Rate Of Electron Flux Research Articles

Flux through mitochondrial redox circuits linked to nicotinamide nucleotide transhydrogenase generates counterbalance changes in energy expenditure

Compensatory changes in energy expenditure occur in response to positive and negative energy balance, but the underlying mechanism remains unclear. Under low energy demand, the mitochondrial electron transport system is particularly sensitive to added energy supply (i.e. reductive stress), which exponentially increases the rate of H2O2 (JH2O2) production. H2O2 is reduced to H2O by electrons supplied by NADPH. NADP+ is reduced back to NADPH by activation of mitochondrial membrane potential-dependent nicotinamide nucleotide transhydrogenase (NNT). The coupling of reductive stress-induced JH2O2 production to NNT-linked redox buffering circuits provides a potential means of integrating energy balance with energy expenditure. To test this hypothesis, energy supply was manipulated by varying flux rate through β-oxidation in muscle mitochondria minus/plus pharmacological or genetic inhibition of redox buffering circuits. Here we show during both non-ADP- and low-ADP-stimulated respiration that accelerating flux through β-oxidation generates a corresponding increase in mitochondrial JH2O2 production, that the majority (∼70-80%) of H2O2 produced is reduced to H2O by electrons drawn from redox buffering circuits supplied by NADPH, and that the rate of electron flux through redox buffering circuits is directly linked to changes in oxygen consumption mediated by NNT. These findings provide evidence that redox reactions within β-oxidation and the electron transport system serve as a barometer of substrate flux relative to demand, continuously adjusting JH2O2 production and, in turn, the rate at which energy is expended via NNT-mediated proton conductance. This variable flux through redox circuits provides a potential compensatory mechanism for fine-tuning energy expenditure to energy balance in real time.

Adapting High-Resolution Respirometry to Glucose-Limited Steady State Mycelium of the Filamentous Fungus Penicillium ochrochloron: Method Development and Standardisation.

Fungal electron transport systems (ETS) are branched, involving alternative NADH dehydrogenases and an alternative terminal oxidase. These alternative respiratory enzymes were reported to play a role in pathogenesis, production of antibiotics and excretion of organic acids. The activity of these alternative respiratory enzymes strongly depends on environmental conditions. Functional analysis of fungal ETS under highly standardised conditions for cultivation, sample processing and respirometric assay are still lacking. We developed a highly standardised protocol to explore in vivo the ETS—and in particular the alternative oxidase—in Penicillium ochrochloron. This included cultivation in glucose-limited chemostat (to achieve a defined and reproducible physiological state), direct transfer without any manipulation of a broth sample to the respirometer (to maintain the physiological state in the respirometer as close as possible to that in the chemostat), and high-resolution respirometry (small sample volume and high measuring accuracy). This protocol was aimed at avoiding any changes in the physiological phenotype due to the high phenotypic plasticity of filamentous fungi. A stable oxygen consumption (< 5% change in 20 minutes) was only possible with glucose limited chemostat mycelium and a direct transfer of a broth sample into the respirometer. Steady state respiration was 29% below its maximum respiratory capacity. Additionally to a rotenone-sensitive complex I and most probably a functioning complex III, the ETS of P. ochrochloron also contained a cyanide-sensitive terminal oxidase (complex IV). Activity of alternative oxidase was present constitutively. The degree of inhibition strongly depended on the sequence of inhibitor addition. This suggested, as postulated for plants, that the alternative terminal oxidase was in dynamic equilibrium with complex IV—independent of the rate of electron flux. This means that the onset of activity does not depend on a complete saturation or inhibition of the cytochrome pathway.

Distinct conformational behaviors of four mammalian dual-flavin reductases (cytochrome P450 reductase, methionine synthase reductase, neuronal nitric oxide synthase, endothelial nitric oxide synthase) determine their unique catalytic profiles.

