Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UNIPROT:Q8NEX9 (reductase)
 26,410 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Antimycin, a specific and highly potent inhibitor of electron transfer in the cytochrome b-c1 segment of the mitochondrial respiratory chain, does not inhibit reduction of cytochrome c1 by succinate in isolated succinate-cytochrome c reductase complex under conditions where the respiratory chain complex undergoes one oxidation-reduction turnover. If a slight molar excess of cytochrome c is added to the isolated reductase complex in the presence of antimycin, there is rapid reduction of one equivalent of c type cytochrome by succinate, after which reduction of the remaining c type cytochrome is inhibited. Antimycin fully inhibits succinate-cytochrome c reductase activity of isolated succinate-cytochrome c reductase complex in which the b-c1 complex undergoes multiple turnovers in a catalytic fashion. In addition, when antimycin is added to isolated reductase complex in the presence of cytochrome c plus cytochrome c oxidase, the inhibitor causes a "crossover" in the steady state level of reduction of the cytochromes b and c1 comparable to this classical effect in mitochondria. On the basis of these results, it is suggested that linear schemes of electron transfer are not adequate to account for the site of antimycin inhibition and the mechanism of electron transfer in the cytochrome b-c1 segment of the respiratory chain. The effects of antimycin are consistent with cyclic electron transfer mechanisms such as the protonmotive Q cycle.
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PMID:Rapid reduction of cytochrome c1 in the presence of antimycin and its implication for the mechanism of electron transfer in the cytochrome b-c1 segment of the mitochondrial respiratory chain. 625 13

Submitochondrial particles from bovine heart in which NADH dehydrogenase is reduced by either addition of NADH and rotenone or by reversed electron transfer generate 0.9 +/- 0.1 nmol of O2-/min per mg of protein at pH 7.4 and at 30 degrees C. When NADH is used as substrate, rotenone, antimycin and cyanide increase O2- production. In NADH- and antimycin-supplemented submitochondrial particles, rotenone has a biphasic effect: it increases O2- production at the NADH dehydrogenase and it inhibits O2- production at the ubiquinone-cytochrome b site. The generation of O2- by the rotenone, the uncoupler carbonyl cyanide rho-trifluoromethoxyphenylhydrazone and oligomycin at concentrations similar to those required to inhibit energy-dependent succinate-NAD reductase. Cyanide did not affect O2- generation at the NADH dehydrogenase, but inhibited O2- production at the ubiquinone-cytochrome b site. Production of O2- at the NADH dehydrogenase is about 50% of the O2- generation but the ubiquinone-cytochrome b area at pH 7.4. Additivity of the two mitochondrial sites of O2- generation was observed over the pH range from 7.0 to 8.8. AN O2- -dependent autocatalytic process that requires NADH, submitochondrial particles and adrenaline is described.
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PMID:Generation of superoxide anion by the NADH dehydrogenase of bovine heart mitochondria. 626 47

A synthetic quinone, 5-n-undecyl-6-hydroxy-4,7-dioxobenzothiazole (UHDBT), inhibits electron transfer reactions in the cytochrome b-c1 segment of the mitochondrial respiratory chain. Addition of UHDBT to isolated succinate-cytochrome c reductase complex has effects on reduction of the cytochromes b and c1 by succinate similar to those which result from removal of the iron-sulfur protein from the b-c1 complex. Thus, UHDBT inhibits reduction of cytochrome c1 by succinate and, if antimycin is added before succinate, UHDBT inhibits reduction of cytochrome b in addition to c1. UHDBT increases the midpoint potential of the iron-sulfur protein of the b-c1 complex from +280 to +350 mV at pH 7.2. The inhibitor also shifts the gx peak in the EPR spectrum of the iron-sulfur protein from g = 1.80 to 1.76 and shifts the gz peak from g = 2.02 to 2.03. It causes only a slight shift in the central gy = 1.90 signal. The efficacy of inhibition of cytochrome c reductase activity of isolated reductase complex by UHDBT appears to depend on the oxidation-reduction poise of some component(s) in the b-c1 complex. Inhibition is decreased and there is an extensive lag in the onset of inhibition under conditions favoring oxidation of the b-c1 complex; inhibition increases and the lag is eliminated under conditions favoring reduction of the b-c1 complex. The titer for inhibition of cytochrome c reductase activity of isolated reductase complex is one UHDBT per b-c1 complex. With reductase complex from which the iron-sulfur protein of the b-c1 complex is reversibly resolved, the titer for inhibition is proportional to the amount of iron-sulfur protein reconstituted to the complex. These results suggest that UHDBT inhibits mitochondrial respiration by binding to the iron-sulfur protein of the b-c1 complex, possibly at a site which is otherwise involved in binding ubiquinone, and that this binding is enhanced when the iron-sulfur protein is reduced.
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PMID:An analogue of ubiquinone which inhibits respiration by binding to the iron-sulfur protein of the cytochrome b-c1 segment of the mitochondrial respiratory chain. 628 79

