Control over the contribution of the mitochondrial membrane potential (DeltaPsi) and proton gradient (DeltapH) to the protonmotive force (Deltap). In silico studies

The protonmotive force across the inner mitochondrial membrane (Deltap) has two components: membrane potential (DeltaPsi) and the gradient of proton concentration (DeltapH). The computer model of oxidative phosphorylation developed previously by Korzeniewski et al. (Korzeniewski, B., Noma, A., and Matsuoka, S. (2005) Biophys. Chem. 116, 145-157) was modified by including the K+ uniport, K+/H+ exchange across the inner mitochondrial membrane, and membrane capacitance to replace the fixed DeltaPsi/DeltapH ratio used previously with a variable one determined mechanistically. The extended model gave good agreement with experimental results. Computer simulations showed that the contribution of DeltaPsi and DeltapH to Deltap is determined by the ratio of the rate constants of the K+ uniport and K+/H+ exchange and not by the absolute values of these constants. The value of Deltap is mostly controlled by ATP usage. The metabolic control over the DeltaPsi/DeltapH ratio is exerted mostly by K+ uniport and K+/H+ exchange in the presence of these processes, and by the ATP usage, ATP/ADP carrier, and phosphate carrier in the absence of them. The K+ circulation across the inner mitochondrial membrane is controlled mainly by K+ uniport and K+/H+ exchange, whereas H+ circulation by ATP usage. It is demonstrated that the secondary K+ ion transport is not necessary for maintaining the physiological DeltaPsi/DeltapH ratio.     

The protonmotive force in bovine heart submitochondrial particles. Magnitude, sites of generation and comparison with the phosphorylation potential.

1. The magnitude of the protonmotive force in respiring bovine heart submitochondrial particles was estimated. The membrane-potential component was determined from the uptake of S14CN-ions, and the pH-gradient component from the uptake of [14C]methylamine. In each case a flow-dialysis technique was used to monitor uptake. 2. With NADH as substrate the membrane potential was approx. 145mV and the pH gradient was between 0 and 0.5 unit when the particles were suspended in a Pi/Tris reaction medium. The addition of the permeant NO3-ion decreased the membrane potential with a corresponding increase in the pH gradient. In a medium containing 200mM-sucrose, 50mM-KCl and Hepes as buffer, the total protonmotive force was 185mV, comprising a membrane potential of 90mV and a pH gradient of 1.6 units. Thus the protonmotive force was slightly larger in the high-osmolarity medium. 3. The phosphorylation potential (= deltaG0' + RT ln[ATP]/[ADP][Pi]) was approx. 43.1 kJ/mol (10.3kcal/mol) in all the reaction media tested. Comparison of this value with the protonmotive force indicates that more than 2 and up to 3 protons must be moved across the membrane for each molecule of ATP synthesized by a chemiosmotic mechanism. 4. Succinate generated both a protonmotive force and a phosphorylation potential that were of similar magnitude to those observed with NADH as substrate. 5. Although oxidation of NADH supports a rate of ATP synthesis that is approximately twice that observed with succinate, respiration with either of these substrates generated a very similar protonmotive force. Thus there seemed to be no strict relation between the size of the protonmotive force and the phosphorylation rate. 6. In the presence of antimycin and/or 2-n-heptyl-4-hydroxyquinoline N-oxide, ascorbate oxidation with either NNN'N'-tetramethyl-p-phenylenediamine or 2,3,5,6-tetramethyl-p-phenylenediamine as electron mediator generated a membrane potential of approx. 90mV, but no pH gradient was detected, even in the presence of NO3-. These data are discussed with reference to the proposal that cytochrome oxidase contains a proton pump.     

