Ca2+ stimulation of the external NADH dehydrogenase in Jerusalem artichoke (Helianthus tuberosum) mitochondria

The Ca2+-Regulation of the Mitochondrial External NADPH Dehydrogenase in Plants Is Controlled by Cytosolic pH

NADPH is a key reductant carrier that maintains internal redox and antioxidant status, and that links biosynthetic, catabolic and signalling pathways. Plants have a mitochondrial external NADPH oxidation pathway, which depends on Ca²⁺ and pH in vitro, but concentrations of Ca²⁺ needed are not known. We have determined the K_0.5(Ca²⁺) of the external NADPH dehydrogenase from Solanum tuberosum mitochondria and membranes of E. coli expressing Arabidopsis thaliana NDB1 over the physiological pH range using O₂ and decylubiquinone as electron acceptors. The K_0.5(Ca²⁺) of NADPH oxidation was generally higher than for NADH oxidation, and unlike the latter, it depended on pH. At pH 7.5, K_0.5(Ca²⁺) for NADPH oxidation was high (≈100 μM), yet 20-fold lower K_0.5(Ca²⁺) values were determined at pH 6.8. Lower K_0.5(Ca²⁺) values were observed with decylubiquinone than with O₂ as terminal electron acceptor. NADPH oxidation responded to changes in Ca²⁺ concentrations more rapidly than NADH oxidation did. Thus, cytosolic acidification is an important activator of external NADPH oxidation, by decreasing the Ca²⁺-requirements for NDB1. The results are discussed in relation to the present knowledge on how whole cell NADPH redox homeostasis is affected in plants modified for the NDB1 gene.

Alternative NAD(P)H dehydrogenases of plant mitochondria

Plant mitochondria have a highly branched electron transport chain that provides great flexibility for oxidation of cytosolic and matrix NAD(P)H. In addition to the universal electron transport chain found in many organisms, plants have alternative NAD(P)H dehydrogenases in the first part of the chain and a second oxidase, the alternative oxidase, in the latter part. The alternative activities are nonproton pumping and allow for NAD(P)H oxidation with varying levels of energy conservation. This provides a mechanism for plants to, for example, remove excess reducing power and balance the redox poise of the cell. This review presents our current understanding of the alternative NAD(P)H dehydrogenases present in plant mitochondria.

PLANT MITOCHONDRIA AND OXIDATIVE STRESS: Electron Transport, NADPH Turnover, and Metabolism of Reactive Oxygen Species

The production of reactive oxygen species (ROS), such as O2- and H2O2, is an unavoidable consequence of aerobic metabolism. In plant cells the mitochondrial electron transport chain (ETC) is a major site of ROS production. In addition to complexes I-IV, the plant mitochondrial ETC contains a non-proton-pumping alternative oxidase as well as two rotenone-insensitive, non-proton-pumping NAD(P)H dehydrogenases on each side of the inner membrane: NDex on the outer surface and NDin on the inner surface. Because of their dependence on Ca2+, the two NDex may be active only when the plant cell is stressed. Complex I is the main enzyme oxidizing NADH under normal conditions and is also a major site of ROS production, together with complex III. The alternative oxidase and possibly NDin(NADH) function to limit mitochondrial ROS production by keeping the ETC relatively oxidized. Several enzymes are found in the matrix that, together with small antioxidants such as glutathione, help remove ROS. The antioxidants are kept in a reduced state by matrix NADPH produced by NADP-isocitrate dehydrogenase and non-proton-pumping transhydrogenase activities. When these defenses are overwhelmed, as occurs during both biotic and abiotic stress, the mitochondria are damaged by oxidative stress.

NAD(P)H-ubiquinone oxidoreductases in plant mitochondria

Plant (and fungal) mitochondria contain multiple NAD(P)H dehydrogenases in the inner membrane all of which are connected to the respiratory chain via ubiquinone. On the outer surface, facing the intermembrane space and the cytoplasm, NADH and NADPH are oxidized by what is probably a single low-molecular-weight, nonproton-pumping, unspecific rotenone-insensitive NAD(P)H dehydrogenase. Exogenous NADH oxidation is completely dependent on the presence of free Ca2+ with a K0.5 of about 1 microM. On the inner surface facing the matrix there are two dehydrogenases: (1) the proton-pumping rotenone-sensitive multisubunit Complex I with properties similar to those of Complex I in mammalian and fungal mitochondria. (2) a rotenone-insensitive NAD(P)H dehydrogenase with equal activity with NADH and NADPH and no proton-pumping activity. The NADPH-oxidizing activity of this enzyme is completely dependent on Ca2+ with a K0.5 of 3 microM. The enzyme consists of a single subunit of 26 kDa and has a native size of 76 kDa, which means that it may form a trimer.

