Oxidoreduction of cytochrome b in the presence of antimycin

1. The effect of oxidizing equivalents on the redox state of cytochrome b in the presence of antimycin has been studied in the presence and absence of various redox mediators.

2. The antimycin-induced extra reduction of cytochrome b is always dependent on the initial presence of an oxidant such as oxygen. After removal of the oxidant this effect remains or is partially (under some conditions even completely) abolished depending on the redox potential of the substrate used and the leak through the antimycin-inhibited site.

3. The increased reduction of cytochrome b induced by oxidant in the presence of antimycin involves all three spectroscopically resolvable b components (b-562, b-566 and b-558.

4. Redox mediators with an actual redox potential of less than 100–170 mV cause the oxidation of cytochrome b reduced under the influence of antimycin and oxidant.

5. Redox titrations of cytochrome b with the succinate/fumarate couple were performed aerobically in the presence of cyanide. In the presence of antimycin two b components are separated potentiometrically, one with an apparent midpoint potential above 80 mV (at pH 7.0), outside the range of the succinate/fumurate couple, and one with an apparent midpoint potential of 40 mV and an n value of 2. In the absence of antimycin cytochrome b titrates essentially as one species with a midpoint potential of 39 mV (at pH 7.0) and n = 1.14.

6. The increased reducibility of cytochrome b induced by antimycin plus oxidant is considered to be the result of two effects: inhibition of oxidation of ferrocytochrome b by ferricytochrome c₁ (the effect of antimycin), and oxidation of the semiquinone form of a two-equivalent redox couple such as ubiquinone/ubiquinol by the added oxidant, leading to a decreased redox potential of the QH₂/QH^• couple and reduction of cytochrome b.     

Cytochrome b and photosynthetic sulfur bacteria

Chromatophores isolated from the purple sulfur bacterium Chromatium and the green sulfur bacterium Chlorobium exhibit absorbance changes in the cytochrome -band region consistent with the presence of a b-type cytochrome. Cytochrome content determined by reduced minus oxidized difference spectra and by heme analysis suggests that each bacterium contains one cytochrome b per molecule of photochemically active bacteriochlorophyll (reaction-center bacteriochlorophyll).

The b-type cytochrome in Chromatium has an -band maximum at 560 nm and a midpoint oxidation-reduction potential of −5 mV at pH 8.0. The b-type cytochrome in Chlorobium has an -band maximum at 564 nm and an apparent midpoint oxidation-reduction potential near −90 mV.

Chromatophores isolated from both Chromatium and Chlorobium cells catalyze a photoreduction of cytochrome b that is enhanced in the presence of antimycin A. Antimycin A and 2-n-heptyl-4-hydroxyquinoline-N-oxide inhibit endogenous (but not phenazine methosulfate-mediated) cyclic photophosphorylation in Chromatium chromatophores and non-cyclic electron flow from Na₂S to NADP in Chlorobium chromatophores. These observations suggest that b-type cytochromes may function in electron transport reactions in photosynthetic sulfur bacteria.     

Electron transfer reactions of cytochrome c confined within erythrocyte ghosts

We have prepared and characterized resealed erythrocyte ghosts in which the only discernible pigment is cytochrome c. The resealed ghosts have the normal orientation and are free of ‘leaky’ species; they are stable and can be maintained at 4°C for many days without lysis.

The internal cytochrome c participates in redox reactions with both soluble and insolubilized cytochrome c present externally, and with external cytochrome b₅. No reaction was observed with plastocyanin, cytochrome c oxidase or NADPH-cytochrome c reductase.

A study has been made of the reaction of the internal cytochrome c with the low molecular weight reductants, ascorbate and glutathione. Complex kinetics are observed with both reagents: with ascorbate the results are best explained by assuming the existence, in the membrane, of a redox-active species able to undergo dedimerization. A protein bound disulfide bond would satisfy the requirement.     

