The Cytochromes of Prototheca zopfii

The respiratory pigments of Prototheca zopfii include seven cytochromes: two c-type cytochromes, a soluble c(549) and a membrane bound c(551); three b-type cytochromes, b(555), b(559) and b(564); and cytochromes a and a₃. Cytochromes a and a₃ could be resolved spectrally in the α-band region by reducing the cells in the presence of methanol and cyanide. Methanol shifted the absorption maximum of cytochrome a from 598 to 603 nanometers and permitted dithionite (or substrate) to reduce the cyanide-cytochrome a₃ complex to give a well defined 595-nanometer absorption band. Methanol did not interfere with CO binding by cytochrome a₃, and CO did not alter the methanol effect on cytochrome a. Azide and cyanide, which partially inhibited exogenous respiration, stimulated endogenous respiration. Frozen steady states of the electron transport chain in the presence of cyanide and azide indicated that the stimulation by these inhibitors was due to an increased autooxidation of one of the b-type cytochromes, possibly b(564).     

Inhibition of Respiration in Prototheca zopfii by Light

Irradiation of cells of Prototheca zopfii with blue light inhibited the respiratory capacity of the cells. The inhibition of respiration was correlated with a photodestruction of cytochrome c(551), cytochrome b(559), and cytochrome a₃. Cytochrome c(549), cytochrome b(555), and cytochrome b(564) were unaffected by the irradiation treatment. The α-band of reduced cytochrome a was shifted from 599 to 603 nm by irradiation, an effect similar to that observed when methanol was added to nonirradiated cells. The presence of oxygen was required during irradiation for both photoinhibition of respiration and photodestruction of the cytochromes. Cytochrome a₃ was protected against photodestruction by cyanide. Photodestruction of these same cytochromes also occurred when washed mitochondria of P. zopfii were irradiated.     

Optical and Potentiometric Study of the b- and c-Type Cytochromes in Mushroom Agaricus bisporus Lge Mitochondria

Differential spectrometry revealed two species for the b-type, as well as for the c-type, cytochromes in mitochondria from Agaricus bisporus Lge. The two b-type components are denoted according to their peak position in the α region at room temperature, i.e. b₅₆₀ and b₅₆₆. The b₅₅₆ component present in all the studied higher plant mitochondria was not detected in the system. At 293 K, the c-type cytochromes exhibit a common α band with a maximum at 550 nanometers. This band is split at 77 K, with peak positions at 547 nanometers (cytochrome c) and 552 nanometers (cytochrome c₁).     

The respiratory chain of Paramecium tetraurelia in wild type and the mutant Cl1 I. Spectral properties and redox potentials

1. Purified mitochondria have been prepared from wild type Paramecium tetraurelia and from the mutant Cl₁ which lacks cytochrome aa₃. Both mitochondrial preparations are characterized by cyanide insensitivity. Their spectral properties and their redox potentials have been studied.2. Difference spectra (dithionite reduced minus oxidized) of mitochondria from wild type P. tetraurelia at 77 K revealed the α peaks of b-type cytochrome(s) at 553 and 557 nm, of c-type cytochrome at 549 nm and a-type cytochrome at 608 nm. Two α peaks at 549 and 545 nm could be distinguished in the isolated cytochrome c at 77 K. After cytochrome c extraction from wild type mitochondria, a new peak at 551 nm was unmasked, probably belonging to cytochrome c₁. The a-type cytochrome was characterized by a split Soret band with maxima at 441 and 450 nm. The mitochondria of the mutant Cl₁ in exponential phase of growth differed from the wild type mitochondria in that cytochrome aa₃ was absent while twice the quantity of cytochrome b was present. In stationary phase, mitochondria of the mutant were characterized by a new absorption peak at 590 nm.3. Cytochrome aa₃ was present at a concentration of 0.3 nmol/mg protein in wild type mitochondria and ubiquinone at a concentration of 8 nmol/mg protein both in mitochondria of the wild type and the mutant Cl₁. Cytochrome aa₃ was more susceptible to heat than cytochromes b and c,c₁.4. CO difference spectra at 77 K revealed two different Co-cytochrome complexes. The first, found only in wild type mitochondria, was a typical CO-cytochrome a₃ complex characterized by peaks at 596 and 435 nm and troughs at 613 and 450 nm. The second, found both in mitochondria of the wild type and the mutant, was a CO-cytochrome b complex with peaks at 567, 539 and 420 nm and a trough at 558-549 nm. Both complexes are photo-dissociable.5. Spectral evidence was obtained for interaction of cyanide with the a-type cytochrome (shift of the α peak at 77 K from 608 to 605 nm), but not with the b-type cytochrome.6. The mid-point potentials of the different cytochromes at neutral pH are as follows: cytochrome aa₃ 235 and 395 mV, cytochrome c,c₁ 233 mV, cytochromes b 120 mV.     