Roles of calcium ions in hyphal tip growth

A role for Ca2+ in the tip growth process of fungal hyphae and other eukaryotic walled cells has been widely explored, following the earlier indications of their importance by Jaffe, Steer, and their colleagues. Analysis of the literature on fungi, with selected comparison with other tip-growing plant cells, shows that the growth rate and morphology of hyphae are sensitive to factors which influence intracellular Ca2+. These factors include variations in extracellular Ca2+ concentrations, Ca2+ ionophores, inhibitors of Ca2+ transport, and calmodulin- and Ca(2+)-binding dyes and buffers introduced into the cytoplasm. The effects of these agents appear to be mediated by a tip-high gradient of cytoplasmic free Ca2+ which is obligatorily present in all critically examined growing tips. Most recent observations agree that the gradient is very steep, declining rapidly within 10 to 20 microns of the tip. This gradient seems to be generated by the combined effects of an influx of Ca2+, via plasma membrane, possibly stretch-activated, channels localized in the hyphal tip, and subapical expulsion or sequestration of these ions. Expulsion probably involves a plasma membrane Ca(2+)-ATPase, but it is not yet possible to differentiate among mitochondria, endoplasmic reticulum, or vacuoles as the dominant sites of sequestration. It is suggested that regulation of the Ca2+ gradient in turn modulates the properties of the actin-based component of the cytoskeleton, which then controls the extensibility, and, possibly, the synthesis of the hyphal apex. Regulatory feedback mechanisms intrinsic to this model of tip growth regulation are briefly discussed, together with suggestions for future experiments which are crucial to its further elucidation and establishment.

Effect of calcium ions and inhibitors on internal NAD(P)H dehydrogenases in plant mitochondria

Both the external oxidation of NADH and NADPH in intact potato (Solanum tuberosum L. cv. Bintje) tuber mitochondria and the rotenone-insensitive internal oxidation of NADPH by inside-out submitochondrial particles were dependent on Ca2+. The stimulation was not due to increased permeability of the inner mitochondrial membrane. Neither the membrane potential nor the latencies of NAD(+)-dependent and NADP(+)-dependent malate dehydrogenases were affected by the addition of Ca2+. The pH dependence and kinetics of Ca(2+)-dependent NADPH oxidation by inside-out submitochondrial particles were studied using three different electron acceptors: O2, duroquinone and ferricyanide. Ca2+ increased the activity with all acceptors with a maximum at neutral pH and an additional minor peak at pH 5.8 with O2 and duroquinone. Without Ca2+, the activity was maximal around pH 6. The Km for NADPH was decreased fourfold with ferricyanide and duroquinone, and twofold with O2 as acceptor, upon addition of Ca2+. The Vmax was not changed with ferricyanide as acceptor, but increased twofold with both duroquinone and O2. Half-maximal stimulation of the NADPH oxidation was found at 3 microM free Ca2+ with both O2 and duroquinone as acceptors. This is the first report of a membrane-bound enzyme inside the inner mitochondrial membrane which is directly dependent on micromolar concentrations of Ca2+. Mersalyl and dicumarol, two potent inhibitors of the external NADH dehydrogenase in plant mitochondria, were found to inhibit internal rotenone-insensitive NAD(P)H oxidation, at the same concentrations and in manners very similar to their effects on the external NAD(P)H oxidation.

The regulation and nature of the cyanide-resistant alternative oxidase of plant mitochondria

In addition to possessing multiple NAD(P)H dehydrogenases, most plant mitochondria contain a cyanide- and antimycin-insensitive alternative terminal oxidase. Although the general characteristics of this terminal oxidase have been known for a considerable number of years, the mechanism by which it is regulated is unclear and until recently there has been relatively little information on its exact nature. In the past 5 years, however, the application of molecular and novel voltametric techniques has advanced our understanding of this oxidase considerably. In this article, we review briefly current understanding on the structure and function of the multiple NADH dehydrogenases and consider, in detail, the nature and regulation of the alternative oxidase. We derive a kinetic model for electron transfer through the ubiquinone pool based on a proposed model for the reduction of the oxidase by quinol and show how this can account for deviations from Q-pool behaviour. We review information on the attempts to isolate and characterise the oxidase and finally consider the molecular aspects of the expression of the alternative oxidase.

