Energy transduction by the reconstituted b-c1 complex from yeast mitochondria. Inhibitory effects of dicyclohexylcarbodiimide

Study of respiratory cytochromes in liposomes

Important findings regarding the structure and function of respiratory cytochromes have been made from the study of these hemeproteins associated to liposomes. These studies contributed to the comprehension of the biological role of these proteins in the electron transfer process, the regulatory mechanisms, the energy transduction mechanisms, the protein sites that interact with mitochondrial membranes and the role played by the non-redox subunits present in the protein complexes of the respiratory chain of eukaryotes. In this chapter, the protocols developed to study cytochrome bc (1) activity in liposomes and the binding of cytochrome c to lipid bilayers is presented . The former protocol was developed to study the mechanism of energy transduction related to the topology of the components of bc (1) complex in the mitochondrial membrane. These studies were done with purified cytochrome bc (1) complexes reconstituted into potassium-loaded vesicles. The latter protocol was developed to study the influence of pH, DeltapH, and DeltaPsi on the interaction of cytochrome c with liposomes that mimic the inner mitochondrial membrane.

Regulation of glycolytic oscillations by mitochondrial and plasma membrane H+-ATPases

We investigated the coupling between glycolytic and mitochondrial membrane potential oscillations in Saccharomyces cerevisiae under semianaerobic conditions. Glycolysis was measured as NADH autofluorescence, and mitochondrial membrane potential was measured using the fluorescent dye 3,3'-diethyloxacarbocyanine iodide. The responses of glycolytic and membrane potential oscillations to a number of inhibitors of glycolysis, mitochondrial electron flow, and mitochondrial and plasma membrane H(+)-ATPase were investigated. Furthermore, the glycolytic flux was determined as the rate of production of ethanol in a number of different situations (changing pH or the presence and absence of inhibitors). Finally, the intracellular pH was determined and shown to oscillate. The results support earlier work suggesting that the coupling between glycolysis and mitochondrial membrane potential is mediated by the ADP/ATP antiporter and the mitochondrial F(0)F(1)-ATPase. The results further suggest that ATP hydrolysis, through the action of the mitochondrial F(0)F(1)-ATPase and plasma membrane H(+)-ATPase, are important in regulating these oscillations. We conclude that it is glycolysis that drives the oscillations in mitochondrial membrane potential.

Mode of Inhibitory Action of Δlac-Acetogenins, a New Class of Inhibitors of Bovine Heart Mitochondrial Complex I

We have revealed that Deltalac-acetogenins, a new class of inhibitors of bovine heart mitochondrial complex I (NADH-ubiquinone oxidoreductase), act differently from ordinary inhibitors such as rotenone and piericidin A [Ichimaru et al. (2005) Biochemistry 44, 816-825]. Since a detailed study of these unique inhibitors might provide new insight into the terminal electron transfer step of the enzyme, we further characterized their inhibitory action using the most potent Deltalac-acetogenin derivative (compound 1). Unlike ordinary complex I inhibitors, 1 had a dose-response curve for inhibition of the reduction of exogenous short-chain ubiquinones that was difficult to explain with a simple bimolecular association model. The inhibitory effect of 1 on ubiquinol-NAD(+) oxidoreductase activity (reverse electron transfer) was much weaker than that on NADH oxidase activity (forward electron transfer), indicating a direction-specific effect. These results suggest that the binding site of 1 is not identical to that of ubiquinone and the binding of 1 to the enzyme secondarily (or indirectly) disturbs the redox reaction of ubiquinone. Using endogenous and exogenous ubiquinone as an electron acceptor of complex I, we investigated the effect of 1 in combination with different ordinary inhibitors on the superoxide production from the enzyme. The results indicated that the level of superoxide production induced by 1 is significantly lower than that induced by ordinary inhibitors probably because of fewer electron leaks from the ubisemiquinone radical to molecular oxygen and that the site of inhibition by 1 is downstream of that by ordinary inhibitors. The unique inhibitory action of hydrophobic Deltalac-acetogenins may be closely associated with the dynamic function of the membrane domain of complex I.

Redox-coupled proton pumping activity in cytochrome b6f, as evidenced by the pH dependence of electron transfer in whole cells of Chlamydomonas reinhardtii

The pH dependence of cytochrome b(6)f catalytic activity has been measured in whole cells of the green alga Chlamydomonas reinhardtii over the 5-8 range. An acid pH slowed the reactions occurring at the lumenal side of the complex (cytochrome b(6) and f reduction) and affected also the rate and amplitude of the slow electrogenic reaction (phase b), which is supposed to reflect transmembrane electron flow in the complex. On the other hand, a direct measurement of the transmembrane electron flow from the kinetics of cytochrome b(6) oxidation revealed no pH sensitivity. This suggests that a substantial fraction of the electrogenicity associated with cytochrome b(6)f catalysis is not due to electron transfer in the b(6) hemes but to a plastoquinol-oxidation-triggered charge movement, in agreement with previous suggestions that a redox-coupled proton pump operates in cytochrome b(6)f complex. The pH dependence of cytochrome b(6)f activity has also been measured in two mutant strains, where the glutamic 78 of the conserved PEWY sequence of subunit IV has been substituted for a basic (E78K) and a polar (E78Q) residue [Zito, F., Finazzi, G., Joliot, P., and Wollman, F.-A. (1998) Biochemistry 37, 10395-10403]. Their comparison with the wild type revealed that this residue plays an essential role in plastoquinol oxidation at low pH, while it is not required for efficient activity at neutral pH. Its involvement in gating the redox-coupled proton pumping activity is also shown.

