Inhibition of mitochondrial electron transport by piericidin A and related compounds

Piericidin A aggravates Tau pathology in P301S transgenic mice

Objective

The P301S mutation in exon 10 of the tau gene causes a hereditary tauopathy. While mitochondrial complex I inhibition has been linked to sporadic tauopathies. Piericidin A is a prototypical member of the group of the piericidins, a class of biologically active natural complex I inhibitors, isolated from streptomyces spp. with global distribution in marine and agricultural habitats. The aim of this study was to determine whether there is a pathogenic interaction of the environmental toxin piericidin A and the P301S mutation.

Methods

Transgenic mice expressing human tau with the P301S-mutation (P301S^+/+) and wild-type mice at 12 weeks of age were treated subcutaneously with vehicle (N = 10 P301S^+/+, N = 7 wild-type) or piericidin A (N = 9 P301S^+/+, N = 9 wild-type mice) at a dose of 0.5 mg/kg/d for a period of 28 days via osmotic minipumps. Tau pathology was measured by stereological counts of cells immunoreative with antibodies against phosphorylated tau (AD2, AT8, AT180, and AT100) and corresponding Western blot analysis.

Results

Piericidin A significantly increased the number of phospho-tau immunoreactive cells in the cerebral cortex in P301S^+/+ mice, but only to a variable and mild extent in wild-type mice. Furthermore, piericidin A led to increased levels of pathologically phosphorylated tau only in P301S^+/+ mice. While we observed no apparent cell loss in the frontal cortex, the synaptic density was reduced by piericidin A treatment in P301S^+/+ mice.

Discussion

This study shows that exposure to piericidin A aggravates the course of genetically determined tau pathology, providing experimental support for the concept of gene-environment interaction in the etiology of tauopathies.

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.

Triton X-100 as a specific inhibitor of the mammalian NADH-ubiquinone oxidoreductase (Complex I)

Triton X-100 inhibits the NADH oxidase and rotenone-sensitive NADH-Q1 reductase activities of bovine heart submitochondrial particles (SMP) with an apparent Ki of 1x10-5 M (pH 8.0, 25 degrees C). The NADH-hexammineruthenium reductase, succinate oxidase, and the respiratory control ratio with succinate as the substrate in tightly coupled SMP are not affected at the inhibitor concentrations below 0.15 mM. The succinate-supported aerobic reverse electron transfer is less sensitive to the inhibitor (Ki=5x10-5 M) than NADH oxidase. Similar to rotenone, limited concentrations of Triton X-100 increase the steady-state level of NAD+ reduction when the nucleotide is added to tightly coupled SMP oxidizing succinate aerobically. Also similar to rotenone, Triton X-100 partially protects Complex I against the thermally induced deactivation and partially activates the thermally deactivated enzyme. The rate of the NADH oxidase inhibition by rotenone is drastically decreased in the presence of Triton X-100 which indicates a competition between these two inhibitors for a common specific binding site. In contrast to rotenone, the inhibitory effect of Triton X-100 is instantly reversed upon dilution of the reaction mixture. The NADH-Q1 reductase activity of SMP is inhibited non-competitively by added Q1 whereas a simple competition between Q1 and the inhibitor is seen for isolated Complex I. The results obtained show that Triton X-100 is a specific inhibitor of the ubiquinone reduction by Complex I and are in accord with our previous findings which suggest that different reaction pathways operate in the forward and reverse electron transfer at this segment of the mammalian respiratory chain.

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.

Interaction of the mitochondrial NADH-ubiquinone reductase with rotenone as related to the enzyme active/inactive transition

