Myeloperoxidase-mediated damage to the succinate oxidase system of Escherichia coli. Evidence for selective inactivation of the dehydrogenase component

Myeloperoxidase, a granule-associated enzyme of neutrophils and monocytes, combines with H2O2 and chloride to form a potent microbicidal system that contributes to phagocyte antimicrobial activity. The nature of the lesion or lesions induced by the myeloperoxidase system which are responsible for the loss of microbial replicative activity (viability) remains unknown. Using Escherichia coli grown to late log or stationary phase under conditions of low aeration with succinate as the sole carbon source, we found that myeloperoxidase-induced loss of microbial viability could be correlated with a decrease in succinate-dependent respiration (succinate oxidase activity). Succinate dehydrogenase activity fell rapidly to undetectable levels during incubation with the myeloperoxidase system, suggesting that damage to the dehydrogenase was a major factor in the loss of oxidase activity. Other components of the succinate oxidase system were resistant to the actions of myeloperoxidase. The ubiquinone-8 and cytochrome components of the respiratory chain remained nearly constant in amount despite reduction of respiration to undetectable levels. However, as expected from the loss of succinate dehydrogenase activity, succinate-ubiquinone reductase and succinate-cytochrome reductase activities were markedly impaired. We propose that the loss of E. coli viability induced by the myeloperoxidase-H2O2-chloride system is due in part to the loss of electron transport function consequent to the oxidation of critical catalytic centers in susceptible dehydrogenases.     

Differential inactivation of Escherichia coli membrane dehydrogenases by a myeloperoxidase-mediated antimicrobial system.

Neutrophil myeloperoxidase, hydrogen peroxide, and chloride constitute a potent antimicrobial system with multiple effects on microbial cytoplasmic membranes. Among these is inhibition of succinate-dependent respiration mediated, principally, through inactivation of succinate dehydrogenase. Succinate-dependent respiration is inhibited at rates that correlate with loss of microbial viability, suggesting that loss of respiration might contribute to the microbicidal event. Because respiration in Escherichia coli can be mediated by dehydrogenases other than succinate dehydrogenase, the effects of the myeloperoxidase system on other membrane dehydrogenases were evaluated by histochemical activity stains of electrophoretically separated membrane proteins. Two bands of succinate dehydrogenase activity proved the most susceptible to inactivation with complete loss of staining activity within 20 min, under the conditions employed. A group with intermediate susceptibility, consisting of lactate, malate, glycerol-3-phosphate, and dihydroorotate dehydrogenases as well as three bands of glucose-6-phosphate dehydrogenase, was almost completely inactivated within 30 min. The relatively resistant group, including the dehydrogenases for glutamate, NADH, and NADPH and the remaining bands of glucose-6-phosphate dehydrogenase, retained substantial amounts of diaphorase activity for up to 60 min of incubation with the myeloperoxidase system. The differential effects of myeloperoxidase on dehydrogenase inactivation could not be correlated with published enzyme contents of flavin or iron-sulfur centers, potential targets of myeloperoxidase-derived oxidants. Despite the relative resistance of NADH dehydrogenase/diaphorase activity to myeloperoxidase-mediated inactivation, electron transport particles prepared from E. coli incubated for 20 min with the myeloperoxidase system lost 55% of their NADH oxidase activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hypochlorous acid and myeloperoxidase-catalyzed oxidation of iron-sulfur clusters in bacterial respiratory dehydrogenases.

