Transport of NAD in Percoll-Purified Potato Tuber Mitochondria: Inhibition of NAD Influx and Efflux by N-4-Azido-2-nitrophenyl-4-aminobutyryl-3'-NAD

Plant mitochondria - past, present and future

The study of plant mitochondria started in earnest around 1950 with the first isolations of mitochondria from animal and plant tissues. The first 35 years were spent establishing the basic properties of plant mitochondria and plant respiration using biochemical and physiological approaches. A number of unique properties (compared to mammalian mitochondria) were observed: (i) the ability to oxidize malate, glycine and cytosolic NAD(P)H at high rates; (ii) the partial insensitivity to rotenone, which turned out to be due to the presence of a second NADH dehydrogenase on the inner surface of the inner mitochondrial membrane in addition to the classical Complex I NADH dehydrogenase; and (iii) the partial insensitivity to cyanide, which turned out to be due to an alternative oxidase, which is also located on the inner surface of the inner mitochondrial membrane, in addition to the classical Complex IV, cytochrome oxidase. With the appearance of molecular biology methods around 1985, followed by genomics, further unique properties were discovered: (iv) plant mitochondrial DNA (mtDNA) is 10-600 times larger than the mammalian mtDNA, yet it only contains approximately 50% more genes; (v) plant mtDNA has kept the standard genetic code, and it has a low divergence rate with respect to point mutations, but a high recombinatorial activity; (vi) mitochondrial mRNA maturation includes a uniquely complex set of activities for processing, splicing and editing (at hundreds of sites); (vii) recombination in mtDNA creates novel reading frames that can produce male sterility; and (viii) plant mitochondria have a large proteome with 2000-3000 different proteins containing many unique proteins such as 200-300 pentatricopeptide repeat proteins. We describe the present and fairly detailed picture of the structure and function of plant mitochondria and how the unique properties make their metabolism more flexible allowing them to be involved in many diverse processes in the plant cell, such as photosynthesis, photorespiration, CAM and C4 metabolism, heat production, temperature control, stress resistance mechanisms, programmed cell death and genomic evolution. However, it is still a challenge to understand how the regulation of metabolism and mtDNA expression works at the cellular level and how retrograde signaling from the mitochondria coordinates all those processes.

Matrix Redox Physiology Governs the Regulation of Plant Mitochondrial Metabolism through Posttranslational Protein Modifications

Mitochondria function as hubs of plant metabolism. Oxidative phosphorylation produces ATP, but it is also a central high-capacity electron sink required by many metabolic pathways that must be flexibly coordinated and integrated. Here, we review the crucial roles of redox-associated posttranslational protein modifications (PTMs) in mitochondrial metabolic regulation. We discuss several major concepts. First, the major redox couples in the mitochondrial matrix (NAD, NADP, thioredoxin, glutathione, and ascorbate) are in kinetic steady state rather than thermodynamic equilibrium. Second, targeted proteomics have produced long lists of proteins potentially regulated by Cys oxidation/thioredoxin, Met-SO formation, phosphorylation, or Lys acetylation, but we currently only understand the functional importance of a few of these PTMs. Some site modifications may represent molecular noise caused by spurious reactions. Third, different PTMs on the same protein or on different proteins in the same metabolic pathway can interact to fine-tune metabolic regulation. Fourth, PTMs take part in the repair of stress-induced damage (e.g., by reducing Met and Cys oxidation products) as well as adjusting metabolic functions in response to environmental variation, such as changes in light irradiance or oxygen availability. Finally, PTMs form a multidimensional regulatory system that provides the speed and flexibility needed for mitochondrial coordination far beyond that provided by changes in nuclear gene expression alone.

