Diphenyleneiodonium inhibits reduction of iron-sulfur clusters in the mitochondrial NADH-ubiquinone oxidoreductase (Complex I)

Mitochondrial requirement for endothelial responses to cyclic strain: implications for mechanotransduction

Mechanical strain triggers a variety of cellular responses, but the underlying mechanotransduction process has not been established. Endothelial cells (EC) respond to mechanical strain by upregulating adhesion molecule expression through a signaling process involving reactive oxygen species (ROS), but the site of their generation is unknown. Mitochondria anchor to the cytoskeleton and could function as mechanotransducers by releasing ROS during cytoskeletal strain. In human umbilical vein EC (HUVEC), ROS production increased 221 ± 17% during 6 h of cyclic strain vs. unstrained controls. Mitochondrial inhibitors diphenylene iodonium or rotenone abrogated this response, whereas inhibitors of nitric oxide (NO) synthase (l-nitroarginine), xanthine oxidase (allopurinol), or NAD(P)H oxidase (apocynin) had no effect. The antioxidants ebselen and diethyldithiocarbamate inhibited the increase in ROS, but the NO scavenger Hb had no effect. Thus strain induces ROS release from mitochondria. In other studies, HUVEC were rendered mitochondria deficient (ρ0EC) to determine the requirement for electron transport in the response to strain. Strain-induced 2′7′-dichlorofluorescein fluorescence was attenuated by >80% in ρ0EC compared with HUVEC (43 ± 7 vs. 221 ± 17%). Treatment with cytochalasin D abrogated strain-induced ROS production, indicating a requirement for the actin cytoskeleton. Cyclic strain (6 h) increased VCAM-1 expression in wild-type but not ρ0EC. Increases in NF-κB activation and VCAM-1 mRNA expression during strain were prevented by antioxidants. These findings demonstrate that mitochondria function as mechanotransducers in endothelium by increasing ROS signaling, which is required for strain-induced increase in VCAM-1 expression via NF-κB.

Cadmium induces caspase-independent apoptosis in liver Hep3B cells: role for calcium in signaling oxidative stress-related impairment of mitochondria and relocation of endonuclease G and apoptosis-inducing factor

Cadmium-induced cellular toxicity has been related to necrosis and/or caspase-dependent apoptosis. In the present study, we show that, on cadmium exposure, the human hepatocarcinoma Hep3B cells undergo caspase-independent apoptosis associated with nuclear translocation of endonuclease G and apoptosis-inducing factor, two mitochondrial apoptogenic proteins. Release of these proteins is likely related to calcium-induced alteration of mitochondrial homeostasis. Indeed, it was first preceded by a rapid and sustained increase in cytoplasmic calcium and then by a coincident loss in mitochondrial membrane potential and production of reactive oxygen species. Bapta-AM (acetoxymethyl ester of 5, 5'-dimethyl-bis (o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid), a calcium chelator, blocked all these events and prevented cadmium-induced apoptosis. Production of reactive oxygen species was inhibited by ruthenium red and rotenone, two mitochondrial inhibitors, and by diphenyleneiodonium, a flavoprotein inhibitor, which also prevented both loss in mitochondrial membrane potential and apoptosis. In addition, Bapta-AM and diphenyleneiodonium were found to almost totally block decreased expression of the mitochondrial anti-apoptotic nuclear factor-kappaB-regulated bcl-x(L) protein in cadmium-treated cells. Taken together, our results show that cadmium induces Hep3B cells apoptosis mainly by calcium- and oxidative stress-related impairment of mitochondria, which probably favors release of apoptosis-inducing factor and endonuclease G.

Synthesis and Inhibitory Action of Novel Acetogenin Mimics with Bovine Heart Mitochondrial Complex I†

Studies on the inhibition mechanism of acetogenins, the most potent inhibitors of complex I, are useful to elucidate the structural and functional features of the terminal electron-transfer step of this enzyme. We synthesized acetogenin mimics that possess two alkyl tails without a gamma-lactone ring, named Deltalac-acetogenin, and examined their inhibitory action on bovine heart mitochondrial complex I. Unexpectedly, the Deltalac-acetogenin carrying two n-undecanyl groups (compound 3) elicited very potent inhibition comparable to that of bullatacin. The inhibitory potency of compound 3 markedly decreased with shortening the length of either or both alkyl tails, indicating that symmetric as well as hydrophobic properties of the inhibitor are important for the inhibition. Both acetylation and deoxygenation of either or both of two OH groups adjacent to the tetrahydrofuran (THF) rings resulted in a significant decrease in inhibitory potency. These structural dependencies of the inhibitory action of Deltalac-acetogenins are in marked contrast to those of ordinary acetogenins. Double-inhibitor titration of steady-state complex I activity showed that inhibition of compound 3 and bullatacin are not additive, though the inhibition site of both inhibitors is downstream of iron-sulfur cluster N2. Our results indicate that the mode of inhibitory action of Deltalac-acetogenins differs from that of ordinary acetogenins. Therefore, Deltalac-acetogenins can be regarded as a novel type of inhibitor acting on the terminal electron-transfer step of complex I.

Flavoprotein autofluorescence imaging of neuronal activation in the cerebellar cortex in vivo

Autofluorescence has been used as an indirect measure of neuronal activity in isolated cell cultures and brain slices, but only to a limited extent in vivo. Intrinsic fluorescence signals reflect the coupling between neuronal activity and mitochondrial metabolism, and are caused by the oxidation/reduction of flavoproteins or nicotinamide adenine dinucleotide (NADH). The present study evaluated the existence and properties of these autofluorescence signals in the cerebellar cortex of the ketamine/xylazine anesthetized mouse in vivo. Surface stimulation of the unstained cerebellar cortex evoked a narrow, transverse beam of optical activity consisting of a large amplitude, short latency increase in fluorescence followed by a longer duration decrease. The optimal wavelengths for this autofluorescence signal were 420-490 nm for excitation and 515-570 nm for emission, consistent with a flavoprotein origin. The amplitude of the optical signal was linearly related to stimulation amplitude and frequency, and its duration was linearly related to the duration of stimulation. Blocking synaptic transmission demonstrated that a majority of the autofluorescence signal is attributed to activating the postsynaptic targets of the parallel fibers. Hypothesized to be the result of oxidation and subsequent reduction of flavoproteins, blocking mitochondrial respiration with sodium cyanide or inactivation of flavoproteins with diphenyleneiodonium substantially reduced the optical signal. This reduction in the autofluorescence signal was accomplished without altering the presynaptic and postsynaptic components of the electrophysiological response. Results from reflectance imaging and blocking nitric oxide synthase demonstrated that the epifluorescence signal is not the result of changes in hemoglobin oxygenation or blood flow. This flavoprotein autofluorescence signal thus provides a powerful tool to monitor neuronal activity in vivo and its relationship to mitochondrial metabolism.

