NADH oxidase of plasma membranes

Medicinally Privileged Natural Chalcones: Abundance, Mechanisms of Action, and Clinical Trials

Chalcones have been utilized for centuries as foods and medicines across various cultures and traditions worldwide. This paper concisely overviews their biosynthesis as specialized metabolites in plants and their significance, potential, efficacy, and possibility as future medicines. This is followed by a more in-depth exploration of naturally occurring chalcones and their corresponding mechanisms of action in human bodies. Based on their mechanisms of action, chalcones exhibit many pharmacological properties, including antioxidant, anti-inflammatory, anticancer, antimalarial, antiviral, and antibacterial properties. Novel naturally occurring chalcones are also recognized as potential antidiabetic drugs, and their effect on the GLUT-4 transporter is investigated. In addition, they are examined for their anti-inflammatory effects, focusing on chalcones used for future pharmaceutical utilization. Chalcones also bind to specific receptors and toxins that prevent bacterial and viral infections. Chalcones exhibit physiological protective effects on the biological degradation of different systems, including demyelinating neurodegenerative diseases and preventing hypertension or hyperlipidemia. Chalcones that are/were in clinical trials have been included as a separate section. By revealing the many biological roles of chalcones and their impact on medicine, this paper underlines the significance of naturally occurring chalcones and their extension to patient care, providing the audience with an index of topic-relevant information.

Isolation and biological activity of natural chalcones based on antibacterial mechanism classification

Bacterial infection, which is still one of the leading causes of death in humans, poses an enormous threat to the worldwide public health system. Antibiotics are the primary medications used to treat bacterial diseases. Currently, the discovery of antibiotics has reached an impasse, and due to the abuse of antibiotics resulting in bacterial antibiotic resistance, researchers have a critical desire to develop new antibacterial agents in order to combat the deteriorating antibacterial situation. Natural chalcones, the flavonoids consisting of two phenolic rings and a three-carbon α, β-unsaturated carbonyl system, possess a variety of biological and pharmacological properties, including anti-cancer, anti-inflammatory, antibacterial, and so on. Due to their potent antibacterial properties, natural chalcones possess the potential to become a new treatment for infectious diseases that circumvents existing antibiotic resistance. Currently, the majority of research on natural chalcones focuses on their synthesis, biological and pharmacological activities, etc. A few studies have been conducted on their antibacterial activity and mechanism. Therefore, this review focuses on the antibacterial activity and mechanisms of seventeen natural chalcones. Firstly, seventeen natural chalcones have been classified based on differences in antibacterial mechanisms. Secondly, a summary of the isolation and biological activity of seventeen natural chalcones was provided, with a focus on their antibacterial activity. Thirdly, the antibacterial mechanisms of natural chalcones were summarized, including those that act on bacterial cell membranes, biological macromolecules, biofilms, and quorum sensing systems. This review aims to lay the groundwork for the discovery of novel antibacterial agents based on chalcones.

An Electrochemical Nanosensor for Monitoring the Dynamics of Intracellular H2 O2 Upon NADH Treatment

Reactive oxygen species (ROS)-based therapeutic strategies play an important role in cancer treatment. However, in situ, real-time and quantitative analysis of intracellular ROS in cancer treatment for drug screening is still a challenge. Herein we report one selective hydrogen peroxide (H₂ O₂ ) electrochemical nanosensor, which is prepared by electrodeposition of Prussian blue (PB) and polyethylenedioxythiophene (PEDOT) onto carbon fiber nanoelectrode. With the nanosensor, we find that the level of intracellular H₂ O₂ increases with NADH treatment and that increase is dose-dependent to the concentration of NADH. High-dose of NADH (above 10 mM) can induce cell death and intratumoral injection of NADH is validated for inhibiting tumor growth in mice. This study highlights the potential of electrochemical nanosensor for tracking and understanding the role of H₂ O₂ in screening new anticancer drug.

The teleos of metallo-reduction and metallo-oxidation in eukaryotic iron and copper trafficking

Eukaryotic cells, whether free-living or organismal, rely on metallo-reductases to process environmental ferric iron and cupric copper prior to uptake. In addition, some free-living eukaryotes (e.g. fungi and algae) couple ferri-reduction to ferro-oxidation, a process catalyzed by a small cohort of multi-copper oxidases; in these organisms, the ferric iron product is a ligand for cell iron uptake via a ferric iron permease. In addition to their support of iron uptake in lower eukaryotes, ferroxidases support ferrous iron efflux in Chordata; in this process the release of the ferrous iron from the efflux transporter is catalyzed by its ferroxidation. Last, ferroxidases also catalyze the oxidation of cuprous copper and, as metallo-oxidases, mirror the dual activity of the metallo-reductases. This Perspective examines the teleos of the yin-yang of this redox cycling of iron and copper in their metabolism.

Inhibitor effect of paricalcitol in rat model of pentylenetetrazol-induced seizures

Vitamin D has various systemic effects on bone metabolism, modulation of the immune system, stabilization of the cell membrane, oxidative stress, inflammation, apoptosis, and various other hormones. Differing from active vitamin D, paricalcitol is a relatively safe VDR agonist due to its relatively few side effects. This study has investigated the anticonvulsant effect of paricalcitol in convulsions induced by pentylenetetrazole (PTZ). 36 male Sprague-Dawley rats were divided randomly into two groups: 18 for EEG recording (PTZ 35 mg/kg) and 18 for behavioral studies (PTZ 70 mg/kg). Forty-five minutes before the PTZ injection, both groups of rats were given 5 and 10 μg/kg of paricalcitol i.p., respectively. Racine convulsion scores, first myoclonic jerk time, spike percentages, and antioxidant status were evaluated in the groups. Our results showed that the Racine's Convulsion Scale (RCS) score significantly dropped in the paricalcitol-treated group, analysis of the first myoclonic jerk (FMJ) latencies demonstrated a significantly longer latency in the paricalcitol-applied group, and spike percentages at EEG recordings significantly decreased with paricalcitol. Moreover, MDA levels were lower and SOD activity were higher in the 5 μg/kg paricalcitol group compared to the saline group; these results were more prominent in 10 μg/kg paricalcitol group. Our study has demonstrated that paricalcitol has protective effects on PTZ-induced convulsions. Based on the SOD and MDA levels in our study, these effects may result from the antioxidant characteristics of paricalcitol.

