Mitochondrial quinone redox states as a marker of mitochondrial metabolism

Mitochondrial and thus cellular energetics are highly regulated both thermodynamically and kinetically. Cellular energetics is of prime importance in the regulation of cellular functions since it provides ATP for their accomplishment. However, cellular energetics is not only about ATP production but also about the ability to re-oxidize reduced coenzymes at a proper rate, such that the cellular redox potential remains at a level compatible with enzymatic reactions. However, this parameter is not only difficult to assess due to its dual compartmentation (mitochondrial and cytosolic) but also because it is well known that most NADH in the cells is bound to the enzymes. In this paper, we investigated the potential relevance of mitochondrial quinones redox state as a marker of mitochondrial metabolism and more particularly mitochondrial redox state. We were able to show that Q2 is an appropriate redox mediator to assess the mitochondrial quinone redox states. On isolated mitochondria, the mitochondrial quinone redox states depend on the mitochondrial substrate and the mitochondrial energetic state (phosphorylating or not phosphorylating). Last but not least, we show that the quinones redox state response allows to better understand the Krebs cycle functioning and respiratory substrates oxidation. Taken together, our results suggest that the quinones redox state is an excellent marker of mitochondrial metabolism.

The Warburg effect and mitochondrial oxidative phosphorylation: Friends or foes?

Cancer cells display an altered energy metabolism, which was proposed to be the root of cancer. This early discovery was done by O. Warburg who conducted one of the first studies of tumor cell energy metabolism. Taking advantage of cancer cells that exhibited various growth rates, he showed that cancer cells display a decreased respiration and an increased glycolysis proportional to the increase in their growth rate, suggesting that they mainly depend on fermentative metabolism for ATP generation. Warburg's results and hypothesis generated controversies that are persistent to this day. It is thus of great importance to understand the mechanisms by which cancer cells can reversibly regulate the two pathways of their energy metabolism as well as the functioning of this metabolism in cell proliferation. In this review, we discuss of the origin of the decrease in cell respiratory rate, whether the Warburg effect is mandatory for an increased cell proliferation rate, the consequences of this effect on two major players of cell energy metabolism that are ATP and NADH, and the role of the microenvironment in the regulation of cellular respiration and metabolism both in cancer cell and in yeast.

Cell energy metabolism: An update

In living cells, growth is the result of coupling between substrate catabolism and multiple metabolic processes that take place during net biomass formation and maintenance processes. During growth, both ATP/ADP and NADH/NAD⁺ molecules play a key role. Cell energy metabolism hence refers to metabolic pathways involved in ATP synthesis linked to NADH turnover. Two main pathways are thus involved in cell energy metabolism: glycolysis/fermentation and oxidative phosphorylation. Glycolysis and mitochondrial oxidative phosphorylation are intertwined through thermodynamic and kinetic constraints that are reviewed herein. Further, our current knowledge of short-term and long term regulation of cell energy metabolism will be reviewed using examples such as the Crabtree and the Warburg effect.

The ROS production induced by a reverse-electron flux at respiratory-chain complex 1 is hampered by metformin

Mitochondrial reactive oxygen species (ROS) production was investigated in mitochondria extracted from liver of rats treated with or without metformin, a mild inhibitor of respiratory chain complex 1 used in type 2 diabetes. A high rate of ROS production, fully suppressed by rotenone, was evidenced in non-phosphorylating mitochondria in the presence of succinate as a single complex 2 substrate. This ROS production was substantially lowered by metformin pretreatment and by any decrease in membrane potential (Delta Phi(m)), redox potential (NADH/NAD), or phosphate potential, as induced by malonate, 2,4-dinitrophenol, or ATP synthesis, respectively. ROS production in the presence of glutamate-malate plus succinate was lower than in the presence of succinate alone, but higher than in the presence of glutamate-malate. Moreover, while rotenone both increased and decreased ROS production at complex 1 depending on forward (glutamate-malate) or reverse (succinate) electron flux, no ROS overproduction was evidenced in the forward direction with metformin. Therefore, we propose that reverse electron flux through complex 1 is an alternative pathway, which leads to a specific metformin-sensitive ROS production.

Contribution of the phosphorylable complex I in the growth phase-dependent respiration of C6 glioma cells in vitro

The energy metabolism of rat C6 glioma cells was investigated as a function of the growth phases. Three-dimensional cultures of C6 cells exhibited diminished respiration and respiratory capacity during the early growth phase, before reaching confluence. This decrease in respiration was neither due to changes in the respiratory complex content nor in the mitochondrial mass per se. Nevertheless, a quantitative correlation was found between cellular respiration and the rotenone-sensitive NADH ubiquinone oxidoreductase (i.e. complex I) activity. Immunoblot analysis showed that phosphorylation of the 18 kDa-subunit of this complex was associated with the growth-phase dependent modulation of complex I and respiratory activity in C6 cells. In addition, by using forskolin or dibutyryl cAMP, short-term activation of protein kinases A of C6 cells correlated with increased phosphorylation of the 18-kDa subunit of complex I, activated NADH ubiquinone oxidoreductase activity and stimulated cellular respiration. These findings suggest that complex I of C6 glioma cells is a key regulating step that modulates the oxidative phosphorylation capacity during growth phase transitions.

Regulation of intracellular chloride concentration in rat lactotrope cells and its relation to the membrane resting potential

Rat lactotrope cells in primary culture exhibit physiological properties closely associated with chloride ions (Cl-) homeostasis. In this work, we studied the regulation of intracellular Cl- concentrations ([Cl-]i) and its relation to the membrane resting potential, using a combination of electrophysiology and spectrofluorimetry. Variations in [Cl-]i resulting from the patch clamp technique, pHi, antagonists of Cl(-)-Ca(2+)-dependent channels, an anion exchanger antagonist, and an antagonist of K(+)-Cl- cotransport were considered with respect to their involvement in membrane potential. We show that: (i) The patch-pipette does not always impose its Cl- concentration. (ii) In rat lactotrope cells, membrane resting potential is partially determined by [Cl-]i. (iii) Besides ion channel activity, electroneutral ion transports (cotransports such as K(+)-Cl- and Na(+)-K(+)-2Cl-) participate actively in maintaining a high [Cl-]i. (iv) Finally, Cl- homeostasis is probably linked to cell energetics.

Interactions between intracellular chloride concentrations, intracellular pH and energetic status in rat lactotrope cells in primary culture

Rat lactotrope cells in primary cultures have a higher intracellular Cl- concentration ([Cl-]i) than that predicted by a passive distribution across the membrane. This suggests that active cellular mechanisms ensure this ionic equilibrium. In this study, we examined the interactions between pHi, [Cl-]i regulation and cell energetics. We analyzed: 1. the interactions between extracellular Cl- concentrations, [Cl-]i and cellular energy; 2. the influence of [Cl-]i on respiratory chain function; 3. the correlation with glycolysis and; 4. the role played by pHi in these cellular mechanisms. We show that low [Cl-]i decreases ATP cell content, ATP/ADP ratio and modify phosphorylative oxidations. ATP production is rather due to the anaerobic pathway of the glucose metabolism than the aerobic one and depends also on other metabolic substrates among which glutamine probably has a special role. Finally, pHi appears as a determinant in the balance between aerobic and anaerobic pathways. These results are discussed in relation to the role of Cl- in normal and pathological (effect of hypoxia on mature and immature neurons) cell situations.

