Studies on the Respiratory Chain-linked Reduced Nicotinamide Adenine Dinucleotide Dehydrogenase

Proteomics-based target identification of natural products affecting cancer metabolism

The Warburg effect, a widely known characteristic of cancer cells, refers to the utilization of glycolysis under aerobic conditions for extended periods of time. Recent studies have revealed that cancer cells are capable of reprogramming their metabolic pathways to meet vigorous metabolic demands. New anticancer drugs that target the complicated metabolic systems of cancer cells are being developed. Identifying the potential targets of novel compounds that affect cancer metabolism may enable the discovery of new therapeutic targets for cancer treatment, and hasten the development of anticancer drugs. Historically, various drug screening techniques such as the analysis of a compound's antiproliferative effect on cancer cells and proteomic methods, that enable target identification have been used to obtain many useful drugs from natural products. Here, we review proteomics-based target identification methods applicable to natural products that affect cancer metabolism.

Quantification of NADH:ubiquinone oxidoreductase (complex I) content in biological samples

Impairments in mitochondrial energy metabolism have been implicated in human genetic diseases associated with mitochondrial and nuclear DNA mutations, neurodegenerative and cardiovascular disorders, diabetes, and aging. Alteration in mitochondrial complex I structure and activity has been shown to play a key role in Parkinson's disease and ischemia/reperfusion tissue injury, but significant difficulty remains in assessing the content of this enzyme complex in a given sample. The present study introduces a new method utilizing native polyacrylamide gel electrophoresis in combination with flavin fluorescence scanning to measure the absolute content of complex I, as well as α-ketoglutarate dehydrogenase complex, in any preparation. We show that complex I content is 19 ± 1 pmol/mg of protein in the brain mitochondria, whereas varies up to 10-fold in different mouse tissues. Together with the measurements of NADH-dependent specific activity, our method also allows accurate determination of complex I catalytic turnover, which was calculated as 104 min-1 for NADH:ubiquinone reductase in mouse brain mitochondrial preparations. α-ketoglutarate dehydrogenase complex content was determined to be 65 ± 5 and 123 ± 9 pmol/mg protein for mouse brain and bovine heart mitochondria, respectively. Our approach can also be extended to cultured cells, and we demonstrated that about 90 × 103 complex I molecules are present in a single human embryonic kidney 293 cell. The ability to determine complex I content should provide a valuable tool to investigate the enzyme status in samples after in vivo treatment in mutant organisms, cells in culture, or human biopsies.

Evaluation of Parent- and Metabolite-Induced Mitochondrial Toxicities Using CYP-Introduced HepG2 cells

Mitochondrial toxicity is an important factor to predict drug-induced liver injury (DILI). Previous studies have focused predominantly on mitochondrial toxicities due to parent forms, and no study has adequately evaluated metabolite-induced mitochondrial toxicity. Moreover, previous studies have used HepG2 cells, which lack many cytochrome P450 (CYP) genes. To overcome this problem, CYP-introduced HepG2 cells were constructed using several gene transfer technologies, including adenoviruses and plasmids. However, these methods only led to a transient expression of CYP genes. In the present study, usefulness of four CYPs introduced-HepG2 (TC-Hep) cells previously constructed through mammalian artificial chromosome technology were examined, especially from the perspective of mitochondrial toxicity. First, we evaluated the effects of known compounds, such as rotenone and flutamide, on mitochondrial toxicity and cell death in TC-Hep cells cultured in galactose conditions. Expectedly, rotenone-induced cell death ameliorated because rotenone was metabolized by CYPs into inactive form(s) and flutamide-induced cell death increased in TC-Hep cells. Second, we evaluated five compounds that caused liver injury in clinical phase and were discontinued during pharmaceutical development. The present in vitro tool suggested that three of the five compounds caused metabolite-induced mitochondrial toxicities. In conclusion, the present in vitro tool could easily and inexpensively detect metabolite-induced mitochondrial toxicity; hence, it can be useful for predicting DILI in preclinical phase.

