Inhibition by capsaicin of NADH-quinone oxidoreductases is correlated with the presence of energy-coupling site 1 in various organisms

Capsaicinoids and Their Effects on Cancer: The "Double-Edged Sword" Postulate from the Molecular Scale

Capsaicinoids are a unique chemical species resulting from a particular biosynthesis pathway of hot chilies (Capsicum spp.) that gives rise to 22 analogous compounds, all of which are TRPV1 agonists and, therefore, responsible for the pungency of Capsicum fruits. In addition to their human consumption, numerous ethnopharmacological uses of chili have emerged throughout history. Today, more than 25 years of basic research accredit a multifaceted bioactivity mainly to capsaicin, highlighting its antitumor properties mediated by cytotoxicity and immunological adjuvancy against at least 74 varieties of cancer, while non-cancer cells tend to have greater tolerance. However, despite the progress regarding the understanding of its mechanisms of action, the benefit and safety of capsaicinoids' pharmacological use remain subjects of discussion, since CAP also promotes epithelial-mesenchymal transition, in an ambivalence that has been referred to as "the double-edge sword". Here, we update the comparative discussion of relevant reports about capsaicinoids' bioactivity in a plethora of experimental models of cancer in terms of selectivity, efficacy, and safety. Through an integration of the underlying mechanisms, as well as inherent aspects of cancer biology, we propose mechanistic models regarding the dichotomy of their effects. Finally, we discuss a selection of in vivo evidence concerning capsaicinoids' immunomodulatory properties against cancer.

N-vanillyl-octanamide represses growth of fungal phytopathogens in vitro and confers postharvest protection in tomato and avocado fruits against fungal-induced decay

Plant diseases caused by pathogenic fungi result in considerable losses in agriculture. The use of fungicides is an important alternative to combat these pathogens, but may affect both the environment and human health. Plants produce many bioactive compounds to defend themselves from biotic challenges and an increasing number of secondary metabolites have been identified, which may be used to control fungal infections. Here, the bioactivity of a synthetic capsaicinoid, N-vanillyl-octanamide, also termed ABX-I, in the growth of five phytopathogenic fungi was assessed in vitro. The compound inhibited growth of Colletotrichum gloeosporioides, Botrytis cinerea, Colletotrichum acutatum, Fusarium sp., and Rhizoctonia solani AG2, while the magnitude of this effect differed from capsaicin. To investigate if ABX-I could effectively protect crops against phytopathogens, fungal challenges were performed in tomato leaves and fruits, as well as avocado fruits co-infiltrated with Botrytis cinerea or Colletotrichum gloeosporioides, respectively. In both tomato leaves and fruits and avocado fruits, ABX-I decreased the fungal damage not only in vegetative but also in edible tissues, and diminished decay symptoms compared with untreated fruits, which were highly sensitive to the pathogens. Furthermore, ABX-I spray application to tomato or avocado plants did not compromise growth and development, whereas it repressed spore germination and growth of C. gloeosporioides, which suggests its potential as an affordable and promising resource to control fungal diseases in the agronomic sector.

Understanding the fate and impact of capsaicin in anaerobic co-digestion of food waste and waste activated sludge

Anaerobic co-digestion is an attractive option to treat food waste and waste activated sludge, which is increasingly applied in real-world situations. As an active component in Capsicum species being substantially present in food waste in many areas, capsaicin has been recently demonstrated to inhibit the anaerobic co-digestion. However, the interaction between capsaicin and anaerobic co-digestion are still poorly understood. This work therefore aims to deeply understand the fate and impact of capsaicin in the anaerobic co-digestion. Experiment results showed that capsaicin was completely degraded in anaerobic co-digestion by hydroxylation, O-demethylation, dehydrogenation and doubly oxidization, respectively. Although methane was proven to be produced from capsaicin degradation, the increase in capsaicin concentration resulted in decrease in methane yield from the anaerobic co-digestion. With an increase of capsaicin from 2 ± 0.7 to 68 ± 4 mg/g volatile solids (VS), the maximal methane yield decreased from 274.6 ± 9.7 to 188.9 ± 8.4 mL/g VS. The mechanic investigations demonstrated that the presence of capsaicin induced apoptosis, probably by either altering key kinases or decreasing the intracellular NAD⁺/NADH ratio, which led to significant inhibitions to hydrolysis, acidogenesis, and methanogenesis, especially acetotrophic methanogenesis. Illumina Miseq sequencing analysis exhibited that capsaicin promoted the populations of complex organic degradation microbes such as Escherichia-Shigella and Fonticella but decreased the numbers of anaerobes relevant to hydrolysis, acidogenesis, and methanogenesis such as Bacteroide and Methanobacterium.

Fungal Seed Pathogens of Wild Chili Peppers Possess Multiple Mechanisms To Tolerate Capsaicinoids

The wild chili pepper Capsicum chacoense produces the spicy defense compounds known as capsaicinoids, including capsaicin and dihydrocapsaicin, which are antagonistic to the growth of fungal pathogens. Compared to other microbes, fungi isolated from infected seeds of C. chacoense possess much higher levels of tolerance of these spicy compounds, having their growth slowed but not entirely inhibited. Previous research has shown capsaicinoids inhibit microbes by disrupting ATP production by binding NADH dehydrogenase in the electron transport chain (ETC) and, thus, throttling oxidative phosphorylation (OXPHOS). Capsaicinoids may also disrupt cell membranes. Here, we investigate capsaicinoid tolerance in fungal seed pathogens isolated from C. chacoense We selected 16 fungal isolates from four ascomycete genera (Alternaria, Colletotrichum, Fusarium, and Phomopsis). Using relative growth rate as a readout for tolerance, fungi were challenged with ETC inhibitors to infer whether fungi possess alternative respiratory enzymes and whether effects on the ETC fully explained inhibition by capsaicinoids. In all isolates, we found evidence for at least one alternative NADH dehydrogenase. In many isolates, we also found evidence for an alternative oxidase. These data suggest that wild-plant pathogens may be a rich source of alternative respiratory enzymes. We further demonstrate that these fungal isolates are capable of the breakdown of capsaicinoids. Finally, we determine that the OXPHOS theory may describe a weak primary mechanism by which dihydrocapsaicin, but not capsaicin, slows fungal growth. Our findings suggest that capsaicinoids likely disrupt membranes, in addition to energy poisoning, with implications for microbiology and human health.IMPORTANCE Plants make chemical compounds to protect themselves. For example, chili peppers produce the spicy compound capsaicin to inhibit pathogen damage and animal feeding. In humans, capsaicin binds to a membrane channel protein, creating the sensation of heat, while in microbes, capsaicin limits energy production by binding respiratory enzymes. However, some data suggest that capsaicin also disrupts membranes. Here, we studied fungal pathogens (Alternaria, Colletotrichum, Fusarium, and Phomopsis) isolated from a wild chili pepper, Capsicum chacoense By measuring growth rates in the presence of antibiotics with known respiratory targets, we inferred that wild-plant pathogens might be rich in alternative respiratory enzymes. A zone of clearance around the colonies, as well as liquid chromatography-mass spectrometry data, further indicated that these fungi can break down capsaicin. Finally, the total inhibitory effect of capsaicin was not fully explained by its effect on respiratory enzymes. Our findings lend credence to studies proposing that capsaicin may disrupt cell membranes, with implications for microbiology, as well as human health.

Characterization of Transplasma Membrane Electron Transport Chain in Wild and Drug-Resistant Leishmania donovani Promastigote and Amastigote

BACKGROUND

Leishmania donovani (L. donovani) is one of the parasites that cause leishmaniasis. The mechanisms by which L. donovani fights against adverse environment and becomes resistant to drugs are not well understood yet.