Spectral study of b cytochromes in yeast mitochondria and intact cells

Three types of b cytochromes are demonstrated in Candida utilis mitochondria. One of these b cytochromes has a symmetrical -band at 561.5 nm at room temperature. This b cytochrome is readily reduced either by anaerobiosis or by cyanide treatment in the presence of glycerol 1-phosphate or succinate both in coupled and uncoupled mitochondria. The second b cytochrome has a double -band at 565 nm and 558 nm. This b cytochrome is readily reduced either by anaerobiosis or by cyanide treatment in the presence of glycerol 1-phosphate or succinate in coupled mitochondria, but in uncoupled mitochondria it is slowly reduced after anaerobiosis and this reduction rate is enhanced by antimycin A addition. Thus the oxidation-reduction state of this cytochrome is energy dependent. The first cytochrome is spectroscopically identified as cytochrome b_K and the second as cytochrome b_T. The third b cytochrome has an -band around 563 nm (b₅₆₃) and is reduced slowly after anaerobiosis in uncoupled mitochondria but faster than the b_T. Further properties of this component are not known. Midpoint potentials of cytochromes b_T, b₅₆₃ and b_K are approximately −50 mV, +5 mV, and +65 mV, respectively.

In intact cells, cytochrome b_T is reduced immediately after anaerobiosis or cyanide treatment, and rapidly oxidized when uncoupler is added. Addition of antimycin A instead of uncoupler to the anaerobic cells causes oxidation of mainly cytochrome b_T while addition of antimycin A to the aerobic cells results in a reduction of the cytochrome b_T.     

The reaction of antimycin with a cytochrome b preparation active in reconstitution of the respiratory chain

1. Succinate-cytochrome c reductase activity was reconstituted by incubating a mixture of succinate dehydrogenase, cytochrome c₁, ubiquinone-10, phospholipid and a preparation of cytochrome b, made by the method of .

2. Preparations of cytochrome b active in reconstitution contained 5–28% native cytochrome b, as adjudged by reducibility with succinate in the reconstituted preparation and by lack of reaction with CO. Preparations of cytochrome b containing no native cytochrome b according to this criterion were inactive in reconstitution.

3. With a fixed amount of cytochrome b, the activity of the reconstituted preparation increased with increasing amounts of cytochrome c₁ until a ratio of about 2b (total): 1c₁ (allowing for the cytochrome c₁ present in the cytochrome b preparation) was reached.

4. The amount of antimycin necessary for maximal inhibition of the reconstituted enzyme is a function of the amount of the cytochrome b and is independent of the amount of cytochrome c₁. It is equal to about one half the amount of native cytochrome b.

5. Preparations of intact or reconstituted succinate-cytochrome c reductase or of cytochrome b completely quench the fluorescence of added antimycin, until an amount of antimycin equal to onehalf the amount of native cytochrome b present was added. Antimycin added in excess of this amount fluoresces with normal intensity. The quenching is only partial in the presence of Na₂S₂O₄. Denatured cytochrome b does not quench the fluorescence.

6. Since preparations of cytochrome b active in reconstitution contained cytochrome c₁ in an amount exceeding one half the amount of native cytochrome b present in the preparation, there is no evidence that native cytochrome b has been resolved from cytochrome c₁. The stimulatory action of cytochrome c₁ may be due to the restoration of a damaged membrane conformation.

7. Based on the assumption that the bc₁ segment of the respiratory chain contains 2b:1c₁:1 antimycin-binding sites, the specific quenching of antimycin fluorescence by binding to cytochrome b enables an accurate determination of the absorbance coefficients of cytochromes b and c₁. These are 25.6 and 20.1 mM⁻¹×cm⁻¹ for the wavelength pairs 563–577 nm and 553–539 nm, respectively, in the difference spectrum reduced minus oxidized.     

Further studies on the NADH oxidase of the cytoplasmic membrane of Mycobacterium tuberculosis

The cytoplasmic membrane of the H₃₇Ra strain of Mycobacterium tuberculosis has been isolated free of cell wall.

