Inhibitors of respiration at energy-coupling site 2 of the respiratory chain

A Fast, Simple, and Affordable Technique to Measure Oxygen Consumption in Living Zebrafish Embryos

In all animal species, oxygen consumption is a key process that is partially impaired in a large number of pathological situations and thus provides informative details on the physiopathology of the disease. In this study, we describe a simple and affordable method to precisely measure oxygen consumption in living zebrafish larvae using a spectrofluorometer and the MitoXpress Xtra Oxygen Consumption Assay. In addition, we used zebrafish larvae treated with mitochondrial respiratory chain inhibitors, antimycin A or rotenone, to verify that our method enables precise and reliable measurements of oxygen consumption.

The antimycin A-sensitive pathway of cyclic electron flow: from 1963 to 2015

Cyclic electron flow has puzzled and divided the field of photosynthesis researchers for decades. This mainly concerns the proportion of its overall contribution to photosynthesis, as well as its components and molecular mechanism. Yet, it is irrefutable that the absence of cyclic electron flow has severe effects on plant growth. One of the two pathways mediating cyclic electron flow can be inhibited by antimycin A, a chemical that has also widely been used to characterize the mitochondrial respiratory chain. For the characterization of cyclic electron flow, antimycin A has been used since 1963, when ferredoxin was found to be the electron donor of the pathway. In 2013, antimycin A was used to identify the PGRL1/PGR5 complex as the ferredoxin:plastoquinone reductase completing the last puzzle piece of this pathway. The controversy has not ended, and here, we review the history of research on this process using the perspective of antimycin A as a crucial chemical for its characterization.

Diuron in water: functional toxicity and intracellular detoxification patterns of active concentrations assayed in tandem by a yeast-based probe

A study on the acute and chronic effects of the herbicide diuron was carried out. The test, basing on a yeast cell probe, investigated the interference with cellular catabolism and possible self-detoxification capacity of Saccharomyces cerevisiae. Aerobic respiration was taken as the toxicological end-point. Percentage interference (%r) with cellular respiration was measured in water by increased dissolved O2 concentration (ppm) after exposure to different doses. Interference was calculated through the comparison of respiratory activity of exposed and non-exposed cells. Short-term and long-term (6 and 24 h respectively) exposures were also considered. The test for short-term exposure gave positive %r values except that for 10-6 M (11.11%, 11.76%, 13.33% and 0% for 10-10 M, 10-8 M, 10-7 M and 10-6 M respectively). In the case of long-term exposure the test showed positive %r values, but less effect than short-term exposure until 10-8 M and much higher at 10-6 M (7.41%, 8.82%, 11.76% and 6.06% for 10-10 M, 10-8 M, 10-7 M and 10-6 M respectively). The findings of aerobic respiration as toxicological end-point were in agreement with known mechanisms of toxicity and intracellular detoxification for both the doses and exposure times employed.

Conformationally linked interaction in the cytochrome bc(1) complex between inhibitors of the Q(o) site and the Rieske iron-sulfur protein

The modified Q cycle mechanism accounts for the proton and charge translocation stoichiometry of the bc(1) complex, and is now widely accepted. However the mechanism by which the requisite bifurcation of electron flow at the Q(o) site reaction is enforced is not clear. One of several proposals involves conformational gating of the docking of the Rieske ISP at the Q(o) site, controlled by the stage of the reaction cycle. Effects of different Q(o)-site inhibitors on the position of the ISP seen in crystals may reflect the same conformational mechanism, in which case understanding how different inhibitors control the position of the ISP may be a key to understanding the enforcement of bifurcation at the Q(o) site (Table 1). Here we examine the available structures of cytochrome bc(1) with different Q(o)-site inhibitors and different ISP positions to look for clues to this mechanism. The effect of ISP removal on binding affinity of the inhibitors stigmatellin and famoxadone suggest a "mutual stabilization" of inhibitor binding and ISP docking, however this thermodynamic observation sheds little light on the mechanism. The cd(1) helix of cytochrome b moves in such a way as to accommodate docking when inhibitors favoring docking are bound, but it is impossible with the current structures to say whether this movement of α-cd(1) is a cause or result of ISP docking. One component of the movement of the linker between E and F helices also correlates with the type of inhibitor and ISP position, and seems to be related to the H-bonding pattern of Y279 of cytochrome b. An H-bond from Y279 to the ISP, and its possible modulation by movement of F275 in the presence of famoxadone and related inhibitors, or its competition with an alternate H-bond to I269 of cytochrome b that may be destabilized by bound famoxadone, suggest other possible mechanisms. This article is part of a Special Issue entitled: Allosteric cooperativity in respiratory proteins.