OBJECTIVE

The present study was designed to evaluate the effects of different regulators on the modulation of Transplasma Membrane Electron Transport (transPMET) systems of susceptible and resistant L. donovani cells.

MATERIALS AND METHODS

Effects of UV, different buffers, and electron transport inhibitors and stimulators on the reduction of α-lipoic acid (ALA), 1,2-naphthoquinone-4-sulphonic acid (NQSA) and ferricyanide were determined.

RESULTS AND DISCUSSION

ALA reductions were inhibited in susceptible, sodium antimony gluconate (SAG)-resistant and paromomycin (PMM)-resistant AG83 amastigote cells, and stimulated in susceptible and SAG-resistant AG83 promastigote cells upon UV exposure. The results indicate that UV irradiation almost oppositely affect ALA reductions in amastigotes and promastigotes. ALA reductions were stimulated in sensitive and inhibited in resistant GE1 amastigotes upon UV exposure. Susceptible amastigotes and promastigotes inhibited, and resistant amastigotes and promastigotes stimulated NQSA reduction under UV irradiation. Thus, susceptible and drug-resistant amastigotes and promastigotes are different in the reduction of ALA. Susceptible and resistant AG83 amastigotes and promastigotes inhibited the ferricyanide reductions upon UV exposure, which indicates, there is no such difference in ferricyanide reductions among susceptible as well as resistant AG83 amastigotes and promastigotes. The reductions of extracellular electron excerptors in susceptible promastigotes requires the availability of Na⁺ and Cl^- ions for maximal activity but susceptible amastigotes are mostly not dependent on the availability of Na⁺ and Cl^- ions. Both in promastigotes and amastigotes, reductions of electron acceptors were strongly inhibited by carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone. Furthermore, antimycin A, rotenone and capsaicin markedly inhibited the reductions of electron acceptors in promastigotes, but not in amastigotes.

CONCLUSION

Results of this study suggest that the transPMET system is functionally different in wild and resistant strains of L. donovani.

An improved cell-permeable fluorogenic substrate as the basis for a highly sensitive test for NAD(P)H quinone oxidoreductase 1 (NQO1) in living cells

NAD(P)H:quinone oxidoreductase 1 (NQO1) is a flavoenzyme upregulated in response to oxidative stress and in some cancers. Its upregulation by compounds has been used as an indicator of their potential anti-cancer properties. In this study we have designed, produced and tested a fluorogenic coumarin conjugate which selectively releases highly fluorescent 4-methylumbelliferone (4-MU) in the presence of NQO1. It was found that measuring 4-MU release rapidly and specifically quantitated NQO1 levels in vitro and in live cells. Both the substrate and its products freely perfused through cell membranes and were non-toxic. The substrate was very specific with low background, and the assay itself could be done in less than 10minutes. This is the first assay to allow the quantitation of NQO1 in live cells which can then be retained for further experiments.

Activation of endogenous TRPV1 fails to induce overstimulation-based cytotoxicity in breast and prostate cancer cells but not in pain-sensing neurons

Vanilloids including capsaicin and resiniferatoxin are potent transient receptor potential vanilloid type 1 (TRPV1) agonists. TRPV1 overstimulation selectively ablates capsaicin-sensitive sensory neurons in animal models in vivo. The cytotoxic mechanisms are based on strong Na(+) and Ca(2+) influx via TRPV1 channels, which leads to mitochondrial Ca(2+) accumulation and necrotic cell swelling. Increased TRPV1 expression levels are also observed in breast and prostate cancer and derived cell lines. Here, we examined whether potent agonist-induced overstimulation mediated by TRPV1 might represent a means for the eradication of prostate carcinoma (PC-3, Du 145, LNCaP) and breast cancer (MCF7, MDA-MB-231, BT-474) cells in vitro. While rat sensory neurons were highly vanilloid-sensitive, normal rat prostate epithelial cells were resistant in vivo. We found TRPV1 to be expressed in all cancer cell lines at mRNA and protein levels, yet protein expression levels were significantly lower compared to sensory neurons. Treatment of all human carcinoma cell lines with capsaicin didn't lead to overstimulation cytotoxicity in vitro. We assume that the low vanilloid-sensitivity of prostate and breast cancer cells is associated with low expression levels of TRPV1, since ectopic TRPV1 expression rendered them susceptible to the cytotoxic effect of vanilloids evidenced by plateau-type Ca(2+) signals, mitochondrial Ca(2+) accumulation and Na(+)- and Ca(2+)-dependent membrane disorganization. Moreover, long-term monitoring revealed that merely the ectopic expression of TRPV1 stopped cell proliferation and often induced apoptotic processes via strong activation of caspase-3 activity. Our results indicate that specific targeting of TRPV1 function remains a putative strategy for cancer treatment.

Energy transducing roles of antiporter-like subunits in Escherichia coli NDH-1 with main focus on subunit NuoN (ND2)

The proton-translocating NADH-quinone oxidoreductase (complex I/NDH-1) contains a peripheral and a membrane domain. Three antiporter-like subunits in the membrane domain, NuoL, NuoM, and NuoN (ND5, ND4 and ND2, respectively), are structurally similar. We analyzed the role of NuoN in Escherichia coli NDH-1. The lysine residue at position 395 in NuoN (NLys(395)) is conserved in NuoL (LLys(399)) but is replaced by glutamic acid (MGlu(407)) in NuoM. Our mutation study on NLys(395) suggests that this residue participates in the proton translocation. Furthermore, we found that MGlu(407) is also essential and most likely interacts with conserved LArg(175). Glutamic acids, NGlu(133), MGlu(144), and LGlu(144), are corresponding residues. Unlike mutants of MGlu(144) and LGlu(144), mutation of NGlu(133) scarcely affected the energy-transducing activities. However, a double mutant of NGlu(133) and nearby KGlu(72) showed significant inhibition of these activities. This suggests that NGlu(133) bears a functional role similar to LGlu(144) and MGlu(144) but its mutation can be partially compensated by the nearby carboxyl residue. Conserved prolines located at loops of discontinuous transmembrane helices of NuoL, NuoM, and NuoN were shown to play a similar role in the energy-transducing activity. It seems likely that NuoL, NuoM, and NuoN pump protons by a similar mechanism. Our data also revealed that NLys(158) is one of the key interaction points with helix HL in NuoL. A truncation study indicated that the C-terminal amphipathic segments of NTM14 interacts with the Mβ sheet located on the opposite side of helix HL. Taken together, the mechanism of H(+) translocation in NDH-1 is discussed.

Roles of subunit NuoK (ND4L) in the energy-transducing mechanism of Escherichia coli NDH-1 (NADH:quinone oxidoreductase)

The bacterial H(+)-translocating NADH:quinone oxidoreductase (NDH-1) catalyzes electron transfer from NADH to quinone coupled with proton pumping across the cytoplasmic membrane. The NuoK subunit (counterpart of the mitochondrial ND4L subunit) is one of the seven hydrophobic subunits in the membrane domain and bears three transmembrane segments (TM1-3). Two glutamic residues located in the adjacent transmembrane helices of NuoK are important for the energy coupled activity of NDH-1. In particular, mutation of the highly conserved carboxyl residue ((K)Glu-36 in TM2) to Ala led to a complete loss of the NDH-1 activities. Mutation of the second conserved carboxyl residue ((K)Glu-72 in TM3) moderately reduced the activities. To clarify the contribution of NuoK to the mechanism of proton translocation, we relocated these two conserved residues. When we shifted (K)Glu-36 along TM2 to positions 32, 38, 39, and 40, the mutants largely retained energy transducing NDH-1 activities. According to the recent structural information, these positions are located in the vicinity of (K)Glu-36, present in the same helix phase, in an immediately before and after helix turn. In an earlier study, a double mutation of two arginine residues located in a short cytoplasmic loop between TM1 and TM2 (loop-1) showed a drastic effect on energy transducing activities. Therefore, the importance of this cytosolic loop of NuoK ((K)Arg-25, (K)Arg-26, and (K)Asn-27) for the energy transducing activities was extensively studied. The probable roles of subunit NuoK in the energy transducing mechanism of NDH-1 are discussed.