NAD+ as the Link Between Oxidative Stress, Inflammation, Caloric Restriction, Exercise, DNA Repair, Longevity, and Health Span

Oxidative stress and decreased DNA damage repair in vertebrates increase with age also due to lowered cellular NAD+. NAD+ depletion may play a major role in the aging process at the cellular level by limiting (1) energy production, (2) DNA repair, and (3) genomic signaling. In this study, we hypothesize that it is not NAD+ as a cofactor in redox reactions and coenzyme in metabolic processes that has the ultimate role in aging, but rather the role of NAD+ in cellular signaling when used as substrate for sirtuins (SIRT1-7 in mammals) and PARPs [Poly(ADP-ribose) polymerases]. Both sirtuins and PARPs influence many transcription factors and can affect gene expression. As a signaling molecule, NAD+ is consumed in the reaction donating ADP-ribose and releasing nicotinamide (NAM) as a by-product. It seems that aging at the cellular level is associated with a decline of NAD+ and that NAD+ restoration can reverse phenotypes of aging by inducing cellular repair and stress resistance. Adequate intracellular NAD+ concentrations may be an important longevity assurance factor, while lowered cellular NAD+ concentration may negatively influence the life span.

Biological effects of the electrostatic field: red blood cell-related alterations of oxidative processes in blood

The aim of this study was to determine activities of pro-/antioxidant enzymes, reactive oxygen species (ROS) content, and oxidative modification of proteins and lipids in red blood cells (RBCs) and blood plasma of rats exposed to electrostatic field (200 kV/m) during the short (1 h) and the long periods (6 day, 6 h daily). Short-term exposure was characterized by the increase of oxidatively damaged proteins in blood of rats. This was strongly expressed in RBC membranes. After long-term action, RBC content in peripheral blood was higher than in control (P < 0.01) and the attenuation of prooxidant processes was shown.

HIGHLIGHTS

External electrostatic field (200 kV/m) alters the balance in pro-/antioxidant processes. We examine oxidative processes in plasma and RBC (hemolysate and membranes). Biological effects of static electric field depend on exposure time. Acute action of electrostatic field (ESF) characterized by activation of the prooxidant processes. Long-term exposure reflected with prevalence of antioxidant activities.

Plasma membrane electron transport in pancreatic β-cells is mediated in part by NQO1

Plasma membrane electron transport (PMET), a cytosolic/plasma membrane analog of mitochondrial electron transport, is a ubiquitous system of cytosolic and plasma membrane oxidoreductases that oxidizes cytosolic NADH and NADPH and passes electrons to extracellular targets. While PMET has been shown to play an important role in a variety of cell types, no studies exist to evaluate its function in insulin-secreting cells. Here we demonstrate the presence of robust PMET activity in primary islets and clonal β-cells, as assessed by the reduction of the plasma membrane-impermeable dyes WST-1 and ferricyanide. Because the degree of metabolic function of β-cells (reflected by the level of insulin output) increases in a glucose-dependent manner between 4 and 10 mM glucose, PMET was evaluated under these conditions. PMET activity was present at 4 mM glucose and was further stimulated at 10 mM glucose. PMET activity at 10 mM glucose was inhibited by the application of the flavoprotein inhibitor diphenylene iodonium and various antioxidants. Overexpression of cytosolic NAD(P)H-quinone oxidoreductase (NQO1) increased PMET activity in the presence of 10 mM glucose while inhibition of NQO1 by its inhibitor dicoumarol abolished this activity. Mitochondrial inhibitors rotenone, antimycin A, and potassium cyanide elevated PMET activity. Regardless of glucose levels, PMET activity was greatly enhanced by the application of aminooxyacetate, an inhibitor of the malate-aspartate shuttle. We propose a model for the role of PMET as a regulator of glycolytic flux and an important component of the metabolic machinery in β-cells.

Trans-plasma membrane electron transport in mammals: functional significance in health and disease

Trans-plasma membrane electron transport (t-PMET) has been established since the 1960s, but it has only been subject to more intensive research in the last decade. The discovery and characterization at the molecular level of its novel components has increased our understanding of how t-PMET regulates distinct cellular functions. This review will give an update on t-PMET, with particular emphasis on how its malfunction relates to some diseases, such as cancer, abnormal cell death, cardiovascular diseases, aging, obesity, neurodegenerative diseases, pulmonary fibrosis, asthma, and genetically linked pathologies. Understanding these relationships may provide novel therapeutic approaches for pathologies associated with unbalanced redox state.

Body weight reducing effect of oral boric acid intake

BACKGROUND

Boric acid is widely used in biology, but its body weight reducing effect is not researched.

METHODS

Twenty mice were divided into two equal groups. Control group mice drank standard tap water, but study group mice drank 0.28mg/250ml boric acid added tap water over five days. Total body weight changes, major organ histopathology, blood biochemistry, urine and feces analyses were compared.

RESULTS

Study group mice lost body weight mean 28.1% but in control group no weight loss and also weight gained mean 0.09% (p<0.001). Total drinking water and urine outputs were not statistically different. Cholesterol, LDL, AST, ALT, LDH, amylase and urobilinogen levels were statistically significantly high in the study group. Other variables were not statistically different. No histopathologic differences were detected in evaluations of all resected major organs.

CONCLUSION

Low dose oral boric acid intake cause serious body weight reduction. Blood and urine analyses support high glucose, lipid and middle protein catabolisms, but the mechanism is unclear.

An archaeal NADH oxidase causes damage to both proteins and nucleic acids under oxidative stress

NADH oxidases (NOXs) catalyze the two-electron reduction of oxygen to H2O2 or four-electron reduction of oxygen to H2O. In this report, we show that an NADH oxidase from Thermococcus profundus (NOXtp) displays two forms: a native dimeric protein under physiological conditions and an oxidized hexameric form under oxidative stress. Native NOXtp displays high NADH oxidase activity, and oxidized NOXtp can accelerate the aggregation of partially unfolded proteins. The aggregates formed by NOXtp have characteristics similar to beta-amyloid and Lewy bodies in neurodegenerative diseases, including an increase of beta-sheet content. Oxidized NOXtp can also bind nucleic acids and cause their degradation by oxidizing NADH to produce H2O2. Furthermore, Escherichia coli cells expressing NOXtp are less viable than cells not expressing NOXtp after treatment with H2O2. As NOXtp shares similar features with eukaryotic cell death isozymes and life may have originated from hyperthermophiles, we suggest that NOXtp may be an ancestor of cell death proteins.