Multidomain enzymes often rely on large conformational motions to function. However, the conformational setpoints, rates of domain motions and relationships between these parameters and catalytic activity are not well understood. To address this, we determined and compared the conformational setpoints and the rates of conformational switching between closed unreactive and open reactive states in four mammalian diflavin NADPH oxidoreductases that catalyze important biological electron transfer reactions: cytochrome P450 reductase, methionine synthase reductase and endothelial and neuronal nitric oxide synthase. We used stopped-flow spectroscopy, single turnover methods and a kinetic model that relates electron flux through each enzyme to its conformational setpoint and its rates of conformational switching. The results show that the four flavoproteins, when fully-reduced, have a broad range of conformational setpoints (from 12% to 72% open state) and also vary 100-fold with respect to their rates of conformational switching between unreactive closed and reactive open states (cytochrome P450 reductase>neuronal nitric oxide synthase>methionine synthase reductase>endothelial nitric oxide synthase). Furthermore, simulations of the kinetic model could explain how each flavoprotein can support its given rate of electron flux (cytochromec reductase activity) based on its unique conformational setpoint and switching rates. The present study is the first to quantify these conformational parameters among the diflavin enzymes and suggests how the parameters might be manipulated to speed or slow biological electron flux.

Intermediate-term periodicities in relativistic solar electron fluences during solar cycles 22 and 23

In this paper, we have investigated the intermediate-term periodicities of the relativistic ( E > 10 MeV) solar electron flares measured by IMP-8 satellite of NASA for the time period of 1986–2001. This period of investigation includes the entire solar cycle 22; ascending, maximum and a part of descending phase of the current solar cycle 23. To determine accurately the occurrence rate of electron flux, we have employed three different spectral decomposition techniques, viz. fast Fourier transformation (FFT); maximum entropy method (MEM) and Lomb–Scargle periodogram analysis method. For solar cycle 22, in the low frequency range, power spectrum analysis exhibits statistically significant periodicities at ∼706, ∼504 and ∼392 days. In the intermediate frequency range, we have found a series of significant periodicities ∼294, ∼221, ∼153, ∼86, ∼73 and ∼66 days. For short term, periodicities of ∼21–23, ∼31 and ∼37 days were found in power spectrum. When solar cycle 23 is considered the significant periodicities are ∼20, ∼23, ∼29, ∼39, ∼54, ∼63, ∼118, ∼133 and ∼154 days. These results provide evidence that the best known Rieger period (∼153 days), appeared in the high energetic electron flux data for cycle 22 and also likely during maxima of cycle 23. The existence of these periodicities has been discussed in the light of earlier results.

Electron Transfer by Neuronal Nitric-oxide Synthase Is Regulated by Concerted Interaction of Calmodulin and Two Intrinsic Regulatory Elements

The nitric-oxide synthases (NOSs) are modular, cofactor-containing enzymes, divided into a heme-containing oxygenase domain and an FMN- and FAD-containing reductase domain. The domains are connected by a calmodulin (CaM)-binding sequence, occupancy of which is required for nitric oxide (NO) production. Two additional CaM-modulated regulatory elements are present in the reductase domains of the constitutive isoforms, the autoregulatory region (AR) and the C-terminal tail region. Deletion of the AR reduces CaM stimulation of electron flow through the reductase domain from 10-fold in wild-type nNOS to 2-fold in the mutant. Deletion of the C terminus yields an enzyme with greatly enhanced reductase activity in the absence of CaM but with activity equivalent to that of wild-type enzyme in its presence. A mutant in which both the AR and C terminus were deleted completely loses CaM modulation through the reductase domain. Thus, transduction of the CaM effect through the reductase domain of nNOS is dependent on these elements. Formation of nitric oxide is, however, still stimulated by CaM in all three mutants. A CaM molecule in which the N-terminal lobe was replaced by the C-terminal lobe (CaM-CC) supported NO synthesis by the deletion mutants but not by wild-type nNOS. We propose a model in which the AR, the C-terminal tail, and CaM interact directly to regulate the conformational state of the reductase domain of nNOS.

Enhanced Electron Flux and Reduced Calmodulin Dissociation May Explain “Calcium-independent” eNOS Activation by Phosphorylation

Bovine endothelial nitric oxide synthase (eNOS) is phosphorylated directly by the protein kinase Akt at serine 1179. Mutation of this residue to the negatively charged aspartate (S1179D eNOS) increases nitric oxide (NO) production constitutively, in the absence of agonist challenge. Here, we examine the potential mechanism of how aspartate at 1179 increases eNOS activity using purified proteins. Examination of NO production and cytochrome c reduction resulted in no substantial changes in the K(m)/EC(50) for L-arginine, calmodulin, and calcium, whereas there was a 2-fold increase in the rate of NO production for S1179D and a 2-4-fold increase in reductase activity (based on cytochrome c reduction). The observed increase in activity for both assays of NOS function indicates that a faster rate of electron flux through the reductase domain is likely the rate-limiting step in NO formation from eNOS. In addition, S1179D eNOS did show an increased resistance to inactivation by EGTA compared with wild type eNOS. These results suggest that a negative charge imposed at serine 1179, either by phosphorylation or by replacement with aspartate, increases eNOS catalytic activity by increasing electron flux at the reductase domain and by reducing calmodulin dissociation from activated eNOS when calcium levels are low.