Mechanism of capsaicin receptor TRPV1-mediated toxicity in pain-sensing neurons focusing on the effects of Na(+)/Ca(2+) fluxes and the Ca(2+)-binding protein calretinin

Transient receptor potential vanilloid subtype 1 (TRPV1) receptor is a pain-sensing, ligand-gated, non-selective cation channel expressed in peripheral sensory neurons. Prolonged activation of TRPV1 by capsaicin leads to cell swelling and formation of membrane blebs in rat dorsal root ganglion (DRG) neurons. Similar results were obtained in NIH3T3 fibroblast cells stably expressing TRPV1. Here, we assessed the contribution of Ca(2+) and Na(+) ions to TRPV1-mediated changes. Cell swelling was caused by a substantial influx of extracellular Na(+) via TRPV1 channels, causing concomitant transport of water. In the absence of extracellular Na(+), the membrane blebbing was completely inhibited, but Ca(2+) influx did not change under these conditions. Na(+) influx was modulated by the intracellular Ca(2+) concentration ([Ca(2+)]i). Elevation of [Ca(2+)]i by ionomycin sensitized/activated TRPV1 channels causing cell swelling in TRPV1-positive cells. In the absence of extracellular Ca(2+), capsaicin caused only little increase in [Ca(2+)]i indicating that the increase in [Ca(2+)]i observed after capsaicin application is derived essentially from extracellular Ca(2+) and not from internal Ca(2+) stores. In the absence of extracellular Ca(2+) also the process of cell swelling was considerably slower. Calretinin is a Ca(2+) buffer protein, which is expressed in a subset of TRPV1-positive neurons. Calretinin decreased the amplitude, but slowed down the decay of Ca(2+) signals evoked by ionomycin. Cells co-expressing TRPV1 and calretinin were less sensitive to TRPV1-mediated, capsaicin-induced volume increases. In TRPV1-expressing NIH3T3 cells, calretinin decreased the capsaicin-induced Ca(2+) and Na(+) influx. Swelling and formation of membrane blebs resulted in impaired plasma membrane integrity finally leading to cell death. Our results hint towards a mechanistic explanation for the apoptosis-independent capsaicin-evoked neuronal loss and additionally reveal a protective effect of calretinin; we propose that the Ca(2+)-buffering capacity of calretinin reduces the susceptibility of calretinin-expressing DRG neurons against cell swelling/death caused by overstimulation of TRPV1 channels. This article is part of a Special Issue entitled:12th European Symposium on Calcium.

Paracoccus denitrificans proton-translocating ATPase: kinetics of oxidative phosphorylation

The initial rates of ATP synthesis catalyzed by tightly coupled Paracoccus denitrificans plasma membrane were measured. The reaction rate was hyperbolically dependent on the substrates, ADP and inorganic phosphate (P(i)). Apparent K(m) values for ADP and P(i) were 7-11 and 60-120 µM, respectively, at saturating concentration of the second substrate (pH 8.0, saturating Mg²(+)). These values were dependent on coupling efficiency. The substrate binding in the ATP synthesis reaction proceeds randomly: K(m) value for a given substrate was independent of the concentration of the other one. A decrease of electrochemical proton gradient by the addition of malonate (when succinate served as the respiratory substrate) or by a decrease of steady-state level of NADH (when NADH served as the respiratory substrate) resulted in a proportional decrease of the maximal rates and apparent K(m) values for ADP and P(i) (double substitution, ping-pong mechanism). The kinetic scheme for ATP synthesis was compared with that described previously for the proton-translocating ATP hydrolysis catalyzed by the same enzyme preparation (T. V. Zharova and A. D. Vinogradov (2006) Biochemistry, 45, 14552-14558).

Kinetics and regulation of mammalian NADH-ubiquinone oxidoreductase (Complex I)

NADH-ubiquinone oxidoreductase (Complex I, European Commission No. 1.6.5.3) is one of the respiratory complexes that generate the proton-motive force required for the synthesis of ATP in mitochondria. The catalytic mechanism of Complex I has not been well understood, due to the complicated structure of this enzyme. Here, we develop a kinetic model for Complex I that accounts for electron transfer from NADH to ubiquinone through protein-bound prosthetic groups, which is coupled to the translocation of protons across the inner mitochondrial membrane. The model is derived based on the tri-bi enzyme mechanism combined with a simple model of the conformational changes associated with proton transport. To study the catalytic mechanism, parameter values are estimated by analyzing kinetic data. The model is further validated by independent data sets from additional experiments, effectively explaining the effect of pH on enzyme activity. Results imply that matrix pH significantly affects the enzyme turnover processes. The overall kinetic analysis demonstrates a hybrid ping-pong rapid-equilibrium random bi-bi mechanism, consolidating the characteristics from previously reported kinetic mechanisms and data.

Interdisciplinary review for correlation between the plant origin capsaicinoids, non-steroidal antiinflammatory drugs, gastrointestinal mucosal damage and prevention in animals and human beings

BACKGROUND

The plant origin capsaicinoids (capsaicin, dihydrocapsaicin, norcapsaicin, dihydrocapsaicin, homocapsaicin, homodihydrocapsaicin) are well known and used as nutritional additive agents in the every day nutritional practice from the last 9,500 years; however, we had have a very little scientifically based knowledge on their chemistry, physiology and pharmacology in animal observations, and in humans up to the mid-twentieth century. Our knowledge about their chemistry, physiology, pharmacology entered to be scientifically based evidence from the year 1980, dominantly in animal observations. The human observations with capsaicin (capsaicinoids), in terms of good clinical practice, have been started only in the last 10-year period (from 1997) in randomized, prospective, multiclinical studies. The name of "capsaicin" used only in the physiological and pharmacological research both in animal experiments and in human observation. The "capsaicin" (as a "chemically" used natural compound) modifies the so-called capsaicin-sensitive afferent nerves depending on their applied doses.

AIMS

The specific action of capsaicin (capsaicinoids) on sensory afferent nerves modifying gastrointestinal (GI) function (under very specific conditions) offers a possibility for the production of an orally applicable drug or for other drug combinations, which can be used in the human medical therapy. The production of new drug is based on the critical interdisciplinary review of the results obtained with capsaicinoids.

MATERIALS AND METHODS

This paper gives an interdisciplinary and critical overview on the chemical, physiological, pharmacological and toxicological actions of the natural origin capsaicinoids (from the point of drug production) under conditions of acute, subacute and chronic administration in animal experiments and human observations, toxicology, pharmacokinetics). This interdisciplinary review covers the following main chapters: (1) physiological and pharmacological research tool by capsaicin in the animals and human beings, (2) capsaicin research in animals (including the acute, subacute toxicology and chronic toxicology metabolism, genotoxicology), (3) capsaicin observation with capsaicin in human beings.

CONCLUSION

(1) The capsaicin used in the physiological and pharmacological observations (in animals and human beings) chemically represents different chemical compounds, which can be obtained from the plants (paprika, chilli, etc.), (2) capsaicinoids are able to modify the capsaicin-sensitive afferent nerves, which have principle roles in the defence of different organs (including the gastrointestinal tract [against the different chemicals, heat, strech, chemical millieu-induced damage], (3) the application of capsaicin (capsaicinoids) can be repeated for the beneficial effects on the gastrointestinal tract as those in animal experiments. After this interdisciplinary and critical review, this paper demonstrates the well-planned research pathways of the discoveries of capsaicinoids from plant chemistry, via physiology, pharmacology and toxicology in animal experiments and human observations.