1. Microsomes were isolated from rabbit fast-twitch and slow-twitch muscle and were separated into heavy and light fractions by centrifugation in a linear (0.3-2m) sucrose density gradient. The membrane origin of microsomal vesicles was investigated by studying biochemical markers of the sarcoplasmic-reticulum membranes and of surface and T-tubular membranes, as well as their freeze-fracture properties. 2. Polyacrylamide-gel electrophoresis showed differences in the Ca(2+)-dependent ATPase/calsequestrin ratio between heavy and light fractions, which were apparently consistent with their respective origin from cisternal and longitudinal sarcoplasmic reticulum, as well as unrelated differences, such as peptides specific to slow-muscle microsomes (mol.wts. 76000, 60000, 56000 and 45000). 3. Freeze-fracture electron microscopy of muscle microsomes demonstrated that vesicles truly derived from the sarcoplasmic reticulum, with an average density of 9nm particles on the concave face of about 3000/mum(2) for both fast and slow muscle, were admixed with vesicles with particle densities below 1000/mum(2). 4. As determined in the light fractions, the sarcoplasmic-reticulum vesicles accounted for 84% and 57% of the total number of microsomal vesicles, for fast and slow muscle respectively. These values agreed closely with the percentage values of Ca(2+)-dependent ATPase protein obtained by gel densitometry. 5. The T-tubular origin of vesicles with a smooth concave fracture face in slow-muscle microsomes is supported by their relative high content in total phospholipid and cholesterol, compared with the microsomes of fast muscle, and by other correlative data, such as the presence of (Na(+)+K(+))-dependent ATPase activity and of low amounts of Na(+)-dependent membrane phosphorylation. 6. Among intrinsic sarcoplasmic-reticulum membrane proteins, a proteolipid of mol.wt. 12000 is shown to be identical in the microsomes of both fast and slow muscle and the Ca(2+)-dependent ATPase to be antigenically and catalytically different, though electrophoretically homogeneous. 7. Basal Mg(2+)-activated ATPase activity was found to be high in light microsomes from slow muscle, but its identification with an enzyme different from the Ca(2+)-dependent ATPase is still not conclusive. 8. Enzyme proteins that are suggested to be specific to slow-muscle longitudinal sarcoplasmic reticulum are the flavoprotein NADH:cytochrome b(5) reductase (mol.wt. 32000), cytochrome b(5) (mol.wt. 17000) and the stearoyl-CoA desaturase, though essentially by criteria of plausibility.
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PMID:Biochemical heterogeneity of skeletal-muscle microsomal membranes. Membrane origin, membrane specificity and fibre types. 628 27

