Oxidation of External NAD(P)H by Mitochondria from Taproots and Tissue Cultures of Sugar Beet (Beta vulgaris)

Potassium channel-oxidative phosphorylation relationship in durum wheat mitochondria from control and hyperosmotic-stressed seedlings

Durum wheat mitochondria (DWM) possess an ATP-inhibited K(+) channel, the plant mitoK(ATP) (PmitoK(ATP) ), which is activated under environmental stress to control mitochondrial ROS production. To do this, PmitoK(ATP) collapses membrane potential (ΔΨ), thus suggesting mitochondrial uncoupling. We tested this point by studying oxidative phosphorylation (OXPHOS) in DWM purified from control seedlings and from seedlings subjected both to severe mannitol and NaCl stress. In severely-stressed DWM, the ATP synthesis via OXPHOS, continuously monitored by a spectrophotometric assay, was about 90% inhibited when the PmitoK(ATP) was activated by KCl. Contrarily, in control DWM, although PmitoK(ATP) collapsed ΔΨ, ATP synthesis, as well as coupling [respiratory control (RC) ratio and ratio between phosphorylated ADP and reduced oxygen (ADP/O)] checked by oxygen uptake experiments, were unaffected. We suggest that PmitoK(ATP) may play an important defensive role at the onset of the environmental/oxidative stress by preserving energy in a crucial moment for cell and mitochondrial bioenergetics. Consistently, under moderate mannitol stress, miming an early stress condition, the channel may efficiently control reactive oxygen species (ROS) generation (about 35-fold from fully open to closed state) without impairing ATP synthesis. Anyway, if the stress significantly proceeds, the PmitoK(ATP) becomes fully activated by decrease of ATP concentration (25-40%) and increase of activators [free fatty acids (FFAs) and superoxide anion], thus impairing ATP synthesis.

Mitochondria-localized NAD biosynthesis by nicotinamide mononucleotide adenylyltransferase in Jerusalem artichoke (Helianthus tuberosus L.) heterotrophic tissues

Current studies in plants suggest that the content of the coenzyme NAD is variable and potentially important in determining cell fate. In cases that implicate NAD consumption, re-synthesis must occur to maintain dinucleotide pools. Despite information on the pathways involved in NAD synthesis in plants, the existence of a mitochondrial nicotinamide mononucleotide adenylyltransferase (NMNAT) activity which catalyses NAD synthesis from nicotinamide mononucleotide (NMN) and ATP has not been reported. To verify the latter assumed pathway, experiments with purified and bioenergetically active mitochondria prepared from tubers of Jerusalem artichoke (Helianthus tuberosus L.) were performed. To determine whether NAD biosynthesis might occur, NMN was added to Jerusalem artichoke mitochondria (JAM) and NAD biosynthesis was tested by means of HPLC and spectroscopically. Our results indicate that JAM contain a specific NMNAT inhibited by Na-pyrophosphate, AMP and ADP-ribose. The dependence of NAD synthesis rate on NMN concentration shows saturation kinetics with K (m) and V (max) values of 82 ± 1.05 μM and 4.20 ± 0.20 nmol min(-1) mg(-1) protein, respectively. The enzyme's pH and temperature dependence were also investigated. Fractionation studies revealed that mitochondrial NMNAT activity was present in the soluble matrix fraction. The NAD pool needed constant replenishment that might be modulated by environmental inputs. Thus, the mitochondrion in heterotrophic plant tissues ensures NAD biosynthesis by NMNAT activity and helps to orchestrate NAD metabolic network in implementing the survival strategy of cells.