Critical roles of subunit NuoH (ND1) in the assembly of peripheral subunits with the membrane domain of Escherichia coli NDH-1

The bacterial proton-translocating NADH:quinone oxidoreductase (NDH-1) consists of two domains, a peripheral arm and a membrane arm. NuoH is a counterpart of ND1, which is one of seven mitochondrially encoded hydrophobic subunits, and is considered to be involved in quinone/inhibitor binding. Sequence comparison in a wide range of species showed that NuoH is comprehensively conserved, particularly with charged residues in the cytoplasmic side loops. We have constructed 40 mutants of 27 conserved residues predicted to be in the cytoplasmic side loops of Escherichia coli NuoH by utilizing the chromosomal DNA manipulation technique and investigated roles of these residues. Mutants of Arg(37), Arg(46), Asp(63), Gly(134), Gly(145), Arg(148), Glu(220), and Glu(228) showed low deamino-NADH-K(3)Fe(CN)(6) reductase activity, undetectable NDH-1 in Blue Native gels, low contents of peripheral subunits (especially NuoB and NuoCD) bound to the membranes, and a significant loss of the membrane potential and proton-pumping function coupled to deamino-NADH oxidation. The results indicated that these conserved residues located in the cytoplasmic side loops are essential for the assembly of the peripheral subunits with the membrane arm. Implications for the involvement of NuoH (ND1) in maintaining the structure and function of NDH-1 are discussed.

A comparative proteomic analysis for capsaicin-induced apoptosis between human hepatocarcinoma (HepG2) and human neuroblastoma (SK-N-SH) cells

The endogenous ROS levels were increased during HepG2 apoptosis, whereas they were decreased during SK-N-SH apoptosis in response to capsaicin treatments. We used 2-DE-based proteomics to analyze the altered protein levels in both cells, with special attention on oxidative stress proteins before and after capsaicin treatments. The 2-DE analysis demonstrated that 23 proteins were increased and 26 proteins were decreased significantly (fold change>1.4) in capsaicin-treated apoptotic HepG2 and SK-N-SH cells, respectively. The distinct effect of capsaicin-induced apoptosis on the expression pattern of HepG2 proteins includes the downregulation of some antioxidant enzymes including aldose reductase (AR), catalase, enolase 1, peroxiredoxin 1, but upregulation of peroxiredoxin 6, cytochrome c oxidase, and SOD2. In contrast, most antioxidant enzymes were increased in SK-N-SH cells in response to capsaicin, where catalase might play a pivotal role in maintenance of low ROS levels in the course of apoptosis. The global gene expression for oxidative stress and antioxidant defense genes revealed that 84 gene expressions were not significantly different in HepG2 cells between control and capsaicin-treated cells. In contrast, a number of oxidative genes were downregulated in SK-N-SH cells, supporting the evidence of low ROS environment in apoptotic SK-N-SH cells after capsaicin treatment. It was concluded that the different relationship between endogenous ROS levels and apoptosis of two cancer cells presumably resulted from complicated expression patterns of many oxidative stress and antioxidant genes, rather than the individual role of some classical antioxidant enzymes such as SOD and catalase.

Differential effects of mitochondrial Complex I inhibitors on production of reactive oxygen species

We have investigated the production of reactive oxygen species (ROS) by Complex I in isolated open bovine heart submitochondrial membrane fragments during forward electron transfer in presence of NADH, by means of the probe 2',7'-Dichlorodihydrofluorescein diacetate. ROS production by Complex I is strictly related to its inhibited state. Our results indicate that different Complex I inhibitors can be grouped into two classes: Class A inhibitors (Rotenone, Piericidin A and Rolliniastatin 1 and 2) increase ROS production; Class B inhibitors (Stigmatellin, Mucidin, Capsaicin and Coenzyme Q(2)) prevent ROS production also in the presence of Class A inhibitors. Addition of the hydrophilic Coenzyme Q(1) as an electron acceptor potentiates the effect of Rotenone-like inhibitors in increasing ROS production, but has no effect in the presence of Stigmatellin-like inhibitors; the effect is not shared by more hydrophobic quinones such as decyl-ubiquinone. This behaviour relates the prooxidant CoQ(1) activity to a hydrophilic electron escape site. Moreover the two classes of Complex I inhibitors have an opposite effect on the increase of NADH-DCIP reduction induced by short chain quinones: only Class B inhibitors allow this increase, indicating the presence of a Rotenone-sensitive but Stigmatellin-insensitive semiquinone species in the active site of the enzyme. The presence of this semiquinone was also suggested by preliminary EPR data. The results suggest that electron transfer from the iron-sulphur clusters (N2) to Coenzyme Q occurs in two steps gated by two different conformations, the former being sensitive to Rotenone and the latter to Stigmatellin.

Mitochondrial production of reactive oxygen species: role of complex I and quinone analogues

Mitochondrial reactive oxygen species (ROS) are mainly produced by the respiratory chain enzymes. The sites for ROS production in mitochondrial respiratory chain are normally ascribed to the activity of Complex I and III. The presence of specific inhibitors modulates reactive oxygen species production in Complex I: inhibitors such as rotenone induce a strong ROS increase, while inhibitors such as stigmatellin prevent it. We have investigated the effect of hydrophilic quinones on Complex I ROS production in presence of different inhibitors. Some short chain quinones are Complex I inhibitors (CoQ2, idebenone and its derivatives), while CoQ1, decylubiquinone~ (DB) and duroquinone (DQ) are good electron acceptors from Complex I. Our results show that the ability of short chain quinones to induce an oxidative stress depends on the site of interaction with Complex I and on their physical-chemical characteristics. We can conclude that hydrophilic quinones may enhance oxidative stress by interaction with the electron escape sites on Complex I while more hydrophobic quinones can be reduced only at the physiological quinone reducing site without reacting with molecular oxygen.

Capsaicin induces heme oxygenase-1 expression in HepG2 cells via activation of PI3K-Nrf2 signaling: NAD(P)H:quinone oxidoreductase as a potential target

Capsaicin (trans-8-methyl-N-vanillyl-6-nonenamide), a major pungent ingredient of red pepper, is reported to have antimutagenic and anticarcinogenic properties. However, the mechanisms underlying its chemoprotective effects remain largely unresolved. In the present study, we found that capsaicin induced expression of heme oxygenase-1 (HO-1) in HepG2 cells. Capsaicin treatment resulted in a transient increase in the phosphorylation of Akt and subsequently nuclear translocation of NF-E2-related factor 2 (Nrf2), enhancing its binding to antioxidant response element (ARE). HepG2 cells treated with capsaicin exhibited increased production of reactive oxygen species (ROS). Prior exposure of cells to N-acetyl-L -cysteine blocked not only the ROS production but also the nuclear translocation of Nrf2 and its ARE binding, as well as HO-1 induction by capsaicin. Immunoblot analysis showed that whereas the level of HO-1 protein was elevated, that of NAD(P)H:quinone oxidoreductase (NQO1) was decreased after the treatment with capsaicin or the inhibitor of NQO1, dicumarol. We hypothesize that quinone metabolites or other reactive forms of capsaicin may bind covalently to NQO1 and thereby inhibit its activity, leading to production of ROS. This, in turn, would trigger the activation of Akt via phosphorylation, increase the nuclear translocation and ARE binding of Nrf2, and upregulate the expression of HO-1.