Binding of the respiratory chain inhibitor antimycin to the mitochondrial bc1 complex: a new crystal structure reveals an altered intramolecular hydrogen-bonding pattern

Antimycin A (antimycin), one of the first known and most potent inhibitors of the mitochondrial respiratory chain, binds to the quinone reduction site of the cytochrome bc1 complex. Structure-activity relationship studies have shown that the N-formylamino-salicyl-amide group is responsible for most of the binding specificity, and suggested that a low pKa for the phenolic OH group and an intramolecular H-bond between that OH and the carbonyl O of the salicylamide linkage are important. Two previous X-ray structures of antimycin bound to vertebrate bc1 complex gave conflicting results. A new structure reported here of the bovine mitochondrial bc1 complex at 2.28 A resolution with antimycin bound, allows us for the first time to reliably describe the binding of antimycin and shows that the intramolecular hydrogen bond described in solution and in the small-molecule structure is replaced by one involving the NH rather than carbonyl O of the amide linkage, with rotation of the amide group relative to the aromatic ring. The phenolic OH and formylamino N form H-bonds with conserved Asp228 of cytochrome b, and the formylamino O H-bonds via a water molecule to Lys227. A strong density, the right size and shape for a diatomic molecule is found between the other side of the dilactone ring and the alphaA helix.

The thankless task of playing genetics with mammalian mitochondrial DNA: a 30-year review

The advances obtained through the genetic tools available in yeast for studying the oxidative phosphorylation (OXPHOS) biogenesis and in particular the role of the mtDNA encoded genes, strongly contrast with the very limited benefits that similar approaches have generated for the study of mammalian mtDNA. Here we review the use of the genetic manipulation in mammalian mtDNA, its difficulty and the main types of mutants accumulated in the past 30 years and the information derived from them. We also point out the need for a substantial improvement in this field in order to obtain new tools for functional genetic studies and for the generation of animal models of mtDNA-linked diseases.

Factors influencing nominal effective concentrations of chemical compounds in vitro: cell concentration

In vitro potency data (e.g. EC(50) values), used to characterise the biological activity of chemicals, are generally based on nominal effective concentrations and thus depend on any factor influencing the availability of a compound. In this study the significance of cell binding for the availability of chemicals in vitro is (i) theoretically investigated by means of a simple equilibrium distribution model and (ii) experimentally examined using a bull sperm assay to measure the cytotoxic potency of selected compounds at different cell concentrations. Compounds were selected either to cover a wide range of hydrophobicity (log K(ow)=2.52-5.69) or to represent modes of cell binding other than partitioning into cellular lipids. With the exception of xylene, the EC(50) values increased with increasing cell concentration. The ratios of EC(50) values determined at about 120 x 10(6) and 15 x 10(6) cells/ml were: pentachlorophenol. 1.2, 1-nitronaphthalene: 1.9, thioridazine: 2.7, dieldrin: 4.1, hexachlorophene: 4.1, digitonin: 5.1, methylmercury chloride: 7.9, antimycin A: 10.1 and p,p'-dichlorodiphenyl dichloroethylene (p,p'-DDE): >19.1. The influence of partitioning into cell lipids was rather well predicted by the equilibrium distribution model, except for p,p'-DDE. The results show that cell binding can significantly affect the availability of compounds in vitro and thus toxic potencies and toxic equivalency factors.

Quinone specificity of complex I

This review considers the interaction of Complex I with different redox acceptors, mainly homologs and analogs of the physiological acceptor, hydrophobic Coenzyme Q. After examining the physical properties of the different quinones and their efficacy in restoring mitochondrial respiration, a survey ensues of the advantages and drawbacks of the quinones commonly used in Complex I activity determination and of their kinetic properties. The available evidence is then displayed on structure-activity relationships of various quinone compounds in terms of electron transfer activity and proton translocation, and the present knowledge is discussed in terms of the nature of multiple quinone-binding sites in the Complex.