Tumor necrosis factor-alpha-induced cytokine-induced neutrophil chemoattractant-1 (CINC-1) production by rat gastric epithelial cells: role of reactive oxygen species and nuclear factor-kappaB

Rat cytokine-induced neutrophil chemoattractant-1 (CINC-1), a counterpart of the human growth-regulated oncogene product (GRO), has been suggested to participate in neutrophil recruitment in an experimental model of gastritis in rat. However, the mechanism(s) involved in regulation of CINC-1 production by the gastric mucosa remains unclear. The aim of this study was to investigate the mechanism(s) of CINC-1 production by rat gastric mucosa in vitro. All experiments were performed using rat normal gastric mucosal cell line (RGM-1). RGM-1s were stimulated with tumor necrosis factor (TNF)-alpha, and CINC-1 mRNA levels (reverse transcription-polymerase chain reaction) and protein secretion (enzyme-linked immunosorbent assay) were assessed. The production of reactive oxygen species (ROS) and nuclear factor (NF)-kappaB activation (translocation to the nuclei) in response to TNF-alpha stimulation was evaluated using fluorescence microscopy in the presence or absence of the inhibitors of mitochondrial electron flow and NF-kappaB activation. Stimulation of RGM-1 cells with TNF-alpha resulted in an increase in intracellular oxidative stress, NF-kappaB translocation to the nuclei, and up-regulation of CINC-1 mRNA and protein, which was prevented by interfering with mitochondria-dependent ROS production and NF-kappaB activation. Taken together, these findings indicate that CINC-1, a counterpart of the human GRO, production by rat gastric epithelial cells in response to TNF-alpha stimulation is an oxidant stress-mediated and NF-kappaB-dependent event.

Human hepatitis B virus-X protein alters mitochondrial function and physiology in human liver cells

The hepatitis B virus-X protein (HBx) regulates fundamental aspects of mitochondrial physiology. We show that HBx down-regulates mitochondrial enzymes involved in electron transport in oxidative phosphorylation (complexes I, III, IV, and V) and sensitizes the mitochondrial membrane potential in a hepatoma cell line. HBx also increases the level of mitochondrial reactive oxygen species and lipid peroxide production. HBx does not activate apoptotic signaling, although it sensitizes hepatoma cells to apoptotic signaling, which is dependent on reactive oxygen species. Increased intrahepatic lipid peroxidation in HBx transgenic mice demonstrated that oxidative injury occurs as a direct result of HBx expression. Therefore, we conclude that mitochondrial dysfunction is a crucial pathophysiological factor in HBx-expressing hepatoma cells and provides an experimental rationale in the investigation of mitochondrial function in rapidly renewed tissues, as in hepatocellular carcinomas.

Protein oxidation in plant mitochondria as a stress indicator

Plant mitochondria produce reactive oxygen species (ROS) as an unavoidable side product of aerobic metabolism, but they have mechanisms for regulating this production such as the alternative oxidase. Once produced, ROS can be removed by several different enzyme systems. Finally, should the first two strategies fail, the ROS produced can act as a signal to the rest of the cell and/or cause damage to DNA, lipids and proteins. Proteins are modified in a variety of ways by ROS, some direct, others indirect e.g. by conjugation with breakdown products of fatty acid peroxidation. Reversible oxidation of cysteine and methionine side chains is an important mechanism for regulating enzyme activity. Mitochondria from both mammalian and plant tissues contain a number of oxidised proteins, but the relative abundance of these post-translationally modified forms is as yet unknown, as are the consequences of the modification for the properties and turnover time of the proteins. Specific proteins appear to be particularly vulnerable to oxidative carbonylation in the matrix of plant mitochondria; these include several enzymes of the Krebs cycle, glycine decarboxylase, superoxide dismutase and heat shock proteins. Plant mitochondria contain a number of different proteases, but their role in removing oxidatively damaged proteins is, as yet, unclear.

Biological chemistry of reactive oxygen and nitrogen and radiation-induced signal transduction mechanisms

In the past few years, nuclear DNA damage-sensing mechanisms activated by ionizing radiation have been identified, including ATM/ATR and the DNA-dependent protein kinase. Less is known about sensing mechanisms for cytoplasmic ionization events and how these events influence nuclear processes. Several studies have demonstrated the importance of cytoplasmic signaling pathways in cytoprotection and mutagenesis. For cytoplasmic signaling, radiation-stimulated reactive oxygen species (ROS) and reactive nitrogen species (RNS) are essential activators of these pathways. This review summarizes recent studies on the chemistry of radiation-induced ROS/RNS generation and emphasizes interactions between ROS and RNS and the relative roles of cellular ROS/RNS generators as amplifiers of the initial ionization events. Cellular mechanisms for regulating ROS/RNS levels are discussed. The mechanisms by which cells sense ROS/RNS are examined in terms of how ROS/RNS modify protein structure and function, for example, interactions with metal-thiol clusters, protein tyrosine nitration, protein cysteine oxidation, S-thiolation and S-nitrosylation. We propose that radiation-induced ROS are the initiators and that nitric oxide (NO*) or derivatives are the effectors activating these signal transduction pathways. In responding to cellular ionization events, the cell converts an oxidative signal to a nitrosative one because ROS are too reactive and unspecific in their reactions for regulatory purposes and the cell is equipped to precisely modulate NO* levels.

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

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

DPI induces mitochondrial superoxide-mediated apoptosis

The iodonium compounds diphenyleneiodonium (DPI) and diphenyliodonium (IDP) are well-known phagocyte NAD(P)H oxidase inhibitors. However, it has been shown that at high concentrations they can inhibit the mitochondrial respiratory chain as well. Since inhibition of the mitochondrial respiratory chain has been shown to induce superoxide production and apoptosis, we investigated the effect of iodonium compounds on mitochondria-derived superoxide and apoptosis. Mitochondrial superoxide production was measured on both cultured cells and isolated rat-heart submitochondrial particles. Mitochondria function was examined by monitoring mitochondrial membrane potential. Apoptotic pathways were studied by measuring cytochrome c release and caspase 3 activation. Apoptosis was characterized by detecting DNA fragmentation on agarose gel and measuring propidium iodide- (PI-) stained subdiploid cells using flow cytometry. Our results showed that DPI could induce mitochondrial superoxide production. The same concentration of DPI induced apoptosis by decreasing mitochondrial membrane potential and releasing cytochrome c. Addition of antioxidants or overexpression of MnSOD significantly reduced DPI-induced mitochondrial damage, cytochrome c release, caspase activation, and apoptosis. These observations suggest that DPI can induce apoptosis via induction of mitochondrial superoxide. DPI-induced mitochondrial superoxide production may prove to be a useful model to study the signaling pathways of mitochondrial superoxide.

Hybrid ubiquinone: novel inhibitor of mitochondrial complex I

We synthesized novel ubiquinone analogs by hybridizing the natural ubiquinone ring (2,3-dimethoxy-5-methyl-1,4-benzoquinone) and hydrophobic phenoxybenzamide unit, and named them hybrid ubiquinones (HUs). The HUs worked as electron transfer substrates with bovine heart mitochondrial succinate-ubiquinone oxidoreductase (complex II) and ubiquinol-cytochrome c oxidoreductase (complex III), but not with NADH-ubiquinone oxidoreductase (complex I). With complex I, they acted as inhibitors in a noncompetitive manner against exogenous short-chain ubiquinones irrespective of the presence of the natural ubiquinone ring. Elongation of the distance between the ubiquinone ring and the phenoxybenzamide unit did not recover the electron accepting activity. The structure/activity study showed that high structural specificity of the phenoxybenzamide moiety is required to act as a potent inhibitor of complex I. These findings indicate that binding of the HUs to complex I is mainly decided by some specific interaction of the phenoxybenzamide moiety with the enzyme. It is of interest that an analogous bulky and hydrophobic substructure can be commonly found in recently registered synthetic pesticides the action site of which is mitochondrial complex I.

