Putting together a plasma membrane NADH oxidase: A tale of three laboratories

The observation that high cellular concentrations of NADH were associated with low adenylate cyclase activity led to a search for the mechanism of the effect. Since cyclase is in the plasma membrane, we considered the membrane might have a site for NADH action, and that NADH might be oxidized at that site. A test for NADH oxidase showed very low activity, which could be increased by adding growth factors. The plasma membrane oxidase was not inhibited by inhibitors of mitochondrial NADH oxidase such as cyanide, rotenone or antimycin. Stimulation of the plasma membrane oxidase by iso-proterenol or triiodothyronine was different from lack of stimulation in endoplasmic reticulum. After 25 years of research, three components of a trans membrane NADH oxidase have been discovered. Flavoprotein NADH coenzyme Q reductases (NADH cytochrome b reductase) on the inside, coenzyme Q in the middle, and a coenzyme Q oxidase on the outside as a terminal oxidase. The external oxidase segment is a copper protein with unique properties in timekeeping, protein disulfide isomerase and endogenous NADH oxidase activity, which affords a mechanism for control of cell growth by the overall NADH oxidase and the remarkable inhibition of oxidase activity and growth of cancer cells by a wide range of anti-tumor drugs. A second trans plasma membrane electron transport system has been found in voltage dependent anion channel (VDAC), which has NADH ferricyanide reductase activity. This activity must be considered in relation to ferricyanide stimulation of growth and increased VDAC antibodies in patients with autism.     

Pyridine nucleotide oxidation by a plasma membrane fraction from red beet (Beta vulgaris L.) storage tissue

The potential role of pyridine nucleotide oxidation in the energization and/or regulation of membrane transport was examined using sealed plasma membrane vesicles isolated from red beet (Beta vulgaris L.) storage tissue. In this system, pyridine nucleotide oxidation, which was enhanced in the presence of ferricyanide, occurred. In the presence or absence of ferricyanide, the oxidation of NADH was several-fold greater than the oxidation of NADPH, indicating that it was the preferred substrate for oxidation in this system. Ferricyanide reduction coupled to NADH oxidation did not require the transmembrane movement of reducing equivalents since ferricyanide incorporated inside the vesicles could not be reduced by NADH added externally to the vesicles, unless the vesicles were made leaky by the addition of 0.05% (v/v) Triton X-100. Using fluorescent probes for the measurement of transmembrane pH gradients and membrane potentials, it was determined that NADH oxidation did not result in the production of a proton electrochemical gradient or have any effect upon the proton electrochemical gradient produced by the plasma membrane H+-ATPase. The oxidation of NADH in the presence of ferricyanide did result in the acidification of the reaction medium. This acidification was unaffected by the addition of Gramicidin D and stimulated by the addition of 0.05% (v/v) Triton X-100, suggesting a scalar (nonvectorial) production of protons in the oxidation/reduction reaction. The results of this study suggest that the oxidation of pyridine nucleotides by plasma membrane vesicles is not related to energization of transport at the plasma membrane or modulation of the activity of the plasma membrane H+-ATPase.     

beta-Oxidation and Glyoxylate Cycle Coupled to NADH: Cytochrome c and Ferricyanide Reductases in Glyoxysomes

Glyoxysomes isolated from castor bean (Ricinus communis L., var Hale) endosperm had NADH:ferricyanide reductase and NADH:cytochrome c reductase activities averaging 720 and 140 nanomole electrons/per minute per milligram glyoxysomal protein, respectively. These redox activities were greater than could be attributed to contamination of the glyoxysomal fractions in which 1.4% of the protein was mitochondrial and 5% endoplasmic reticulum. The NADH:ferricyanide reductase activity in the glyoxysomes was greater than the palmitoyl-coenzyme A (CoA) oxidation activity which generated NADH at a rate of 340 nanomole electrons per minute per milligram glyoxysomal protein. Palmitoyl-CoA oxidation could be coupled to ferricyanide or cytochrome c reduction. Complete oxidation of palmitoyl-CoA, yielding 14 nanomole electrons/per nanomole palmitoyl-CoA, was demonstrated with the acceptors, NAL, cytochrome c, and ferricyanide. Malate was also oxidized by glyoxysomes, if acetyl-CoA, ferricyanide, or cytochrome c was present. Glyoxysomal NADH:ferricyanide reductase activity has the capacity to support the combined rates of NADH generation by β-oxidation and the glyoxylate cycle.     