Hypochlorous acid and related oxidants derived from myeloperoxidase-catalyzed reactions contribute to the microbicidal activities of phagocytosing neutrophils and monocytes. Microbial iron-sulfur (Fe/S) clusters have been suggested as general targets of myeloperoxidase-derived oxidations, but no susceptible Fe/S site has yet been identified. In this study, the effects of HOCl and myeloperoxidase-catalyzed peroxidation of chloride ion upon EPR-detectable Fe/S clusters in Escherichia coli and Pseudomonas aeruginosa were examined. Increasing amounts of oxidant produced progressive loss of signal amplitudes from the S-1 and S-3 Fe/S clusters of succinate:ubiquinone oxidoreductase in respiring membrane fragments. These changes were compared to loss of microbial viability, succinate uptake rates, succinate dehydrogenase activity and succinate-dependent respiration. The amounts of oxidant required to destroy Fe/S clusters exceeded the amounts required to kill organisms or inhibit respiratory function by factors of four or five. Power saturation characteristics of the S-1 signal indicated that the S-2 signal was also resistant to modification, even in highly oxidized membranes. Loss of succinate-dependent respiration was closely associated with HOCl and myeloperoxidase-mediated microbicidal activity against P. aeruginosa and was also an early event in the oxidant-mediated metabolic dysfunctions of E. coli. However, these effects were not caused by the destruction of the Fe/S clusters within the succinate:ubiquinone oxidoreductase. Rather, the major respiration-inhibiting lesion(s) appeared to reside at points in the respiratory chain between the Fe/S clusters and the ubiquinone reductase site.     

Role of ubiquinone in the mitochondrial generation of hydrogen peroxide.

Antimycin-inhibited bovine heart submitochondrial particles generate O2- and H2O2 with succinate as electron donor. H2O2 generation involves the action of the mitochondrial superoxide dismutase, in accordance with the McCord & Fridovich [(1969) j. biol. Chem. 244, 6049-6055] reaction mechanism. Removal of ubiquinone by acetone treatment decreases the ability of mitochondrial preparations to generate O2- and H2O2, whereas supplementation of the depleted membranes with ubiquinone enhances the peroxide-generating activity in the reconstituted membranes. Addition of superoxide dismutase to ubiquinone-reconstituted membranes is essential in order to obtain maximal rates of H2O2 generation since the acetone treatment of the membranes apparently inactivates (or removes) the mitochondrial superoxide dismutase. Parallel measurements of H2O2 production, succinate dehydrogenase and succinate-cytochrome c reductase activities show that peroxide generation by ubiquinone-supplemented membranes is a monotonous function of the reducible ubiquinone content, whereas the other two measured activities reach saturation at relatively low concentrations of reducible quinone. Alkaline treatment of submitochondrial particles causes a significant decrease in succinate dehydrogenase activity and succinate-dependent H2O2 production, which contrasts with the increase of peroxide production by the same particles with NADH as electron donor. Solubilized succinate dehydrogenase generates H2O2 at a much lower rate than the parent submitochondrial particles. It is postulated that ubisemiquinone (and ubiquinol) are chiefly responsible for the succinate-dependent peroxide production by the mitochondrial inner membrane.     

The role of Sdh4p Tyr-89 in ubiquinone reduction by the Saccharomyces cerevisiae succinate dehydrogenase

Succinate dehydrogenase (complex II or succinate:ubiquinone oxidoreductase) is a tetrameric, membrane-bound enzyme that catalyzes the oxidation of succinate and the reduction of ubiquinone in the mitochondrial respiratory chain. Two electrons from succinate are transferred one at a time through a flavin cofactor and a chain of iron-sulfur clusters to reduce ubiquinone to an ubisemiquinone intermediate and to ubiquinol. Residues that form the proximal quinone-binding site (Q_P) must recognize ubiquinone, stabilize the ubisemiquinone intermediate, and protonate the ubiquinone to ubiquinol, while minimizing the production of reactive oxygen species. We have investigated the role of the yeast Sdh4p Tyr-89, which forms a hydrogen bond with ubiquinone in the Q_P site. This tyrosine residue is conserved in all succinate:ubiquinone oxidoreductases studied to date. In the human SDH, mutation of this tyrosine to cysteine results in paraganglioma, tumors of the parasympathetic ganglia in the head and neck. We demonstrate that Tyr-89 is essential for ubiquinone reductase activity and that mutation of Tyr-89 to other residues does not increase the production of reactive oxygen species. Our results support a role for Tyr-89 in the protonation of ubiquinone and argue that the generation of reactive oxygen species is not causative of tumor formation.     