Carboxyl residues in the iron-sulfur protein are involved in the proton pumping activity of P. denitrificans bc(1) complex

A study is presented on chemical modification of the three subunit Paracoccus denitrificans bc(1) complex. N-(Ethoxycarbonyl)-2-ethoxy-1,2-dihydroquinoline (EEDQ) treatment caused a loss of the proton pumping activity of liposome-reconstituted bc(1) complex. A similar effect, which is referred to as the decoupling effect, resulted upon reaction of N,N'-dicyclohexylcarbodiimide (DCCD) with the complex. Direct measurement of the binding of EEDQ to the complex subunits, performed in the presence of the fluorescent hydrophobic nucleophile 4'-[(aminoacetamido)methyl]fluorescein (AMF), showed that the iron-sulfur protein (ISP) and cytochrome c(1) were labeled by EEDQ, whereas cytochrome b was not. Tryptic digestion and sequencing analysis of the fluorescent fragment of the ISP revealed this to consist of a segment with six acidic residues, among which the highly conserved aspartate 160 is present. Analogous experiments on DCCD binding showed that all the three subunits of the complex were labeled. However, DCCD concentration dependence of carboxyl residue modification in the individual subunits and of proton pumping activity showed that the decrease of the H(+)/e(-) ratio correlated only with the modification of the ISP. Tryptic digestion of labeled ISP and sequencing analysis of the fluorescent fragment gave results superimposable upon those obtained with EEDQ. Chymotryptic digestion and sequencing analysis of the single fluorescent fragment of cytochrome b showed that this fragment contained glutamate 174 and aspartate 187. We conclude that, in the P. denitrificans bc(1) complex, carboxyl residues in cytochrome b do not appear to be critically involved in the proton pump mechanism of the complex.

Substituting leucine for alanine-86 in the tether region of the iron-sulfur protein of the cytochrome bc1 complex affects the mobility of the [2Fe2S] domain

Mutating three conserved alanine residues in the tether region of the iron-sulfur protein of the yeast cytochrome bc(1) complex resulted in 22-56% decreases in enzymatic activity [Obungu et al. (2000) Biochim. Biophys. Acta 1457, 36-44]. The activity of the cytochrome bc(1) complex isolated from A86L was decreased 60% compared to the wild-type without loss of heme or protein and without changes in the 2Fe2S cluster or proton-pumping ability. The activity of the bc(1) complex from mutant A92R was identical to the wild-type, while loss of both heme and activity was observed in the bc(1) complex isolated from mutant A90I. Computer simulations indicated that neither mutation A86L nor mutation A92R affects the alpha-helical backbone in the tether region; however, the side chain of the leucine substituted for Ala-86 interacts with the side chain of Leu-89. The Arrhenius plot for mutant A86L was apparently biphasic with a transition observed at 17-19 degrees C and an activation energy of 279.9 kJ/mol below 17 degrees C and 125.1 kJ/mol above 17 degrees C. The initial rate of cytochrome c(1) reduction was lowered 33% in mutant A86L; however, the initial rate of cytochrome b reduction was unaffected, suggesting that movement of the tether region of the iron-sulfur protein is necessary for maximum rates of enzymatic activity. Substituting a leucine for Ala-86 impedes the unwinding of the alpha-helix and hence movement of the tether.

DCCD inhibits the reactions of the iron-sulfur protein in Rhodobacter sphaeroides chromatophores

N,N'-dicyclohexylcarbodiimide (DCCD) has been reported to inhibit proton translocation by cytochrome bc(1) and b(6)f complexes without significantly altering the rate of electron transport, a process referred to as decoupling. To understand the possible role of DCCD in inhibiting the protonogenic reactions of cytochrome bc(1) complex, we investigated the effect of DCCD modification on flash-induced electron transport and electrochromic bandshift of carotenoids in Rb. sphaeroides chromatophores. DCCD has two distinct effects on phase III of the electrochromic bandshift of carotenoids reflecting the electrogenic reactions of the bc(1) complex. At low concentrations, DCCD increases the magnitude of the electrogenic process because of a decrease in the permeability of the membrane, probably through inhibition of F(o)F(1). At higher concentrations (>150 microM), DCCD slows the development of phase III of the electrochromic shift from about 3 ms in control preparations to about 23 ms at 1.2 mM DCCD, without significantly changing the amplitude. DCCD treatment of chromatophores also slows down the kinetics of flash-induced reduction of both cytochromes b and c, from 1.5-2 ms in control preparations to 8-10 ms at 0.8 mM DCCD. Parallel slowing of the reduction of both cytochromes indicates that DCCD treatment modifies the reaction of QH(2) oxidation at the Q(o) site. Despite the similarity in the kinetics of both cytochromes, the onset of cytochrome c re-reduction is delayed 1-2 ms in comparison to cytochrome b reduction, indicating that DCCD inhibits the delivery of electrons from quinol to heme c(1). We conclude that DCCD treatment of chromatophores leads to modification of the rate of Q(o)H(2) oxidation by the iron-sulfur protein (ISP) as well as the donation of electrons from ISP to c(1), and we discuss the results in the context of the movement of ISP between the Q(o) site and cytochrome c(1).

