conductance from penguin muscle mitochondria demonstrating that avUCP plays no role in the basal proton leak.

Production of endogenous matrix superoxide from mitochondrial complex I leads to activation of uncoupling protein 3

Superoxide generated using exogenous xanthine oxidase indirectly activates an uncoupling protein (UCP)-mediated proton conductance of the mitochondrial inner membrane. We investigated whether endogenous mitochondrial superoxide production could also activate proton conductance. When respiring on succinate, rat skeletal muscle mitochondria produced large amounts of matrix superoxide. Addition of GDP to inhibit UCP3 markedly inhibited proton conductance and increased superoxide production. Both superoxide production and the GDP-sensitive proton conductance were suppressed by rotenone plus an antioxidant. Thus, endogenous superoxide can activate the proton conductance of UCP3, which in turn limits mitochondrial superoxide production. These observations provide a departure point for studies under more physiological conditions.

Effect of caloric restriction on mitochondrial reactive oxygen species production and bioenergetics: reversal by insulin

To gain insight into the antiaging mechanisms of caloric restriction (CR), mitochondria from liver tissue of male Brown Norway rats were used to study the effects of CR and insulin on mitochondrial reactive oxygen species production and bioenergetics. As assessed by hydrogen peroxide measurement, CR resulted in a decrease in the production rate of reactive oxygen species. This decrease was attributed to a decrease in protonmotive force in mitochondria from the CR animals. The decrease in protonmotive force resulted from an increase in proton leak activity and a concomitant decrease in substrate oxidation activity. Each of these effects of CR was reversed by subjecting CR animals to 2 wk of insulin treatment. To achieve continuous and stable insulin delivery, animals were placed under temporary halothane anesthesia and miniosmotic pumps were implanted subcutaneously. To gain further insight into how CR and insulin exerted its effects on mitochondrial bioenergetics, the effects of CR and insulin were quantified using modular metabolic control analysis. This analysis revealed that the effects of CR were transmitted through different reaction branches of the bioenergetic system, and insulin reversed the effects of CR by acting through the same branches. These results provide a plausible mechanism by which mitochondrial reactive oxygen species production is lowered by CR and a complete description of the effects of CR on mitochondrial bioenergetics. They also indicate that these changes may be due to lowered insulin concentrations and altered insulin signaling in the CR animal.

Nitric oxide produced in rat liver mitochondria causes oxidative stress and impairment of respiration after transient hypoxia

Nitric oxide (NO) is produced in mammals by different isoforms of NO synthase (NOS), including the constitutive mitochondrial enzyme (mtNOS). Here we demonstrate that the concentration of NO resulting from a mitochondrial NOS activity increases under hypoxic conditions in isolated rat liver mitochondria. We show that mitochondrially derived NO mediates the impairment of active (state 3) respiration as measured in the presence of the substrates glutamate and malate after reoxygenation. Simultaneously, NO induces oxidative stress in mitochondria, characterized by an increase in the amount of protein carbonyls and a decrease in glutathione (GSH). Both the accumulation of oxidative stress markers during and the impaired respiration after reoxygenation were prevented by blocking NO production with the NOS inhibitor L-NAME. These observations suggest that mitochondria are exposed to high amounts of NO generated by a mitochondrial NOS upon hypoxia/reoxygenation. Such increased NO levels, in turn, inhibit mitochondrial respiration and may cause oxidative stress that leads to irreversible impairment of mitochondria.

Superoxide and hydrogen peroxide production by Drosophila mitochondria

Drosophila melanogaster is a key model organism for genetic investigation of the role of free radicals in aging, but biochemical understanding is lacking. Superoxide production by Drosophila mitochondria was measured fluorometrically as hydrogen peroxide, using its dependence on substrates, inhibitors, and added superoxide dismutase to determine sites of production and their topology. Glycerol 3-phosphate dehydrogenase and center o of complex III in the presence of antimycin had the greatest maximum capacities to generate superoxide on the cytosolic side of the inner membrane. Complex I had significant capacity on the matrix side. Center i of complex III, cytochrome c, and complex IV produced no superoxide. Native superoxide generation by isolated mitochondria was also measured without added inhibitors. There was a high rate of superoxide production with sn-glycerol 3-phosphate as substrate; two-thirds mostly from glycerol 3-phosphate dehydrogenase on the cytosolic side and one-third on the matrix side from complex I following reverse electron transport. There was little superoxide production from any site with NADH-linked substrate. Superoxide production by complex I following reverse electron flow from glycerol 3-phosphate was particularly sensitive to membrane potential, decreasing 70% when potential decreased 10 mV, showing that mild uncoupling lowers superoxide production in the matrix very effectively.

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%.

Bioenergetic analysis of peroxisome proliferator-activated receptor gamma coactivators 1alpha and 1beta (PGC-1alpha and PGC-1beta) in muscle cells

Peroxisome proliferator-activated receptor gamma coactivator (PGC)-1alpha is a coactivator of nuclear receptors and other transcription factors that regulates several components of energy metabolism, particularly certain aspects of adaptive thermogenesis in brown fat and skeletal muscle, hepatic gluconeogenesis, and fiber type switching in skeletal muscle. PGC-1alpha has been shown to induce mitochondrial biogenesis when expressed in muscle cells, and preliminary analysis has suggested that this molecule may specifically increase the fraction of uncoupled versus coupled respiration. In this paper, we have performed detailed bioenergetic analyses of the function of PGC-1alpha and its homolog PGC-1beta in muscle cells by monitoring simultaneously oxygen consumption and membrane potential. Cells expressing PGC-1alpha or PGC-1beta display higher proton leak rates at any given membrane potential than control cells. However, cells expressing PGC-1alpha have a higher proportion of their mitochondrial respiration linked to proton leak than cells expressing PGC-1beta. Although these two proteins cause a similar increase in the expression of many mitochondrial genes, PGC-1beta preferentially induces certain genes involved in the removal of reactive oxygen species, recently recognized as activators of uncoupling proteins. Together, these data indicate that PGC-1alpha and PGC-1beta profoundly alter mitochondrial metabolism and suggest that these proteins are likely to play different physiological functions.

