Electron and proton transport across the plasma membrane

Nanosecond Pulsed Electric Fields (nsPEFs) Modulate Electron Transport in the Plasma Membrane and the Mitochondria

Nanosecond pulsed electric fields (nsPEFs) are a pulsed power technology known for ablating tumors, but they also modulate diverse biological mechanisms. Here we show that nsPEFs regulate trans-plasma membrane electron transport (tPMET) rates in the plasma membrane redox system (PMRS) shown as a reduction of the cell-impermeable, WST-8 tetrazolium dye. At lower charging conditions, nsPEFs enhance, and at higher charging conditions inhibit tPMET in H9c2 non-cancerous cardiac myoblasts and 4T1-luc breast cancer cells. This biphasic nsPEF-induced modulation of tPMET is typical of a hormetic stimulus that is beneficial and stress-adaptive at lower levels and damaging at higher levels. NsPEFs also attenuated mitochondrial electron transport system (ETS) activity (O2 consumption) at Complex I when coupled and uncoupled to oxidative phosphorylation. NsPEFs generated more reactive oxygen species (ROS) in mitochondria (mROS) than in the cytosol (cROS) in non-cancer H9c2 heart cells but more cROS than mROS in 4T1-luc cancer cells. Under lower charging conditions, nsPEFs support glycolysis while under higher charging conditions, nsPEFs inhibit electron transport in the PMRS and the mitochondrial ETS producing ROS, ultimately causing cell death. The impact of nsPEF on ETS presents a new paradigm for considering nsPEF modulation of redox functions, including redox homeostasis and metabolism.

Interaction effect of fungicide and chitosan on non-target lichenized fungi

Excessive use of plant growth stimulants and pesticides is currently a considerable problem, especially in agriculture, horticulture, and arboriculture. Understanding the impacts of these compounds and their combinations on non-target organisms is crucial to minimize unintended consequences, while maintaining their use in plant protection. The aim of this study was to test how long-term spraying with different solutions of natural biostimulator chitosan, synthetic fungicide Switch 62.5 WG, and their combinations affects the physiology of epiphytic lichen Xanthoria parietina naturally occurring in fruit orchards and farmlands. We showed that fungicides composed of fludioxionil and cypronidil, as well as the combined use of such fungicides together with chitosan, can cause the considerable impairment of lichen physiology, and these disturbances relate to both algal and fungal partners of the symbiotic association. This negative effect was especially visible in the loss of cell membrane integrity, the high level of membrane lipid peroxidation, and changes in chlorophyll fluorescence parameters on the last day of the experiment. The combined use of these agents also leads to clear disturbances in the functioning of the mitochondrial respiratory chain, which was manifested by increased NADH dehydrogenase activity, while the use of these compounds separately led to a decrease in the activity of this enzyme. We concluded that the regular use of these agents in fruit tree cultivation may cause serious ecological consequences for epiphytic lichen communities as a result of the death of lichen thalli. This study suggests that the impact of some plant protection agents, both individually and in combinations, merits further attention in terms of their impact on non-target fungi.

Insights into the New Cancer Therapy through Redox Homeostasis and Metabolic Shifts

Modest levels of reactive oxygen species (ROS) are necessary for intracellular signaling, cell division, and enzyme activation. These ROS are later eliminated by the body’s antioxidant defense system. High amounts of ROS cause carcinogenesis by altering the signaling pathways associated with metabolism, proliferation, metastasis, and cell survival. Cancer cells exhibit enhanced ATP production and high ROS levels, which allow them to maintain elevated proliferation through metabolic reprograming. In order to prevent further ROS generation, cancer cells rely on more glycolysis to produce ATP and on the pentose phosphate pathway to provide NADPH. Pro-oxidant therapy can induce more ROS generation beyond the physiologic thresholds in cancer cells. Alternatively, antioxidant therapy can protect normal cells by activating cell survival signaling cascades, such as the nuclear factor erythroid 2-related factor 2 (Nrf2)-Kelch-like ECH-associated protein 1 (Keap1) pathway, in response to radio- and chemotherapeutic drugs. Nrf2 is a key regulator that protects cells from oxidative stress. Under normal conditions, Nrf2 is tightly bound to Keap1 and is ubiquitinated and degraded by the proteasome. However, under oxidative stress, or when treated with Nrf2 activators, Nrf2 is liberated from the Nrf2-Keap1 complex, translocated into the nucleus, and bound to the antioxidant response element in association with other factors. This cascade results in the expression of detoxifying enzymes, including NADH-quinone oxidoreductase 1 (NQO1) and heme oxygenase 1. NQO1 and cytochrome b5 reductase can neutralize ROS in the plasma membrane and induce a high NAD⁺/NADH ratio, which then activates SIRT1 and mitochondrial bioenergetics. NQO1 can also stabilize the tumor suppressor p53. Given their roles in cancer pathogenesis, redox homeostasis and the metabolic shift from glycolysis to oxidative phosphorylation (through activation of Nrf2 and NQO1) seem to be good targets for cancer therapy. Therefore, Nrf2 modulation and NQO1 stimulation could be important therapeutic targets for cancer prevention and treatment.