Aspartate-187 of cytochrome b is not needed for DCCD inhibition of ubiquinol: cytochrome c oxidoreductase in Rhodobacter sphaeroides chromatophores

N,N'-dicyclohexylcarbodiimide (DCCD) has been reported to inhibit steady-state proton translocation by cytochrome bc(1) and b(6)f complexes without significantly altering the rate of electron transport, a process referred to as decoupling. In chromatophores of the purple bacterium Rhodobacter sphaeroides, this has been associated with the specific labeling of a surface-exposed aspartate-187 of the cytochrome b subunit of the bc(1) complex [Wang et al. (1998) Arch. Biochem. Biophys. 352, 193-198]. To explore the possible role of this amino acid residue in the protonogenic reactions of cytochrome bc(1) complex, we investigated the effect of DCCD modification on flash-induced electron transport and the electrochromic bandshift of carotenoids in Rb. sphaeroides chromatophores from wild type (WT) and mutant cells, in which aspartate-187 of cytochrome b (Asp(B187)) has been changed to asparagine (mutant B187 DN). The kinetics and amplitude of phase III of the electrochromic shift of carotenoids, reflecting electrogenic reactions in the bc(1) complex, and of the redox changes of cytochromes and reaction center, were similar (+/- 15%) in both WT and B187DN chromatophores. DCCD effectively inhibited phase III of the carotenoid bandshift in both B187DN and WT chromatophores. The dependence of the kinetics and amplitude of phase III of the electrochromic shift on DCCD concentration was identical in WT and B187DN chromatophores, indicating that covalent modification of Asp(B187) is not specifically responsible for the effect of DCCD-induced effects of cytochrome bc(1) complex. Furthermore, no evidence for differential inhibition of electrogenesis and electron transport was found in either strain. We conclude that Asp(B187) plays no crucial role in the protonogenic reactions of bc(1) complex, since its replacement by asparagine does not lead to any significant effects on either the electrogenic reactions of bc(1) complex, as revealed by phase III of the electrochromic shift of carotenoids, or sensitivity of turnover to DCCD.

Generation of superoxide anion by succinate-cytochrome c reductase from bovine heart mitochondria

Production of superoxide anion (O-2), measured as the chemiluminescence of the 2-methyl-6-(p-methoxyphenyl)-3, 7-dihydroimidazo[1,2-a]pyrazin-3-one hydrochloride (MCLA)-O-2 adduct, was observed during electron transfer from succinate to cytochrome c by reconstituted succinate-cytochrome c reductase-phospholipid vesicles replenished with succinate dehydrogenase. Addition of carbonyl cyanide p-trifluoromethoxyphenylhydrazone or detergent to the reconstituted reductase-phospholipid vesicles abolished O-2 production, suggesting that O-2 generation is caused by the membrane potential generated during electron transfer through the cytochrome bc1 complex. Production of O-2 was also observed during electron transfer from succinate to cytochrome c by antimycin-treated reductase, in which approximately 99.7% of the reductase activity was inhibited. The rate of O-2 production was closely related to the rate of antimycin-insensitive cytochrome c reduction. Factors affecting antimycin-insensitive reduction of cytochrome c also affected O-2 production and vice versa. When the oxygen concentration in the system was decreased, the rate of O-2 production and cytochrome c reduction by antimycin-treated reductase decreased. When the concentrations of MCLA and cytochrome c were increased, the rate of O-2 production and cytochrome c reduction by antimycin-treated reductase increased. The rate of antimycin-insensitive cytochrome c reduction was sensitive to Qo site inhibitors such as 5-undecyl-6-hydroxy-4,7-dioxobenzothiazole. These results indicate that generation of O-2 during the oxidation of ubiquinol by the cytochrome bc1 complex results from a leakage of the second electron of ubiquinol from its Q cycle electron transfer pathway to interact with oxygen. The electron-leaking site is located at the reduced cytochrome b566 or ubisemiquinone of the Qo site because addition of MCLA to antimycin-treated cytochrome bc1 complex, in the presence of catalytic amounts of succinate-cytochrome c reductase, delayed cytochrome b reduction by succinate. In the presence of oxidized cytochrome c, purified succinate dehydrogenase also catalyzed oxidation of succinate to generate O-2. When succinate dehydrogenase was reconstituted with the bc1 particles to form succinate-cytochrome c reductase, the production of O-2 diminished. These results suggest that reduced FAD of succinate dehydrogenase is the electron donor for oxygen to produce O-2 in the absence of their immediate electron acceptor and in the presence of cytochrome c.

In vivo analysis of the effect of dicyclohexylcarbodiimide on electron and proton transfers in cytochrome bf complex of Chlorella sorokiniana

The effect of N,N'-dicyclohexylcarbodiimide (DCCD) on electron and proton transfers within the cytochrome (cyt) bf complex has been analyzed in living cells of the green algae Chlorella sorokiniana under anaerobic conditions. DCCD induces a partial decoupling of the protomotive Q-cycle, in agreement with the conclusions of Wang and Beattie (1991) Arch. Biochem. Biophys. 291, 363-370. In the presence of 20 microM DCCD, we observe the development of a lag phase in the kinetics of the slow electrogenic phase associated with electron and proton transfers within the cyt bf complex. In the same conditions, the initial rate of cyt b and cyt f reduction is decreased by about 30%. We propose that in the absence of DCCD, a transmembrane movement of proton is coupled to the oxidation of plastoquinol at site Qo. In the presence of 20 microM DCCD, this redox-coupled proton pump is inhibited, and the kinetics of phase b and cyt b reduction become close to that predicted on the basis of a pure Q-cycle process. In agreement with this hypothesis, we observe that upon a weak-flash excitation, two charges are translocated through the membrane in addition to the charge translocated at the level of photosystem I. Part of this large electrogenic phase could be associated with the translocation of a proton from the stroma to the lumen. A tentative mechanism is discussed that remains in the frame of the Q-cycle but accounts for an additional proton-pumping process or for the partial decoupling observed in the presence of DCCD, as well.