Reactivity of the Bacillus subtilis succinate dehydrogenase complex with quinones.

The succinate dehydrogenase isolated from Bacillus subtilis was found to catalyze the oxidation of succinate with hydrophilic quinones. Either naphthoquinones or benzoquinones served as acceptors. The enzyme activity increased with the redox potential of the quinone. The highest turnover number was commensurate with that of the bacterial succinate respiration in vivo. The succinate dehydrogenase was similarly active in fumarate reduction with quinols. The highest activity was obtained with the most electronegative quinol. The fumarate reductase isolated from Wolinella succinogenes catalyzed succinate oxidation with quinones and fumarate reduction with the corresponding quinols at activities similar to those of the B. subtilis enzyme. Succinate oxidation by the lipophilic quinones, ubiquinone or vitamin K-1, was monitored as cytochrome c reduction using proteoliposomes containing succinate dehydrogenase together with the cytochrome bc1 complex. The activity with ubiquinone or vitamin K-1 was commensurate with the succinate respiratory activity of bacteria or of the bacterial membrane fraction. The results suggest that menaquinone is involved in the succinate respiration of B. subtilis, although its redox potential is unfavorable.     

The role of Sdh4p Tyr-89 in ubiquinone reduction by the Saccharomyces cerevisiae succinate dehydrogenase

Succinate dehydrogenase (complex II or succinate:ubiquinone oxidoreductase) is a tetrameric, membrane-bound enzyme that catalyzes the oxidation of succinate and the reduction of ubiquinone in the mitochondrial respiratory chain. Two electrons from succinate are transferred one at a time through a flavin cofactor and a chain of iron-sulfur clusters to reduce ubiquinone to an ubisemiquinone intermediate and to ubiquinol. Residues that form the proximal quinone-binding site (Q(P)) must recognize ubiquinone, stabilize the ubisemiquinone intermediate, and protonate the ubiquinone to ubiquinol, while minimizing the production of reactive oxygen species. We have investigated the role of the yeast Sdh4p Tyr-89, which forms a hydrogen bond with ubiquinone in the Q(P) site. This tyrosine residue is conserved in all succinate:ubiquinone oxidoreductases studied to date. In the human SDH, mutation of this tyrosine to cysteine results in paraganglioma, tumors of the parasympathetic ganglia in the head and neck. We demonstrate that Tyr-89 is essential for ubiquinone reductase activity and that mutation of Tyr-89 to other residues does not increase the production of reactive oxygen species. Our results support a role for Tyr-89 in the protonation of ubiquinone and argue that the generation of reactive oxygen species is not causative of tumor formation.     

Succinate modulation of H2O2 release at NADH:ubiquinone oxidoreductase (Complex I) in brain mitochondria

Complex I (NADH:ubiquinone oxidoreductase) is responsible for most of the mitochondrial H2O2 release, both during the oxidation of NAD-linked substrates and during succinate oxidation. The much faster succinate-dependent H2O2 production is ascribed to Complex I, being rotenone-sensitive. In the present paper, we report high-affinity succinate-supported H2O2 generation in the absence as well as in the presence of GM (glutamate/malate) (1 or 2 mM of each). In brain mitochondria, their only effect was to increase from 0.35 to 0.5 or to 0.65 mM the succinate concentration evoking the semi-maximal H2O2 release. GM are still oxidized in the presence of succinate, as indicated by the oxygen-consumption rates, which are intermediate between those of GM and of succinate alone when all substrates are present together. This effect is removed by rotenone, showing that it is not due to inhibition of succinate influx. Moreover, alpha-oxoglutarate production from GM, a measure of the activity of Complex I, is decreased, but not stopped, by succinate. It is concluded that succinate-induced H2O2 production occurs under conditions of regular downward electron flow in Complex I. Succinate concentration appears to modulate the rate of H2O2 release, probably by controlling the hydroquinone/quinone ratio.     