Reducing strength prevailing at root surface of plants promotes reduction of Ag+ and generation of Ag(0)/Ag2O nanoparticles exogenously in aqueous phase

Potential of root system of plants from wide range of families to effectively reduce membrane impermeable ferricyanide to ferrocyanide and blue coloured 2,6-dichlorophenol indophenol (DCPIP) to colourless DCPIPH₂ both under non-sterile and sterile conditions, revealed prevalence of immense reducing strength at root surface. As generation of silver nanoparticles (NPs) from Ag⁺ involves reduction, present investigations were carried to evaluate if reducing strength prevailing at surface of root system can be exploited for reduction of Ag⁺ and exogenous generation of silver-NPs. Root system of intact plants of 16 species from 11 diverse families of angiosperms turned clear colorless AgNO₃ solutions, turbid brown. Absorption spectra of these turbid brown solutions showed silver-NPs specific surface plasmon resonance peak. Transmission electron microscope coupled with energy dispersive X-ray confirmed the presence of distinct NPs in the range of 5–50 nm containing Ag. Selected area electron diffraction and powder X-ray diffraction patterns of the silver NPs showed Bragg reflections, characteristic of crystalline face-centered cubic structure of Ag⁰ and cubic structure of Ag₂O. Root system of intact plants raised under sterile conditions also generated Ag⁰/Ag₂O-NPs under strict sterile conditions in a manner similar to that recorded under non-sterile conditions. This revealed the inbuilt potential of root system to generate Ag⁰/Ag₂O-NPs independent of any microorganism. Roots of intact plants reduced triphenyltetrazolium to triphenylformazon and impermeable ferricyanide to ferrocyanide, suggesting involvement of plasma membrane bound dehydrogenases in reduction of Ag⁺ and formation of Ag⁰/Ag₂O-NPs. Root enzyme extract reduced triphenyltetrazolium to triphenylformazon and Ag⁺ to Ag⁰ in presence of NADH, clearly establishing potential of dehydrogenases to reduce Ag⁺ to Ag⁰, which generate Ag⁰/Ag₂O-NPs. Findings presented in this manuscript put forth a novel, simple, economically viable and green protocol for synthesis of silver-NPs under ambient conditions in aqueous phase, using root system of intact plants.

Microbial volatile-induced accumulation of exceptionally high levels of starch in Arabidopsis leaves is a process involving NTRC and starch synthase classes III and IV

Microbial volatiles promote the accumulation of exceptionally high levels of starch in leaves. Time-course analyses of starch accumulation in Arabidopsis leaves exposed to fungal volatiles (FV) emitted by Alternaria alternata revealed that a microbial volatile-induced starch accumulation process (MIVOISAP) is due to stimulation of starch biosynthesis during illumination. The increase of starch content in illuminated leaves of FV-treated hy1/cry1, hy1/cry2, and hy1/cry1/cry2 Arabidopsis mutants was many-fold lower than that of wild-type (WT) leaves, indicating that MIVOISAP is subjected to photoreceptor-mediated control. This phenomenon was inhibited by cordycepin and accompanied by drastic changes in the Arabidopsis transcriptome. MIVOISAP was also accompanied by enhancement of the total 3-phosphoglycerate/Pi ratio, and a two- to threefold increase of the levels of the reduced form of ADP-glucose pyrophosphorylase. Using different Arabidopsis knockout mutants, we investigated the impact in MIVOISAP of downregulation of genes directly or indirectly related to starch metabolism. These analyses revealed that the magnitude of the FV-induced starch accumulation was low in mutants impaired in starch synthase (SS) classes III and IV and plastidial NADP-thioredoxin reductase C (NTRC). Thus, the overall data showed that Arabidopsis MIVOISAP involves a photocontrolled, transcriptionally and post-translationally regulated network wherein photoreceptor-, SSIII-, SSIV-, and NTRC-mediated changes in redox status of plastidial enzymes play important roles.

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.

A role for Na+/H+ exchangers and intracellular pH in regulating vitamin C-driven electron transport across the plasma membrane

Ascorbate (vitamin C) is the major electron donor to a tPMET (transplasma membrane electron transport) system that was originally identified in human erythrocytes. This plasma membrane redox system appears to transfer electrons from intracellular ascorbate to extracellular oxidants (e.g. non-transferrin-bound iron). Although this phenomenon has been observed in nucleated cells, its mechanism and regulation are not well understood. In the present study we have examined both facets of this phenomenon in K562 cells and primary astrocyte cultures. Using ferricyanide as the analytical oxidant we demonstrate that tPMET is enhanced by dehydroascorbate uptake via facilitative glucose transporters, and subsequent accumulation of intracellular ascorbate. Additionally, we demonstrate that this stimulation is not due to ascorbate that is released from the cells, but is dependent only on a restricted intracellular pool of the vitamin. Substrate-saturation kinetics suggest an enzyme-catalysed reaction across the plasma membrane by an as-yet-unidentified reductase that relies on extensive recycling of intracellular ascorbate. Inhibition of ascorbate-stimulated tPMET by the NHE (Na+/H+-exchanger) inhibitors amiloride and 5-(N-ethyl-N-isopropyl)amiloride, which is diminished by bicarbonate, suggests that tPMET activity may be regulated by intracellular pH. In support of this hypothesis, tPMET in astrocytes was significantly inhibited by ammonium chloride-pulse-induced intracellular acidification, whereas it was significantly stimulated by bicarbonate-induced intracellular alkalinization. These results suggest that ascorbate-dependent tPMET is enzyme-catalysed and is modulated by NHE activity and intracellular pH.

Involvement of plasma membrane redox systems in hormone action

Reactive oxygen species (ROS) is the common name used to describe the partially reduced forms of molecular oxygen that may be generated in cells during oxidative metabolism. They are normally considered to be toxic, and cells possess various defence systems to protect themselves including antioxidant enzymes and low molecular weight antioxidants like vitamin C and vitamin E. However, it is now clear that small amounts of ROS also act as messenger molecules in cell signal transduction pathways; the plasma membrane of eukaryotic cells in particular contains a variety of different ROS-producing oxidases and reductases, of which the best characterized are the superoxide-producing NADPH oxidases. It has been known for many years that membrane redox activity can be changed rapidly by various hormones and growth factors, but the molecular mechanisms involved and the physiological importance of this phenomenon have only recently begun to be unveiled. This review summarizes the state of the art on plasma membrane-based ROS signalling in the pathways of insulin, steroid and thyroid hormones and growth factors. The apparent paradox of ROS being essential biomolecules in the regulation of cellular functions, but also toxic by-products of metabolism, may be important for the pharmacological application of natural and synthetic antioxidants.