NADH is specifically channeled through the mitochondrial porin channel in Saccharomyces cerevisiae

In many kinds of permeabilized cells, the restriction of metabolite diffusion by a mitochondrial porin "closed state" has been shown to control the respiration rate. However, since in isolated mitochondria the porin appears to be always "open," the physiological relevance of a putative regulation via this channel status is now a subject of discussion. In Saccharomyces cerevisiae, in which some of the NADH dehydrogenase active sites are facing the intermembrane space, this regulatory mechanism might play an important role for the regulation of the cytosolic redox status. Using permeabilized spheroplasts from wild-type and porin-deficient mutant, we show that the NADH produced in the cytosol is channeled to the mitochondrial NADH dehydrogenases through a metabolic network involving the porin channel. Thus, the control exerted by the porin (i.e., "open" or "closed" state) seems to be determined through its participation or not in organized metabolic networks.

Mitochondrial respiratory chain adjustment to cellular energy demand

Because adaptation to physiological changes in cellular energy demand is a crucial imperative for life, mitochondrial oxidative phosphorylation is tightly controlled by ATP consumption. Nevertheless, the mechanisms permitting such large variations in ATP synthesis capacity, as well as the consequence on the overall efficiency of oxidative phosphorylation, are not known. By investigating several physiological models in vivo in rats (hyper- and hypothyroidism, polyunsaturated fatty acid deficiency, and chronic ethanol intoxication) we found that the increase in hepatocyte respiration (from 9.8 to 22.7 nmol of O(2)/min/mg dry cells) was tightly correlated with total mitochondrial cytochrome content, expressed both per mg dry cells or per mg mitochondrial protein. Moreover, this increase in total cytochrome content was accompanied by an increase in the respective proportion of cytochrome oxidase; while total cytochrome content increased 2-fold (from 0.341 +/- 0.021 to 0.821 +/- 0.024 nmol/mg protein), cytochrome oxidase increased 10-fold (from 0.020 +/- 0.002 to 0.224 +/- 0.006 nmol/mg protein). This modification was associated with a decrease in the overall efficiency of the respiratory chain. Since cytochrome oxidase is well recognized for slippage between redox reactions and proton pumping, we suggest that this dramatic increase in cytochrome oxidase is responsible for the decrease in the overall efficiency of respiratory chain and, in turn, of ATP synthesis yield, linked to the adaptive increase in oxidative phosphorylation capacity.

Aging and oxidative stress: studies of some genes involved both in aging and in response to oxidative stress

Aging is a complex physiological phenomenon and several theories have been developed about its origin. Among such theories, the 'mitochondrial theory of aging' has been supported by numerous studies and reviews. Cell oxidative damage, in particular the accumulation of mtDNA mutations, is determined by the rate of reactive oxygen species production and degradation induced by the antioxidant defense systems. In this review, data from our laboratory and from the recent literature have been examined to provide arguments that reinforce the crucial role of mitochondria in aging. Various genes that affect life span have been described in numerous organisms. Some of them encode signal transduction proteins and participate in the regulation of mitochondrial metabolism.

Participation of acetaldehyde dehydrogenases in ethanol and pyruvate metabolism of the yeast Saccharomyces cerevisiae

This work was undertaken to clarify the role of acetaldehyde dehydrogenases in Saccharomyces cerevisiae metabolism during growth on respiratory substrates. Until now, there has been little agreement concerning the ability of mutants deleted in gene ALD4, encoding mitochondrial acetaldehyde dehydrogenase, to grow on ethanol. Therefore we constructed mutants in two parental strains (YPH499 and W303-1a). Some differences appeared in the growth characteristics of mutants obtained from these two parental strains. For these experiments we used ethanol, pyruvate or lactate as substrates. Mitochondria can oxidize lactate into pyruvate using an ATP synthesis-coupled pathway. The ald4Delta mutant derived from the YPH499 strain failed to grow on ethanol, but growth was possible for the ald4Delta mutant derived from the W303-1a strain. The co-disruption of ALD4 and PDA1 (encoding subunit E1alpha of pyruvate dehydrogenase) prevented the growth on pyruvate for both strains but prevented growth on lactate only in the double mutant derived from the YPH499 strain, indicating that the mutation effects are strain-dependent. To understand these differences, we measured the enzyme content of these different strains. We found the following: (a) the activity of cytosolic acetaldehyde dehydrogenase in YPH499 was relatively low compared to the W303-1a strain; (b) it was possible to restore the growth of the mutant derived from YPH499 either by addition of acetate in the media or by introduction into this mutant of a multicopy plasmid carrying the ALD6 gene encoding cytosolic acetaldehyde dehydrogenase. Therefore, the lack of growth of the mutant derived from the YPH499 strain seemed to be related to the low activity of acetaldehyde oxidation. Therefore, when cultured on ethanol, the cytosolic acetaldehyde dehydrogenase can partially compensate for the lack of mitochondrial acetaldehyde dehydrogenase only when the activity of the cytosolic enzyme is sufficient. However, when cultured on pyruvate and in the absence of pyruvate dehydrogenase, the cytosolic acetaldehyde dehydrogenase cannot compensate for the lack of the mitochondrial enzyme because the mitochondrial form produces intramitochondrial NADH and consequently ATP through oxidative phosphorylation.

Protein oxidation in G0 cells of Saccharomyces cerevisiae depends on the state rather than rate of respiration and is enhanced in pos9 but not yap1 mutants

Immunodetection of protein carbonyl groups demonstrates that growth arrest elicited by carbon or nitrogen starvation causes an increased oxidation of proteins in Saccharomyces cerevisiae. Mutant analysis suggests that the response regulator Pos9p is involved in mitigating self-inflicted oxidative damages in G(0) cells, whereas Yap1p is primarily required in growing cells. The data also suggest that oxidation of target proteins is not a priori an effect of arrest of cell division or nutrient depletion and cannot be explained by the respiratory activity alone nor a high ratio of catabolic/anabolic activity in G(0) cells. Instead, we observed that starvation elicits a transition in the respiratory state (from phosphorylating to nonphosphorylating respiration) and that this transition is associated with a stepwise increase in protein oxidation. During carbon starvation, this transition and increase in oxidation occurs immediately as the carbon source is depleted, growth is arrested, and the respiratory rate falls drastically. In contrast, during nitrogen starvation and excess carbon the respiratory state transition and stepwise increase in protein oxidation are markedly delayed and occur long after the nitrogen source has been depleted and division and growth-arrested. Oxidation in G(0) cells could be enhanced by treating cells with low concentrations of antimycin A and attenuated with myxothiazol, indicating that protein oxidation is intimately linked to reactive oxygen species generated by semiquinones of the Q-cycle. Thus, the work presented suggests that the degree of coupling in the mitochondrial respiratory apparatus rather then the overall rate of respiration affects the degree of protein oxidation in nondividing yeast cells.

Regulation of cytochrome c oxidase by adenylic nucleotides. Is oxidative phosphorylation feedback regulated by its end-products?