O(2) sensing in hypoxic pulmonary vasoconstriction: the mitochondrial door re-opens

The identity of the O(2) sensor underlying the hypoxic pulmonary vasoconstriction (HPV) response has been sought for more than 50 years. Recently, the mitochondria have again come into sharp focus as the cellular organelle responsible for triggering the events that culminate in pulmonary artery constriction. Studies from different laboratories propose two disparate models to explain how mitochondria react to a decrease in P(O(2)). One model proposes that hypoxia slows or inhibits mitochondrial electron transport resulting in the accumulation of reducing equivalents and a decrease in the generation of reactive oxygen species (ROS). This is proposed to activate a redox-sensitive pathway leading to pulmonary vasoconstriction. A second and opposing model suggests that hypoxia triggers a paradoxical increase in mitochondrial ROS generation. This increase would then lead to the activation of an oxidant-sensitive signaling transduction pathway leading to HPV. This article summarizes the potential involvement of mitochondria in these two very different models.

Mitochondrial superoxide production during oxalate-mediated oxidative stress in renal epithelial cells

Crystals of calcium oxalate monohydrate (COM) in the renal tubule form the basis of most kidney stones. Tubular dysfunction resulting from COM-cell interactions occurs by mechanism(s) that are incompletely understood. We examined the production of reactive oxygen intermediates (ROI) by proximal (LLC-PK1) and distal (MDCK) tubular epithelial cells after treatment with COM (25-250 microg/ml) to determine whether ROI, specifically superoxide (O(2)(*-)), production was activated, and whether it was sufficient to induce oxidative stress. Employing inhibitors of cytosolic and mitochondrial systems, the source of ROI production was investigated. In addition, intracellular glutathione (total and oxidized), energy status (ATP), and NADH were measured. COM treatment for 1-24 h increased O(2)(*-) production 3-6-fold as measured by both lucigenin chemiluminescence in permeabilized cells and dihydrorhodamine fluorescence in intact cells. Using selective inhibitors we found no evidence of cytosolic production. The use of mitochondrial probes, substrates, and inhibitors indicated that increased O(2)(*-) production originated from mitochondria. Treatment with COM decreased glutathione (total and redox state), indicating a sustained oxidative insult. An increase in NADH in COM-treated cells suggested this cofactor could be responsible for elevating O(2)(*-) generation. In conclusion, COM increased mitochondrial O(2)(*-) production by epithelial cells, with a subsequent depletion of antioxidant status. These changes may contribute to the reported cellular transformations during the development of renal calculi.

Expression of a functionally active gp91phox-containing neutrophil-type NAD(P)H oxidase in smooth muscle cells from human resistance arteries: regulation by angiotensin II

A major source of vascular smooth muscle cell (VSMC) superoxide is NAD(P)H oxidase. However, the molecular characteristics and regulation of this enzyme are unclear. We investigated whether VSMCs from human resistance arteries (HVSMCs) possess a functionally active, angiotensin II (Ang II)-regulated NAD(P)H oxidase that contains neutrophil oxidase subunits, including p22phox, gp91phox, p40phox, p47phox, and p67phox. mRNA expression of gp91phox homologues, nox1 and nox4, was also assessed in HVSMCs, human aortic smooth muscle cells, and rat VSMCs. HVSMCs were obtained from resistance arteries from gluteal biopsies of healthy subjects. gp91phox and nox4, but not nox1, were detected in HVSMCs. Nox1 and nox4, but not gp91phox, were expressed in human aortic smooth muscle cells and rat VSMCs. All NAD(P)H oxidase subunits were present in HVSMCs as detected by reverse transcriptase-polymerase chain reaction and immunoblotting. Ang II increased NAD(P)H oxidase subunit abundance. These effects were inhibited by cycloheximide. Acute Ang II stimulation (10 to 15 minutes) increased p47phox serine phosphorylation and induced p47phox and p67phox translocation. This was associated with NAD(P)H oxidase activation. In cells transfected with gp91phox antisense oligonucleotides, Ang II-mediated actions were abrogated. NADPH-induced superoxide generation was reduced by gp91ds-tat and apocynin, inhibitors of p47phox-gp91phox interactions. Our results suggest that HVSMCs possess a functionally active gp91phox-containing neutrophil-like NAD(P)H oxidase. Ang II regulates the enzyme by inducing phosphorylation of p47phox, translocation of cytosolic subunits, and de novo protein synthesis. These novel findings provide insight into the molecular regulation of NAD(P)H oxidase by Ang II in HVSMCs. Furthermore, we identify differences in gp91phox homologue expression in VSMCs from rats and human small and large arteries.

Role of mitochondrial oxidant generation in endothelial cell responses to hypoxia

Endothelial cells increase their secretion of the cytokine interleukin-6 (IL-6) during hypoxia, which then acts in an autocrine fashion to increase the permeability of cell monolayers. These responses are attenuated by antioxidants, suggesting that reactive oxygen species (ROS) participate in signaling in hypoxic endothelium. We tested whether mitochondria are responsible for these ROS in human umbilical vein endothelial cells exposed to hypoxia. Oxidation of the probe 2', 7'-dichlorodihydrofluorescein to fluorescent dichlorofluorescein or the probe dihydroethidium was used to assess oxidant signaling, whereas permeability was assessed by using transendothelial electrical resistance. Hypoxia elicited increases in dichlorofluorescein and dihydroethidium fluorescence that were abrogated by the mitochondrial electron transport (ET) inhibitors rotenone (2 micromol/L) and diphenyleneiodonium (5 micromol/L). The same ET inhibitors also attenuated hypoxia-induced increases in nuclear factor-kappaB (NF-kappaB) activation, although they did not abrogate NF-kappaB activation in response to endotoxin (lipopolysaccharide). ET inhibition also abolished the hypoxia-induced increases in IL-6 mRNA expression, hypoxia-stimulated IL-6 secretion into the media, and the hypoxia-induced increases in transendothelial electrical resistance of human umbilical vein endothelial cell monolayers. By contrast, the above responses to hypoxia were not significantly affected by treatment with the NAD(P)H oxidase inhibitor apocynin (30 micromol/L), the xanthine oxidase inhibitor allopurinol (100 micromol/L), or the NO synthase inhibitor N-nitro-L-arginine (100 micromol/L). We conclude that ROS signals originating from the mitochondrial ET chain trigger the increase in NF-kappaB activation, the transcriptional activation of IL-6, the secretion of IL-6 into the cell culture media, and the increases in endothelial permeability observed during hypoxia.

Mitochondrial injury, oxidative stress, and antioxidant gene expression are induced by hepatitis C virus core protein

BACKGROUND & AIMS

The mechanisms of liver injury in chronic hepatitis C virus (HCV) infection are poorly understood. Indirect evidence suggests that oxidative stress and mitochondrial injury play a role. The aim of this study was to determine if the HCV core protein itself alters mitochondrial function and contributes to oxidative stress.

METHODS

HCV core protein was expressed in 3 different cell lines, and reactive oxygen species (ROS) and lipid peroxidation products were measured.