Dicyclohexylcarbodiimide inhibits proton pumping in ubiquinol:cytochrome c oxidoreductase of Rhodobacter sphaeroides and binds to aspartate-187 of cytochrome b

In recent studies we reported that dicyclohexylcarbodiimide (DCCD) inhibited proton translocation in ubiquinol:cytochrome c oxidoreductase (cytochrome bc1 complex) from yeast mitochondria where it was bound to aspartate-160 of cytochrome b. In the current study, we report that DCCD and its fluorescent analogue, N-cyclohexyl-N'-[4-(dimethylamino)naphthyl]-carbodiimide (NCD-4), inhibit 50-60% proton pumping in the cytochrome bc1 complex of the bacterium Rhodobacter sphaeroides with a 20% inhibition of electron transfer activity. Radioactive DCCD is bound exclusively to cytochrome b at aspartate-187, which is located at the C-terminal region of the CD loop connecting membrane-spanning helices C and D of cytochrome b. Fluorescent studies with NCD-4 revealed that aspartate-187 is located in a mildly hydrophobic pocket in the bc1 complex at a distance of 2-3 A from the surface of the membrane.

Chemical modification of the bovine mitochondrial bc1 complex reveals critical acidic residues involved in the proton pumping activity

Bovine heart ubiquinol-cytochrome c reductase (bc1 complex) was modified with N-(ethoxycarbonyl)-2-ethoxy-1,2-dihydroquinoline (EEDQ), which is a selective reagent for buried carboxyl groups. EEDQ treatment caused a loss of the proton pumping activity of liposome-reconstituted bc1 complex, without effect on the passive proton conductivity of the proteoliposomes. Although the decoupling effect produced on proton translocation was similar to that elicited by N,N'-dicyclohexylcarbodiimide (DCCD) modification of cytochrome b and subunit IX, EEDQ modified different subunits, namely the Core protein II and the iron-sulfur protein (ISP). A time-dependent increase of the labeling of both subunits was observed which was kinetically comparable with the decrease of the H+/e- ratio. Trypsin treatment of the complex showed that the EEDQ-modified carboxyl group in the ISP belongs to the protruding moiety of the protein, holding the Fe/S cluster. The results obtained show that critical acidic residues, located in different subunits of the bc1 complex, at both sides of the membrane, contribute to its proton pumping activity.

Potential induced redox reactions in mitochondrial and bacterial cytochrome b-c1 complexes

Purified cytochrome b-c1 complexes from beef heart mitochondria and Rhodobacter sphaeroides were reconstituted into potassium-loaded asolectin liposomes for studies of the energy-dependent electron transfer reactions within the complexes. Both complexes in a ubiquinone-sufficient state exhibit antimycin-sensitive reduction of cytochromes b (both low and high potential ones) upon induction of a diffusion potential by valinomycin in the presence of ascorbate. Addition of N,N,N',N'-tet-ramethyl-p-phenylenediamine (TMPD) to the ascorbate-reduced potassium-loaded asolectin proteoliposomes resulted in reduction of cytochrome b262. Upon addition of valinomycin, the induced diffusion potential caused a partial reoxidation of cytochrome b562 and partial reduction of cytochrome b566 in beef heart cytochrome b-c1 complex in the presence of antimycin and/or myxothiazol. Surprisingly, when ubiquinone-depleted beef heart cytochrome b-c1 complex liposomes were treated under the same conditions, no cytochrome b566 reduction was observed but only the oxidation of cytochrome b562, and the oxidation was not oxygen-dependent. We explain this effect by b566, iron-sulfur protein short-circuiting under these conditions, assuming that both antimycin and myxothiazol markedly affect subunit b conformation. The electrochemical midpoint potential of heme b566 appears to be significantly higher than that of heme b562 in the presence of myxothiazol, which cannot be accounted for only by the potential-driven electron transfer between these two hemes plus the shift in chemical midpoint potentials caused by myxothiazol. A model for energy coupling consistent with structural findings by Ohnishi et al. (Ohnishi, T., Schagger, H., Meinhardt, S. W., LoBrutto, R., Link, T. A., and von Jagow, G. (1989) J. Biol. Chem. 264, 735-744) is presented. This model is a compromise between pure "redox-loop" and pure "proton-pump" mechanisms. Reoxidation of high potential heme b is observed in an antimycin- or antimycin plus myxothiazol-inhibited, ascorbate plus TMPD-prereduced R. sphaerodies b-c1 complex, upon membrane potential development, suggesting that a similar electron transfer mechanism is also operating in the bacterial complex.