A critique of the use of inhibitors to estimate partitioning of electrons between mitochondrial respiratory pathways in plants

The contribution of individual plant mitochondrial respiratory pathways to total respiration is commonly assessed by titration with specific inhibitors of different components in the branched electron transport chain. A pathway's contribution is equal to the activity when the other branch is blocked by an inhibitor multiplied by the degree (0‐1.0) to which this activity is engaged when both pathways are operating. According to Bahr and Bonner (1973. J. Biol. Chem. 218: 3441–3445) the plot of the activities of identical titrations, one performed in the absence and the other in the presence of a specific inhibitor of the other branch of the respiratory chain, yields a straight line whose slope indicates the engagement of the titrated pathway during uninhibited respiration. An initial slope of zero may occur if electron flux is diverted between pathways during titrations. However, beyond the breakpoint (representing the point of pathway saturation), a straight line is obtained with a slope representing engagement. This technique assumes that the kinetics of inhibiting a specific component of the respiratory chain are independent of the absolute rate of electron flux through the total pathway. To test this assumption, the activity of respiratory pathways in isolated soybean (Glycine max [L]. Merr. cv. Stevens) mitochondria was titrated with specific inhibitors of the cytochrome and alternative oxidases. Under these conditions, the electron flux through a given pathway was manipulated by poising the rate of succinate oxidation with the succinate dehydrogenase inhibitor malonate. Construction of activity plots in the presence versus absence of malonate failed to result in straight lines for either KCN (when titrating the cytochrome pathway) or salicylhydroxamic acid (when titrating the alternative pathway). Rather, the resultant plots were always curvilinear whenever the activity in the presence of malonate divided by the activity in the absence of malonate was less than 1.0. In no case could the real engagement of the pathway be precisely estimated from the titration data. Titrations of cytochrome pathway activity in isolated potato tuber (Solanum tuberosum L. cv. Sabago and Canabex) mitochondria (which lack the alternative oxidase) showed that as the inhibitor concentration was increased, so did the reduction status of the ubiquinone pool, to a new steady state. The dependence of inhibition kinetics on the rate of flux through the pathway, and the increase in ubiquinone pool reduction upon KCN addition, are explained in terms of the elasticity of component enzymes as outlined in the theory of metabolic control analysis. The implications of this finding for the use of titrations to estimate engagement of plant respiratory pathways are discussed.

On the mechanism of control of photosynthetic electron transport by phosphorylation

FEBS LettersVolume 69, Issue 1-2 p. 81-85 Full-length articleFree Access On the mechanism of control of photosynthetic electron transport by phosphorylation Donald Ort, Donald Ort Department of Biological Sciences, Purdue University, West Lafayette, In. 47909, USASearch for more papers by this author Donald Ort, Donald Ort Department of Biological Sciences, Purdue University, West Lafayette, In. 47909, USASearch for more papers by this author First published: October 15, 1976 https://doi.org/10.1016/0014-5793(76)80658-8Citations: 11AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat References 1 B. Rumberg, U. Siggel, Naturwissenschaften, 55, (1968), 77– 79. 2 E. Bamberger, H. Rottenberg, H. Avron, Eur. J. Biochem., 34, (1973), 557– 563. 3 A. Trebst, W. Briggs Ann. Review of Plant Physiol. 25, (1974), Annual Review Inc Palo Alto, Calif 423– 458. 4 D. Ort, R. Dilley, Biochim. Biophys. Acta, (1976), in press 5 D. Ort, R. Dilley, N. Good, Biochim. Biophys. Acta, (1976), in press 6 S. Izawa, G. Hind, Biochim. Biophys. Acta, 143, (1967), 377– 390. 7 W. Schliephake, W. Junge, H. Witt, Z. Naturforsch., 23b, (1968), 1571– 1578. 8 S. Izawa, J. Gould, D. Ort, P. Felker, N. Good, Biochim. Biophys. Acta, 305, (1973), 119– 128. 9 A. Trebst, S. Reimer, Biochim. Biophys. Acta, 305, (1973), 129– 139. 10 J. Gould, D.R. Ort, Biochim. Biophys. Acta, 325, (1973), 157– 166. 11 A. Trebst, E. Harth, W. Draber, Z. Naturforsch, 25b, (1970), 1157– 1159. 12 B. Kok, P. Joliot, M. McGloin, H. Metzner Progress in Photosynthesis Research II, (1969), Internat. Union Biol. Sci. Tubingen 1042– 1056. 13 R. Gaensslen, R. McCarty, Anal. Biochem., 48, (1972), 504– 514. 14 M. Black, R. Brandt, Anal. Biochem., 58, (1974), 246– 254. 15 M. Avron, B. Chance, Brookhaven Symposia in Biol., 19, (1966), 149– 160. Citing Literature Volume69, Issue1-2October 15, 1976Pages 81-85 ReferencesRelatedInformation