These membrane preparations contain very small quantities of cytochromes c, b and cytochrome oxidase. The cytochrome c is not extracted by any method attempted. The cytochrome b is reducible only by dithionite and is believed not to be involved in the direct transfer of electrons during the oxidation of NADH by these preparations. The NADH oxidase activity of the membrane is inhibited by high concentrations of cyanide and also by 2-(n-heptyl)-4-hydroxyquinoline-N-oxide (HQNO). The cytochrome oxidase of the membrane contains both cytochromes a and a₃ and is present in low concentrations relative to cytochrome c. The cytochrome a₃ component was identified by characteristic complexes with both CO and cyanide and shows a γ-band absorption maximum at a slightly lower wavelength than the cytochrome oxidase of mammalian mitochondria (442 nm vs. 445 nm). The functional activity of the cytochrome oxidase is indicated by the inhibition of reoxidation of reduced cytochromes c and a in the presence of cyanide.     

Reducibility of cytochromes b in aerobic beef-heart mitochondria treated with antimycin

Under anaerobic conditions cytochrome b in beef-heart mitochondria is partially reduced in the presence of NADH, whereas other cytochromes are completely reduced. Addition of antimycin together with oxygen under these conditions causes an immediate reduction of cytochromes b-558, b-562 and b-566 and oxidation of cytochrome c. During the subsequent transient aerobic steady state cytochromes b-558 and b-566 are rapidly re-oxidized without changes in redox state of cytochrome c, but cytochrome b-562 remains reduced. When oxygen is consumed by the leak through or around the antimycin-inhibition site, cytochrome b-562 becomes oxidized with concomitant reduction of cytochrome c.

The cytochromes b in lyophilized beef-heart mitochondria are more readily accessible to electrons from NADH, and in the presence of antimycin and NADH a complete and stable reduction is obtained under both aerobic and anaerobic conditions. Gradual addition of rotenone under these conditions causes re-oxidation of cytochromes b in which oxidation of cytochromes b-558 and b-566 precedes that of cytochrome b-562.

It is concluded that (1) the effect of antimycin in the presence of oxygen involves all three cytochromes b, (2) the reducibility of the cytochromes b in the aerobic steady state of antimycin-treated mitochondria is dependent upon the potential of the substrate redox couple registered on the cytochromes, and (3) the midpoint potential of cytochrome b-562 in the presence of antimycin is higher than that of cytochrome b-558 or b-566.     

Cytochrome c1 and b-type cytochrome with two heme centers in the photosynthetic membranes from Rhodopseudomonas palustris

The photosynthetic membranes from Rhodopseudomonas palustris contained one species of membrane-bound c-type cytochrome, presumably cytochrome c₁, and a b-type cytochrome with two heme centers. The molecular weight and midpoint potential of cytochrome c₁ were 30000 and 275 mV, respectively. The peak of the reduced-minus-oxidized difference spectrum of cytochrome c₁ was at 552 nm. Molecular weight of the b-type cytochrome was 32000 and the cytochrome had two midpoint potentials of 60 mV and −55 mV. The peaks of the reduced-minus-oxidized difference spectra of the high and low midpoint potential heme centers were at 560 and 562 nm, respectively. These results suggested that there was a cytochrome b-c₁ complex in Rps. palustris.     

Energy transduction in photosynthetic bacteria. XI. Further resolution of cytochromes of b type and the nature of the CO-sensitive oxidase present in the respiratory chain of Rhodopseudomonas capsulata

1. In membranes prepared from dark grown cells of Rhodopseudomonas capsulata, five cytochromes of b type (E′₀ at pH 7.0 +413±5, +270±5, +148±5, +56±5 and −32±5 mV) can be detected by redox titrations at different pH values. The midpoint potentials of only three of these cytochromes (b₁₄₈, b₅₆, and b₋₃₂) vary as a function of pH with a slope of 30 mV per pH unit.