Structure of inhibitor-bound mammalian complex I

Respiratory complex I (NADH:ubiquinone oxidoreductase) captures the free energy from oxidising NADH and reducing ubiquinone to drive protons across the mitochondrial inner membrane and power oxidative phosphorylation. Recent cryo-EM analyses have produced near-complete models of the mammalian complex, but leave the molecular principles of its long-range energy coupling mechanism open to debate. Here, we describe the 3.0-Å resolution cryo-EM structure of complex I from mouse heart mitochondria with a substrate-like inhibitor, piericidin A, bound in the ubiquinone-binding active site. We combine our structural analyses with both functional and computational studies to demonstrate competitive inhibitor binding poses and provide evidence that two inhibitor molecules bind end-to-end in the long substrate binding channel. Our findings reveal information about the mechanisms of inhibition and substrate reduction that are central for understanding the principles of energy transduction in mammalian complex I.

The respiratory complex I (NADH:ubiquinone oxidoreductase) is a large redox-driven proton pump that initiates respiration in mitochondria. Here, the authors present the 3.0 Å cryo-EM structure of complex I from mouse heart mitochondria with the ubiquinone-analogue inhibitor piericidin A bound in the active site and with kinetic measurements and MD simulations they further show that this inhibitor acts competitively against the native ubiquinone-10 substrate.

The unique chemistry and biology of the piericidins

The piericidin family of microbial metabolites features a 4-pyridinol core linked with a methylated polyketide side chain. Piericidins are exclusively produced by actinomycetes, especially members of the genus Streptomyces. The close structural similarity with coenzyme Q renders the piericidins important NADH-ubiquinone oxidoreductase (complex I) inhibitors in the mitochondrial electron transport chain. Because of the significant activities of the piericidins, which include insecticidal, antimicrobial and antitumor effects, total syntheses of the piericidins were developed using various synthetic strategies. The biosynthetic origin of this class has also been the subject of investigation. This review covers the isolation and structure determination of the natural piericidins, their chemical modification, the total syntheses of natural and unnatural analogs, their biosynthesis, and reported biological activities together with structure-activity relationships. Given the fundamental biology of this class of metabolites, the piericidin family will likely continue to attract attention as biological probes of important biosynthetic processes.

Mitochondria as oxidative signaling organelles in T-cell activation: physiological role and pathological implications

Early scientific reports limited the cell biological role of reactive oxygen species (ROS) to the cause of pathological damage. However, extensive research performed over the last decade led to a wide recognition of intracellular oxidative/redox signaling as a crucial mechanism of homeostatic regulation. Amongst different cellular processes known to be influenced by redox signaling, T-cell activation is one of the most established. Numerous studies reported an indispensible role for ROS as modulators of T-cell receptor-induced transcription. Nevertheless, mechanistic details regarding signaling pathways triggered by ROS are far from being delineated. The nature and interplay between enzymatic sources involved in the generation of "oxidative signals" are also a matter of ongoing research. In particular, active participation of the mitochondrial respiratory chain as ROS producer constitutes an intriguing issue with various implications for bioenergetics of activated T cells as well as for T-cell-mediated pathologies. The aim of the current review is to address these interesting concepts.

Redox-dependent change of nucleotide affinity to the active site of the mammalian complex I

A very potent and specific inhibitor of mitochondrial NADH:ubiquinone oxidoreductase (complex I), a derivative of NADH (NADH-OH) has recently been discovered (Kotlyar, A. B., Karliner, J. S., and Cecchini, G. (2005) FEBS Lett. 579, 4861-4866). Here we present a quantitative analysis of the interaction of NADH-OH and other nucleotides with oxidized and reduced complex I in tightly coupled submitochondrial particles. Both the rate of the NADH-OH binding and its affinity to complex I are strongly decreased in the presence of succinate. The effect of succinate is completely reversed by rotenone, antimycin A, and uncoupler. The relative affinity of ADP-ribose, a competitive inhibitor of NADH oxidation, is also shown to be significantly affected by enzyme reduction (KD of 30 and 500 microM for oxidized and the succinate-reduced enzyme, respectively). Binding of NADH-OH is shown to abolish the succinate-supported superoxide generation by complex I. Gradual inhibition of the rotenone-sensitive uncoupled NADH oxidase and the reverse electron transfer activities by NADH-OH yield the same final titration point (approximately 0.1 nmol/mg of protein). The titration of NADH oxidase appears as a straight line, whereas the titration of the reverse reaction appears as a convex curve. Possible models to explain the different titration patterns for the forward and reverse reactions are briefly discussed.