A kinetic study on ubiquinol-cytochrome c reductase (EC 1.10.2.2) has been undertaken either in situ in KCN-inhibited mitochondria and submitochondrial particles, or in the isolated cytochrome b-c1 complex using ubiquinol-1 and exogenous cytochrome c as substrates. The steady-state two-substrate kinetics of the reductase appears to follow a general sequential mechanism, allowing calculation of a Km for ubiquinol-1 of 13.4 microM in mitochondria and of 24.6 microM in the isolated cytochrome b-c1 complex. At low concentrations of cytochrome c, however, the titrations as a function of quinol concentration appear biphasic both in mitochondria and in submitochondrial particles containing trapped cytochrome c inside the vesicle space, fitting two apparent Km values for ubiquinol-1. Relatively high antimycin-sensitive rates of ubiquinol-1-cytochrome c reductase have been found in submitochondrial particles: both the Vmax and the Km for ubiquinol-1 are, however, affected by the overall orientation of the particle preparation, i.e., by the reactivity of cytochrome c with its proper site. The turnover numbers corrected for particle orientation with respect to cytochrome c interaction are at least 2-fold higher in submitochondrial particles than in mitochondria. This is particularly evident using inside-out particles containing trapped cytochrome c in the vesicle space (and therefore reacting with its physiological site). A diffusion step for the quinol substrate appears to be rate limiting in mitochondria and can be removed by addition of deoxycholate, suggesting that the oxidation site of ubiquinol may be more exposed to the matrix side of the inner mitochondrial membrane.
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PMID:Kinetics of ubiquinol-1-cytochrome c reductase in bovine heart mitochondria and submitochondrial particles. 629 57

The triphasic course previously reported for the reduction of cytochrome b in the succinate-cytochrome c reductase by either succinate or duroquinol has been shown to be dependent on the redox state of the enzyme preparation. Prior reduction with increasing concentrations of ascorbate leads to partial reduction of cytochrome c1, and a gradual decrease in the magnitude of the oxidation phase of cytochrome b. At an ascorbate concentration sufficient to reduce cytochrome c1 almost completely, the reduction of cytochrome b by either succinate or duroquinol becomes monophasic. Owing to the presence of a trace amount of cytochrome oxidase in the reductase preparation employed, the addition of cytochrome c makes electron flow from substrate to oxygen possible. Under such circumstances, the addition of a limited amount of either succinate or duroquinol leads to a multiphasic reduction and oxidation of cytochrome b. After the initial three phases as described previously, cytochrome b becomes oxidized before cytochrome c1 when the limited amount of added substrate is being used up. However, at the end of the reaction when cytochrome c1 is being rapidly oxidized, cytochrome b becomes again reduced. The above observations support a cyclic scheme of electron flow in which the reduction of cytochrome b proceeds by two different routes and its oxidation controlled by the redox state of a component of the respiratory chain.
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PMID:Multiphasic oxidation-reduction of cytochrome b in the succinate-cytochrome c reductase. 629 71

Ubiquinol: cytochrome c reductase was isolated from Neurospora mitochondria as a protein-detergent complex and dissociated by mild salt treatment. Three parts were obtained and characterized. Firstly, a complex containing the subunits III (cytochrome b), IV (cytochrome c1), VI, VII, VIII and IX; secondly, a complex containing the subunits I and II; and thirdly, the single subunit V (iron-sulphur subunit). Membrane crystals were prepared from the cytochrome bc1 subunit complex and by combining tilted electron microscopic views of the crystals, a low-resolution three-dimensional structure was calculated. This structure was compared to that of the whole cytochrome reductase (previously determined by electron microscopy of membrane crystals). Protein density absent from the structure of the subunit complex was then attributed to the missing subunits according to their size and shape and their association with the phospholipid bilayer.
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PMID:Structural studies of cytochrome reductase. Subunit topography determined by electron microscopy of membrane crystals of a subcomplex. 630 89