The interaction of rotenone with active ('pulsed') and thermally de-activated ('resting') membrane-bound Complex I (Kotlyar, A.B. and Vinogradov, A.D. (1990) Biochim. Biophys. Acta 1019, 151-158) as revealed by inhibition of NADH-ubiquinone- and ubiquinol-NAD+ reductase activities was studied. Ki = 1 x 10(-9) M, k(on) = 5 x 10(7) M-1 min-1 and k(off) = 0.02 min-1 (inhibitory effect of rotenone on NADH oxidation) and Ki = 2 x 10(-8) M (inhibition of reverse electron transfer) were determined for pulsed enzyme. The equilibrium between de-activated and active enzyme is reached (K approximately 100) after the slow strongly temperature-dependent de-activation process has completed. Rotenone partially prevents and reverses the enzyme de-activation. About two order of magnitude difference in affinity of rotenone to the active and de-activated forms of the enzyme was demonstrated. The strong difference in rotenone sensitivity of the direct and reverse reactions can not be accounted for delta mu H(+)-dependence of rotenone binding. We propose that two rotenone-specific inhibitory sites exist in Complex I: one is involved in NADH oxidation by ubiquinone and the other is operating in ubiquinol-NAD+ reductase reaction. The affinities of rotenone for both sites are strongly altered upon the slow enzyme active/inactive transition.

Comparison of the inhibitory action of synthetic capsaicin analogues with various NADH-ubiquinone oxidoreductases

Capsaicin is a new naturally occurring inhibitor of proton-pumping NADH-ubiquinone oxidoreductase (NDH-1), that competitively acts against ubiquinone. A series of capsaicin analogues was synthesized to examine the structural factors required for the inhibitory action and to probe the structural property of the ubiquinone catalytic site of various NADH-ubiquinone reductases, including non-proton-pumping enzyme (NDH-2), from bovine heart mitochondria, potato tuber (Solanum tuberosum, L) mitochondria and Escherichia coli (GR 19N) plasma membranes. Some synthetic capsaicins were fairly potent inhibitors of each of the three NDH-1 compared with the potent rotenone and piericidin A. Synthetic capsaicin analogues inhibited all three NDH-1 activities in a competitive manner against an exogenous quinone. The modification both of the substitution pattern and of the number of methoxy groups on the benzene ring, which may be superimposable on the quinone ring of ubiquinone, did not drastically affect the inhibitory potency. In addition, alteration of the position of dipolar amide bond unit in the molecule and chemical modifications of this unit did not change the inhibitory potency, particularly with bovine heart and potato tuber NDH-1. These results might be explained assuming that the ubiquinone catalytic site of NDH-1 is spacious enough to accommodate a variety of structurally different capsaicin analogues in a dissimilar manner. Regarding the moiety corresponding to the alkyl side chain, a rigid diphenyl ether structure was more inhibitory than a flexible alkyl chain. Structure-activity studies and molecular orbital calculations suggested that a bent form is the active conformation of capsaicin analogues. On the other hand, poor correlations between the inhibitory potencies determined with the three NDH-1 suggested that the structural similarity of the ubiquinone catalytic sites of these enzymes is rather poor. The sensitivity to the inhibition by synthetic capsaicins remarkably differed between NDH-1 and NDH-2, supporting the notion that the sensitivity against capsaicin inhibition correlates well with the presence of an energy coupling site in the enzyme (Yagi, T. (1990) Arch. Biochem. Biophys. 281, 305-311). It is noteworthy that several synthetic capsaicins discriminated between NDH-1 and NDH-2 much better than natural capsaicin.

The 1,2,3-benzothiadiazoles. A new type of compound acting on coupling site I, in rat liver mitochondria

1. In rat liver mitochondria, 6-chloro-1,2,3-benzothiadiazole inhibited ADP phosphorylation and Ca2+-transport when the energy required for these processes came from the oxidation of NAD-linked substrates. The inhibition was characterized by substantial reduction in oxygen consumption, H+-movement and disappearance of acceptor control ratio. 2. When the substrate oxidized was succinate, depending on the 6-chloro-1,2,3-benzothiadiazole concn., little or no effect was observed on ADP phosphorylation and Ca2+-transport. 3. The results suggest that 6-chloro-1,2,3-benzothiadiazole can block site I at low concn., but at higher concn. can affect site I and site II, although site I is always more affected.

