Inhibition of NADH-ubiquinone reductase activity by N,N'-dicyclohexylcarbodiimide and correlation of this inhibition with the occurrence of energy-coupling site 1 in various organisms

The NADH-ubiquinone reductase activity of the respiratory chains of several organisms was inhibited by the carboxyl-modifying reagent N,N'-dicyclohexylcarbodiimide (DCCD). This inhibition correlated with the presence of an energy-transducing site in this segment of the respiratory chain. Where the NADH-quinone reductase segment involved an energy-coupling site (e.g., in bovine heart and rat liver mitochondria, and in Paracoccus denitrificans, Escherichia coli, and Thermus thermophilus HB-8 membranes), DCCD acted as an inhibitor of ubiquinone reduction by NADH. By contrast, where energy-coupling site 1 was absent (e.g., in Saccharomyces cerevisiae mitochondria and Bacillus subtilis membranes), there was no inhibition of NADH-ubiquinone reductase activity by DCCD. In the bovine and P. denitrificans systems, DCCD inhibition was pseudo first order with respect to incubation time, and reaction order with respect to inhibitor concentration was close to unity, indicating that inhibition resulted from the binding of one inhibitor molecule per active unit of NADH-ubiquinone reductase. In the bovine NADH-ubiquinone reductase complex (complex I), [14C]DCCD was preferentially incorporated into two subunits of molecular weight 49,000 and 29,000. The time course of labeling of the 29,000 molecular weight subunit with [14C]DCCD paralleled the time course of inhibition of NADH-ubiquinone reductase activity.

Effects of 1-methyl-4-phenyl-1,2,5,6-tetrahydropyridine and its metabolite 1-methyl-4-phenylpyridine on acetylcholine synthesis in synaptosomes from rat forebrain

1-Methyl-4-phenyl-1,2,5,6-tetrahydropyridine (MPTP) and its metabolite, 1-methyl-4-phenylpyridine (MPP+), have been shown to cause a number of lesions in dopaminergic pathways of the nigro-striatal region of the brain. However, data on the effects of these neurotoxins on other aspects of brain metabolism are scarce. The data presented here show that MPTP and MPP+ inhibit glucose oxidation via the tricarboxylic acid cycle, and acetylcholine synthesis in synaptosomal preparations from rat forebrain. Monoamine oxidase B inhibitors (e.g., pargyline, MDL 72145) relieve the inhibition caused by MPTP but not MPP+. The inhibitory effects of MPP+ on glucose oxidation and acetylcholine synthesis are a consequence of the decreased glucose metabolism in synaptosomes and are consistent with its role as an inhibitor of the Complex I (NADH-CoQ reductase) of the mitochondrial respiratory chain.

Inhibition of mitochondrial NADH-ubiquinone oxidoreductase activity by 1-methyl-4-phenylpyridinium ion

Effect of 1-methyl-4-phenylpyridinium ion (MPP+) on the activity of NADH-ubiquinone oxidoreductase was studied using mitochondria prepared from rat brains. At first, inhibition of oxygen consumption by MPP+ with pyruvate + malate or glutamate + malate as substrates was confirmed polarographically using a Clark-type oxygen electrode. Then, activity of NADH-ubiquinone oxidoreductase in the same samples used in polarography was assayed. Incubation of mitochondria with 0.05 mM of MPP+ together with glutamate, malate and ADP resulted in approximately 50% inhibition of NADH-ubiquinone oxidoreductase activity. Significance of the results was discussed with respect to the mechanism of neuronal degeneration by MPP+.

Mechanism of inhibition of mitochondrial enzymatic complex I-III by adriamycin derivatives

We demonstrate here that complex I-III of bovine heart mitochondrial membrane is inhibited by adriamycin derivatives. This inhibition is a cardiolipin-dependent process. This lipid, specific to the inner mitochondrial membrane, has been shown previously to interact specifically with adriamycin in model membranes (Goormaghtigh, E., Chatelain, P., Caspers, J. and Ruysschaert, J.-M. (1980) Biochim. Biophys. Acta 597, 1-14) and in mitochondrial membranes (Cheneval, D., Müller, M., Toni, R., Ruetz, S. and Carafoli, E. (1985) J. Biol. Chem. 260, 13003-13007). The differential scanning calorimetry data indicate that, in multilamellar liposomes, the formation of antibiotic-cardiolipin complexes induces a clustering of cardiolipin molecules. Conformational analysis of the antibiotic-cardiolipin complexes suggests that plane-plane interactions between the antibiotics aromatic moieties stabilize this complex formation. Possible mechanisms of inactivation of complex I-III by adriamycin are proposed.