A model of antimycin A binding based on structure-activity studies of synthetic antimycin A analogues

The structural factors of antimycin A molecule required for inhibitory action were studied using newly synthesized antimycin A derivatives with bovine heart submitochondrial particles, in order to probe the interaction between antimycin A and its binding site. In particular, we focused upon the roles of the amide bond bridge, which connects the salicylic acid and dilactone ring moieties, and the 3-formylamino group in the salicylic acid moiety. The lack of formation of an intramolecular hydrogen-bond between phenolic OH and amide carbonyl groups resulted in a remarkable loss of the activity (by four orders of magnitude), indicating that this hydrogen-bond is essential for the inhibition. This result suggested that both the phenolic OH and the carbonyl groups form a hydrogen-bond with some residues at a fixed conformation. In addition, the inhibitory potency was remarkably decreased by N-methylation of the amide bond moiety, indicating that the NH group might function in hydrogen-bond interaction with the binding site. The N-methylation of 3-formylamino group also resulted in a decrease in the activity, probably due to a loss of the rotational freedom of this functional group. Molecular orbital calculation studies with respect to the conformation of the 3-formylamino group indicated that this group takes an active conformation when the formyl carbonyl projects to the opposite side of the phenolic OH group. Based upon a series of structure-activity studies of synthetic antimycin A analogues, we propose a tentative model for antimycin A binding in its binding cavity.

Structural factors of antimycin A molecule required for inhibitory action

A series of antimycin A analogues was synthesized by modifying the salicylic acid moiety, whereas the portion of the molecule corresponding to the natural dilactone-ring moiety was fixed as di-n-octyl L-glutamate. To probe the structure of the antimycin A binding site, the structural factors of the salicylic acid moiety required for inhibitory action were examined by means of structure-activity studies with intact rat-liver mitochondria and the cytochrome bc1 complex isolated from bovine heart mitochondria. As suggested earlier (Rieske, J.S. (1976) Biochim. Biophys. Acta 456, 195-247), the phenolic OH was very important for inhibition. For the derivatives which do not possess a formylamino group in the 3-position (ortho to the phenolic OH), the inhibitory activity tended to increase as the electron-withdrawing property of the substituent increased, i.e., as the acidity of the phenolic OH group increased. This indicates that the acidity of the phenolic OH is an important factor governing inhibition. While the electron-withdrawing property of the formylamino group itself is rather poor, 3-formylamino derivatives elicited potent activity. The conformation of the 3-formylamino group was also found to be a very important factor in establishing inhibitory activity. In addition, the bulkier the moiety corresponding to the 3-formylamino group, the lower the activity. These results demonstrate that the presence of the 3-formylamino group, and its proper conformation, are needed for a close fitting of antimycin A to its binding domain. Although the inhibitors that lack a 3-formylamino group retained fairly potent activity, their effects on the reduction of cytochromes b and c1 were somewhat different from those of natural antimycin A, indicating that the 3-formylamino group is essential for inhibitor binding to the cytochrome bc1 complex in the same manner as natural antimycin A. It is concluded that both the 3-formylamino group and the phenolic OH of antimycin A make important contributions to specific interactions with the amino acid residues of the cytochrome b.

Mitochondrial cytochrome b: evolution and structure of the protein

Cytochrome b is the central redox catalytic subunit of the quinol: cytochrome c or plastocyanin oxidoreductases. It is involved in the binding of the quinone substrate and it is responsible for the transmembrane electron transfer by which redox energy is converted into a protonmotive force. Cytochrome b also contains the sites to which various inhibitors and quinone antagonists bind and, consequently, inhibit the oxidoreductase. Ten partial primary sequences of cytochrome b are presented here and they are compared with sequence data from over 800 species for a detailed analysis of the natural variation in the protein. This sequence information has been used to predict some aspects of the structure of the protein, in particular the folding of the transmembrane helices and the location of the quinone- and heme-binding pockets. We have observed that inhibitor sensitivity varies greatly among species. The comparison of inhibition titrations in combination with the analysis of the primary structures has enabled us to identify amino acid residues in cytochrome b that may be involved in the binding of the inhibitors and, by extrapolation, quinone/quinol. The information on the quinone-binding sites obtained in this way is expected to be both complementary and supplementary to that which will be obtained in the future by mutagenesis and X-ray crystallography.