Target identification of drug induced mitochondrial toxicity using immunocapture based OXPHOS activity assays

Mitochondrial dysfunction has been shown to be a pharmacotoxicological response to a variety of currently-marketed drugs. In order to reduce attrition due to mitochondrial toxicity, high throughput-applicable screens are needed for early stage drug discovery. We describe, here, a set of immunocapture based assays to identify compounds that directly inhibit four of the oxidative phosphorylation (OXPHOS) complexes: I, II, IV, and V. Intra- and inter-assay variation were determined and specificity tested by using classical mitochondrial inhibitors. Twenty drugs, some with known mitochondrial toxicity and others with no known mitochondrial liability, were studied. Direct inhibition of one or more of the OXPHOS complexes was identified for many of the drugs. Novel information was obtained for several drugs including ones with previously unknown effects on oxidative phosphorylation. A major advantage of the immunocapture approach is that it can be used throughout drug screening from early compound evaluation to clinical trials.

Novel role for mitochondria: protein kinase Ctheta-dependent oxidative signaling organelles in activation-induced T-cell death

Reactive oxygen species (ROS) play a key role in regulation of activation-induced T-cell death (AICD) by induction of CD95L expression. However, the molecular source and the signaling steps necessary for ROS production are largely unknown. Here, we show that the proximal T-cell receptor-signaling machinery, including ZAP70 (zeta chain-associated protein kinase 70), LAT (linker of activated T cells), SLP76 (SH2 domain-containing leukocyte protein of 76 kDa), PLCgamma1 (phospholipase Cgamma1), and PKCtheta (protein kinase Ctheta), are crucial for ROS production. PKCtheta is translocated to the mitochondria. By using cells depleted of mitochondrial DNA, we identified the mitochondria as the source of activation-induced ROS. Inhibition of mitochondrial electron transport complex I assembly by small interfering RNA (siRNA)-mediated knockdown of the chaperone NDUFAF1 resulted in a block of ROS production. Complex I-derived ROS are converted into a hydrogen peroxide signal by the mitochondrial superoxide dismutase. This signal is essential for CD95L expression, as inhibition of complex I assembly by NDUFAF1-specific siRNA prevents AICD. Similar results were obtained when metformin, an antidiabetic drug and mild complex I inhibitor, was used. Thus, we demonstrate for the first time that PKCtheta-dependent ROS generation by mitochondrial complex I is essential for AICD.

A novel strong competitive inhibitor of complex I

Alkaline incubation of NADH results in the formation of a very potent inhibitor of complex I (NADH:ubiquinone oxidoreductase). Mass spectroscopy (molecular mass equal to 696) and absorption spectroscopy suggest that the inhibitor is derived from attachment of two oxygen atoms to the nicotinamide moiety of NADH. The inhibitor is competitive with respect to NADH with a K(i) of about 10(-8)M. The inhibitor efficiently suppresses NADH-oxidase, NADH-artificial acceptor reductase, and NADH-quinone reductase reactions catalyzed by submitochondrial particles, as well as the reactions catalyzed by either isolated complex I or the three subunit flavoprotein fragment of complex I.

The mitochondrial and prokaryotic proton-translocating NADH:ubiquinone oxidoreductases: similarities and dissimilarities of the quinone-junction sites

The catalytic properties of the rotenone-sensitive NADH:ubiquinone reductase (Complex I) in bovine heart submitochondrial particles and in inside-out vesicles derived from Paracoccus denitrificans and Rhodobacter capsulatus were compared. The prokaryotic enzymes catalyze the NADH oxidase and NADH:quinone reductase reactions with similar kinetic parameters as those for the mammalian Complex I, except for lower apparent affinities for the substrates--nucleotides. Unidirectional competitive inhibition of NADH oxidation by ADP-ribose, previously discovered for submitochondrial particles, was also evident for tightly coupled P. denitrificans vesicles, thus suggesting that a second, NAD(+)-specific site is present in the simpler prokaryotic enzyme. The inhibitor sensitivity of the forward and reverse electron transfer reactions was compared. In P. denitrificans and Bos taurus vesicles different sensitivities to rotenone and Triton X-100 for the forward and reverse electron transfer reactions were found. In bovine heart preparations, both reactions showed the same sensitivity to piericidin, and the inhibition was titrated as a straight line. In P. denitrificans, the forward and reverse reactions show different sensitivity to piericidin and the titrations of both activities were curvilinear with apparent I(50) (expressed as mole of inhibitor per mole of enzyme) independent of the enzyme concentration. This behavior is explained by a model involving two different sites rapidly interacting with piericidin within the hydrophobic phase.