Selective inhibition of mitochondrial NADH-ubiquinone reductase (Complex I) by an alkyl polyoxyethylene ether

The detergent mono-n-dodecyl octaoxyethylene ether tightly bound to mitochondrial electron-transport particles and below its critical micellar concentration inhibited the NADH oxidase activity, but not the succinate oxidase activity. The result indicates that the inhibition site is in the Complex I segment. The detergent inhibited rotenone-sensitive NADH-ubiquinone reductase activity, but not NADH-ferricyanide reductase activity, of isolated Complex I. Partial removal of phospholipids from Complex I from 18.8% (w/w) to 14.5% significantly decreased its susceptibility to the inhibitor as well as to rotenone. These results show that the binding site of the detergent responsible for the inhibition lies between the NADH dehydrogenase of flavoprotein and ubiquinone in Complex I and that the binding of the detergent to the site requires phospholipids.

Vitamin K antagonism of coumarin anticoagulation. A dehydrogenase pathway in rat liver is responsible for the antagonistic effect

In the liver, it appears that there are two different pathways for vitamin K reduction. One pathway is irreversibly inhibited by coumarin anticoagulant drugs. The other pathway has been shown in the present study to be composed of enzymes that are not effected by physiological 'in vivo' concentrations of these drugs. This pathway appears to be responsible for the antidotal effect of vitamin K in overcoming coumarin poisoning. In rat liver the pathway has been shown to be composed of DT-diaphorase (EC.1.6.99.2) and a microsomal dehydrogenase(s). The activity of the microsomal dehydrogenase(s) was 3.6-fold higher with NADH than with NADPH present in the test system. It appears that this enzyme is the physiologically important enzyme in the pathway. In contrast with DT-diaphorase, this enzyme(s) is shown to be tightly associated with the mirosomal membrane. The enzyme(s) is not identical with either of the quinone-reducing enzymes cytochrome P-450 reductase or cytochrome-b5 reductase. Our data thus postulate the existence of an as-yet-unidentified microsomal dehydrogenase that appears to have an important function in the pathway.

Lapachol inhibition of DT-diaphorase (NAD(P)H:quinone dehydrogenase)

Lapachol has been found to be a potent inhibitor of the enzyme DT-Diaphorase. Inhibition is competitive versus NADH, Ki = 0.15 microM. Lapachol was not a good substrate for cytochrome P450 reductase, thus inhibition of DT-Diaphorase should not promote its metabolism via radical generating pathways. DT-Diaphorase has been used to test a lapachol affinity chromatography column designed for purification of another coumarin anticoagulant and lapachol sensitive enzyme, vitamin K epoxide reductase.

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.

Uncoupler-inhibitor titrations of ATP-driven reverse electron transfer in bovine submitochondrial particles provide evidence for direct interaction between ATPase and NADH:Q oxidoreductase

From the chemiosmotic hypothesis it follows that no change is expected in potency of an uncoupler to inhibit an energy-driven reaction in an energy-transducing membrane if the energy-requiring part of the reaction, the so-called secondary proton pump, is partially inhibited by a specific, tightly bound inhibitor. An increase in potency upon inhibition of the primary pump may be expected, due to a lower rate of the total proton flow that can be used by the secondary pump and dissipated by the uncoupler. Contrary to this prediction several uncouplers (S13, SF6847, 2,4-dinitrophenol, valinomycin + nigericin) show an increase in uncoupling efficiency in ATP-driven reverse electron transfer (reversal) upon inhibition of the secondary pump in this reaction, the NADH:Q oxidoreductase, by rotenone. The increase in uncoupling efficiency is proportional to the decrease in the rate of reversal, that is to the decrease in concentration of active secondary pump. Similarly, upon inhibition of the primary pump, the ATPase, with oligomycin, an increase in uncoupling efficiency was found, also proportional to the decrease in the rate of reversal. When the pore-forming uncoupler gramicidin was used, no change in uncoupling potency was found upon inhibition of NADH:Q oxidoreductase. Inhibition of the ATPase, however, resulted in a proportionally lower uncoupling titre for gramicidin, just as was found for S13 in the presence of oligomycin. A difference was also found in the relative concentrations of S13 and gramicidin required to stimulate ATP hydrolysis or to inhibit reversal. The amount of S13 needed to stimulate ATP hydrolysis was clearly higher than the amount needed to inhibit reversal. On the contrary, the titre of gramicidin for both actions was about the same. To explain these results we propose that gramicidin uncouples via dissipation of the bulk delta mu H+, whereas the carrier-type uncouplers preferentially interfere with the direct energy transduction between the ATPase and redox enzymes. This is in accordance with the recently developed collision hypothesis.