Adriamycin affects plasma membrane redox functions

Adriamycin (Doxorubicin) stimulates NADH oxidase activity in liver plasma membrane, but does not cause NADH oxidase activity to appear where it is not initially present, as in erythrocyte membrane. NADH dehydrogenase from rat liver and erythrocyte plasma membranes shows similar adriamycin effects with other electron acceptors. Both NADH ferricyanide reductase and vanadate-stimulated NADH oxidation are inhibited by adriamycin, as is a cyanide insensitive ascorbate oxidase activity, whereas NADH cytochrome c reductase is not affected. The effects may contribute to the growth inhibitory (control) and/or deleterious effects of adriamycin. It is clear that adriamycin effects on the plasma membrane dehydrogenase involve more than a simple catalysis of superoxide formation.     

Studies on NADH(NADPH)-cytochrome c reductase (FMN-containing) from yeast: steady-state kinetic properties of the flavoenzyme from top-fermenting ale yeast

A study of the steady-state kinetics of NADH(NADPH)-cytochrome c reductase (FMN-containing) from ale yeast (M. S. Johnson and S. A. Kuby (1985) J. Biol. Chem. 260, 12341-12350) has led to a postulated three-substrate random-ordered hybrid mechanism, where NAD(P)H and FMN add randomly and very likely in a steady-state fashion, followed by an ordered addition of cytochrome c. Kinetic parameters have been derived from this mechanism. Arrhenius plots showed large differences between NADH and NADPH, as the substrate-reductant. Menadione accelerated cytochrome c reduction and also O2 uptake, but vitamin K1 and coenzyme Q10 were ineffective as electron mediators, possibly as a result of their insolubility. With NADPH as the substrate-reductant, the order of the rate of reduction of electron acceptors was ferricyanide greater than DCIP greater than cytochrome c greater than oxygen; with menadione, the specificity sequence was cytochrome c greater than ferricyanide greater than DCIP greater than oxygen. With NADH, the order was ferricyanide greater than cytochrome c greater than oxygen greater than DCIP, which changed to cytochrome c greater than ferricyanide greater than oxygen greater than DCIP on addition of menadione. Cytochrome b5 was also reduced in the absence of oxygen. No transhydrogenase activity was observed, but the reduced thionicotinamide analogs of NADH and NADPH acted as substrates. Superoxide dismutase inhibited cytochrome c reduction in air by 50%, but O2-. was not necessary for cytochrome c reduction, as evidenced by the increase in rate in the absence of O2. The product of the reaction with oxygen appeared to be H2O2.     

Plasma Membrane Redox System in Guard Cell Protoplasts of Pea (Pisum sativum L.)

Guard cell protoplasts (GCP) from leaves of pea (Pisum sativum)were capable of reducing/oxidizing the membrane impermeableelectron carriers, ferricyanide/NADH. The redox activity ofGCP required the presence of both ferricyanide and NADH, althoughsome ferricyanide reduction occurred even in the absence ofNADH. The GCP preferred NADH to NADPH during ferricyanide reductionand the reduction was slow with DCPIP or cytochrome c. A stoichiometryof about 2 existed between moles of ferricyanide reduced andNADH oxidized by GCP. The redox activities of GCP were severaltimes greater than those of mesophyll protoplasts from pea leaves.The ferricyanide reduction or NADH oxidation by GCP was unaffectedby abscisic acid or sodium orthovanadate and fusicoccin indicatingthe non-involvement of plasma membrane ATPase in these redoxreactions.The redox activities were markedly inhibited by chloroquineor 8-hydroxyquinoline. The findings are discussed in relationto the possible regulatory role of a guard cell plasma membraneredox system in stomatal function. Key words: Plasma membrane redox system, mesophyll protoplasts, pea, guard cell protoplasts, stomatal function     

NADH-Linked Ferricyanide and Cytochrome c Reduction Activities in Corn Root Plasma Membrane

Corn root plasma membrane catalyzed NADH reduction of ferricyanideand cytochrome c over a wide pH range. At pH 7.5, apparent K_msof NADH-cytochrome c pair were significantly lower than thoseof NADH-ferricyanide pair. FMN and polylysine respectively enhancedthe reduction of ferricyanide and cytochrome c. Yet, polyaspartatedecreased the ferricyanide reduction. NADH oxidation observedin the presence of both ferricyanide and cytochrome c was significantlyslower than the sum of rates obtained with individual acceptors.The results suggest that the membrane may contain differentbut not totally independent reduction sites for cytochrome cand ferricyanide. (Received April 13, 1993; Accepted August 23, 1993)     