The Mechanism of Ethylene and Cyanide Action in Triggering the Rise in Respiration in Potato Tubers

Ethylene and cyanide induce a sharp increase in respiration in potato tubers (Solanum tuberosum, var. Russet) attended by changes in the glycolytic intermediates which indicate that both gases enhance glycolysis. The level of sucrose also increases in response to both treatments. The data are taken to indicate that both cyanide and ethylene either activate or affect the link between the conventional electron transport chain and the cyanide-insensitive path. It is further proposed that this activation may well be the primary event leading to the rise in respiration. Ethylene increases the level of adenosine 5'-triphosphate and it is suggested that because of the 4- to 6-fold increase in the rate of electron flux through site I, which continues to operate in the over-all cyanide-insensitive path, the absolute levels of adenosine 5'-triphosphate will also be expected to increase in the presence of cyanide. The increase in sucrose content is considered to be the consequence of the rise in adenosine 5'-triphosphate concentration.

Inhibition by cyanide of the respiratory chain oxidases of Escherichia coli

The kinetics of inhibition by KCN of NADH oxidation in respiratory particles from Escherichia coli could be related to the relative amounts of cytochromes d and o which were present. Particles which contained higher levels of cytochrome d relative to cytochrome o were less sensitive to inhibition by cyanide. When cyanide reacted with the respiratory particles, the absorption bands of reduced cytochrome d at 442 and 628 nm in the reduced plus cyanide minus reduced difference spectrum were eliminated, as also were the bands at 423, 428, and 555 nm of b- and/or c-type cytochromes. Cyanide appeared to react with the oxidized form of cytochrome d to eliminate its α-band absorption with a second-order rate constant of 0.011 m −1 sec −1 for the rate of formation of cyanocytochrome d in the absence of added substrate. Under turnover conditions using NADH as substrate, the rate constant was 0.58 m −1 sec −1. This value is close to that determined from cyanide inhibition of NADH oxidase activity. The magnitude of the second-order rate constant for the formation of cyanocytochrome d was directly related to the rate of electron flux through cytochrome d. It is suggested that an intermediate species formed during the normal oxidation-reduction cycle of cytochrome d reacts with cyanide.

Studies on the energy coupling sites of photophosphorylation. III. The different effects of methylamine and ADP plus phosphate on electron transport through coupling sites I and II in isolated chloroplasts