The respiratory electron transport system of heterotrophically-grown Rhodopseudomonas palustris

The enzymatic activities and the cytochrome components of the respiratory chain were investigated with membrane fractions from chemoheterotrophically grown Rhodopseudomonas palustris. Whereas the level of electron transfer carriers was not distinctly affected by a change of the culture conditions, the potential activities of the enzymes were clearly increased when the cells were grown aerobically. Reduced-minus oxidized difference spectra of the membrane fractions prepared from dark aerobically grown cells revealed the presence of three b-type cytochromes b ₅₆₁, b ₅₆₀ and b ₅₅₈, and at least two c-type cytochromes c ₅₅₆ and c ₂ as electron carriers in the electron transfer chain. Cytochrome of a-type could not be detected in these membranes. Reduced plus CO minus reduced difference spectra of the membrane fractions were indicative of cytochrome o, which may be equivalent to cytochrome b ₅₆₀, appearing in substrate-reduced minus oxidized difference spectra. Cytochrome o was found to be the functional terminal oxidase. CO difference spectra of the high speed supernatant fraction indicated the presence of cytochrome c′. Succinate and NADH reduced the same types of cytochromes. However, a considerable amount of cytochrome b ₅₆₁ with associated β and γ bands at 531 and 429 nm, respectively, was reducible by succinate, but not by NADH. A substantial fraction of the membrane-bound b-type cytochrome was non-substrate reducible and was found in dithionite-reduced minus substrate-reduced spectra. Cytochrome c ₂ may be localized in a branch of the electron transport system, with the branch-point at the level of ubiquinone. The separate pathways rejoined at a common terminal oxidase. Two terminal oxidases with different KCN sensitivity were present in the respiratory chain, one of which was sensitive to low concentrations of KCN and was connected with the cytochrome chain. The other terminal oxidase which was inhibited only by high concentrations of cyanide was located in a branched pathway, through which the electrons could flow from ubiquinone to oxygen bypassing the cytochrome chain.     

The respiratory chain components of higher plant mitochondria

Tightly coupled mitochondria have been prepared from a variety of plant sources: white potato (Solanum tuberosum), Jerusalem artichoke (Heliantus tuberosus), cauliflower buds (Brassica oleracea), and mung bean hypocotyls (Phaseolus aureus). Mitochondria with no appreciable coupling were also prepared from skunk cabbage spadices (Symplocarpus foetidus).

Room temperature difference spectra show that these mitochondria are very similar in the qualitative and quantitative composition of their electron carriers. The different cytochromes are present in the amounts of 0.1 to 0.3 mμmole per mg of mitochondrial protein. The molar ratios of the different electron carriers are, on the average: 0.7:0.7:1.0:3 to 4:10 to 15 respectively for cytochrome aa₃, cytochromes b, cytochromes c, flavoproteins, and pyridine nucleotides.

From low temperature difference spectra carried out under particular experimental conditions, it can be deduced that these mitochondria contain 3 b cytochromes whose α bands are located at 552, 557, and 561 mμ, and 2 c cytochromes, one of which, a c₁-like cytochrome, is firmly bound to the mitochondrial membrane. Cytochrome oxidase can be optically resolved into its 2 components a and a₃.

For all kinds of mitochondria, the rates of oxidation of succinate are similar as well as the turnover of cytochrome oxidase (50-70 sec⁻¹), regardless of the metabolic activities of the tissues. The number of mitochondria per cell appears to be the controlling factor of the intensity of tissue respiration.