NADH dehydrogenase from bovine neutrophil membranes. Purification and properties

A membrane-associated NADH dehydrogenase from beef neutrophils was purified to homogeneity, using detergent (cholate plus Triton X-100) extraction and chromatography on DEAE-Sepharose CL-6B, agarose-hexane-NAD, and hydroxylapatite. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed an apparent subunit molecular weight of 17,500, but the enzyme was highly aggregated (Mr greater than 450,000) in nondenaturing gels containing 0.1% Triton X-100. The protein band in nondenaturing gels was also stained for activity using NADH and nitro blue tetrazolium. The enzyme showed greatest electron acceptor activity with ferricyanide (100%), followed by cytochrome c (3.5%), dichloroindophenol (2.7%), and cytochrome b5 (0.34%). No activity was seen with oxygen. The Km values for NADH and ferricyanide were 18 and 9.5 microM, respectively, and NAD+ was a weak competitive inhibitor (Ki = 118 microM). No activity was seen with NADPH. No effects were seen with mitochondrial respiratory inhibitors such as azide, cyanide, or rotenone, but p-chloromercuribenzoate was strongly inhibitory and N-ethylmaleimide was weakly inhibitory. No free flavin was detectable in enzyme preparations. Based upon kinetic, physical, and inhibition properties, this NADH dehydrogenase differs from those previously described in microsomes and erythrocyte plasma membrane.     

NADH oxidase of liver plasma membrane stimulated by diferric transferrin and neoplastic transformation induced by the carcinogen 2-acetylaminofluorene

NADH oxidase of purified plasma membranes (electron transfer from NADH to oxygen) was stimulated by the growth factor diferric transferrin. This stimulation was of an activity not inhibited by cyanide and was not seen in plasma membranes prepared from hyperplastic nodules from liver of animals fed the hepatocarcinogen, 2-acetylaminofluorene, nor was it due to reduction of iron associated with diferric transferrin. With plasma membranes from nodules, the activity was already elevated and the added transferrin was without effect. The stimulation by diferric transferrin did not correlate with the absence of transferrin receptors which were increased at the nodule plasma membranes. With liver plasma membranes, the stimulation by diferric transferrin raised the plasma membrane NADH oxidase specific activity to approximately that of the nodule plasma membranes. In contrast to NADH oxidase, which was markedly stimulated by the diferric transferrin, NADH ferricyanide oxidoreductase or reduction of ferric ammonium citrate by liver plasma membranes was approximately equal to or slightly greater than that of the nodule plasma membrane and unaffected by diferric transferrin. The results suggest the possibility of coupling of NADH oxidase activity to a growth factor response in mammalian cells as observed previously for this enzyme in another system.     

Ascorbate free-radical reduction by glyoxysomal membranes

Glyoxysomal membranes from germinating castor bean (Ricinus communis L. cv Hale) endosperm contain an NADH dehydrogenase. This enzyme can utilize extraorganellar ascorbate free-radical as a substrate and can oxidize NADH at a rate which can support intraglyoxysomal demand for NAD⁺. NADH:ascorbate free-radical reductase was found to be membrane-associated, and the activity remained in the membrane fraction after lysis of glyoxysomes by osmotic shock, followed by pelleting of the membranes. In whole glyoxysomes, NADH:ascorbate free-radical reductase, like NADH:ferricyanide reductase and unlike NADH:cytochrome c reductase, was insensitive to trypsin and was not inactivated by Triton X-100 detergent. These results suggest that ascorbate free-radical is reduced by the same component which reduces ferricyanide in the glyoxysomal membrane redox system. NADH:ascorbate free-radical reductase comigrated with NADH:ferricyanide and cytochrome c reductases when glyoxy-somal membranes were solubilized with detergent and subjected to rate-zonal centrifugation. The results suggest that ascorbate free-radical, when reduced to ascorbate by membrane redox system, could serve as a link between glyoxysomal metabolism and other cellular activities.     