Redox activity ofHydrodictyon reticulatum plasmalemma vesicles

Plasmalemma vesicles with preserved redox activity were prepared from nets ofHydrodictyon reticulatum. Since the walls are mechanically very resistant, a combination of partial cell-wall enzyme digestion and ultrasonic homogenization had to be used for the disruption of cells. To isolate the plasma-membrane-enriched microsomal fraction separation in an aqueous two-phase polymer system was found to be most suitable. The right-side-out vesicles reduced added hexacyanoferrate(III) by electrons supplied either by a transmembrane flow or from external NADH.

Erythrocyte plasma membrane redox system in human aging

Eukaryotic cells display a plasma membrane redox system (PMRS) that transfers electrons from intracellular substrates to extracellular electron acceptors. The physiologic importance of PMRS is still not fully understood. The authors have carried out studies to determine the activity of PMRS in human erythrocytes as a function of age and correlate the activity with total plasma antioxidant capacity in an effort to understand the role of PMRS in human aging. The study was carried out on 80 normal healthy subjects of both genders between the ages of 18 and 85 years. The activity of erythrocyte PMRS was estimated by following the reduction of ferricyanide. The total antioxidant capacity of the plasma was estimated in terms of Ferric Reducing Ability of Plasma (FRAP) values. A significant (p < 0.0001) positive correlation (r = 0.7797) is observed between PMRS activity of erythrocytes and human age. There is an age-dependent decrease in total plasma antioxidant capacity measured in terms of FRAP values. A highly significant correlation is observed between PMRS activity and plasma FRAP values. The authors hypothesize that the increased PMRS in erythrocytes during aging may be a protective mechanism of the system for efficient extracellular DHA reduction and ascorbate recycling under condition of increased oxidative stress.

Up-regulation of plasma membrane-associated redox activities in neuronal cells lacking functional mitochondria

Mitochondria-deficient cells (rho(o) cells) survive through enhanced glycolytic metabolism in the presence of pyruvate and uridine. The plasma membrane redox system (PMRS) contains several NAD(P)H-related enzymes and plays a key role in maintaining the levels of NAD(+)/NADH and reduced coenzyme Q. In this study, rho(o) cells were used to investigate how the PMRS is regulated under conditions of mitochondrial dysfunction. rho(o) cells exhibited a lower oxygen consumption rate and higher levels of lactate than parental cells, and were more sensitive to glycolysis inhibitors (2-deoxyglucose and iodoacetamide) than control cells. However, they were more resistant to H(2)O(2), consistent with increased catalase activity and decreased oxidative damage (protein carbonyls and nitrotyrosine). PM-associated redox enzyme activities were enhanced in rho(o) cells compared to those in control cells. Our data suggest that all PMRS enzymes and biomarkers tested are closely related to the ability of the PMs to maintain redox homeostasis. These results illustrate that an up-regulated PM redox activity can protect cells from oxidative stress as a result of an improved antioxidant capacity, and suggest a mechanism by which neurons adapt to conditions of impaired mitochondrial function.

The plasma membrane redox system in aging

Oxidative stress over time leads to the accumulation of damaged macromolecules and to profound physiological changes that are associated with several age-related diseases. The plasma membrane redox system (PMRS) appears to attenuate oxidative stress acting as a compensatory mechanism during the aging process. The PMRS appears to play a protective role during mitochondrial dysfunction to provide cells with a survival mechanism by lowering oxidative stress. The PMRS accomplishes this by producing more NAD(+) for glycolytic ATP production via transfer of electrons from intracellular reducing equivalents to extracelluar acceptors. Ubiquinone and alpha-tocopherol are key antioxidant molecules in the plasma membrane that are affected by aging and can be up-regulated by dietary interventions such as calorie restriction (CR). Up-regulation of PMRS activity leads to cell survival and membrane homeostasis under stress conditions and during calorie restriction. Further studies of the PMRS may provide not only additional information on the mechanisms involved in aging and CR, but may provide therapeutic targets for the prevention and treatment of age-related diseases.

Distinct trans-plasma membrane redox pathways reduce cell-impermeable dyes in HeLa cells

Trans-plasma membrane electron transport (tPMET) in mammalian cells has been demonstrated using artificial cell-impermeable dyes, but the extent to which reduction of these dyes involves distinct pathways remains unclear. Here we compare the properties of three commonly used dyes, WST-1, FeCN and DCIP. The presence of an intermediate electron carrier (mPMS or CoQ(1)) was obligatory for WST-1 reduction, whereas FeCN and DCIP were reduced directly. FeCN reduction was, however, greatly enhanced by CoQ(1), whereas DCIP was unaffected. Superoxide dismutase (SOD) and aminooxyacetate (AOA), a malate/aspartate shuttle inhibitor, strongly inhibited WST-1 reduction and reduced DCIP reduction by 40-60%, but failed to affect FeCN reduction, indicating involvement of mitochondrial TCA cycle-derived NADH and a possible role for superoxide in WST-1 but not FeCN reduction. Reduction of all three substrates was similarly inhibited by dicoumarol, diphenyleneiodonium and capsaicin. These results demonstrate that WST-1 FeCN and DCIP are reduced by distinct tPMET pathways.

Mouse liver plasma membrane redox system activity is altered by aging and modulated by calorie restriction

Caloric restriction (CR) is known as the only non-genetic method proven to slow the rate of aging and extend lifespan in animals. Free radicals production emerges from normal metabolic activity and generates the accumulation of oxidized macromolecules, one of the main characteristics of aging. Due to its central role in cell bioenergetics, a great interest has been paid to CR-induced modifications in mitochondria, where CR has been suggested to decrease reactive oxygen species production. The plasma membrane contains a trans-membrane redox system (PMRS) that provides electrons to recycle lipophilic antioxidants, such as α-tocopherol and coenzyme Q (CoQ), and to modulate cytosolic redox homeostasis. In the present study, we have investigated age differences in the PMRS in mouse liver and their modulation by CR. Aging induced a decrease in the ratio of CoQ10/CoQ9 and α-tocopherol in liver PM from AL-fed mice that was attenuated by CR. CoQ-dependent NAD(P)H dehydrogenases highly increased in CR old mice liver PMs. On the other hand, the CoQ-independent NADH-FCN reductase activity increased in AL-fed animals; whereas, in mice under CR this activity did not change during aging. Our results suggest that liver PMRS activity changes during aging and that CR modulates these changes. By this mechanism CR maintains a higher antioxidant capacity in liver PM of old animals by increasing the activity of CoQ-dependent reductases. Also, the putative role of PMRS in the modulation of redox homeostasis of cytosol is implicated.