Activation of the plant mitochondrial potassium channel by free fatty acids and acyl-CoA esters: a possible defence mechanism in the response to hyperosmotic stress

The effect of free fatty acids (FFAs) and acyl-CoA esters on K(+) uptake was studied in mitochondria isolated from durum wheat (Triticum durum Desf.), a species that has adapted well to the semi-arid Mediterranean area and possessing a highly active mitochondrial ATP-sensitive K(+) channel (PmitoK(ATP)), that may confer resistance to environmental stresses. This was made by swelling experiments in KCl solution under experimental conditions in which PmitoK(ATP) activity was monitored. Linoleate and other FFAs (laurate, palmitate, stearate, palmitoleate, oleate, arachidonate, and the non-physiological 1-undecanesulphonate and 5-phenylvalerate), used at a concentration (10 μM) unable to damage membranes of isolated mitochondria, stimulated K(+) uptake by about 2-4-fold. Acyl-CoAs also promoted K(+) transport to a much larger extent with respect to FFAs (about 5-12-fold). In a different experimental system based on safranin O fluorescence measurements, the dissipation of electrical membrane potential induced by K(+) uptake via PmitoK(ATP) was found to increase in the presence of 5-phenylvalerate and palmitoyl-CoA, both unable to elicit the activity of the Plant Uncoupling Protein. This result suggests a direct activation of PmitoK(ATP). Stimulation of K(+) transport by FFAs/acyl-CoAs resulted in a widespread phenomenon in plant mitochondria from different mono/dicotyledonous species (bread wheat, barley, triticale, maize, lentil, pea, and topinambur) and from different organs (root, tuber, leaf, and shoot). Finally, an increase in mitochondrial FFAs up to a content of 50 nmol mg(-1) protein, which was able to activate PmitoK(ATP) strongly, was observed under hyperosmotic stress conditions. Since PmitoK(ATP) may act against environmental/oxidative stress, its activation by FFAs/acyl-CoAs is proposed to represent a physiological defence mechanism.

Plant inner membrane anion channel (PIMAC) function in plant mitochondria

To date, the existence of the plant inner membrane anion channel (PIMAC) has been shown only in potato mitochondria, but its physiological role remains unclear. In this study, by means of swelling experiments in K(+) and ammonium salts, we characterize a PIMAC-like anion-conducting pathway in mitochondria from durum wheat (DWM), a monocotyledonous species phylogenetically far from potato. DWM were investigated since they possess a very active potassium channel (PmitoK(ATP)), so implying a very active matching anion uniport pathway and, possibly, a coordinated function. As in potato mitochondria, the electrophoretic uptake of chloride and succinate was inhibited by matrix [H(+)], propranolol, and tributyltin, and was insensitive to Mg(2+), N,N'-dicyclohexylcarbodiimide (DCCD) and mercurials, thus showing PIMAC's existence in DWM. PIMAC actively transports dicarboxylates, oxodicarboxylates, tricarboxylates and Pi. Interestingly, a novel mechanism of swelling in ammonium salts of isolated plant mitochondria is reported, based on electrophoretic anion uptake via PIMAC and ammonium uniport via PmitoK(ATP). PIMAC is inhibited by physiological compounds, such as ATP and free fatty acids, by high electrical membrane potential (Delta Psi), but not by acyl-CoAs or reactive oxygen species. PIMAC was found to cooperate with dicarboxylate carrier by allowing succinate uptake that triggers succinate/malate exchange in isolated DWM. Similar results were obtained using mitochondria from the dicotyledonous species topinambur, so suggesting generalization of results. We propose that PIMAC is normally inactive in vivo due to ATP and Delta Psi inhibition, but activation may occur in mitochondria de-energized by PmitoK(ATP) (or other dissipative systems) to replace or integrate the operation of classical anion carriers.

Functional coexpression of the mitochondrial alternative oxidase and uncoupling protein underlies thermoregulation in the thermogenic florets of skunk cabbage

Two distinct mitochondrial energy dissipating systems, alternative oxidase (AOX) and uncoupling protein (UCP), have been implicated as crucial components of thermogenesis in plants and animals, respectively. To further clarify the physiological roles of AOX and UCP during homeothermic heat production in the thermogenic skunk cabbage (Symplocarpus renifolius), we identified the thermogenic cells and performed expression and functional analyses of these genes in this organism. Thermographic analysis combined with in situ hybridization revealed that the putative thermogenic cells surround the stamens in the florets of skunk cabbage and coexpress transcripts for SrAOX, encoding Symplocarpus AOX, and SrUCPb, encoding a novel UCP that lacks a fifth transmembrane segment. Mitochondria isolated from the thermogenic florets exhibited substantial linoleic acid (LA)-inducible uncoupling activities. Moreover, our results demonstrate that LA is capable of inhibiting the mitochondrial AOX pathway, whereas the proportion of pyruvate-stimulated AOX capacity was not significantly affected by LA. Intriguingly, the protein expression levels for SrAOX and SrUCPb were unaffected even when the ambient air temperatures increased from 10.3 degrees C to 23.1 degrees C or from 8.3 degrees C to 24.9 degrees C. Thus, our results suggest that functional coexpression of AOX and UCP underlies the molecular basis of heat production, and that posttranslational modifications of these proteins play a crucial role in regulating homeothermic heat production under conditions of natural ambient temperature fluctuations in skunk cabbage.