Decoupling of the bc1 complex in S. cerevisiae; point mutations affecting the cytochrome b gene bring new information about the structural aspect of the proton translocation

Four mutations in the mitochondrial cytochrome b of S. cerevisiae have been characterized with respect to growth capacities, catalytic properties, ATP/2e- ratio, and transmembrane potential. The respiratory-deficient mutant G137E and the three pseudo-wild type revertants E137 + I147F, E137 + C133S, and E137 + N256K were described previously (Tron and Lemesle-Meunier, 1990; Di Rago et al., 1990a). The mutant G137E is unable to grow on respiratory substrates but its electron transfer activity is partly conserved and totally inhibited by antimycin A. The secondary mutations restore the respiratory growth at variable degree, with a phosphorylation efficiency of 12-42% as regards the parental wild type strain, and result in a slight increase in the various electron transfer activities at the level of the whole respiratory chain. The catalytic efficiency for ubiquinol was slightly (G137E) or not affected (E137 + I147F, E137 + C133S, and E137 + N256K) in these mutants. Mutation G137E induces a decrease in the ATP/2e- ratio (50% of the W.T. value) and transmembrane potential (60% of the W.T. value) at the bc1 level, whereas the energetic capacity of the cytochrome oxidase is conserved. Secondary mutations I147F, C133S, and N256K partly restore the ATP/2e- ratio and the transmembrane potential at the bc1 complex level. The results suggest that a partial decoupling of the bc1 complex is induced by the cytochrome b point mutation G137E. In the framework of the protonmotive Q cycle, this decoupling can be explained by the existence of a proton wire connecting centers P and N in the wild type bc1 complex which may be amplified or uncovered by the G137E mutation when the bc1 complex is functioning.

Topographical organization of cytochrome b6 in the thylakoid membrane of spinach chloroplasts determined by fluorescence studies with N-cyclohexyl-N'-[4-(dimethylamino)naphthyl]carbodiimide

In a recent study [Wang & Beattie (1992) Biochemistry 31, 8445-8459], we reported that dicyclohexylcarbodiimide (DCCD) was bound to either aspartate-155 or glutamate-166 localized in an amphiphilic, non-membrane-spanning, helix of cytochrome. Moreover, DCCD inhibits proton translocation in a cytochrome bf complex reconstituted into proteoliposomes without significant inhibition of electron transfer, suggesting that the helix containing aspartate-155 and glutamate-166 may play a role in proton movements. In order to explore the environment of this amphiphilic helix, we employed a fluorescent derivative of DCCD, N-cyclohexyl-N'-[4-(dimethylamino)naphthyl]carbodiimide (NCD-4). After incubation of NCD-4 with a cytochrome bf complex isolated from spinach chloroplasts, a fluorescent compound was formed with a 331-nm excitation peak and 440-nm emission peak. NCD-4 was selectively bound to cytochrome b6 and inhibited proton translocation with only a minimal inhibitory effect on electron transfer in the cytochrome bf complex reconstituted into proteoliposomes. Exhaustive digestion of the NCD-4-labeled cytochrome b6 with trypsin resulted in the formation of a single 6-kDa fluorescent peptide with similar properties to the peptide labeled with radioactive DCCD. The fluorescence of NCD-4 bound to the cytochrome bf complex reconstituted into proteoliposomes was quenched by CAT-16, an amphiphilic spin label that intercalates at the membrane surface, as well as by nitroxide derivatives of stearic acid in the order 5-doxylstearic acid > 7-doxylstearic acid > 12-doxylstearic acid. At higher concentrations, the hydrophilic membrane-impermeant quenchers, CAT-1 and D-569, also quenched the fluorescence of NCD-4.(ABSTRACT TRUNCATED AT 250 WORDS)

A proposed pathway of proton translocation through the bc complexes of mitochondria and chloroplasts

The cytochrome bc complexes of the electron transport chain from a wide variety of organisms generate an electrochemical proton gradient which is used for the synthesis of ATP. Proton translocation studies with radiolabeled N,N'-dicyclohexylcarbodiimide (DCCD), the well-established carboxyl-modifying reagent, inhibited proton-translocation 50-70% with minimal effect on electron transfer in the cytochrome bc1 and cytochrome bf complexes reconstituted into liposomes. Subsequent binding studies with cytochrome bc1 and cytochrome bf complexes indicate that DCCD specifically binds to the subunit b and subunit b6, respectively, in a time and concentration dependent manner. Further analyses of the results with cyanogen bromide and protease digestion suggest that the probable site of DCCD binding is aspartate 160 of yeast cytochrome b and aspartate 155 or glutamate 166 of spinach cytochrome b6. Moreover, similar inhibition of proton translocating activity and binding to cytochrome b and cytochrome b6 were noticed with N-cyclo-N-(4-dimethylamino-napthyl)carbodiimide (NCD-4), a fluorescent analogue of DCCD. The spin-label quenching experiments provide further evidence that the binding site for NCD-4 on helix cd of both cytochrome b and cytochrome b6 is localized near the surface of the membrane but shielded from the external medium.

Triple inhibitor titrations support the functionality of the dimeric character of mitochondrial ubiquinol-cytochrome c oxidoreductase

The ubiquinol-2 or duroquinol oxidoreductase activity of mitochondrial ubiquinol-cytochrome c oxidoreductase was titrated with combinations of antimycin, myxothiazol and N,N'-dicyclohexylcarbodiimide (DCCD). A statistical model has been developed that can predict the activity of the complex treated with all possible combinations of these inhibitors. On the basis of the measured titration curves the model had to accommodate interaction between the two promoters of the complex. The titrations confirm that treatment with DCCD results in the modification of a certain site in one of the two promoters of the bc1 dimer, thereby blocking one antimycin A binding site without inhibiting electron transfer. Modification of both antimycin A binding sites of the dimer is apparently required for inhibition of electron transfer through the complex, just as modification of both myxothiazol-binding sites is required for full inhibition. The conclusion can be drawn that mitochondrial ubiquinol-cytochrome c oxidoreductase is a functional dimer, consisting of electrically interacting protomers.