Inhibition of mitochondrial electron transport by hydrophilic metal chelators. Determination of dehydrogenase topography

The topography of the inner mitochondrial membrane was investigated using inhibitors of electron transport on preparations of beef heart mitochondria and electron transport particles of opposite orientation. Reductions of juglone, ferricyanide, indophenol, coenzyme Q, duroquinone, and cytochrome c by NADH are inhibited to different extents on both sides of the membrane by the impermeant hydrophilic chelators bathophenanthroline sulfonate and orthophenanthroline. The extent of inhibition for each acceptor increased in the order given. At least two chelator-sensitive sites are present on each membrane face between the flavoprotein and coenzyme Q and a chelator-sensitive site is present on the matrix face between the sites of coenzyme Q and duroquinone interaction. Duroquinol oxidation in mitochondria only is stimulated by bathophenanthroline sulfonate. Juglone reduction is stimulated in electron transport particles (only) by p-hydroxymercuribenzenesulfonate, but after mercurial treatment, juglone reduction in both particles and mitochondria is more sensitive to bathophenanthroline sulfonate. Succinate dehydrogenase components are inhibited by hydrophilic orthophenanthroline or bathophenanthroline sulfonate in mitochondria only. Electron flow between the dehydrogenases of succinate and NADH occurs via a chelator-sensitive site located on the matrix face of the membrane. Inter-complex electron flow is prevented by rotenone or thenoyltrifluoroacetone. The lack of succinate-indophenol reductase inhibition by bathophenanthroline sulfonate in the presence of rotenone or thenoyltrifluoroacetone indicates that the rotenone-sensitive site may be located on the matrix face and demonstrates that electrons flow between the NADH and succinate dehydrogenases via a hydrophilic chelator and rotenone-thenoyltrifluoroacetone-sensitive site on the matrix face of the membrane. Inhibiton by hydrophilic chelators only in mitochondria indicates that succinate dehydrogenase as well as NADH dehydrogenase has a transmembranous orientation.

The activation of non-phosphorylating electron transport by adenine nucleotides in Jerusalem-artichoke (Helianthus tuberosus) mitochondria

In isolated plant mitochondria the oxidation of both succinate and exogenous NADH responded in the expected manner to the addition of ADP or uncoupling agents, and the uncoupled rate of respiration was often in excess of the rate obtained in the presence of ADP. However, the oxidation of NAD+-linked substrates responded in a much more complex manner to the addition of ADP or uncoupling agents such as carbonyl cyanide p-trifluoromethoxyphenylhydrazone to mitochondria oxidizing pyruvate plus malate failed to result in a reliable stimulation; this uncoupled rate could be stimulated by adding AMP or ADP in the presence of oligomycin or bongkrekic acid. Spectrophometric measurements showed that the addition of AMP or ADP resulted in the simultaneous oxidation of endogenous nicotinamide nucleotide and the reduction of cytochrome b. ADP was only effective in bringing about these changes in redox state in the presence of Mg2+ whereas AMP did not require Mg2+. It was concluded that AMP activated the flow of electrons from endogenous nicotinamide nucleotide to cytochrome b, possible at the level of the internal NADH dehydrogenase.

Synthesis of plastoquinone analogs and inhibition of photosynthetic and mammalian enzyme systems

New 5-hydroxy- and 5-chloro-6-alkyl-1,4-benzoquinones with one or two methyl groups on the nucleus were synthesized as potential antimetabolites of plastoquinones for biological research on photosynthetic and mammalian enzyme systems; the primary emphasis was on photosynthesis.2,3-Dimethyl-5-hydroxy-6-phytyl-1,4-benzoquinone completely inhibited in chloroplasts the water-dependent electron transport, but photosystem I was insensitive to this analog. The data are consistent with the interpretation that this analog inhibits electron transport in the chain prior to the site of electron donation from the ascorbate-dichlorophenolindophenol couple. Concentrations of 70 muM and 120 muM of this analog caused about 50 and 100% inhibition, respectively, of cyclic photophosphorylation.2,3-Dimethyl-5-hydroxy-6-phytyl-1,4-benzoquinone is a new type of inhibitor of photosynthetic electron transport that specifically inhibits the rate-limiting step between photosystems I and II. Structurally related analogs caused inhibitions in the range of 50-100% in chloroplasts. These analogs showed marginal inhibition in mitochondrial coenzyme Q(10)-oxidase systems from beef heart.