RESULTS

Core expression uniformly increased ROS. In 2 inducible expression systems, core protein also increased lipid peroxidation products and induced antioxidant gene expression as well. A mitochondrial electron transport inhibitor prevented the core-induced increase in ROS. A fraction of the expressed core protein localized to the mitochondria and was associated with redistribution of cytochrome c from mitochondrial to cytosolic fractions. Sensitivity to oxidative stress was also seen in HCV transgenic mice in which increased intrahepatic lipid peroxidation products occurred in response to carbon tetrachloride.

CONCLUSIONS

Oxidative injury occurs as a direct result of HCV core protein expression both in vitro and in vivo and may involve a direct effect of core protein on mitochondria. These results provide new insight into the pathogenesis of hepatitis C and provide an experimental rationale for investigation of antioxidant therapy.

Increased generation of superoxide by angiotensin II in smooth muscle cells from resistance arteries of hypertensive patients: role of phospholipase D-dependent NAD(P)H oxidase-sensitive pathways

OBJECTIVE

We tested the hypothesis that increased responsiveness of phospholipase D (PLD) to angiotensin II (Ang II) is associated with increased oxidative stress and exaggerated growth responses in vascular smooth muscle cells (VSMC) from untreated essential hypertensive patients.

DESIGN

VSMCs from peripheral resistance arteries of normotensive and hypertensive subjects were studied. Production of reactive oxygen species (ROS) was measured with the fluoroprobe 5-(and 6)-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate (CM-H2DCFDA). PLD and reduced nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) oxidase were assessed with the inhibitors, dihydro-D-erythro-sphingosine (sphinganine) and diphenylene iodinium (DPI), respectively, and protein kinase C (PKC) effects were determined using chelerythrine chloride and calphostin C. PLD activity was measured by the transphosphatidylation assay.

RESULTS

Ang II increased the CM-H2DCFDA fluorescence signal, derived predominantly from H2O2. Ang II-induced generation of DPI-inhibitable ROS was significantly enhanced in cells from hypertensives compared with normotensives (Emax = 72 +/- 2 versus 56.9 +/- 1.8 fluorescence units, P< 0.01). PLD inhibition attenuated Ang II-induced ROS generation, with greater effects in the hypertensive group than the normotensive group (delta = 42 +/- 3.3 versus 21 +/- 2 units). PKC inhibition partially decreased Ang II-elicited signals. Ang II-stimulated PLD activity and DNA and protein synthesis were significantly greater in cells from hypertensives than normotensives. These effects were normalized by DPI and sphinganine.

CONCLUSIONS

Our results suggest that in essential hypertension enhanced oxidative stress and augmented growth-promoting actions of Ang II are associated with increased activation of PLD-dependent pathways. These processes may contribute to vascular remodeling in hypertension.

Model for hypoxic pulmonary vasoconstriction involving mitochondrial oxygen sensing

We tested whether mitochondria function as the O(2) sensor underlying hypoxic pulmonary vasoconstriction (HPV). In buffer-perfused rat lungs, rotenone, myxothiazol, and diphenyleneiodonium, which inhibit mitochondria in the proximal region of the electron transport chain (ETC), abolished HPV without attenuating the response to U46619. Cyanide and antimycin A inhibit electron transfer in the distal region of the ETC, but they did not abolish HPV. Cultured pulmonary artery (PA) myocytes contract in response to hypoxia or to U46619. The hypoxic response was abolished while the response to U46619 was maintained in mutant (rho(0)) PA myocytes lacking a mitochondrial ETC. To test whether reactive oxygen species (ROS) derived from mitochondria act as signaling agents in HPV, the antioxidants pyrrolidinedithiocarbamate and ebselen and the Cu,Zn superoxide dismutase inhibitor diethyldithiocarbamate were used. These abolished HPV without affecting contraction to U46619, suggesting that ROS act as second messengers. In cultured PA myocytes, oxidation of intracellular 2',7'-dichlorofluorescin diacetate (DCFH) dye increased under 2% O(2), indicating that myocytes increase their generation of H(2)O(2) during hypoxia. This was attenuated by myxothiazol, implicating mitochondria as the source of increased ROS during HPV. These results indicate that mitochondrial ATP is not required for HPV, that mitochondria function as O(2) sensors during hypoxia, and that ROS generated in the proximal region of the ETC act as second messengers in the response.

Surfactant protein D regulates NF-kappa B and matrix metalloproteinase production in alveolar macrophages via oxidant-sensitive pathways

Targeted ablation of the surfactant protein D (SP-D) gene caused progressive pulmonary emphysema associated with pulmonary infiltration by foamy alveolar macrophages (AMs), increased hydrogen peroxide production, and matrix metalloproteinase (MMP)-2, -9, and -12 expression. In the present study, the mechanisms by which SP-D influences macrophage MMP activity were assessed in AMs from SP-D(-/-) mice. Tissue lipid peroxides and reactive carbonyls were increased in lungs of SP-D(-/-) mice, indicating oxidative stress. Immunohistochemical staining of AMs from SP-D(-/-) mice demonstrated that NF-kappaB was highly expressed and translocated to the nucleus. Increased NF-kappaB binding was detected by EMSA in nuclear extracts of AMs isolated from SP-D(-/-) mice. Antioxidants N-acetylcysteine and pyrrolidine dithiocarbamate inhibited MMP production by AMs from SP-D(-/-) mice. To assess whether increased oxidant production influenced NF-kappaB activation and production of MMP-2 and -9, AMs from SP-D(-/-) mice were treated with the NADPH oxidase inhibitors diphenylene iodonium chloride and apocynin. Inhibition of NADPH oxidase suppressed NF-kappaB binding by nuclear extracts and decreased production of MMP-2 and 9 in AMs from SP-D(-/-) mice. SN-50, a synthetic NF-kappaB-inhibitory peptide, decreased MMP production by AMs from SP-D(-/-) mice. Oxidant production and reactive oxygen species were increased in lungs of SP-D(-/-) mice, in turn activating NF-kappaB and MMP expression. SP-D plays an unexpected inhibitory role in the regulation of NF-kappaB in AMs.

PLANT MITOCHONDRIA AND OXIDATIVE STRESS: Electron Transport, NADPH Turnover, and Metabolism of Reactive Oxygen Species

The production of reactive oxygen species (ROS), such as O2- and H2O2, is an unavoidable consequence of aerobic metabolism. In plant cells the mitochondrial electron transport chain (ETC) is a major site of ROS production. In addition to complexes I-IV, the plant mitochondrial ETC contains a non-proton-pumping alternative oxidase as well as two rotenone-insensitive, non-proton-pumping NAD(P)H dehydrogenases on each side of the inner membrane: NDex on the outer surface and NDin on the inner surface. Because of their dependence on Ca2+, the two NDex may be active only when the plant cell is stressed. Complex I is the main enzyme oxidizing NADH under normal conditions and is also a major site of ROS production, together with complex III. The alternative oxidase and possibly NDin(NADH) function to limit mitochondrial ROS production by keeping the ETC relatively oxidized. Several enzymes are found in the matrix that, together with small antioxidants such as glutathione, help remove ROS. The antioxidants are kept in a reduced state by matrix NADPH produced by NADP-isocitrate dehydrogenase and non-proton-pumping transhydrogenase activities. When these defenses are overwhelmed, as occurs during both biotic and abiotic stress, the mitochondria are damaged by oxidative stress.