2. In the presence of a Co/N₂ mixture, the apparent E′₀ of cytochrome b₂₇₀ shifts markedly towards higher potentials (+355 mV); a similar but less pronounced shift is apparent also for cytochrome b₁₅₀. The effect of CO on the midpoint potential of cytochrome b₂₇₀ is absent in the respiration deficient mutant M6 which possesses a specific lesion in the CO-sensitive segment of the branched respiratory chain present in the wild type strain.

3. Preparations of spheroplasts with lysozyme digestion lead to the release of a large amount of cytochrome c₂ and of virtually all cytochrome cc′. These preparations show a respiratory chain impaired in the electron pathway sensitive to low KCN concentration, in agreement with the proposed role of cytochrome c₂ in this branch; on the contrary, the activity of the CO-sensitive branch remains unaffected, indicating that neither cytochrome c₂ nor the CO-binding cytochrome cc′ are involved in this pathway.

4. Membranes prepared from spheroplasts still possess a CO-binding pigment characterized by maxima at 420.5, 543 and 574 nm and minima at 431, 560 nm in CO-difference spectra and with an band at 562.5 nm in reduced minus oxidized difference spectra. This membrane-bound cytochrome, which is coincident with cytochrome b₂₇₀, can be classified as a typical cytochrome “o” and considered the alternative CO-sensitive oxidase.     

Antimycin-insensitive mutants of Candida utilis. II. The effects of antimycin on cytochrome b

1. Cytochrome b-562 is more reduced in submitochondrial particles of mutant 28 during the aerobic steady-state respiration with succinate than in particles of the wild type. When anaerobiosis is reached, the reduction of cytochrome b is preceded by a rapid reoxidation in the mutant. A similar reoxidation is observed in the wild type in the presence of low concentrations of antimycin.

2. In contrast to the wild type, inhibition of electron transport in the mutant has a much higher antimycin titre than effects on cytochromes b (viz., aerobic steadystate reduction; reduction in the presence of substrate, cyanide and oxygen; the ‘red shift’ and lowering of E′₀ of cytochrome b-562). Moreover, the titration curve of electron transport is hyperbolic whereas the curves for the reduction are sigmoidal. The conclusion is, that in both mutant and wild type, the actions of antimycin on electron transport and cytochromes b are separable.

3. The red shift in the mutant is more extensive than in the wild type.

4. Cytochrome b-558 and cytochrome b-566 (that absorbs in mutant and wild type at 564.5 nm) do not respond simultaneously to addition of antimycin, indicating that they are two separate cytochromes.

5. The difference between the effect of antimycin on electron transport and cytochromes b reduction is also found in intact cells of the mutant.

6. A model is suggested for the wild-type respiratory chain in which (i) the cytochromes b lie, in an uncoupled system, out of the main electron-transfer chain, (ii) antimycin induces a conformation change in QH₂-cytochrome c reductase resulting in effects on cytochrome b and inhibition of electron transport, (iii) a second antimycinbinding site with low affinity to the antibiotic is present, capable of inhibiting electron transport.     

Low-temperature spectral properties of the respiratory chain cytochromes of mitochondria from Crithidia fasciculata

Highly purified mitochondrial preparations from the trypanosomatid hemoflagellate, Crithidia fasciculata (A.T.C.C. No.11745), were examined by low-temperature difference spectroscopy. The cytochrome a+a₃ maximum of hypotonically-treated mitochondria reduced with succinate, was shifted from 605 nm at room temperature to 601 nm at 77 °K. The Soret maximum, found at 445 nm at 23 °C, was split at 77 °K into two approximately equally absorbing species with maxima at 438 and 444 nm. A prominent shoulder observed at 590 nm with hypotonically-treated mitochondria was not present in spectra of isotonic controls.

