A link between transport and plasma membrane redox system(s) in carrot cells

Oxidoreductases in plant plasma membranes

Electron transporting oxidoreductases at biological membranes mediate several physiological processes. While such activities are well known and widely accepted as physiologically significant for other biological membranes, oxidoreductase activities found at the plasma membrane of plants are still being neglected. The ubiquity of the oxidoreductases in the plasma membrane suggests that the activity observed is of major importance in fact up to now no plant without redox activity at the plasmalemma is known. Involvement in proton pumping, membrane energization, ion channel regulation, iron reduction, nutrient uptake, signal transduction, and growth regulation has been proposed. However, positive proof for one of the numerous theories about the physiological function of the system is still missing. Evidence for an involvement in signalling and regulation of growth and transport activities at the plasma membrane is strong, but the high activity of the system displayed in some experiments also suggests function in defense against pathogens.

Properties of a transplasma membrane redox system of Phanerochaete chrysosporium

A transplasma membrane redox system of Phanerochaete chrysosporium was studied using ferricyanide, a membrane-impermeable electron acceptor. Rates of reduction were dependent upon initial ferricyanide concentration and mycelial mass. Specific activities of 12 +/- 2 nmol/min/mg mycelia (dry wt) were consistently obtained using nutrient-sufficient mycelia at pH 8.0 and 10 mM ferricyanide. Upon nutrient limitation (either carbon or nitrogen), activity decreased. Reduction was inhibited by carbonyl cyanide m-chloromethoxyphenyl hydrazone, 2,4-dinitrophenol, and sodium azide but not by potassium cyanide at 100 nmol/mg mycelia. Ferricyanide reduction and proton export rates increased with pH above the physiological pH for the fungus. The stimulation in proton exported by the addition of ferricyanide was equal to the rate of ferricyanide reduced at pH 8.0 when Hepes buffer was used. The relevance of these findings with regard to the physiological pH optimum of the fungus and the metabolism of pollutants by this fungus is discussed.

Transplasma membrane electron transport in plants

The presence of transplasma membrane electron transport in a variety of plant cells and tissues is reported. It is now agreed that this property of eukaryotic cells is of ubiquitous nature. Studies with highly purified plasma membranes have established the presence of electron transport enzymes. Two types of activities have been identified. One, termed "Standard" reductase, is of general occurrence. The other, inducible under iron deficiency and relatively more active, is "Turbo" reductase. However, the true nature of components participating in electron transport and their organization in the plasma membrane is not known. The electron transport is associated with proton release and uses intracellular NAD(P)H as substrate. The electron flow leads to changes in intracellular redox status, pH, and metabolic energy. The responsiveness of this system to growth hormones is also observed. These findings suggest a role for electron flow across the plasma membrane in cell growth and regulation of ion transport. Involvement of this system in many other cellular functions is also argued.

Generation of superoxide anion and hydrogen peroxide at the surface of plant cells

In addition to well-known cell wall peroxidases, there is now evidence for the presence of this enzyme at the plasma membrane of the plant cells (surface peroxidase). Both are able to catalyze, through a chain of reactions involving the superoxide anion, the oxidation of NADH to generate hydrogen peroxide. The latter is oxidized by other wall-bound peroxidases to convert cinnamoyl alcohols into radical forms, which, then polymerize to generate lignin. However, there are other enzymes at the surface of plasma membranes capable of generating hydrogen peroxide (cell wall polyamine oxidase), superoxide anion (plasma membrane Turbo reductase), or both (plasma membrane flavoprotein?). These enzymes utilize NAD(P)H as a substrate. The Turbo reductase and the flavoprotein catalyze the univalent reduction of Fe3+ and then of O2 to produce Fe2+ and O2-., respectively. The superoxide anion, in the acidic environment of the cell wall, may then dismutate to H2O2. These superoxide anion- and hydrogen peroxide-generating systems are discussed in relation to their possible involvement in physiological and pathological processes in the apoplast of plant cells.

Redox reactions of tonoplast and plasma membranes isolated from soybean hypocotyls by free-flow electrophoresis

Redox reactions were studied in more than 90% pure tonoplast and plasma membranes isolated by free-flow electrophoresis from soybean (Glycine max) hypocotyls. Both types of membrane contained a b-type cytochrome (alpha max = 561 nm) and a noncovalently bound flavin, two possible components of a transmembrane electron-transport chain. Isolated tonoplast and plasma membranes reduced ferricyanide, indophenol and various iron complexes with NADH or NADPH as electron donors. The redox activity was inhibited in tonoplast membranes by about 60% by 10 microM p-chloromercuribenzene sulfonate, 8% by 500 microM lanthanum nitrate and 10% by 100 microM nitrophenyl acetate. In contrast, the redox activity of isolated plasma membranes was inhibited by about 60% by 500 microM lanthanum nitrate or 100 microM nitrophenyl acetate, but only 25% by 10 microM p-chloromercuribenzene sulfonate. The results show that both tonoplast and plasma membranes of soybean contain active electron-transport systems, but that the two systems respond differently to inhibitors.

Oxidation of reduced pyridine nucleotides by plasma membranes of soybean hypocotyl

Highly purified plasma membranes isolated from soybean hypocotyls by free-flow electrophoresis or by a two-phase polymer separation system oxidize reduced pyridine nucleotides, NADH or NADPH, at rates of 2-5 nanomoles/mg protein/min. These rates are not influenced by mitochondrial inhibitors or by inhibitors of the alternate respiratory pathway. The NADH oxidase has a Km of 200 microM NADH. The enzyme activity is stimulated by Ca2+ and Mg2+ ions. The function of this enzyme is unknown at present, but it may represent a redox-controlled proton pump linked to acidification.

Transmembrane ferricyanide reduction in carrot cells

Carrot cells (Daucus carota) grown in tissue culture are capable of reducing the non-permeable electron acceptor, ferricyanide, with concomitant proton extrusion from the cell. Optimum conditions for transmembrane ferricyanide reduction include a pH of 7.0-7.5 in a medium containing 10 mM each KCl, NaCl and CaCl2. Data are shown to prove that transmembrane ferricyanide reduction is an enzymatic process. It does not depend on the secretion of phenolics from the cell within the time limits of the assay (10 min). The presence of broken cells and cell fragments are excluded on the basis of stimulation or only slight inhibition by mitochondrial inhibitors. However, transmembrane ferricyanide reduction by carrot cells is inhibited about 50% by various glycolysis inhibitors, which are presumed to reduce the internal levels of NADH. Treatment of cells with p-diazoniumbenzenesulfonic acid, a non-permeant membrane modifying agent, also inhibits transmembrane ferricyanide reduction more than 90%. The data presented support the existence of a transplasma membrane redox system in carrot cells.

Reduction of extracellular potassium ferricyanide by transmembrane NADH: (acceptor) oxidoreductase of human erythrocytes

Reduction of extracellular ferricyanide by intact erythrocytes proceeds by a membrane bound, NADH-dependent reaction. It is depressed by a glycolysis inhibitor and a non penetrable sulfhydryl reagent, and activated by dehydroascorbate. Dehydroascorbate activation cannot be accounted for by release of reducing equivalents from the cells. It is concluded that the observed reaction is brought about by transmembrane NADH-acceptor oxidoreductase with donor binding at the inner and acceptor binding at the outer cell surface.