NADH oxidase activity of rat liver plasma membrane activated by guanine nucleotides

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.

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.

Human aging and global function of coenzyme Q10

In this paper, we review two parts of our recent work on human skeletal muscle. The first part mainly describes changes occurring during aging, whereas the second part discusses the functions of coenzyme Q10 (CoQ10), particularly in relation to the aging process. During the lifetime of an individual, mtDNA undergoes a variety of mutation events and rearrangements. These mutations and their consequent bioenergenic decline, together with nuclear DNA damage, contribute to the reduced function of cells and organs, especially in postmitotic tissues. In skeletal muscle, this functional decline can be observed by means of changes with age in fiber type profile and the reduction in the number and size of the muscle fibers. In addition to the functions of coenzyme Q10 as an electron carrier in the respiratory chain and as an antioxidant, CoQ10 has been shown to regulate global gene expression in skeletal muscle. We hypothesize that this regulation is achieved via superoxide formation with H2O2 as a second messenger to the nucleus.

Reduction of transition metals by human (THP-1) monocytes is enhanced by activators of protein kinase C

Macrophages oxidize low density lipoprotein (LDL) by enzymatic and non-enzymatic mechanisms; however, it is evident that macrophage reduction of transition metals can accelerate LDL oxidation in vitro, and possibly in vivo. Distinct cellular pathways contribute to this process, including trans-plasma membrane electron transport (TPMET), and production of free thiols or superoxide. Here, we explore the role of protein kinase C (PKC) in regulating transition metal reduction by each of these redox-active pathways, in human (THP-1) monocytes. We demonstrate that PKC agonists and/or inhibitors modulate reduction of transition metals by monocytes: both thiol-independent (direct) and thiol-dependent (indirect) pathways for transition metal reduction are enhanced by PKC activation, suggesting a potential strategy for therapeutic intervention.

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.

NADH oxidase activity of soybean plasma membranes inhibited by submicromolar concentrations of ATP

The activity of an auxin-stimulated NADH oxidase activity from soybean hypocotyls was inhibited by submicromolar concentrations of ATP. Auxins are plant growth regulators that increase the rate of cell enlargement in plant stems. A synthetic auxin, 2,4-dichlorophenoxyacetic acid (2,4-D), was used. The inhibition was half maximal at 1 nM ATP and was not observed with other nucleotides and nucleosides. The inhibition was the result of an increase in the Km for NADH from about 60 microM to > 100 microM and was noncompetitive. The decrease in Km due to ATP was enhanced by the addition of 1 microM 2,4-D. The Vmax of the plasma membrane NADH oxidase was approximately doubled (1.5-2.8-fold) by ATP and by 1 microM 2,4-D. No further increase in the Vmax was observed by the combination of 1 nM to 0.1 mM ATP in the presence of 1 microM 2,4-D. The results demonstrate a response of the NADH oxidase activity of isolated vesicles of soybean plasma membranes to ATP distinct from that observed previously with other nucleotide di- and triphosphates. The results are suggestive either of control of the cell surface NADH oxidase by phosphorylation or a direct response to ATP binding at nanomolar concentrations of ATP.

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.

NADH oxidase activity from sera altered by capsaicin is widely distributed among cancer patients

A cancer-specific form of NADH oxidase inhibited or stimulated by 1 or 100 microM capsaicin (8-methyl-N-vanillyl-6-noneamide) is present in sera from cancer patients. The capsaicin-inhibited NADH oxidase activity appears to be absent from sera of individuals free of cancer. The capsaicin-inhibited activity is present both in freshly collected sera and in sera stored frozen for varying periods of time. For the latter, an assay was carried out under renaturing conditions in the presence of NADH and reduced glutathione followed by dilute hydrogen peroxide. Inhibition was half maximal at about 1 microM capsaicin. The capsaicin-inhibited activity was found in sera over a broad spectrum of cancer patients including patients with solid cancers (e.g., breast, prostate, lung, ovarian) as well as with leukemias and lymphomas.

Differential response of the NADH oxidase of plasma membranes of rat liver and hepatoma and HeLa cells to thiol reagents