The cytochrome b maxima observed in the presence of succinate plus antimycin A were shifted from the 431 and 561 nm positions observed at 23 °C to 427 and 557 nm at 77 °K. Multiple b cytochromes were not apparent.

Unlike other soluble c-type cytochromes, the maximum of cytochrome c₅₅₅ was not shifted at 77 °K although it was split to give a 551 nm shoulder adjacent to the 555 nm maximum. This lack of a low-temperature blue shift was true for partially purified hemoprotein preparations as well as in situ in the mitochondrial membrane.

Using cytochrome c₅₅₅-depleted mitochondria, a cytochrome c₁ pigment was observed with a maximum at 420 nm and multiple maxima at 551, 556, and 560 nm. After extraction of non-covalently bound heme, the pyridine hemochromogen difference spectrum of cytochrome c₅₅₅-depleted preparations exhibited an maximum at 553 nm at room temperature.

The reduced rate of succinate oxidation by cytochrome c₅₅₅-depleted mitochondria and the ferricyanide requirement for the reoxidation of cytochrome c₁, even in the presence of antimycin, indicated that cytochrome c₅₅₅-mediated electron transfer between cytochromes c₁ and a+a₃ in a manner analagous to that of cytochrome c in mammalian mitochondria.     

Cyclic AMP induces a transient alkalinization in Dictyostelium

In a wide range of cell types, stimulus-response coupling is accompanied by a rise in cytoplasmic pH (pH_i). It is shown that stimulation of developing Dictyostelium discoideum cells with the chemoattractant cAMP also results in a rise in pH_i. About 1.5 min after stimulation, pH_i starts increasing from pH_i7.45 to pH_i7.60, as is revealed independently by two different pH null-point methods. The rise in pH_i is transient, also with a persistent stimulus, and effectively inhibited by diethylstilbestrol (DES), strongly suggesting that the rise in pH_i is accomplished by the DES-sensitive plasma membrane proton pump which has been demonstrated in D. discoideum.     

Respiratory cytochromes of Euglena gracilis

1. 1. Difference spectra of whole cells and of a particulate fraction of a streptomycin-bleached strain of Euglena gracilis showed the presence of a b-type cytochrome, cytochrome b (561 Euglena), and an a-type cytochrome, cytochrome a-type (609 Euglena). The cytochromes were characterized by pyridine hemochromogen formation and were found associated with a particulate fraction enriched with mitochondria.

2. 2. Both b-type and a-type cytochromes were reduced by succinate, oxidized by oxygen and reacted with a soluble c-type cytochrome, cytochrome c-type (556 Euglena), in reversible oxidation-reduction reactions. The steady-state level of reduction for each cytochrome was 92, 22 and 5% of the anaerobic level for the b-type, c-type and a-type cytochrome, respectively.

3. 3. Oxidation of c-type and a-type cytochromes was completely inhibited by cyanide, although respiration of a particulate fraction was only 60% inhibited by the same concentration of cyanide. Antimycin A inhibited respiration by up to 70% but completely inhibited reduction of the c-type cytochrome.

4. 4. The data suggest that electron transfer in the respiratory pathway of Euglena involves the b-, c- and a-type cytochrome in a direct sequence. The cyanide and antimycin A-insensitive oxidation pathway is considered to involve a more direct oxidation of the b-type cytochrome.

Circular dichroism of cytochrome oxidase, cytochrome b566, and cytochrome c in beef heart mitochondrial membrane fragments

1. Circular dichroism spectra of the cytochromes in membrane fragments derived from sonicated beef heart mitochondria have been obtained in the wavelength region 400–480 nm in which the major absorbance maxima of the heme prosthetic groups are found.

2. 2. Cytochrome oxidase in the mitochondrial membrane fragments has a band of positive ellipticity at 426 nm in the oxidized form and a pronounced band of positive ellipticity at 445 nm in the reduced form. The reduced-minus-oxidized difference molar ellipticity at 445 nm, Δ[θ]₄₄₅ is 3.0·10⁵ degree·cm⁻²·dmole⁻¹ heme a for membrane-bound oxidase compared to 1.6·10⁵ degree·cm⁻²·dmole⁻¹ heme a for the purified oxidase. The membrane-bound oxidase in the reduced form also appears to have a band of negative ellipticity at 426 nm not found in the purified oxidase.