Plasma membrane redox and control of sirtuin

We consider possible contributions of plasma membrane redox systems to Aging control by sirtuin (SIR). Reported changes in plasma membrane redox introduced by calorie restriction (CR) may lead to activation of SIR. The most obvious effect would lie in the increase of NAD+ as a result of NADH oxidation. So the question arises, do the observed changes herald an increase in NADH oxidase under CR? The other possibility is an increase in expression of SIR by activation of plasma membrane oxidase. Previous experiments have shown that activation of the plasma membrane redox system can increase cellular NAD+ concentration. The plasma membrane redox systems are also involved in control of protein kinase activity through oxygen radical generation. This activity may be related to control of SIR expression.

Plasma membrane oxidoreductase activity in cultured cells in relation to mitochondrial function and oxidative stress

Dichlorophenol indophenol (DCIP) reduction by intracellualr pyridine nucleotides was investigated in two different lines of cultured cells characterized by enhanced production of reacive oxygen species (ROS) with respect to suitable controls. The first line denominated XTC-UC1 was derived from a metastasis of an oxyphilic thyroid tumor characterized by mitochondrial hyperplasia and compared with a line (B-CPAP) derived from a papillary thyroid carcinoma with normal mitochondrial mass. The second line (170 MN) was a cybrid line derived from rho0 cells from an osteosarcoma line (143B) fused with platelets from a patient with a nucleotide 9957 mutation in mitochondrial DNA (encoding for cytochrome c oxidase subunit III) in comparison with the parent 143B line. The experimental lines had no major decreases of electron transfer activities with respect to the controls; both of them, however, exhibited an increased peroxide production. The XTC-UC1 cell line exhibited enhanced activity with respect to control of dicoumarol-sensitive DCIP reduction, identified with membrane bound DT-diaphorase, whereas dicoumarol insensitive DCIP reduction was not significantly changed. On the other hand the mtDNA mutated cybrids exhibited a strong increase of both dicoumarol sensitive and insensitive DCIP reduction. The results suggest that enhanced oxidative stress and not deficient respiratory activity per se is the stimulus triggering over-expression of plasma membrane oxidative enzymes.

Transplasma membrane electron transport: enzymes involved and biological function

The notion of transmembrane electron transport is usually associated with mitochondria and chloroplasts. However, since the early 1970s, it has been known that this phenomenon also occurs at the level of the plasma membrane. Ever since, evidence has accumulated for the existence of a plethora of transplasma membrane electron transport enzymes. In this review, we discuss the various enzymes known, their molecular characteristics and their biological functions.

A simplified dehydrogenase enzyme assay in contaminated sediment using 2-(p-Iodophenyl)-3(p-nitrophenyl)-5-phenyl tetrazolium chloride

2-(p-Iodophenyl)-3(p-nitrophenyl)-5-phenyl tetrazolium chloride (INT) accepts electrons from dehydrogenase enzymes and is reduced to a red-colored formazan (INTF), which can be quantified by colorimetric analysis. Use of previously published methods for this technique was unsuccessful due to background chemical reactions from high levels of polycyclic aromatic hydrocarbons (PAHs) and metals in the sediments. A modified method using acetonitrile extraction of the INTF was efficient and did not chemically reduce INT. This activity method is simple, quick, inexpensive and precise.

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.

Enhanced activity of the plasma membrane oxidoreductase in circulating lymphocytes from insulin-dependent diabetes mellitus patients

Circulating human lymphocytes contain a transmembrane oxidoreductase (PMOR) capable of reducing dichlorophenol indophenol (DCIP) by endogenous reductants, presumably NADH. Membranes from lymphocytes obtained from buffy coats contain a NADH DCIP reductase having a K(m) of about 1 microM and almost insensible to dicoumarol. The PMOR of lymphocytes from insulin-dependent diabetic patients is higher than that from age-matched controls and, in addition, has a dicoumarol-sensitive component, lacking in most controls, presumably due to membrane association of DT-diaphorase. The increase of PMOR in diabetes is likely due to overexpression of the enzyme, in view of the very low K(m) for NADH indicating that, in intact cells, the enzyme is practically saturated with the reductant substrate.

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.

Biochemical functions of coenzyme Q10

Coenzyme Q is well defined as a crucial component of the oxidative phosphorylation process in mitochondria which converts the energy in carbohydrates and fatty acids into ATP to drive cellular machinery and synthesis. New roles for coenzyme Q in other cellular functions are only becoming recognized. The new aspects have developed from the recognition that coenzyme Q can undergo oxidation/reduction reactions in other cell membranes such as lysosomes. Golgi or plasma membranes. In mitochondria and lysosomes, coenzyme Q undergoes reduction/oxidation cycles during which it transfers protons across the membrane to form a proton gradient. The presence of high concentrations of quinol in all membranes provides a basis for antioxidant action either by direct reaction with radicals or by regeneration of tocopherol and ascorbate. Evidence for a function in redox control of cell signaling and gene expression is developing from studies on coenzyme Q stimulation of cell growth, inhibition of apoptosis, control of thiol groups, formation of hydrogen peroxide and control of membrane channels. Deficiency of coenzyme Q has been described based on failure of biosynthesis caused by gene mutation, inhibition of biosynthesis by HMG coA reductase inhibitors (statins) or for unknown reasons in ageing and cancer. Correction of deficiency requires supplementation with higher levels of coenzyme Q than are available in the diet.