The potato tuber mitochondrial proteome

Mitochondria are called the powerhouses of the cell. To better understand the role of mitochondria in maintaining and regulating metabolism in storage tissues, highly purified mitochondria were isolated from dormant potato tubers (Solanum tuberosum 'Folva') and their proteome investigated. Proteins were resolved by one-dimensional gel electrophoresis, and tryptic peptides were extracted from gel slices and analyzed by liquid chromatography-tandem mass spectrometry using an Orbitrap XL. Using four different search programs, a total of 1,060 nonredundant proteins were identified in a quantitative manner using normalized spectral counts including as many as 5-fold more "extreme" proteins (low mass, high isoelectric point, hydrophobic) than previous mitochondrial proteome studies. We estimate that this compendium of proteins represents a high coverage of the potato tuber mitochondrial proteome (possibly as high as 85%). The dynamic range of protein expression spanned 1,800-fold and included nearly all components of the electron transport chain, tricarboxylic acid cycle, and protein import apparatus. Additionally, we identified 71 pentatricopeptide repeat proteins, 29 membrane carriers/transporters, a number of new proteins involved in coenzyme biosynthesis and iron metabolism, the pyruvate dehydrogenase kinase, and a type 2C protein phosphatase that may catalyze the dephosphorylation of the pyruvate dehydrogenase complex. Systematic analysis of prominent posttranslational modifications revealed that more than 50% of the identified proteins harbor at least one modification. The most prominently observed class of posttranslational modifications was oxidative modifications. This study reveals approximately 500 new or previously unconfirmed plant mitochondrial proteins and outlines a facile strategy for unbiased, near-comprehensive identification of mitochondrial proteins and their modified forms.

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.

Regulation of plant glycine decarboxylase by s-nitrosylation and glutathionylation

Mitochondria play an essential role in nitric oxide (NO) signal transduction in plants. Using the biotin-switch method in conjunction with nano-liquid chromatography and mass spectrometry, we identified 11 candidate proteins that were S-nitrosylated and/or glutathionylated in mitochondria of Arabidopsis (Arabidopsis thaliana) leaves. These included glycine decarboxylase complex (GDC), a key enzyme of the photorespiratory C(2) cycle in C3 plants. GDC activity was inhibited by S-nitrosoglutathione due to S-nitrosylation/S-glutathionylation of several cysteine residues. Gas-exchange measurements demonstrated that the bacterial elicitor harpin, a strong inducer of reactive oxygen species and NO, inhibits GDC activity. Furthermore, an inhibitor of GDC, aminoacetonitrile, was able to mimic mitochondrial depolarization, hydrogen peroxide production, and cell death in response to stress or harpin treatment of cultured Arabidopsis cells. These findings indicate that the mitochondrial photorespiratory system is involved in the regulation of NO signal transduction in Arabidopsis.

Molecular identification and functional characterization of Arabidopsis thaliana mitochondrial and chloroplastic NAD+ carrier proteins

The Arabidopsis thaliana L. genome contains 58 membrane proteins belonging to the mitochondrial carrier family. Two mitochondrial carrier family members, here named AtNDT1 and AtNDT2, exhibit high structural similarities to the mitochondrial nicotinamide adenine dinucleotide (NAD(+)) carrier ScNDT1 from bakers' yeast. Expression of AtNDT1 or AtNDT2 restores mitochondrial NAD(+) transport activity in a yeast mutant lacking ScNDT. Localization studies with green fluorescent protein fusion proteins provided evidence that AtNDT1 resides in chloroplasts, whereas only AtNDT2 locates to mitochondria. Heterologous expression in Escherichia coli followed by purification, reconstitution in proteoliposomes, and uptake experiments revealed that both carriers exhibit a submillimolar affinity for NAD(+) and transport this compound in a counter-exchange mode. Among various substrates ADP and AMP are the most efficient counter-exchange substrates for NAD(+). Atndt1- and Atndt2-promoter-GUS plants demonstrate that both genes are strongly expressed in developing tissues and in particular in highly metabolically active cells. The presence of both carriers is discussed with respect to the subcellular localization of de novo NAD(+) biosynthesis in plants and with respect to both the NAD(+)-dependent metabolic pathways and the redox balance of chloroplasts and mitochondria.

Involvement of matrix NADP turnover in the oxidation of NAD-linked substrates by pea leaf mitochondria