Molecular cloning and characterization of a candidate human growth-related and time-keeping constitutive cell surface hydroquinone (NADH) oxidase

ENOX (ECTO-NOX) proteins are growth-related cell surface proteins that catalyze both hydroquinone or NADH oxidation and protein disulfide-thiol interchange and exhibit both prion-like and time-keeping (clock) properties. The two enzymatic activities they catalyze alternate to generate a regular period of 24 min in length. Here we report the cloning, expression, and characterization of a human candidate constitutive ENOX (CNOX or ENOX1) protein. The gene encoding this 643 amino acid long protein is located on chromosome 13 (13q 14.11). Functional motifs previously identified by site-directed mutagenesis in a cancer-associated ENOX (tNOX or ENOX2) as adenine nucleotide or copper binding along with essential cysteines are present, but the drug-binding motif (EEMTE) sequence of ENOX2 is absent. The activities of the recombinant protein expressed in Escherichia coli were not affected by capsaicin, EGCg, and other ENOX2-inhibiting substances. The purified recombinant protein bound ca. 2 mol of copper/mol of protein. Bound copper was necessary for activity. H260 and H579 were required for copper binding as confirmed by site-directed mutagenesis, loss of copper-binding capacity, and resultant loss of enzymatic activity. Addition of melatonin phased the 24 min period such that the next complete period began exactly 24 min after the melatonin addition as appears to be characteristic of ENOX1 activities in general. Oxidative activity was exhibited with both NAD(P)H and reduced coenzyme Q as substrate. Concentrated solutions of the purified candidate ENOX1 protein irreversibly formed insoluble aggregates, devoid of enzymatic activity, resembling amyloid.

Quinone and oxyradical scavenging properties of N-acetylcysteine prevent dopamine mediated inhibition of Na+, K+-ATPase and mitochondrial electron transport chain activity in rat brain: implications in the neuroprotective therapy of Parkinson's disease

Dopamine oxidation products such as H2O2 and reactive quinones have been held responsible for various toxic actions of dopamine, which have implications in the aetiopathogenesis of Parkinson's disease. This study has shown that N-acetylcysteine (0.25-1 mm) is a potent scavenger of both H2O2 and toxic quinones derived from dopamine and it further prevents dopamine mediated inhibition of Na+,K+-ATPase activity and mitochondrial respiratory chain function. The quinone scavenging ability of N-acetylcysteine is presumably related to its protective effect against dopamine mediated inhibition of mitochondrial respiratory chain activity. However, both H2O2 scavenging and quinone scavenging properties of N-acetylcysteine probably account for its protective effect against Na+,K+-ATPase inhibition induced by dopamine. The results have important implications in the neuroprotective therapy of sporadic Parkinson's disease since inactivation of mitochondrial respiratory activity and Na+,K+-ATPase may trigger intracellular damage pathways leading to the death of nigral dopaminergic neurons.

Study the effect of Vitamin K on intracellular NAD level in yeast by fluorescence spectrum

The intracellular NAD level plays a pivotal role in numerous biological processes such as rhythm, senescence, cancer and death. The study of the intracellular NAD level has been one of the "hotspots" in biomedical research. We investigated the effect of Vitamin K on intracellular NAD level in yeast by fluorescence spectrum in this paper. Plasma membrane redox system of yeast was found to be greatly promoted by the addition of Vitamin K(3) or Vitamin K(1). Ferricyanide reduction catalyzed by Vitamin K was accompanied by the decrease in intracellular NADH concentration and the increase in intracellular NAD level of yeast cells.

The coexistence of the oxidative and reductive systems in roots: the role of plasma membranes

Different components of the plasma membrane bound and associated redox system, which participate in the energy transfer from the predominantly reducing intercellular environment to the extracellular oxidizing environment, are reviewed. Special attention is given to plant root cells. An analysis of the plasma membrane-associated redox components, such as the cytochromes, quinones, and different types of oxidoreductases (dehydrogenases, oxidases, peroxidases, and superoxide dismutases), is made, as well as their coupling with naturally occurring extracellular substrates, such as oxygen and its reactive forms, phenols, ascorbate, nitrate, ferric ion, and organic acids. The participation of different free radical species in most of the plasma membrane-bound redox reactions is documented.

Structural changes revealed by Fourier transform infrared and circular dichroism spectroscopic analyses underlie tNOX periodic oscillations

A recurring pattern of spectral changes indicative of periodic changes in the proportion of beta-structure and a-helix of a recombinant ECTO-NOX fusion protein of tNOX, with a cellulose binding domain peptide, was demonstrated by Fourier transform infrared (FTIR) and circular dichroism (CD) spectroscopic analyses. The pattern of structural changes correlated with oscillatory patterns of enzymatic activities exhibited by the protein previously interpreted as indicative of a clock function. The pattern consisted of a repeating pattern of oscillations with a period length of 21 min with five maxima (two separated by 5 min and 3 separated by 4 to 4.5 min) within each 21 min repeat. Oscillatory patterns were not obvious in comparable FTIR or CD spectra of albumin, ribonuclease or concanavalin A. The period length was constant at 5, 15, 25, 35 and 45 degrees C (temperature compensated) and oscillations occurred independently of substrate presence. Spectra obtained in deuterium oxide yielded a longer period length of 26 min both for oscillations in enzymatic activity and absorbance ratios determined by FTIR. Taken together the findings suggest that the regular patterns of oscillations exhibited by the ECTO-NOX proteins are accompanied by recurrent global changes in the conformation of the protein backbone that directly modulate enzymatic activity.