A proposed mechanism for the lowering of mitochondrial electron leak by caloric restriction

Caloric restriction (CR) of laboratory rodents, which extends their maximum lifespan, only transiently reduces the specific metabolic rate of highly oxidative tissues. However, superoxide production by mitochondria of those tissues is greatly reduced by CR. This is probably a major contributor to the slowed aging seen in CR, but its mechanism is unknown. Here it is proposed that the major metabolic shift enabling reduced superoxide production is a diversion of much of the electron flux generated by glycolysis and the TCA cycle away from its usual destination, Complex I, and to the plasma membrane redox system. The cell's ATP synthesis capacity is thereby diminished, but so is its ATP demand, due to reduced turnover of the Na+/K+-ATPase. Direct tests of this hypothesis are proposed.

Pulmonary reduction of an intravascular redox polymer

Pulmonary endothelial cells in culture reduce external electron acceptors via transplasma membrane electron transport (TPMET). In studying endothelial TPMET in intact lungs, it is difficult to exclude intracellular reduction and reducing agents released by the lung. Therefore, we evaluated the role of endothelial TPMET in the reduction of a cell-impermeant redox polymer, toluidine blue O polyacrylamide (TBOP(+)), in intact rat lungs. When added to the perfusate recirculating through the lungs, the venous effluent TBOP(+) concentration decreased to an equilibrium level reflecting TBOP(+) reduction and autooxidation of its reduced (TBOPH) form. Adding superoxide dismutase (SOD) to the perfusate increased the equilibrium TBOP(+) concentration. Kinetic analysis indicated that the SOD effect could be attributed to elimination of the superoxide product of TBOPH autooxidation rather than of superoxide released by the lungs, and experiments with lung-conditioned perfusate excluded release of other TBOP(+) reductants in sufficient quantities to cause significant TBOP(+) reduction. Thus the results indicate that TBOP(+) reduction is via TPMET and support the utility of TBOP(+) and the kinetic model for investigating TPMET mechanisms and their adaptations to physiological and pathophysiological stresses in the intact lung.

Dual role of plasma membrane electron transport systems in defense

Because oxidative stress is one of the main sources of severe cellular damage, cells have different defense weapons against reactive oxygen species. Ubiquitous plasma membrane redox systems play a role in defense against oxidative stress damage. On the other hand, a tightly controlled and localized production of reactive oxygen species by a plasma membrane NADPH oxidase can be used as a potent microbicidal weapon. This dual, prooxidant and antioxidant role of plasma membrane electron transport systems in defense is studied and discussed.

Oxygen-sensing persistent sodium channels in rat hippocampus

1. Persistent sodium channel activity was recorded before and during hypoxia from cell-attached and inside-out patches obtained from cultured hippocampal neurons at a pipette potential (Vp) of +30 mV. Average mean current (IU) of these channels was very low under normoxic conditions and was similar in cell-attached and excised inside-out patches (-0.018 +/- 0.010 and -0.025 +/- 0.008 pA, respectively, n = 24). 2. Hypoxia increased the activity of persistent sodium channels in 10 cell-attached patches (IU increased from -0. 026 +/- 0.016 pA in control to -0.156 +/- 0.034 pA during hypoxia, n = 4, P = 0.013). The increased persistent sodium channel activity was most prominent at a VP between +70 and +30 mV (membrane potential, Vm = -70 to -30 mV) and could be blocked by lidocaine, TTX or R56865 (n = 5). Sodium cyanide (NaCN, 5 mM; 0.5-5 min) increased persistent sodium channel activity in cell-attached patches (n = 3) in a similar manner. 3. Hypoxia also increased sodium channel activity in inside-out patches from hippocampal neurons. Within 2-4 min of exposure to hypoxia, I had increased 9-fold to -0. 18 +/- 0.04 pA (n = 21, P = 0.001). Sodium channel activity increased further with longer exposures to hypoxia. 4. The hypoxia-induced sodium channel activity in inside-out patches could be inhibited by exposure to 10-100 microM lidocaine applied via the bath solution (I = -0.03 +/- 0.01 pA, n = 8) or by perfusion of the pipette tip with 1 microM TTX (I = -0.01 +/- 0.01 pA, n = 3). 5. The reducing agent dithiothreitol (DTT, 2-5 mM) rapidly abolished the increase in sodium channel activity caused by hypoxia in excised patches (I = -0.01 +/- 0.01 pA, n = 4). Similarly, reduced glutathione (GSH, 5-20 mM) also reversed the hypoxia-induced increase in sodium channel activity (IU = -0.02 +/- 0.02 pA, n = 5). 6. These results suggest that persistent sodium channels in neurons can sense O2 levels in excised patches of plasma membrane. Hypoxia triggers an increase in sodium channel activity. The redox reaction involved in increasing the sodium channel activity probably occurs in an auxiliary regulatory protein, co-localized in the plasma membrane.

Phorboid 20-homovanillates induce apoptosis through a VR1-independent mechanism

BACKGROUND

Vanilloids, such as capsaicin and resiniferatoxin (RTX), are recognized at the cell surface by vanilloid receptor type 1 (VR1), which has recently been cloned. VR1 mediates the effects of capsaicin and RTX in VR1-expressing cells, but vanilloids can induce apoptosis through a pathway not mediated by VR1. Phorboid 20-homovanillates can be used to investigate cell death induced by vanilloids.

RESULTS

12,13-Diacylphorbol-20 homovanillates were prepared by the sequential esterification of the natural polyol. Phorbol 12-phenylacetate 13-acetate 20-homovanillate (PPAHV) induced apoptosis in Jurkat cells to the same extent as RTX. Apoptosis was preceded by an increase in intracellular reactive oxygen species and by the loss of mitochondrial transmembrane potential. PPAHV-induced apoptosis was mediated by a pathway involving caspase-3 activation and was initiated at the S phase of the cell cycle. The cell-death pathway triggered by VR1 activation was studied in 293T cells transfected with the cloned rat vanilloid receptor. In this system, capsaicin and PPAHV induced cell death by an apparent necrotic mechanism, which was selectively inhibited by the competitive vanilloid receptor antagonist capsazepine. Interestingly, phorbol-12, 13-bisnonanoate-20-homovanillate, an analogue of PPAHV, induced cell death in VR1-transfected cells but could not trigger apoptosis in the Jurkat cell line.