Cytochrome c oxidase, which catalyzes an irreversible step of the respiratory chain, is one of the rate-controlling steps of oxidative phosphorylation on isolated mitochondria. The rate of electron transfer through the complex is primarily controlled by the associated thermodynamic forces, i.e., the span in redox potential between oxygen and cytochrome c and the protonmotive force. However, the electron flux also depends on the various kinetic effectors, including adenylic nucleotides. Although the number of binding sites for ATP and ADP on cytochrome oxidase is still a matter of debate, experiments performed on the solubilized and reconstituted enzyme provide strong functional evidence that the mammalian cytochrome c oxidase binds adenylic nucleotides on both sides of the inner membrane. These effects include modification in cytochrome c affinity, allosteric inhibition and changes in proton pumping efficiency. Immunological studies have pointed out the role of subunit IV and that of an ATP-binding protein, subunit VIa, in these kinetic regulations. In yeast, the role of the nuclear-encoded subunits in assembly and regulation of the cytochrome c oxidase has been further substantiated by using gene-disruption analysis. Using a subunit VIa-null mutant, the consequences of the ATP regulation on oxidative phosphorylation have been further investigated on isolated mitochondria. Taken together, the data demonstrate that there are multiple regulating sites for ATP on the yeast cytochrome oxidase with respect to the location (matrix versus cytosolic side), kinetic effect (activation versus inhibition) and consequence on the flow-force relationships. The question is therefore raised as to the physiological meaning of such feedback regulation of the respiratory chain by ATP in the control and regulation of cellular energy metabolism.

Mitochondrial ROS metabolism: modulation by uncoupling proteins

Most of the oxygen consumed by aerobic organisms is reduced to water by the enzyme cytochrome c oxidase in the terminal reaction of the mitochondrial respiratory chain. A significant proportion of the oxygen molecules are converted to superoxide anion radicals by complexes I and III via a nonenzymatic process. A cascade of enzymes, some of them inside the mitochondria themselves, scavenges superoxide anions in order to protect cells from oxidative damage induced by reactive oxygen species (ROS). Unfortunately, the quantification of the fluxes of mitochondrial ROS inside living cells is currently almost impossible, and this in turn limits our knowledge. Presently, the involvement of mitochondrial ROS can only be demonstrated by indirect strategies and among them knockout techniques are the most convincing. The yield of superoxide generation and subsequently ROS production depend mostly on oxygen concentration but can be efficiently modulated by mitochondrial uncoupling. This role could be assumed in part by one of the Uncoupling Proteins (UCPs). These proteins have coenzyme Q as an obligatory partner and we present here the hypothesis of UCPs as a crucial element of the respiratory chain. ROS have been mostly involved in degenerative processes including ageing. More recently, numerous studies point out the role of ROS as true intracellular second messengers. A putative role of mitochondrial ROS as the sensing element of energy metabolism is discussed here. We propose that UCPs could play a central role in modulation of ROS-dependent signalling pathways and metabolic sensing via the modulation of ROS generation.

Allosteric activation of pyruvate kinase via NAD+ in rat liver cells

In isolated rat hepatocytes, it has previously been reported that a rise in the ATP content induces a proportional increase in cytosolic NAD+ concentration [Devin, A., Guérin, B. & Rigoulet, M. (1997) FEBS Lett. 410, 329-332]. This occurs under physiological conditions such as various substrates or different energetic states. To investigate the effect of a physiological rise in cytosolic [NAD+] per se on glycolysis and gluconeogenesis, an increase in [NAD+] induced by exogenous nicotinamide addition was obtained without a change in redox potential, ATP/ADP ratio and ATP concentration. Using dihydroxyacetone as substrate, we found that an increase in cytosolic [NAD+] decreases gluconeogenesis and enhances glycolysis without significant alteration of dihydroxyacetone consumption rate. These modifications are the consequence of an allosteric activation of pyruvate kinase via cytosolic NAD+ content. Thus, in addition to the well-known thermodynamic control of glycolysis by pyridine-nucleotide redox status, our study points to a new mechanism of glycolytic flux regulation by NAD+ concentration at the level of pyruvate kinase activity.

Cytosolic redox metabolism in aerobic chemostat cultures of Saccharomyces cerevisiae

Cytosolic redox balance has to be maintained in order to allow an enduring cellular metabolism. In other words, NADH generated in the cytosol has to be re-oxidized back to NAD(+). Aerobically this can be done by respiratory oxidation of cytosolic NADH. However, NADH is unable to cross the mitochondrial inner membrane and mechanisms are required for conveying cytosolic NADH to the mitochondrial electron transport chain. At least two such systems have proved to be functional in S. cerevisiae, the external NADH dehydrogenase (Luttik et al., 1998; Small and McAlister-Henn, 1998) and the G3P shuttle (Larsson et al., 1998). The aim of this investigation was to study the regulation and performance of these two systems in a wild-type strain of S. cerevisiae using aerobic glucose- and nitrogen-limited chemostat cultures. The rate of cytosolic NADH formation was calculated and as expected there was a continuous increase with increasing dilution rate. However, measurements of enzyme activities and respiratory activity on isolated mitochondria revealed a diminishing capacity at elevated dilution rates for both the external NADH dehydrogenase and the G3P shuttle. This suggests that adjustment of in vivo activities of these systems to proper levels is not achieved by changes in amount of protein but rather by, for example, activation/inhibition of existing enzymes. Adenine nucleotides are well-known allosteric regulators and both the external NADH and the G3P shuttle were sensitive to inhibition by ATP. The most severe inhibition was probably on the G3P shuttle, since one of its member proteins, Gpdp, turned out to be exceptionally sensitive to ATP. The external NADH dehydrogenase is suggested as the main system employed for oxidation of cytosolic NADH. The G3P shuttle is proposed to be of some importance at low growth rates and perhaps its real significance is only expressed during starvation conditions.

The mitochondrial consequences of uncoupling intact cells depend on the nature of the exogenous substrate

In isolated mitochondria the consequences of oxidative phosphorylation uncoupling are well defined, whereas in intact cells various effects have been described. Uncoupling liver cells with 2,4-dinitrophenol (DNP) in the presence of dihydroxyacetone (DHA) and ethanol results in a marked decrease in mitochondrial transmembrane electrical potential (DeltaPsi), ATP/ADP ratios and gluconeogenesis (as an ATP-utilizing process), whereas the increased oxidation rate is limited and transient. Conversely, when DHA is associated with octanoate or proline, DNP addition results in a very large and sustained increase in oxidation rate, whereas the decreases in DeltaPsi, ATP/ADP ratios and gluconeogenesis are significantly less when compared with DHA and ethanol. Hence significant energy wastage (high oxidation rate) by uncoupling is achieved only with substrates that are directly oxidized in the mitochondrial matrix. Conversely in the presence of substrates that are first oxidized in the cytosol, uncoupling results in a profound decrease in mitochondrial DeltaPsi and ATP synthesis, whereas energy wastage is very limited.

Mitochondrial adaptation to in vivo polyunsaturated fatty acid deficiency: increase in phosphorylation efficiency

Polyunsaturated fatty acid (PUFA) deficiency affects respiratory rate both in isolated mitochondria and in hepatocytes, an effect that is normally ascribed to major changes in membrane composition causing, in turn, protonophoriclike effects. In this study, we have compared the properties of hepatocytes isolated from PUFA-deficient rats with those from control animals treated with concentrations of the protonophoric uncoupler 2,4-dinitrophenol (DNP). Despite identical respiratory rate and in situ mitochondrial membrane potential (delta psi), mitochondrial and cytosolic ATP/ADP-Pi ratios were significantly higher in PUFA-deficient cells than in control cells treated with DNP. We show that PUFA-deficient cells display an increase of phosphorylation efficiency, a higher mitochondrial ATP/ADP-Pi ratio being maintained despite the lower delta psi. This is achieved by (1) decreasing mitochondrial Pi accumulation, (2) increasing ATP synthase activity, and (3) by increasing the flux control coefficient of adenine nucleotide translocation. As a consequence, oxidative phosphorylation efficiency was only slightly affected in PUFA-deficient animals as compared to protonophoric uncoupling (DNP). Thus, the energy waste induced by PUFA deficiency on the processes that generate the proton motive force (pmf) is compensated in vivo by powerful adaptive mechanisms that act on the processes that use the pmf to synthesize ATP.