Adriamycin and DT-diaphorase

It has been suggested that DT-diaphorase (EC 1.6.99.2) can oppose the cytotoxic effect of the antineoplastic drug adriamycin. In this study, adriamycin was tested as a substrate for purified DT-diaphorase from rat liver. Purified DT-diaphorase was unable to use adriamycin as substrate. Also, the drug did not have any significant inhibitory effect on the enzyme.

Inhibition of energy-transducing reactions by 8-nitreno-ATP covalently bound to bovine heart submitochondrial particles: direct interaction between ATPase and redox enzymes

The photoaffinity label 8-azido-ATP has been used to study the effect of inhibition of ATP synthase on ATP-driven reverse electron transfer from succinate to NAD+ ('reversal'), succinate- and NADH-driven ATP synthesis and ATP-Pi exchange. In reversal, where ATPase functions as primary proton pump, inactivation by covalently bound nitreno-ATP results in an inhibition that is proportional to the inactivation of ATP hydrolysis, or, consequently, with the concentration of inactivated ATP synthases. Up to 60% inactivation of the reversal rate does not lead to a decrease in delta mu H+. Inhibition of ATP synthase as secondary proton pump results in case of NADH-driven ATP synthesis in a proportional inhibition, but with succinate as substrate ATP synthesis is less than proportionally inhibited, compared with inactivation of ATP hydrolysis. Inhibition of one of the primary pumps of NADH-driven ATP synthesis, the NADH:Q oxidoreductase, with rotenone also resulted in an inhibition of the rate of ATP synthesis proportional to that of the NADH oxidation. ATP-Pi exchange is much more affected than ATP hydrolysis by photoinactivation with 8-azido-ATP. Contrary to reversal and NADH-driven ATP synthesis the rate of ATP-Pi exchange does not depend linearly, but quadratically on the concentration of active ATP synthases. The observed proportional relationships between inhibition of the primary or secondary pump and the inhibition of the overall energy-transfer reactions do not support the existence of a pool intermediate in energy-transduction reactions. However, the results are consistent with a direct transfer of energy from redox enzymes to ATP synthase and vice versa.

Mitochondrial translation of subunits of the rotenone-sensitive NADH:ubiquinone reductase in Neurospora crassa

The rotenone sensitive NADH:ubiquinone was isolated from mitochondria of Neurospora crassa as a monodisperse preparation with the apparent mol. wt. in Triton solution of 0.9 X 10(6). The enzyme is composed of at least 22 subunits with apparent mol. wts. in SDS between 70 and 11 kd. Six of the subunits with the mol. wts. 70, 48, 37, 25, 22 and 18 kd were radioactively labelled in the enzyme isolated from cells which had incorporated [35S]methionine in the presence of cycloheximide. These subunits are synthesized in the mitochondria. Eleven subunits were radioactively labelled in the enzyme from cells which had incorporated [35S]methionine in the presence of chloramphenicol. These subunits are synthesized in the cytoplasm. The site of translation of the other subunits could not be established by the pulse-labelling technique. The assignment of the mitochondrially synthesized subunits to unidentified reading frames on the mitochondrial DNA is discussed.