HQNO-sensitive NADH:quinone oxidoreductase of Bacillus cereus KCTC 3674

The enzymatic properties of NADH:quinone oxidoreductase were examined in Triton X-100 extracts of Bacillus cereus membranes by using the artificial electron acceptors ubiquinone-1 and menadione. Membranes were prepared from B. cereus KCTC 3674 grown aerobically on a complex medium and oxidized with NADH exclusively, whereas deamino-NADH was determined to be poorly oxidized. The NADH oxidase activity was lost completely by solubilization of the membranes with Triton X-100. However, by using the artificial electron acceptors ubiquinone-1 and menadione, NADH oxidation could be observed. The activities of NADH:ubiquinone-1 and NADH:menadione oxidoreductase were enhanced approximately 8-fold and 4-fold, respectively, from the Triton X-100 extracted membranes. The maximum activity of FAD-dependent NADH:ubiquinone-1 oxidoreductase was obtained at about pH 6.0 in the presence of 0.1M NaCl, while the maximum activity of FAD-dependent NADH:menadione oxidoreductase was obtained at about pH 8.0 in the presence of 0.1 M NaCl. The activities of the NADH:ubiquinone-1 and NADH:menadione oxidoreductase were very resistant to such respiratory chain inhibitors as rotenone, capsaicin, and AgNO(3), whereas these activities were sensitive to 2-heptyl-4-hydroxyquinoline-N-oxide (HQNO). Based on these results, we suggest that the aerobic respiratory chain-linked NADH oxidase system of B. cereus KCTC 3674 possesses an HQNO-sensitive NADH:quinone oxidoreductase that lacks an energy coupling site containing FAD as a cofactor.

Respiratory system of Gluconacetobacter diazotrophicus PAL5 Evidence for a cyanide-sensitive cytochrome bb and cyanide-resistant cytochrome ba quinol oxidases

In highly aerobic environments, Gluconacetobacter diazotrophicus uses a respiratory protection mechanism to preserve nitrogenase activity from deleterious oxygen. Here, the respiratory system was examined in order to ascertain the nature of the respiratory components, mainly of the cyanide sensitive and resistant pathways. The membranes of G. diazotrophicus contain Q(10), Q(9) and PQQ in a 13:1:6.6 molar ratios. UV(360 nm) photoinactivation indicated that ubiquinone is the electron acceptor for the dehydrogenases of the outer and inner faces of the membrane. Strong inhibition by rotenone and capsaicin and resistance to flavone indicated that NADH-quinone oxidoreductase is a NDH-1 type enzyme. KCN-titration revealed the presence of at least two terminal oxidases that were highly sensitive and resistant to the inhibitor. Tetrachorohydroquinol was preferentially oxidized by the KCN-sensitive oxidase. Neither the quinoprotein alcohol dehydrogenase nor its associated cytochromes c were instrumental components of the cyanide resistant pathway. CO-difference spectrum and photodissociation of heme-CO compounds suggested the presence of cytochromes b-CO and a(1)-CO adducts. Air-oxidation of cytochrome b (432 nm) was arrested by concentrations of KCN lower than 25 microM while cytochrome a(1) (442 nm) was not affected. A KCN-sensitive (I(50)=5 microM) cytochrome bb and a KCN-resistant (I(50)=450 microM) cytochrome ba quinol oxidases were separated by ion exchange chromatography.

Preliminary evidence on existence of transplasma membrane electron transport in Entamoeba histolytica trophozoites: a key mechanism for maintaining optimal redox balance

Entamoeba histolytica, an amitochondriate parasitic protist, was demonstrated to be capable of reducing the oxidized form of alpha-lipoic acid, a non permeable electron acceptor outside the plasma membrane. This transmembrane reduction of non permeable electron acceptors with redox potentials ranging from -290 mV to +360 mV takes place at neutral pH. The transmembrane reduction of non permeable electron acceptors was not inhibited by mitochondrial electron transport inhibitors such as antimycin A, rotenone, cyanide and azide. However, a clear inhibition with complex III inhibitor, 2-(n-heptyl)-4-hydroxyquinoline-N-oxide; modifiers of sulphydryl groups and inhibitors of glycolysis was revealed. The iron-sulphur centre inhibitor thenoyltrifluoroacetone failed to inhibit the reduction of non permeable electron acceptors whereas capsaicin, an inhibitor of energy coupling NADH oxidase, showed substantial inhibition. p-trifluromethoxychlorophenylhydrazone, a protonophore uncoupler, resulted in the stimulation of alpha-lipoic acid reduction but inhibition in oxygen uptake. Mitochondrial electron transport inhibitors substantially inhibited the oxygen uptake in E. histolytica. Transmembrane reduction of alpha-lipoic acid was strongly stimulated by anaerobiosis and anaerobic stimulation was inhibited by 2-(n-heptyl)-4-hydroxyquinoline-N-oxide. Transmembrane redox system of E. histolytica was also found to be sensitive to UV irradiation. All these findings clearly demonstrate the existence of transplasma membrane electron transport system in E. histolytica and possible involvment of a naphthoquinone coenzyme in transmembrane redox of E. histolytica which is different from that of mammalian host and therefore can provide a novel target for future rational chemotherapeutic drug designing.

Cis-unsaturated fatty acids stimulate reactive oxygen species generation and lipid peroxidation in human spermatozoa

CONTEXT

Defective sperm function is the largest defined cause of human infertility; however, the etiology of this condition is poorly understood. Although oxidative stress is acknowledged as a key contributor to this pathology, there are also data indicating that defective human spermatozoa contain abnormally high amounts of cis-unsaturated fatty acids. This study investigated whether a causative relationship exists between these two attributes of impaired semen quality.

OBJECTIVE

The objective of this study was to determine whether polyunsaturated fatty acids can induce oxidative stress in human spermatozoa.

METHOD

Dihydroethidium and SYTOX Green were used in conjunction with flow cytometry and HPLC to investigate reactive oxygen species (ROS) generation by human spermatozoa after fatty acid exposure.

RESULTS

Arachidonic acid (AA) induced a time- and dose-dependent increase in ROS generation by human spermatozoa that led to the promotion of peroxidative damage and a loss of sperm motility. This effect could not be blocked with inhibitors of the cyclooxygenase or lipoxygenase pathways of AA metabolism, rotenone, protein kinase C antagonists, or known inhibitors of plasma membrane redox systems. However, ROS generation could be triggered with other cis-unsaturated fatty acids including linoleic and docosahexaenoic acids. Saturated fatty acids, methyl esters of unsaturated fatty acids, or other amphiphiles were all ineffective. However in a cell-free system, AA could trigger a redox signal via mechanisms that were profoundly disrupted by diphenylene iodonium, a flavoprotein inhibitor.

CONCLUSIONS

The presence of excess unsaturated fatty acids in defective human spermatozoa may precipitate the oxidative stress encountered in male infertility.

Construction of a deletion strain and expression vector for the Escherichia coli NADH:ubiquinone oxidoreductase (Complex I)

Complex I of Escherichia coli is encoded by 13 consecutive genes, called the nuo operon. A chromosomal deletion of all nuo genes has been achieved by homologous recombination. A vector that encodes all of the nuo genes has been constructed, and it expresses a functional enzyme.