Brain mitochondria are primed by moderate Ca2+ rise upon hypoxia/reoxygenation for functional breakdown and morphological disintegration

In animal models, brain ischemia causes changes in respiratory capacity, mitochondrial morphology, and cytochrome c release from mitochondria as well as a rise in cytosolic Ca2+ concentration. However, the causal relationship of the cellular processes leading to mitochondrial deterioration in brain has not yet been clarified. Here, by applying various techniques, we used isolated rat brain mitochondria to investigate how hypoxia/reoxygenation and nonphysiological Ca2+ concentrations in the low micromolar range affect active (state 3) respiration, membrane permeability, swelling, and morphology of mitochondria. Either transient hypoxia or a micromolar rise in extramitochondrial Ca2+ concentration, given as a single insult alone, slightly decreased active respiration. However, the combination of both insults caused devastating effects. These implied almost complete loss of active respiration, release of both NADH and cytochrome c, and rupture of mitochondria, as shown by electron microscopy. Mitochondrial respiration deteriorated even in the presence of cyclosporin A, documenting that membrane permeabilization occurred independent of mitochondrial permeability transition pore. Ca2+ has to enter the mitochondrial matrix in order to mediate this mitochondrial injury, because blockade of the mitochondrial Ca2+-transport system by ruthenium red in combination with CGP37157 completely prevented damage. Furthermore, protection of respiration from Ca2+-mediated damage by the adenine nucleotide ADP, but not by AMP, during hypoxia/reoxygenation is consistent with the delayed susceptibility of brain mitochondria to prolonged hypoxia, which is observed in vivo.

Mitochondrial functional state in clonal pancreatic beta-cells exposed to free fatty acids

Excessive free fatty acid (FFA) exposure represents a potentially important diabetogenic condition that can impair insulin secretion from pancreatic beta-cells. Because mitochondrial oxidative phosphorylation is a main link between glucose metabolism and insulin secretion, in the present work we investigated the effects of the FFA oleate (OE) on mitochondrial function in the clonal pancreatic beta-cell line, MIN6. Both the long term (72 h) and short term (immediately after application) impact of OE exposure on beta-cells was investigated. After 72 h of exposure to OE (0.4 mm, 0.5% bovine serum albumin) cells were washed and permeabilized, and mitochondrial function (respiration, phosphorylation, membrane potential formation, production of reactive oxygen species) was measured in the absence or presence of OE. MIN6 cells exposed to OE for 72 h showed impaired glucose-stimulated insulin secretion and decreased cellular ATP. Mitochondria in OE-exposed cells retained normal functional characteristics in FFA-free medium; however, they were significantly more sensitive to the acute uncoupling effect of OE treatment. The mitochondria of OE-exposed cells displayed increased depolarization caused by acute OE treatment, which is attributable to the elevation in the FFA-transporting function of uncoupling protein 2 and the dicarboxylate carrier. These cells also had an increased production of reactive oxygen species in complex I of the mitochondrial respiratory chain that could be activated by FFA. A high level of reduction of respiratory complex I augmented acute FFA-induced uncoupling in a way compatible with activation of mitochondrial uncoupling protein by intramitochondrial superoxide. A stronger augmentation was observed in OE-exposed cells. Together, these events may underlie FFA-induced depression of the ATP/ADP ratio in beta-cells, which accounts for the defective glucose-stimulated insulin secretion associated with lipotoxicity.

Reversible glutathionylation of complex I increases mitochondrial superoxide formation

Increased production of reactive oxygen species (ROS) by mitochondria is involved in oxidative damage to the organelle and in committing cells to apoptosis or senescence, but the mechanisms of this increase are unknown. Here we show that ROS production by mitochondrial complex I increases in response to oxidation of the mitochondrial glutathione pool. This correlates with thiols on the 51- and 75-kDa subunits of complex I forming mixed disulfides with glutathione. Glutathionylation of complex I increases superoxide production by the complex, and when the mixed disulfides are reduced, superoxide production returns to basal levels. Within intact mitochondria oxidation of the glutathione pool to glutathione disulfide also leads to glutathionylation of complex I, which correlates with increased superoxide formation. In this case, most of this superoxide is converted to hydrogen peroxide, which can then diffuse into the cytoplasm. This mechanism of reversible mitochondrial ROS production suggests how mitochondria might regulate redox signaling and shows how oxidation of the mitochondrial glutathione pool could contribute to the pathological changes that occur to mitochondria during oxidative stress.

Tissue-specific depression of mitochondrial proton leak and substrate oxidation in hibernating arctic ground squirrels

A significant proportion of standard metabolic rate is devoted to driving mitochondrial proton leak, and this futile cycle may be a site of metabolic control during hibernation. To determine if the proton leak pathway is decreased during metabolic depression related to hibernation, mitochondria were isolated from liver and skeletal muscle of nonhibernating (active) and hibernating arctic ground squirrels (Spermophilus parryii). At an assay temperature of 37 degrees C, state 3 and state 4 respiration rates and state 4 membrane potential were significantly depressed in liver mitochondria isolated from hibernators. In contrast, state 3 and state 4 respiration rates and membrane potentials were unchanged during hibernation in skeletal muscle mitochondria. The decrease in oxygen consumption of liver mitochondria was achieved by reduced activity of the set of reactions generating the proton gradient but not by a lowered proton permeability. These results suggest that mitochondrial proton conductance is unchanged during hibernation and that the reduced metabolism in hibernators is a partial consequence of tissue-specific depression of substrate oxidation.