Unidirectional effect of lauryl sulfate on the reversible NADH:ubiquinone oxidoreductase (Complex I)

Lauryl sulfate inhibits the Deltamu;(H)(+)-dependent reverse electron transfer reactions catalyzed by NADH:ubiquinone oxidoreductase (Complex I) in coupled bovine heart submitochondrial particles and in vesicles derived from Paracoccus denitrificans. The inhibitor affects neither NADH oxidase (coupled or uncoupled) nor NADH:ferricyanide reductase and succinate oxidase activities at the concentrations that selectively prevent the succinate-supported, rotenone-sensitive NAD(+) or ferricyanide reduction. Possible uncoupling effects of the inhibitor are ruled out: in contrast to oligomycin and gramicidin, which increases and decreases the rate of the reverse electron transfer, respectively, in parallel with their coupling and uncoupling effects, lauryl sulfate does not affect the respiratory control ratio. A mechanistic model for the unidirectional effect of lauryl sulfate on the Complex I catalyzed oxidoreduction is proposed.

The insecticide target in the PSST subunit of complex I

Current insecticides have been selected by sifting and winnowing hundreds of thousands of synthetic chemicals and natural products to obtain commercial preparations of optimal effectiveness and safety. This process has often ended up with compounds of high potency as inhibitors of the electron transport chain and more specifically of complex I (NADH:ubiquinone oxidoreductase). Many classes of chemicals are involved and the enzyme is one of the most complicated known, with 43 subunits catalyzing electron transfer from NADH to ubiquinone through flavin mononucleotide and up to eight iron-sulfur clusters. We used a potent photoaffinity ligand, (trifluoromethyl)diazirinyl[3H]pyridaben, to localize the insecticide target to a single high-affinity site in the PSST subunit that couples electron transfer from iron-sulfur cluster N2 to ubiquinone. Most importantly, all of the potent complex I-inhibiting pesticides, despite their great structural diversity, compete for this same specific binding domain in PSST. Finding their common mode of action and target provides insight into shared toxicological features and potential selection for resistant pests.

Catalytic activity of NADH-ubiquinone oxidoreductase (complex I) in intact mitochondria. evidence for the slow active/inactive transition

The mammalian purified dispersed NADH-ubiquinone oxidoreductase (Complex I) and the enzyme in inside-out submitochondrial particles are known to be the slowly equilibrating mixture of the active and de-activated forms (Vinogradov, A. D. (1998) Biochim. Biophys. Acta 1364, 169-185). We report here the phenomenon of slow active/de-active transition in intact mitochondria where the enzyme is located within its natural environment being exposed to numerous mitochondrial matrix proteins. A simple procedure for permeabilization of intact mitochondria by channel-forming antibiotic alamethicin was worked out for the "in situ" assay of Complex I activity. Alamethicin-treated mitochondria catalyzed the rotenone-sensitive NADH-quinone reductase reaction with exogenousely added NADH and quinone-acceptor at the rates expected if the enzyme active sites would be freely accessible for the substrates. The matrix proteins were retained in alamethicin-treated mitochondria as judged by their high rotenone-sensitive malate-cytochrome c reductase activity in the presence of added NAD(+). The sensitivity of Complex I to N-ethylmaleimide and to the presence of Mg(2+) was used as the diagnostic tools to detect the presence of the de-activated enzyme. The NADH-quinone reductase activity of alamethicin-treated mitochondria was sensitive to neither N-ethylmaleimide nor Mg(2+). After exposure to elevated temperature (37 degrees C, the conditions known to induce de-activation of Complex I) the enzyme activity became sensitive to the sulfhydryl reagent and/or Mg(2+). The sensitivity to both inhibitors disappeared after brief exposure of the thermally de-activated mitochondria with malate/glutamate, NAD(+), and cytochrome c (the conditions known for the turnover-induced reactivation of the enzyme). We conclude that the slow active/de-active Complex I transition is a characteristic feature of the enzyme in intact mitochondria and discuss its possible physiological significance.