Characterization of the redox components of transplasma membrane electron transport system from Leishmania donovani promastigotes

An investigation has been made of the points of coupling of four nonpermeable electron acceptors e.g., alpha-lipoic acid (ALA), 5,5'-dithiobis (2-nitroaniline-N-sulphonic acid) (DTNS), 1,2-naphthoquinone-4-sulphonic acid (NQSA) and ferricyanide which are mainly reduced via an interaction with the redox sites present in the plasma membrane of Leishmania donovani promastigotes. ALA, DTNS, NQSA and ferricyanide reduction and part of O2 reduction is shown to take place on the exoplasmic face of the cell, for it is affected by external pH and agents that react with the external surface. Redox enzymes of the transplasma membrane electron transport system orderly transfer electron from one redox carrier to the next with the molecular oxygen as the final electron acceptor. The redox carriers mediate the transfer of electrons from metabolically generated reductant to nonpermeable electron acceptors and oxygen. At a pH of 6.4, respiration of Leishmania cells on glucose substrate shut down almost completely upon addition of an uncoupler FCCP and K+-ionophore valinomycin. The most pronounced effects on O2 uptake were obtained by treatment with antimycin A, 2-heptadecyl-4-hydroxyquinone-N-oxide, paracholoromercuribenzene sulphonic acid and trifluoperazine. Relatively smaller effects were obtained by treatment with potassium cyanide. Inhibition observed with respect to the reduction of the electron acceptors ALA, DTNS, NQSA and ferricyanide was not similar in most cases. The redox chain appears to be branched at several points and it is suggested that this redox chain incorporate iron-sulphur center, b-cytochromes, cyanide insensitive oxygen redox site, Na+ and K+ channel, capsaicin inhibited energy coupling site and trifluoperazine inhibited energy linked P-type ATPase. We analyzed the influence of ionic composition of the medium on reduction of electron acceptors in Leishmania donovani promastigotes. Our data suggest that K+ have some role for ALA reduction and Na+ for ferricyanide reduction. No significant effects were found with DTNS and NQSA reduction when Na+ or K+ was omitted from the medium. Stimulation of ALA, DTNS, NQSA and ferricyanide reduction was obtained by omitting Cl- from the medium. We propose that this redox system may be an energy source for control of membrane function in Leishmania cells.

Enzymatic properties of the membrane-bound NADH oxidase system in the aerobic respiratory chain of Bacillus cereus

Membranes prepared from Bacillus cereus KCTC 3674, grown aerobically on a complex medium, oxidized NADH exclusively, whereas deamino-NADH was little oxidized. The respiratory chain-linked NADH oxidase exhibited an apparent K(m) value of approximately 65 microM for NADH. The maximum activity of the NADH oxidase was obtained at about pH 8.5 in the presence of 0.1 M KCl (or NaCl). Respiratory chain inhibitor 2-heptyl-4-hydroxyquinoline-N-oxide (HQNO) inhibited the activity of the NADH oxidase by about 90% at a concentration of 40 microM. Interestingly, rotenone and capsaicin inhibited the activity of the NADH oxidase by about 60% at a concentration of 40 microM and the activity was also highly sensitive to Ag(+).

Mitochondria: A novel target for the chemoprevention of cancer

The mitochondria have emerged as a novel target for anticancer chemotherapy. This tenet is based on the observations that several conventional and experimental chemotherapeutic agents promote the permeabilization of mitochondrial membranes in cancerous cells to initiate the release of apoptogenic mitochondrial proteins. This ability to engage mitochondrial-mediated apoptosis directly using chemotherapy may be responsible for overcoming aberrant apoptosis regulatory mechanisms commonly encountered in cancerous cells. Interestingly, several putative cancer chemopreventive agents also possess the ability to trigger apoptosis in transformed, premalignant, or malignant cells in vitro via mitochondrial membrane permeabilization. This process may occur through the regulation of Bcl-2 family members, or by the induction of the mitochondrial permeability transition. Thus, by exploiting endogenous mitochondrial-mediated apoptosis-inducing mechanisms, certain chemopreventive agents may be able to block the progression of premalignant cells to malignant cells or the dissemination of malignant cells to distant organ sites as means of modulating carcinogenesis in vivo. This review will examine cancer chemoprevention with respect to apoptosis, carcinogenesis, and the proapoptotic activity of various chemopreventive agents observed in vitro. In doing so, I will construct a paradigm supporting the notion that the mitochondria are a novel target for the chemoprevention of cancer.

Effects of new ubiquinone-imidazo[2,1-b]thiazoles on mitochondrial complex I (NADH-ubiquinone reductase) and on mitochondrial permeability transition pore

In this work we describe the synthesis of a series of imidazo[2,1-b]thiazoles and 2,3-dihydroimidazo[2,1-b]thiazoles connected by means of a methylene bridge to CoQ(0). These compounds were tested as specific inhibitors of the NADH:ubiquinone reductase activity in mitochondrial membranes. The imidazothiazole system when bound to the quinone ring in place of the isoprenoid lateral side chain, may increase the inhibitory effect (with an IC(50) for NADH-Q(1) activity ranging between 0.25 and 0.96 microM) whereas the benzoquinone moiety seems to lose the capability to accept electrons from complex I as indicated by very low maximal velocity elicited by the compounds tested. Moreover the low rotenone sensitivity for almost all of these compounds suggests that they are only partially able to interact with the physiological ubiquinone-reduction site. The compounds were investigated for the capability of increasing the permeability transition of the inner mitochondrial membrane in isolated mitochondria. Unlike CoQ(0), which is considered a mitochondrial membrane permeability transition inhibitor, the new compounds were inducers.

Cell surface oxygen consumption by mitochondrial gene knockout cells

Mitochondrial gene knockout (rho(0)) cells that depend on glycolysis for their energy requirements show an increased ability to reduce cell-impermeable tetrazolium dyes by electron transport across the plasma membrane. In this report, we show for the first time, that oxygen functions as a terminal electron acceptor for trans-plasma membrane electron transport (tPMET) in HL60rho(0) cells, and that this cell surface oxygen consumption is associated with oxygen-dependent cell growth in the absence of mitochondrial electron transport function. Non-mitochondrial oxygen consumption by HL60rho(0) cells was extensively inhibited by extracellular NADH and NADPH, but not by NAD(+), localizing this process at the cell surface. Mitochondrial electron transport inhibitors and the uncoupler, FCCP, did not affect oxygen consumption by HL60rho(0) cells. Inhibitors of glucose uptake and glycolysis, the ubiquinone redox cycle inhibitors, capsaicin and resiniferatoxin, the flavin centre inhibitor, diphenyleneiodonium, and the NQO1 inhibitor, dicoumarol, all inhibited oxygen consumption by HL60rho(0) cells. Similarities in inhibition profiles between non-mitochondrial oxygen consumption and reduction of the cell-impermeable tetrazolium dye, WST-1, suggest that both systems may share a common tPMET pathway. This is supported by the finding that terminal electron acceptors from both pathways compete for electrons from intracellular NADH.

Role of vanilloid receptors in the capsaicin-mediated induction of iNOS in PC12 cells

The vanilloid receptor 1(VR1) is a nonselective cation channel that is activated by pungent vanilloid compound, extracellular protons, or noxious heat. mRNA of VR1 and vanilloid receptor 1-like receptor (VRL1) were expressed in PC12 cells, and only VRI mRNA was detected in glioma and A10 cell lines. VRI protein was demonstrated in PC12 cells by immunocytochemistry and Western blotting. Capsaicin (CPS), the VRI receptor agonist, led to an increase in intracellular calcium ion, and this effect was blocked by pretreatment with VR1 receptor antagonist capsazepin (CPZ). Treatment of PC12 cells with low concentration of CPS (5-50 microM) increased reactive oxygen species (ROS) production, and inducible nitric oxide synthase (iNOS) was expressed after CPS treatment for 24 h. These CPS-induced changes are inhibited by pretreatment of CPZ. These findings suggest that CPS-induced iNOS expression through the VR1 and/or VRL1-mediated pathway, and this may explain the CPS-mediated physiological and pathological effects in neuron system.

