Synthesis and Inhibition Mechanism of Δlac-Acetogenins, a Novel Type of Inhibitor of Bovine Heart Mitochondrial Complex I

Total synthesis of squafosacin F: stereodivergent approach to mono-tetrahydrofuran acetogenins

Annonaceous acetogenins have a wide range of potential biological activities. The development of simple and diversity-oriented approaches to their synthesis is therefore important. We have achieved the first total synthesis of squafosacin F and assigned its absolute configuration. The key steps were an acid-mediated tandem intramolecular double cyclization to build the hydroxy-flanked mono-tetrahydrofuran core and decoration with the desired functionalities of the target natural product via highly stereoselective reactions.

The first total synthesis of squafosacin F was achieved via construction of the hydroxy-flanked mono-tetrahydrofuran by acid-mediated tandem cyclization as a key step.

Formal synthesis of cis-solamin: acid-catalyzed one-step construction of 2,5-disubstituted tetrahydrofuran

A divergent strategy has been used for the concise and efficient enantioselective formal synthesis of Annonaceous acetogenin cis-solamin. Our synthetic strategy comprises concise preparation of the diepoxyester via an 11-membered silaketal constructed by ring-closing metathesis after the dimerization of chiral epoxides, and uses an acid-catalyzed tandem intramolecular SN2-like reaction to construct the threo-cis-threo configuration of the tetrahydrofuran-diol moiety.

Acetogenins from Annonaceae

In recent decades, annonaceous acetogenins have become highly studied plant secondary metabolites in terms of their isolation, structure elucidation, synthesis, biological evaluation, mechanism of action, and toxicity. The aim of the present contribution is to summarize chemical and biological reports published since 1997 on annonaceous acetogenins and synthetic acetogenin mimics. The compounds are considered biologically in terms of their cytotoxicity for cancer cell lines, neurotoxicity, pesticidal effects, and miscellaneous activities.

Critical role of a methyl group on the γ-lactone ring of annonaceous acetogenins in the potent inhibition of mitochondrial complex I

C34-epi and C34-epi-C35-trifluoro analogues of solamin, a mono-THF annonaceous acetogenin, were synthesized. Their inhibitory activity, along with previously synthesized analogues (C35-fluoro, C35-difluoro, and C35-trifluorosolamins), against bovine mitochondrial NADH-ubiquinone oxidoreductase (complex I) was determined. The present study revealed that the methyl group on the γ-lactone moiety is critical to the potent inhibition of complex I by natural acetogenins.

The deactive form of respiratory complex I from mammalian mitochondria is a Na+/H+ antiporter

In mitochondria, complex I (NADH:ubiquinone oxidoreductase) uses the redox potential energy from NADH oxidation by ubiquinone to transport protons across the inner membrane, contributing to the proton-motive force. However, in some prokaryotes, complex I may transport sodium ions instead, and three subunits in the membrane domain of complex I are closely related to subunits from the Mrp family of Na(+)/H(+) antiporters. Here, we define the relationship between complex I from Bos taurus heart mitochondria, a close model for the human enzyme, and sodium ion transport across the mitochondrial inner membrane. In accord with current consensus, we exclude the possibility of redox-coupled Na(+) transport by B. taurus complex I. Instead, we show that the "deactive" form of complex I, which is formed spontaneously when enzyme turnover is precluded by lack of substrates, is a Na(+)/H(+) antiporter. The antiporter activity is abolished upon reactivation by the addition of substrates and by the complex I inhibitor rotenone. It is specific for Na(+) over K(+), and it is not exhibited by complex I from the yeast Yarrowia lipolytica, which thus has a less extensive deactive transition. We propose that the functional connection between the redox and transporter modules of complex I is broken in the deactive state, allowing the transport module to assert its independent properties. The deactive state of complex I is formed during hypoxia, when respiratory chain turnover is slowed, and may contribute to determining the outcome of ischemia-reperfusion injury.