The calorimetric-respirometric ratio is an on-line marker of enthalpy efficiency of yeast cells growing on a non-fermentable carbon source

Although on-line calorimetry has been widely used to detect transitions in global metabolic activity during the growth of microorganisms, the relationships between oxygen consumption flux and heat production are poorly documented. In this work, we developed a respirometric and calorimetric approach to determine the enthalpy efficiency of respiration-linked energy transformation of isolated yeast mitochondria and yeast cells under growing and resting conditions. On isolated mitochondria, the analysis of different phosphorylating and non-phosphorylating steady states clearly showed that the simultaneous measurements of heat production and oxygen consumption rates can lead to the determination of both the enthalpy efficiency and the ATP/O yield of oxidative phosphorylation. However, these determinations were made possible only when the net enthalpy change associated with the phosphorylating system was different from zero. On whole yeast cells, it is shown that the simultaneous steady state measurements of the heat production and oxygen consumption rates allow the enthalpy growth efficiency (i.e. the amount of energy conserved as biomass compared to the energy utilised for complete catabolism plus anabolism) to be assessed. This method is based on the comparison between the calorimetric-respirometric ratio (CR ratio) determined under growth versus resting conditions during a purely aerobic metabolism. Therefore, in contrast to the enthalpy balance approach, this method does not rely on the exhaustive and tedious determinations of the metabolites and elemental composition of biomass. Thus, experiments can be performed in the presence of non-limiting amounts of carbon substrate, an approach which has been successfully applied to slow growing cells such as yeast cells expressing wild-type or a mutant rat uncoupling protein-1.

Reconstitution of ethanolic fermentation in permeabilized spheroplasts of wild-type and trehalose-6-phosphate synthase mutants of the yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, TPS1-encoded trehalose-6-phosphate synthase (TPS) exerts an essential control on the influx of glucose into glycolysis, presumably by restricting hexokinase activity. Deletion of TPS1 results in severe hyperaccumulation of sugar phosphates and near absence of ethanol formation. To investigate whether trehalose 6-phosphate (Tre6P) is the sole mediator of hexokinase inhibition, we have reconstituted ethanolic fermentation from glucose in permeabilized spheroplasts of the wild-type, tps1Delta and tps2Delta (Tre6P phosphatase) strains. For the tps1Delta strain, ethanol production was significantly lower and was associated with hyperaccumulation of Glu6P and Fru6P. A tps2Delta strain shows reduced accumulation of Glu6P and Fru6P both in intact cells and in permeabilized spheroplasts. These results are not consistent with Tre6P being the sole mediator of hexokinase inhibition. Reconstitution of ethanolic fermentation in permeabilized spheroplasts with glycolytic intermediates indicates additional target site(s) for the Tps1 control. Addition of Tre6P partially shifts the ethanol production rate and the metabolite pattern in permeabilized tps1Delta spheroplasts to those of the wild-type strain, but only with glucose as substrate. This is observed at a very high ratio of glucose to Tre6P. Inhibition of hexokinase activity by Tre6P is less efficiently counteracted by glucose in permeabilized spheroplasts compared to cell extracts, and this effect is largely abolished by deletion of TPS2 but not TPS1. In permeabilized spheroplasts, hexokinase activity is significantly lower in a tps2Delta strain compared to a wild-type strain and this difference is strongly reduced by additional deletion of TPS1. These results indicate that Tps1-mediated protein-protein interactions are important for control of glucose influx into yeast glycolysis, that Tre6P inhibition of hexokinase might not be competitive with respect to glucose in vivo and that also Tps2 appears to play a role in the control of hexokinase activity.

Chronic ethanol ingestion increases efficiency of oxidative phosphorylation in rat liver mitochondria

The efficiency of oxidative phosphorylation was compared between rats chronically fed with ethanol and controls. (i) Results showed that the liver mitochondria state 4 respiratory rate was strongly inhibited, while the corresponding proton-motive force was not affected; (ii) the cytochrome oxidase content and activity were decreased and (iii) the oxidative-phosphorylation yield was increased in the ethanol exposed group. Furthermore, oxidative phosphorylation at coupling site II was not affected by ethanol. Cytochrome oxidase inhibition by sodium-azide mimicked the effects of ethanol intoxication in control mitochondria. This indicates that the decrease in cytochrome oxidase activity induced by ethanol intoxication directly increases the efficiency of oxidative phosphorylation.

Growth of the yeast Saccharomyces cerevisiae on a non-fermentable substrate: control of energetic yield by the amount of mitochondria

The purpose of this study was to investigate the long-term control of ATP synthesis during the course of Saccharomyces cerevisiae batch grown on lactate, a purely respiratory substrate. For this, we used a respirometric and on-line calorimetric approach to analyse the energetic balances and the control of energetic metabolism during growth. Enthalpic growth yields assessed by enthalpy balance (taking account of substrate consumption, by-product accumulation, biomass formation and heat dissipation) remained constant during the entire exponential growth. Moreover, at the same time, a parallel decrease in basal respiratory rate and enthalpy flux occurred. It is shown that the decrease in respiration corresponds to a decrease in the amount of mitochondria per cell but not to a change of steady state of oxidative phosphorylation. Taking into account the part of energy used for maintenance, it can be concluded that mitochondria by themselves are the major heat dissipative system in a fully aerobic metabolism, and that the decrease in the amount of mitochondria when growth rate decreases leads to an enthalpic growth yield constant.

Dimethylbiguanide inhibits cell respiration via an indirect effect targeted on the respiratory chain complex I

We report here a new mitochondrial regulation occurring only in intact cells. We have investigated the effects of dimethylbiguanide on isolated rat hepatocytes, permeabilized hepatocytes, and isolated liver mitochondria. Addition of dimethylbiguanide decreased oxygen consumption and mitochondrial membrane potential only in intact cells but not in permeabilized hepatocytes or isolated mitochondria. Permeabilized hepatocytes after dimethylbiguanide exposure and mitochondria isolated from dimethylbiguanide pretreated livers or animals were characterized by a significant inhibition of oxygen consumption with complex I substrates (glutamate and malate) but not with complex II (succinate) or complex IV (N,N,N',N'-tetramethyl-1, 4-phenylenediamine dihydrochloride (TMPD)/ascorbate) substrates. Studies using functionally isolated complex I obtained from mitochondria isolated from dimethylbiguanide-pretreated livers or rats further confirmed that dimethylbiguanide action was located on the respiratory chain complex I. The dimethylbiguanide effect was temperature-dependent, oxygen consumption decreasing by 50, 20, and 0% at 37, 25, and 15 degrees C, respectively. This effect was not affected by insulin-signaling pathway inhibitors, nitric oxide precursor or inhibitors, oxygen radical scavengers, ceramide synthesis inhibitors, or chelation of intra- or extracellular Ca(2+). Because it is established that dimethylbiguanide is not metabolized, these results suggest the existence of a new cell-signaling pathway targeted to the respiratory chain complex I with a persistent effect after cessation of the signaling process.