Fine structural localization of alkaloid synthesis in endoplasmic reticulum of submergedClaviceps purpurea

Cyanide-insensitive mitochondria from Saccharomycopsis lipolytica possess an exogenous NADH dehydrogenase, located outside the inner mitochondrial membrane, and linked to coupling site II. These mitochondria are able to oxidize exogenous NADH via two pathways: (1) a cyanide- and antimycin-sensitive pathway, or cytochrome pathway, and (2) a cyanide- and antimycin-insensitive pathway, or alternative pathway. Although the oxidation of exogenous NADH through the cytochrome pathway occurs with an ATP/0 ratio tending to 2, it proceeds, per molecule of NADH oxidized, with the apparent ejection in the outer medium of only 3 protons instead of 4 protons, as is the case with glycerol 3-phosphate as control substrate, but leaves 1 hydroxyl ion in the outer medium after decay of the protonmotive force. These properties were used to demonstrate the non electrogenic function of the alternative pathway. Indeed, the oxidation of exogenous NADH via the alternative pathway proceeds without apparent ejection of protons, although this oxidation generates an electron flux in the alternative pathway as demonstrated by the net appearance in the outer medium of 1 hydroxyl ion per atom of oxygen reduced, appearance which proves sensitive to benhydroxamic acid, a specific inhibitor of the alternative pathway. The non electrogenicity of the alternative pathway is accompanied by the absence of ATP synthesis as expected from Mitchell's chemiosmotic model. The absence of energy conservation when the electron transfer proceeds via the alternative pathway is not the result of an uncoupling property of an active alternative pathway, as the oxidation of malate plus pyruvate via coupling site I and the alternative pathway occurs with an ATP/0 ratio tending to 1.     

Variable proton conductance of submitochondrial particles.

The relationship between the rate of substrate oxidation and the protonmotive force (electrochemical proton gradient) generated by bovine heart submitochondrial particles has been examined. Unexpectedly, oxidation of succinate generated a higher protonmotive force than the oxidation of NADH, although the rate of proton translocation across the membrane was inferred to be considerably lower with succinate as substrate. The data suggest that the flow of electrons through site 1 of the respiratory chain may increase the conductance of the mitochondrial membrane for protons. Upon reduction of the rate of succinate oxidation by titration with malonate, the protonmotive force remained essentially constant until the extent of inhibition was greater than 75%. The general conclusion from this work is that a constant passive membrane conductance for protons cannot be assumed.     

Thermodynamic control of electron flux through mitochondrial cytochrome bc1 complex.

The redox states of exogenously added ubiquinone-2 and cytochrome c, and the protonmotive force (delta p) of rat liver mitochondria were measured as the respiration rate was titrated with the uncoupler carbonyl cyanide p-trifluoromethoxyphenyl-hydrazone. The force ratio delta Eh/delta p across the bc1 complex was close to 1:1 in State 4, indicating an H+/e- stoichiometry of 1:1 for the cytochrome bc1 complex, excluding protons moved by pool ubiquinone. Assuming a constant stoichiometry the rate of electron transport increased linearly with the disequilibrium (delta Eh - delta p) across the complex.     

Mitochondrial protonophoric activity induced by a thyromimetic fatty acid analogue

Calcium-dependent uncoupling of liver mitochondrial oxidative phosphorylation by a non-metabolizable long chain fatty acyl analogue was compared with uncoupling induced by in vivo thyroid hormone treatment. beta,beta'-Methyl-substituted hexadecane alpha, omega-dioic acid (Medica 16) is reported here to induce a saturable 20-30% decrease in liver mitochondrial DeltaPsi, DeltapH and protonmotive force which proceeds in the presence of added Ca(2+) to cyclosporin A-sensitive mitochondrial permeabilization. Ca(2+)-dependent uncoupling by Medica 16 was accompanied by atractylate-enhanced, bongkrekic-inhibited activation of mitochondrial Ca(2+) efflux. The direct mitochondrial effect exerted in vitro by Medica 16 is similar to that induced by in vivo thyroid hormone treatment. Hence, the thyromimetic protonophoric activity of Medica 16 and the uncoupling activity of TH converge onto components of the mitochondrial permeabilization transition pore.     