Inhibition of electron transport of rat-liver mitochondria by synthesized antimycin A analogs

A series of antimycin A analogs was synthesized by replacement of a dilactone-ring moiety of natural antimycin A by various alkyl, substituted phenyl, substituted diphenyl ether, or amino acid ester groups. The structure-inhibitory activity relationship was studied with rat-liver mitochondria to identify roles of the dilactone-ring moiety in the inhibitor binding to a Qi reaction center of cytochrome bc1 complex. All derivatives caused further reduction of cytochrome b reduced by succinate and the oxidant-induced reduction, showing that the derivatives inhibited electron transport by interacting with a Qi reaction center. The inhibition tended to increase as the hydrophobicity of the inhibitor increased. The mode of binding of inhibitor molecules to a Qi center, which was reflected in, for example, a sigmoidal titration curve for respiratory inhibition and a time-dependent change in inhibitory activity, varied depending on structure. These results suggested that the role of the dilactone-ring moiety of antimycin A may be not only to support hydrophobic interaction with the binding domain by increasing the hydrophobicity of the molecule, as proposed earlier, but also to regulate close fitting of the salicylic acid moiety to the binding domain.

Cytochrome b of fish mitochondria is strongly resistant to funiculosin, a powerful inhibitor of respiration

We report here some unusual properties of ubiquinol: cytochrome c reductase of eel and other fish mitochondria. The turnover rate of the reductase is clearly higher than in mammalian mitochondria and the binding constant for ubiquinone seems to be larger than in other vertebrates. Additionally, the reductase activity of fish mitochondria is resistant to some powerful inhibitors that bind to cytochrome b, in particular to funiculosin. After sequencing most of the gene of eel cytochrome b and comparing the deduced amino acid sequence with that of other fish and animals, we hypothesize that the decreased binding of funiculosin could be due to a few amino acid replacements in the third and fourth transmembrane helix of the protein. In particular, the presence of methionine instead of alanine at position 125 seems to be largely responsible for the strong resistance to funiculosin and also to the partial resistance to myxothiazol in all fish mitochondria. Correlations between some residue substitutions in cytochrome b and the different effects of funiculosin in different species are also considered.

EPR characterization of the cytochrome b-c1 complex from Rhodobacter sphaeroides

EPR characteristics of cytochrome c1, cytochromes b-565 and b-562, the iron-sulfur cluster, and an antimycin-sensitive ubisemiquinone radical of purified cytochrome b-c1 complex of Rhodobacter sphaeroides have been studied. The EPR specra of cytochrome c1 shows a signal at g = 3.36 flanked with shoulders. The oxidized form of cytochrome b-562 shows a broad EPR signal at g = 3.49, while oxidized cytochrome b-565 shows a signal at g = 3.76, similar to those of two b cytochromes in the mitochondrial complex. The distribution of cytochromes b-565 and b-562 in the isolated complex is 44 and 56%, respectively. Antimycin and 2,5-dibromo-3-methyl-6-isopropyl-1,4-benzoquinone (DBMIB) have little effect on the g = 3.76 signal, but they cause a slight downfield and upfield shifts of the g = 3.49 signal, respectively. 5-Undecyl-6-hydroxyl-4,7-dioxobenzothiazole (UHDBT) shifts the g = 3.49 signal downfield to g = 3.56 and sharpens the g = 3.76 signal slightly. Myxothiazol causes an upfield shift of both g = 3.49 and g = 3.76 signals. EPR characteristics of the reduced iron-sulfur cluster in bacterial cytochrome b-c1 complex are: gx = 1.8 with a small shoulder at g = 1.76, gy = 1.89 and gz = 2.02, similar to those observed with the mitochondrial enzyme. The gx = 1.8 signal decreased and the shoulder increased concurrently as the redox potential decreased, indicating that the environment of the iron-sulfur cluster is sensitive to the redox state of the complex. UHDBT sharpens the gz and and shifts it downfield from g = 2.02 to 2.03, and shifts gx upfield from g = 1.80 to 1.78. UHDBT also causes an upfield shift of gy but to a much lesser extent compared to the other two signals. Addition of DBMIB causes a downfield shift of the gy from 1.89 to 1.94 and broadens the gx signal with an upfield to g = 1.75. Myxothiazol and antimycin show little effect on the gy and gz signals, but they broaden and shift the gx signal upfield to g = 1.74. However, the myxothiazol effect is partially reversed by UHDBT. An antimycin-sensitive ubisemiquinone radical was detected in the cytochrome b-c1 complex. At pH 8.4, the antimycin-sensitive ubisemiquinone radical has a maximal concentration of 0.66 mol per mol complex at 100 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Protection by glycine of proximal tubules from injury due to inhibitors of mitochondrial ATP production