A mitochondrial pyruvate dehydrogenase bypass in the yeast Saccharomyces cerevisiae

Spheroplasts of the yeast Saccharomyces cerevisiae oxidize pyruvate at a high respiratory rate, whereas isolated mitochondria do not unless malate is added. We show that a cytosolic factor, pyruvate decarboxylase, is required for the non-malate-dependent oxidation of pyruvate by mitochondria. In pyruvate decarboxylase-negative mutants, the oxidation of pyruvate by permeabilized spheroplasts was abolished. In contrast, deletion of the gene (PDA1) encoding the E1alpha subunit of the pyruvate dehydrogenase did not affect the spheroplast respiratory rate on pyruvate but abolished the malate-dependent respiration of isolated mitochondria. Mutants disrupted for the mitochondrial acetaldehyde dehydrogenase gene (ALD7) did not oxidize pyruvate unless malate was added. We therefore propose the existence of a mitochondrial pyruvate dehydrogenase bypass different from the cytosolic one, where pyruvate is decarboxylated to acetaldehyde in the cytosol by pyruvate decarboxylase and then oxidized by mitochondrial acetaldehyde dehydrogenase. This pathway can compensate PDA1 gene deletion for lactate or respiratory glucose growth. However, the codisruption of PDA1 and ALD7 genes prevented the growth on lactate, indicating that each of these pathways contributes to the oxidative metabolism of pyruvate.

ATP-regulation of cytochrome oxidase in yeast mitochondria: role of subunit VIa

The role of the nuclear-encoded subunit VIa in the regulation of cytochrome oxidase by ATP was investigated in isolated yeast mitochondria. As the subunit VIa-null strain possesses a fully active and assembled cytochrome oxidase, multiple ATP-regulating sites were characterized with respect to their location and their kinetic effect: (a) intra-mitochondrial ATP inhibited the complex IV activity of the null strain, whereas the prevailing effect of ATP on the wild-type strain, at low ionic strength, was activation on the cytosolic side of complex IV, mediated by subunit VIa. However, at physiological ionic strength (i.e. approximately 200 mM), activation by ATP was absent but inhibition was not impaired; (b) in ethanol-respiring mitochondria, when the electron flux was modulated using a protonophoric uncoupler, the redox state of aa3 cytochromes varied with respect to activation (wild-type) or inhibition (null-mutant) of the cytochrome oxidase by ATP; (c) consequently, the control coefficient of cytochrome oxidase on respiratory flux, decreased (wild-type) or increased (null-mutant) in the presence of ATP; (d) considering electron transport from cytochrome c to oxygen, the response of cytochrome oxidase to its thermodynamic driving force was increased by ATP for the wild-type but not for the mutant subunit. Taken together, these findings indicate that at physiological concentration, ATP regulates yeast cytochrome oxidase via subunit-mediated interactions on both sides of the inner membrane, thus subtly tuning the thermodynamic and kinetic control of respiration. This study opens up new prospects for understanding the feedback regulation of the respiratory chain by ATP.

Activation and deactivation of F0F1-ATPase in yeast mitochondria

The regulation of membrane-bound proton F0F1 ATPase by the protonmotive force and nucleotides was studied in yeast mitochondria. Activation occurred in whole mitochondria and the ATPase activity was measured just after disrupting the membranes with Triton X-100. Deactivation occurred either in whole mitochondria uncoupled with FCCP, or in disrupted membranes. No effect of Triton X-100 on the ATPase was observed, except a slow reactivation observed only in the absence of MgADP. Both AMPPNP and ATP increased the ATPase deactivation rate, thus indicating that occupancy of nucleotidic sites by ATP is more decisive than catalytic turnover for this process. ADP was found to stimulate the energy-dependent ATPase activation. ATPase deactivated at the same rate in uncoupled and disrupted mitochondria This suggests that deactivation is not controlled by rebinding of some soluble factor, like IF1, but rather by the conversion of the F1.IF1 complex into an inactive form.

[Cellular energy metabolism: physiologic and pathologic aspects]

Cellular homeostasis requires permanent energy production and consumption. Adenosine triphosphate (ATP) is the major energy component for the cell. Its synthesis occurs mainly in mitochondria where the oxidative phosphorylations realise the coupling between oxygen consumption and phosphorylation of adenosine diphosphate. The anaerobic production of ATP plays an important role in the intermediary metabolism. The enzymatic complexes of the mitochondrial respiratory chain are energy transducers acting as proton pumps. In cardiomyocytes, the phosphocreatine circuit allows a substrate channelling between mitochondria and myofibrils. This metabolic compartmentation explains the difficulties of studying energetic metabolism in the beating heart and the lack of correlation between cardiac function and the usual energy parameters. Mitochondria are a potential site of action of anaesthetic agents. Lipophilic local anaesthetics impair cellular energy metabolism and mitochondrial ATP production. Such effects could be associated with toxic effects of these molecules. NMR or near-infrared spectroscopy are non invasive techniques for monitoring energetic metabolism in vivo. Clinical applications are developed for analysing brain, muscle or cardiac function in physiological and pathological conditions.

Energetics of swelling in isolated hepatocytes: a comprehensive study

Cell swelling is now admitted as being a new principle of metabolic control but little is known about the energetics of cell swelling. We have studied the influence of hypo- or hyperosmolarity on both isolated hepatocytes and isolated rat liver mitochondria. Cytosolic hypoosmolarity on isolated hepatocytes induces an increase in matricial volume and does not affect the myxothiazol sensitive respiratory rate while the absolute value of the overall thermodynamic driving force over the electron transport chain increases. This points to an increase in kinetic control upstream the respiratory chain when cytosolic osmolarity is decreased. On isolated rat liver mitochondria incubated in hypoosmotic potassium chloride media, energetic parameters vary as in cells and oxidative phosphorylation efficiency is not affected. Cytosolic hyperosmolarity induced by sodium co-transported amino acids, per se, does not affect either matrix volume or energetic parameters. This is not the case in isolated rat liver mitochondria incubated in sucrose hyperosmotic medium. Indeed, in this medium, adenine nucleotide carrier is inhibited as the external osmolarity increases, which lowers the state 3 respiration close to state 4 level and consequently leads to a decrease in oxidative phosphorylation efficiency. When isolated rat liver mitochondria are incubated in KCl hyperosmotic medium, state 3 respiratory rate, matrix volume and membrane electrical potential vary as a function of time. Indeed, matrix volume is recovered in hyperosmotic KCl medium and this recovery is dependent on Pi-Kentry. State 3 respiratory rate increases and membrane electrical potential difference decreases during the first minutes of mitochondrial incubation until the attainment of the same value as in isoosmotic medium. This shows that matrix volume, flux and force are regulated as a function of time in KCl hyperosmotic medium. Under steady state, neither matrix volume nor energetic parameters are affected. Moreover, NaCl hyperosmotic medium allows matrix volume recovery but induces a decrease in state 3 respiratory flux. This indicates that potassium is necessary for both matrix volume and flux recovery in isolated mitochondria. We conclude that hypoosmotic medium induces an increase in kinetic control both upstream and on the respiratory chain and changes the oxidative phosphorylation response to forces. At steady state, hyperosmolarity, per se, has no effect on oxidative phosphorylation in either isolated hepatocytes or isolated mitochondria incubated in KCl medium. Therefore, potassium plays a key role in matrix volume, flux and force regulation.