Ca2+-binding and Ca2+-independent respiratory NADH and NADPH dehydrogenases of Arabidopsis thaliana

Type II NAD(P)H:quinone oxidoreductases are single polypeptide proteins widespread in the living world. They bypass the first site of respiratory energy conservation, constituted by the type I NADH dehydrogenases. To investigate substrate specificities and Ca(2+) binding properties of seven predicted type II NAD(P)H dehydrogenases of Arabidopsis thaliana we have produced them as T7-tagged fusion proteins in Escherichia coli. The NDB1 and NDB2 enzymes were found to bind Ca(2+), and a single amino acid substitution in the EF hand motif of NDB1 abolished the Ca(2+) binding. NDB2 and NDB4 functionally complemented an E. coli mutant deficient in endogenous type I and type II NADH dehydrogenases. This demonstrates that these two plant enzymes can substitute for the NADH dehydrogenases in the bacterial respiratory chain. Three NDB-type enzymes displayed distinct catalytic profiles with substrate specificities and Ca(2+) stimulation being considerably affected by changes in pH and substrate concentrations. Under physiologically relevant conditions, the NDB1 fusion protein acted as a Ca(2+)-dependent NADPH dehydrogenase. NDB2 and NDB4 fusion proteins were NADH-specific, and NDB2 was stimulated by Ca(2+). The observed activity profiles of the NDB-type enzymes provide a fundament for understanding the mitochondrial system for direct oxidation of cytosolic NAD(P)H in plants. Our findings also suggest different modes of regulation and metabolic roles for the analyzed A. thaliana enzymes.

The transcript levels of two plant mitochondrial uncoupling protein (pUCP)-related genes are not affected by hyperosmotic stress in durum wheat seedlings showing an increased level of pUCP activity

Etiolated early seedlings of durum wheat submitted to moderate and severe salt (NaCl) and osmotic (mannitol) stress showed no relevant increase of both transcript levels of two plant uncoupling protein (pUCP)-related genes and maximal pUCP activity in purified mitochondria (which estimates protein level); contrarily, pUCP functioning due to endogenous free fatty acids strongly increased. These results show that pUCP activation under hyperosmotic stress may be due to modulation of pUCP reaction rather than to an increased protein synthesis. Finally, a properly developed method, based on a single membrane potential measurement, to evaluate both pUCP maximal activity and functioning, is reported.

Seawater stress applied at germination affects mitochondrial function in durum wheat (Triticum durum) early seedlings

Seawater stress effects on mitochondrial ATP synthesis and membrane potential (ΔΨ) were investigated in germinating durum wheat seedlings under moderate (22% seawater osmolarity, -0.62 MPa) and severe (37% seawater osmolarity, -1.04 MPa) stress. To estimate the osmotic component of salt stress, mannitol solutions (0.25 and 0.42 m) iso-osmotic with the saline ones were used. Moderate stress intensity only delayed mean germination time (MGT), whereas higher seawater osmolarity reduced germination percentage as well. In contrast, Na⁺ and Cl^- accumulation showed a sharp increase under moderate stress and only a small further increase under severe stress, which was more pronounced for Cl^-. Only severe stress significantly damaged succinate-dependent oxidative phosphorylation, which may be related to the stress-induced alteration in inner mitochondrial membrane permeability, as indicated by changes in ΔΨ profiles. Proline-dependent oxidative phosphorylation, however, was inhibited under moderate stress. This suggests the occurrence of an adaptation mechanism leading to proline accumulation as an osmoprotectant. Moreover, both the osmotic and the toxic components of seawater stress were detrimental to oxidative phosphorylation. Damage to germination and MGT, in contrast, were mainly caused by osmotic stress. Therefore, mitochondrial function appears to be a more sensitive target of toxic stress than growth. In conclusion, the effects of seawater stress on mitochondrial ATP synthesis vary in relation to the substrate oxidised and stress level, inducing both adaptive responses and damage.