Bioenergetics of the heart at high altitude: environmental hypoxia imposes profound transformations on the myocardial process of ATP synthesis

The low concentration of O2 in the thin air at high altitude is undoubtedly the reason for the remarkable modifications in the structure and function of the heart, lung, and blood of humans permanently living under these conditions. The effect of natural hypoxia on the energy metabolism of the cell is however not well understood. Here we study the proces of ATP synthesis in the heart of guinea pigs native to high altitude (4500 m) as compared with those native to sea level. The following are the novel findings of this study. (1) The rates and extents of ATP synthesis in the presence of low concentrations of ADP (<30 microM) are significantly higher at high altitude than at sea level. (2) The Hill coefficient, i.e. the degree of cooperativity between the three catalytic sites of the ATP synthase, is lower at high altitude (n = 1.36) than at sea level (n = 1.94). (3) Both, the affinity for ADP and the fractional occupancy of the catalytic sites by ATP, are higher at high altitude than at sea level but the P50, i.e. the concentration of ADP at which 50% of the catalytic sites are filled with ADP and/or ATP, is the same (approximately 74.7 microM). (4) In the physiological range of ADP concentrations, the phosphorylation potential deltaGp is significantly higher at high altitude than at sea level. It is concluded that the molecular mechanism of energy transduction is profoundly modified at high altitude in order to readily and efficiently generate ATP in the presence of low concentrations of O2 and ADP.

Cytochrome c oxidase: the mechanistic significance of structural H+ in energy transduction

Changes in the bulk-phase concentration of O2 and H+ associated with the reduction of O2 to water are simultaneously determined in reactions catalyzed by fully reduced cytochrome c oxidase both isolated and embedded in liposomes. Consistent with the polyphasic kinetics of electron transfer through the oxidase, the time course of O2 consumption and H+ translocation exhibit the following novel characteristics: (1) The uptake of scalar protons (Hm+), the ejection of vectorial protons (Hv+), and the consumption of O2, all proceed in a kinetically polyphasic process. (2) During thefirst phase of the reaction the rates of O2 uptake and H+ transfer are extremely fast and compatible with the rates of electron flow through the oxidase. (3) The Km of the oxidase for O2 is close to 75 microM, the same for O2 consumption and scalar H+ uptake. The Vmax of O2 reduction to water in reactions catalyzed by the isolated enzyme is, at least, 0.5 x 10(4) s(-1). (4) The extent of vectorial H+ ejection by cytochrome c oxidase embedded in liposomes is an exponential function dependent on both enzyme concentration and extent of O2 consumption. (5) The H+/O stoichiometry of H+ ejection is a variable that may reach a maximum value of 4.0 only when the enzyme undergoes net oxidation at extremely high enzyme/O2 molar ratios. It is postulated that the generation of useful energy at the level of cytochrome c oxidase depends not only on the number of molecules of O2 reduced to water but also on the extent and state of reduction and/or protonation of the enzyme.

Primary causes of decreased mitochondrial oxygen consumption during metabolic depression in snail cells

Cells isolated from the hepatopancreas of estivating snails (Helix aspersa) have strongly depressed mitochondrial respiration compared with controls. Mitochondrial respiration was divided into substrate oxidation (which produces the mitochondrial membrane potential) and ATP turnover and proton leak (which consume it). The activity of substrate oxidation (and probably ATP turnover) decreased, whereas the activity of proton leak remained constant in estivation. These primary changes resulted in a lower mitochondrial membrane potential in hepatopancreas cells from estivating compared with active snails, leading to secondary decreases in respiration to drive ATP turnover and proton leak. The respiration to drive ATP turnover and proton leak decreased in proportion to the overall decrease in mitochondrial respiration, so that the amount of ATP turned over per O2 consumed remained relatively constant and aerobic efficiency was maintained in this hypometabolic state. At least 75% of the total response of mitochondrial respiration to estivation was caused by primary changes in the kinetics of substrate oxidation, with only 25% or less of the response occurring through primary effects on ATP turnover.

The basal proton conductance of skeletal muscle mitochondria from transgenic mice overexpressing or lacking uncoupling protein-3

The ability of native uncoupling protein-3 (UCP3) to uncouple mitochondrial oxidative phosphorylation is controversial. We measured the expression level of UCP3 and the proton conductance of skeletal muscle mitochondria isolated from transgenic mice overexpressing human UCP3 (UCP3-tg) and from UCP3 knockout (UCP3-KO) mice. The concentration of UCP3 in UCP3-tg mitochondria was approximately 3 microg/mg protein, approximately 20-fold higher than the wild type value. UCP3-tg mitochondria had increased nonphosphorylating respiration rates, decreased respiratory control, and approximately 4-fold increased proton conductance compared with the wild type. However, this increased uncoupling in UCP3-tg mitochondria was not caused by native function of UCP3 because it was not proportional to the increase in UCP3 concentration and was neither activated by superoxide nor inhibited by GDP. UCP3 was undetectable in mitochondria from UCP3-KO mice. Nevertheless, UCP3-KO mitochondria had unchanged respiration rates, respiratory control ratios, and proton conductance compared with the wild type under a variety of assay conditions. We conclude that uncoupling in UCP3-tg mice is an artifact of transgenic expression, and that UCP3 does not catalyze the basal proton conductance of skeletal muscle mitochondria in the absence of activators such as superoxide.

An Escherichia coli cytotoxin increases superoxide anion generation via rac in epithelial cells

Cytotoxic Necrotizing Factor 1 (CNF1) is a protein toxin from Escherichia coli that induces the activation of Rho, Rac, and Cdc42 GTPases, all involved in actin reorganization. Rac plays a further role in oxidase function. In epithelial cells, CNF1 has been reported to induce a phagocytic-like behavior in terms of a ruffle-driven ingestion of large material. We herein show that CNF1-activated epithelial cells may exert additional cell responses typical of professional phagocytes following stimulation, i.e., an increase in oxygen consumption and the generation of superoxide anions. Such effects were triggered by the contact of latex beads with epithelial cells and were significantly augmented by CNF1-induced Rac activation. Altogether our data indicate that Rac, one of the targets of CNF1, plays a pivotal role in these phenomena, suggesting the involvement in epithelial cells of a Rac-dependent NADPH-oxidase complex similar to that employed by professional phagocytes.