We have determined whether glycine or glutathione can protect rabbit proximal tubules damaged by chemical inhibitors of oxidative phosphorylation: antimycin A, rotenone, cyanide, oligomycin, or carbonyl cyanide m-chlorophenylhdrazone (CCCP). All the agents severely depleted cell ATP levels within 15 min and caused lethal cell injury, as quantified by lactate dehydrogenase (LDH) release. Glycine and glutathione largely prevented this injury without altering the primary effects of the inhibitors on tubule respiration or the depletion of ATP. Buthionine sulfoximine and 1,3-bis(2-chloroethyl)-1-nitrosourea decreased cell glutathione but did not prevent the protective effects of either glycine or glutathione in tubules treated with rotenone. Protection was sustained during both a 15-min exposure and a 45-min postwash period irrespective of whether the wash removed the agent or mitochondrial function recovered. Cysteine uniquely induced a dramatic recovery of mitochondrial function in tubules washed after treatment with CCCP. These data 1) demonstrate that the cytoprotective effects of glycine previously seen during hypoxia extend to other tubule lesions characterized by severe ATP depletion, 2) emphasize the actions of glycine to preserve cell structural integrity in spite of sustained severe impairment of ATP-generating processes in proximal tubules, and 3) indicate that it is glycine rather than intracellular or extracellular glutathione which mediates protection.

Mutational analysis of the mouse mitochondrial cytochrome b gene

The protonmotive cytochrome b protein of the mitochondrial bc1 respiratory chain complex contains two reactions centers, designated Qo and Qi, which can be distinguished by the effects of different inhibitors. The nucleotide sequences have been determined of the mitochondrial cytochrome b genes from a series of mouse cell mutants selected for increased inhibitor resistance. Each mutant contains a single nucleotide change which results in an amino acid substitution. When the proximity of the altered amino acid residues to the histidines involved in heme ligation is considered, the results support a model for cytochrome b folding in which there are eight transmembrane domains rather than the nine of the Widger-Saraste model. Replacement of the Gly38 residue by valine results in resistance to the Qi inhibitors antimycin A and funiculosin but not 2-n-heptyl-hydroxyquinoline-N-oxide. Based upon sequence comparisons of mitochondrial and bacterial cytochrome b and chloroplast b6 proteins, the region of the molecule involved in antimycin binding is as highly conserved as those domains involved in heme ligation. It is suggested that the antimycin binding domain of cytochrome b is involved in forming the Qi reaction center. Alterations of the Gly142 and Thr147 residues result in resistance to myxothiazol and stimatellin, respectively. While both inhibitors block the Qo reaction center, the two mutations do not confer cross-resistance to each other. This region of cytochrome b is the most highly conserved during evolution and these inhibitor binding sites probably occur within the protein domain constituting the Qo reaction center. In addition, there is a less conserved region of the protein, defined by the Leu294 residue, which may function in binding the hydrophobic portions of Qo inhibitors.

The role of phospholipids in the binding of antimycin to yeast Complex III

Treatment of yeast Complex III with hexane or repeated fractionation with ammonium sulfate-cholate abolishes the ability of antimycin to bind to the complex. Antimycin binding is partially restored by addition of phospholipids to the depleted complex; this action of phospholipids can be enhanced by including Q6 in the reconstitution mixture.

On the oxidation pathways of the mitochondrial bc1 complex from beef heart. Effects of various inhibitors

We have investigated the oxidation of the reduced ubiquinol:cytochrome c reductase (bc1 complex) isolated from beef heart mitochondria. The oxidation of cytochrome c1 by both potassium ferricyanide and cytochrome c in the ascorbate-reduced bc1 complex is not a first-order process. This is taken as evidence that cytochrome c1 is in rapid equilibrium with the Rieske iron-sulphur center. Among the several inhibitors tested, only 5-n-undecyl-6-hydroxy-4,7-dioxobenzothiazole and stigmatellin are seen to affect this redox equilibrium between the high-potential centers of the beef heart bc1 complex. The oxidation of cytochrome b by cytochrome c in both the succinate-reduced and the fully reduced bc1 complex is blocked by all the inhibitors tested. This inhibition occurs simultaneously with an acceleration in the oxidation of cytochrome c1, even after extraction of the endogenous ubiquinone which is present in the bc1 preparation. Almost complete extraction of ubiquinone from the bc1 complex has no effect upon the rapid phase of cytochrome b oxidation, nor does it alter the inhibition of cytochrome b oxidation by the various inhibitors. The oxidation of cytochrome b by exogenous ubiquinones is stimulated by myxothiazol and partially inhibited by antimycin. However, the addition of both these inhibitors together completely blocks the oxidation of cytochrome b by quinones. In contrast, the simultaneous addition of antimycin and myxothiazol has no such synergistic effect upon the oxidation of cytochrome b by cytochrome c. Our data show that intramolecular electron transfer from cytochrome(s) b to the Rieske iron-sulphur center can take place in the bc1 complex without involvement of endogenous ubiquinone-10. This electron pathway is sensitive to all the inhibitors of the enzyme.