Separation of respiratory reactions in Rhodospirillum rubrum: Inhibition studies with 2-hydroxydiphenyl

1. Respiration of chemotrophically and phototrophically grown Rhodospirillum rubrum is inhibited by 2-hydroxydiphenyl.2. Membrane-bound NADH oxidase and NADH: cytochrome c reductase are inhibited also. The inhibitor constant for both reactions (K_i) is 0.075±0.012 mM. NADH dehydrogenase is not inhibited significantly.3. The inhibition of succinate:cytochrome c reductase is associated for chemotrophic membranes with K_i = 0.22±0.03 mM and for phototrophic membranes with K_i = 0.49±0.09 mM. Succinate dehydrogenase is not affected by 2-hydroxydiphenyl.4. Cytochrome oxidase is inhibited only slightly.5. While NADH-dependent reactions in both phototrophic and chemotrophic membranes are inhibited maximally more than 95%, succinate-dependent reactions can be inhibited more than 95% only in chemotrophic membranes. In photo-trophic membranes the maximum inhibition of succinate-dependent reactions is about 70%.6. The type of inhibition in both cases 2 and 3 is non-competitive.7. While the reduction of b-type cytochrome is inhibited by 2-hydroxydiphenyl, the degree of ubiquinone reduction is not influenced. The data suggest that the site of inhibition is localized between ubiquinone and cytochrome b.8. Implications of these data for the respiratory electron transport system in R. rubrum are discussed.     

On the sidedness of the ubiquinone redox cycle. Kinetic studies in mitochondrial membranes

The inhibition of NADH oxidation but not of succinate oxidation by the low ubiquinone homologs UQ-2 and UQ-3 is not due to a lower rate of reduction of ubiquinone by NADH dehydrogenase: experiments in submitochondrial particles and in pentane-extracted mitochondria show that UQ-3 is reduced at similar rates using either NADH or succinate as substrates. The fact that reduced UQ-3 cannot be reoxidized when reduced by NADH but can be reoxidized when reduced by succinate may be explained by a compartmentation of ubiquinone.Using reduced ubiquinones as substrates of ubiquinol oxidase activity in intact mitochondria and in submitochondrial particles we found that ubiquinol-3 is oxidized at higher rates in submitochondrial particles than in mitochondria. The initial rates of ubiquinol oxidation increased with increasing lengths of isoprenoid side chains in mitochondria, but decreased in submitochondrial particles. These findings suggest that the site of oxidation of reduced ubiquinone is on the matrix side of the membrane; reduced ubiquinones may reach their oxidation site in mitochondria only crossing the lipid bilayer: the rate of diffusion of ubiquinol-3 is presumably lower than that of ubiquinol-7 due to the differences in hydrophobicity of the two quinones.     

Reconstitution of pyrroloquinoline quinone-dependent D-glucose oxidase respiratory chain of Escherichia coli with cytochrome o oxidase.

D-Glucose dehydrogenase is a pyrroloquinoline quinone-dependent primary dehydrogenase linked to the respiratory chain of a wide variety of bacteria. The enzyme exists in the membranes of Escherichia coli, mainly as an apoenzyme which can be activated by the addition of pyrroloquinoline quinone and magnesium. Thus, membrane vesicles of E. coli can oxidize D-glucose to gluconate and generate an electrochemical proton gradient in the presence of pyrroloquinoline quinone. The D-glucose oxidase-respiratory chain was reconstituted into proteoliposomes, which consisted of two proteins purified from E. coli membranes, D-glucose dehydrogenase and cytochrome o oxidase, and E. coli phospholipids containing ubiquinone 8. The electron transfer rate during D-glucose oxidation and the membrane potential generation in the reconstituted proteoliposomes were almost the same as those observed in the membrane vesicles when pyrroloquinoline quinone was added. The results demonstrate that the quinoprotein, D-glucose dehydrogenase, can reduce ubiquinone 8 directly within phospholipid bilayer and that the D-glucose oxidase system of E. coli has a relatively simple respiratory chain consisting of primary dehydrogenase, ubiquinone 8, and a terminal oxidase.     