Distinct Ca2+ thresholds determine cytochrome c release or permeability transition pore opening in brain mitochondria

In diseases associated with neuronal degeneration, such as Alzheimer's or cerebral ischemia, the cytosolic Ca2+ concentration ([Ca2+]cyt) is pathologically elevated. It is still unclear, however, under which conditions Ca2+ induces either apoptotic or necrotic neuronal cell death. Studying respiration and morphology of rat brain mitochondria, we found that extramitochondrial [Ca2+] above 1 M causes reversible release of cytochrome c, a key trigger of apoptosis. This event was NO-independent but required Ca2+ influx into the mitochondrial matrix. The mitochondrial permeability transition pore (PTP), widely thought to underlie cytochrome c release, was not involved. In contrast to noncerebral tissue, only relatively high [Ca2+] (is approximately equal to 200 M) opened PTP and ruptured mitochondria. Our findings might reflect a fundamental mechanism to protect postmitotic neuronal tissue against necrotic devastation and inflammation.

Processes contributing to metabolic depression in hepatopancreas cells from the snail Helix aspersa

Cells isolated from the hepatopancreas of the land snail Helix aspersa strongly depress respiration both immediately in response to lowered P(O2) (oxygen conformation) and, in the longer term, during aestivation. These phenomena were analysed by dividing cellular respiration into non-mitochondrial and mitochondrial respiration using the mitochondrial poisons myxothiazol, antimycin and azide. Non-mitochondrial respiration accounted for a surprisingly large proportion, 65+/−5 %, of cellular respiration in control cells at 70 % air saturation. Non-mitochondrial respiration decreased substantially as oxygen tension was lowered, but mitochondrial respiration did not, and the oxygen-conforming behaviour of the cells was due entirely to the oxygen-dependence of non-mitochondrial oxygen consumption. Non-mitochondrial respiration was still responsible for 45+/−2 % of cellular respiration at physiological oxygen tension. Mitochondrial respiration was further subdivided into respiration used to drive ATP turnover and respiration used to drive futile proton cycling across the mitochondrial inner membrane using the ATP synthase inhibitor oligomycin. At physiological oxygen tensions, 34+/−5 % of cellular respiration was used to drive ATP turnover and 22+/−4 % was used to drive proton cycling, echoing the metabolic inefficiency previously observed in liver cells from mammals, reptiles and amphibians. The respiration rate of hepatopancreas cells from aestivating snails was only 37 % of the control value. This was caused by proportional decreases in non-mitochondrial and mitochondrial respiration and in respiration to drive ATP turnover and to drive proton cycling. Thus, the fraction of cellular respiration devoted to different processes remained constant and the cellular energy balance was preserved in the hypometabolic state.

Impact of endotoxin on UCP homolog mRNA abundance, thermoregulation, and mitochondrial proton leak kinetics

Linking tissue uncoupling protein (UCP) homolog abundance with functional metabolic outcomes and with expression of putative genetic regulators promises to better clarify UCP homolog physiological function. A murine endotoxemia model characterized by marked alterations in thermoregulation was employed to examine the association between heat production, UCP homolog expression, and mitochondrial proton leak ("uncoupling"). After intraperitoneal lipopolysaccharide (LPS, approximately 6 mg/kg) injection, colonic temperature (T(c)) in adult female C57BL6/J mice dropped to a nadir of approximately 30 degrees C by 8 h, preceded by a four- to fivefold drop in liver UCP2 and UCP5/brain mitochondrial carrier protein 1 mRNA levels, with no change in their hindlimb skeletal muscle (SKM) expression. SKM UCP3 mRNA rose fivefold during development of hypothermia and was correlated with an LPS-induced increase in plasma free fatty acid concentration. UCP2 and UCP5 transcripts recovered about three- to sixfold in both tissues starting at 6-8 h, preceding a recovery of T(c) between 16 and 24 h. SKM UCP3 followed an opposite pattern. Such results are not consistent with an important influence of UCP3 in driving heat production but do not preclude a role for UCP2 or UCP5 in this process. The transcription coactivator PGC-1 displayed a transient LPS-evoked rise (threefold) or drop (two- to fivefold) in SKM and liver expression, respectively. No differences between control and LPS-treated mouse liver or SKM in vitro mitochondrial proton leak were evident at time points corresponding to large differences in UCP homolog expression.

The synthesis and antibacterial activity of totarol derivatives. Part 3: Modification of ring-B

Ring-B derivatization of totarol (1) afforded the series of compounds 2-22 which were screened in vitro against: beta-lactamase-positive and high level gentamycin-resistant Enterococcus faecalis, penicillin-resistant Streptococcus pneumoniae, methicillin-resistant Staphylococcus aureus (MRSA), and multiresistant Klebsiella pneumoniae. Several of the derivatives retained much of the antibacterial activity of totatol against the first three of these organisms (all gram-positive), but none was more active. The gram-negative Klebsiella was resistant to all compounds examined. Totarol (1) was shown to uncouple oxidative phosphorylation in isolated mitochondria at 50 microM.

Intrinsic metabolic depression in cells isolated from the hepatopancreas of estivating snails

Many animals across the phylogenetic scale are routinely capable of depressing their metabolic rate to 5-15% of that at rest, remaining in this state sometimes for years. However, despite its widespread occurrence, the biochemical processes associated with metabolic depression remain obscure. We demonstrate here the development of an isolated cell model for the study of metabolic depression. The isolated cells from the hepatopancreas (digestive gland) of the land snail (Helix aspersa) are oxygen conformers; i.e., their rate of respiration depends on pO(2). Cells isolated from estivating snails show a stable metabolic depression to 30% of control (despite the long and invasive process of cell isolation) when metabolic rate at the physiological pH and pO(2) of the hemolymph of estivating snails is compared with metabolic rate at the physiological pH and pO(2) of the hemolymph of control snails. When the extrinsic effects of pH and pO(2) are excluded, the intrinsic metabolic depression of the cells from estivating snails is still to below 50% of control snails. The in vitro effect of pO(2) on metabolic rate is independent of pH and state (awake or estivating), but the effects of pH and state significantly interact. This suggests that pH and state change affect metabolic depression by similar mechanisms but that the metabolic depression by hypoxia involves a separate mechanism.