Chlortetracycline and the transmembrane potential of the inner membrane of plant mitochondria

The oxidation of NADH or succinate by Jerusalem-artichoke (Helianthus tuberosus L.) mitochondria in the presence of chlortetracycline induced an increase in chlortetracycline fluorescence. Any treatment that prevented the formation of a transmembrane potential (as monitored by changes in safranine absorbance, A511-A533), e.g. uncoupling with carbonyl cyanide p-trifluoromethoxyphenylhydrazone, inhibition of dehydrogenase activity or electron transport, anaerobiosis or depletion of substrate, prevented the increase in chlortetracycline fluorescence or caused it to disappear. Changes in chlortetracycline fluorescence were always slower than changes in the safranine absorbance. The increase in chlortetracycline fluorescence caused by succinate oxidation had an excitation maximum at 393 nm, indicating that a Ca2+-chlortetracycline complex was involved. The increase in fluorescence was observed even in the presence of EDTA, which removes all external bivalent cations, indicating that internal Ca2+ is mobilized. Although NADH and succinate oxidations gave the same membrane potential and qualitatively had the same effect on chlortetracycline fluorescence, NADH oxidation caused a much larger (over 3-fold) increase in chlortetracycline fluorescence than did succinate oxidation. It is possible that this is connected with the Ca2+-dependence of NADH oxidation. In the presence of 2 mM external Ca2+, chlortetracycline collapsed the transmembrane potential and uncoupled succinate and duroquinone oxidation.

Immunological analysis of plant mitochondrial NADH dehydrogenases

Plant mitochondrial NADH dehydrogenases were analysed by two immunological strategies. The first exploited an antiserum raised to a preparation of SDS-solubilized mitochondrial-inner-membrane particles. By using a combination of activity-immunoprecipitation and crossed immunoelectrophoresis, it was shown that Triton X-100-solubilized membranes contain at least three immunologically distinct NADH dehydrogenases. Two of these were subsequently isolated by line immunoelectrophoresis and analysed for polypeptide composition: one contained three polypeptides with molecular masses of 75, 62 and 41 kDa; the other was a single polypeptide with a molecular mass of 53 kDa. The other approach was to probe plant mitochondrial membranes with antibodies raised to a purified preparation of ox heart rotenone-sensitive NADH dehydrogenase and subunits thereof. Cross-reactions were observed with the subunit-specific antisera against the 30 and 49 kDa ox heart proteins. However, the molecular masses of the equivalent polypeptides in plant mitochondria are slightly lower, at 27 and 46 kDa respectively.

The regulation of exogenous NAD(P)H oxidation in spinach (Spinacia oleracea) leaf mitochondria by pH and cations

Essentially chlorophyll-free mitochondria were isolated from green leaves of spinach (Spinacia oleracea L. cv. Viking II). Uncoupled oxidation of exogenous NADPH (1 mM) to oxygen had an optimum at pH 6.0, and activity was relatively low at pH 7.0, even in the presence of 1 mM-CaCl2. There was a proportional increase in the apparent Km for NADPH with decreasing H+ concentrations, suggesting that NADPH protonated on the 2'-phosphate group was the true substrate. Exogenous NADH was oxidized by oxygen with an optimum at pH 6.9. Under low-cation conditions, EGTA or EDTA (both 1 mM) had no effect on the Vmax. of NADH oxidation, although the removal of bivalent cations from the membrane surface by the chelators could be observed by use of 9-aminoacridine fluorescence. In contrast, under high-cation conditions, chelators lowered the Vmax. by about 50%, probably due to a better approach of the negatively charged chelators to the negative membrane surface than under low-cation conditions. In a low-cation medium, the Vmax. of NADH oxidation was increased by about 50% by the addition of cations. This was caused by a lowering of the size of the negative surface potential through charge screening. In contrast with other cations, La3+ inhibited NADH oxidation, possibly through binding to lipids essential for NADH oxidation. The apparent Km for NADH varied 6-fold in response to changes in the size of the surface potential, suggesting that the approach of the negatively charged NADH to the active site is hampered by the negative surface potential. The results demonstrate that the spinach leaf cell can regulate the mitochondrial NAD(P)H oxidation through several mechanisms: the pH; the cation concentration in general; and the concentration of Ca2+ in particular. The results also emphasize the importance of electrostatic considerations when investigating the kinetic behaviour of membrane-bound enzymes.

Electrostatic surface properties of plasmalemma vesicles from oat and wheat roots. Ion binding and screening investigated by 9-aminoacridine fluorescence

Right-side-out and sealed plasmalemma vesicles were isolated from roots of spring wheat (Triticum aestivum L. cv. Drabant) and oat (Avena sativa L. cv. Brighton) by two-phase partition in a medium containing sucrose (0.25 mol l(-1)). Oat root plasmalemma vesicles were discovered to contain a strongly fluorescent compound with an emission maximum at 418 nm. The surface potential of the membranes was monitored by 9-aminoacridine fluorescence and the effect of protein concentration, mannitol versus sucrose, absence of osmoticum, concentrations of salt, and titrations with chelators investigated. It is concluded that i) protein concentrations of less than 50 μg ml(-1) for oat and 100 μg ml(-1) for wheat plasmalemma vesicles should be used to avoid serious problems with non-linearity of response of 9-aminoacridine fluorescence, ii) mannitol can be used instead of sucrose as the osmoticum, iii) the vesicles were ruptured in the absence of osmoticum allowing us to monitor both sides of the membranes, iv) plasmalemma vesicles from oat roots are more negative than vesicles from wheat roots, and v) oat and wheat root plasmalemma vesicles are isolated with about the same amounts of bound Ca(2+) and Mg(2+). These bound divalent cations may not, however, reflect the in-vivo conditions since the tissues were homogenised in the presence of ethylenediaminetetraacetic acid.