Alteration of inner-membrane components and damage to electron-transfer activities of bovine heart submitochondrial particles induced by NADPH-dependent lipid peroxidation.

We investigated the changes of the inner-membrane components and the electron-transfer activities of bovine heart submitochondrial particles induced by the lipid peroxidation supported by NADPH in the presence of ADP-Fe3+. Most of the polyunsaturated fatty acids were lost as a result of the peroxidation, and phospholipids were changed to polar species. Ubiquinone was also modified to polar substances as the peroxidation proceeded. Sodium dodecyl sulphate/polyacrylamide-gel electrophoresis showed the disappearance of 27000-Mr and 30000-Mr proteins and the appearance of highly polymerized substances. Flavins and cytochromes were not diminished, but the respiratory activity was lost. The reactions of NADH oxidase and NADH-cytochrome c reductase were most sensitive to the peroxidation, followed by those of succinate oxidase and succinate-cytochrome c reductase. Succinate dehydrogenase and duroquinol-cytochrome c reductase were inactivated by more extensive peroxidation, but cytochrome c oxidase was only partially inactivated. NADH-ferricyanide reductase was not inactivated. The pattern of the inactivation indicated that the lipid peroxidation affected the electron transport intensively between NADH dehydrogenase and ubiquinone, and moderately at the succinate dehydrogenase step and between ubiquinone and cytochrome c.     

D-lactate oxidation and generation of the proton electrochemical gradient in membrane vesicles from Escherichia coli GR19N and in proteoliposomes reconstituted with purified D-lactate dehydrogenase and cytochrome o oxidase

The respiratory chain in the cytochrome d deficient mutant Escherichia coli GR19N is a relatively simple, linear system consisting of primary dehydrogenases, ubiquinone 8, cytochrome b-556, and cytochrome o oxidase. By use of right-side-out and inside-out membrane vesicles from this strain, various oxidase activities and the generation of the H+ electrochemical gradient were studied. Oxidation of ubiquinol 1 or N,N,-N',N'-tetramethyl-p-phenylenediamine, which donate electrons directly to the terminal oxidase, generates a H+ electrochemical gradient comparable to that observed during D-lactate oxidation. In contrast, D-lactate/ubiquinone 1 or D-lactate/ferricyanide oxidoreductase activity does not appear to generate a membrane potential, suggesting that electron flow from D-lactate dehydrogenase to ubiquinone is not electrogenic. Moreover, proteoliposomes reconstituted with purified D-lactate dehydrogenase, ubiquinone 8, and purified cytochrome o catalyze D-lactate and ubiquinol 1 oxidation and generate a H+ electrochemical gradient similar to that observed in membrane vesicles. Strikingly, in inside-out vesicles, NADH oxidation generates a H+ electrochemical gradient that is very significantly greater than that produced by either D-lactate or ubiquinol 1; furthermore, NADH/ubiquinone 1 and NADH/ferricyanide oxidoreductase activities are electrogenic. It is suggested that the only component between D-lactate dehydrogenase or ubiquinol and oxygen in GR19N membranes that is directly involved in the generation of the H+ electrochemical gradient is cytochrome o, which functions as a "half-loop" (i.e., the oxidase catalyzes the scalar release of 2 H+ from ubiquinol on the outer surface of the membrane.(ABSTRACT TRUNCATED AT 250 WORDS)     

The Carboxin-Binding Site on Paracoccus denitrificans Succinate:Quinone Reductase Identified by Mutations

Succinate:quinone reductase catalyzes electron transfer from succinate to quinone in aerobic respiration. Carboxin is a specific inhibitor of this enzyme from several different organisms. We have isolated mutant strains of the bacterium Paracoccus denitrificans that are resistant to carboxin due to mutations in the succinate:quinone reductase. The mutations identify two amino acid residues, His228 in SdhB and Asp89 in SdhD, that most likely constitute part of a carboxin-binding site. This site is in the same region of the enzyme as the proposed active site for ubiquinone reduction. From the combined mutant data and structural information derived from Escherichia coli and Wolinella succinogenes quinol:fumarate reductase, we suggest that carboxin acts by blocking binding of ubiquinone to the active site. The block would be either by direct exclusion of ubiquinone from the active site or by occlusion of a pore that leads to the active site.     