Rosmarinic acid inhibits the formation of reactive oxygen and nitrogen species in RAW264.7 macrophages

Antioxidant action of Rosmarinic acid (Ros A), a natural phenolic ingredient in many Lamiaceae herbs such as Perilla frutescens, sage, basil and mint, was analyzed in relation to the Ikappa-B activation in RAW264.7 macrophages. Ros A inhibited nitric oxide (NO) production and inducible nitric oxide synthase (iNOS) protein synthesis induced by lipopolysaccharide (LPS), and also effectively suppressed phorbol 12-myristate 13-acetate (PMA)-induced superoxide production in RAW264.7 macrophages in a dose-dependent manner. Peroxynitrite-induced formation of 3-nitrotyrosine in bovine serum albumin and RAW264.7 macrophages were also inhibited by Ros A. Moreover, Western blot analysis demonstrated that LPS-induced phosphorylation of Ikappa-Balpha was abolished by Ros A. Ros A can act as an effective protector against peroxynitrite-mediated damage, and as a potent inhibitor of superoxide and NO synthesis; the inhibition of the formation of reactive oxygen and nitrogen species are partly based on its ability to inhibit the serine phosphorylation of Ikappa-Balpha.

Multiple proteins with single activities or a single protein with multiple activities: the conundrum of cell surface NADH oxidoreductases

Reduction of the cell-impermeable tetrazolium salt WST-1 has been used to characterise two plasma membrane NADH oxidoreductase activities in human cells. The trans activity, measured with WST-1 and the intermediate electron acceptor mPMS, utilises reducing equivalents from intracellular sources, while the surface activity, measured with WST-1 and extracellular NADH, is independent of intracellular metabolism. Whether these two activities involve distinct proteins or are inherent to a single protein is unclear. In this work, we have attempted to address this question by examining the relationship between the trans and surface WST-1-reducing activities and a third well-characterised family of cell surface oxidases, the ECTO-NOX proteins. Using blue native-polyacrylamide gel electrophoresis, we have identified a complex in the plasma membranes of human 143B osteosarcoma cells responsible for the NADH-dependent reduction of WST-1. The dye-reducing activity of the 300 kDa complex was attributed to a 70 kDa NADH oxidoreductase activity that cross-reacted with antisera against the ECTO-NOX protein CNOX. Differences in enzyme activities and inhibitor profiles between the WST-1-reducing NADH oxidoreductase enzyme in the presence of NADH or mPMS and the ECTO-NOX family are reconciled in terms of the different purification methods and assay systems used to study these proteins.

Spontaneous generation of reactive oxygen species in the mixture of cyanide and glycerol

Reactive oxygen species are involved in tumor promotion or apoptosis. In assaying prooxidant or antioxidant activities, cyanide has been commonly used as an inhibitor of mitochondrial oxidases, peroxidases, or Cu,Zn-superoxide dismutase, which have an influence on intracellular levels of reactive oxygen species. It has also been used to chemically mimic hypoxia. On the other hand, glycerol has been widely used as a stabilizer of various enzymes. In particular, glycerol is required to maintain the enzymatic activities of membrane-bound NAD(P)H oxidases extracted from surrounding phospholipids. Since both cyanide and glycerol are relatively inert, they have been used concomitantly regardless of any mutual interference. In this study, we demonstrate that a mixture of glycerol and cyanide reduced cytochrome c and nitroblue tetrazolium, both of which are superoxide anion indicators. The mixture also enhanced the production of superoxide anion in the presence of redox-cycling compounds. Superoxide production by the mixture was confirmed by electron spin resonance spectra. Moreover, the mixture induced lipid peroxidation and hemolysis in human erythrocytes. These results suggest that cyanide and glycerol should be used carefully in reaction systems used to measure superoxide production or antioxidant activity. However, sucrose and sodium azide in combination do not produce such artifacts and thus may be used as an alternative.

Tetrazolium dyes as tools in cell biology: new insights into their cellular reduction

Tetrazolium salts have become some of the most widely used tools in cell biology for measuring the metabolic activity of cells ranging from mammalian to microbial origin. With mammalian cells, fractionation studies indicate that the reduced pyridine nucleotide cofactor, NADH, is responsible for most MTT reduction and this is supported by studies with whole cells. MTT reduction is associated not only with mitochondria, but also with the cytoplasm and with non-mitochondrial membranes including the endosome/lysosome compartment and the plasma membrane. The net positive charge on tetrazolium salts like MTT and NBT appears to be the predominant factor involved in their cellular uptake via the plasma membrane potential. However, second generation tetrazolium dyes that form water-soluble formazans and require an intermediate electron acceptor for reduction (XTT, WST-1 and to some extent, MTS), are characterised by a net negative charge and are therefore largely cell-impermeable. Considerable evidence indicates that their reduction occurs at the cell surface, or at the level of the plasma membrane via trans-plasma membrane electron transport. The implications of these new findings are discussed in terms of the use of tetrazolium dyes as indicators of cell metabolism and their applications in cell biology.

Involvement of NADPH oxidase-mediated generation of reactive oxygen species in the apototic cell death by capsaicin in HepG2 human hepatoma cells

Although capsaicin (8-methyl-N-vanillyl-6-nonenamide), a pungent ingredient in a variety of red peppers of the genus Capsicum, has been shown to induce apoptotic cell death in many cancer cells, the exact mechanism of this action of capsaicin is not completely understood. In this study, we investigated the possible mediation of the NADPH oxidase-modulated production of reactive oxygen species (ROS) in the apoptotic mechanism of capsaicin in HepG2 human hepatoblastoma cells. Capsaicin induced apoptotic cell death in a time- and dose-dependent manner. Capsaicin at the concentration of inducing apoptosis also markedly increased the level of ROS. The capsaicin-induced generation of ROS and apoptosis was significantly suppressed by treatment with antioxidants, DPPD and tocopherol. In addition, inhibitors of NADPH oxidase, diphenylene iodonium, apocynin and neopterine, profoundly blocked the capsaicin-induced ROS generation and apoptosis. The expression of Rac1N17, a dominant negative mutant of Rac1, also significantly inhibited the capsaicin-induced apoptosis. Activation of nuclear factor-kappaB, a transcription factor essentially involved in ROS-induced apoptosis, was also observed by treatment with capsaicin. Collectively, these results suggest that the NADPH oxidase-mediated generation of ROS may be essentially involved in the mechanism of capsaicin-induced apoptosis in HepG2 cells. These results further suggest that capsaicin may be a valuable agent for the therapeutic intervention of human hepatomas.