Ascorbate-reduced horse heart cytochrome c reduces photo-oxidized bacterial reaction centres with a second-order rate constant of (5-8) X 10(8) M-1 X s-1 at an ionic strength of 50 mM. In the absence of cytochrome c, the cytochrome c1 in the ubiquinol:cytochrome c oxidoreductase is oxidized relatively slowly (k = 3.3 X 10(5) M-1 X s-1). Ferrocytochrome c binds specifically to ascorbate-reduced reductase, with a Kd of 0.6 microM, and only the free cytochrome c molecules are involved in the rapid reduction of photo-oxidized reaction centres. The electron transfer between ferricytochrome c and ferrocytochrome c1 of the reductase is rapid, with a second-order rate constant of 2.1 X 10(8) M-1 X s-1 at an ionic strength of 50 mM. The rate of electron transfer from the Rieske iron-sulphur cluster to cytochrome c1 is even more rapid. The cytochrome b of the ubiquinol:cytochrome c oxidoreductase can be reduced by electrons from the reaction centres through two pathways: one is sensitive to antimycin and the other to myxothiazol. The amount of cytochrome b reduced in the absence of antimycin is dependent on the redox potential of the system, but in no case tested did it exceed 25% of the amount of photo-oxidized reaction centres.
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PMID:Kinetics of flash-induced electron transfer between bacterial reaction centres, mitochondrial ubiquinol:cytochrome c oxidoreductase and cytochrome c. 631 49

In a system containing reaction centres isolated from Rhodopseudomonas sphaeroides mutant R26, and variable amounts of horse-heart cytochrome c and bovine-heart mitochondrial QH2: cytochrome c oxidoreductase in a medium containing 2 mM ascorbate and 0.1 microM phenazine methosulphate, electron transfer was induced by a single flash. Three distinct phases of electron transfer can be distinguished: the first event is the oxidation of cytochrome c, and this is followed by an equilibration between cytochrome c, cytochrome c1 and the Rieske [2Fe-2S] cluster. The actual rates of these processes depend on the concentrations of cytochrome c and the reductase. The slower third phase is the oxidation of ubiquinol, which can follow two pathways: one sensitive to antimycin and one sensitive to myxothiazole. The antimycin-sensitive pathway (t1/2 approximately equal to 10 ms) is an equilibration between the Q/QH2 couple and cytochrome b, but may also include a direct reduction of cytochrome b by the QB of the reaction centres. The myxothiazole-sensitive pathway is a coupled reduction of cytochrome b and the Rieske [2Fe-2S] cluster which rapidly equilibrates with cytochromes c1 and c. Both pathways are sensitive to 7-(n-heptadecyl)mercapto-6-hydroxy-5,8-quinoline quinone, but with different affinities. In the absence of inhibitors the initial reduction of cytochrome b (via both pathways) is followed by a net oxidation which is the resultant of a continuing reduction (together with the reduction of the Rieske [2Fe-2S] cluster) and an oxidation (via the antimycin-sensitive site) by quinone. The results are discussed in the light of linear and cyclic models proposed to explain electron transfer between cytochromes b and c. It is concluded that only the Q-cycle model fits the present experimental data.
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PMID:Flash-induced electron transfer through mitochondrial QH2: cytochrome c oxidoreductase in the presence of bacterial reaction centres and cytochrome c. Analysis of subsequent processes and effect of inhibitors. 632 66

The superoxide (O2.-)-forming enzyme NADPH oxidase from pig neutrophils was solubilized and partially purified by gel-filtration chromatography. The purification procedure allowed the separation of NADPH oxidase activity from NADH-dependent cytochrome c reductase and 2,6-dichlorophenol-indophenol reductase activities. O2.-forming activity was co-purified with cytochrome b-245 and was associated with phospholipids. However, active fractions endowed with cytochrome b were devoid of ubiquinone and contained only little FAD. The cytochrome b/FAD ratio was 1.13:1 in the crude solubilized extract and increased to 18.95:1 in the partially purified preparations. Most of FAD was associated with fractions containing NADH-dependent oxidoreductases. These results are consistent with the postulated role of cytochrome b in O2.-formation by neutrophil NADPH oxidase, but raise doubts about the participation of flavoproteins in this enzyme activity.
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PMID:Composition of partially purified NADPH oxidase from pig neutrophils. 643 85


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