Alternative Oxidase Activity and the Ubiquinone Redox Level in Soybean Cotyledon and Arum Spadix Mitochondria during NADH and Succinate Oxidation

In Arum and soybean (Glycine max L.) mitochondria, the dependence of the alternative oxidase activity on the redox level of ubiquinone, with NADH and succinate as substrates, was studied, using a voltametric procedure to measure the ubiquinone redox poise in the mitochondrial membrane. The results showed that when the enzyme was activated by pyruvate the relationship between the alternative oxidase rate and the redox state of the ubiquinone pool was the same for both NADH and succinate oxidations. In the absence of pyruvate the alternative oxidase had an apparent lower affinity for ubiquinol. This was more marked with NADH than with succinate and was possibly due to pyruvate production during succinate oxidation or to an activation of the alternative oxidase by succinate itself. In Arum spadix (unlike soybean cotyledon) mitochondria, succinate oxidation via the alternative oxidase maintained the ubiquinone pool in a partially reduced state (60%), whereas NADH oxidation kept it almost completely reduced. Previous data comparing mitochondria from thermogenic and nonthermogenic tissues have not examined the full range of ubiquinone redox levels in both tissues, leading to the suggestion that the activity of alternative oxidase for Arum was different from nonthermogenic tissues. When the complete range of redox states of ubiquinone is used and the oxidase is fully activated, the alternative oxidase from thermogenic tissue (Arum) behaves similarly to that of nonthermogenic tissue (soybean).     

Ferulenol specifically inhibits succinate ubiquinone reductase at the level of the ubiquinone cycle

The natural compound ferulenol, a sesquiterpene prenylated coumarin derivative, was purified from Ferula vesceritensis and its mitochondrial effects were studied. Ferulenol caused inhibition of oxidative phoshorylation. At low concentrations, ferulenol inhibited ATP synthesis by inhibition of the adenine nucleotide translocase without limitation of mitochondrial respiration. At higher concentrations, ferulenol inhibited oxygen consumption. Ferulenol caused specific inhibition of succinate ubiquinone reductase without altering succinate dehydrogenase activity of the complex II. This inhibition results from a limitation of electron transfers initiated by the reduction of ubiquinone to ubiquinol in the ubiquinone cycle. This original mechanism of action makes ferulenol a useful tool to study the physiological role and the mechanism of electron transfer in the complex II. In addition, these data provide an additional mechanism by which ferulenol may alter cell function and demonstrate that mitochondrial dysfunction is an important determinant in Ferula plant toxicity.     

Malate oxidation by mitochondrial succinate:ubiquinone-reductase

Succinate:ubiquinone reductase was shown to catalyze the oxidation of L- and D-stereoisomers of malate by artificial electron acceptors and ubiquinone. The rate of malate oxidation by succinate:ubiquinone reductase is by two orders of magnitude lower than that for the natural substrate--succinate. The values of kinetic constants for the oxidation of D- and L-stereoisomers of malate are equal to: V infinity = 0.1 mumol/min/mg protein, Km = 2 mM and V infinity = 0.05 mumol/min/mg protein, Km = 2 mM, respectively. The malate dehydrogenase activity is fully inhibited by the inhibitors of the dicarboxylate-binding site of the enzyme, i.e., N-ethylmaleimide and malonate and is practically insensitive to carboxin, a specific inhibitor of the ubiquinone-binding center. The enol form of oxaloacetate was shown to be the product of malate oxidation by succinate:ubiquinone reductase. The kinetics of inhibition of the enzyme activity by the ketone and enol forms of oxaloacetate was studied. Both forms of oxaloacetate effectively inhibit the succinate:ubiquinone reductase reaction.     