1. 1. The reduction of lipophilic (Class III) oxidants such as oxidized p-phenylenediamine consists of two components. One component requires both Photosystem II and Photosystem I and includes both sites of energy coupling associated with noncyclic electron transport. The second component requires only Photosystem II and includes only the site of energy coupling located before plastoquinone (Site II). When oxidized p-phenylenediamine is being reduced by both pathways, the overall rate of electron transport is stimulated by the addition of ADP plus phosphate or the uncoupler methylamine. However, if the Photosystem I component of oxidized p-phenylenediamine reduction is eliminated by a low concentration of the plastoquinone-antagonist dibromothymoquinone, the stimulation of electron transport by ADP plus phosphate or methylamine is also abolished, although the remaining Photosystem II-dependent electron transport remains firmly coupled to phosphorylation ( via coupling Site II). These results indicate that coupling Site II, unlike the well-known rate-limiting coupling site between plastoquinone and cytochrome f (Site I), does not exert any control over the rate of associated electron transport. 2. 2. When substituted p-benzoquinones ( e.g. 2,5-dimethyl- p-benzoquinone) or quinonediimides ( e.g. p-phenylenediimine) are used as Class III acceptors in conjunction with dibromothymoquinone, a small but significant stimultation of electron transport by ADP plus phosphate is observed. However, it can be shown that this stimulation does not arise from coupling Site II but rather is due to a low rate of electron flux through coupling Site I even in the presence of dibromothymoquinone. Apparently the p-benzoquinones can catalyze an electron “bypass” around the dibromothymoquinone-induced block at plastoquinone, possibly by substituting partially for the natural electron carrier. If this bypass electron flow is blocked at plastocyanin by KCN treatment, the stimulation of electron transport by ADP plus phosphate is eliminated, although a high rate of phosphorylation (from Site II only) remains. 3. 3. These results provide strong evidence that a profound difference exists between the two sites of energy coupling associated with non-cyclic electron transport in isolated chloroplasts. That is, the rate of electron flow through coupling Site I, which is the rate-determining step in the Hill reaction, is strictly regulated by phosphorylating conditions, whereas the rate of electron flux through coupling Site II is independent of phosphorylating conditions. 4. 4. A model is presented which accounts for the lack of control over electron transport exhibited by coupling Site II. It is postulated that Site II is coupled to an essentially irreversible electron transport step, so that conditions which affect the phosphorylation reaction would have no effect on the rate of electron transport through the coupling site. Two essentially irreversible reactions, closely associated with Photosystem II—the water-splitting reaction and the System II photoact itself—are discussed as possible locations for coupling Site II.

Control of the Removal of Reducing Equivalents from the Cytosol in Perfused Rat Liver

Abstract The rat liver perfusion previously employed was adapted to a flow-through system in which samples of effluent perfusion medium were collected for measurement of metabolic products at 2-min intervals by means of a fraction collector. Overnight fasted rats were used to avoid complications of glucose formation from endogenous glycogen. Since xylitol conversion to glucose and lactate by the liver is associated with the production of NADH in the cytosol, the rates of production of these metabolic end products can be used to provide a measure of the rate of NADH production and utilization. Xylitol metabolism was predominantly cyanide sensitive showing an obligatory involvement of mitochondrial respiration for removal of reducing equivalents from the cytosol. The participation of various shuttles for the transport of reducing equivalents into the mitochondria was investigated by infusion of the oxidized or reduced partners of cytosolic or mitochondrial NAD-linked dehydrogenases (e.g. ethanol, β-hydroxybutyrate), or specific inhibitors such as rotenone, amobarbital, cyanide, and the transaminase inhibitor aminooxyacetate. The influence of flux in the mitochondrial electron transport chain on the rate of removal of NADH from the cytosol was studied by infusion of 2,4-dinitrophenol to produce an uncoupling of oxidative phosphorylation, ornithine plus ammonia to provide an energy drain on mitochondrial ATP, and artificial electron acceptors to bypass rate-limiting electron transport steps. The studies show that xylitol metabolism was not controlled by the activity of NAD-xylitol dehydrogenase but by the rate of reoxidation of NADH. This process was regulated by the rate of transfer of reducing equivalents into the mitochondria and by the rate of electron flux to oxygen. Evidence was obtained for the participation of both NAD- and flavin-linked shuttles in the over-all transport of reducing equivalents from cytosol to mitochondria. The flavin-linked shuttle may be identified with the α-glycerophosphate shuttle which operates by virtue of the abnormally high cytosolic α-glycerophosphate concentration produced during xylitol metabolism. Involvement of the NAD-linked malate-aspartate shuttle was shown by the sensitivity of glucose formation from xylitol to aminooxyacetate and β-hydroxybutyrate. The relevance of these findings to the control of ethanol utilization by the liver is discussed.

The metabolism of pyruvate by bovine-adrenal-cortex mitochondria.