Determination of metabolite compartition in hepatic cells by varying the redox state in vivo.

In the present study, metabolite (lactate, pyruvate, glycerol 3-phosphate, dihydroxyacetone phosphate) concentrations were measured in various redox states. The mathematical relations between metabolite concentrations in various redox states were expressed algebraically and studied. Models which provided separate lactate/pyruvate (L/P) and glycerol 3-phosphate/dihydroxyacetone phosphate (G/D) spaces correspond to the experimental results in the case of "reductants" (e.g. ethanol, acetaldehyde, dihydroxyacetone and acetate) or of "oxidizing agents" (e.g. pyruvate) of the cytosolic NAD-NADH. Crotonate injection caused an oxidation of cytosolic redox couples, but no separation of the lactate/pyruvate space from the glycerol 3-phosphate/dihydroxyacetone phosphate space may necessarily be inferred. Furthermore, the following statements could be made in both first cases: (i.e. of "reductants" and "oxidizing agents"): (a) Redox couples in L/P space and in G/D space (together L/P-G/D system) are in equilibrium; (b) Redox-equivalent transport from the L/P space to the G/D space is not subject to any velocity-limiting mechanism; (c) Substrates which transports redox-equivalents into and from the L/P-G/D system reach concentrations to values, which are in a linear relation to each other in this system; (d) It is possible that these substrates are regenerated in another system which is also in equilibrium and subject to statement c.     

Top-down control analysis of the effect of temperature on ectotherm oxidative phosphorylation

Top-down control and elasticity analysis was conducted on mitochondria isolated from the midgut of the tobacco hornworm (Manduca sexta) to assess how temperature affects oxidative phosphorylation in a eurythermic ectotherm. Oxygen consumption and protonmotive force (measured as membrane potential in the presence of nigericin) were monitored at 15, 25, and 35 degrees C. State 4 respiration displayed a Q(10) of 2.4-2.7 when measured over two temperature ranges (15-25 degrees C and 25-35 degrees C). In state 3, the Q(10)s for respiration were 2.0 and 1.7 for the lower and higher temperature ranges, respectively. The kinetic responses (oxygen consumption) of the substrate oxidation system, proton leak, and phosphorylation system increased as temperature rose, although the proton leak and substrate oxidation system showed the greatest thermal sensitivity. Whereas there were temperature-induced changes in the activities of the oxidative phosphorylation subsystems, there was no change in the state 4 membrane potential and little change in the state 3 membrane potential. Top-down control analysis revealed that control over respiration did not change with temperature. In state 4, control of respiration was shared nearly equally by the proton leak and the substrate oxidation system, whereas in state 3 the substrate oxidation system exerted over 90% of the control over respiration. The proton leak and phosphorylation system account for <10% of the temperature-induced change in the state 3 respiration rate. Therefore, when the temperature is changed, the state 3 respiration rate is altered primarily because of temperature's effect on the substrate oxidation system.     

The protonmotive force in phosphorylating membrane vesicles from Paracoccus denitrificans. Magnitude, sites of generation and comparison with the phosphorylation potential