Experimental observations on the structure and function of mitochondrial complex III that are unresolved by the protonmotive ubiquinone-cycle hypothesis

The current model of the protonmotive ubiquinone cycle as applied to mitochondrial ubiquinol-cytochrome c reductase complex (Complex III) is able to explain a number of previously puzzling observations concerning electron-transfer and proton translocating functions of the complex. However, a number of pertinent experimental observations concerning the structure and function of this complex cannot as yet be incorporated into the present version of the ubiquinone cycle. The yet unresolved problems of electron transfer uncovered by these observations include some kinetic and thermodynamic problems, uncertainties in the binding site(s) and mode of binding of ubiquinol and inhibitors, the observed multiple spectroscopic, electrochemical, and kinetic forms of cytochromes b, iron-sulfur protein, and cytochrome c1, the multiple and overlapping effects of inhibitors, and the functional role of conformational changes in the complex. It is concluded that although the Q cycle is a valuable base for the design of future experiments, its mechanism must be reconciled with the above uncertainties as well as with the accumulated evidence that Complex III can exist in two or more interchangeable forms, exhibiting different properties with respect to electron-transfer pathways, inhibitor binding, and spectral and electrochemical properties of the electron-carrier subunits.

Differential labeling of the subunits of respiratory complex III with [3H]succinic anhydride, [14C]succinic anhydride, and p-diazobenzene-[35S]sulfonate

Exposure of antimycin-treated Complex III (ubiquinol-cytochrome c reductase) purified from bovine heart mitochondria to [3H]succinic anhydride plus [35S]p-diazobenzenesulfonate (DABS) resulted in somewhat uniform relative labeling of the eight measured subunits of the complex by [3H]succinic anhydride. In contrast, relative labeling by [35S]DABS was similar to [3H]succinic anhydride for the subunits of high molecular mass, i.e., core proteins, cytochromes, and the iron-sulfur protein, but greatly reduced for the polypeptides of molecular mass below 15 kDa. With Complex II depleted in the iron-sulfur protein the relative labeling of core protein I by exposure of the complex to [3H]succinic anhydride was significantly enhanced, whereas labeling of the polypeptides represented by SDS-PAGE bands 7 and 8 was significantly inhibited. Dual labeling of the subunits of Complex III by 14C- and 3H-labeled succinic anhydride before and after dissociation of the complex by sodium dodecyl sulfate, respectively, was measured with the complex in its oxidized, reduced, and antimycin-inhibited states. Subunits observed to be most accessible or reactive to succinic anhydride were core protein II, the iron-sulfur protein, and polypeptides of SDS-PAGE bands 7,8, and 9. Two additional polypeptides of molecular masses 23 and 12kDa, not normally resolved by gel-electrophoresis, were detected. Reduction of the complex resulted in a significant change of 14C/3H labeling ratio of core protein only, whereas treatment of the complex with antimycin resulted in decreases in 14C/3H labeling ratios of core proteins I and II, cytochrome c1, and a polypeptide of molecular mass 13kDa identified as an antimycin-binding protein.

Detection of antimycin-binding subunits of complex III by photoaffinity-labeling with an azido derivative of antimycin

Deformamidoazidoantimycin A (DAA), a photoactive derivative of antimycin A containing an azido group substituting for the formamido group attached to the phenyl ring, was synthesized. The ultraviolet spectrum of DAA was almost identical to that of antimycin A, indicating little alteration of the electronic structure of the substituted phenyl ring by the azido substitution. However, the inhibitory effectiveness of DAA toward ubiquinol-cytochrome c reductase (Complex III) purified from bovine heart (Ki = ca. 0.5 microM) was considerably less than that of antimycin (Ki less than or equal to 3 pM), indicating a direct rather than a supporting role of the formamido group in the inhibitory activity of antimycin. Exposure of purified Complex III to [3H]DAA plus ultraviolet light caused a major labeling by tritium of SDS-PAGE band 7 (m = 13 kDa by SDS-PAGE) and lesser but significant labeling of bands 3, 6, 8, and 9. Pretreatment of Complex III with antimycin greatly suppressed the labeling of bands 5, 6, and 7 but caused an apparent increased labeling of bands 8 and 9 by [3H]DAA, respectively. The labeling of band 7 by [3H]DAA also was strongly suppressed by reduction of Complex III by either sodium borohydride or ascorbate. Based on magnitude of labeling by [3H]DAA and the degree of suppression of labeling by antimycin, the protein of band 7 qualified as the principal component for specific binding of antimycin with the protein of band 6 (m = 16 kDa) showing a lesser but significant amount of specific binding.