Reactive oxygen species produced by NAD(P)H oxidase inhibit apoptosis in pancreatic cancer cells

One reason why pancreatic cancer is so aggressive and unresponsive to treatments is its resistance to apoptosis. We report here that reactive oxygen species (ROS) are a prosurvival, antiapoptotic factor in pancreatic cancer cells. Human pancreatic adenocarcinoma MIA PaCa-2 and PANC-1 cells generated ROS, which was stimulated by growth factors (serum, insulin-like growth factor I, or fibroblast growth factor-2). Growth factors also stimulated membrane NAD(P)H oxidase activity in these cells. Both intracellular ROS and NAD(P)H oxidase activity were inhibited by antioxidants tiron and N-acetylcysteine and the inhibitor of flavoprotein-dependent oxidases, diphenylene iodonium, but not by inhibitors of various other ROS-generating enzymes. Using Rho(0) cells deficient in mitochondrial DNA, we showed that a nonmitochondrial NAD(P)H oxidase is a major source of growth factor-induced ROS in pancreatic cancer cells. Among proteins that have been implicated in NAD(P)H oxidase activity, MIA PaCa-2 and PANC-1 cells do not express the phagocytic gp91(phox) subunit but express several nonphagocytic oxidase (NOX) isoforms. Transfection with Nox4 antisense oligonucleotide inhibited NAD(P)H oxidase activity and ROS production in MIA PaCa-2 and PANC-1 cells. Inhibiting ROS with the antioxidants, Nox4 antisense, or MnSOD overexpression all stimulated apoptosis in pancreatic cancer cells as measured by internucleosomal DNA fragmentation, phosphatidylserine externalization, cytochrome c release, and effector caspase activation. The results show that growth factor-induced ROS produced by NAD(P)H oxidase (probably Nox4) protect pancreatic cancer cells from apoptosis. This mechanism may play an important role in pancreatic cancer resistance to treatment and thus represent a novel therapeutic target.

Amiloride inhibition of the proton-translocating NADH-quinone oxidoreductase of mammals and bacteria

The proton-translocating NADH-quinone oxidoreductase in mitochondria (complex I) and bacteria (NDH-1) was shown to be inhibited by amiloride derivatives that are known as specific inhibitors for Na(+)/H(+) exchangers. In bovine submitochondrial particles, the effective concentrations were about the same as those for the Na(+)/H(+) exchangers, whereas in bacterial membranes the inhibitory potencies were lower. These results together with our earlier observation that the amiloride analogues prevent labeling of the ND5 subunit of complex I with a fenpyroximate analogue suggest the involvement of ND5 in H(+) (Na(+)) translocation and no direct involvement of electron carriers in H(+) (Na(+)) translocation.

CNS neurons express two distinct plasma membrane electron transport systems implicated in neuronal viability

Trans-plasma membrane electron transport is critical for maintaining cellular redox balance and viability, yet few, if any, investigations have studied it in intact primary neurons. In this investigation, extracellular reduction of 2,6-dichloroindophenol (DCIP) and ferricyanide (FeCN) were measured as indicators of trans-plasma membrane electron transport by chick forebrain neurons. Neurons readily reduced DCIP, but not FeCN unless CoQ(1), an exogenous ubiquinone analog, was added to the assays. CoQ(1) stimulated FeCN reduction in a dose-dependent manner but had no effect on DCIP reduction. Reduction of both substrates was totally inhibited by epsilon-maleimidocaproic acid (MCA), a membrane-impermeant thiol reagent, and slightly inhibited by superoxide dismutase. Diphenylene iodonium, a flavoenzyme inhibitor, completely inhibited FeCN reduction but had no affect on DCIP reduction, suggesting that these substrates are reduced by distinct redox pathways. The relationship between plasma membrane electron transport and neuronal viability was tested using the inhibitors MCA and capsaicin. MCA caused a dose-dependent decline in neuronal viability that closely paralleled its inhibition of both reductase activities. Similarly capsaicin, a NADH oxidase inhibitor, induced a rapid decline in neuronal viability. These results suggest that trans-plasma membrane electron transport helps maintain a stable redox environment required for neuronal viability.

Riot control agents: pharmacology, toxicology, biochemistry and chemistry

The desired effect of all riot control agents is the temporary disablement of individuals by way of intense irritation of the mucous membranes and skin. Generally, riot control agents can produce acute site-specific toxicity where sensory irritation occurs. Early riot control agents, namely, chloroacetophenone (CN) and chlorodihydrophenarsazine (DM), have been replaced with 'safer' agents such as o-chlorobenzylidene malononitrile (CS) and oleoresin of capsicum (OC). Riot control agents are safe when used as intended: however, the widespread use of riot control agents raises questions and concerns regarding their health effects and safety. A large margin exists between dosages that produce harassment and dosages likely to cause adverse health effects for modern riot control agents such as CS and dibenz[b,f]1 : 4-oxazepine (CR). Yet, despite the low toxicity of modern riot control agents, these compounds are not entirely without risk. The risk of toxicity increases with higher exposure levels and prolonged exposure durations. Ocular, pulmonary and dermal injury may occur on exposure to high levels of these substances, and exposure to riot control agents in enclosed spaces may produce significant toxic effects. Reported deaths are few involving riot control agents, and then only under conditions of prolonged exposure and high concentrations. Recently, concern has focused on the deaths resulting from law enforcement use of OC, a riot control agent generally regarded as safe because it is a natural product. As with other xenobiotics, not enough is known concerning the long-term/chronic effects of riot control agents. Clearly, there is considerable need for additional research to define and delineate the biological and toxicological actions of riot control agents and to illuminate the full health consequences of these compounds as riot control agents.

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.

Capsaicin-induced depolarisation of mitochondria in dorsal root ganglion neurons is enhanced by vanilloid receptors

Capsaicin, a pungent ingredient of hot chilli peppers, triggered Ca(2+) influx in dorsal root ganglion (DRG) neurons, which express specific vanilloid receptors of type 1, with ED(50)<100 nM. An increase in capsaicin concentration to 10 microM inhibited Ca(2+) clearance from the cytosol, but did not affect the amplitude of intracellular Ca(2+) elevation. In DRG neurons, 10 microM capsaicin also produced a significant drop in mitochondrial membrane potential (Deltapsi), as measured with the mitochondria-specific potentiometric fluorescent dye JC-1. Similar loss of mitochondrial potential upon application of capsaicin was observed in non-neuronal primary (human lymphocytes) and transformed (human myeloid leukaemia cell line, HL-60) cells. The EC(50) values for capsaicin-induced mitochondrial depolarisation were 6.9 microM (DRG neurons), 200 microM (human lymphocytes) and 150 microM (HL-60 cells). Removal of extracellular Ca(2+) or an application of the antioxidant trolox attenuated capsaicin-induced dissipation of Deltapsi in DRG neurons, but not in human lymphocytes and HL-60 cells. Rotenone, an inhibitor of complex I of the mitochondrial respiratory chain, and oligomycin, an inhibitor of F(0)F(1)-ATPase, significantly enhanced the mitochondrial depolarisation produced by capsaicin in DRG neurons. In human lymphocytes and HL-60 cells, only oligomycin potentiated the effect of capsaicin. From our results, we suggest that, in DRG neurons and non-neuronal cells, capsaicin dissipates Deltapsi, possibly due to a direct inhibition of complex I of the mitochondrial respiratory chain. The presence of vanilloid receptor-1 in DRG neurons makes their mitochondria 20-30-fold more sensitive to the depolarising effect of capsaicin compared with non-neuronal cells lacking vanilloid receptor-1. The higher sensitivity of DRG neurons to capsaicin may underlie a selective neurotoxicity of capsaicin towards sensory neurons.