     

Occurrence of cytochrome aa3 in Anacystis nidulans

The cytochrome content of membrane fragments prepared from the bluegreen alga (cyanobacterium) Anacystis nidulans was examined by difference spectrophotometry. Two b-type cytochromes and a hitherto unknown cytochrome a could be characterized. In the reduced-minus-oxidised difference spectra the a-type cytochrome showed an α-band at 605 nm and a γ-band at 445 nm. These bands shifted to 590 and 430 nm, respectively, in CO difference spectra. NADPH, NADH and ascorbate reduced the cytochrome through added horse heart cytochrome c as electron mediator. In presence of KCN the reduced-minus-oxidised spectrum showed a peak at 600 nm and a trough at 604 nm. Photoaction spectra of O₂ uptake and of horse heart cytochrome c oxidation by CO-inhibited membranes showed peaks at 590 and 430 nm. These findings are consistent with cytochrome aa₃ being the predominant respiratory cytochrome c oxidase in Anacystis nidulans.     

The participation of cytochromes in the process of nitrate respiration in Klebsiella (Aerobacter) aerogenes

1. The participation of cytochromes in the membrane-bound, nitrate and oxygen respiratory systems of Klebsiella (Aerobacter) aerogenes has been investigated. The membrane preparations contained the NADH, succinate, lactate and formate oxidase systems, and in addition a high respiratory nitrate reductase activity.2. Difference spectra indicated the presence of cytochromes b, a₁, d, and o. Cytochromes of the c-type could not be detected in these membranes. Both cytochrome b content and respiratory nitrate reductase activity were the highest in bacteria grown anaerobically in the presence of nitrate.3. Cytochrome b was the only cytochrome which, after being reduced by NADH, could be partially reoxidized anaerobically in the presence of nitrate. Furthermore, nitrate caused a lower aerobic steady state reduction only of cytochrome b.4. NADH oxidase and NADH-linked respiratory nitrate reductase activities were both inhibited by antimycin A, 2-n-heptyl-4-hydroxyquinoline-N-oxide and KCN. NADH oxidase activity was selectively inhibited by CO, while azide was found to inhibit only the respiratory nitrate reductase. In the presence of azide, nitrate did not affect the level of reduction of cytochrome b.5. The evidence presented suggests that cytochrome b is a carrier in the electron transport systems to both nitrate and oxygen; from cytochrome b branching occurs, with one branch linked to the respiratory nitrate reductase and one branch linked to oxidase systems, containing the cytochromes a₁, d and o.     

Properties of Ascaris muscle mitochondria. I. Cytochromes

1. The cytochrome system in Ascaris muscle mitochondria was further characterized using purer preparations.2. Difference spectra (at 22 °C and ?196 °C) of the mitochondrial preparations using succinate and ascorbate plus N,N,N′,N′-tetramethyl-p-phenylenediamine show that Ascaris muscle mitochondria contain cytochromes c₁, c and aa₃, and also at least three b-type cytochromes. The b-type cytochrome is the predominant component.3. Cytochrome c and Ascaris cytochrome b-560 can be extracted from the mitochondrial preparations with 150 mM KCl, leaving the membrane-bound cytochromes c₁, b and aa₃ in the KCl residue.     

Measurement of cytochromes in human skeletal muscle mitochondria,isolated from fresh and frozen stored muscle specimens

Mitochondria were isolated from small muscle biopsy specimens, and the cytochrome content was calculated from the reduced minus oxidized difference spectrum recorded at room temperature.From the difference spectra obtained after reduction of the cytochromes with dithionite it is concluded that human mitochondrial suspensions are contaminated with hemoglobin or myoglobin.The cytochrome content calculated after reduction of the cytochromes with succinate plus KCN is lower than that obtained after reduction with dithionite, indicating incomplete reduction of the cytochromes by the former method.Storage of muscle tissue at −70°C before isolation of mitochondria results in a loss of cytochrome c + c₁ from these mitochondria.     

Action de l'antimycine a sur l'activité succinoxydase des mitochondries du tubercule de Pomme de Terre,et comportement des cytochromes de type b