CONCLUSIONS

Vanilloids can induce cell death through different signalling pathways. The cell death induced in a VR1-independent manner has the hallmark of apoptosis, whereas the cell death mediated by vanilloids binding to VR1 is seemingly necrotic. Phorboid homovanillates that have antitumour and anti-inflammatory activities but lack the undesirable side effects of the natural vanilloids could be developed as potential drugs.

Toluidine blue O and methylene blue as endothelial redox probes in the intact lung

There is increasing evidence that the redox activities of the pulmonary endothelial surface may have important implications for the function of both lungs and blood. Because of the inherent complexity of intact organs, it can be difficult to study these activities in situ. Given the availability of appropriate indicator probes, the multiple-indicator dilution (MID) method is one approach for dealing with some aspects of this complexity. Therefore, the objectives of the present study were to 1) evaluate the potential utility of two thiazine redox indicators, methylene blue (MB) and toluidine blue O (TBO), as MID electron acceptor probes for in situ pulmonary endothelium and 2) develop a mathematical model of the pulmonary disposition of these indicators as a tool for quantifying their reduction on passage through the lungs. Experiments were carried out using isolated rabbit lungs perfused with physiological salt solution with or without plasma albumin over a range of flow rates. A large fraction of the injected TBO disappeared from the perfusate on passage through the lungs. The reduction of its oxidized, strongly polar, relatively hydrophilic blue form to its colorless, highly lipophilic reduced form was revealed by the presence of the reduced form in the venous effluent when plasma albumin was included in the perfusate. MB was also lost from the perfusate, but the fraction was considerably smaller than for TBO. A distributed-in-space-and-time model was developed to estimate the reduction rate parameter, which was approximately 29 and 1.0 ml/s for TBO and MB, respectively, and almost flow rate independent for both indicators. The results suggest the utility particularly of TBO as an electron acceptor probe for MID studies of in situ pulmonary endothelium and of the model for quantitative evaluation of the data.

Plasma membrane NADH-oxidoreductase system: a critical review of the structural and functional data

The observation in the early 1970s that ferricyanide can replace transferrin as a growth factor highlighted the major role plasma membrane proteins can play within a mammalian cell. Ferricyanide, being impermeant to the cell, was assumed to act at the level of the plasma membrane. Since that time, several enzymes isolated from the plasma membrane have been described, which, using NADH as the intracellular electron donor, are capable of reducing ferricyanide. However, their exact modes of action, and their physiological substrates and functions have not been solved to date. Numerous hypotheses have been proposed for the role of such redox enzymes within the plasma membrane. Examples include the regulation of cell signaling, cell growth, apoptosis, proton pumping, and ion channels. All of these roles may be a result of the function of these enzymes as cellular redox sensors. The emergence of many diverse roles for ferricyanide utilizing redox enzymes present in the plasma membrane might also, in part, be due to the numerous redox enzymes present within the membrane; the poor molecular characterization of the enzymes may be the reason for some of the diverging results reported in the literature as various researchers may be working on different enzymes. Here we review the diverse proposals given for structure and function to the plasma membrane NADH-oxidoreductase system(s) with a specific focus on those enzyme activities which can couple ferricyanide and NADH. Although they are still ill-defined enzymes, evidence is rising that they are of utmost significance for cellular regulation.

Cell membrane redox systems and transformation

Cell membrane redox systems carry electrons from intracellular donors and transport them to extracellular acceptors. This phenomenon appears to be universal. Numerous reviews have emphasized not only the bioenergetic mechanisms of redox systems but also the antioxidant defense mechanisms in which they participate. Moreover, significant progress has been made in the modulation of the membrane redox systems on cell proliferation. Because membrane redox systems play a key role in the regulation of cell growth, they need to be somehow linked into the signaling pathways resulting in either controlled or unregulated growth by both internal and external signals. Ultimately, these sequential events lead to either normal cell proliferation or cancer cell formation. However, much less is known about the involvement of membrane redox in transformation or tumorgenesis. In this review, the facts and ideas are summarized concerning the redox systems and tumorgenesis in several aspects, such as the regulation of cell growth and the effect on cell differentiation and on signaling pathways. In addition, information on a unique tumor-associated nicotinamide adenine dinucleotide (NADH) oxidase (tNOX) protein is reviewed.

Effect of lactacidosis on cell volume and intracellular pH of astrocytes

Acute traumatic or ischemic cerebral lesions are associated with tissue acidosis leading to cytotoxic brain edema, predominantly affecting astrocytes. Glial swelling from acidosis is believed to be the attempt of cells to maintain a physiological intracellular pH (pHi). However, this concept, potentially important for the development of new treatment strategies for cytotoxic brain edema, has not been validated experimentally. In the present study, cell volume and pHi of astrocytes were measured simultaneously in vitro. Exposure of suspended astrocytes to levels of acidosis found in vivo during ischemia and trauma (pH 6.8-6.2) led to a maximal increase in cell volume of 121.2% after 60 min (n = 5, p < 0.05) and to immediate intracellular acidification close to extracellular levels (pH 6.2, n = 5, p < 0.05). Inhibition of membrane transporters responsible for pHi regulation (0.1 mM amiloride for the Na+/H+ antiporter or 1 mM SITS for HCO3- -dependent transporters) inhibited cell swelling from acidosis but did not affect the profound intracellular acidification. In addition, acidosis-induced cell swelling and intracellular acidification were partly prevented by the addition of ZnCl2 (0.1 mM), an inhibitor of selective proton channels not yet described in astrocytes (n = 5, p < 0.05). In conclusion, these data demonstrate that glial swelling from acidosis is not a cellular response to defend the normal pHi, as had been thought. If these results obtained in vitro are transferable to in vivo conditions, the development of blood-brain barrier-permeable agents for the inhibition of acidosis-induced cytotoxic edema might be therapeutically useful, since they do not enhance intracellular acidosis and thus cell damage.