NADH oxidase activity of mitochondrial apoptosis-inducing factor

Apoptosis-inducing factor (AIF) is a mitochondrial flavoprotein, which translocates to the nucleus during apoptosis and causes chromatin condensation and large scale DNA fragmentation. Here we report the biochemical characterization of AIF's redox activity. Natural AIF purified from mitochondria and recombinant AIF purified from bacteria (AIFDelta1-120) exhibit NADH oxidase activity, whereas superoxide anion (O(2)(-)) is formed. AIFDelta1-120 is a monomer of 57 kDa containing 1 mol of noncovalently bound FAD/mol of protein. ApoAIFDelta1-120, which lacks FAD, has no NADH oxidase activity. However, native AIFDelta1-120, apoAIFDelta1-120, and the reconstituted (FAD-containing) holoAIFDelta1-120 protein exhibit a similar apoptosis-inducing potential when microinjected into the cytoplasm of intact cells. Inhibition of the redox function, by external addition of superoxide dismutase or covalent derivatization of FAD with diphenyleneiodonium, failed to affect the apoptogenic function of AIFDelta1-120 assessed on purified nuclei in a cell-free system. Conversely, blockade of the apoptogenic function of AIFDelta1-120 with the thiol reagent para- chloromercuriphenylsulfonic acid did not affect its NADH oxidase activity. Altogether, these data indicate that AIF has a marked oxidoreductase activity which can be dissociated from its apoptosis-inducing function.

Identification of Two Tetranuclear FeS Clusters on the Ferredoxin-Type Subunit of NADH:Ubiquinone Oxidoreductase (Complex I)

The proton-translocating NADH:ubiquinone oxidoreductase of respiratory chains (complex I) contains one flavin mononucleotide and five EPR-detectable iron-sulfur clusters as redox groups. Because of the number of conserved motifs typical for binding iron-sulfur clusters and the high content of iron and acid-labile sulfide of complex I preparations, it is predicted that complex I contains additional clusters which have not yet been detected by EPR spectroscopy. To search for such clusters, we used a combination of UV/vis and EPR spectroscopy to study complex I from Neurospora crassa and Escherichia coli adjusted to distinct redox states. We detected a UV/vis redox difference spectrum characterized by negative absorbances at 325 and 425 nm that could not be assigned to the known redox groups. Redox titration was used to determine the pH-independent midpoint potential to be -270 mV, being associated with the transfer of two electrons. Comparison with UV/vis difference spectra obtained from complex I fragments and related enzymes showed that this group is localized on subunit Nuo21.3c of the N. crassa or NuoI of the E. coli complex I, respectively. This subunit (the bovine TYKY) belongs to a family of 8Fe-ferredoxins which contain two tetranuclear iron-sulfur clusters as redox groups. We detected EPR signals in a fragment of complex I which we attribute to the novel FeS clusters of complex I.

Involvement of matrix NADP turnover in the oxidation of NAD-linked substrates by pea leaf mitochondria

The involvement of the internal rotenone-insensitive NADPH dehydrogenase on the inner surface of the inner mitochondrial membrane [NDin(NADPH)] in the oxidation of strictly NAD+-linked substrates by pea (Pisum sativum L.) leaf mitochondria was measured. As estimated by the inhibition caused by 5 µM diphenyleneiodonium (DPI) in the presence of rotenone to inhibit complex I, the activity of NDin(NADPH) during glycine oxidation (measured both as O2 uptake and as CO2 release) was 40-50 nmol mg-1 protein min-1. No significant activity of NDin(NADPH) could be detected during the oxidation of 2-oxoglutarate, another strictly NAD+-linked substrate; this was possibly due to its relatively low oxidation rate. Control experiments showed that, even at 125 µM, DPI had no effect on the activity of glycine decarboxylase complex (GDC) and lipoamide dehydrogenase. The relative activity of complex I, NDin(NADPH), and NDin(NADH) during glycine oxidation, estimated using rotenone and DPI, differed depending on the pyridine nucleotide supply in the mitochondrial matrix. This was shown by loading the mitochondria with NAD+ and NADP+, both of which were taken up by the organelle. We conclude that the involvement of NADP turnover during glycine oxidation is not due to the direct production of NADPH by GDC but is an indirect result of this process. It probably occurs via the interconversion of NADH to NADPH by the two non-energy-linked transhydrogenase activities recently identified in plant mitochondria.

Dominant cell death induction by extramitochondrially targeted apoptosis-inducing factor

The complete AIF cDNA comprising the amino-terminal mitochondrial localization sequence (MLS) and the oxidoreductase domain has been fused in its carboxyl terminus to enhanced green fluorescent protein (GFP), thereby engineering an AIF-GFP fusion protein that is selectively targeted to the mitochondrial intermembrane space. Upon induction of apoptosis, the AIF-GFP protein translocates together with cytochrome c (Cyt-c) to the extramitochondrial compartment. Microinjection of recombinant AIF leads to the release of AIF-GFP and Cyt-c-GFP, indicating that ectopic AIF can favor permeabilization of the outer mitochondrial membrane. These mitochondrial effects of AIF are caspase independent, whereas the Cyt-c-microinjection induced translocation of AIF-GFP and Cyt-c-GFP is suppressed by the pan-caspase inhibitor Z-VAD.fmk. Upon prolonged culture, transfection-enforced overexpression of AIF results in spontaneous translocation of AIF-GFP from mitochondria, nuclear chromatin condensation, and cell death. These effects are caspase independent and do not rely on the oxidoreductase function of AIF. Spontaneous AIF-GFP translocation and subsequent nuclear apoptosis can be retarded by overexpression of a Bcl-2 protein selectively targeted to mitochondria, but not by a Bcl-2 protein targeted to the endoplasmic reticulum. Overexpression of a mutant AIF protein in which the MLS has been deleted (AIF Delta 1-100) results in the primary cytosolic accumulation of AIF. AIF Delta 1-100-induced cell death is suppressed by neither Z-VAD.fmk or by Bcl-2. Thus, extramitochondrially targeted AIF is a dominant cell death inducer.

Reactive oxygen species generated at mitochondrial complex III stabilize hypoxia-inducible factor-1alpha during hypoxia: a mechanism of O2 sensing

During hypoxia, hypoxia-inducible factor-1alpha (HIF-1alpha) is required for induction of a variety of genes including erythropoietin and vascular endothelial growth factor. Hypoxia increases mitochondrial reactive oxygen species (ROS) generation at Complex III, which causes accumulation of HIF-1alpha protein responsible for initiating expression of a luciferase reporter construct under the control of a hypoxic response element. This response is lost in cells depleted of mitochondrial DNA (rho(0) cells). Overexpression of catalase abolishes hypoxic response element-luciferase expression during hypoxia. Exogenous H(2)O(2) stabilizes HIF-1alpha protein during normoxia and activates luciferase expression in wild-type and rho(0) cells. Isolated mitochondria increase ROS generation during hypoxia, as does the bacterium Paracoccus denitrificans. These findings reveal that mitochondria-derived ROS are both required and sufficient to initiate HIF-1alpha stabilization during hypoxia.