Ferricyanide Reduction by Guard Cell Protoplasts

Guard cell protoplasts of Commelina communis L. reduced exogenousferricyanide at pH values lower than 5?0; upon addition of NADH,reduction of ferricyanide by guard cell protoplasts was stimulatedover the pH range 4?0 to 9?0 with two peaks of activity at pH5?0 and between pH 8?0 and pH 9?0. Calcium chloride (1?0 molm^–3) and MgCl2 (1?0 mol m^–3) increased the NADH-stimulatedreduction of ferricyanide. Superoxide dismutase and cyanidehad little effect on the NADH-stimulated reduction of ferricyanideby guard cell protoplasts, but, salicylhydroxamic acid completelyinhibited this activity. The NADH-stimulated reduction of ferricyanidealso occurred in the cell-free supernatant. Horseradish peroxidasedid not reduce ferricyanide in the absence of NADH over a broadrange of pH (4?0 to 9?0). However, in the presence of NADH,horseradish peroxidase reduced ferricyanide over the pH range5?0 to 9?0 with maximal activity at pH 8?0. The NADH-stimulatedreduction of ferricyanide by horseradish peroxidase showed similarproperties to those observed with guard cell protoplasts. Mannitol,superoxide dismutase, and cyanide did not inhibit the NADH-stimulatedreduction of ferricyanide by horseradish peroxidase; SHAM, however,completely inhibited the reduction of ferricyanide by horseradishperoxidase. Catalase inhibited the NADH-stimulated reductionof ferricyanide by horseradish peroxidase by 20%, while absenceof oxygen in the assay medium stimulated this activity over60%. We propose that the reduction of ferricyanide in the presenceof NADH by guard cell protoplasts, can be explained in termsof peroxidase activity associated with the plasma membrane andsecreted to the extracellular medium. However, the capacityof guard cell protoplasts to reduce ferricyanide at acid pHvalues where little peroxidase activity occurs may indicatethe presence of a plasma membrane redox system in guard cellsof C. communis. Key words: Commelina, guard cell protoplasts, ferricyanide reduction, peroxidase, redox system     

NADH-ascorbate free radical and -ferricyanide reductase activities represent different levels of plasma membrane electron transport

Plasma membranes isolated from rat liver by two-phase partition exhibited dehydrogenase activities for ascorbate free radical (AFR) and ferricyanide reduction in a ratio of specific activities of 1 : 40. NADH-AFR reductase could not be solubilized by detergents from plasma membrane fractions. NADH-AFR reductase was inhibited in both clathrin-depleted membrane and membranes incubated with anti-clathrin antiserum. This activity was reconstituted in plasma membranes in proportion to the amount of clathrin-enriched supernatant added. NADH ferricyanide reductase was unaffected by both clathrin-depletion and antibody incubation and was fully solubilized by detergents. Also, wheat germ agglutinin only inhibited NADH-AFR reductase. The findings suggest that NADH-AFR reductase and NADH-ferricyanide reductase activities of plasma membrane represent different levels of the electron transport chain. The inability of the NADH-AFR reductase to survive detergent solubilization might indicate the involvement of more than one protein in the electron transport from NADH to the AFR but not to ferricyanide.     

Electron transport in the membrane of lutoids from the latex of Hevea brasiliensis.

1. An antimycin-insensitive NADH-cytochrome c oxidoreductase (E.C. 1.6.99.3) activity can be demonstrated in the membrane of lutoids isolated from the latex of Hevea brasiliensis. This electron transport system can also use ferricyanide as an electron acceptor, but is unable to oxidize NADPH. 2. Two beta-type cytochromes are present in the membranes. Cytochrome beta563 is partially reduced by NADH and ascorbate, but is not reducible by NADPH. It shows a double peak at 555 and 561 nm at 77 degrees K. A second cytochrome, cytochrome beta561, seems to be reducible by hydrosulfite only. 3. In the reduced state, these cytochromes do not combine with CO. The occurrence of cytochrome P-450 could not be demonstrated. 4. The role of the NADH oxidation system is considered in relation to the biosynthesis of polyisoprene compounds in the latex.     