Plasma membrane NADH-oxidoreductase in cells carrying mitochondrial DNA G11778A mutation and in cells devoid of mitochondrial DNA (rho0)

The mammalian plasma membrane (PM) NADH-oxidoreductase (PMOR) system is a multi-enzyme complex located in the plasma membrane of all eukaryotic cells, harboring at least two distinct activities, the plasma membrane NADH-ferricyanide reductase and the NADH-oxidase. To assess the behaviour of the two activities of the PMOR system, we measured the NADH-ferricyanide reductase and NADH-oxidase activities in fibroblast cell lines derived from patients carrying a mitochondrial DNA (mtDNA) G11778A mutation. We also measured the two activities in other cell lines, the HL-60 and HeLa (S3) lines, as well as in rho0 cells (cells devoid of mtDNA) generated from those lines and the fibroblast cells. These rho0 cells consequently lack oxidative phosphorylation and rely on anaerobic glycolysis for their ATP need. We have proposed that in rho0 cells, at least in part, up-regulation of the PMOR is a necessity to maintain the NAD+/NADH ratio, and a pre-requisite for cell growth and viability. We show here that the PM NADH-ferricyanide reductase activity was up-regulated in HL-AV2 (HL-60 rho0) cell lines, but not in the other rho0 and mtDNA mutant lines. The plasma membrane NADH oxidase activity was found to be up-regulated in both HL-AV2 and HeLa rho0 cell lines, but not significantly in the fibroblast rho0 and G11778A lines.

Adriamycin tolerance in human mesothelioma lines and cell surface NADH oxidase

Adriamycin tolerant human mesothelioma cell lines derived from a single tumor prior to either chemotherapy or radiation therapy and a susceptible cell line were investigated. Not only was growth resistant to low doses of adriamycin but an unusual pattern of resistance was encountered in which cells seemed to better tolerate high adriamycin doses than intermediate doses. The differential growth susceptibility of the tolerant lines compared to A549 lung carcinoma and the bimodal dose response correlated with differences in the specific activity of a plasma membrane-associated NADH oxidase (NOX). Plasma membrane fractions of high purity were isolated by aqueous two-phase partition and assayed directly. The NADH oxidase activity of the plasma membranes for the susceptible cell line was maximally inhibited by 1 microM adriamycin whereas the NADH oxidase activity of the tolerant lines was less and was maximally inhibited by 0.1 microM adriamycin with 1 and 10 microM adriamycin being less inhibitory than 0.1 microM adriamycin. The findings suggest a relationship between the growth response to adriamycin of the adriamycin tolerant mesothelioma lines and the activity of the plasma membrane-associated NADH oxidase activity of the cell surface in these cell lines.

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.

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.

Purification and characterization of a doxorubicin-inhibited NADH-quinone (NADH-ferricyanide) reductase from rat liver plasma membranes

Plasma membrane-associated redox systems play important roles in regulation of cell growth, internal pH, signal transduction, apoptosis, and defense against pathogens. Stimulation of cell growth and stimulation of the redox system of plasma membranes are correlated. When cell growth is inhibited by antitumor agents such as doxorubicin, capsaicin, and antitumor sulfonylureas, redox activities of the plasma membrane also are inhibited. A doxorubicin-inhibited NADH-quinone reductase was characterized and purified from plasma membranes of rat liver. First, an NADH-cytochrome b(5) reductase, which was doxorubicin-insensitive, was removed from the plasma membranes by the lysosomal protease, cathepsin D. After removal of the NADH-cytochrome b(5) reductase, the plasma membranes retained a doxorubicin-inhibited NADH-quinone reductase activity. The enzyme, with an apparent molecular mass of 57 kDa, was purified 200-fold over the cathepsin D-treated plasma membranes. The purified enzyme had also an NADH-coenzyme Q(0) reductase (NADH: external acceptor (quinone) reductase; EC 1.6.5.) activity. Partial amino acid sequence of the enzyme showed that it was unique with no sequence homology to any known protein. Antibody against the enzyme (peptide sequence) was produced and affinity-purified. The purified antibody immunoprecipitated both the NADH-ferricyanide reductase activity and NADH-coenzyme Q(0) reductase activity of plasma membranes and cross-reacted with human chronic myelogenous leukemia K562 cells and doxorubicin-resistant human chronic myelogenous leukemia K562R cells. Localization by fluorescence microscopy showed that the reaction was with the external surface of the plasma membranes. The doxorubicin-inhibited NADH-quinone reductase may provide a target for the anthracycline antitumor agents and a candidate ferricyanide reductase for plasma membrane electron transport.

Preferential inhibition of the plasma membrane NADH oxidase (NOX) activity by diphenyleneiodonium chloride with NADPH as donor

The cell-surface NADH oxidase (NOX) protein of plant and animal cells will utilize both NADH and NADPH as reduced electron donors for activity. The two activities are distinguished by a differential inhibition by the redox inhibitor diphenyleneiodonium chloride (DPI). Using both plasma membranes and cells, activity with NADPH as donor was markedly inhibited by DPI at submicromolar concentrations, whereas with NADH as donor, DPI was much less effective or had no effect on the activity. The possibility of the inhibition being the result of two different enzymes was eliminated by the use of a recombinant NOX protein. The findings support the concept that NOX proteins serve as terminal oxidases for plasma membrane electron transport involving cytosolic reduced pyridine nucleotides as the natural electron donors and with molecular oxygen as the electron acceptor.

Plasma membrane NADH oxidase of maize roots responds to gravity and imposed centrifugal forces

NADH oxidase activities measured with excised roots of dark-grown maize (Zea mays) seedlings and with isolated plasma membrane vesicles from roots of dark-grown maize oscillated with a regular period length of 24 min and were inhibited by the synthetic auxin 2,4-dichlorophenoxyacetic [correction of dichorophenoxyacetic] acid. The activities also responded to orientation with respect to gravity and to imposed centrifugal forces. Turning the roots upside down resulted in stimulation of the activity with a lag of about 10 min. Returning the sections to the normal upright position resulted in a return to initial rates. The activity was stimulated reversibly to a maximum of about 2-fold with isolated plasma membrane vesicles, when subjected to centrifugal forces of 25 to 250 x g for 1 to 4 min duration. These findings are the first report of a gravity-responsive enzymatic activity of plant roots inhibited by auxin and potentially related to the gravity-induced growth response.