Calcium regulation of oxidative phosphorylation in rat skeletal muscle mitochondria

Activation of oxidative phosphorylation by physiological levels of calcium in mitochondria from rat skeletal muscle was analysed using top-down elasticity and regulation analysis. Oxidative phosphorylation was conceptually divided into three subsystems (substrate oxidation, proton leak and phosphorylation) connected by the membrane potential or the protonmotive force. Calcium directly activated the phosphorylation subsystem and (with sub-saturating 2-oxoglutarate) the substrate oxidation subsystem but had no effect on the proton leak kinetics. The response of mitochondria respiring on 2-oxoglutarate at two physiological concentrations of free calcium was quantified using control and regulation analysis. The partial integrated response coefficients showed that direct stimulation of substrate oxidation contributed 86% of the effect of calcium on state 3 oxygen consumption, and direct activation of the phosphorylation reactions caused 37% of the increase in phosphorylation flux. Calcium directly activated phosphorylation more strongly than substrate oxidation (78% compared to 45%) to achieve homeostasis of mitochondrial membrane potential during large increases in flux.

Quantitative investigation of mitochondrial function in single rat hippocampal slices: a novel application of high-resolution respirometry and laser-excited fluorescence spectroscopy

Highly sensitive techniques are needed for the quantitative determination of mitochondrial oxidative phosphorylation function in single rat hippocampal slices or isolated hippocampal subfields. We determined the oxygen consumption of single hippocampal slices or subfields applying high-resolution respirometry adapted for slice measurements and measured the redox state of mitochondrial NAD(P)H in single hippocampal slices by laser-excited fluorimetry. These methods allow the sensitive detection of two parameters of mitochondrial oxidative phosphorylation which depend on supply of substrates and respiratory chain function.

Aerotaxis and other energy-sensing behavior in bacteria

Energy taxis is widespread in motile bacteria and in some species is the only known behavioral response. The bacteria monitor their cellular energy levels and respond to a decrease in energy by swimming to a microenvironment that reenergizes the cells. This is in contrast to classical Escherichia coli chemotaxis in which sensing of stimuli is independent of cellular metabolism. Energy taxis encompasses aerotaxis (taxis to oxygen), phototaxis, redox taxis, taxis to alternative electron acceptors, and chemotaxis to a carbon source. All of these responses share a common signal transduction pathway. An environmental stimulus, such as oxygen concentration or light intensity, modulates the flow of reducing equivalents through the electron transport system. A transducer senses the change in electron transport, or possibly a related parameter such as proton motive force, and initiates a signal that alters the direction of swimming. The Aer and Tsr proteins in E. coli are newly recognized transducers for energy taxis. Aer is homologous to E. coli chemoreceptors but unique in having a PAS domain and a flavin-adenine dinucleotide cofactor that is postulated to interact with a component of the electron transport system. PAS domains are energy-sensing modules that are found in proteins from archaea to humans. Tsr, the serine chemoreceptor, is an independent transducer for energy taxis, but its sensory mechanism is unknown. Energy taxis has a significant ecological role in vertical stratification of microorganisms in microbial mats and water columns. It plays a central role in the behavior of magnetotactic bacteria and also appears to be important in plant-microbe interactions.

Effect of local anesthetic ropivacaine on isolated rat liver mitochondria

Ropivacaine is a new long-acting aminoamide local anesthetic with a reduced systemic and cardiac toxicity. Since the latter seems to be related, at least partially, to an interference with mitochondrial energy transduction, the effect of ropivacaine on the metabolism of rat liver mitochondria was studied. Ropivacaine alone exhibited little effect on mitochondrial metabolism, whereas effects were strongly enhanced by tetraphenylboron (TPB-) anion. At low drug concentrations, state 4 respiration was stimulated and mitochondrial membrane potential collapsed. At higher concentrations, state 4 and uncoupled respiration were inhibited by impairment of electron transfer from NAD- and flavine adenine dinucleotide-linked substrates to the respiratory chain. The fact that TPB- increased drug effects indicated that stimulation of respiration was due to dissipation of the electrochemical proton gradient caused by its electrophoretic uptake, although a classical uncoupling mechanism cannot be excluded. The mechanism for the lower toxicity of ropivacaine in vivo was ascribed to low liposolubility leading to reduced access to the mitochondrial membrane, resulting in a minimal perturbation of mitochondrial metabolism.

Effect of adenine nucleotide pool size in mitochondria on intramitochondrial ATP levels

Net adenine nucleotide transport into and out of the mitochondrial matrix via the ATP-Mg/Pi carrier is activated by micromolar calcium concentrations in rat liver mitochondria. The purpose of this study was to induce net adenine nucleotide transport by varying the substrate supply and/or extramitochondrial ATP consumption in order to evaluate the effect of the mitochondrial adenine nucleotide pool size on intramitochondrial adenine nucleotide patterns under phosphorylating conditions. Above 12 nmol/mg protein, intramitochondrial ATP/ADP increased with an increase in the mitochondrial adenine nucleotide pool. The relationship between the rate of respiration and the mitochondrial ADP concentration did not depend on the mitochondrial adenine nucleotide pool size up to 9 nmol ADP/mg mitochondrial protein. The results are compatible with the notion that net uptake of adenine nucleotides at low energy states supports intramitochondrial ATP consuming processes and energized mitochondria may lose adenine nucleotides. The decrease of the mitochondrial adenine nucleotide content below 9 nmol/mg protein inhibits oxidative phosphorylation. In particular, this could be the case within the postischemic phase which is characterized by low cytosolic adenine nucleotide concentrations and energized mitochondria.