NF- kappa B independent suppression of endothelial vascular cell adhesion molecule-1 and intercellular adhesion molecule-1 gene expression by inhibition of flavin binding proteins and superoxide production

Oxidation-reduction (redox) coupled mechanisms play an important role in the regulation of cell surface adhesion molecule expression. In endothelial cells membrane-bound NADH/NADPH oxidase is a significant source of intracellular superoxide (O(2)(-)) production. We explored the role of flavin containing proteins such as NADH/NADPH oxidase in the induction of vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1) gene expression in human aortic endothelial cells (HAECs) and human dermal microvascular endothelial cells (HMECs). Treatment of HAECs by tumor necrosis factor- alpha (TNF- alpha, 100 U/ml) for 1 h induced a 31% increase in O(2)(-)production within 5 min as determined by lucigenin chemiluminescence analysis of whole cells (n=4, P<0.05). Pretreatment with the NADH/NADPH oxidase inhibitor diphenylene iodonium (DPI, 40 microm) for 1 h inhibited O(2)(-)production. DPI also inhibited TNF and LPS-induced VCAM-1 and ICAM-1 cell surface expression and TNF- alpha, LPS, or IL-1 beta induced VCAM-1 and ICAM-1 mRNA accumulation. However, DPI did not inhibit TNF- alpha -induced activation of nuclear NF- kappa B-like binding activity in HAECs and HMECs. Furthermore, DPI did not inhibit TNF- alpha induced transactivation of NF- kappa B-driven VCAM-1 and HIV-LTR promoter gene constructs in transiently transfected HMECs. These data suggest that flavin binding proteins such as NADH/NADPH oxidase can regulate VCAM-1 gene expression independent of NF- kappa B. Furthermore, intracellular O(2)(-)generation is not necessary for NF- kappa B activation or for transactivation of NF- kappa B driven promoters.

Role of oxidants in NF-kappa B activation and TNF-alpha gene transcription induced by hypoxia and endotoxin

The transcription factor NF-kappa B stimulates the transcription of proinflammatory cytokines including TNF-alpha. LPS (endotoxin) and hypoxia both induce NF-kappa B activation and TNF-alpha gene transcription. Furthermore, hypoxia augments LPS induction of TNF-alpha mRNA. Previous reports have indicated that antioxidants abolish NF-kappa B activation in response to LPS or hypoxia, which suggests that reactive oxygen species (ROS) are involved in NF-kappa B activation. This study tested whether mitochondrial ROS are required for both NF-kappaB activation and the increase in TNF-alpha mRNA levels during hypoxia and LPS. Our results indicate that hypoxia (1.5% O2) stimulates NF-kappa B and TNF-alpha gene transcription and increases ROS generation as measured by the oxidant sensitive dye 2',7'-dichlorofluorescein diacetate in murine macrophage J774.1 cells. The antioxidants N-acetylcysteine and pyrrolidinedithiocarbamic acid abolished the hypoxic activation of NF-kappa B, TNF-alpha gene transcription, and increases in ROS levels. Rotenone, an inhibitor of mitochondrial complex I, abolished the increase in ROS signal, the activation of NF-kappa B, and TNF-alpha gene transcription during hypoxia. LPS stimulated NF-kappa B and TNF-alpha gene transcription but not ROS generation in J774.1 cells. Rotenone, pyrrolidinedithiocarbamic acid, and N-acetylcysteine had no effect on the LPS stimulation of NF-kappa B and TNF-alpha gene transcription, indicating that LPS activates NF-kappa B and TNF-alpha gene transcription through a ROS-independent mechanism. These results indicate that mitochondrial ROS are required for the hypoxic activation of NF-kappa B and TNF-alpha gene transcription, but not for the LPS activation of NF-kappa B.

The F420H2 dehydrogenase from Methanosarcina mazei is a Redox-driven proton pump closely related to NADH dehydrogenases

The F(420)H(2) dehydrogenase is part of the energy conserving electron transport system of the methanogenic archaeon Methanosarcina mazei Gö1. Here it is shown that cofactor F(420)H(2)-dependent reduction of 2-hydroxyphenazine as catalyzed by the membrane-bound enzyme is coupled to proton translocation across the cytoplasmic membrane, exhibiting a stoichiometry of 0.9 H(+) translocated per two electrons transferred. The electrochemical proton gradient thereby generated was shown to drive ATP synthesis from ADP + P(i). The gene cluster encoding the F(420)H(2) dehydrogenase of M. mazei Gö1 comprises 12 genes that are referred to as fpoA, B, C, D, H, I, J, K, L, M, N, and O. Analysis of the deduced amino acid sequences revealed that the enzyme is closely related to proton translocating NADH dehydrogenases of respiratory chains from bacteria (NDH-1) and eukarya (complex I). Like the NADH-dependent enzymes, the F(420)H(2) dehydrogenase is composed of three subcomplexes. The gene products FpoA, H, J, K, L, M, and N are highly hydrophobic and are homologous to subunits that form the membrane integral module of NDH-1. FpoB, C, D, and I have their counterparts in the amphipathic membrane-associated module of NDH-1. Homologues to the hydrophilic NADH-oxidizing input module are not present in M. mazei Gö1. Instead, the gene product FpoF may be responsible for F(420)H(2) oxidation and may function as the electron input part. Thus, the F(420)H(2) dehydrogenase from M. mazei Gö1 resembles eukaryotic and bacterial proton translocating NADH dehydrogenases in many ways. The enzyme from the methanogenic archaeon functions as a NDH-1/complex I homologue and is equipped with an alternative electron input unit for the oxidation of reduced cofactor F(420) and a modified output module adopted to the reduction of methanophenazine.

Two separate transhydrogenase activities are present in plant mitochondria

Inside-out submitochondrial particles from both potato tubers and pea leaves catalyze the transfer of hydride equivalents from NADPH to NAD(+) as monitored with a substrate-regenerating system. The NAD(+) analogue acetylpyridine adenine dinucleotide is also reduced by NADPH and incomplete inhibition by the complex I inhibitor diphenyleneiodonium (DPI) indicates that two enzymes are involved in this reaction. Gel-filtration chromatography of solubilized mitochondrial membrane complexes confirms that the DPI-sensitive TH activity is due to NADH-ubiquinone oxidoreductase (EC 1.6.5.3, complex I), whereas the DPI-insensitive activity is due to a separate enzyme eluting around 220 kDa. The DPI-insensitive TH activity is specific for the 4B proton on NADH, whereas there is no indication of a 4A-specific activity characteristic of a mammalian-type energy-linked TH. The DPI-insensitive TH may be similar to the soluble type of transhydrogenase found in, e.g., Pseudomonas. The presence of non-energy-linked TH activities directly coupling the matrix NAD(H) and NADP(H) pools will have important consequences for the regulation of NADP-linked processes in plant mitochondria.