Oxidative stress and vascular damage in hypertension

Oxidative stress, a state of excessive reactive oxidative species activity, is associated with vascular disease states such as hypertension. In this review, we discuss the recent advances in the field of reactive oxidative species-mediated vascular damage in hypertension. These include the identification of redox-sensitive tyrosine kinases, the characterization of enzymatic sources of superoxide production in human blood vessels, and their relationship with vascular damage in atherosclerosis and hypertension. Finally, recent developments in the search for strategies to attenuate vascular oxidative stress are reviewed.

Induction of apoptosis by capsaicin in A172 human glioblastoma cells

Capsaicin induced apoptosis of A172 human glioblastoma cells in a time- and dose-dependent manner. Neither capsazepine, a vanilloid receptor antagonist, nor bis-(o-aminophenoxy)-ethane-N,N,N', N'-tetraacetic acid/acetoxymethyl ester (BAPTA/AM), an intracellular Ca(2+) chelator, significantly inhibited the capsaicin-induced apoptosis, although capsaicin increased intracellular Ca(2+) level. Capsaicin markedly reduced the basal generation of reactive oxygen species (ROS) and lipid peroxidation. Exogenous application of H(2)O(2) significantly prevented the cells from the apoptosis by capsaicin. Treatment with N-acetyl cysteine alone induced both reduction of the basal production of ROS and apoptosis. Taken together, these results suggest that capsaicin induced apoptosis in A172 cells and that vanilloid receptors and intracellular Ca(2+) may not be involved in the apoptotic mechanism of capsaicin. Reduction of the basal generation of ROS may play a role in the induction of apoptosis by capsaicin.

Inhibition of vascular NADH/NADPH oxidase activity by thiol reagents: lack of correlation with cellular glutathione redox status

Vascular NAD(P)H oxidase activity contributes to oxidative stress. Thiol oxidants inhibit leukocyte NADPH oxidase. To assess the role of reactive thiols on vascular oxidase, rabbit iliac/carotid artery homogenates were incubated with distinct thiol reagents. NAD(P)H-driven enzyme activity, assessed by lucigenin (5 or 250 microM) luminescence, was nearly completely (> 97%) inhibited by the oxidant diamide (1mM) or the alkylator p-chloromercuryphenylsulfonate (pCMPS, 0.5mM). Analogous inhibition was also shown with EPR spectroscopy using DMPO as a spin trap. The oxidant dithionitrobenzoic acid (0.5mM) inhibited NADPH-driven signals by 92% but had no effect on NADH-driven signals. In contrast, the vicinal dithiol ligand phenylarsine oxide (PAO, 1 microM) induced minor nonsignificant inhibition of NADPH-driven activity, but significant stimulation of NADH-triggered signals. The alkylator N-ethyl maleimide (NEM, 0.5mM) or glutathione disulfide (GSSG, 3mM) had no effect with each substrate. Coincubation of N-acetylcysteine (NAC, 3mM) with diamide or pCMPS reversed their inhibitory effects by 30-60%, whereas NAC alone inhibited the oxidase by 52%. Incubation of intact arterial rings with the above reagents disclosed similar results, except that PAO became inhibitor and NAC stimulator of NADH-driven signals. Notably, the cell-impermeant reagent pCMPS was also inhibitory in whole rings, suggesting that reactive thiol(s) affecting oxidase activity are highly accessible. Since lack of oxidase inhibition by NEM or GSSG occurred despite significant cellular glutathione depletion, change in intracellular redox status is not sufficient to account for oxidase inhibition. Moreover, the observed differences between NADPH and NADH-driven oxidase activity point to complex or multiple enzyme forms.

Preferential inhibition by (-)-epigallocatechin-3-gallate of the cell surface NADH oxidase and growth of transformed cells in culture

A drug-responsive and cancer-specific NADH oxidase of the mammalian plasma membrane, constitutively activated in transformed cells, was inhibited preferentially in HeLa and human mammary adenocarcinoma by the naturally-occurring catechin of green tea, (-)-epigallocatechin-3-gallate (EGCg). With cells in culture, EGCg preferentially inhibited growth of HeLa and mammary adenocarcinoma cells compared with growth of mammary epithelial cells. Inhibited cells became smaller, and cell death was accompanied by a condensed and fragmented appearance of the nuclear DNA as revealed by fluorescence microscopy with 4',6-diamidino-2-phenylindole, suggestive of apoptosis. Mammary epithelial cells recovered from EGCg treatment even at 50 microM, whereas growth of HeLa and mammary adenocarcinoma cells was inhibited by EGCg at concentrations as low as 1 microM with repeated twice-daily additions and did not recover from treatment with 50 microM EGCg. The findings correlate inhibition of cell surface NADH oxidase activity and inhibition of growth with EGCg-induced apoptosis.

Applications of aqueous two-phase partition to isolation of membranes from plants: a periodic NADH oxidase activity as a marker for right side-out plasma membrane vesicles

Phase separations using standardized mixtures of polyethylene glycol, dextran and potassium phosphate are used widely to prepare highly purified plasma membranes from plants and in the preparation of chloroplast subfractions. Other uses include the removal of right side-out plasma membrane vesicles as contaminants from Golgi apparatus, endoplasmic reticulum and tonoplast (vacuole membrane) fractions and separation of right side-out and inside-out plasma membrane vesicles. The higher degree of separation between plasma membranes into the upper phase and internal membranes into the lower phase is in large measure due to the fact that only plasma membranes are oriented cytoplasmic side in. Most other membranes are oriented cytoplasmic side-out. This property extends to separations of right side-out and inside-out plasma membrane vesicles and to the separation of right side-out and inside-out sub-mitochondrial particles. The inside-out vesicles partition into the lower phase whereas the right side-out vesicles remain in the upper phase. The lack of efficacy of aqueous two-phase partitioning in other types of membrane separations is apparently due to the fact that surface characteristics that may distinguish different internal membranes are not located at the cytosolic membrane surface. At present there are no direct enzymatic markers for right side-out plasma membrane vesicles from plants. Demonstrations of sidedness and estimates of fraction purity are based on measurements of latency of marker enzymes, e.g., ATPases, at the cytosolic surface. This report describes a periodic NADH oxidase as an enzyme marker for right side-out plasma membrane vesicles not requiring detergent disruptions of vesicles for measurement of activity.