Essential structural factors of annonaceous acetogenins as potent inhibitors of mitochondrial complex I

The annonaceous acetogenins are the most potent of the known inhibitors of bovine heart mitochondrial complex I. These inhibitors act, at the terminal electron transfer step of the enzyme, in a similar way to the usual complex I inhibitors, such as piericidin A and rotenone; however, structural similarities are not apparent between the acetogenins and these known complex I inhibitors. A systematic set of isolated natural acetogenins was prepared and examined for their inhibitory actions with bovine heart mitochondrial complex I to identify the essential structural factors of these inhibitors for the exhibition of potent activity. Despite their very potent activity, the structural requirements of the acetogenins are not particularly rigid and remain somewhat ambiguous. The most common structural units, such as adjacent bis-tetrahydrofuran (THF) rings and hydroxyl groups in the 4- and/or 10-positions, were not essential for exhibiting potent activity. The stereochemistry surrounding the THF rings, surprisingly, seemed to be unimportant, which was corroborated by an exhaustive conformational space search analysis, indicating that the model compounds, with different stereochemical arrangements around the THF moieties, were in fairly good superimposition. Proper length and flexibility of the alkyl spacer moiety, which links the THF and the alpha, beta-unsaturated gamma-lactone ring moieties, were essential for the potent activity. This probably results from some sort of specific conformation of the spacer moiety which regulates the two ring moieties to locate into an optimal spatial position on the enzyme. It is, therefore, suggested that the structural specificity of the acetogenins, required for optimum inhibition, differs significantly from that of the common complex I inhibitors in which essential structural units are compactly arranged and conveniently defined. The structure-activity profile for complex I inhibition is discussed in comparison with those for other biological activities.

Inhibitors of NADH-ubiquinone reductase: an overview

This article provides an updated overview of the plethora of complex I inhibitors. The inhibitors are presented within the broad categories of natural and commercial compounds and their potency is related to that of rotenone, the classical inhibitor of complex I. Among commercial products, particular attention is dedicated to inhibitors of pharmacological or toxicological relevance. The compounds that inhibit the NADH-ubiquinone reductase activity of complex I are classified according to three fundamental types of action on the basis of available evidence and recent insights: type A are antagonists of the ubiquinone substrate, type B displace the ubisemiquinone intermediate, and type C are antagonists of the ubiquinol product.

Structure-activity relationships of some complex I inhibitors

A wide variety of complex I inhibitors act at or close to the ubiquinone reduction site. Identification of the structural factors required for exhibiting inhibitory actions on the basis of structure-activity relationships is useful to elucidate the manner in which inhibitors interact with the enzyme. This review summarizes studies on the structure-activity relationship of rotenoids, piericidins, capsaicins, pyridinium-type inhibitors and modern synthetic agrochemicals acting at mitochondrial complex I.

Insecticidal quinazoline derivatives with (trifluoromethyl)diazirinyl and azido substituents as NADH:ubiquinone oxidoreductase inhibitors and candidate photoaffinity probes

Two candidate photoaffinity probes are designed from 4-substituted quinazolines known to be potent insecticides/acaricides and NADH:ubiquinone oxidoreductase inhibitors acting at or near the rotenone site. 4-(11-Azidoundecyl-2-amino)quinazoline, based on the undecylamino analog SAN 548A as a prototype, was synthesized in 18% overall yield from ethyl 10-undecenoate by oxidation of the terminal double bond, reductive amination, coupling to 4-chloroquinazoline, and functional group manipulation of the terminal ethyl ester to an alcohol, a mesylate and finally nucleophilic displacement with azide ions. 4-(4-(3-(Trifluoromethyl)-3H-diazirin-3-yl)phenethoxy)quinaz oline [the (trifluoromethyl)diazirinyl analog of fenazaquin insecticide/acaricide] was prepared from 4-bromophenethyl alcohol in 31% overall yield by first introducing the trifluoromethylketone moiety followed by its conversion to the (trifluoromethyl)-diazirine and finally coupling to 4-chloroquinazoline as above. Both candidate photoaffinity probes have the inhibitory potency of rotenone (IC50 of 3-4 nM in each case). The azidoundecylamino compound has inadequate photoreactivity whereas that of the (trifluoromethyl)diazirinyl analog is ideal at 350 nm. Radiosynthesis of the latter photoaffinity ligand included introduction of the diazirinyl moiety as the carbene precursor, oxidation of (trifluoromethyl)diazirinylphenethyl alcohol to the corresponding acid with Jones' reagent, and reduction of the phenacetyl chloride intermediate with sodium borotritide to incorporate tritium.