Transcription of arcA and rpoS during growth of Salmonella typhimurium under aerobic and microaerobic conditions

Physiology of the exponential and stationary phase of growth, under both aerobic and microaerobic conditions, of Salmonella typhimurium and its isogenic mutants nuoG::Km, cydA::TnphoA, DeltaarcA and DeltarpoS was studied using luxAB transcriptional fusions with the rpoS and arcA genes. In the wild-type strain, rpoS transcription was greater under aerobic than under microaerobic conditions, whereas transcription of arcA was suppressed by aerobiosis. Under aerobic conditions, no interaction between NuoG, CydA, ArcA and RpoS was detected. Under microaerobic conditions, rpoS was suppressed in the nuoG mutant as compared with the wild-type strain, but it was overexpressed in the cydA and arcA mutants. A deletion in the rpoS gene, on the other hand, resulted in non-restricted, increased arcA expression in stationary-phase cultures under microaerobic conditions. Based on the rpoS transcription in the nuoG mutant the authors propose that the decrease in the NADH:NAD ratio that occurs when carbon sources become limiting serves as a signal for increased rpoS transcription, while active respiration catalysed by CydA and controlled by ArcA downregulates rpoS transcription. When, finally, the RpoS-controlled stationary phase of growth is reached, arcA is suppressed in an RpoS-dependent fashion. Transition into stationary phase under microaerobic conditions is thus controlled by coordinated action of the RpoS and ArcA regulators, depending on subtle changes in the environment.

The natural product capsaicin inhibits photosynthetic electron transport at the reducing side of photosystem II and purple bacterial reaction center: structural details of capsaicin binding

Capsaicin, a natural quinone analog, was found to block electron transport, in both plant photosystem II (PSII) and bacterial reaction center (RC) from Rhodobacter sphaeroides, at the QB site. The mode of action of capsaicin was investigated by O2 evolution measurements and fluoresence induction studies in the case of PSII, and flash-induced absorbance spectroscopy in the case of the bacterial RC. Structural details of capsaicin binding to the bacterial RC complex were determined by X-ray crystallographic analysis.

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.

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.

Two NADH:ubiquinone oxidoreductases of Azotobacter vinelandii and their role in the respiratory protection

Initial steps of the Azotobacter vinelandii respiratory chain have been studied on the inside-out subcellular vesicles. Two NADH:ubiquinone oxidoreductases were revealed: (i) proton-motive, capsaicin-sensitive and oxidizing dNADH as well as NADH enzyme and (ii) enzyme non-coupled to the energy conservation, capsaicin-resistant and oxidizing only NADH. The level of the oxidoreductases strongly depends upon [O2] and [NH3] in the growth medium. Increase in [O2] results in lowering of the coupled-enzyme level and in rise of the non-coupled one. Exclusion of NH3 from the growth medium increases the level of the non-coupled enzyme whereas that of the coupled enzyme remains constant. The O2-linked control of NADH:ubiquinone oxidoreductases requires CydR, a Fnr-like regulatory protein. Summarizing the above observations with those made in this group on the terminal steps of the A. vinelandii respiratory chains, one can assume that the respiratory protection of nitrogenase could be carried out by co-operation of the non-coupled NADH:ubiquinone oxidoreductase and the "partially coupled" quinoloxidase of the bd-type. Efficiency of this chain seems to be five-fold lower than that of the usual proton-motive chain (the coupled NADH:ubiquinone oxidoreductase, the Q-cycle and cytochrome oxidase of the o-type) which is also present in A. vinelandii and operates at low [O2].

Rotenone-insensitive internal NADH-quinone oxidoreductase of Saccharomyces cerevisiae mitochondria: the enzyme expressed in Escherichia coli acts as a member of the respiratory chain in the host cells

The NDI1 gene encodes the internal rotenone-insensitive NADH-quinone oxidoreductase localized in the inner mitochondrial membranes of Saccharomyces cerevisiae. The T7 tag-fused mature NDI1 was overexpressed in Escherichia coli. The overexpressed NDI1 was exclusively found in the membrane fraction. The NDI1-overexpressed membranes showed significantly increased activities of NADH oxidase and NADH-ubiquinone-1 (UQ1) reductase when compared with the control membranes. Flavone, which is a specific inhibitor of the S. cerevisiae NDI1, inhibited almost completely NADH oxidase and NADH-UQ1 reductase activities of NDI1-overexpressed membranes but scarcely inhibited these activities of the control membranes. In addition, the NADH oxidase activity of the NDI1-overexpressed membranes was also inhibited by KCN as well as the control membranes. These results indicate that the overexpressed NDI1 worked as a member of the respiratory chain in the host cells, even though E. coli membranes are different from S. cerevisiae inner mitochondrial membranes in terms of quinones and lipid composition.

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

A wide variety of N-methylpyridinium and quinolinium cationic inhibitors of mitochondrial complex I was synthesized to develop potent and specific inhibitors acting selectively at one of the two proposed ubiquinone binding sites of this enzyme (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). N-Methyl-2-n-dodecyl-3-methylquinolinium (MQ18) inhibited electron transfer of complex I at under microM order regardless of whether exogenous or endogenous ubiquinone was used as an electron acceptor. The presence of tetraphenylboron (TPB-) potentiated the inhibition by MQ18 in a different way depending upon the molar ratio of TPB- to MQ18. In the presence of a catalytic amount of TPB-, the inhibitory potency of MQ18 was remarkably enhanced, and the extent of inhibition was almost complete. The presence of equimolar TPB- partially reactivated the enzyme activity, and the inhibition was saturated at an incomplete level (approximately 50%). These results are explained by the proposed dual binding sites model for ubiquinone (cited above). The inhibition behavior of MQ18 for proton pumping activity was similar to that for electron transfer activity. The good correlation of the inhibition behavior for the two activities indicates that both ubiquinone binding sites contribute to redox-driven proton pumping. On the other hand, N-methyl-4-[2-methyl-3-(p-tert-butylphenyl)]propylpyridinium (MP6) without TPB- brought about approximately 50% inhibition at 5 microM, but the inhibition reached a plateau at this level over a wide range of concentrations. Almost complete inhibition was readily obtained at low concentrations of MP6 in the presence of TPB-. Thus MP6 appears to be a selective inhibitor of one of the two ubiquinone binding sites. With a combined use of MP6 and 2,3-diethoxy-5-methyl-6-geranyl-1,4-benzoquinone, we also provided kinetic evidence for the existence of two ubiquinone binding sites.

Quinolones and their N-oxides as inhibitors of mitochondrial complexes I and III

4(1H)-quinolones (2-alkyl- (1), 2-alkyl-3-methyl- (2), 2-methyl-3-alkyl- (3), 1-hydroxy-2-methyl-3-alkyl- (4) and 1-hydroxy-2-alkyl- (5)) with n-alkyl side chains varying from C5 to C17 have been synthesized and tested for biological activity in mitochondrial complexes. Whereas all quinolones were efficient inhibitors of electron transport in the cytochrome b/c1-complex from either beef heart or Rhodospirillum rubrum, in complex I from beef heart quinolones 1 and 2 only were highly active. In a Quantitative Structure-Activity Relationship (QSAR) inhibitory activity in the cytochrome b/c1-complexes could be correlated to the physicochemical parameters lipophilicity pi and/or to STERIMOL L. Maximal inhibitory potency was achieved at a carbon chain length of 12-14 A. Oxidant-induced reduction of cytochrome b established that some quinolones are inhibitors of the Qp rather than the Qn site.

H+/e- stoichiometry for NADH dehydrogenase I and dimethyl sulfoxide reductase in anaerobically grown Escherichia coli cells

Anaerobically grown Escherichia coli cells were shown to acidify the reaction medium in response to oxygen or dimethyl sulfoxide (DMSO) pulses, with the H+/e- stoichiometry being close to 2.5 and 1.5, respectively. In the presence of the NADH dehydrogenase I (NDH-I) inhibitor 8-methyl-N-vanillyl-6-nonenamide (capsaicin) or in mutants lacking NDH-I, this ratio decreased to 1 for O2 and to 0 for DMSO. These data suggest that (i) the H+/e- stoichiometry for E. coli NDH-I is at least 1.5 and (ii) the DMSO reductase does not generate a proton motive force.