NADH oxidase activity of plasma membranes from rat hepatoma and HeLa cells responded to thiol reagents in a manner different from that of plasma membranes of liver. Specifically, the NADH oxidase activity of plasma membranes of HeLa cells was inhibited by submicromolar concentrations of the thiol reagents p-chloromercuribenzoate (PCMB), N-ethylmaleimide (NEM), or 5,5'-dithiobis-(2-nitrophenylbenzoic acid) (DTNB), whereas that of the rat liver plasma membranes was unaffected or stimulated over a wide range of concentrations extending into the millimolar range. With some hepatoma preparations, the NADH oxidase activity of hepatoma plasma membranes was stimulated rather than inhibited by PCMB, whereas with all preparations of hepatoma plasma membranes, NEM and DTNB stimulated the activity. In contrast, NADH oxidase activity of rat liver plasma membrane was largely unaffected over the same range of PCMB concentrations that either stimulated or inhibited with rat hepatoma or HeLa cell plasma membranes. Dithiothreitol and glutathione stimulated NADH oxidase activity of plasma membranes of rat liver and hepatoma but inhibited that of HeLa plasma membranes. The findings demonstrate a difference between the NADH oxidase activity of normal rat liver plasma membranes of rat hepatoma and HeLa cell plasma membranes in addition to the differential response to growth factors and hormones reported previously (Bruno et al., 1992). Results are consistent with a structural modification of a NADH oxidase activity involving thiol groups present in plasma membranes of rat hepatoma and HeLa cells but absent or inaccessible with plasma membranes of rat liver.

Hormone- and growth factor-stimulated NADH oxidase

An NADH oxidase activity of animal and plant plasma membrane is described that is stimulated by hormones and growth factors. In plasma membranes of cancer cells and tissues, the activity appears to be constitutively activated and no longer hormone responsive. With drugs that inhibit the activity, cells are unable to grow although growth inhibition may be more related to a failure of the cells to enlarge than to a direct inhibition of mitosis. The hormone-stimulated activity in plasma membranes of plants and the constitutively activated NADH oxidase in tumor cell plasma membranes is inhibited by thiol reagents whereas the basal activity is not. These findings point to a thiol involvement in the action of the activated form of the oxidase. NADH oxidase oxidation by Golgi apparatus of rat liver is inhibited by brefeldin A plus GDP. Brefeldin A is a macrolide antibiotic inhibitor of membrane trafficking. A model is presented where the NADH oxidase functions as a thiol-disulfide oxidoreductase activity involved in the formation and breakage of disulfide bonds. The thiol-disulfide interchange is postulated as being associated with physical membrane displacement as encountered in cell enlargement or in vesicle budding. The model, although speculative, does provide a basis for further experimentation to probe a potential function for this enzyme system which, under certain conditions, exhibits a hormone- and growth factor-stimulated oxidation of NADH.

Inhibition by brefeldin A of NADH oxidation activity of rat liver Golgi apparatus accelerated by GDP

Reduced pyridine nucleotide has been reported to enhance cell-free transfer of membrane material from a radiolabeled Golgi apparatus donor fraction from rat liver to an acceptor fraction consisting of inside-out plasma vesicles immobilized on nitrocellulose [(1992) Biochim. Biophys. Acta 1107, 131]. As part of a continuing effort to identify NADH-requiring enzymes in the Golgi apparatus which may be important to membrane trafficking, highly purified fractions of Golgi apparatus from rat liver were tested for their ability to oxidize NADH and the inhibition of the oxidation of NADH by brefeldin A. The isolated Golgi apparatus fractions were found to oxidize NADH with a specific activity comparable to that of the plasma membrane of rat liver. The activity was inhibited by brefeldin A and this inhibition was augmented by GDP. At near optimal concentrations of 7 microM brefeldin A and 1 microM GDP, the activity was > 90% inhibited. Brefeldin A inhibition of NADH oxidation by the Golgi apparatus was time-dependent and GDP appeared to accelerate the inhibition by brefeldin A.

Coenzyme Q10, plasma membrane oxidase and growth control

The plasma membrane of eukaryotic cells contains an NADH oxidase which can transfer electrons across the membrane. This oxidase is controlled by hormones, growth factors and other ligands which bind to receptors in the plasma membrane. Oncogenes also affect activity of the oxidase. Natural serum components such as diferric transferrin and ceruloplasmin which stimulate proliferation also stimulate membrane oxidase activity. Additional growth factors can be required to complement the proliferative effect. Electron transport across the plasma membrane can be measured by the reduction of impermeable electron acceptors, such as ferricyanide, which also stimulate cell growth. The oxidants activate growth-related signals such as cytosolic alkalinization and calcium mobilization. Antiproliferative agents such as adriamycin and retinoic acid inhibit the plasma membrane electron transport. Flavin, Coenzyme Q and an iron chelate on the cell surface are apparent electron carriers for the transmembrane electron transport. Coenzyme Q10 stimulates cell growth, and Coenzyme Q analogs such as capsaicin and chloroquine reversibly inhibit both growth and transmembrane electron transport. Addition of iron salts to the depleted cells restores activity and growth. The ligand-activated oxidase in the plasma membrane introduces a new basis for control of signal transduction in cells. The redox state of the quinone in the oxidase is proposed to control tyrosine kinase either by generation of H2O2 or redox-induced conformational change.