A kinetic study of the effect of antimycin A on succinate oxidase from plant mitochondria produced sigmoidal curves for the reduction of cytochromes b₅₆₀ and b₅₈₅ and for the inhibition of succinate oxidase. In the stationary state the interaction of the various components of the respiratory chain (flavins, ubiquinone, cytochromes…) occurs in a sequential mode which allows the application of simple equations utilizing rate constants and cytochrome concentrations. In these equations, it is assumed that there exists an excess of ubiquinone relative to other components of the respiratory chain as suggested by Kröger & Klingenberg (1970) and that the reoxidation of b cytochromes is fast. The inhibition by antimycin A, characterized by non-linear inhibition curves for succin-oxidase and inhibitor fixation in complex III on a component other than cytochrome c₁, is interpreted in terms of this model. This hypothesis presupposes the existence of an F factor between cytochrome b₅₆₀ and cytochrome c₁ as suggested by other authors. Utilizing these equations, theoretical curves for the inhibition of the reduction of cytochrome b₅₆₀ have been constructed and the results agree with the experimental data. The kinetic behavior of cytochrome b₅₆₆ during the induction of anaerobiosis suggests that it is not directly involved in the electron transfer chain but rather is either in thermodynamic equilibrium with cytochrome b₅₆₀ or in a shunt between cytochrome b₅₆₀ and factor F. From the experimental data, an equation is derived for the inhibition of the reduction of cytochrome b₅₆₆ by antimycin A. The actual effects of ATP and mClCCP on succinoxidase agree well with those predicted by the model.     

Respiratory Electron Transport Systems of Aquatic Fungi. I. Leptomitus lacteus and Apodachlya punctata

The electron transport systems of 2 species of aquatic fungi, Leptomitus lacteus and A podachlya punctata, contained cytochrome a-a₃ (605 mμ), 2 b type cytochromes (564 and 557 mμ), c type cytochrome (551 mμ), and flavoprotein, but they appeared to lack cytochrome c₁. Reduced-minus-oxidized difference spectra and difference spectra in the presence of antimycin A or cyanide were used to characterize these systems. Studies with the electron microscope revealed that hyphae of Leptomitus lacteus contained numerous, conspicuous mitochondria with tubular cristae.     

The electron-transport chains of the obligate methylotroph Methylophilus methylotrophus

The cytochrome complement of Methylophilus methylotrophus and its respiratory properties were determined during batch culture and in continuous culture under conditions of methanol-, nitrogen- and O₂-limitation. About 35% of the cytochrome c produced by the bacteria was released into the growth medium, and of the remaining cytochrome c about half was membrane-bound and half was soluble. Two cytochromes c were always present on membranes (redox potentials 375mV and 310mV), and these probably correspond to the soluble cytochromes c described previously [Cross & Anthony (1980) Biochem. J. 192, 421–427]. A third minor component of cytochrome c (midpoint potential 356mV) was only detected on membranes of methanol-limited bacteria. M. methylotrophus always contained two membrane-bound cytochromes b with α-band absorption maxima of about 556 and 563nm (measured at room temperature) and midpoint potentials of 110 and 60mV respectively. There appeared to be relatively more of the cytochrome b₅₆₃ in methanol-limited bacteria. A third b-type cytochrome with an α-band absorption maximum at 558 (at 77K) reacted with CO and had a high midpoint redox potential (260mV); it is thus a potential oxidase and hence is called cytochrome o. The roles of these cytochromes in electron transport were confirmed by investigating the patterns of respiratory inhibition. It is proposed that two cytochromes are physiological oxidases: cytochrome a+a₃, which is present only in methanol-limited conditions, and the cytochrome o, which is induced 10-fold in conditions of methanol excess. Schemes for electron transport from methanol and NAD(P)H to O₂ in M. methylotrophus under various limitations are proposed. Spectra and potentiometric titrations of cytochromes in whole cells and membranes of M. methylotrophus grown under various nutrient limitations have been deposited as Supplementary Publication SUP 50111 (10 pages) at the British Library Lending Division, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1978) 169, 5.     

The electron transport systems of Fasciola hepatica mitochondria.

The electron transport systems of Fasciola hepatica mitochondria were investigated spectrophotometrically at room temperature and at −196°. The mitochondria were found to contain substrate reducible a-, b- and c-type cytochromes. All of the cytochrome components of the classical mammalian type of respiratory chain were present, although the concentration of cytochromes aa₃ was low. In addition to the mammalian type of respiratory chain, the Fasciola mitochondria contained a substrate reducible b-type cytochrome component (557 nm) which included a CO reactive o-type cytochrome. The results suggest that F. hepatica mitochondria contain a branched electron transport system including a mammalian type of chain and involving two terminal oxidases and at least two b-type cytochromes.     

