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

How to Use Respiratory Chain Inhibitors in Toxicology Studies-Whole-Cell Measurements

Mitochondrial electron transport chain (ETC) inhibition is a phenomenon interesting in itself and serves as a tool for studying various cellular processes. Despite the fact that searching the term “rotenone” in PubMed returns more than 6900 results, there are many discrepancies regarding the directions of changes reported to be caused by this RTC inhibitor in the delicate redox balance of the cell. Here, we performed a multifaceted study of the popular ETC inhibitors rotenone and antimycin A, involving assessment of mitochondrial membrane potential and the production of hydrogen peroxide and superoxide anions at cellular and mitochondrial levels over a wide range of inhibitor concentrations (1 nmol/dm³–100 µmol/dm³). All measurements were performed with whole cells, with accompanying control of ATP levels. Antimycin A was more potent in hindering HepG2 cells’ abilities to produce ATP, decreasing ATP levels even at a 1 nmol/dm³ concentration, while in the case of rotenone, a 10,000-times greater concentration was needed to produce a statistically significant decrease. The amount of hydrogen peroxide produced in the course of antimycin A biological activity increased rapidly at low concentrations and decreased below control level at a high concentration of 100 µmol/dm³. While both inhibitors influenced cellular superoxide anion production in a comparable manner, rotenone caused a greater increase in mitochondrial superoxide anions compared to a modest impact for antimycin A. IC50 values for rotenone and antimycin A with respect to HepG2 cell survival were of the same order of magnitude, but the survival curve of cells treated with rotenone was clearly biphasic, suggesting a concentration-dependent mode of biological action. We propose a clear experimental setup allowing for complete and credible analysis of the redox state of cells under stress conditions which allows for better understanding of the effects of ETC inhibition.

An Overview of the Chemical Characteristics, Bioactivity and Achievements Regarding the Therapeutic Usage of Acetogenins from Annona cherimola Mill

Annona cherimola Mill., or the custard apple, is one of the species belonging to the Annonaceae family, is widely used in traditional medicine, and has been reported to be a valuable source of bioactive compounds. A unique class of secondary metabolites derived from this family are Annonaceous acetogenins, lipophilic polyketides considered to be amongst the most potent antitumor compounds. This review provides an overview of the chemical diversity, isolation procedures, bioactivity, modes of application and synthetic derivatives of acetogenins from A. cherimola Mill.

An adverse outcome pathway for parkinsonian motor deficits associated with mitochondrial complex I inhibition

Epidemiological studies have observed an association between pesticide exposure and the development of Parkinson's disease, but have not established causality. The concept of an adverse outcome pathway (AOP) has been developed as a framework for the organization of available information linking the modulation of a molecular target [molecular initiating event (MIE)], via a sequence of essential biological key events (KEs), with an adverse outcome (AO). Here, we present an AOP covering the toxicological pathways that link the binding of an inhibitor to mitochondrial complex I (i.e., the MIE) with the onset of parkinsonian motor deficits (i.e., the AO). This AOP was developed according to the Organisation for Economic Co-operation and Development guidelines and uploaded to the AOP database. The KEs linking complex I inhibition to parkinsonian motor deficits are mitochondrial dysfunction, impaired proteostasis, neuroinflammation, and the degeneration of dopaminergic neurons of the substantia nigra. These KEs, by convention, were linearly organized. However, there was also evidence of additional feed-forward connections and shortcuts between the KEs, possibly depending on the intensity of the insult and the model system applied. The present AOP demonstrates mechanistic plausibility for epidemiological observations on a relationship between pesticide exposure and an elevated risk for Parkinson's disease development.

Stereochemistry and Mechanism of Undecylprodigiosin Oxidative Carbocyclization to Streptorubin B by the Rieske Oxygenase RedG

The prodiginines are a group of specialized metabolites that share a 4-methoxypyrrolyldipyrromethene core structure. Streptorubin B is a structurally remarkable member of the prodiginine group produced by Streptomyces coelicolor A3(2) and other actinobacteria. It is biosynthesized from undecylprodigiosin by an oxidative carbocyclization catalyzed by the Rieske oxygenase-like enzyme RedG. Undecylprodigiosin derives from the RedH-catalyzed condensation of 2-undecylpyrrole and 4-methoxy-2, 2'-bipyrrole-5-carboxaldehyde (MBC). To probe the mechanism of the RedG-catalyzed reaction, we synthesized 2-(5-pentoxypentyl)-pyrrole, an analogue of 2-undecylpyrrole with an oxygen atom next to the site of C-C bond formation, and fed it, along with synthetic MBC, to Streptomyces albus expressing redH and redG. This resulted in the production of the 6'-oxa analogue of undecylprodigiosin. In addition, a small amount of a derivative of this analogue lacking the n-pentyl group was produced, consistent with a RedG catalytic mechanism involving hydrogen abstraction from the alkyl chain of undecylprodigiosin prior to pyrrole functionalization. To investigate the stereochemistry of the RedG-catalyzed oxidative carbocyclization, [7'-(2)H](7'R)-2-undecylpyrrole and [7'-(2)H](7'S)-2-undecylpyrrole were synthesized and fed separately, along with MBC, to S. albus expressing redH and redG. Analysis of the extent of deuterium incorporation into the streptorubin B produced in these experiments showed that the pro-R hydrogen atom is abstracted from C-7' of undecylprodigiosin and that the reaction proceeds with inversion of configuration at C-7'. This contrasts sharply with oxidative heterocyclization reactions catalyzed by other nonheme iron-dependent oxygenase-like enzymes, such as isopenicillin N synthase and clavaminate synthase, which proceed with retention of configuration at the carbon center undergoing functionalization.