1. The magnitude of the protonmotive force in phosphorylating membrane vesicles from Paracoccus denitrificans was estimated. The membrane potential component was determined from the uptake of S(14)CN(-), and the transmembrane pH gradient component from the uptake of [(14)C]methylamine. In each case a flow-dialysis technique was used to monitor uptake. 2. With NADH as substrate, the membrane potential was about 145mV and the pH gradient was below 0.5 pH unit. The membrane potential was decreased by approx. 15mV during ATP synthesis, and was abolished on addition of carbonyl cyanide p-trifluoromethoxyphenylhydrazone. In the presence of KCl plus valinomycin the membrane potential was replaced by a pH gradient of 1.5 units. 3. Succinate oxidation generated a membrane potential of approx. 125mV and the pH gradient was below 0.5 pH unit. Oxidation of ascorbate (in the presence of antimycin) with either 2,3,5,6-tetramethyl-p-phenylenediamine or NNN'N'-tetramethyl-p-phenylenediamine as electron mediator usually generated a membrane potential of approx. 90mV. On occasion, ascorbate oxidation did not generate a membrane potential, suggesting that the presence of a third energy-coupling site in P. denitrificans vesicles is variable. 4. With NADH or succinate as substrate, the phosphorylation potential (DeltaG(p)=DeltaG(0)'+RTln[ATP]/ [ADP][P(i)]) was approx. 53.6kJ/mol (12.8kcal/mol). Comparison of this value with the protonmotive force indicates that more than 3 protons need to be translocated via the adenosine triphosphatase of P. denitrificans for each molecule of ATP synthesized by a chemiosmotic mechanism. In the presence of 10mm-KNO(3) the protonmotive force was not detectable (<60mV) but DeltaG(p) was not altered. This result may indicate either that there is no relationship between the protonmotive force and DeltaG(p), or that for an unidentified reason the equilibration of SCN(-) or methylamine with the membrane potential and the pH gradient is prevented by NO(3) (-) in this system.     

Control of oxidative phosphorylation during insect metamorphosis

The midgut of the tobacco hornworm (Manduca sexta) is a highly aerobic tissue that is destroyed and replaced by a pupal epithelium at metamorphosis. To determine how oxidative phosphorylation is altered during the programmed death of the larval cells, top-down control analysis was performed on mitochondria isolated from the midguts of larvae before and after the commitment to pupation. Oxygen consumption and protonmotive force (measured as membrane potential in the presence of nigericin) were monitored to determine the kinetic responses of the substrate oxidation system, proton leak, and phosphorylation system to changes in the membrane potential. Mitochondria from precommitment larvae have higher respiration rates than those from postcommitment larvae. State 4 respiration is controlled by the proton leak and the substrate oxidation system. In state 3, the substrate oxidation system exerted 90% of the control over respiration, and this high level of control did not change with development. Elasticity analysis, however, revealed that, after commitment, the activity of the substrate oxidation system falls. This decline may be due, in part, to a loss of cytochrome c from the mitochondria. There are no differences in the kinetics of the phosphorylation system, indicating that neither the F(1)F(0) ATP synthase nor the adenine nucleotide translocase is affected in the early stages of metamorphosis. An increase in proton conductance was observed in mitochondria isolated from postcommitment larvae, indicating that membrane area, lipid composition, or proton-conducting proteins may be altered during the early stages of the programmed cell death of the larval epithelium.     

The effect of noradrenaline on glyceride synthesis and oxidative metabolism in vitro in the brown fat of newborn rabbits

1. The effect of noradrenaline on the synthesis of glyceride from [U-¹⁴C]glucose and on gas exchange in the brown fat of newborn rabbits in vitro was investigated. 2. The specific radioactivity of l-glycerol 3-phosphate was lower than that of lactate, presumably because glycerol derived from glyceride was rephosphorylated by glycerokinase. 3. In the basal state more than 25% of the total respiration was due to pyruvate oxidation. Noradrenaline stimulated glyceride synthesis and total respiration without changing the proportion of the total respiration due to pyruvate oxidation. 4. The extra ADP released by noradrenaline stimulation of glyceride synthesis could not have supported more than 2% of the observed increase in substrate oxidation if mitochondria from brown-fat-cells remain fully coupled in the stimulated state, but could have supported about one-third of the observed increase if they become uncoupled in the presence of noradrenaline.     

The control of tricarboxylate-cycle oxidations in blowfly flight muscle. The steady-state concentrations of citrate, isocitrate 2-oxoglutarate and malate in flight muscle and isolated mitochondria.