Coenzyme Q(10) in submicron-sized dispersion improves development, hatching, cell proliferation, and adenosine triphosphate content of in vitro-produced bovine embryos

Coenzyme Q(10) (CoQ(10)) is an essential component of the plasma membrane ion transporter (PMIT) system and of the electron transport chain in the inner mitochondrial membrane. Because of its intrinsic functions in cell growth and energy metabolism (ATP synthesis), and its protective effects against oxidative stress, CoQ(10) is a good candidate for supporting growth of cells in culture. However, because of its quinone structure, CoQ(10) is extremely lipophilic and practically insoluble in water. We used a specific technology to prepare a submicron-sized dispersion of CoQ(10), inhibiting re-crystallization by a stabilizer. This dispersion, which exhibits a very large specific surface area for drug dissolution, was tested as a supplement for the in vitro culture of bovine embryos in a chemically defined system. The rate of early cleavage of embryos (5- to 8-cell stages) was evaluated 66 h postinsemination (hpi) and was highest in medium supplemented with 30 or 100 microM CoQ(10) (66.5 +/- 0.8% and 68.7 +/- 1.1%, respectively) and lowest in 10 microM CoQ(10) (55.3 +/- 0.8%). The proportions of oocytes developing to blastocysts by 186 hpi were 19.0 +/- 0.6% and 25.2 +/- 0.3% in medium supplemented with 10 microM and 30 microM CoQ(10), respectively, and were significantly (p < 0.001) higher than those obtained with the equivalent amounts of stabilizer (9.9 +/- 0.4% and 11.3 +/- 0.4%). In the presence of 30 microM CoQ(10), significantly (p < 0.001) more blastocysts hatched by 210 hpi than in the equivalent amount of stabilizer (31.8 +/- 1.3 vs. 8.4 +/- 2.2). Expanded blastocysts produced in the presence of 30 microM CoQ(10) had significantly (p < 0.01) more inner cell mass cells and trophectoderm cells, and a significantly (p < 0.001) increased ATP content as compared to expanded blastocysts produced in the presence of the corresponding amount of stabilizer. Our results show that noncrystalline CoQ(10) in submicron-sized dispersion supports the development and viability of bovine embryos produced in a chemically defined culture system.

Is ascorbic acid an antioxidant for the plasma membrane?

Ascorbic acid, or vitamin C, is a primary antioxidant in plasma and within cells, but it can also interact with the plasma membrane by donating electrons to the alpha-tocopheroxyl radical and a trans-plasma membrane oxidoreductase activity. Ascorbate-derived reducing capacity is thus transmitted both into and across the plasma membrane. Recycling of alpha-tocopherol by ascorbate helps to protect membrane lipids from peroxidation. However, neither the mechanism nor function of the ascorbate-dependent oxidoreductase activity is known. This activity has typically been studied using extracellular ferricyanide as an electron acceptor. Whereas an NADH:ferricyanide reductase activity is evident in open membranes, ascorbate is the preferred electron donor within cells. The oxidoreductase may be a single membrane-spanning protein or may only partially span the membrane as part of a trans-membrane electron transport chain composed of a cytochrome or even hydrophobic antioxidants such as alpha-tocopherol or ubiquinol-10. Further studies are needed to elucidate the structural components, mechanism, and physiological significance of this activity. Proposed functions for the oxidoreductase include stimulation of cell growth, reduction of the ascorbate free radical outside cells, recycling of alpha-tocopherol, reduction of lipid hydroperoxides, and reduction of ferric iron prior to iron uptake by a transferrin-independent pathway.

Carcinogenesis and the plasma membrane

Presented is a two-stage hypothesis of carcinogenesis based on: (1) plasma membrane defects that produce abnormal electron and proton efflux; and (2) electrical uncoupling of cells through loss of intercellular communication. These changes can be induced by a wide variety of stimuli including chemical carcinogens, oncoviruses, inherited and/or acquired genetic defects, and epigenetic abnormalities. The resulting loss of electron/proton homeostasis leads to decreased transmembrane potential, electrical microenvironment alterations, decreased extracellular pH, and increased intracellular pH. This produces a positive feedback loop to enhance and sustain the proton/electron efflux and loss of intercellular communication. Low transmembrane potential is functionally related to rapid cell cycling, changes in membrane structure, and malignancy. Intracellular alkalinization affects a variety of pH-sensitive systems including glycolysis, DNA synthesis, DNA transcription and DNA repair, and promotes genetic instability, accounting for the accumulation of genetic defects seen in malignancy. The abnormal microenvironment results in the selective survival and proliferation of malignant cells at the expense of contiguous normal cell populations.

Ascorbate-mediated transplasma membrane electron transport in pulmonary arterial endothelial cells

Pulmonary endothelial cells are capable of reducing certain electron acceptors at the luminal plasma membrane surface. Motivation for studying this phenomenon comes in part from the expectation that it may be important both as an endothelial antioxidant defense mechanism and in redox cycling of toxic free radicals. Pulmonary arterial endothelial cells in culture reduce the oxidized forms of thiazine compounds that have been used as electron acceptor probes for studying the mechanisms of transplasma membrane electron transport. However, they reduce another commonly studied electron acceptor, ferricyanide, only very slowly by comparison. In the present study, we examined the influence of ascorbate [ascorbic acid (AA)] and dehydroascorbate [dehydroascorbic acid (DHAA)] on the ferricyanide and thiazine reductase activities of the bovine pulmonary arterial endothelial cell surface. The endothelial cells were grown on microcarrier beads so that the reduction of ferricyanide and methylene blue could be studied colorimetrically in spectrophotometer cuvettes and in flow-through cell columns. The ferricyanide reductase activity could be increased 80-fold by adding DHAA to the medium, with virtually no effect on methylene blue reduction. The DHAA effect persisted after the DHAA was removed from the medium. AA also stimulated the ferricyanide reductase activity but was less potent, and the relative potencies of AA and DHAA correlated with their relative rates of uptake by the cells. The results are consistent with the hypothesis that AA is an intracellular electron donor for an endothelial plasma membrane ferricyanide reductase and that the stimulatory effect of DHAA is the result of increasing intracellular AA. Adding sufficient DHAA to markedly increase extracellular ferricyanide reduction had little effect on the plasma membrane methylene blue reductase activity, suggesting that pulmonary arterial endothelial cells have at least two separate transplasma membrane electron transport systems.