Prominent role of DT-diaphorase as a cellular mechanism reducing chromium(VI) and reverting its mutagenicity

Rat liver postmitochondrial (S-12) fractions accounted for the bulk of the activity of whole cell homogenates in reducing chromium(VI) and accordingly in decreasing its mutagenicity. Both cytosolic (S-105) and microsomal fractions concurred to this process, which in all subcellular preparations tested was selectively induced by phenobarbital and especially by Aroclor 1254, but not by 3-methylcholanthrene. Cytosolic fractions were markedly more efficient in reducing chromium(VI) than microsomal fractions recovered from the same amount of tissue (liver or lung), although the latter preparations had a higher specific activity. The microsomal activity was exclusively NADPH dependent. A minor part of the cytosolic reduction was determined by nonenzymatic components, notably by some electron donors and chiefly by reduced glutathione, which proved to reduce chromium(VI) at physiological concentrations. However, also in cytosolic fractions, the most important contribution to chromium reduction was enzyme catalyzed, as shown by the following properties: thermolability; requirement for exogenous NADH or NADPH [supplied as such or in the form of a NADPH-generating system (S-9 mix)]; and saturation by chromium(VI). The likely involvement of DT-diaphorase in this metabolic process is supported by several findings, including its sharp pH dependence and its partial suppression by known inhibitors of this enzyme protein, such as p-chloromercuribenzoate, L-thyroxine, and dicumarol (which conversely did not counteract the metabolic deactivation of the other direct-acting mutagens 2-methoxy-6-chloro-9-[3-(2-chloroethyl)aminopropylamino]acridine 2HCl and epichlorohydrin). Similarly, cytosolic reduction of chromium(VI) was partially inhibited by selective metabolic depletors of both coenzymes of DT-diaphorase, i.e., NADPH and NADH. Pretreatment of rats with enzyme inducers (phenobarbital and 3-methylcholanthrene) stimulated the activity of DT-diaphorase in liver cytosolic fractions. A dramatic stimulation (35 to 40 times over untreated controls) was produced by Aroclor 1254, which also coinduced the liver cytosolic activity of enzymes involved in the glucose 6-phosphate-dependent pathway of both nicotinamide-adenine-dinucleotide phosphate and glutathione reduction (glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, and glutathione reductase). In the lung cytosol, a slight yet significant stimulation of some of these enzyme activities was determined by the daily intratracheal instillations of high doses of chromium(VI) itself for 4 weeks, a condition which has been found to enhance the pulmonary metabolism of this metal ion.

Experimentally induced defects of mitochondrial metabolism in rat skeletal muscle. Biological effects of the NADH: coenzyme Q reductase inhibitor diphenyleneiodonium

An animal model for the human condition of mitochondrial myopathy has been established and characterized physiologically and biochemically. The NADH: coenzyme Q reductase inhibitor diphenyleneiodonium [Bloxham (1979) Biochem. Soc. Trans. 7, 103-106] was either infused acutely in vivo into rat hind limb or injected chronically into rats. Both modes of delivery resulted in a reduced muscle oxidative capacity and increased fatigue. Analysis of muscle metabolites by h.p.l.c. and 31P-n.m.r. indicated that ATP concentrations were similar to control values during periods of stimulation and these were maintained by the phosphocreatine pool. During the recovery period after muscle stimulation in the experimental animals the muscle pH remained depressed and the rate of phosphocreatine synthesis was markedly delayed as compared with controls. Factors thought to be involved in the fatigue response are discussed in relation to this model.

The interaction of yeast Complex III with some respiratory inhibitors

We have examined the effects of eight inhibitors of the bovine-heart mitochondrial Complex III on the catalytic activity of the analogous complex from yeast mitochondria. All eight compounds were inhibitory, with potent inhibition being obtained with antimycin, myxothiazol and UHDBT (5-N-undecyl-6-hydroxy-4,7-dioxobenzothiazole). These three inhibitors, and also funiculosin, have been further studied by characterizing their effects on the visible absorbance, magnetic circular dichroism and EPR spectra of the complex and also on the potentiometric properties of the individual metal centers present in the complex. All four inhibitors had little or no effect on either the absorbance or magnetic circular dichroism spectra. Funiculosin produced a change in the EPR lineshape of the iron-sulfur cluster; EPR spectra recorded at 12 K also revealed complete reduction of cytochrome b-562 by ascorbate. UHDBT also changed the lineshape of the iron-sulfur cluster and this change could be partially reversed by myxothiazol. Neither antimycin nor myxothiazol affected the iron-sulfur cluster and produced only small changes in the EPR absorption envelope of the b cytochromes. Both funiculosin and UHDBT raised the midpoint potential of the iron-sulfur cluster, by about 150 and 70 mV, respectively. Only UHDBT changed the potential of c1, lowering it by about 30 mV. Funiculosin raised the potential of b-562 by about 30 mV, while myxothiazol had no effect; the other two compounds produced only small changes. All four compounds had only small effects on the midpoint potential of b-566. The relative contributions of the two b cytochromes to the magnetic circular dichroism amplitudes could be changed by the addition of inhibitors, even though the absolute magnetic circular dichroism spectra of oxidized and reduced complex were unaffected.