1. Blowfly (Phormia regina) flight-muscle mitochondria were allowed to oxidize pyruvate under a variety of experimental conditions, and determinations of the citrate, isocitrate, 2-oxoglutarate and malate contents of both the mitochondria and the incubation medium were made. For each intermediate a substantial portion of the total was present within the mitochondria. 2. Activation of respiration by either ADP or uncoupling agent resulted in a decreased content of citrate and isocitrate and an increased content of 2-oxoglutarate and malate when the substrate was pyruvate, APT and HCO3 minus. Such a decrease in citrate content was obscured when the substrate was pyruvate and proline owing to a large rise in the total content of tricarboxylate-cycle intermediates in the presence of proline and ADP. 3. An experiment involving oligomycin and uncoupling agent demonstrated that the ATP/ADP ratio is the main determinant of flux through the tricarboxylate cycle, with the redox state of nicotinamide nucleotide being of lesser importance. 4. Addition of ADP and Ca-2+ to activate the oxidation of both glycerol 3-phosphate and pyruvate, simulating conditions on initiation of flight, gave a decrease in citrate and isocitrate and an increase in 2-oxoglutarate and malate content. 5. There was a good correlation between these results with isolated flight-muscle mitochondria and the changes found in fly thoraces after 30s and 2 mihorax. 6. It is concluded that NAD-isocitrate dehydrogenase (EC 1.1.1.41) controls the rate of pyruvate oxidation in both resting fly flight muscle in vivo and isolated mitochondria in state 4 (nomenclature of Change & Williams, 1955).     

Anacardic acid-mediated changes in membrane potential and pH gradient across liposomal membranes.

We have previously shown that anacardic acid has an uncoupling effect on oxidative phosphorylation in rat liver mitochondria using succinate as a substrate (Life Sci. 66 (2000) 229-234). In the present study, for clarification of the physicochemical characteristics of anacardic acid, we used a cyanine dye (DiS-C3(5)) and 9-aminoacridine (9-AA) to determine changes of membrane potential (DeltaPsi) and pH difference (DeltapH), respectively, in a liposome suspension in response to the addition of anacardic acid to the suspension. The anacardic acid quenched DiS-C3(5) fluorescence at concentrations higher than 300 nM, with the degree of quenching being dependent on the log concentration of the acid. Furthermore, the K(+) diffusion potential generated by the addition of valinomycin to the suspension decreased for each increase in anacardic acid concentration used over 300 nM, but the sum of the anacardic acid- and valinomycin-mediated quenching was additively increasing. This indicates that the anacardic acid-mediated quenching was not due simply to increments in the K(+) permeability of the membrane. Addition of anacardic acid in the micromolar range to the liposomes with DeltaPsi formed by valinomycin-K(+) did not significantly alter 9-AA fluorescence, but unexpectedly dissipated DeltaPsi. The DeltaPsi preformed by valinomycin-K(+) decreased gradually following the addition of increasing concentrations of anacardic acid. The DeltaPsi dissipation rate was dependent on the pre-existing magnitude of DeltaPsi, and was correlated with the logarithmic concentration of anacardic acid. Furthermore, the initial rate of DeltapH dissipation increased with logarithmic increases in anacardic acid concentration. These results provide the evidence for a unique function of anacardic acid, dissimilar to carbonylcyanide p-trifluoromethoxyphenylhydrazone or valinomycin, in that anacardic acid behaves as both an electrogenic (negative) charge carrier driven by DeltaPsi, and a 'proton carrier' that dissipates the transmembrane proton gradient formed.     

Simulation of state 4 --> state 3 transition in isolated mitochondria

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

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.     