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

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

Ang II-stimulated superoxide production is mediated via phospholipase D in human vascular smooth muscle cells

Intracellular signaling events that mediate the long-term effects of Ang II in vascular smooth muscle cells are unclear, but oxidative stress may play an important role. This study examined the ability of Ang II to generate reactive oxygen species and investigated the putative role of phospholipase D (PLD)-dependent signaling pathways for its production in human vascular smooth muscle cells. In addition, we assessed whether redox-sensitive pathways influence Ang II-stimulated cell growth. Primary and low-passage cells (passages 1 to 4) derived from resistance arteries of subcutaneous gluteal biopsies from healthy subjects were studied. Oxidative stress was measured with the fluorescent probe 5-(and 6)-chloromethyl-2', 7'-dichlorodihydrofluorescein diacetate (CM-H(2)DCFDA) (8 micromol/L), and the role of PLD was assessed with the PLD inhibitor D-erythro-sphingosine, dihydro (sphinganine) (10 micromol/L). To determine whether NADH/NADPH oxidase contributes to production of reactive oxygen species, Ang II-stimulated cells were pretreated with the specific flavoprotein inhibitor diphenylene iodinium (DPI) (10 micromol/L). DNA and protein synthesis were determined by [(3)H]thymidine and [(3)H]leucine incorporation, respectively. Ang II increased CM-H(2)DCFDA fluorescence, and this was inhibited by catalase (350 U/mL), indicating that the fluorescence signal was derived predominantly from H(2)O(2). Ang II dose-dependently increased H(2)O(2) production (E(max)=57.6+/-1.7 nmol/L, pD(2)=7.7+/-0.06) and PLD activation (E(max)=207+/-3.3% of control, pD(2)=7.7+/-0.5). H(2)O(2) effects were evident within 1 hour, and maximal PLD activation occurred within 40 minutes after stimulation. DPI inhibited (P<0.01) Ang II-stimulated responses. PLD inhibition significantly attenuated (P<0.05) Ang II-elicited H(2)O(2) production (E(max)=29+/-5 nmol/L). DPI and sphinganine inhibited Ang II-induced DNA and protein synthesis. These data indicate that in vascular smooth muscle cells from human peripheral resistance arteries, Ang II increases H(2)O(2) generation via PLD-dependent, NADH/NADPH oxidase-sensitive pathways. These cascades may function as second messengers in long-term Ang II-mediated growth-signaling events.

Novel reactions involved in energy conservation by methanogenic archaea

Methanogenic archaea of the order Methanosarcinales which utilize C(1) compounds such as methanol, methylamines or H(2)+CO(2), employ two novel membrane-bound electron transport systems generating an electrochemical proton gradient: the H(2):heterodisulfide oxidoreductase and the F(420)H(2):heterodisulfide oxidoreductase. The systems are composed of the heterodisulfide reductase and either a membrane-bound hydrogenase or a F(420)H(2) dehydrogenase which is functionally homologous to the proton-translocating NADH dehydrogenase. Cytochromes and the novel electron carrier methanophenazine are also involved. In addition, the methyl-H(4)MPT:HS-CoM methyltransferase is bioenergetically relevant. The enzyme couples methyl group transfer with the translocation of sodium ions and seems to be present in all methanogens. The proton-translocating systems with the participation of cytochromes and methanophenazine have been found so far only in the Methanosarcinales.

NADPH oxidase inhibition does not interfere with low PO2 transduction in rat and rabbit CB chemoreceptor cells

The aim of the present work was to elucidate the role of NADPH oxidase in hypoxia sensing and transduction in the carotid body (CB) chemoreceptor cells. We have studied the effects of several inhibitors of NADPH oxidase on the normoxic and hypoxia-induced release of [3H]catecholamines (CA) in an in vitro preparation of intact CB of the rat and rabbit whose CA deposits have been labeled by prior incubation with the natural precursor [3H]tyrosine. It was found that diphenyleneiodonium (DPI; 0.2-25 microM), an inhibitor of NADPH oxidase, caused a dose-dependent release of [3H]CA from normoxic CB chemoreceptor cells. Contrary to hypoxia, DPI-evoked release was only partially Ca2+ dependent. Concentrations of DPI reported to produce full inhibition of NADPH oxidase in the rat CB did not prevent the hypoxic release response in the rat and rabbit CB chemoreceptor cells, as stimulation with hypoxia in the presence of DPI elicited a response equaling the sum of that produced by DPI and hypoxia applied separately. Neopterin (3-300 microM) and phenylarsine oxide (0.5-2 microM), other inhibitors of NADPH oxidase, did not promote release of [3H]CA in normoxic conditions or affect the response elicited by hypoxia. On the basis of effects of neopterin and phenylarsine oxide, it is concluded that NADPH oxidase does not appear to play a role in oxygen sensing or transduction in the rat and rabbit CB chemoreceptor cells in vitro and, in the context of the present study, that DPI effects are not related to NADPH oxidase inhibition.

Inhibition of membrane-bound electron transport of the methanogenic archaeon Methanosarcina mazei Gö1 by diphenyleneiodonium

The proton translocating electron transport systems (F420H2:heterodisulfide oxidoreductase and H2:heterodisulfide oxidoreductase) of Methanosarcina mazei Gö1 were inhibited by diphenyleneiodonium chloride (DPI) indicated by IC50 values of 20 nmol DPI.mg-1 protein and 45 nmol DPI.mg-1 protein, respectively. These effects are due to a complex interaction of DPI with key enzymes of the electron transport chains. It was found that 2-hydroxyphenazine-dependent reactions as catalyzed by F420-nonreducing hydrogenase, F420H2 dehydrogenase and heterodisulfide reductase were inhibited. Interestingly, the H2-dependent methylviologen reduction and the heterodisulfide reduction by reduced methylviologen as catalyzed by the hydrogenase and the heterodisulfide reductase present in washed membranes were unaffected by DPI, respectively. Analysis of the redox behavior of membrane-bound cytochromes indicated that DPI inhibited CoB-S-S-CoM-dependent oxidation of reduced cytochromes and H2-dependent cytochrome reduction. Membrane-bound and purified F420H2 dehydrogenase were inhibited by DPI irrespectively whether methylviologen + metronidazole or 2-hydroxyphenazine were used as electron acceptors. Detailed examination of 2-hydroxy-phenazine-dependent F420H2-oxidation revealed that DPI is a competitive inhibitor of the enzyme, indicated by the Km value for 2-hydroxyphenazine, which increased from 35 microm to 100 microm in the presence of DPI. As DPI and phenazines are structurally similar with respect to their planar configuration we assume that the inhibitor is able to bind to positions where interaction between phenazines and components of the electron transport systems take place. Thus, electron transfer from reduced 2-hydroxyphenazine to cytochrome b2 as part of the heterodisulfide reductase and from H2 to cytochrome b1 as subunit of the membrane-bound hydrogenase is affected in the presence of DPI. In case of the F420H2 dehydrogenase electron transport from FAD or from FeS centers to 2-hydroxyphenazine is inhibited.

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.

Localization of the Enzyme System Involved in Anaerobic Reduction of Azo Dyes by Sphingomonas sp. Strain BN6 and Effect of Artificial Redox Mediators on the Rate of Azo Dye Reduction

The effect of different artificial redox mediators on the anaerobic reduction of azo dyes by Sphingomonas sp. strain BN6 or activated sludge was investigated. Reduction rates were greatly enhanced in the presence of sulfonated anthraquinones. For strain BN6, the presence of both cytoplasmic and membrane-bound azo reductase activities was shown.

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.