Generation of low-level chemiluminescence during the metabolism of 1-naphthol by rat liver microsomes

The metabolism of 1-naphthol in rat liver microsomal fractions supplemented with NADPH is accompanied by low-level chemiluminescence which reflects the formation of molecular excited states. Photoemission consists of two phases which both are dependent on microsomal protein and 1-naphthol concentration. The involvement of cytochrome P-450 in the microsomal metabolism of 1-naphthol was indicated by an inhibition of chemiluminescence by aminopyrine or metyrapone. Oxygen is required for light emission. Whereas phase I is hardly influenced by superoxide dismutase, phase II is suppressed. Chemiluminescence was not associated with malondialdehyde accumulation, in contrast to NADPH-dependent lipid peroxidation in microsomal fractions in the absence of 1-naphthol. Phase I of chemiluminescence appears to directly reflect cytochrome P-450-dependent hydroxylation, and phase II is attributed to redox cycling of products arising from these reactions, e.g. the 1,4- and/or 1,2-naphthoquinones as oxidation products of the corresponding dihydroxynaphthalenes.

Lapachol inhibition of vitamin K epoxide reductase and vitamin K quinone reductase

Lapachol [2-hydroxy-3-(3-methyl-2-butenyl)-1,4-naphthoquinone] has been shown to be a potent inhibitor of both vitamin K epoxide reductase and the dithiothreitol-dependent vitamin K quinone reductase of rat liver microsomes in vitro. These observations explain the anticoagulant activity of lapachol previously observed in both rats and humans. Lapachol inhibition of the vitamin K epoxide and quinone reductases resembled coumarin anticoagulant inhibition, and was observed in normal strain but not in warfarin-resistant strain rat liver microsomes. This similarity of action suggests that the lactone functionality of the coumarins is not critical for their activity. The initial-velocity steady-state inhibition patterns for lapachol inhibition of the solubilized vitamin K epoxide reductase were consistent with tight binding of lapachol to the oxidized form of the enzyme, and somewhat lower affinity for the reduced form. It is proposed that lapachol assumes a 4-enol tautomeric structure similar to that of the 4-hydroxy coumarins. These structures are analogs of the postulated hydroxyvitamin K enolate intermediate bound to the oxidized form of the enzyme in the chemical reaction mechanism of vitamin K epoxide reductase, thus explaining their high affinity.

Inhibition of mitochondrial NADH:ubiquinone oxidoreductase by ethoxyformic anhydride

The NADH:ubiquinone, but not the NADH:ferricyanide, reductase activity of mitochondrial complex I (NADH:ubiquinone oxidoreductase) is inhibited by incubation of the enzyme at pH 6.0 and 0 degree C with ethoxyformic anhydride (EFA), and the inhibition is partially reversed by subsequent incubation of EFA-treated complex I with hydroxylamine. These results and spectral changes of EFA-treated complex I in the u.v. region are consistent with modification of essential histidyl or tyrosyl residues between the primary NADH dehydrogenase and the site of ubiquinone reduction. Treatment of complex I with EFA in the presence of high concentrations of Seconal or Demerol did not protect against EFA inactivation, suggesting that the site of EFA modification may not be the same as the inhibiton sites of Seconal and Demerol. However, the presence of NADH during incubation of complex I with EFA greatly enhanced the inhibition rate, indicating that the reduced conformation of complex I is more susceptible to attack by EFA.

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).