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.

Effects of fatty acids, nucleotides and reactive oxygen species on durum wheat mitochondria

Linoleic acid (LA) and other fatty acids added to respiring durum wheat mitochondria (DWM) were found to cause a remarkable membrane potential (deltaPsi) decrease, as monitored by measuring safranin fluorescence. The rate of deltaPsi decrease showed (i) saturation dependence on LA concentration; (ii) fatty acid specificity; (iii) inhibition by externally added ATP, GDP, GTP and Mg(2+) and (iv) sigmoid dependence upon initial DeltaPsi, thus suggesting the existence of an active plant mitochondrial uncoupling protein (PUMP) in mitochondria from monocotyledonous species (durum wheat, Triticum durum Desf.). Surprisingly, the rate of the linoleate dependent DeltaPsi decrease was found to be activated by reactive oxygen species (ROS) (hydrogen peroxide and superoxide anion) and, moreover, linoleate proved to lower the mitochondrial generation of superoxide anion. These results suggest that ROS can activate PUMP, thus protecting the cell against mitochondrial ROS production.

The existence of the K(+) channel in plant mitochondria

In this study, evidence is given that a number of isolated coupled plant mitochondria (from durum wheat, bread wheat, spelt, rye, barley, potato, and spinach) can take up externally added K(+) ions. This was observed by following mitochondrial swelling in isotonic KCl solutions and was confirmed by a novel method in which the membrane potential decrease due to externally added K(+) is measured fluorimetrically by using safranine. A detailed investigation of K(+) uptake by durum wheat mitochondria shows hyperbolic dependence on the ion concentration and specificity. K(+) uptake electrogenicity and the non-competitive inhibition due to either ATP or NADH are also shown. In the whole, the experimental findings reported in this paper demonstrate the existence of the mitochondrial K(+)(ATP) channel in plants (PmitoK(ATP)). Interestingly, Mg(2+) and glyburide, which can inhibit mammalian K(+) channel, have no effect on PmitoK(ATP). In the presence of the superoxide anion producing system (xanthine plus xanthine oxidase), PmitoK(ATP) activation was found. Moreover, an inverse relationship was found between channel activity and mitochondrial superoxide anion formation, as measured via epinephrine photometric assay. These findings strongly suggest that mitochondrial K(+) uptake could be involved in plant defense mechanism against oxidative stress due to reactive oxygen species generation.

Purification and properties of NAD(P)H: (quinone-acceptor) oxidoreductase of sugarbeet cells

NAD(P)H:(quinone-acceptor) oxidoreductase [NAD(P)H-QR], a plant cytosolic protein, was purified from cultured sugarbeet cells by a combination of ammonium sulfate fractionation, FPLC Superdex 200 gel filtration, Q-Sepharose anion-exchange chromatography, and a final Blue Sepharose CL-6B affinity chromatography with an NADPH gradient. The subunit molecular mass is 24 kDa and the active protein (94 kDa) is a tetramer. The isoelectric point is 4.9. The enzyme was characterized by ping-pong kinetics and extremely elevated catalytic capacity. It prefers NADPH over NADH as electron donor (kcat/Km ratios of 1.7 x 10(8) M-1 S-1 and 8.3 x 10(7) M-1 S-1 for NADPH and NADH, respectively, with benzoquinone as electron acceptor). The acridone derivative 7-iodo-acridone-4-carboxylic acid is an efficient inhibitor (I0.5 = 5 x 10(-5) M), dicumarol is weakly inhibitory. The best acceptor substances are hydrophilic, short-chain quinones such as ubiquinone-0 (Q-0), benzoquinone and menadione, followed by duroquinone and ferricyanide, whereas hydrophobic quinones, cytochrome c and oxygen are reduced at negligible rates at best. Quinone acceptors are reduced by a two-electron reaction with no apparent release of free semiquinonic intermediates. This and the above properties suggest some relationship of NAD(P)H-QR to DT-diaphorase, an animal flavoprotein which, however, has distinct structural properties and is strongly inhibited by dicumarol. It is proposed that NAD(P)H-QR by scavenging unreduced quinones and making them prone to conjugation may act in plant tissues as a functional equivalent of DT-diaphorase.