Deficiencies of NADH and succinate dehydrogenases in degenerative diseases and myopathies

This paper examines the experimental foundations of reports in the literature on mitochondrial diseases involving Complexes I and II of the respiratory chain. Many of the reports may be questioned on the basis of the assay conditions used which disregard established knowledge of the precautions required for valid activity measurements. In addition, some findings are open to question because of the experimental material chosen for the study, such as the measurement of NADH oxidase activity in platelets in Parkinson's disease, which affects selectively the dopamine neurons, or the use of autopsy material stored for prolonged periods during which post-mortem changes may have occurred. Deficiencies claimed to involve several components of the respiratory chain may reflect indirect effects, such as defects in the synthesis of iron-sulfur clusters or in the availability of iron, rather than mutations in the genes coding for the deficient enzymes. Nevertheless, there are a few instances reported of Complex II deficiency free from such criticisms. As to Complex I, idiopathic Parkinsonism appears to involve a documentable decline in the activity of this enzyme. Using the model system provided by N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), which produces biochemical, pharmacological, and clinical syndromes closely resembling Parkinsonism, the etiology of the disease is examined.

Comparison of the inhibitory action of natural rotenone and its stereoisomers with various NADH-ubiquinone reductases

Two stereoisomers of natural rotenone (5'alpha-epirotenone and 5'beta-epirotenone) were synthesized to identify the stereochemical factor of rotenone required for the inhibition and also to probe the structure of the rotenone binding site. The inhibitory action of the stereoisomers was compared with that of rotenone using NADH-ubiquinone reductases from bovine heart submitochondrial particles (SMP), potato tubers (Solanum tuberosum L.) SMP and Escherichia coli (GR19N) membranes. With respect to bovine heart SMP, it was found that the bent form of rotenone is essential for the activity. The modification of the E-ring moiety also affected both the inhibitory potency and the pattern of inhibition. These results indicated that the rotenone-binding site recognizes the whole molecular structure (or shape) of rotenone in a strict sense. Rotenone and 5'beta-epirotenone inhibited the NADH-ubiquinone reductase of bovine heart SMP in a noncompetitive manner against exogenous quinones. In contrast, the inhibition pattern of 5'alpha-epirotenone varied from noncompetitive to competitive as the concentration of quinone increased. These results suggest that rotenone binds close to, but not at a site identical to, the location for ubiquinone in the ubiquinone-catalytic reaction site, whereas the 5'alpha-epirotenone-binding site overlaps that for ubiquinone due to a structural modification of E-ring moiety. Furthermore, the complex inhibition pattern of 5'alpha-epirotenone suggests that there are two quinone-binding sites in NADH-ubiquinone reductase. In contrast, the order of the inhibitory potencies of the three inhibitors with proton-pumping NADH-ubiquinone reductase of potato SMP was the same as that observed for the bovine enzyme. This suggests that the structure of rotenone-binding sites (or ubiquinone-binding sites) of these enzymes are similar. It was further demonstrated that 5'alpha-epirotenone inhibits quinone binding to both proton-pumping and non-proton-pumping NADH-ubiquinone reductases of potato SMP in a competitive manner. With respect to the proton-pumping NADH-ubiquinone reductase of the E. coli membrane, the sensitivity of the enzyme to the inhibitor was remarkably decreased and the difference in the inhibitory potencies of the three inhibitors became ambiguous. In addition, the inhibition pattern of the three inhibitors was competitive against quinone. These results indicated that, contrary to the mammalian enzyme, only part of the rotenone molecule is recognized by the quinone-binding site of this enzyme.

The reaction sites of rotenone and ubiquinone with mitochondrial NADH dehydrogenase

This article summarizes recent studies in the authors' and other laboratories of selective inhibitors acting at the 'rotenone' site and at the Q binding site in the NADH-Q oxidoreductase segment of the respiratory chain. A wide array of inhibitors act at the rotenone site to block electron flux from the enzyme to the Q pool. Using evidence from studies with rotenone, piericidin A, and analogs of the neurotoxic N-methyl-4-phenylpyridinium, we have proposed two binding sites for these inhibitors, both of which must be occupied for complete inhibition of NADH oxidation.