Energetic and morphological plasticity of C6 glioma cells grown on 3-D support; effect of transient glutamine deprivation

The energetic metabolism of rat C6 glioma cells has been investigated as a function of the proliferative and differentiation states under three-dimensional (3-D) growing conditions on microcarrier beads. First, the transient deprivation of glutamine from the culture medium induced a marked decrease in the growth rate and a differentiation of C6 cells through the oligodendrocytic phenotype. Second, the respiratory capacity of the C6 cells during short-term subcultures with or without glutamine continuously declined as a function of the cell density, in part due to the mitochondrial content decrease. During the transition from the early exponential to the plateau growth phase in glutamine-containing medium, the oxygen consumption rate per single cell decreased concomitantly with a decrease in the glucose consumption and lactate production rates. This phenomenon led to a sixfold decrease in the total ATP production flux, without significantly affecting the cellular ATP/ADP ratio, thus indicating that some ATP-consuming processes were simultaneously suppressed during C6 proliferation. In glutamine-free medium, the cellular ATP/ADP ratio transiently increased due to growth arrest and to a reduced ATP turnover. Moreover, the results indicated that glutamine is not an essential respiratory substrate for rat C6 glioma under short-term glutamine deprivation. Worth noting was the high contribution of the mitochondrial oxidative phosphorylation toward the total ATP synthesis (about 80%), regardless of the proliferation or the differentiation status of the C6 cells.

Characterization of the yeast mitochondria unselective channel: a counterpart to the mammalian permeability transition pore?

Large and unselective permeabilities through the inner membrane of yeast mitochondria have been observed for more than 20 years, but the characterization of these permeabilities, leading to hypothesize the existence of a large-conductance unselective channel in yeast inner mitochondrial membrane, was done only recently by several groups. This channel has been tentatively identified as a yeast counterpart to the mammalian permeability transition pore, the crucial role of which is now well-documented in physiopathological phenomena, such as Ca2+ homeostasis, ischemic damages, or programmed cell death. The aim of this review is to make a point on the known characteristics of this yeast mitochondrial unselective channel (YMUC) and to analyze whether or not it can be considered as a "yeast permeability transition pore."

Oxidative phosphorylation in intact hepatocytes: quantitative characterization of the mechanisms of change in efficiency and cellular consequences

Two mechanisms may affect the yield of the oxidative phosphorylation pathway in isolated mitochondria: (i) a decrease in the intrinsic coupling of the proton pumps (H+/2e- or H+/ATP), and (ii) an increase in the inner membrane conductance (proton or cation leak). Hence three kinds of modifications can occur and each of them have been characterized in isolated rat liver mitochondria (see preceding chapter by Rigoulet et al.). In intact isolated hepatocytes, these modifications are linked to specific patterns of bioenergetic parameters, i.e. respiratory flux, mitochondrial redox potential, DY, and phosphate potential. (1) The increase in H+/ATP stoichiometry of the mitochondrial ATP synthase, as induced by almitrine [20], leads to a decrease in mitochondrial and cytosolic ATP/ADP ratios without any change in the protonmotive force nor in the respiratory rate or redox potential. (2) In comparison to carbohydrate, octanoate metabolism by beta-oxidation increases the proportion of electrons supplied at the second coupling site of the respiratory chain. This mimics a redox slipping. Octanoate addition results in an increased respiratory rate and mitochondrial NADH/NAD ratio while protonmotive force and phosphate potential are almost unaffected. The respiratory rate increase is associated with a decrease in the overall apparent thermodynamic driving force (2deltaE'o - ndeltap) which confirms the 'redox-slipping-like' effect. (3) An increase in proton conductance as induced by the protonophoric uncoupler 2,4-dinitrophenol (DNP) leads to a decrease, as expected, in the mitochondrial NADH/NAD and ATP/ ADP ratios and in deltapsi while respiratory rate is increased. Thus, each kind of modification (proton leak, respiratory chain redox slipping or increase in H+/ATP stoichiometry of ATPase) is related to a specific set of bioenergetic parameters in intact cells. Moreover, these patterns are in good agreement with the data found in isolated mitochondria. From this work, we conclude that quantitative analysis of four bioenergetic parameters (respiration rate, mitochondrial NADH/ NAD ratio, protonmotive force and mitochondrial phosphate potential) gives adequate tools to investigate the mechanism by which some alterations may affect the yield of the oxidative phosphorylation pathway in intact cells.

Yeast mitochondrial metabolism: from in vitro to in situ quantitative study

In this work, we first compared yeast mitochondrial oxidative metabolism at different levels of organization: whole cells (C), spheroplasts (S), permeabilized spheroplasts (PS) or isolated mitochondria (M). At present, S are more suitable for use than C for biochemical techniques such as fast extraction of metabolites and permeabilization. We show here that respiratory rates of S with various substrates are similar to C, which demonstrate that they are adapted to yeast bioenergetic studies. It appeared from ethanol metabolism +/- NAD+ or NADH respiratory rates on PS that ethanol metabolism was largely cytosolic; moreover, the activity of NADH dehydrogenase was lesser in the case of PS than in S. By comparing PS and M, the biggest difference concerned the respiratory rates of pyruvate and pyruvate-malate, which were much lower for M. Thus mitochondria preparation caused an unidentified loss involved directly in pyruvate metabolism. When the respiratory rate was lowered as a consequence of a high kinetic control of oxidative activity upstream from the respiratory chain, a similar correlation between the increase in ATP/O and decrease in respiratory rate was observed. So, the intrinsic uncoupling of proton pumps is not a particularity of M. Secondly, we demonstrate the existence of a mechanism of retarded diffusion in yeast similar to that already observed in permeabilized mammalian cells for ADP. Such a mechanism also occurs in yeast for several respiratory substrates: the K0.5 for each substrate toward the respiration rate in PS always exceeds that for M. It is proposed that such a discrepancy is due to a restriction of metabolite movement across the outer mitochondrial membrane in permeabilized cells, i.e. regulation of the substrate permeability through porin channels. In the porin-deficient yeast mutant, the K0.5 for NADH is not significantly different in either M or PS and is comparable to that of the parent strain PS. This result confirms that this retarded diffusion is essentially due to the opening-closing of the porin channel.

Quantitative analysis of some mechanisms affecting the yield of oxidative phosphorylation: dependence upon both fluxes and forces

The purpose of this work was to show how the quantitative definition of the different parameters involved in mitochondrial oxidative phosphorylation makes it possible to characterize the mechanisms by which the yield of ATP synthesis is affected. Three different factors have to be considered: (i) the size of the different forces involved (free energy of redox reactions and ATP synthesis, proton electrochemical difference); (ii) the physical properties of the inner mitochondrial membrane in terms of leaks (H+ and cations); and finally (iii) the properties of the different proton pumps involved in this system (kinetic properties, regulation, modification of intrinsic stoichiometry). The data presented different situations where one or more of these parameters are affected, leading to a different yield of oxidative phosphorylation. (1) By manipulating the actual flux through each of the respiratory chain units at constant protonmotive force in yeast mitochondria, we show that the ATP/O ratio decreases when the flux increases. Moreover, the highest efficiency was obtained when the respiratory rate was low and almost entirely controlled by the electron supply. (2) By using almitrine in different kinds of mitochondria, we show that this drug leads to a decrease in ATP synthesis efficiency by increasing the H+/ATP stoichiometry ofATP synthase (Rigoulet M et al. Biochim Biophys Acta 1018: 91-97, 1990). Since this enzyme is reversible, it was possible to test the effect of this drug on the reverse reaction of the enzyme i.e. extrusion of protons catalyzed by ATP hydrolysis. Hence, we are able to prove that, in this case, the decrease in efficiency of oxidative phosphorylation is due to a change in the mechanistic stoichiometry of this proton pump. To our knowledge, this is the first example of a modification in oxidative phosphorylation yield by a change in mechanistic stoichiometry of one of the proton pumps involved. (3) In a model of polyunsaturated fatty acid deficiency in rat, it was found that non-ohmic proton leak was increased, while ohmic leak was unchanged. Moreover, an increase in redox slipping was also involved, leading to a complex picture. However, the respective role of these two mechanisms may be deduced from their intrinsic properties. For each steady state condition, the quantitative effect of these two mechanisms in the decrease of oxidative phosphorylation efficiency depends on the values of different fluxes or forces involved. (4) Finally the comparison of the thermokinetic data in view of the three dimensional-structure of some pumps (X-ray diffraction) also gives some information concerning the putative mechanism of coupling (i.e. redox loop or proton pump) and their kinetic control versus regulation of mitochondrial oxidative phosphorylation.