D-lactate oxidation and generation of the proton electrochemical gradient in membrane vesicles from Escherichia coli GR19N and in proteoliposomes reconstituted with purified D-lactate dehydrogenase and cytochrome o oxidase

The respiratory chain in the cytochrome d deficient mutant Escherichia coli GR19N is a relatively simple, linear system consisting of primary dehydrogenases, ubiquinone 8, cytochrome b-556, and cytochrome o oxidase. By use of right-side-out and inside-out membrane vesicles from this strain, various oxidase activities and the generation of the H+ electrochemical gradient were studied. Oxidation of ubiquinol 1 or N,N,-N',N'-tetramethyl-p-phenylenediamine, which donate electrons directly to the terminal oxidase, generates a H+ electrochemical gradient comparable to that observed during D-lactate oxidation. In contrast, D-lactate/ubiquinone 1 or D-lactate/ferricyanide oxidoreductase activity does not appear to generate a membrane potential, suggesting that electron flow from D-lactate dehydrogenase to ubiquinone is not electrogenic. Moreover, proteoliposomes reconstituted with purified D-lactate dehydrogenase, ubiquinone 8, and purified cytochrome o catalyze D-lactate and ubiquinol 1 oxidation and generate a H+ electrochemical gradient similar to that observed in membrane vesicles. Strikingly, in inside-out vesicles, NADH oxidation generates a H+ electrochemical gradient that is very significantly greater than that produced by either D-lactate or ubiquinol 1; furthermore, NADH/ubiquinone 1 and NADH/ferricyanide oxidoreductase activities are electrogenic. It is suggested that the only component between D-lactate dehydrogenase or ubiquinol and oxygen in GR19N membranes that is directly involved in the generation of the H+ electrochemical gradient is cytochrome o, which functions as a "half-loop" (i.e., the oxidase catalyzes the scalar release of 2 H+ from ubiquinol on the outer surface of the membrane.(ABSTRACT TRUNCATED AT 250 WORDS)     

The reduced nicotinamide adenine dinucleotide "oxidase" of Acholeplasma laidlawii membranes.

An NADH dehydrogenase possessing a specific activity 3-5 times that of membrane-bound enzyme was obtained by extraction of Acholeplasma laidlawii membranes with 9.0% ethanol at 43 degrees C. This dehydrogenase contained only trace amounts of iron (suggesting an uncoupled respiration), a flavin ratio of 1:2 FAD to FMN and 30-40% lipid. Its resistance to sedimentation is probably due to the high flotation density of the lipids. It efficiently utilized ferricyanide, menadione and dichlorophenol indophenol as electron acceptors, but not O2, ubiquinone Q10 or cytochrome c. Lineweaver-Burk plots of the dehydrogenase were altered to linear functions upon extraction with 9.0% ethanol. A secondary site of ferricyanide reduction could not be explained by the presence of cytochromes, which these membranes lack. In comparison to other respiratory chain-linked NADH dehydrogenases in cytochrome-containing respiratory chains, this dehydrogenase was characterized by similar Km's with ferricyanide, dichlorophenol indophenol, menadione as electron acceptors, but considerably smaller V's with ferricyanide, dichlorophenol indophenol, menadione as electron acceptors, and smaller specific activities. It was not stimulated or reactivated by the addition of FAD, FMN, Mg2+, cysteine or membrane lipids, and was less sensitive to respiratory inhibitors than unextracted enzyme. The ineffectiveness of ADP stimulation on O2 uptake, the insensitivity to oligomycin and the very low iron content of A. laidlawii membranes were considered in relation to conservation of energy by these cells. Some kinetic properties of the dehydrogenation, the uniquely high glycolipid content and apparently uncoupled respiration at Site I were noteworthy characteristics of this NADH dehydrogenase from the truncated respiratory chain of A. laidlawii.     

Purification and Identification of a Plasma Membrane Associated Electron Transport Protein from Maize (Zea mays L.) Roots

Plasma membranes isolated from three-day-old maize (Zea mays L.) roots by aqueous two-phase partitioning were used as starting material for the purification of a novel electron transport enzyme. The detergent-solubilized enzyme was purified by dyeligand affinity chromatography on Cibacron blue 3G-A-agarose. Elution was achieved with a gradient of 0 to 30 micromolar NADH. The purified protein fraction exhibited a single 27 kilodalton silver nitrate-stained band on sodium dodecyl sulfate polyacrylamide gel electrophoretograms. Staining intensity correlated with the enzyme activity profile when analyzed in affinity chromatography column fractions. The enzyme was capable of accepting electrons from NADPH or NADH to reduce either ferricyanide, juglone, duroquinone, or cytochrome c, but did not transfer electrons to ascorbate free-radical or nitrate. The high degree of purity of plasma membranes used as starting material as well as the demonstrated insensitivity to mitochondrial electron transport inhibitors confirmed the plasma membrane origin of this enzyme. The purified reductase was stimulated upon prolonged incubation with flavin mononucleotide suggesting that the enzyme may be a flavoprotein. Established effectors of plasma membrane electron transport systems had little effect on the purified enzyme, with the exception of the sulfhydryl inhibitor p-chloromercuriphenyl-sulfonate, which was a strong inhibitor of ferricyanide reducing activity.     