3. 3. When reduced with succinate in the presence of cyanide and oxygen, cytochrome oxidase in the membrane fragments has a positive band at 442 nm very similar to that observed with the purified oxidase.

4. 4. Cytochrome c, which has a positive band at 426 nm in the purified form when reduced, appears to have a negative band at this wavelength in the mito-chondrial membrane fragments which contributes to the pronounced negative band at 426 nm observed in the membrane fragments reduced with succinate in anaerobiosis. There is no evidence for a contribution to the CD spectra of the membrane fragments from cytochrome c₁ or from cytochrome b₅₆₁ in either the oxidized or the reduced form.

5. 5. Cytochrome b₅₆₆ in the mitochondrial membrane fragments has no detectable CD spectrum in the oxidized form, but has a small positive band at 427 nm and a small negative band at 436 nm in the reduced form. The same CD spectrum is observed with cytochrome b₅₆₆ reduced with succinate in the presence of antimycin A or 2-heptyl-4-hydroxyquinoline-N-oxide. The same increase in positive ellipticity is observed at 427 nm in the mitochondrial membrane fragments, treated with oligomycin to restore energy coupling, when cytochrome b₅₆₆ is reduced with succinate in the energized membrane, as is observed in the inhibitor-treated membrane fragments. The absence of a pronounced conformational change in cytochrome b₅₆₆ on energization, as revealed by its CD spectrum, favors the concept that its reduction by succinate in the energized state is due to reversed electron transport rather than an intrinsic shift in the cytochrome's midpoint redox potential.

A kinetic completion of the cyclic photosynthetic electron pathway of Rhodopseudomonas sphaeroides: Cytochrome b-cytochrome c2 oxidation-reduction

In Rhodopseudomonas sphaeroides, following a single-turnover flash of light, cytochrome c₂ is oxidized by reaction center bacteriochlorophyll, and a cytochrome b is reduced by the primary electron acceptor, probably via ubiquinone. In this report we show that, in the uncoupled state, the rate of re-oxidation of the cytochrome b is identical to the rate of reduction of the cytochrome c₂, a kinetic completion of the cyclic photosynthetic electron transport system.     

The role of a cytochrome c-552-cytochrome c complex in the oxidation of sulfide in Chromatium vinosum

The sulfide:cytochrome c oxidoreductase activity of the flavocytochrome c-522 from the purple sulfur bacterium Chromatium vinosum has been investigated. The oxidized sulfur product of the sulfide:cytochrome c reductase activity has been shown to be elemental sulfur. Cytochrome c-552 has been found to form a stable complex with horse heart cytochrome c that appears to be held together by electrostatic interactions. The stability of this complex and the sulfide:cytochrome c reductase activity of cytochrome c-552 are both ionic strength dependent, with maximal rates of cytochrome c reduction and extent of complex formation occurring over the same ionic strength range. Trifluoroacetylated cytochrome c is not reduced in the presence of cytochrome c-552 and sulfide, nor does it form a complex with cytochrome c-552. These results suggest the possible involvement of cytochrome c lysine residues in complex formation. Cytochrome c-552 migrates with an anomalously high apparent molecular weight on gel filtration columns equilibrated with low ionic strength buffers, suggesting the possibility of conformational changes or dimerization of the protein. However, complexation of cytochrome c-552 with cytochrome c still occurs at low ionic strength.     