Exploring the binding site of delta(lac)-acetogenin in bovine heart mitochondrial NADH-ubiquinone oxidoreductase

Biochemical characterization of the inhibition mechanism of Deltalac-acetogenins synthesized in our laboratory indicated that they are a new type of inhibitor of bovine heart mitochondrial NADH-ubiquinone oxidoreductase (complex I) [Murai, M., et al. (2006) Biochemistry 45, 9778-9787]. To identify the binding site of Deltalac-acetogenins with a photoaffinity labeling technique, we synthesized a photoreactive Deltalac-acetogenin ([(125)I]diazinylated Deltalac-acetogenin, [(125)I]DAA) which has a small photoreactive diazirine group attached to a pharmacophore, the bis-THF ring moiety. Characterization of the inhibitory effects of DAA on bovine complex I revealed unique features specific to, though not completely the same as those of, the original Deltalac-acetogenin. Using [(125)I]DAA, we carried out photoaffinity labeling with bovine heart submitochondrial particles. Analysis of the photo-cross-linked protein by Western blotting and immunoprecipitation revealed that [(125)I]DAA binds to the membrane subunit ND1 with high specificity. The photo-cross-linking to the ND1 subunit was suppressed by an exogenous short-chain ubiquinone (Q(2)) in a concentration-dependent manner. Careful examination of the fragmentation patterns of the cross-linked ND1 generated by limited proteolysis using lysylendopeptidase, endoprotease Asp-N, or trypsin and their changes in the presence of the original Deltalac-acetogenin strongly suggested that the cross-linked residues are located at two different sites in the third matrix-side loop connecting the fifth and sixth transmembrane helices.

Medicinal chemistry of Annonaceous acetogenins: design, synthesis, and biological evaluation of novel analogues

Most Annonaceous acetogenins are characterized by between one and three THF ring(s) with one or two flanking hydroxyl group(s) in the center of a C32/34 fatty acid, and the terminal carboxylic acid is combined with a 2-propanol unit to form an alpha,beta-unsaturated gamma-lactone. While many studies have addressed the properties and synthesis of natural acetogenins due to their attractive biological activities and unique structural features, a number of analogues have also been described. This review covers the design, synthesis, and biological evaluation of acetogenin analogues.

Exploring the binding site of acetogenin in the ND1 subunit of bovine mitochondrial complex I

125I-labeled (trifluoromethyl)phenyldiazirinyl acetogenin, [125I]TDA, a photoaffinity labeling probe of acetogenin, photo-cross-links to the ND1 subunit of bovine heart mitochondrial NADH-ubiquinone oxidoreductase (complex I) with high specificity [M. Murai, A. Ishihara, T. Nishioka, T. Yagi, and H. Miyoshi, (2007) The ND1 subunit constructs the inhibitor binding domain in bovine heart mitochondrial complex I, Biochemistry 46 6409-6416.]. To identify the binding site of [125I]TDA in the ND1 subunit, we carried out limited proteolysis of the subunit cross-linked by [125I]TDA using various proteases and carefully analyzed the fragmentation patterns. Our results revealed that the cross-linked residue is located within the region of the 4th to 5th transmembrane helices (Val144-Glu192) of the subunit. It is worth noting that an excess amount of short-chain ubiquinones such as ubiquinone-2 (Q2) and 2-azido-Q2 suppressed the cross-linking by [125I]TDA in a concentration-dependent way. Although the question of whether the binding sites for ubiquinone and different inhibitors in complex I are identical remains to be answered, the present study provided, for the first time, direct evidence that an inhibitor (acetogenin) and ubiquinone competitively bind to the enzyme. Considering the present results along with earlier photoaffinity labeling studies, we propose that not all inhibitors acting at the terminal electron transfer step of complex I necessarily bind to the ubiquinone binding site itself.

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.

Function of the alkyl side chains of Δlac-acetogenins in the inhibitory effect on mitochondrial complex I (NADH-ubiquinone oxidoreductase)

We synthesized a series of Deltalac-acetogenins in which the two alkyl side chains were systematically modified, and examined their inhibitory effect on bovine heart mitochondrial complex I (NADH-ubiquinone oxidoreductase). The results revealed that the physicochemical properties of the side chains, such as the balance of hydrophobicity and the width (or bulkiness) of the chains, are important structural factors for a potent inhibitory effect of amphiphilic Deltalac-acetogenins. This is probably because such properties decide the precise location of the hydrophilic bis-THF ring moiety in the enzyme embedded in the inner mitochondrial membrane.

Remarkable substituent effect: beta-aminosquamocin, a potent dual inhibitor of mitochondrial complexes I and III

The introduction of a primary amine function on the terminal alpha,beta-unsaturated lactone of squamocin 1, a common structural hallmark of annonaceous acetogenins, shifted this specific inhibitor of mitochondrial complex I into a potent dual inhibitor of complexes I and III. The mechanism of action of beta-aminosquamocin 2, against these two respiratory targets, is studied and discussed in view of current structure-activity relationship knowledge in the acetogenin series.