The effect of delta mu H+ on the interaction of rotenone with complex I of submitochondrial particles

The inhibition by rotenone of the forward (NADH-oxidase) and reverse (delta mu H(+)-dependent succinate-NAD+ reductase activities of submitochondrial vesicles was measured. The inhibition of NADH-oxidase, measured in the presence of uncoupler, followed a monophasic inhibition curve with Ki < or = 2 nM. The reverse electron flow was only partially (40%) inhibited at these rotenone concentrations. The rest of the activity was less sensitive to the inhibitor (Ki approximately 30 nM). The lower affinity for the inhibitor of the reverse electron flow is a consequence of enhanced rate of rotenone dissociation caused by the high delta mu H+ value required for this reaction. The analysis of the results indicates that the AS-SMP preparation consists of two subpopulations: one with a relatively low degree of coupling, which exhibits high sensitivity to rotenone and the other which is highly coupled with lower affinity to the inhibitor.

Relation of superoxide generation and lipid peroxidation to the inhibition of NADH-Q oxidoreductase by rotenone, piericidin A, and MPP+

The addition of NADH to submitochondrial particles inhibited by agents which interrupt electron transport from NADH-Q oxidoreductase (Complex I) to Q10 (rotenone, piericidin A, and MPP+) results in superoxide formation and lipid peroxidation. A study of the quantitative relations now shows that oxyradical formation does not appear to be the direct result of the inhibition. Although tetraphenyl boron (TPB) greatly enhances the inhibition by MPP+, it has no effect on O2. formation or lipid peroxidation. When submitochondrial particles completely inhibited by rotenone or piericidin A are treated with bovine serum albumin to remove spuriously bound inhibitor molecules without affecting those bound at the specific inhibition site, NADH-Q activity remains inhibited and lipid peroxidation occurs but superoxide formation ceases. Thus oxyradical formation may be the result of the binding of inhibitors at sites in the membrane other than those related to the inhibition of electron transport.

Interaction of 1-methyl-4-phenylpyridinium ion (MPP+) and its analogs with the rotenone/piericidin binding site of NADH dehydrogenase

Nigrostriatal cell death in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced Parkinson's disease results from the inhibition of mitochondrial respiration by 1-methyl-4-phenylpyridinium (MPP+). MPP+ blocks electron flow from NADH dehydrogenase to coenzyme Q at or near the same site as do rotenone and piericidin and protects against binding of and loss of activity due to these inhibitors. The 4'-analogs of MPP+ showed increasing affinity for the site with increasing length of alkyl chain, with the lowest Ki, for 4'-heptyl-MPP+, being 6 microM. The 4'-analogs compete with rotenone for the binding site in a concentration-dependent manner. They protect the activity of the enzyme from inhibition by piericidin in parallel to preventing its binding, indicating that the analogs and piericidin bind at the same inhibitory site(s). The optimum protection, however, was afforded by 4'-propyl-MPP+. The lesser protection by the more lipophilic MPP+ analogs with longer alkyl chains may involve a different orientation in the hydrophobic cleft, allowing rotenone and piericidin to still bind even when the pyridinium cation is in a position to interrupt electron flow from NADH to coenzyme Q.

New evidence for the dimeric nature of NADH:Q oxidoreductase in bovine-heart submitochondrial particles

The initial velocity of NADH oxidation by bovine-heart submitochondrial particles was measured at pH 8.0 after pretreatment of these particles with different amounts of the inhibitor piericidine A together with 0.035 mM NADH. The amount of piericidine A required to fully inhibit the NADH oxidation activity extrapolated to exactly 1.0 per Fe-S cluster 2 of NADH:Q oxidoreductase. When no reducing equivalents from NADH were present during the pretreatment, this ratio was 1.2. The difference is explained by assuming that NADH:Q oxidoreductase binds piericidine A more effectively in the reduced state than in the oxidized state. It was also found that after Q10-extraction and reincorporation of submitochondrial particles, the amount of piericidine A required to fully inhibit the NADH oxidation activity of the particles increased with the amount of Q10 present during reincorporation. This is explained by assuming that binding of piericidine A, to the inhibitory site of NADH:Q oxidoreductase requires Q10. When 0.035 mM NADPH instead of NADH was present during the pretreatment of submitochondrial particles with piericidine A, the amount of inhibitor per cluster 2 required to fully inhibit the initial NADH-oxidation activity extrapolated to 0.5. This result strongly suggests that NADH:Q oxidoreductase is a functional dimer.