Probing the ubiquinone reduction site of mitochondrial complex I using novel cationic inhibitors

Synthesis and characterization of new piperazine-type inhibitors for mitochondrial NADH-ubiquinone oxidoreductase (complex I)

The mode of action of Deltalac-acetogenins, strong inhibitors of bovine heart mitochondrial complex I, is different from that of traditional inhibitors such as rotenone and piericidin A [Murai, M., et al. (2007) Biochemistry 46 , 6409-6416]. As further exploration of these unique inhibitors might provide new insights into the terminal electron transfer step of complex I, we drastically modified the structure of Deltalac-acetogenins and characterized their inhibitory action. In particular, on the basis of structural similarity between the bis-THF and the piperazine rings, we here synthesized a series of piperazine derivatives. Some of the derivatives exhibited very potent inhibition at nanomolar levels. The hydrophobicity of the side chains and their balance were important structural factors for the inhibition, as is the case for the original Deltalac-acetogenins. However, unlike in the case of the original Deltalac-acetogenins, (i) the presence of two hydroxy groups is not crucial for the activity, (ii) the level of superoxide production induced by the piperazines is relatively high, (iii) the inhibitory potency for the reverse electron transfer is remarkably weaker than that for the forward event, and (iv) the piperazines efficiently suppressed the specific binding of a photoaffinity probe of natural-type acetogenins ([ (125)I]TDA) to the ND1 subunit. We therefore conclude that the action mechanism of the piperazine series differs from that of the original Deltalac-acetogenins. The photoaffinity labeling study using a newly synthesized photoreactive piperazine ([ (125)I]AFP) revealed that this compound binds to the 49 kDa subunit and an unidentified subunit, not ND1, with a frequency of approximately 1:3. A variety of traditional complex I inhibitors as well as Deltalac-acetogenins suppressed the specific binding of [ (125)I]AFP to the subunits. The apparent competitive behavior of inhibitors that seem to bind to different sites may be due to structural changes at the binding site, rather than occupying the same site. The meaning of the occurrence of diverse inhibitors exhibiting different mechanisms of action is discussed in light of the functionality of the membrane arm of complex I.

New antitumoral acetogenin 'Guanacone type' derivatives: isolation and bioactivity. Molecular dynamics simulation of diacetyl-guanacone

We describe herein the isolation and semisynthesis of four acetogenin derivatives (1-4) as well as their ability to inhibit the mitochondrial respiratory chain and several tumor cell lines. In addition, four nanoseconds (ns) of MD simulation of compound 4, in a fully hydrated POPC bilayer, is reported.

Drugs targeting mitochondrial functions to control tumor cell growth

Mitochondria, the power houses of the cell, are at the cross-road of many cellular pathways. They play a central role in energy metabolism, regulate calcium flux and are implicated in apoptosis. Mitochondrial dysfunctions have been associated with various physiopathological disorders, especially neurodegenerative diseases and cancer. Structurally diverse pharmacological agents have shown direct effects on mitochondria ultra-structures and functions, either at the DNA level or upon targeting proteins located in the inner or outer mitochondrial membranes. The brief review deals with the molecular targets and mechanisms of action of chemically diverse small molecules acting on specific mitochondrial loci, such as the respiratory chain, DNA biogenesis, potassium channels, the Bcl-2 protein and the permeability transition pores (PTP). Drugs, which specifically compromise the structural and functional integrity of mitochondria, may provide novel opportunities to combat cancer cell proliferation, providing that these molecules can be selectively delivered to tumor sites. Different examples reported here show that mitochondrial insult or failure can rapidly lead to inhibition of cell survival and proliferation. Mitochondrial impairment may be a successful anti-cancer strategy.

MPTP as a mitochondrial neurotoxic model of Parkinson's disease

1-Methy-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is a potent neurotoxin extensively used to model Parkinson's disease (PD). A cascade of deleterous events, in which mitochondria play a pivotal role, drives MPTP neurotoxicity. How mitochondria are affected by MPTP and how their defect contributes to the demise of dopaminergic neurons in this model of PD are discussed in this review.

Characterization of inhibitor binding sites of mitochondrial complex I using fluorescent inhibitor

Recent progress in complex I research suggests that a wide variety of complex I inhibitors share a common large binding domain with partially overlapping sites. To verify this concept, we carried out real-time displacement tests of a fluorescent ligand with various competitors using a novel quinazoline-type inhibitor (aminoquinazoline, AQ). In the presence of an excess amount of the competitors, the binding of AQ to the enzyme was completely suppressed, being in line with the concept mentioned above. However, AQ bound to the enzyme was not displaced by subsequent addition of an increasing amount of competitors in the concentration range expected from the relative magnitude of the K(d) values of AQ and competitors, rather, much higher concentrations of the competitors were needed to displace bound AQ. These results cannot be explained merely by the premise of a common or partially overlapping binding site(s) between AQ and competitors. On the other hand, double-inhibitor titration of steady state complex I activity suggested that additivity of inhibition is not necessarily observed for all pairs of complex I inhibitors. Our results are discussed in light of the cooperativity of the inhibitor binding sites.