Top-down control analysis of ATP turnover, glycolysis and oxidative phosphorylation in rat hepatocytes

Control analysis was used to analyse the internal control of rat hepatocyte metabolism. The reactions of the cell were grouped into nine metabolic blocks linked by five key intermediates. The blocks were glycogen breakdown, glucose release, glycolysis, lactate production, NADH oxidation, pyruvate oxidation, mitochondrial proton leak, mitochondrial phosphorylation and ATP consumption. The linking intermediates were intracellular glucose-6-phosphate, pyruvate and ATP levels, cytoplasmic NADH/NAD ratio and mitochondrial membrane potential. The steady-state fluxes through the blocks and the levels of the intermediates were measured in the absence and presence of specific effectors of hepatocyte metabolism. Application of the multiple modulation approach gave the kinetic responses of each block to each intermediate (the elasticities). These were then used to calculate all of the control coefficients, which describe the degree of control each block had over the level of each intermediate, and over the rate of each process. Within this full description of control, many different interactions could be identified. One key finding was that the processes that consumed ATP had only 35% of the control over the rate of ATP consumption. Instead, the reactions that produced ATP exerted the most control over ATP consumption rate; particularly important were mitochondrial phosphorylation (30% of control) and glycolysis (19%). The rate of glycolysis was positively controlled by the glycolytic enzymes themselves (66% of control) and by ATP consumption (47%). Mitochondrial production of ATP, including oxidative, proton leak and phosphorylation processes, had negative control over glycolysis (-26%; the Pasteur effect). In contrast, glycolysis had little control over the rate of ATP production by the mitochondria (-10%; the Crabtree effect). Control over the flux through the mitochondrial phosphorylation block was shared between pyruvate oxidation (23%), ATP consumption (28%) and the mitochondrial phosphorylation block itself (64%).

Induction of permeability transition in pancreatic mitochondria by cerulein in rats

Hyperstimulation with cholecystokinin analogue cerulein induces a mild edematous pancreatitis in rats. There is evidence for a diminished energy metabolism of acinar cells in this experimental model. The aim of this study was to demonstrate permeability transition of the mitochondrial inner membrane as an early change in mitochondrial function and morphology. As functional parameters, the respiration and membrane potential of mitochondria isolated from control and cerulein-treated animals were measured, and changes in volume and morphology were investigated by swelling experiments and electron microscopy. Five hours after the first injection of cerulein, the leak respiration was nearly doubled and the resting membrane potential was decreased by about 17 mV. These alterations were reversed by extramitochondrial ADP or did not occur when cyclosporin A was added to the mitochondrial incubation. A considerable portion of the mitochondria isolated from cerulein-treated animals was swollen and showed dramatic changes in morphology such as a wrinkled outer membrane and the loss of a distinct cristae structure. These data provide evidence for the opening of the mitochondrial permeability transition pore at an early stage of cerulein induced pancreatitis. This suggests that the permeability transition is an initiating event for lysis of individual mitochondria and the initiation of apoptosis and/or necrosis, as had been shown to occur in this experimental model.

Interaction of nitric oxide donors and ascorbic acid on D-[3H] aspartate efflux from rat striatal slices

There are conflicting reports in the literature concerning the neuroprotective effect of ascorbic acid on excitotoxic processes in which excessive glutamate release and nitric oxide are supposed to be major factors. To study the influence of ascorbate on the nitric oxide modulated glutamate release rat striatal slices, preloaded with the tritiated glutamate analog D-aspartate, were used. The high potassium-induced efflux of D-[3H]aspartate was concentration dependently stimulated by the nitric oxide donors sodium nitroprusside, S-nitroso-N-acetylpenicillamine (SNAP) or 5-amino 3-morpholinyl-1,2,3-oxadiazolium chloride (SIN-1), as well as by solutions of gaseous nitric oxide and, interestingly, by cyanide. Only the stimulation of D-[3H]aspartate release by SNAP and nitroprusside was affected by ascorbate in terms of a highly significant potentiation. Ascorbate was shown to exert its effect primarily by influencing the decomposition of these nitric oxide donors rather than by a direct interaction of ascorbate with nitric monoxide on glutamate release.

A perifusion system to control oxygen concentration in cell suspensions

A system is described for the perifusion of cells with a perifusate containing oxygen at concentrations defined by the user. The metabolic competency of cells in this system was assessed by measuring total ATP turnover for human platelets. Human platelets, which are small (radius of 1-2 micron) anucleate cells involved in blood clotting, maintained ATP turnover over a 4-h period at 37 degrees C. Oxygen concentration in this system was controlled by adjusting the proportions of air-saturated and nitrogen-saturated perifusates. Examples of oxygen consumption and lactate output as a function of oxygen concentration are described.

Measurement of volumetric (OUR) and determination of specific (qO2) oxygen uptake rates in animal cell cultures

Oxygen is a key substrate in animal cell metabolism. It has been reported that the oxygen uptake rate (OUR) is a good indicator of cellular activity, and even under some conditions, a good indicator of the number of viable cells. The measurement of OUR is difficult due to many different reasons. In particular, the very low specific consumption rate (0.2 x 10(-12) mol cell h-1), the sensitivity of the cells to variations in dissolved oxygen concentration and the difficulty to provide oxygen without damaging the cells are problems which must be taken into account for the development of OUR measurement methods. Different solutions based on an oxygen balance on either the liquid phase or around the entire reactor, and with a variable or stable concentration of dissolved oxygen have been reported. The accuracy of the OUR measurements and the required analytical devices are very different from method to method.