Direct evidence for the presence of two external NAD(P)H dehydrogenases coupled to the electron transport chain in plant mitochondria

Exogenous NADPH oxidation by purified mitochondria from both potato tuber and Arum maculatum spadix was completely and irreversibly inhibited by sub-micromolar diphenyleneiodonium (DPI), while exogenous NADH oxidation was inhibited to only a small degree. Addition of DPI caused the collapse of the membrane potential generated by NADPH oxidation, while the potential generated by NADH was unaffected. We conclude that there are two distinct enzymes on the outer surface of the inner membrane of plant mitochondria, one specific for NADH, the other relatively specific for NADPH, with both enzymes linked to the electron transport chain.

Functional molecular aspects of the NADH dehydrogenases of plant mitochondria

There are multiple routes of NAD(P)H oxidation associated with the inner membrane of plant mitochondria. These are the phosphorylating NADH dehydrogenase, otherwise known as Complex I, and at least four other nonphosphorylating NAD(P)H dehydrogenases. Complex I has been isolated from beetroot, broad bean, and potato mitochondria. It has at least 32 polypeptides associated with it, contains FMN as its prosthetic group, and the purified enzyme is sensitive to inhibition by rotenone. In terms of subunit complexity it appears similar to the mammalian and fungal enzymes. Some polypeptides display antigenic similarity to subunits from Neurospora crassa but little cross-reactivity to antisera raised against some beef heart complex I subunits. Plant complex I contains eight mitochondrial encoded subunits with the remainder being nuclear-encoded. Two of these mitochondrial-encoded subunits, nad7 and nad9, show homology to corresponding nuclear-encoded subunits in Neurospora crassa (49 and 30 kDa, respectively) and beef heart CI (49 and 31 kDa, respectively), suggesting a marked difference between the assembly of CI from plants and the fungal and mammalian enzymes. As well as complex I, plant mitochondria contain several type-II NAD(P)H dehydrogenases which mediate rotenone-insensitive oxidation of cytosolic and matrix NADH. We have isolated three of these dehydrogenases from beetroot mitochondria which are similar to enzymes isolated from potato mitochondria. Two of these enzymes are single polypeptides (32 and 55 kDa) and appear similar to those found in maize mitochondria, which have been localized to the outside of the inner membrane. The third enzyme appears to be a dimer comprised of two identical 43-kDa subunits. It is this enzyme that we believe contributes to rotenone-insensitive oxidation of matrix NADH. In addition to this type-II dehydrogenases, several observations suggest the presence of a smaller form of CI present in plant mitochondria which is insensitive to rotenone inhibition. We propose that this represents the peripheral arm of CI in plant mitochondria and may participate in nonphosphorylating matrix NADH oxidation.

Effects of 3,5-Dibromo-4-Hydroxybenzonitrile (Bromoxynil) on Bioenergetics of Higher Plant Mitochondria (Pisum sativum)

The herbicide bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) was tested on mitochondria from etiolated pea (Pisum sativum L. cv Alaska) stems. This compound when used at micromolar concentrations ([almost equal to]20 [mu]M) inhibited malate- and succinate-dependent respiration by intact mitochondria but not oxidation of exogenously added NADH. Bromoxynil did not affect the activities of the succinic and the internal NADH dehydrogenases. Analyses of the effects induced by this herbicide on the membrane potential, [delta]pH, matrix Ca2+ movements, and dicarboxylate transport demonstrated that bromoxynil is likely to act as an inhibitor of the dicarboxylate carrier. In addition, bromoxynil caused a mild membrane uncoupling at concentrations [greater than or equal to]20 [mu]M. No effect on the ATPase activity was observed.