DCCD binds to cytochrome b6 of a cytochrome bf complex isolated from spinach chloroplasts and inhibits proton translocation

Radiolabeled N,N'-dicyclohexylcarbodiimide (DCCD) was bound selectively in a time- and concentration-dependent manner to cytochrome b6 of an enzymatically active cytochrome bf complex isolated from spinach chloroplasts. Maximum labeling of cytochrome b6 was observed with 30 nmol DCCD per nmol cytochrome b6 in the cytochrome bf complex incubated for 30-60 min at 12 degrees C. After incubation of the cytochrome bf complex with DCCD under these conditions, the rate of proton ejection in the complex reconstituted into liposomes was decreased approximately 65-70% when compared to controls; however, under these same conditions the rate of electron transfer through either the soluble bf complex or the complex reconstituted into liposomes was only decreased around 20%. These results suggest that the mechanism of proton translocation through the cytochrome bf complex of spinach chloroplasts is similar to that of the cytochrome bc1 complex from yeast mitochondria in which proton pumping but not electron transfer is also inhibited by DCCD (D. S. Beattie and A. Villalobo, 1982, J. Biol. Chem. 257, 14,745-14,752).

Extracellular generation of active oxygen species catalyzed by exogenous menadione in yeast cell suspension

Luminol chemiluminescence was observed by addition of menadione to yeast cell suspension and was amplified 1000-fold by further addition of Fe-complex. Catalase, superoxide dismutase and ceruloplasmin had inhibitory effects on luminol chemiluminescence, indicating the extracellular generation of active oxygens (H2O2 and O2-) and reduction of Fe-complex. The generation of H2O2 and reduction of Fe-complex were mainly dependent on the activity of NADH: menadione oxidoreductase in the plasma membrane and cytosol fractions. Both luminol chemiluminescence and H2O2 production were sensitive to the inhibitory effects of proton conductor, ionophorous antibiotics and ATPase inhibitor rather than the inhibitors of the mitochondria electron transport system. The incubation of glucose with yeast cells caused a parallel increase in luminol chemiluminescence, H2O2 production and intracellular NADH concentration. These facts suggest that menadione-catalyzed H2O2 production and chemiluminescence are used as the indicators of cell activity to keep the NADH concentration and NADH: menadione oxidoreductase activity which may be sensitive to the change in pH and ion concentrations.

Time and concentration dependence of the dicyclohexylcarbodiimide inhibition of proton movements in the cytochrome bc1 complex from yeast mitochondria reconstituted into proteoliposomes

The cytochrome bc1 complex was isolated from yeast mitochondria solubilized with the detergent dodecyl maltoside and reconstituted into proteoliposomes to measure electrogenic proton pumping. Optimal respiratory control ratios of 4.0, obtained after addition of the uncoupler CCCP, and H+/e- ratios of 1.6 were obtained when the proteoliposomes were prepared with egg yolk phosphatidylcholine supplemented with cardiolipin. Moreover, it was critical to remove excess dodecyl maltoside in the final concentrated preparation prior to reconstitution to prevent loss of enzymatic activity. The rate of electrogenic proton pumping, the respiratory control ratios, and the H+/e- ratios were decreased by incubation of the cytochrome bc1 complex with dicyclohexylcarbodiimide (DCCD) in a time and concentration dependent manner. Maximum inhibitions were observed when 50 nmol DCCD per nmol of cytochrome b were incubated for 30 min at 12 degrees C with the intact cytochrome bc1 complex. Under these same conditions maximum labeling of cytochrome b with [14C] DCCD was reported in a previous study [Beattie et al. (1984). J. Biol. Chem. 259, 10562-10532] consistent with a role for cytochrome b in electrogenic proton movements.

Dicyclohexylcarbodiimide as inducer of mitochondrial Ca2+ release

The effect of the alkylating reagent dicyclohexylcarbodiimide (DCCD) on mitochondrial Ca2+ content was studied. The results obtained indicate that DCCD at a concentration of 100 microM induces mitochondrial Ca2+ efflux. This reaction is accompanied by an increasing energy drain on the system, stimulation of oxygen consumption, and mitochondrial swelling. These DCCD effects can be partially suppressed by supplementing the incubation medium with 1 mM phosphate. By electrophoretic analysis on polyacrylamide-sodium dodecyl sulfate, it was found that DCCD binds to a membrane component with an Mr of 20 to 29 kDa.

Hexokinase-binding properties of the mitochondrial VDAC protein: inhibition by DCCD and location of putative DCCD-binding sites

The outer mitochondrial membrane receptor for hexokinase binding has been identified as the VDAC protein, also known as mitochondrial porin. The ability of the receptor to bind hexokinase is inhibited by pretreatment with dicyclohexylcarbodiimide (DCCD). At low concentrations, DCCD inhibits hexokinase binding by covalently labeling the VDAC protein, with no apparent effect on VDAC channel-forming activity. The stoichiometry of [14C]-DCCD labeling is consistent with one to two high-affinity DCCD-binding sites per VDAC monomer. A comparison between the sequence of yeast VDAC and a conserved sequence found at DCCD-binding sites of several membrane proteins showed two sites where the yeast VDAC amino acid sequence appears to be very similar to the conserved DCCD-binding sequence. Both of these sites are located near the C-terminal end of yeast VDAC (residues 257-265 and 275-283). These results are consistent with a model in which the C-terminal end of VDAC is involved in binding to the N-terminal end of hexokinase.