Trypanosoma brucei brucei: the catabolism of glycolytic intermediates by digitonin-permeabilized bloodstream trypomastigotes and some aspects of regulation of anaerobic glycolysis

1. The production of pyruvate, glycerol and glycerol-3-phosphate by intact and digitonin-permeabilized Trypanosoma brucei brucei has been studied with glucose or the glycolytic intermediates as substrates. 2. Under aerobic conditions hexosephosphates gave maximal glycolysis in the presence of 40-60 micrograms digitonin/10(8) trypanosomes while the triosephosphates gave it at 20-30 micrograms digitonin/10(8) trypanosomes. 3. In the presence of salicylhydroxamic acid, and the glycolytic intermediates, permeabilized trypanosomes produced equimolar amounts of pyruvate and glycerol-3-phosphate and no glycerol. Under the same conditions, glucose catabolism produced glycerol in addition to pyruvated and glycerol-3-phosphate. 4. In the presence of salicylhydroxamic acid and ATP or ADP intact trypanosomes produced equimolar amounts of pyruvate and (glycerol plus glycerol-3-phosphate) with glucose as substrate. 5. A carrier for ATP and ADP at the glycosomal membrane is implicated. 6. It is apparent that glycerol formation is regulated by the ATP/ADP ratio and that it needs intact glycosomal membrane and the presence of glucose.     

Kinetics of glycerol uptake by the perfused rat liver. Membrane transport, phosphorylation and effect on NAD redox level.

The kinetics of glycerol uptake by the perfused rat liver were determined according to a model which includes membrane transport, intracellular phosphorylation and competitive inhibition of glycerol phosphorylation by L-glycerol 3-phosphate. The membrane transport obeys first-order kinetics at concentrations below 10 mM in the affluent medium. The K-m of the glycerol phosphorylation was 10 muM and the K-i of the L-glycerol 3-phosphate inhibition was 50 muM. The maximum activity (V) was 3.70 mumoles/min per g liver wet wt. These results are similar to in vitro kinetics of the glycerol kinase, except that K-i was found to be somewhat lower in the intact organ. At low glycerol concentrations, a steep concentration gradient exists across the liver cell membrane. The increase in the lactate to pyruvate concentration ratio during glycerol metabolism is related to the actual concentration of L-glycerol 3-phosphate, not to the rate of glycerol uptake.     

The mechanism of transmembrane delta muH+ generation in mitochondria by cytochrome c oxidase.

In rat liver mitochondria treated with rotenone, N-ethylmaleimide or oligomycin the expected alkalinization caused by proton consumption for aerobic oxidation of ferrocyanide was delayed with respect to ferrocyanide oxidation, unless carbonyl cyanide p-trifluoromethoxyphenylhydrazone was present. 2. When valinomycin or valinomycin plus antimycin were also present, ferricyanide, produced by oxidation of ferrocyanide, was re-reduced by hydrogenated endogenous reductants. Under these circumstances the expected net proton consumption caused by ferrocyanide oxidation was preceded by transient acidification. It is shown that re-reduction of formed ferricyanide and proton release derive from rotenone- and antimycin-resistant oxidation of endogenous reductants through the proton-translocating segments of the respiratory chain on the substrate side of cytochrome c. The number of protons released per electron flowing to ferricyanide varied, depending on the experimental conditions, from 3.6 to 1.5. 3. The antimycin-insensitive re-reduction of ferricyanide and proton release from mitochondria were strongly depressed by 2-n-heptyl-4-hydroxyquinoline N-oxide. This shows that the ferricyanide formed accepts electrons passing through the protonmotive segments of the respiratory chain at the level of cytochrome c and/or redox components of the cytochrome b-c1 complex situated on the oxygen side of the antimycin-inhibition site. Dibromothymoquinone depressed and duroquinol enhanced, in the presence of antimycin, the proton-release process induced by ferrocyanide respiration. Both quinones enhanced the rate of scalar proton production associated with ferrocyanide respiration, but lowered the number of protons released per electron flowing to the ferricyanide formed. 4. Net proton consumption caused by aerobic oxidation of exogenous ferrocytochrome c by antimycin-supplemented bovine heart mitochondria was preceded by scalar proton release, which was included in the stoicheiometry of 1 proton consumed per mol of ferrocytochrome c oxidized. This scalar proton production was associated with transition of cytochrome c from the reduced to the oxidized form and not to electron flow along cytochrome c oxidase. 5. It is concluded that cytochrome c oxidase only mediates vectorial electron flow from cytochrome c at the outer side to protons that enter the oxidase from the matrix side of the membrane. In addition to this consumption of protons the oxidase does not mediate vectorial proton translocation.     