1 The metabolism of pyruvate by isolated bovine-adrenal-cortex mitochondria has been studied. The rate of pyruvate oxidation depends on the availability of oxaloacetate. Oxalo-acetate is formed from malate but malate cannot function in a catalytic manner in the Krebs cycle, because some is oxidised to pyruvate via malic enzyme activity with resulting diminution of the oxaloacetate pool. 2 An additional source of oxaloacetate is provided by low levels of pyruvate carboxylase. In the presence of ATP and bicarbonate, the principal initial product of pyruvate metabolism is citrate. Reducing equivalents for 11β-hydroxylation of deoxycorticosterone may be provided by oxidation of citrate. 3 When oxaloacetate is supplied by malate oxidation, the rate of citrate formation in the presence of fluorocitrate is controlled by the rate of electron flux through either the respiratory chain or the hydroxylation chain, and is increased by addition of ADP on the one hand, or deoxy-corticosterone on the other. This appears to be due to reoxidation of the pyridine nucleotides resulting in increased availability of oxaloacetate for citrate formation. Equilibration of NAD+/NADH and NADP+/NADPH is achieved by transhydrogenase activity but this activity is not energy-linked under the conditions of these experiments. 4 In the absence of fluorocitrate, the stimulation of citrate formation brought about by initiation of 11β-hydroxylation results in increased Krebs cycle activity. In the transition from state 4 to state 3 respiration, there is an additional activation of citrate oxidation, which is interpreted to be due to activation of NAD+-linked isocitrate dehydrogenase on addition of ADP. 5 Addition of both deoxycorticosterone and ADP together to the mitochondria results in inhibition of state 3 electron flux through the respiratory chain, with concomitant inhibition of ATP formation. The rate of 11β-hydroxylation is not affected. Inhibition of ATP formation can be achieved by a variety of steroids and correlates with inhibition of NADH oxidase.

Effect of various inducers on electron transport system associated with drug metabolism by liver microsomes

The metabolism of a wide variety of drugs and other compounds is catalyzed by liver microsomal enzyme systems involving cytochrome P-450 and NADPH-cytochrome c reductase. The sequence of events in these reactions is presumably as follows: The drug substrate reacts with the oxidized form of cytochrome P-450 to form a complex which is reduced either directly by NADPH-cytochrome c reductase or indirectly via an unidentified electron carrier. The reduced cytochrome P-450-substrate complex then reacts with oxygen to form an “active oxygen” complex, which decomposes with the formation of the oxidized drug and oxidized cytochrome P-450. Since the rate of reduction of the cytochrome P-450-substrate is considerably slower than the rate of oxidation of the complex under steady-state conditions, most of the cytochrome P-450-substrate complex is in the oxidized form. The rate of electron flux through the system, however, may be modified by altering the amount of cytochrome P-450-substrate complex, or the rate at which it is reduced. Inducers of drug metabolism may be classified according to their effects on the various components of the system. Phenobarbital-like inducers cause increases in NADPH-cytochrome c reductase and cytochrome P-450, and thus increases in the amount of cytochrome P-450-substrate complex and its rate of reduction. Polycyclic hydrocarbons, such as methylcholanthrene, induce the formation of a variant of cytochrome P-450, which has different affinities for the various drug substrates than does the usual form. This kind of inducer does not increase either NADPH-cytochrome c reductase or the rate of cytochrome P-450 reduction; indeed in some experiments they decrease the rate of cytochrome P-450 reduction per cytochrome P-450. In female rats, spironolactone and perhaps other anabolic steroids increase NADPH-cytochrome c reductase activity and the rate of cytochrome P-450-substrate reduction, but have little or no effect on the amount of cytochrome P-450. The relationship between the electron flux and drug metabolism in liver microsomes depends on the substrate. For example, the demethylation of ethylmorphine and NADPH oxidation (or cytochrome P-450 reduction) approaches the theoretical 1:1 relationship, whereas, the rate of hydroxylation of hexobarbital may be considerably less than the substrate induced rate of NADPH oxidation (and cytochrome P-450 reduction).

Rate of ubiquinone oxidation in electron transport particles reduced by succinate

The half-time for oxidation of reduced ubiquinone in electron transport particles (ETP) made anaerobic with succinate is 3–6 msec and is comparable to that for cytochrome c which lies between 2 and 3 msec in these particles. The half-time for oxidation of reduced cytochrome b is much greater, being in the range of 60–100 msec. These results imply that ubiquinone and cytochrome b form two branches of electron transport between succinate and cytochrome c, but that most of the electron transport occurs through ubiquinone. Previous work had showed that ubiquinone is peripheral to the NADH oxidase pathway in these particles (4). Comparison of these rates in ETP with rates of electron transport through cytochrome b and ubiquinone in intact mitochondria (15) and in submitochondrial particles derived by sonication (16) indicates that increasing disruption of mitochondrial structure decreases the rate of electron flux through cytochrome b and increases the rate through ubiquinone. Assignment to ubiquinone of a central function in electron transport, based on experiments involving disruption and fractionation of the electron transport chain. would appear to be unjustified.