The effect of calmodulin on the interaction of carbodiimides with the purified human erythrocyte (Ca2+ + Mg2+)-ATPase

The activity of the solubilized and purified (Ca2+ + Mg2+)-ATPase from human erythrocyte membranes was inhibited by N,N'-dicyclohexylcarbodiimide in a concentration-dependent manner. The carbodiimide prevented formation of the phosphorylated intermediate during the catalytic cycle of the enzyme. Treatment of the enzyme with N,N'-dicyclohexyl[14C]carbodiimide resulted in the formation of a 14C-labelled polypeptide corresponding to the enzyme monomer (molecular weight 136,000). The tryptic fragmentation of this 14C-labelled enzyme resulted in the formation of three major 14C-labelled fragments with molecular weights of 58,000, 36,500 and 23,000, the latter two probably representing transmembrane and calmodulin-binding domains of the enzyme, respectively. In the absence of calmodulin, 6.7 molecules of N,N'-dicyclohexyl[14C]carbodiimide covalently bound to each molecule of Ca2+-ATPase; in the presence of calmodulin, the number of molecules of carbodiimide bound was 13.1. The binding of N,N'-dicyclohexylcarbodiimide to the (Ca2+ + Mg2+)-ATPase greatly reduced its ability to bind to a calmodulin-agarose gel.

Restoration of high-potential cytochrome b-564 by integration of baker's yeast complex III into liposomes

Cytochrome b-564 in isolated complex III from baker's yeast mitochondria exhibits the midpoint redox potential proper to the low-potential couple (+60 mV, pH 7.2). Incorporation of the complex into liposomes promotes total conversion to the high-potential couple (+170 mV, pH 7.2). The reconstituted system shows electrogenic proton translocation, which is inhibited by the uncoupler CCCP. Deenergizing treatments result, moreover, in reversal of the redox potential change. These results support our previous proposal that cytochrome b-564 acts as a transducer of redox energy into acid-base energy in the complex III region of the respiratory chain.

Proton pump-linked Mg2+-ATPase activity in isolated rat liver lysosomes

ATPase activity in highly purified rat liver lysosome preparations was evaluated in the presence of other membrane cellular ATPase inhibitors, and compared with lysosome ATP-driven proton translocating activity. Replacement of 5 mM Mg2+ with equimolar Ca2+ brought about a 50% inhibition in divalent cation-dependent ATPase activity, and an 80% inactivation of ATP-linked lysosomal H+ pump activity. In the presence of optimal concentrations of Ca2+ and Mg2+, ATPase activity was similar to that seen in an Mg2+ medium. Mg2+-dependent ATPase activity was greatly inhibited (from 70 to 80%) by the platinum complexes; cis-didimethylsulfoxide dichloroplatinum(II) (CDDP) at approximately 90 microM and cis-diaminedichloroplatinum(II) at twofold higher concentrations. Less inhibition, about 30 and 45%, was obtained with N,N'-dicyclohexylcarbodiimide and N-ethylmaleimide, and the maximal effect occurred in the 50-100 microM and 0.1-1.5 mM ranges, respectively. The concentration dependence of inhibition by the above drugs was determined for both proton pumping and ATPase activities, and half-maximal inhibition concentration of each activity was found at nearly similar values. A micromolar concentration of carbonylcyanide p-trifluoromethoxyphenylhydrazone (FCCP) prevented ATP from setting up a pH gradient across the lysosomal membranes, but stimulated Mg2+-ATPase activity significantly. ATPase activity in Ca2+ medium was also inhibited by CDDP and stimulated by FCCP, but both effects were two- to threefold less than those observed in Mg2+ medium. FCCP failed to stimulate ATPase activity in a CDDP-supplemented medium, thus suggesting that the same ATPase activity fraction was sensitive to both CDDP and FCCP. Mg2+-ATPase activity, like the proton pump, was anion dependent. The lowest activity was recorded in a F-medium, and increased in the order of F- less than SO2-4 less than Cl- approximately equal to Br-. The CDDP-sensitive ATPase activity observed, supported by Mg2+ and less so by Ca2+, may be related to lysosome proton pump activity.

Calcium-dependent inhibition of the erythrocyte Ca2+ translocating ATPase by carbodiimides

The ATP hydrolytic activity of the solubilized and purified Ca2+-translocating ATPase from human erythrocyte plasma membrane was strongly inhibited by the nonpolar compound, N,N'-dicyclohexylcarbodiimide, both in the presence and in the absence of calmodulin. However, the more water-soluble carbodiimides, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and 1-cyclohexyl-3-(2-morpholinoethyl)carbodiimide had little inhibitory effect on the enzyme. The inhibitory effect of N,N'-dicyclohexylcarbodiimide was most pronounced at acid pH, and declined sharply at alkaline pH values. In addition, the optimum pH for the enzyme activity also shifted to more alkaline values in the presence of the carbodiimide. Calcium ion appears to favor the inhibition induced by the carbodiimide, in contrast to the observed protection by Ca2+ in the sarcoplasmic reticulum Ca2+-translocating ATPase. N,N'-Dicyclohexylcarbodiimide also dramatically decreased the stimulatory effect of calmodulin on the activity of the enzyme.