Effects of bc1-site electron transfer inhibitors on the absorption spectra of mitochondrial cytochromes b

Changes are described that are brought about by antimycin, NoHOQnO, funiculosin, myxothiazol and mucidin in the alpha-, beta- and gamma-absorption bands of reduced and oxidized cytochromes b in the isolated complex bc1 form beef heart mitochondria. The inhibitors can be divided into 2 groups. Antimycin, funiculosin and NoHOQnO are likely to shift the spectrum of b-562 and compete for specific binding with complex bc1, with each other but not with myxothiazol and mucidin. The spectral effects of the latter two inhibitors are more difficult to interpret and may involve contributions not only from b-562 but from b-566 as well. The existence of 2 independent inhibitor binding-sites in the complex bc1 corroborates the Q-cycle hypothesis.

Effects of dibromothymoquinone on the structure and function of the mitochondrial bc1 complex

We have investigated in detail the effects of dibromothymoquinone (2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, DBMIB) on the ubiquinol-cytochrome c reductase (cytochrome bc1 complex) from bovine heart mitochondria. The inhibitory action of DBMIB on the steady-state activity of the bc1 complex is related to the specific binding of the quinone to the purified enzymatic complex. At concentrations higher than 10 mol per mol of the enzyme, DBMIB is able to stimulate an antimycin-insensitive reduction of cytochrome c catalyzed by the bc1 complex. In accordance with kinetic data showing a competition by endogenous ubiquinone in the inhibitory action, DBMIB can be considered as a product-like inhibitor of the ubiquinol-cytochrome c reductase activity. The site of specific binding of dibromothymoquinone in the bc1 complex enables it to interact with the iron-sulphur center of the enzyme, as indicated by changes induced in the EPR spectrum of the center. However, the inhibitor also directly interacts with cytochrome b, promoting a fast chemical oxidation of the reduced heme center. In spite of these effects, DBMIB has been found not to exert significant effects on the first turnover of the fully oxidized bc1 complex, as monitored by the rapid reduction of both cytochromes b and c1 by ubiquinol-1. In the presence of antimycin, only a stimulation of cytochrome c1 reduction, in parallel to an enhanced cytochrome b reoxidation, is observed. Moreover, DBMIB does not affect the oxidant-induced extra cytochrome b reduction in the presence of antimycin. On the basis of the evidences suggesting a competition with the endogenous ubiquinone in the redox cycle of the bc1 complex, a model is proposed for the mechanism of DBMIB inhibition. Such model can also explain at the molecular level the redox bypass induced by dibromothymoquinone in the whole respiratory chain (Degli Esposti, M., Rugolo, M. and Lenaz, G. (1983) FEBS Lett. 156, 15-19).

Mammalian mitochondrial mutants selected for resistance to the cytochrome b inhibitors HQNO or myxothiazol

Mouse LA9 cell lines were selected for increased resistance to either HQNO or myxothiazol, inhibitors of electron transport which bind to the mitochondrial cytochrome b protein. Two phenotypically distinguishable HQNO-resistant mutants were recovered while the myxothiazol-resistant isolates had a common phenotype. All three mutant phenotypes were transmitted cytoplasmically in cybrid crosses. Biochemical studies further established that for all three mutant types, resistance at the cellular level was paralleled by an increase in inhibitor resistance of mitochondrial succinate-cytochrome c oxidoreductase, the respiratory complex containing cytochrome b. As with the previously described mitochondrial antimycin-resistant mutant, the initial biochemical and genetic studies indicated that these mutations occur within the mitochondrial cytochrome b gene. This conclusion was strongly supported by the results of mtDNA restriction fragment analyses in which it was found that one HQNO-resistant mutant had undergone a small insertion or duplication in the apocytochrome b gene. Finally, all four mitochondrial cytochrome b mutants have been analyzed in both cell plating studies and succinate-cytochrome c oxidoreductase assays to determine the pattern of cross-resistance to inhibitors of cytochrome b other than the one used for selection.