Effect of the nonionic detergent Triton X-100 on mitochondrial succinate-oxidizing enzymes

Specific activities of succinate:coenzyme Q reductase, ubiquinone:cytochrome c reductase, cytochrome oxidase, succinate:cytochrome c reductase, succinate oxidase, and ubiquinol oxidase have been measured in rat liver mitochondria in the presence of Triton X-100. The last three activities are much more sensitive to Triton X-100 than the first ones; the data suggest that the electron transport chain components cannot react with each other in the presence of the detergent. At least in the case of succinate:cytochrome c reductase, reconstitution of the detergent-treated membranes with externally added phospholipids reverses the inhibition produced by Triton X-100. These results support the idea that the respiratory chain components diffuse at random in the plane of the inner mitochondrial membrane; the main effect of the detergent would be to impair lateral diffusion by decreasing the area of lipid bilayer. When detergent-treated mitochondrial suspensions are centrifuged in order to separate the solubilized from the particulate material, only the first three enzyme activities mentioned above are found in the supernatants. After centrifugation, a latent ubiquinol:cytochrome c oxidase activity becomes apparent, whereas the same centrifugation process produces inhibition of cytochrome c oxidase in the presence of certain Triton X-100 concentrations. These effects could be due either to a selective solubilization of regulatory or catalytic subunits or to a conformational change of the enzyme-detergent complex.     

Electron transport in Paracoccus halodenitrificans and the role of ubiquinone

The membrane-bound NADH oxidase of Paracoccus halodenitrificans was inhibited by dicoumarol, 2-n-heptyl-4-hydroxy-quinoline-N-oxide (HQNO), and exposure to ultraviolet light (at 366 nm). When the membranes were extracted with n-pentane, NADH oxidase activity was lost. Partial restoration was achieved by adding the ubiquinone fraction extracted from the membranes. Succinate oxidation was not inhibited by dicoumarol or HQNO, but was affected by ultraviolet irradiation or n-pentane extraction. However, the addition of the ubiquinone fraction to the membranes extracted with n-pentane did not restore enzyme activity. These observations suggested that NADH and succinate were not oxidized through a common ubiquinone pool.     

The role of ubiquinone in the respiratory chain of Acetobacter xylinum

1. Whole cells of Acetobacter xylinum were found to contain a quinone of the ubiquinone (coenzyme Q) group. The quinone was isolated from the cells and crystallized. It was identified by its physical, chemical and spectroscopic properties as a ubiquinone with 10 isoprene units (ubiquinone-10). No naphthaquinone was detected in the cells. 2. Cell-free extracts prepared by means of a French pressure cell were separated into three fractions by differential centrifugation. The ubiquinone was located predominantly in the particulate fraction sedimenting at 33000g, which also contained most of the NADH oxidase and malate oxidase activities. The concentration of ubiquinone-10 in extracts was similar to that of the flavoproteins and about three times the concentration of the individual cytochromes. 3. Aerobic incubations of crude extracts with either NADH or malate resulted in reduction of the endogenous ubiquinone-10 to steady-state concentrations of 55 and 40% of the total quinone respectively. In the presence of cyanide more than 95% of the endogenous ubiquinone-10 was reduced by either NADH or malate. 4. The initial rate of reduction of endogenous ubiquinone-10 by malate and the rate of ubiquinol oxidation, in A. xylinum extracts, were found to be compatible with the overall rate of malate oxidation with oxygen. 5. The effects of various respiratory inhibitors on the oxidation-reduction reactions of the endogenous quinone indicate that its position on the respiratory chain is between the malate flavoprotein dehydrogenase and the cytochrome chain.