Studies of cytochrome synthesis in rat liver

The incorporation of radioactive amino acids and of δ-amino[2,3-³H₂]laevulinate into rat liver cytochromes b₅ and c and cytochrome oxidase has been examined with and without protein-synthesis inhibitors. Cycloheximide promptly inhibits labelling of both haem and protein for cytochrome c in parallel fashion. Although incorporation of ¹⁴C-labelled amino acid into microsomal cytochrome b₅ is also rapidly inhibited, cycloheximide incompletely inhibits haem labelling of cytochrome b₅ and cytochrome a+a₃, and inhibition occurs only after repeated antibiotic injections. The possibility of apo-protein pools, or of haem exchange, with a rapidly renewed `free' haem pool, is considered. Consistent with this model is the observation of non-enzymic haem exchange in vitro between cytochrome b₅ and methaemoglobin. Chloramphenicol, injected intravenously over 5h, results in a 20–40% decrease in incorporation of δ-amino[2,3-³H₂]laevulinate into haem a+a₃ and haem of cytochromes b₅ and c. With the dosage schedule of chloramphenicol studied, amino acid labelling of total liver protein and of cytochrome c was not inhibited. Similarly, ferrochelatase activity was not decreased.     

Mitochondrial respiratory chain of Tetrahymena pyriformis. The thermodynamic and spectral properties

The mitochondria isolated from the ciliate protozoon Tetrahymena pyriformis carry an oxidative phosphorylation with P/O ratio of 2 for succinate oxidation and P/O ratio of 3 for the oxidation of the NAD-linked substrates. The respiration is more than 90% inhibited with 1 mM cyanide while antimycin A and rotenone inhibit at concentrations of 1000-fold higher than those effective in mammalian mitochondria.Using a combination of spectral studies and potentiometric titrations, the components of the respiratory chain were identified and characterized with respect to the values of their half-reduction potentials. In the cytochrome bc₁ region of the chain a cytochrome c was present with an E_m7.2 of 0.225 V and two components with absorption maxima at 560 nm and the half-reduction potential values of ?0.065 and ?0.15 V at pH 7.2. The cytochrome with the more positive half-reduction potential was identified as the analogue of the cytochrome(s) b present in mitochondria of higher organisms, while the cytochrome with the more negative half-reduction potential was tentatively identified as cytochrome o. In addition ubiquinone was present at a concentration of approx. 4 nmol per mg mitochondrial protein.In the spectral region where cytochromes a absorb at least three cytochromes were found. A cytochrome with an absorption maximum at 593 nm and a midpoint potential of ?0.085 V at pH 7.2 was identified as cytochrome a₁. The absorption change at 615–640 nm, attributed usually to cytochrome a₂ was resolved into two components with E_m7.2 values of 0.245 and 0.345 V. It is concluded that the terminal oxidase in Tetrahymena pyriformis mitochondria is cytochrome a₂ which in its two-component structure resembles cytochrome aa₃.     

Biochemical heterogeneity of rat liver mitochondria separated by rate zonal centrifugation

1. Rat liver mitochondria were separated on the basis of their sedimentation coefficients in an iso-osmotic gradient of Ficoll–sucrose by rate zonal centrifugation. The fractions (33, each of 40ml) were collected in order of decreasing density. Fractions were analysed by spectral analysis to determine any differences in the concentrations of the cytochromes and by enzyme analyses to ascertain any differences in the activities of NADH dehydrogenase, succinate dehydrogenase and α-glycerophosphate dehydrogenase. 2. When plotted as% of the highest specific concentration, the contents of cytochrome a+a₃ and cytochrome c+c₁ were constant in all fractions but cytochrome b was only 65% of its maximal concentration in fraction 7 and increased with subsequent fractions. As a result, the cytochrome b/cytochrome a+a₃ ratio almost doubled between fractions 7 and 25 whereas the cytochrome c+c₁/cytochrome a+a₃ ratio was unchanged. 3. Expression of the dehydrogenase activities as% of highest specific activity showed the following for fractions 6–26: NADH dehydrogenase activity remained fairly constant in all fractions; succinate dehydrogenase activity was 62% in fraction 6 and increased steadily to its maximum in fraction 18 and then decreased; the activity of α-glycerophosphate dehydrogenase was only 53% in fraction 6 and increased slowly to its peak in fractions 22 and 24. 4. These differences did not result from damaged or fragmented mitochondria or from microsomal contamination. 5. These results demonstrate that isolated liver mitochondria are biochemically heterogeneous. The importance of using a system for separating biochemically different mitochondria in studies of mitochondrial biogenesis is discussed.