Surface oxidase and oxidative stress propagation in aging

This report summarizes new evidence for a plasma-membrane-associated hydroquinone oxidase designated as CNOX (constitutive plasma membrane NADH oxidase) that functions as a terminal oxidase for a plasma membrane oxidoreductase (PMOR) electron transport chain to link the accumulation of lesions in mitochondrial DNA to cell-surface accumulations of reactive oxygen species. Previous considerations of plasma membrane redox changes during aging have lacked evidence for a specific terminal oxidase to catalyze a flow of electrons from cytosolic NADH to molecular oxygen (or to protein disulfides). Cells with functionally deficient mitochondria become characterized by an anaerobic metabolism. As a result, NADH accumulates from the glycolytic production of ATP. Elevated PMOR activity has been shown to be necessary to maintain the NAD(+)/NADH homeostasis essential for survival. Our findings demonstrate that the hyperactivity of the PMOR system results in an NADH oxidase (NOX) activity capable of generating reactive oxygen species at the cell surface. This would serve to propagate the aging cascade both to adjacent cells and to circulating blood components. The generation of superoxide by NOX forms associated with aging is inhibited by coenzyme Q and provides a rational basis for the anti-aging activity of circulating coenzyme Q.

The plasma membrane NADH oxidase of soybean has vitamin K(1) hydroquinone oxidase activity

Isolated plasma membrane vesicles and the plasma membrane NADH oxidase partially purified from soybean plasma membrane vesicles exhibited a cyanide-insensitive vitamin K(1) hydroquinone oxidase activity with isolated plasma membrane vesicles. Reduced vitamin K(1) (phylloquinol) was oxidized at a rate of about 10 nmol/min/mg protein as determined by reduced vitamin K(1) reduction or oxygen consumption. The K(m) for reduced K(1) was 350 microM. With the partially purified enzyme, reduced vitamin K(1) was oxidized at a rate of about 600 nmol/min/mg protein and the K(m) was 400 microM. When assayed in the presence of 1 mM KCN, activities of both plasma membrane vesicles and of the purified protein were stimulated (0.1 microM) or inhibited (0.1 mM) by the synthetic auxin growth factor 2, 4-dichlorophenoxyacetic acid. The findings suggest the potential participation of the plasma membrane NADH oxidase as a terminal oxidase of plasma membrane electron transport from cytosolic NAD(P)H via reduced vitamin K(1) to acceptors (molecular oxygen or protein disulfides) at the cell surface.

Cell-surface NAD(P)H-oxidase: relationship to trans-plasma membrane NADH-oxidoreductase and a potential source of circulating NADH-oxidase

The surface of mammalian cells faces an oxidizing environment that has the potential to damage proteins, lipids, and carbohydrates to which it is exposed. In contrast, the cytoplasm is reducing and its redox state is tightly regulated. Trans-plasma membrane oxidoreductases that shift electrons from cytosolic NADH to external electron acceptors such as oxygen are widely involved in cellular redox control. They reduce oxygen to water and may generate reactive oxygen species such as superoxide and hydrogen peroxide. In addition, external NAD(P)H-oxidases have been demonstrated on intact cells and as eluted proteins, but the relationship between trans-plasma membrane NADH-oxidoreductases and cell-surface NAD(P)H-oxidases is not known. To investigate further the relationship between plasma membrane NAD(P)H-oxidoreductases, and to gain insight into the physiological functions of these redox active membrane proteins, we have adapted a simple colorimetric assay for measuring the trans-plasma membrane NADH-oxidoreductase activity of viable cells to measure NAD(P)H-oxidase at the cell surface in real time. Using the cell-impermeable tetrazolium salt WST-1 in the presence of NADH or NADPH, but in the absence of an intermediate electron acceptor, we show that cell-surface NAD(P)H-oxidase is widely expressed on mammalian cells, being more abundant on rapidly proliferating cells than on resting neutrophils and spleen cells. The ratio of cofactor dependence of NAD(P)H-oxidase (NADH:NADPH) varied widely between different cells (0.7-5.2), suggesting a family of cell surface oxidases or that the activity of these enzymes may be modulated in various ways. Comparison of NAD(P)H-oxidase on the surface of viable cells with trans-membrane NADH-oxidoreductase, measured with WST-1 in the presence of 1-methoxy PMS, showed that cell-surface NAD(P)H-oxidase was differentially inhibited by the cell-impermeable thiol-blocking agent pCMBS, but was unaffected or stimulated by other thiol blocking agents. Capsaicin, which inhibits trans-plasma membrane NADH-oxidoreductase activity, stimulated surface NAD(P)H-oxidase. Metabolic inhibitors had little effect on surface NAD(P)H-oxidase activity but inhibited trans-plasma membrane activity. These results do not support the view the surface NAD(P)H-oxidase is a terminal oxidase for trans-plasma membrane NADH-oxidoreductase.

High-capacity redox control at the plasma membrane of mammalian cells: trans-membrane, cell surface, and serum NADH-oxidases

The high capacity of proliferating mammalian cells to transfer electrons from cytosolic NADH to extracellular electron acceptors like oxygen is poorly understood and not widely recognized. Nevertheless, trans-plasma membrane electron transport (plasma membrane redox control) probably ranks alongside the Na+/H+ antiport system (pH control) and glucose transport in facilitating cellular responses to physiological stimuli. These plasma membrane transport systems are acutely responsive to receptor ligation by growth factors, polypeptide hormones, and other cell activators. A novel tetrazolium-based cell proliferation assay that we have shown to measure an NADH-oxidoreductase component of the trans-plasma membrane electron transport system has allowed direct comparisons with NADH:ferricyanide-oxidoreductase and respiratory burst NADPH-oxidoreductase. In addition, an NAD(P)H-oxidase at the cell surface and an NADH-oxidase activity in body fluids can be measured by modifying the basic cell proliferation assay. As determined by reduction of the cell-impermeable tetrazolium reagent, WST-1, electron transfer across the plasma membrane of dividing cells can exceed that of fully activated human peripheral blood neutrophils. Cellular reduction of WST-1 is dependent on the presence of an intermediate electron acceptor and is inhibited by superoxide dismutase (SOD) and by oxygen, implying indirect involvement of superoxide in WST-1 reduction. Cell-surface NAD(P)H-oxidase and serum NADH-oxidase are shown to be distinct from trans-plasma membrane NADH-oxidoreductase by their differential sensitivity to capsaicin and pCMBS. The glycolytic metabolism of cancer cells may be linked to changes in trans-plasma membrane NADH:WST-1-oxidoreductase activity and to increased serum NADH-oxidase in cancer.