Mitochondrial respiratory chain of Tetrahymena pyriformis The properties of submitochondrial particles and the soluble b and c type pigments

Submitochondrial particles isolated from Tetrahymena pyriformis contain essentially the same redox carriers as those present in parental mitochondria: at pH 7.2 and 22 °C there are two b-type pigments with half-reduction potentials of −0.04 and −0.17 V, a c-type cytochrome with a half reduction potential of 0.215 V, and a two-component cytochrome a₂ with E_m7.2 of 0.245 and 0.345 V.

EPR spectra of the aerobic submitochondrial particles in the absence of substrate show the presence of low spin ferric hemes with g values at 3.4 and 3.0, a high spin ferric heme with g = 6, and a g = 2.0 signal characteristic of oxidized copper. In the reduced submitochondrial particles signals of various iron-sulfur centers are observed.

Cytochrome c₅₅₃ is lost from mitochondria during preparation of the submitochondrial particles. The partially purified cytochrome c₅₅₃ is a negatively charged protein at neutral pH with an E_m7.2 of 0.25 V which binds to the cytochrome c-depleted Tetrahymena mitochondria in the amount of 0.5 nmol/mg protein with a K_D of 0.8 · 10⁻⁶ M. Reduced cytochrome c₅₅₃ serves as an efficient substrate in the reaction with its own oxidase. The EPR spectrum of the partially purified cytochrome c₅₅₃ shows the presence of a low spin ferric heme with the dominant resonance signal at g = 3.28.

A pigment with an absorption maximum at 560 nm can be solubilized from the Tetrahymena cells with butanol. This pigments has a molecular weight of approx. 18 000, and E_m7.2 of −0.17 V and exhibits a high spin ferric heme signal at g = 6.     

Kinetics of the electrogenic step and cytochrome b6 and f redox changes in chloroplasts: Evidence for a Q cycle

The spectroscopic measurements of the slow phase of the electrochromic effect and the redox kinetics of cytochrome b₆ and f provide strong evidence that a Q cycle operates in chloroplasts under conditions of non-cyclic electron transport. The effect of HQNO and DBMIB on the extent and kinetics of these light-induced changes places several constraints on the mechanism of quinol oxidation by the cyt. b/f—FeS complex: for each electron removed from the cyt. b/f—FeS complex by P700 an additional charge is transferred across the membrane; the cyclic pathway of electrons involved in quinol oxidation by the cyt. b/f—FeS complex includes at least one of the two b₆ cytochromes; the electrogenic step associated with quinol oxidation is subsequent to the reduction of at least one cytochrome b₆ quinol oxidation may proceed in a stepwise manner, with the first electron going to cytochrome b₆ and the second electron going to the FeS center and cytochrome f.     

The electron transport system of Acetobacter suboxydans with particular reference to cytochrome o

1. The nature and distribution of the electron transport system of Acetobacter suboxydans (ATCC 621) has been investigated, with particular reference to cytochrome o.

2. A highly active membrane-bound electron transport system has been demonstrated, and functional roles suggested for ubiquinone, two c-type cytochromes ( peaks at 549 and 553 nm at — 196°), and two b-type cytochromes ( peaks at 558 and 564 nm at — 196°).

3. Evidence is presented suggesting that both the b-type cytochromes may be terminal oxidases of the cytochrome o type, and that cytochrome o (558) has an O₂ affinity approx. 10 times greater than cytochrome o (565), and a CO affinity only half as great.     

Interaction of antimycin with cytochrome b-561: A study in secretory granules and in plasma membrane isolated from chromaffin cells of bovine adrenal medulla

Cytochrome b-561 in chromaffin granules interacts with antimycin and its -peak shifts 1 nm towards red. When chromaffin granules were treated with Triton X-100 antimycin no effect was observed. Cytochrome b-561 is located in the plasma membrane isolated from the chromaffin cells. The plasma membrane b-561 does not seem to interact with antimycin. A number of NADH or NADPH (acceptor) oxidoreductase activity has been observed in isolated plasma membrane providing clues to the origin of plasma membrane dehydrogenase. The possible role of cytochrome b561 in secretory granules other than its accredited energy conserving electron transport property is projected.