Partial purification and properties of the external NADH dehydrogenase from cuckoo-pint (Arum maculatum) mitochondria

The external NADH dehydrogenase has been purified from Arum maculatum (cuckoo-pint) mitochondria by phosphate washing, extraction with deoxycholate, ion-exchange and gel-filtration chromatography. Sodium dodecyl sulphate/polyacrylamide-gel electrophoresis shows, when the gel is silver-stained, that the purified enzyme contains two major bands of Mr 78 000 and 65 000 and a minor one of Mr about 76 000. It is not possible at present to determine which of these, or which combination, constitutes the dehydrogenase. The enzyme contains non-covalently bound FAD and a small amount of FMN. Since the conditions of purification lead to considerable loss of flavin and possibly iron-sulphur centres, it is not possible to decide with certainty whether the enzyme is a flavo- or ferroflavo-protein. The enzyme has been distinguished from the other NADH dehydrogenases on the basis of its substrate specificity, its capability of reducing electron acceptors such as ubiquinone-1 and 2,6-dichlorophenol-indophenol and its sensitivity towards Ca2+, EGTA and dicoumarol.

Purification and characterization of the rotenone-insensitive NADH dehydrogenase of mitochondria from Arum maculatum

The non-ionic detergent lauryl dimethylamine N-oxide (LDAO) has been used to extract the NADH dehydrogenases of Arum maculatum mitochondria. Affinity chromatography on 5'-ADP-Sepharose 4B was used to separate the rotenone-sensitive (complex I) NADH dehydrogenase from the rotenone-insensitive NADH dehydrogenase. An 18-fold purification of the rotenone-insensitive NADH dehydrogenase was achieved. The enzyme is specific for NADH with optimal activity around pH 7.2. The apparent Km for NADH is 28 microM, with dichloroindophenol as acceptor at pH 7.2. The rotenone-insensitive NADH dehydrogenase appears to be a flavoprotein and no iron-sulphur centres were detected by electron spin resonance spectroscopy.

Trifluoperazine inhibition of electron transport and adenosine triphosphatase in plant mitochondria

Trifluoperazine inhibits ADP-stimulated respiration in mung bean (Phaseolus aureus) mitochondria when either NADH, malate, or succinate serve as substrates (IC50 values of 56, 59, and 55 microM, respectively). Succinate:ferricyanide oxidoreductase activity of these mitochondria was inhibited to a similar extent. The oxidation of ascorbate/TMPD was also sensitive to the phenothiazine (IC50 = 65 microM). Oxidation of exogenous NADH was inhibited by trifluoperazine even in the presence of excess EGTA [ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid] (IC50 = 60 microM), indicating an interaction with the electron transport chain rather than with the dehydrogenase itself. In contrast, substrate oxidation in Voodoo lily (Sauromatum guttatum) mitochondria was relatively insensitive to the phenothiazine. The results suggest the bc1 complex to be a major site of inhibition. The membrane potential of energized mung bean mitochondria was depressed by micromolar concentrations of trifluoperazine, suggesting an effect on the proton-pumping capability of these mitochondria. Membrane-bound and soluble ATPases were equally sensitive to trifluoperazine (IC50 of 28 microM for both), implying the site of inhibition to be on the F1. Inhibition of the soluble ATPase was not affected by EGTA, CaCl2, or exogenous calmodulin. Trifluoperazine inhibition of electron transport and phosphorylation in plant mitochondria appears to be due to an interaction with a protein of the organelle that is not calmodulin.

Evidence for a Ca2+ gradient across the plasma membrane of wheat protoplasts

Fluxes of Ca2+ across the plasma membrane of isolated wheat protoplasts have been measured both as net accumulation and as uptake under steady-state conditions. The ATPase inhibitors, orthovanadate and diethylstibesterol, and the divalent cation ionophore, A23187, were all found to enhance net Ca2+ accumulation by protoplasts. The uptake of Ca2+ under steady-state conditions was also stimulated by A23187 but relatively unaffected by a range of plant hormones or by red or far red light. Light treatments were compared to dark controls with protoplasts isolated from etiolated wheat. The results suggest that plant cells maintain a Ca2+ gradient across their plasma membrane but it appears not to be under phytochrome control.