A new electrogenic step in the ubiquinol:cytochrome c2 oxidoreductase complex of Rhodopseudomonas sphaeroides

Myxothiazol, an inhibitor of the ubiquinol oxidase site of the ubiquinol:cytochrome c2 oxidoreductase complex, has been shown in the present work to inhibit a part of the electrogenic process indicated by phase III of the carotenoid change, in addition to the part of the change inhibited by antimycin. This finding shows that there is an antimycin-insensitive, but myxothiazol-sensitive portion of the slow phase, which indicates the existence of an electrogenic event within the ubiquinol:cytochrome c2 oxidoreductase complex, in addition to that linked to oxidation of cytochrome b-561 which has been previously characterized. Redox titrations show that the appearance of the new electrogenic step is correlated with the amount of cytochrome b-561 available in the oxidized form before the flash. The rate of the antimycin-insensitive and myxothiazol-sensitive portion of the carotenoid change correlates well with the rate of reduction of cytochrome b-561. No carotenoid change associated with reduction of cytochrome b-566 was seen. These findings suggest that the newly identified electrogenic process is linked to electron transfer between cytochrome b-566 and b-561. Calculations of the contribution of this new electrogenic step to the total electrogenic event within the complex show that electrons passing from cytochrome b-566 to cytochrome b-561 pass about 35-50% of the distance across the whole membrane.

Potent and selective hydroxynaphthoquinone inhibitors of mitochondrial electron transport in Eimeria tenella (Apicomplexa: Coccidia)

Novel hydroxynaphthoquinones have been shown to be potent and selective inhibitors of mitochondrial electron transport in the protozoan Eimeria tenella, inhibiting at concentrations of 10(-10) to 10(-11)M. The primary site of electron transport inhibition has been localized to the ubiquinol-cytochrome c reductase span of the respiratory chain, whereas a secondary site of inhibition occurs in the NADH- and succinate-ubiquinone reductase complexes. Inhibition at the primary site is selective for the E. tenella enzyme; inhibition at the secondary sites is comparable in both E. tenella and chick (Gallus gallus) liver mitochondria. Hydroxynaphthoquinone inhibition of chick liver succinate-cyto-chrome c reductase was fully reversible by addition of the exogenous ubiquinone-2 analogue, 6-decyl-2,3-dimethoxy-5-methyl-1,4-benzoquinone; inhibition of the corresponding E. tenella enzyme was not reversed by this ubiquinone. E. tenella lines made resistant to the anticoccidial agents decoquinate or clopidol showed no cross-resistance to the hydroxynaphthoquinones, either at the level of electron transport or in vivo.

Characterization of the cytochrome d terminal oxidase complex of Escherichia coli using polyclonal and monoclonal antibodies

The cytochrome d terminal oxidase complex is one of two terminal oxidases in the aerobic respiratory chain of Escherichia coli. Previous work has shown by dodecyl sulfate-polyacrylamide gel electrophoresis that this enzyme contains two subunits (I and II) and three cytochrome components, b558 , a1, and d. Reconstitution studies have demonstrated that the enzyme functions as a ubiquinol-8 oxidase and catalyzes an electrogenic reaction, i.e. turnover is accompanied by a charge separation across the membrane bilayer. In this paper, monoclonal and polyclonal antibodies were used to obtain structural information about the cytochrome d complex. It is shown that antibodies directed against subunit I effectively inhibit ubiquinol-1 oxidation by the purified enzyme in detergent, whereas antibodies which bind to subunit II have no effect on quinol oxidation. The oxidation rate of N,N,N',N'-tetramethyl-p-phenylenediamine, in contrast, is unaffected by antisubunit I antibodies, but is inhibited by antibodies against subunit II. It is concluded that the quinol oxidation site is on subunit I, previously shown to be the cytochrome b558 component of the complex, and that N,N,N',N'-tetramethyl-p-phenylenediamine oxidation occurs at a secondary site on subunit II. The antibodies were also used to analyze the results of a protein cross-linking experiment. Dimethyl suberimidate was used to cross-link the subunits of purified, solubilized oxidase. Immunoblot analysis of the products of this cross-linking clearly indicate that subunit II probably exists as a dimer within the complex. Finally, it is shown that the purified enzyme contains tightly bound lipopolysaccharide. This was revealed after discovering that one of the monoclonal antibodies raised against the purified complex is actually directed against lipopolysaccharide. The significance of this finding is not known.