Morpholine vapour inhalation and interactions of simultaneous nitrite intake. Biochemical effects on rat spinal cord axons and skeletal muscle

Male Wistar rats exposed intermittently to 300 ppm (12.5 mumol/l) morpholine vapour 5 days a week for 6 h daily during 4-15 weeks displayed an increasing brain solvent content analyzed by a sensitive gas chromatographic method. Rats drinking water which contained 150 mg NaNO2/l during the vapour exposure period showed initially larger brain solvent concentrations which began to decrease after 8 weeks. Fat solvent concentrations were a fraction of those detected in brain of the solvent-exposed animals or of those in the combined exposure. Nitrite exposure alone caused decreased spinal cord axon acetylcholine esterase activity after 8 weeks while this effect was noted in the combined exposure only at 8 weeks disappearing later on. Axonal succinate dehydrogenase activity was below the controls in the combined exposure throughout the study, and combination also caused persistent increase in the muscle creatine kinase activity. The morpholine-induced effects were less remarkable. The results point at pharmacokinetic interaction between the solvent and nitrite with its own effects on energy metabolism. No N-nitrosomorpholine was found although other metabolic interactions could not be excluded.

Inhibition of the mitochondrial bc1 complex by dibromothymoquinone

We have studied the effects of dibromothymoquinone (DBMIB) in various redox activities of the succinate-cytochrome c span of the mitochondrial respiratory chain. At concentrations higher than 50 mol/mol of cytochrome c1 the inhibitor produces a bypass of electron transfer on the substrate side of the bc1 complex, because of its autooxidation capability. This induces an artifactual overestimation of the real inhibition titer of the redox activity of this enzyme, which has been found to be 3-6 mol/mol of cytochrome c1 by following the ubiquinol-cytochrome c reductase activity. This action is reversed by addition of excess of sulphydryl compounds like cysteine.

Mitochondrial genetics of mammalian cells: a mouse antimycin-resistant mutant with a probable alteration of cytochrome b

Mouse LA9 antimycin-resistant mutants (ANT-R) were isolated and characterized. Genetic analyses established that this phenotype is encoded within the mtDNA: (1) the ANT-R phenotype showed frequent mitotic segregation and reassortment in hybrid clonal lines; (2) it was transmitted directly in cybrid crosses; and (3) it was cotransmitted in cybrid crosses with the mitochondrial CAP-R marker. Furthermore, the genetic studies suggested that the LA9 CAP-R ANT-R cells were heteroplasmic and contained at least two mtDNA genotypes, cap-r ant-s and cap-s ant-r. Cellular respiration of the ANT-R mutant was markedly more resistant to inhibition by antimycin than that of the parental ANT-S cells. The increased resistance of cellular respiration was entirely accounted for by an increase in the resistance of mitochondrial succinate-cytochrome c oxidoreductase to antimycin inhibition. There was no detectable change in the specific activity of the oxidoreductase in mitochondria of resistant ANT-R cells nor in the sensitivity of the complex to three other specific inhibitors of the complex: TTFA, myxothiazol, and HQNO. Taken together, these studies indicate that the ANT-R phenotype is most likely encoded within the mitochondrial cytochrome b gene and, more specifically, within an antimycin binding domain.

Effects of extended peroral borate ingestion on rat liver and brain

2-Month-old male Wistar rats were given sodium tetraborate in drinking water (3 g/l). Cerebral succinate dehydrogenase activity increased after 10 and 14 weeks of exposure. Increased RNA concentration and increased acid proteinase activity in brain occurred after 14 weeks. NADPH-cytochrome c reductase activity and cytochrome b5 content decreased in the liver microsomal fraction after 10 and 14 weeks. A reduction in the cytochrome P-450 concentration was detected at 14 weeks. The results support the hypothesis that borate anion exerts its toxic action by interfering with flavin metabolism in flavoprotein-dependent pathways.

Dose-dependent effects of peroral dimethylformamide administration on rat brain

Three-month-old Wistar rats were given dimethylformamide in their drinking water at three concentrations. Succinate dehydrogenase activity decreased at the two higher doses in brain after 2 or 7 weeks. Decreased glutathione concentration occurred at the highest dose. Cerebral azoreductase activity was below the control range after 7 weeks at all doses. Glial cell succinate dehydrogenase activity was below the control range in all animals. No qualitative changes in the spinal cord axon protein composition were detected. It is postulated that formic acid generated in the dimethylformamide metabolism might have led to a significant derangement of cerebral energy metabolism.