Energetics of swelling in isolated hepatocytes: a comprehensive study

Cell swelling is now admitted as being a new principle of metabolic control but little is known about the energetics of cell swelling. We have studied the influence of hypo- or hyperosmolarity on both isolated hepatocytes and isolated rat liver mitochondria. Cytosolic hypoosmolarity on isolated hepatocytes induces an increase in matricial volume and does not affect the myxothiazol sensitive respiratory rate while the absolute value of the overall thermodynamic driving force over the electron transport chain increases. This points to an increase in kinetic control upstream the respiratory chain when cytosolic osmolarity is decreased. On isolated rat liver mitochondria incubated in hypoosmotic potassium chloride media, energetic parameters vary as in cells and oxidative phosphorylation efficiency is not affected. Cytosolic hyperosmolarity induced by sodium co-transported amino acids, per se, does not affect either matrix volume or energetic parameters. This is not the case in isolated rat liver mitochondria incubated in sucrose hyperosmotic medium. Indeed, in this medium, adenine nucleotide carrier is inhibited as the external osmolarity increases, which lowers the state 3 respiration close to state 4 level and consequently leads to a decrease in oxidative phosphorylation efficiency. When isolated rat liver mitochondria are incubated in KCl hyperosmotic medium, state 3 respiratory rate, matrix volume and membrane electrical potential vary as a function of time. Indeed, matrix volume is recovered in hyperosmotic KCl medium and this recovery is dependent on Pi-Kentry. State 3 respiratory rate increases and membrane electrical potential difference decreases during the first minutes of mitochondrial incubation until the attainment of the same value as in isoosmotic medium. This shows that matrix volume, flux and force are regulated as a function of time in KCl hyperosmotic medium. Under steady state, neither matrix volume nor energetic parameters are affected. Moreover, NaCl hyperosmotic medium allows matrix volume recovery but induces a decrease in state 3 respiratory flux. This indicates that potassium is necessary for both matrix volume and flux recovery in isolated mitochondria. We conclude that hypoosmotic medium induces an increase in kinetic control both upstream and on the respiratory chain and changes the oxidative phosphorylation response to forces. At steady state, hyperosmolarity, per se, has no effect on oxidative phosphorylation in either isolated hepatocytes or isolated mitochondria incubated in KCl medium. Therefore, potassium plays a key role in matrix volume, flux and force regulation.

Flux-force relationships in intact cells: a helpful tool for understanding the mechanism of oxidative phosphorylation alterations?

On isolated mitochondria, numerous studies of the relationships between fluxes and their associated forces have led to the description of some properties of the oxidative phosphorylation pathway. However whether such an approach can be applied to understanding the actual situation in intact living cells needs further consideration. In this study on isolated hepatocytes, we describe the dependence of the respiratory rate on the three thermodynamic forces linked to oxidative phosphorylation (i.e. the redox span over the respiratory chain, the electrical potential difference across the inner mitochondrial membrane and the free energy of ATP synthesis reaction). Even if this description is phenomenological and some objections may be raised regarding the relevance of such a bulk-phase force estimation, we present some results showing that the study of flux-force relationships in intact cells may be a helpful approach for understanding the mechanisms by which oxidative phosphorylation activity is changed.

The importance of the glycerol 3-phosphate shuttle during aerobic growth of Saccharomyces cerevisiae

Maintenance of a cytoplasmic redox balance is a necessity for sustained cellular metabolism. Glycerol formation is the only way by which Saccharomyces cerevisiae can maintain this balance under anaerobic conditions. Aerobically, on the other hand, several different redox adjustment mechanisms exist, one of these being the glycerol 3-phosphate (G3P) shuttle. We have studied the importance of this shuttle under aerobic conditions by comparing growth properties and glycerol formation of a wild-type strain with that of gut2 delta mutants, lacking the FAD-dependent glycerol 3-phosphate dehydrogenase, assuming that the consequent blocking of G3P oxidation is forcing the cells to produce glycerol from G3P. To impose different demands on the redox adjustment capability we used various carbon sources having different degrees of reduction. The results showed that the shuttle was used extensively with reduced substrate such as ethanol, whereas the more oxidized substrates lactate and pyruvate, did not provoke any activity of the shuttle. However, the absence of a functional G3P shuttle did not affect the growth rate or growth yield of the cells, not even during growth on ethanol. Presumably, there must be alternative systems for maintaining a cytoplasmic redox balance, e.g. the so-called external NADH dehydrogenase, located on the outer side of the inner mitochondrial membrane. By comparing the performance of the external NADH dehydrogenase and the G3P shuttle in isolated mitochondria, it was found that the former resulted in high respiratory rates but a comparably low P/O ratio of 1.2, whereas the shuttle gave low rates but a high P/O ratio of 1.7. Our results also demonstrated that of the two isoforms of NAD-dependent glycerol 3-phosphate dehydrogenase, only the enzyme encoded by GPD1 appeared important for the shuttle, since the enhanced glycerol production that occurs in a gut2 delta strain proved dependent on GPD1 but not on GPD2.

Energetics of isolated hepatocyte swelling induced by sodium co-transported amino acids

This study was designed to investigate the energetics of isolated rat hepatocyte swelling due to sodium-cotransported amino acid accumulation in a medium containing either glucose or octanoate as basal substrate. We show that the size of the increase in cytosolic volume is directly correlated with the total amino acid accumulation, which depends on the difference of electrical potential across the plasma membrane. Such a change in cell volume, with either glucose or octanoate, does not modify the mitochondrial volume. Addition of sodium-cotransported amino acids for which the metabolism was avoided showed that the rise in cell volume, per se, did not change the respiratory rate, deltap, or phosphate potential in either mitochondrial or cytosolic compartments. Conversely, the large increase in oxidative phosphorylation flux was due to the metabolism of amino acids as a consequence of a rise in electron supply for the respiratory chain rather than an increase in cellular ATP demand, as indicated by the increase in cytosolic phosphate potential. Moreover, although we confirm that octanoate addition largely increases the respiratory rate by a process different from uncoupling, we observed that the same overall thermodynamic driving force through the respiratory chain and the same mitochondrial or cytosolic phosphate potential were maintained for much higher oxygen consumption when octanoate was present. We propose that these octanoate effects are due to a decrease in the actual protons/2 electrons stoichiometry as a consequence of a shift in electron supply toward a two-coupling site instead of a three-coupling site. The change in the FADH2/NADH formation flux ratio in either fatty acid or carbohydrate oxidation explains such results.