Transmembrane redox signaling activates NF-kappaB in macrophages

In macrophages, NF-kappaB can be activated by H2O2 generated by the respiratory burst or added exogenously. The mechanism of H2O2 signaling may involve changes in the cellular redox state or a redox reaction at the plasma membrane; however, the site of H2O2 action cannot be readily ascertained because of its membrane permeability. Ferricyanide, a nonpermeable redox active anion, activated NF-kappaB in the macrophage cell line, J774A.1. In contrast with exogenous H2O2, activation by ferricyanide did not correlate with net oxidation of NAD(P)H or glutathione, suggesting that a transplasma membrane redox reaction itself was the first signaling process in NF-kappaB activation.

Multifunctional plasma membrane redox systems

All the biological membranes contain oxidoreduction systems actively involved in their bioenergetics. Plasma membrane redox systems seem to be ubiquitous and they have been related to several important functions, including not only their role in cell bioenergetics, but also in cell defense through the generation of reactive oxygen species, in iron uptake, in the control of cell growth and proliferation and in signal transduction. In the last few years, an increasing number of mechanistic and molecular studies have deeply widened our knowledge on the function of these plasma membrane redox systems. The aim of this review is to summarize what is currently known about the components and physiological roles of these systems.

The diversity of coenzyme Q function

Coenzyme Q is uniquely designed as an electron and proton carrier within the lipid phase of membranes. It now appears that this unique chemistry has diverse application to important functions in all cellular membranes. The first function of coenzyme Q was defined in the energy transduction process in mitochondria. New studies show that the presence of coenzyme Q in other cellular membranes has dynamic rather than passive significance. Coenzyme Q functions in the plasma membrane electron transport involved in activation of signalling protein kinases related to gene activation for cellular proliferation. Furthermore, the antioxidant potential of the reduced coenzyme Q is now taken on a new significance in the evidence that the reduced quinone can act to maintain tocopherol in the reduced state in membranes and ascorbate reduced both inside and outside the cell.

Effectors of the mammalian plasma membrane NADH-oxidoreductase system. Short-chain ubiquinone analogues as potent stimulators

In the presence of effectors variations in the two recognized activities of the plasma membrane NADH-oxidoreductase system were studied in separate, specific in vitro assays. We report here that ubiquinone analogues that contain a short, less hydrophobic side chain than coenzyme Q-10 dramatically stimulate the NADH-oxidase activity of isolated rat liver plasma membranes whereas they show no effect on the reductase activity of isolated membranes. If measured in assays of the NADH:ferricyanide reductase of living cultured cells these compounds have only a limited effect; the oxidase activity of whole cells is not measurable in our hands. We have furthermore identified selective inhibitors of both enzyme activities. In particular, the NADH-oxidase activity can be significantly inhibited by structural analogues of ubiquinone, such as capsaicin and resiniferatoxin. The NADH:ferricyanide reductase, on the other hand, is particularly sensitive to pCMBS, indicating the presence of a sulfhydryl group of groups at its active site. The identification of these specific effectors of the different enzyme activities of the PMOR yields further insights into the function of this system.

Effects of protein kinase C and phosphoprotein phosphatase modulators on Ehrlich cell plasma membrane redox system activity

Diacyl glycerols and phorbol esters, which activate protein kinases C, stimulated Ehrlich ascites tumor cell ferricyanide reductase activity. On the contrary, selective inhibition of active protein kinases C with bis-indolyl maleimide did not change the rate of ferricyanide reduction by Ehrlich cells. Selective inhibitors of phosphoprotein phosphatases, okadaic acid and cyclosporin A, also stimulated plasma membrane redox system. Taking all these data together, protein kinases or phosphoprotein phosphatases seemed to be involved in the multiple and complex regulation of Ehrlich cell plasma membrane redox system.

The plasma membrane NADH-diaphorase is active during selective phases of the cell cycle in mouse neuroblastoma cell line NB41A3. Its relation to cell growth and differentiation

Plasma membrane oxidoreductases have been described in all cells and use extracellular impermeant electron acceptors (DCIP, Ferricyanide) that are reduced by NADH. They appear to regulate the overall cell activity in response to oxidative stress from the cellular environment. An NADH-DCIP reductase has been described at the plasma membrane of NB41A3, a neuroblastoma cell line (Zurbriggen and Dryer (1993) Biochim. Biophys. Acta 1183, 513-520) whose activation with extracellular impermeant substrates promotes cell growth. Elutriation was performed to separate cells and the various fractions were analysed for enzyme activity on intact cells combined with flow cytometry. These studies showed that the enzyme is mostly induced and activated during the G1 and during the G2/M-phases. These observations were further corroborated with specific inhibitors of the cell cycle. A three-fold increase in enzyme activity was observed in the presence of alpha-amanitin, a specific cell cycle inhibitor of the G1-phase. Taxol, a specific inhibitor of the M-phase, also induces a significant increase in enzyme activity. FACS analysis of taxol -treated and alpha-amanitin-treated cells corroborated these data. The cells have been synchronized and the enzyme activity was measured at different time intervals. An activity increase was observed after ca. 2-3 h, that corresponds to a raise in the M-phase, according to FACS data. Furthermore, NTera-2 cells - a human neuroblastoma cell line that differentiates into fully mature neurones in the presence of retinoic acid - exhibit a 50% decrease in the enzyme activity during the G0-phase upon differentiation, compared to undifferentiated cells. Together the data presented in this paper show that this plasma membrane NADH-diaphorase affects cell growth and differentiation and is strongly modulated at various phases of the cell cycle.