Is there sufficient experimental evidence to consider the mitochondrial cytochrome bc1 complex a proton pump? Probably no

The electron flow through the cytochrome bc1 complex of the mitochondrial respiratory chain is accompanied by vectorial proton translocation, though the mechanism of the latter phenomenon has not yet been clarified. Several proposed hypotheses are briefly presented and discussed here. Recently, a number of papers have appeared claiming the existence of a proton pump in the enzyme mainly on the basis of the interaction of the complex with N,N'-dicyclohexylcarbodiimide. These data are reviewed here with the aim of showing their ability to fit multiple interpretations. This together with some other arguments leads to the conclusion that a proton pump in the mitochondrial bc1 complex has not yet been demonstrated.

Further studies on the binding of DCCD to cytochrome B and subunit VIII of complex III isolated from beef heart mitochondria

Complex III (the cytochrome b-c1 complex) from beef heart mitochondria was incubated with [14C]DCCD for various periods of time. The polypeptide profile of the complex was compared in both stained gels and their autoradiograms when three different methods were used to terminate the reaction. Precipitation with ammonium sulfate resulted in the formation of a new band with an apparent molecular weight of 39,000 in both incubated samples and the zero time controls. Reisolation of the complex by centrifugation through 10% sucrose or by precipitation with trichloroacetic acid did not result in any changes in the appearance of the subunit peptides of the complex. Subunit III (cytochrome b) and subunit VIII were the only bands labeled after termination of the reaction by centrifugation through sucrose, while both ammonium sulfate and trichloroacetic precipitation resulted in nonspecific labeling of several other subunits of the complex and increased labeling of subunit VIII relative to subunit III. Preincubation of the complex with antimycin prior to treatment with [14C]DCCD resulted in a 50% decrease in the binding of DCCD to both cytochrome b and subunit VIII. Furthermore, treatment of the complex III with DCCD resulted in a change in the red shift observed after antimycin or myxothiazol addition to the dithionite-reduced complex resulting in a broad peak with no sharp maximum. These results provide further confirmation that DCCD binds preferentially to cytochrome b and subunit VIII of complex III from beef heart mitochondria and suggest that cytochrome b may play a role in proton translocation.

Bypasses of the antimycin a block of mitochondrial electron transport in relation to ubisemiquinone function

Two different bypasses around the antimycin block of electron transport from succinate to cytochrome c via the ubiquinol-cytochrome c oxidoreductase of intact rat liver mitochondria were analyzed, one promoted by N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) and the other by 2,6-dichlorophenolindophenol (DCIP). Both bypasses are inhibited by myxothiazol, which blocks electron flow from ubiquinol to the Rieske iron-sulfur center, and by 2-hydroxy-3-undecyl-1,4-naphthoquinone, which inhibits electron flow from the iron-sulfur center to cytochrome c1. In the bypass promoted by TMPD its oxidized form (Wurster's blue) acts as an electron acceptor from some reduced component prior to the antimycin block, which by exclusion of other possibilities is ubisemiquinone. In the DCIP bypass its reduced form acts as an electron donor, by reducing ubisemiquinone to ubiquinol; reduced DCIP is regenerated again at the expense of either succinate or ascorbate. The observations described are consistent with and support current models of the Q cycle. Bypasses promoted by artificial electron carriers provide an independent approach to analysis of electron flow through ubiquinol-cytochrome c oxidoreductase.

Redox-linked proton translocation in the b-c1 complex from beef-heart mitochondria reconstituted into phospholipid vesicles. Studies with chemical modifiers of amino acid residues

Possible involvement of polypeptides of b-c1 complex of beef-heart mitochondria in its redox and protonmotive activity has been investigated, by means of chemical modification of amino acid residues in the soluble as well as in the phospholipid-reconstituted b-c1 complex. Treatment of the enzyme with tetranitromethane (C(NO2)4) or with ethoxyformic anhydride (EFA), that modify reversibly tyrosyl and hystidyl residues respectively, resulted in a marked inhibition of electron transport from reduced quinols to cytochrome c. This was accompanied, in b-c1 reconstituted into phospholipid vesicles, by a parallel inhibition of respiratory-linked proton translocation; the H+/e- stoichiometry remained unchanged. Treatment of b-c1 complex with DCCD, that specifically modifies carboxylic groups of glutammic or aspartic residues caused a marked depression of proton translocation in b-c1 vesicles, under conditions where the rate of electron flow in the coupled state, was enhanced. As a consequence the H+/e- stoichiometry was lowered. SDS gel electrophoresis and [14C]DCCD-labelling of the polypeptides of the b-c1 complex showed a major binding of 14C-DCCD to the 8-kDa subunit of the complex and possible cross-linking, induced by DCCD treatment, of polypeptide(s) in the 8-kDa band and the 12-kDa band, with the Fe-s protein of the complex, with the appearance of a new polypeptide band with an apparent molecular mass of about 40 kDa. Involvement of polypeptides of low molecular mass, for which no functional role was so far described, and possibly of the Fe-S protein in the redox-linked proton translocation in b-c1 complex is suggested.