Non-haem iron and the dissociation of piericidin A sensitivity from site 1 energy conservation in mitochondria from Torulopsis utilis

1. The aerobic incubation of iron-deficient Torulopsis utilis cells for 12h under non-growing conditions results in the recovery by mitochondria of the previously absent site 1 energy conservation and sensitivity to piericidin A. 2. The recovery of piericidin A sensitivity but not site 1 is prevented by the presence of cycloheximide (100mug/ml) in the medium used for aerobic incubation of the cells. Rotenone sensitivity behaved similarly. 3. Chloramphenicol, erythromycin and tetracycline were without effect on the recovery of site 1 and piericidin A sensitivity. 4. Inclusion of (59)Fe in the growth medium can be used as the basis for a highly sensitive assay for non-haem iron. 5. Iron-limited growth of T. utilis lowers the concentration of both non-haem iron and acid-labile sulphide of submitochondrial particles by over 20-fold compared with the ;normal' situation with iron-supplemented glycerol-limited growth. 6. Increases in the non-haem iron and acid-labile sulphide concentrations of submitochondrial particles occur when site 1 and piericidin A sensitivity are recovered. The increase is approximately halved by the presence of cycloheximide. 7. The non-haem iron of T. utilis submitochondrial particles does not exchange with added iron. 8. Continuous culture of T. utilis at the transition between glycerol- and iron-limitation results in cells where mitochondria possess site 1 energy conservation but lack piericidin A sensitivity. 8. It is concluded, in contrast with widely held views to the opposite, that energy conservation at site 1 does not require electron flow to proceed through a piericidin A- or rotenone-sensitive route. 9. Restriction of the iron supplied to growing T. utilis to a concentration just above that required for growth limitation demonstrates that a 10- to 20-fold decrease of the ;normal' non-haem iron concentration of both cells and mitochondria is without effect on the growth yield per unit of carbon source. Submitochondrial particles prepared from such iron-restricted but otherwise functionally normal cells have a non-haem iron concentration of about 0.5-0.8ng-atom/mg of protein. It is concluded that the concentration of iron-sulphur protein required for normal function by the respiratory chain is close to the concentrations of cytochromes and flavoproteins.

The function of ubiquinone in Escherichia coli

1. The function of ubiquinone in Escherichia coli was studied by using whole cells and membrane preparations of normal E. coli and of a mutant lacking ubiquinone. 2. The mutant lacking ubiquinone, strain AN59 (Ubi(-)), when grown under aerobic conditions, gave an anaerobic type of growth yield and produced large quantities of lactic acid, indicating that ubiquinone plays a vital role in electron transport. 3. NADH and lactate oxidase activities in membranes from strain AN59 (Ubi(-)) were greatly impaired and activity was restored by the addition of ubiquinone (Q-1). 4. Comparison of the percentage reduction of flavin, cytochrome b(1) and cytochrome a(2) in the aerobic steady state in membranes from the normal strain (AN62) and strain AN59 (Ubi(-)) and the effect of respiratory inhibitors on these percentages in membranes from strain AN62 suggest that ubiquinone functions at more than one site in the electron-transport chain. 5. Membranes from strain AN62, in the absence of substrate, showed an electron-spin-resonance signal attributed to ubisemiquinone. The amount of reduced ubiquinone (50%) found after rapid solvent extraction is consistent with the existence of ubiquinone in membranes as a stabilized ubisemiquinone. 6. The effects of piericidin A on membranes from strain AN62 suggest that this inhibitor acts at the ubiquinone sites: thus inhibition of electron transport is reversed by ubiquinone (Q-1); the aerobic steady-state oxidation-reduction levels of flavins and cytochrome b(1) in the presence of the inhibitor are raised to values approximating those found in the membranes of strain AN59 (Ubi(-)); the inhibitor rapidly eliminates the electron-spin-resonance signal attributed to ubisemiquinone and allows slow oxidation of endogenous ubiquinol in the absence of substrate and prevents reduction of ubiquinone in the presence of substrate. It is concluded that piericidin A separates ubiquinone from the remainder of the electron-transport chain. 7. A scheme is proposed in which ubisemiquinone, complexed to an electron carrier, functions in at least two positions in the electron-transport sequence.