Conserved amino acid residues of the NuoD segment important for structure and function of Escherichia coli NDH-1 (complex I)

The NuoD segment (homologue of mitochondrial 49 kDa subunit) of the proton-translocating NADH:quinone oxidoreductase (complex I/NDH-1) from Escherichia coli is in the hydrophilic domain and bears many highly conserved amino acid residues. The three-dimensional structural model of NDH-1 suggests that the NuoD segment, together with the neighboring subunits, constitutes a putative quinone binding cavity. We used the homologous DNA recombination technique to clarify the role of selected key amino acid residues of the NuoD segment. Among them, residues Tyr273 and His224 were considered candidates for having important interactions with the quinone headgroup. Mutant Y273F retained partial activity but lost sensitivity to capsaicin-40. Mutant H224R scarcely affected the activity, suggesting that this residue may not be essential. His224 is located in a loop near the N-terminus of the NuoD segment (Gly217–Phe227) which is considered to form part of the quinone binding cavity. In contrast to the His224 mutation, mutants G217V, P218A, and G225V almost completely lost the activity. One region of this loop is positioned close to a cytosolic loop of the NuoA subunit in the membrane domain, and together they seem to be important in keeping the quinone binding cavity intact. The structural role of the longest helix in the NuoD segment located behind the quinone binding cavity was also investigated. Possible roles of other highly conserved residues of the NuoD segment are discussed.

Synthesis and Characterization of Series of Soft-Template Agents for Mesoporous Materials

Quaternary ammonium surfactants 3a–d, with diether linkages in the backbone between hydrocarbon chains and the ammonium head groups, were synthesized as potential soft-template agents for the preparation of mesoporous materials. Varying the length of their carbon chains was expected to control the pore sizes of the corresponding mesoporous materials prepared. The critical micelle concentration (CMC) and γcmc (surface tension of the CMC) of these surfactants were measured by surface tension method.

An Efficient Synthesis of Enantiomerically Pure trans-N1,N2-dimethylcyclohexane-1,2-diamine

A new pathway is described to produce a highly optically pure isomer of trans-N1,N2-dimethylcyclohexane-1,2-diamine through four simple steps. Reaction of cyclohexene oxide with aqueous methylamine, followed by cyclisation with Mitsunobu reagent and ring-opening reactions gave rac-trans- N1,N2-dimethylcyclohexane-1,2-diamine, and the enantiomers were obtained via a kinetic resolution using tartaric acid.

Sulfonyl fluoride inhibitors of fatty acid amide hydrolase

Sulfonyl fluorides are known to inhibit esterases. Early work from our laboratory has identified hexadecyl sulfonylfluoride (AM374) as a potent in vitro and in vivo inhibitor of fatty acid amide hydrolase (FAAH). We now report on later generation sulfonyl fluoride analogs that exhibit potent and selective inhibition of FAAH. Using recombinant rat and human FAAH, we show that 5-(4-hydroxyphenyl)pentanesulfonyl fluoride (AM3506) has similar inhibitory activity for both the rat and the human enzyme, while rapid dilution assays and mass spectrometry analysis suggest that the compound is a covalent modifier for FAAH and inhibits its action in an irreversible manner. Our SAR results are highlighted by molecular docking of key analogs.

Exploring interactions between the 49 kDa and ND1 subunits in mitochondrial NADH-ubiquinone oxidoreductase (complex I) by photoaffinity labeling

Quinazolines are strong inhibitors of NADH-ubiquinone oxidoreductase (complex I) from bovine heart mitochondria. Using a photoreactive quinazoline, [(125)I]AzQ, and bovine heart submitochondrial particles (SMPs), we demonstrated previously that [(125)I]AzQ binds at the interface of the 49 kDa and ND1 subunits in complex I; it labeled a site in the N-terminal (Asp41-Arg63) region of the 49 kDa subunit, suggesting that this region contacts the ND1 subunit [Murai, M., et al. (2009) Biochemistry 48, 688-698]. The labeled region of ND1 could not be identified because it is highly hydrophobic, and the SMPs did not yield sufficient amounts of labeled protein. Here, we describe how photoaffinity labeling of isolated complex I by [(125)I]AzQ yielded sufficient material for identification of the labeled region of the ND1 subunit. The inhibition of the isolated enzyme by AzQ is comparable to that of SMPs. Our results reveal that the labeled site in ND1 is between Asp199 and Lys262, mostly likely in the third matrix loop that connects the fifth and sixth transmembrane helices. Thus, our results reveal new information about the interface between the hydrophilic and hydrophobic domains of complex I, a region that is thought to be important for ubiquinone reduction and energy transduction.

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.