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.

Mitochondria make a come back

This review attempts to summarize our present state of knowledge of mitochondria in relation to a number of areas of biology, and to indicate where future research might be directed. In the evolution of eukaryotic cells mitochondria have for a long time played a prominent role. Nowadays their integration into many activities of a cell, and their dynamic behavior as subcellular organelles within a cell and during cell division are a major focus of attention. The crystal structures of the major complexes of the electron transport chain (except complex I) have been established, permitting increasingly detailed analyses of the important mechanism of proton pumping coupled to electron transport. The mitochondrial genome and its replication and expression are beginning to be understood in considerable detail, but more questions remain with regard to mutations and their repair, and the segregation of the mtDNA in oogenesis and development. Much emphasis and a large effort have recently been devoted to understand the role of mitochondria in programmed cell death (apoptosis). The understanding of their central role in mitochondrial diseases is a major achievement of the past decade. Finally, various drugs have traditionally played a part in understanding biochemical mechanisms within mitochondria; the repertoire of drugs with novel and interesting targets is expanding.

Probing the ubiquinone reduction site in bovine mitochondrial complex I using a series of synthetic ubiquinones and inhibitors

Studies of the structure-activity relationships of ubiquinones and specific inhibitors are helpful to probe the structural and functional features of the ubiquinone reduction site of bovine heart mitochondrial complex I. Bulky exogenous short-chain ubiquinones serve as sufficient electron acceptors from the physiological ubiquinone reduction site of bovine complex I. This feature is in marked contrast to other respiratory enzymes such as mitochondrial complexes II and III. For various complex I inhibitors, including the most potent inhibitors, acetogenins, the essential structural factors that markedly affect the inhibitory potency are not necessarily obvious. Thus, the loose recognition by the enzyme of substrate and inhibitor structures may reflect the large cavity like structure of the ubiquinone (or inhibitor) binding domain in the enzyme. On the other hand, several phenomena are difficult to explain by a simple one-catalytic site model for ubiquinone.

Definition of crucial structural factors of acetogenins, potent inhibitors of mitochondrial complex I

Some natural acetogenins are the most potent inhibitors of bovine heart mitochondrial complex I. These compounds are characterized by two functional units (i.e. hydroxylated tetrahydrofuran (THF) and alpha,beta-unsaturated gamma-lactone ring moieties) separated by a long alkyl spacer. To elucidate which structural factors of acetogenins including their active conformation are crucial for the potent inhibitory effect, we synthesized a series of novel acetogenin analogues possessing bis-THF rings. The present study clearly demonstrated that the natural gamma-lactone ring is not crucial for the potent inhibition, although this moiety is the most common structural unit among a large number of natural acetogenins and has been suggested to be the only reactive species that directly interacts with the enzyme (Shimada et al., Biochemistry 37 (1998) 854-866). The presence of free hydroxy group(s) in the adjacent bis-THF rings was favorable, but not essential, for the potent activity. This was probably because high polarity (or hydrophilicity), rather than hydrogen bond-donating ability, around the bis-THF rings is required to retain the inhibitor in the active conformation. Interestingly, length of the alkyl spacer proved to be a very important structural factor for the potent activity, the optimal length being approximately 13 carbon atoms. The present study provided further strong evidence for the previous proposal (Kuwabara et al., Eur. J. Biochem. 267 (2000) 2538-2546) that the gamma-lactone and THF ring moieties act in a cooperative manner on complex I with the support of some specific conformation of the spacer.

D-beta-hydroxybutyrate protects neurons in models of Alzheimer's and Parkinson's disease

The heroin analogue 1-methyl-4-phenylpyridinium, MPP(+), both in vitro and in vivo, produces death of dopaminergic substantia nigral cells by inhibiting the mitochondrial NADH dehydrogenase multienzyme complex, producing a syndrome indistinguishable from Parkinson's disease. Similarly, a fragment of amyloid protein, Abeta(1-42), is lethal to hippocampal cells, producing recent memory deficits characteristic of Alzheimer's disease. Here we show that addition of 4 mM d-beta-hydroxybutyrate protected cultured mesencephalic neurons from MPP(+) toxicity and hippocampal neurons from Abeta(1-42) toxicity. Our previous work in heart showed that ketone bodies, normal metabolites, can correct defects in mitochondrial energy generation. The ability of ketone bodies to protect neurons in culture suggests that defects in mitochondrial energy generation contribute to the pathophysiology of both brain diseases. These findings further suggest that ketone bodies may play a therapeutic role in these most common forms of human neurodegeneration.