Two NADH:ubiquinone oxidoreductases of Azotobacter vinelandii and their role in the respiratory protection

Initial steps of the Azotobacter vinelandii respiratory chain have been studied on the inside-out subcellular vesicles. Two NADH:ubiquinone oxidoreductases were revealed: (i) proton-motive, capsaicin-sensitive and oxidizing dNADH as well as NADH enzyme and (ii) enzyme non-coupled to the energy conservation, capsaicin-resistant and oxidizing only NADH. The level of the oxidoreductases strongly depends upon [O2] and [NH3] in the growth medium. Increase in [O2] results in lowering of the coupled-enzyme level and in rise of the non-coupled one. Exclusion of NH3 from the growth medium increases the level of the non-coupled enzyme whereas that of the coupled enzyme remains constant. The O2-linked control of NADH:ubiquinone oxidoreductases requires CydR, a Fnr-like regulatory protein. Summarizing the above observations with those made in this group on the terminal steps of the A. vinelandii respiratory chains, one can assume that the respiratory protection of nitrogenase could be carried out by co-operation of the non-coupled NADH:ubiquinone oxidoreductase and the "partially coupled" quinoloxidase of the bd-type. Efficiency of this chain seems to be five-fold lower than that of the usual proton-motive chain (the coupled NADH:ubiquinone oxidoreductase, the Q-cycle and cytochrome oxidase of the o-type) which is also present in A. vinelandii and operates at low [O2].

Alpha- and beta- alkyl-substituted eicosapentaenoic acids: incorporation into phospholipids and effects on prostaglandin H synthase and 5-lipoxygenase

Alpha-ethyl-, alpha-methyl- and beta-methyl eicosapentaenoic acid (EPA) were prepared and their incorporation into cell lipids and effects on eicosanoid synthesis compared with EPA and docosahexaenoic acid (DHA). alpha- and beta-methyl EPA were incorporated into hepatocyte triacylglycerols as efficiently as EPA, whereas lesser amounts were found in phospholipids. alpha-ethyl EPA was not incorporated into phospholipids but small amounts were detected in triacylglycerol. All derivatives inhibited the synthesis of arachidonic acid, although less efficiently than EPA and DHA. The derivatives were poor substrates of prostaglandin H (PGH) synthase and 5-lipoxygenase, and they all inactivated PGH synthase. In isolated platelets, alpha-methyl EPA was a stronger inhibitor of TxB2 production than EPA, alpha-ethyl- and beta-methyl EPA. All derivatives were stronger inducers of peroxisomal beta-oxidation than EPA and DHA. This increased induction probably is a consequence of the blocked mitochondrial beta-oxidation of the derivatives.

Characterization of mitochondrial electron-transfer in Leishmania mexicana

Some general features of the respiratory chain and respiratory control were characterized in coupled mitochondrial preparations from Leishmania mexicana promastigotes. O2 uptake was sensitive to the electron-transfer inhibitors rotenone, flavone, malonate, 4,4,4-trifluoro-1-(2-thienyl) 1.3 butanedione (TTFA), antimycin A, 2n-nonyl-4-hydroxyquinoline-N-oxide (HQNO), myxothiazol, cyanide and azide. A high concentration of rotenone (60 microM) was required to inhibit O2 uptake effectively. Difference spectra revealed the presence of cytochromes (a + a3), b and c. Respiratory control was stimulated 2-fold by ADP with different exogenous oxidizable substrates. Calculated ADP/O ratios were consistent with the notion that ascorbate/N,N,N',N'-tetramethylphenylenediamine (TMPD)-linked and FAD-linked respiration proceeds, respectively, with one third and two thirds of the ATP producing capacity of NADH-linked respiration. State 3 was suppressed by the ATP synthase inhibitors oligomycin and aurovertin and by the adenine nucleotide translocator inhibitors atractyloside and carboxy atractyloside. The protonophore carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP) provoked state 3u respiration. The mitochondrial preparation was capable of Ca2+ uptake and Ca2+ stimulated respiration. Data obtained suggests strongly that mitochondrial complexes I, II, III and IV are present in a major pathway of electron-transfer and that oxidative phosphorylation might proceed with high bioenergetic efficiency.

Oxygen dependence of mitochondrial function measured by high-resolution respirometry in long-term hypoxic rats

Respiration and oxidative phosphorylation were investigated in tightly coupled mitochondria isolated from liver and heart of rats submitted to a simulated altitude of 4,400 m for 14-15 mo and their corresponding controls at sea level. High-resolution respirometry was utilized to determine the apparent Michaelis-Menten constant for ADP and O2 (K(m)-ADP and K(m)-O2, respectively), the latter under active and resting states of mitochondrial respiration. The K(m)-O2 in mitochondria isolated from normoxic rats was higher for active (state 3) than for resting (state 4) respiration; the values decreased from 1.5 and 1.7 to 0.25 and 0.30 microM in heart and liver mitochondria, respectively. The K(m)-O2 values found in the active state suggest a role for the normally occurring intracellular PO2 range reported in the literature in the regulation of cellular respiration. No changes were found in the ADP or O2 dependence of respiration in the mitochondria isolated from long-term acclimatized rats compared with their controls, indicating that the intrinsic properties and the efficiency of mitochondria do not change as a consequence of adaptation to hypoxia.

Liver energy metabolism during hibernation in the golden-mantled ground squirrel,Spermophilus lateralis

Large changes in ATP production capacities and rates have been reported in mammalian hibernators throughout the different stages of the hibernation cycle. In this study we showed that total extractable liver [ATP], [ADP], and [ATP]/[ADP] do not differ among summer normothermic, hibernating, and aroused golden-mantled ground squirrels, Spermophilus lateralis, indicating that metabolism remains well balanced throughout the hibernation cycle. This implies that rates of ATP consumption must be down-regulated during deep hibernation in order to maintain this balance. Despite this, basal oxygen-consumption rates [Formula: see text] of hepatocytes isolated from hibernating, aroused, and summer cold-acclimated ground squirrels were 22.4–35.1% higher than those from ground squirrels in the summer normothermic condition when measured at 37 °C. The relatively high hepatocyte [Formula: see text] may help to minimize interbout arousal times, reducing energy demands during the hibernation season. At 7 °C, hepatocyte [Formula: see text] values do not differ among the four groups; however, the Q10for hepatocyte [Formula: see text] is significantly lower for the summer group, suggesting lower temperature sensitivity. Despite the seasonal changes in thyroid hormone status known to occur in scuirid hibernators, the proportion of hepatocyte [Formula: see text] attributed to Na+,K+-ATPase, estimated by inhibition with 1 mM ouabain, is only around 15% and does not differ among hibernation/seasonal conditions.