Heterogeneity of the sn-glycerol 3-phosphate pool in isolated hepatocytes, demonstrated by the use of deuterated glycerols and ethanol.

Hepatocytes were isolated from female rats and incubated with [1,1,3,3-2H4]glycerol or [2-2H]glycerol. The deuterium excess in phosphatidylcholines, sn-glycerol 3-phosphate and other organic acids was determined by g.l.c./mass spectrometry. The unlabelled fraction of the major phosphatidylcholines decreased exponentially, and the turnover was not changed by the presence of ethanol. The relative contribution of the two deuterated glycerols was about the same in the major phosphatidylcholine as in sn-glycerol 3-phosphate, indicating that formation by acylation of dihydroxyacetone phosphate is insignificant. [1,1,3,3-2H4]Glycerol had lost deuterium to a larger extent when it was incorporated in the phosphatidylcholine than when it was incorporated in sn-glycerol-3-phosphate, indicating that the phosphatidylcholines are formed from a separate pool of sn-glycerol 3-phosphate. Deuterium at C-2 was transferred between sn-glycerol 3-phosphate molecules to about 25%. Ethanol decreased the extent of deuterium transfer, the extent of glycerol uptake and the loss of deuterium at C-1 and C-3 in sn-glycerol 3-phosphate. The results indicate that the oxidation to dihydroxyacetone phosphate was inhibited by the NADH formed during ethanol oxidation. [2-2H]Glycerol also labelled an alcohol dehydrogenase substrate, malate and lactate, indicating oxidation of sn-glycerol 3-phosphate in the cytosol. The two acids appeared to be formed in reductions with different pools of NADH.     

Role of mitochondrial calcium transport in the control of substrate oxidation

This paper reviews the model of the control of mitochondrial substrate oxidation by Ca²⁺ ions. The mechanism is the activation by Ca²⁺ of four mitochondrial dehydrogenases, viz: glycerol 3-phosphate dehydrogenase, the pyruvate dehydrogenase multienzyme complex (PDH), NAD-linked isocitrate dehydrogenase (NAD-IDH) and 2-oxoglutarate dehydrogenase (OGDH). This results in the increase, or near-maintenance, of mitochondrial NADH/NAD ratios in the activated state, depending upon the tissue and the degree of "downstream" activation by Ca²⁺, likely at the level of the F₁F₀ ATP-ase. Higher values of the redox span of the respiratory chain allow for greatly increased fluxes through oxidative phosphorylation with a minimal drop in protonmotive force and phosphorylation potential. As PDH, NAD-IDH and OGDH are all located within the inner mitochondrial membrane, it is changes in matrix free Ca²⁺ ( [Ca²⁺]m ) which act as a signal to these activities. In this article, we review recent work in which ([Ca²⁺]m) is measured in cells and tissues, using different techniques, with special emphasis on the question of the degree of damping of ([Ca²⁺]m) relative to changes in cytosol free Ca²⁺ in cells with rapid transients in cytosol Ca²⁺, e.g. cardiac myocytes. Further, we put forward the point of view that the failure of mitochondrial energy transduction to keep pace with cellular energy needs in some forms of heart failure may involve a failure of ([Ca²⁺]m) to be raised adequately to allow the activation of the dehydrogenases. We present new data to show that this is so in cardiac myocytes isolated from animals suffering from chronic, atreptozocin-induced diabetes. This raises the possibility of therapy based upon partial inhibition of mitochondrial Ca²⁺ efflux pathways, thereby raising ([Ca²⁺]m) at a given, time-average value of cytosol free Ca²⁺.