Design syntheses and mitochondrial complex I inhibitory activity of novel acetogenin mimics

Some natural acetogenins are the most potent inhibitors of mitochondrial complex I. These compounds are characterized by two functional units [i.e. hydroxylated tetrahydrofuran (THF) and alpha, beta-unsaturated gamma-lactone ring moieties] separated by a long alkyl spacer. To elucidate which structural factors of acetogenins, including their active conformation, are crucial for the potent inhibitory activity we synthesized a novel bis-acetogenin and its analogues possessing two gamma-lactone rings connected to bis-THF rings by flexible alkyl spacers. The inhibitory potency of the bis-acetogenin with bovine heart mitochondrial complex I was identical to that of bullatacin, one of the most potent natural acetogenins. This result indicated that one molecule of the bis-acetogenin does not work as two reactive inhibitors, suggesting that a gamma-lactone and the THF ring moieties act in a cooperative manner on the enzyme. In support of this, either of the two ring moieties synthesized individually showed no or very weak inhibitory effects. Moreover, combined use of the two ring moieties at various molar ratios exhibited no synergistic enhancement of the inhibitory potency. These observations indicate that both functional units work efficiently only when they are directly linked by a flexible alkyl spacer. Therefore, some specific conformation of the spacer must be important for optimal positioning of the two units in the enzyme. Furthermore, the alpha,beta-unsaturated gamma-lactone, the 4-OH group in the spacer region, the long alkyl tail attached to the THF unit and the stereochemistry surrounding the hydroxylated bis-THF rings were not crucial for the activity, although these are the most common structural features of natural acetogenins. The present study provided useful guiding principles not only for simplification of complicated acetogenin structure, but also for further wide structural modifications of these molecules.

Origin of selective inhibition of mitochondrial complex I by pyridinium-type inhibitor MP-24

Positively charged pyridiniums are unique inhibitors to probe the structural and functional properties of the ubiquinone reduction site of bovine heart mitochondrial complex I. In this study, we synthesized a series of neutral as well as pyridinium analogues of MP-24 (N-methyl-4-[2-methyl-2-(p-tert-butylbenzyl)propyl]pyridinium), a selective inhibitor of one of the two proposed binding sites of these pyridinium-type inhibitors of complex I (H. Miyoshi et al., J. Biol. Chem. 273 (1998) 17368-17374), to elucidate the origin of its selectivity. Inhibitory potencies of all neutral and pyridinium analogues with tetraphenylboron (TPB(-)), which forms an ion-pair with pyridiniums, were comparable, although the degrees of selective inhibition by pyridiniums without TPB(-) were entirely different. In contrast to MP-24, the dose-response curves of nonselective pyridiniums and all neutral analogues were not affected by incubation conditions. These results strongly suggested that the process of the inhibitor passage to the binding sites is responsible for the selective inhibition.

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.

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.

Specificity of pyridinium inhibitors of the ubiquinone reduction sites in mitochondrial complex I

Dual binding sites for pyridinium-type inhibitors in bovine heart mitochondrial complex I have been proposed (Gluck, M. R., Krueger, M. J., Ramsay, R. R., Sablin, S. O., Singer, T. P., and Nicklas, W. J. (1994) J. Biol. Chem. 269, 3167-3174). The marked biphasic nature of the dose-response curve for inhibition of the enzyme by MP-6(N-methyl-4-[2-(p-tert-butylbenzyl)propyl]pyridinium) makes this compound the first selective inhibitor of the two sites (Miyoshi, H., Inoue, M., Okamoto, S., Ohshima, M., Sakamoto, K., and Iwamura, H. (1997) J. Biol. Chem. 272, 16176-16183). Modifications of the structure of MP-6 show that a tert-butyl group on the benzene ring, a methyl group attached to the pyridine nitrogen atom, para-substitution pattern in the pyridine ring, and the presence of a branched structure in the spacer moiety are important for the selective inhibition. On the basis of the structural specificity, we synthesized a selective inhibitor, MP-24 (N-methyl-4-[2-methyl-2-(p-tert-butylbenzyl)propyl]pyridinium), which elicits greater selectivity. Characterization of the inhibitory behavior of MP-24 provided further strong evidence for the dual binding sites model.

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.