Coenzyme Q versus hypertension: does CoQ decrease endothelial superoxide generation?

Reports from several research groups--including two small double-blind clinical studies--indicate that supplemental coenzyme Q10 (CoQ) is moderately effective as a treatment for hypertension, in humans and in animals. Its efficacy is associated with a decrease in total peripheral resistance, and appears to reflect a direct impact of CoQ on the vascular wall. A reasonable interpretation of these findings is that CoQ is acting as an antagonist of vascular superoxide--either scavenging it, or suppressing its synthesis. By improving the efficiency of shuttle mechanisms that transfer high-energy electrons from the cytoplasm to the mitochondrial respiratory chain, CoQ may decrease cytoplasmic NADH levels and thereby diminish the reductive power that drives superoxide synthesis in endothelium and vascular smooth muscle. If CoQ therapy does indeed lower vascular superoxide levels, it can be expected to decrease the atherothrombotic risk associated with hypertension, and may have broader utility in the management of disorders characterized by endotheliopathy.

Use of dipyridyl-dithio substrates to measure directly the protein disulfide-thiol interchange activity of the auxin stimulated NADH: protein disulfide reductase (NADH oxidase) of soybean plasma membranes

Dipyridyl-dithio substrates were cleaved by isolated vesicles of plasma membranes prepared from etiolated hypocotyls of soybean. The cleavage was stimulated by auxins at physiological concentrations. The substrates utilized were principally 2,2'-dithiodipyridine (DTP) and 6,6'-dithiodinicotinic acid (DTNA). The DTP generated 2 moles of 2-pyridinethione whereas the 6,6'-dithiodinicotinic acid generated 2 moles of 6-nicotinylthionine. Both products absorbed at 340 nm. The auxin herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D) stimulated the activity approximately 2-fold to a maximum at about 10 microM. Concentrations of 2,4-D greater than 100 microM inhibited the activity. Indole-3-acetic acid stimulated the activity as well. The growth-inactive auxin, 2,3-dichlorophenoxyacetic acid (2,3-D), was without effect. DTNA cleavage correlated with oxidation of NADH and reduction of protein disulfide bonds reported earlier in terms of location at the external plasma membrane surface, absolute specific activity, pH dependence and auxin specificity. The dipyridyl-dithio substrates provide, for the first time, a direct measure of the disulfide-thiol interchange activity of the protein previously measured only indirectly as an auxin-dependent ability of isolated plasma membrane vesicles to restore activity to scrambled and inactive RNase.

Stereocontrolled Synthesis of Key Advanced Intermediates toward Simplified Acetogenin Analogues

The stereo- and enantiocontrolled synthesis of substituted beta-hydroxy ethers based on glycol and catechol bearing an alkyne group and a series of substituents is reported. These substrates were designed to mimic the bis-THF array of annonaceous acetogenins and to provide an access to simplified and modified analogues. The key steps of the synthesis involve the condensation of the nonracemic mesylate of solketal with ethylene glycol and catechol, followed by an alkylation with a glycidyl derivative. Under appropriate conditions, the reaction is completely stereoselective and allows the synthesis of all the diastereomers. After the epoxide was opened with triethylsilylacetylene, the second epoxide was unmasked and reacted with a series of alkyl, aryl, amine, and alcohol reagents. A series of 28 analogues was prepared having a glycol or a catechol core, a stereodefined configuration of the flanking hydroxyl groups, and an acetylenic appendage suitable for a coupling to a lactone-bearing fragment.

Semisynthesis and cytotoxicity of amino acetogenins and derivatives

Semisynthetic derivatives were prepared from two natural annonaceous acetogenins, rolliniastatin-1 and squamocin, and their cytotoxicity was evaluated. Amino derivatives show decreased bioactivity. Isorolliniastatin-1 was found to be much less toxic than rolliniastatin-1 after intraperitoneal administration to mice, although the in vitro cytotoxicity of both compounds was comparable.

Reduction of a tetrazolium salt, CTC, by intact HepG2 human hepatoma cells: subcellular localisation of reducing systems

Cell-mediated reduction of tetrazolium salts, including MTT, XTT, MTS, NBT, NTV, INT, in the presence or absence of intermediate electron carriers is used as a convenient test for animal or bacterial cell viability. Bioreduction of tetrazolium is considered an alternative to a clonogenic assay and a thymidine incorporation assay. However, correlation between clonogenic potential and capacity to reduce tetrazolium has not been demonstrated convincingly. Moreover, despite a wide use of tetrazolium viability assays, the mechanism and subcellular localisation of reducing systems or species in viable intact cells have not been fully elucidated. We report evidence indicating that a tetrazolium salt CTC can be reduced in the presence as well as in the absence of an electron carrier by viable HepG2 human hepatoma cells. CTC-formazan is formed within or at the outer surface of plasma membranes. We hypothesise that in the presence of an electron carrier the electron donors active in the reduction of CTC are located in the intracellular compartment, as well as in plasma membranes. However, in the absence of an electron carrier, the reduction occurs primarily via a plasma membrane-associated enzymatic system or species.

Plasma membrane NADH oxidase is gravi-responsive

NADH oxidase activities measured with intact tissue sections and with isolated plasma membrane vesicles from etiolated hypocotyls of soybean (Glycine max) respond to gravity and imposed centrifugal forces. The response is one of inhibition of activity with tissue sections lying flat for 20 min or less at 1 x g and one of stimulation with times of lying flat of 30 min or longer at 1 x g. Turning the tissue sections upside down resulted in stimulation of the activity with a lag of about 30 min. Returning the sections to the normal upright position resulted in a return to initial rates with a lag of less than 20 min. Both the stimulated and non-stimulated activities oscillate with a period of 24 min, precluding a more precise analyses of lag times. The activity was stimulated reversibly to a maximum of about 2-fold both in tissue sections and in isolated plasma membrane vesicles when subjected to centrifugal forces of 10 to 400 x g for 0.5 to 4 min duration. The findings are the first description of a gravi-responsive enzymatic activity related to the growth response in plants.