Neutrophil-associated inflammatory responses in rats are inhibited by phenylarsine oxide

NADPH oxidase is a phagocyte-specific enzyme which produces O2- and so initiates a cascade of reactive oxygen species formation. Inflammatory diseases involve overproduction of reactive oxygen species which induce tissue damage. Phenylarsine oxide has been described previously as a complete and direct inhibitor of NADPH oxidase in vitro that acts by covalently binding to vicinal thiol groups of a membrane-associated component of the enzyme. In the present work, the potential anti-inflammatory effect of phenylarsine oxide was tested on two experimental models in rats, carrageenan-induced paw oedema and lipopolysaccharide-mediated lung inflammation. Intraperitoneal injection of phenylarsine oxide reduced (i) reactive oxygen species production by rat phagocytes, (ii) neutrophil infiltration into the lung after inhalation of lipopolysaccharide and (iii) neutrophil-dependent oedema induced by carrageenan in hindpaws. We conclude that phenylarsine oxide has anti-inflammatory properties which are probably exerted by its ability to inhibit neutrophil NADPH oxidase-dependent reactive oxygen species production. The present work provides the basis for the development of new anti-inflammatory, arsenic-free agents reacting at the phenylarsine oxide site, which seems to be the Achilles' heel of NADPH oxidase.

p22phox is a critical component of the superoxide-generating NADH/NADPH oxidase system and regulates angiotensin II-induced hypertrophy in vascular smooth muscle cells

Superoxide anion formation is vital to the microbicidal activity of phagocytes. Recently, however, there is accumulating evidence that it is also involved in cell growth in vascular smooth muscle cells (VSMCs). We have shown that the hypertrophic agent angiotensin II stimulates superoxide production by activating the membrane-bound NADH/NADPH oxidase and that inhibition of this oxidase attenuates vascular hypertrophy. However, the molecular identity of this oxidase in VSMCs is unknown. We have recently cloned the cytochrome b558 alpha-subunit, p22(phox) (one of the key electron transfer elements of the NADPH oxidase in phagocytes), from a rat VSMC cDNA library, but its role in VSMC oxidase activity remains unclarified. Here we report that the complete inhibition of p22(phox) mRNA expression by stable transfection of antisense p22(phox) cDNA into VSMCs results in a decrease in cytochrome b content, which is accompanied by a significant inhibition of angiotensin II-stimulated NADH/NADPH-dependent superoxide production, subsequent hydrogen peroxide production, and [3H]leucine incorporation. We provide the first evidence that p22(phox) is a critical component of superoxide-generating vascular NADH/NADPH oxidase and suggest a central role for this oxidase system in vascular hypertrophy.

Disruption of the nuclear gene encoding the 20.8-kDa subunit of NADH: ubiquinone reductase of Neurospora mitochondria

The nuclear gene coding for the 20.8-kDa subunit of the membrane arm of respiratory chain NADH: ubiquinone reductase (Complex I) from Neurospora crassa, nuo-20.8, was localized on linkage group I of the fungal genome. A genomic DNA fragment containing this gene was cloned and a duplication was created in a strain of N. crassa by transformation. To generate RIP (repeat-induced point) mutations in the duplicated sequence, the transformant was crossed with another strain carrying an auxotrophic marker on chromosome I. To increase the chance of finding an isolate with a non-functional nuo-20.8 gene, random progeny from the cross were selected against this auxotrophy since RIP of the target gene will only occur in the nucleus carrying the duplication. Among these, we isolated and characterised a mutant strain that lacks the 20.8 kDa mitochondrial protein, indicating that this cysteine-rich polypeptide is not essential. Nevertheless, the absence of the 20.8-kDa subunit prevents the full assembly of complex I. It appears that the peripheral arm and two intermediates of the membrane arm of the enzyme are still formed in the mutant mitochondria. The NADH: ubiquinone reductase activity of sonicated mitochondria from the mutant is rotenone insensitive. Electron microscopy of mutant mitochondria does not reveal any alteration in the structure or numbers of the organelles.

Activation of an H2O2-generating NADH oxidase in human lung fibroblasts by transforming growth factor beta 1

The cellular source(s) and mechanisms of generation of reactive oxygen species (ROS) in nonphagocytic cells stimulated by cytokines are unclear. In this study, we demonstrate that transforming growth factor beta 1 (TGF-beta 1, 1 ng/ml) induces the release of H2O2 from human lung fibroblasts within 8 h following exposure to this cytokine. Elevation in H2O2 release peaked at 16 h (approximately 22 pmol/min/10(6) cells) and gradually declined to undetectable levels at 48 h after TGF-beta 1 treatment. NADH consumption by these cells was stimulated by TGF-beta 1 while that of NADPH remained unchanged. NADPH oxidase activity as measured by diphenyliodonium (DPI)-inhibitable NADH consumption in TGF-beta 1-treated cells followed a time course similar to that of H2O2 release. DPI, an inhibitor of the NADPH oxidase complex of neutrophils and other flavoproteins, also inhibited the TGF-beta 1-induced H2O2 production. Inhibitors of other enzymatic systems involving flavoproteins that may be responsible for the production of H2O2 in these cells, including xanthine oxidase, nitric oxide synthase, and both mitochondrial and microsomal electron transport systems, failed to inhibit TGF-beta 1-induced NADH oxidation and H2O2 production. The delay (> 4 h) following TGF-beta 1 exposure along with the inhibition of this process by cycloheximide and actinomycin D suggest the requirement of new protein synthesis for induction of NADH oxidase activity in TGF-beta 1-stimulated fibroblasts.

Direct evidence for the presence of two external NAD(P)H dehydrogenases coupled to the electron transport chain in plant mitochondria

Exogenous NADPH oxidation by purified mitochondria from both potato tuber and Arum maculatum spadix was completely and irreversibly inhibited by sub-micromolar diphenyleneiodonium (DPI), while exogenous NADH oxidation was inhibited to only a small degree. Addition of DPI caused the collapse of the membrane potential generated by NADPH oxidation, while the potential generated by NADH was unaffected. We conclude that there are two distinct enzymes on the outer surface of the inner membrane of plant mitochondria, one specific for NADH, the other relatively specific for NADPH, with both enzymes linked to the electron transport chain.

Carbocyanine dyes with long alkyl side-chains: broad spectrum inhibitors of mitochondrial electron transport chain activity

Certain indocarbocyanine, thiacarbocyanine, and oxacarbocyanine dyes possessing short alkyl side-chains (one to five carbons) are potent inhibitors of mammalian mitochondrial NADH-ubiquinone reductase (EC 1.6.99.3) activity (Anderson et al., Biochem Pharmacol 41: 677-684, 1991; Anderson et al., Biochem Pharmacol 45: 691-696, 1993; Anderson et al., Biochem Pharmacol 45: 2115-2122, 1993), and act similarly to rotenone. This study examines the inhibitory capacities of twelve other carbocyanine dyes (six indocarbocyanines, four oxacarbocyanines, and two thiacarbocyanines) possessing long alkyl side-chains (seven to eighteen carbons with both saturated and unsaturated side-chains) on mitochondrial NADH, succinate and cytochrome c oxidase activities. Three of the indocarbocyanines inhibited electron transport chain activity, while three were non-inhibitory. Two of the oxacarbocyanines also inhibited electron transport chain activity, while the other two were without effect. Both the thiacarbocyanines were non-inhibitory. In contrast to previous studies, the long alkyl side-chain carbocyanines exhibited a broad spectrum of inhibition of respiratory chain activity, affecting either oxidation of all three substrates or of NADH and cytochrome c, rather than specific inhibition of mitochondrial NADH-ubiquinone reductase activity, indicating that there could be multiple binding sites for these compounds. The five inhibitory long side-chain carbocyanines also inhibited reduction of ferricyanide and coenzyme Q1 by NADH, using submitochondrial particles, but not when tested with purified complex I, indicating that the mitochondrial inner membrane was an integral component in their inhibitory capacity. No general correlation of side-chain length or degree of unsaturation and inhibitory capacity was discernible.