Heneicosapentaenoate (21:5n-3): its incorporation into lipids and its effects on arachidonic acid and eicosanoid synthesis

6,9,12,15,18-Heneicosapentaenoic acid (21:5n-3) (HPA), present in small amounts in fish oils, has been prepared by chemical elongation of eicosapentaenoic acid (EPA) and its biological properties compared with EPA and docosahexaenoic acid (DHA). All the double bonds of HPA are displaced one carbon away from the carboxyl group when compared to EPA. HPA is incorporated into phospholipids and into triacylglycerol in cell culture to a similar extent as EPA and DHA. HPA is a stronger inhibitor of the conversion of alpha-linoleic acid and dihomo-gamma-linolenic acid to arachidonic acid (AA) in hepatoma cells than are EPA, DHA, and AA. HPA is a poor substrate for prostaglandin H synthase and for 5-lipoxygenase, but it inactivates prostaglandin H synthase as rapidly as do AA, EPA, and DHA. HPA inhibits thromboxane synthesis in isolated platelets as efficiently as EPA. EPA, HPA, and DHA are all weak inducers of acyl-CoA oxidase in hepatoma cells. Therefore, since fish oils contain only small amounts of HPA, it is unlikely that this fatty acid is of particular significance for the biological effects of these oils, possibly with the exception that it is a strong inhibitor of AA synthesis.

Rubisco: its role in photorespiration

The release of CO 2 during photosynthesis that is due to the production and metabolism of glycollic acid is usually regarded as outward evidence for the wasteful process of photorespiration in plants. In the light, glycollic acid is produced almost entirely as a result of the oxygenase activity of ribulose bisphosphate carboxylaseoxygenase (Rubisco). Metabolism of the glycollic acid not only releases recently assimilated carbon back into the atmosphere but also uses a considerable amount of energy to recycle remaining carbon from the glycollate to intermediates of the photosynthetic carbon reduction cycle. Furthermore, nitrogen from amino acids is released as ammonia during the metabolism of glycollate; some further energy is needed for this ammonia to be reassimilated. The oxygenation of ribulose bisphosphate is competitive with carboxylation and it appears to be the relative concentrations of oxygen and carbon dioxide present in cells containing the enzyme that mainly determine the relative rates of the two reactions in leaves. Systems which concentrate carbon dioxide in photosynthetic cells decrease the extent of photorespiration in C 4 species, certain algae and cyanobacteria. However, carboxylases from different species also vary considerably in their relative capacities to catalyse carboxylation and oxygenation of ribulose bisphosphate under standard conditions. This variation allows some hope that photorespiration might be decreased without recourse to energydependent systems for increasing cellular CO 2 concentrations.

Method for measuring a comprehensive energy budget in a proliferating cell system over multiple cell cycles

Isolated cell systems are now being used very effectively to study a range of important biochemical questions, but their energy metabolism has never been comprehensively investigated. We have developed a system, using J2E cells, which enables us to measure total ATP turnover and the contribution of various fuels and pathways to this total in a dynamic, proliferating preparation. Cells are cultured in 500 ml airtight glass containers which enables (1) the measurement of oxygen consumption, (2) the collection and measurement of 14CO2 production from labelled fuels, and (3) the measurement of metabolite utilization and production. Data on cell numbers are then used to produce a curve of cell number vs. time, the area under which (cell numbers x hour) is used as a base by which all measurements and experiments are compared. To our knowledge this is the first time a comprehensive energy budget has been measured in a proliferating cell system over a period that covers multiple cell cycles.

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.

Energy metabolism, enzymatic flux capacities, and metabolic flux rates in flying honeybees

Honeybees rely primarily on the oxidation of hexose sugars to provide the energy required for flight. Measurement of VCO2 (equal to VO2, because VCO2/VO2 = 1.0 during carbohydrate oxidation) during flight allowed estimation of steady-state flux rates through pathways of flight muscle energy metabolism. Comparison of Vmax values for flight muscle hexokinase, phosphofructokinase, citrate synthase, and cytochrome c oxidase with rates of carbon and O2 flux during flight reveal that these enzymes operate closer to Vmax in the flight muscles of flying honeybees than in other muscles previously studied. Possible mechanistic and evolutionary implications of these findings are discussed.

Altered mitochondrial function in fibroblasts containing MELAS or MERRF mitochondrial DNA mutations

A number of human diseases are caused by inherited mitochondrial DNA mutations. Two of these diseases, MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes) and MERRF (myoclonic epilepsy and ragged-red fibres), are commonly caused by point mutations to tRNA genes encoded by mitochondrial DNA. Here we report on how these mutations affect mitochondrial function in primary fibroblast cultures established from a MELAS patient containing an A to G mutation at nucleotide 3243 in the tRNA(Leu(UUR) gene and a MERRF patient containing an A to G mutation at nucleotide 8344 in the tRNA(Lys) gene. Both mitochondrial membrane potential and respiration rate were significantly decreased in digitonin-permeabilized MELAS and MERRF fibroblasts respiring on glutamate/malate. A similar decrease in mitochondrial membrane potential was found in intact MELAS and MERRF fibroblasts. The mitochondrial content of these cells, estimated by stereological analysis of electron micrographs and from measurement of mitochondrial marker enzymes, was similar in control, MELAS and MERRF cells. Therefore, in cultured fibroblasts, mutation of mitochondrial tRNA genes leads to assembly of bioenergetically incompetent mitochondria, not to an alteration in their amount. However, the cell volume occupied by secondary lysosomes and residual bodies in the MELAS and MERRF cells was greater than in control cells, suggesting increased mitochondrial degradation in these cells. In addition, fibroblasts containing mitochondrial DNA mutations were 3-4-fold larger than control fibroblasts. The implications of these findings for the pathology of mitochondrial diseases are discussed.