Kinetic properties of purified sheep lung microsomal NADH-cytochrome b5 reductase.

1. Lung NADH-cytochrome b5 reductase was saturated with its artificial substrate, potassium ferricyanide at approximately 0.1 mM ferricyanide concentration, and the activity of the lung enzyme was inhibited by the higher concentrations of potassium ferricyanide. Ferricyanide at 0.5 and 1.0 mM inhibited the activity of the enzyme by about 20 and 61% respectively. The apparent Km value was calculated as 13.7 microM potassium ferricyanide and 4.3 microM NADH. 2. The Michaelis constants for cytochrome b5 and NADH were determined to be 1.67 and 7.7 microM from the Lineweaver-Burk plots. These results demonstrate that affinity of the lung reductase for its natural substrate is almost 10 times higher than that for potassium ferricyanide. 3. Addition of non-ionic detergent stimulated the rate of reductase-catalyzed reduction of lung cytochrome b5 up to 8.2-fold. 4. Kinetic studies performed with lung reductase by varying NADH and cytochrome b5 concentrations at different fixed concentrations at cytochrome b5 or NADH showed a series of parallel lines indicating a "ping-pong" type of kinetic mechanism for interaction of NADH and cytochrome b5 with lung cytochrome b5 reductase.     

Redox activity at the surface of oat root cells

Electron transport activity at the cell surface of intact oat seedlings (Avena sativa L. cv Garry) was examined by measuring the oxidation and/or reduction of agents in the medium bathing the roots. Oxidation of NADH with or without added electron acceptors and reduction of ferricyanide by an endogenous electron donor were detected. The activities appear to be due to electron transfer at, or across, the plasma membrane and not due to reagent uptake or leakage of oxidants or reductants. NADH-ferricyanide oxidoreductase activity was also detected in plasma membrane-enriched preparations from Avena roots. Based on redox responses to pH, various ions, and to a variety of electron donors and acceptors, the results indicate that more than one electron transport system is present at the plasma membrane.     

Orientation of electron transport activities in the membrane of intact glyoxysomes isolated from castor bean endosperm

Intact glyoxysomes were isolated from castor bean endosperm on isometric Percoll gradients. The matrix enzyme, malate dehydrogenase, was 80% latent in the intact glyoxysomes. NADH:ferricyanide and NADH:cytochrome c reductase activities were measured in intact and deliberately broken organelles. The latencies of these redox activities were found to be about half the malate dehydrogenase latency. Incubation of intact organelles with trypsin eliminated NADH:cytochrome c reductase activity, but did not affect NADH:ferricyanide reductase activity. NADH oxidase and transhydrogenase activities were negligible in isolated glyoxysomes. Mersalyl and Cibacron blue 3GA were potent inhibitors of NADH:cytochrome c reductase. Quinacrine, Ca²⁺ and Mg²⁺ stimulated NADH:cytochrome c reductase activity in intact glyoxysomes. The data suggest that some electron donor sites are on the matrix side and some electron acceptor sites are on the cytosolic side of the membrane.     

Evidence for coenzyme Q function in transplasma membrane electron transport

Transplasma membrane electron transport activity has been associated with stimulation of cell growth. Coenzyme Q is present in plasma membranes and because of its lipid solubility would be a logical carrier to transport electrons across the plasma membrane. Extraction of coenzyme Q from isolated rat liver plasma membranes decreases the NADH ferricyanide reductase and added coenzyme Q10 restores the activity. Piericidin and other analogs of coenzyme Q inhibit transplasma membrane electron transport as measured by ferricyanide reduction by intact cells and NADH ferricyanide reduction by isolated plasma membranes. The inhibition by the analogs is reversed by added coenzyme Q10. Thus, coenzyme Q in plasma membrane may act as a transmembrane electron carrier for the redox system which has been shown to control cell growth.