Response of isolated rat liver mitochondria to variation of external osmolarity in KCl medium: regulation of matrix volume and oxidative phosphorylation

When isolated rat liver mitochondria are incubated in KCI medium, matrix volume, flux, and forces in both hypo- and hyperosmolarity are time-dependent. In hypoosmotic KCl medium, matrix volume is regulated via the K+/H+ exchanger. In hyperosmotic medium, the volume is regulated in such a manner that at steady state, which is reached within 4 min, it is maintained whatever the hyperosmolarity. This regulation is Pi- and deltamuH+-dependent, indicating Pi-K salt entry into the matrix. Under steady state, hyperosmolarity has no effect on isolated rat liver mitochondria energetic parameters such as respiratory rate, proton electrochemical potential difference, and oxidative phosphorylation yield. Hypoosmolarity decreases the NADH/NAD+ ratio, state 3 respiratory rate, and deltamuH+, while oxidative phosphorylation yield is not significantly modified. This indicates kinetic control upstream the respiratory chain. This study points out the key role of potassium on the regulation of matrix volume, flux, and forces. Indeed, while matrix volume is regulated in NaCl hyperosmotic medium, flux and force restoration in hyperosmotic medium occurs only in the presence of external potassium.

Cytosolic NAD+ content strictly depends on ATP concentration in isolated liver cells

By focusing on the question of the thermodynamic relationships involved in the regulation of biological energy conversion, bioenergetic studies usually consider the free pyridine and adenine nucleotide rather than their total pools, in either cytosol or mitochondria. In this study, we report a new observation that, at steady state, nicotinamide nucleotide content is increased by a rise in the ATP content of the whole cell under physiological conditions. It is a straight line relationship when only NAD+ and ATP are considered. When regarding the compartmentation of this phenomenon, it appears that the linear relationship between [NAD+] and [ATP] occurs only in the cytosol. Such a dependence could be a supplementary mechanism of regulation between various metabolic pathways in the liver cell.

Control of oxidative phosphorylation in rat liver mitochondria: effect of ionic media

The aim of this work was to compare oxidative phosphorylation activity and its kinetic control on isolated rat liver mitochondria in either various ionic or sucrose isoosmotic media. Whatever the ionic medium, state 3 and uncoupled state respiratory rates were higher in ionic than in sucrose media, although state 4 respiration rate remained constant. Moreover, under isoosmotic conditions, the salt concentration necessary for half state 3 stimulation depends on the cation involved: for inorganic cations, these K0.5 values increased, as did the absolute value of hydratation enthalpy. The ATP/O ratio did not vary in any medium and matrix volume was about 20% increased in ionic media. JO2 versus delta p relationships were left-shifted in ionic media compared to sucrose medium: for the same respiratory rate, the protonmotive force maintained was lesser in ionic media. However, the relationship between JO2 and delta p is unique whatever the ionic medium under study. In ionic media compared to the sucrose medium, kinetic control was increased on one of the protonmotive force generating systems (cytochrome c oxidase) and decreased on one of the protonmotive force dissipating systems (adenine nucleotide translocator), even if the fluxes increased.

The yield of oxidative phosphorylation is controlled both by force and flux

Dissipation of energy during oxidative phosphorylation may be due to two distinct mechanisms: passive permeability to protons and/or cations (leak) or decrease in the efficiency of some proton pumps (slip). Whatever the mechanism involved, it is admitted that the wastage depends on the protonmotive force. However, the most relevant question in physiology is to determine whether other factors contribute or not to this efficiency. By comparing phosphorylating (high respiratory flux) or non phosphorylating (low respiratory flux) states at similar protonmotive force, we have shown that the wastage is higher in phosphorylating than in non-phosphorylating conditions. This strongly argues for the fact that the flux of oxidative phosphorylation is an important parameter in the control of the yield of this major energetic pathway.

Regulation of intracellular chloride concentration in rat lactotrophs: possible role of mitochondria

Increasing evidence is accumulating for the involvement of chloride ions in the stimulus-secretion coupling of pituitary cells. We show that the mean intracellular chloride concentration [Cl-]i of rat lactotroph cells maintained in culture is high, close to 60 mM (59.4 mM), using the Cl- sensitive fluorescent probe 6-methoxy-N-(3-sulfopropyl) quinolinium (SPQ), coupled with whole-cell patch-clamp recordings. We demonstrate that this high level is correlated with the presence of mitochondrial stores of Cl- as shown by the release of Cl- in response to various metabolic inhibitors. We determine that CCP (50 microM) induces a mean [Cl-]i increase of 15.8+/-5.8 mM, using combined electrophysiology and microspectrofluorimetry methods. These data strongly suggest that cell metabolism, including the mitochondrial function, modulate [Cl-]i.

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

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

Effect of polyunsaturated fatty acids deficiency on oxidative phosphorylation in rat liver mitochondria

Liver mitochondria isolated from controls or polyunsaturated fatty acid (PUFA) deficient rats were studied for oxidative phosphorylation. A PUFA-deficient diet led to a dramatic change in the fatty acid composition of mitochondrial lipid content, similar to that reported in the literature. Besides the changes in lipid composition, mitochondrial volume was enlarged (+45% in state 4 and two-fold in state 3). State 4 respiration was increased together with a decrease in protonmotive force. The non-ohmicity of the relationship between non-phosphorylating respiration and protonmotive force was more pronounced in the PUFA-deficient group. State 3 oxygen consumption as well as the rate of ATP synthesis showed no difference between the two groups, whereas the protonmotive force decreased substantially in mitochondria from PUFA-deficient animals. In contrast, ATP/O ratios were decreased in the PUFA-deficient group when determined at subsaturating ADP concentration. Taken together, these results are in agreement with both an increased non-ohmic proton leak and an increased redox slipping. The relative importance of these two effects on the overall efficiency of oxidative phosphorylation depends on both the rate of oxidative phosphorylation and the maintained protonmotive force. Hence, in isolated mitochondria the respective role of each effect may vary between state 4 and state 3.

Dependence of flux size and efficiency of oxidative phosphorylation on external osmolarity in isolated rat liver mitochondria: role of adenine nucleotide carrier

The aim of this work was a thermodynamic and kinetic study of the influence of varying external osmolarity on overall oxidative phosphorylations in isolated rat liver mitochondria. When external osmolarity is increased from 100 to 400 mosM by using a non-penetrant sugar: (i) matrix volume diminishes, (ii) state 3 respiratory rate decreases when state 4 slightly varies, (iii) states 3 and 4 protonmotive force and NAD(P)H level increase, whereas oxidative phosphorylation efficiency (ATP/O) decreases. Indeed, respiratory flux versus protonmotive force relationships depend on the osmolarity considered: the lower the external osmolarity, the higher the span of overall driving force necessary for the same respiratory rate. To further investigate the mechanism of the decrease in respiratory and ATP synthesis flux leading to a lowering in oxidative phosphorylation efficiency, we determined the adenine nucleotide carrier control coefficient on respiratory and ATP synthesis rates respectively. The main result is that the adenine nucleotide carrier control coefficient on respiratory rate decreases, and conversely that adenine nucleotide carrier control on ATP synthesis rate increases, from iso- to hyperosmolarity. Furthermore, whatever the osmolarity, when state 3 respiratory rate is titrated with carboxyatractyloside, the same relationship is observed between ATP/O ratio and respiratory flux. From many previous studies, it has been shown that an increase in external osmolarity and a consequent decrease in matrix volume inhibits almost all mitochondrial proton pumps (coupling site 1 and 2 of respiratory chain, ATPase) in different ways. In this work, we show that in phosphorylating mitochondria, the adenine nucleotide carrier plays a key role: its inhibition as the external osmolarity increases lowers the state 3 respiration close to state 4 level and consequently leads to a decrease in oxidative phosphorylation efficiency.