The involvement of the internal rotenone-insensitive NADPH dehydrogenase on the inner surface of the inner mitochondrial membrane [NDin(NADPH)] in the oxidation of strictly NAD+-linked substrates by pea (Pisum sativum L.) leaf mitochondria was measured. As estimated by the inhibition caused by 5 µM diphenyleneiodonium (DPI) in the presence of rotenone to inhibit complex I, the activity of NDin(NADPH) during glycine oxidation (measured both as O2 uptake and as CO2 release) was 40-50 nmol mg-1 protein min-1. No significant activity of NDin(NADPH) could be detected during the oxidation of 2-oxoglutarate, another strictly NAD+-linked substrate; this was possibly due to its relatively low oxidation rate. Control experiments showed that, even at 125 µM, DPI had no effect on the activity of glycine decarboxylase complex (GDC) and lipoamide dehydrogenase. The relative activity of complex I, NDin(NADPH), and NDin(NADH) during glycine oxidation, estimated using rotenone and DPI, differed depending on the pyridine nucleotide supply in the mitochondrial matrix. This was shown by loading the mitochondria with NAD+ and NADP+, both of which were taken up by the organelle. We conclude that the involvement of NADP turnover during glycine oxidation is not due to the direct production of NADPH by GDC but is an indirect result of this process. It probably occurs via the interconversion of NADH to NADPH by the two non-energy-linked transhydrogenase activities recently identified in plant mitochondria.

Effect of High Physiological Temperatures on NAD+ Content of Green Leaf Mitochondria (Apparent Inhibition of Glycine Oxidation)

We observed a rapid decline in the rate of glycine oxidation by purified pea (Pisum sativum L.) leaf mitochondria preincubated at 40[deg]C for 2 min. In contrast, exogenous NADH and succinate oxidations were not affected by the heat treatment. We first demonstrated that the inhibition of glycine oxidation was not attributable to a direct effect of high temperatures on glycine decarboxylase/serine hydroxymethyltransferase. We observed that (a) addition of NAD+ to the incubation medium resulted in a resumption of glycine-dependent O2 uptake by intact mitochondria, (b) addition of NAD+ to the suspending medium prevented the decline in the rate of glycine-dependent O2 consumption by pea leaf mitochondria incubated at 40[deg]C, (c) NAD+ concentration in the matrix space collapses within only 5 min of warm temperature treatment, and (d) mitochondria treated with the NAD+ analog N-4-azido-2-nitrophenyl-4-aminobutyryl-3[prime]-NAD+ retained high rates of glycine-dependent O2 uptake after preincubation at 40[deg]C. Therefore, we conclude that the massive and rapid efflux of NAD+, leading to the apparent inhibition of glycine oxidation, occurs through the specific NAD+ carrier present in the inner membrane of plant mitochondria. Finally, our data provide further evidence that NAD+ is not firmly bound to the inner membrane.

Comparison of the Kinetic Behavior toward Pyridine Nucleotides of NAD-Linked Dehydrogenases from Plant Mitochondria

In this article we compare the kinetic behavior toward pyridine nucleotides (NAD(+), NADH) of NAD(+)-malic enzyme, pyruvate dehydrogenase, isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase, and glycine decarboxylase extracted from pea (Pisum sativum) leaf and potato (Solanum tuberosum) tuber mitochondria. NADH competitively inhibited all the studied dehydrogenases when NAD(+) was the varied substrate. However, the NAD(+)-linked malic enzyme exhibited the weakest affinity for NAD(+) and the lowest sensitivity for NADH. It is suggested that NAD(+)-linked malic enzyme, when fully activated, is able to raise the matricial NADH level up to the required concentration to fully engage the rotenone-resistant internal NADH-dehydrogenase, whose affinity for NADH is weaker than complex I.

Effects of Helminthosporium maydis Race T Toxin on Electron Transport in Susceptible Corn Mitochondria and Prevention of Toxin Actions by Dicyclohexylcarbodiimide

The effect of Helminthosporium maydis race T toxin on electron transport in susceptible cytoplasmic male-sterile Texas corn (Zea mays L.) mitochondria was investigated, using dichlorophenol indophenol and ferricyanide as electron acceptors. Succinate-dependent electron transport was stimulated by the toxin, consistent with the well described increase in membrane permeability induced by the toxin. Malate-dependent electron transport was inhibited. This inhibition of electron transport increased as a function of time of exposure to the toxin. Mitochondria from normal-fertile (N) corn were not affected by the toxin. Both the inhibition of electron transport and the increase in ion permeability, such as dissipation of membrane potential and Ca(2+) gradients, induced by the toxin in T corn was prevented by N,N'-dicyclohexylcarbodiimide, a hydrophobic carbodiimide. A water-soluble carbodiimide, 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide, was ineffective in preventing dissipation of membrane potential by the toxin. These results suggest that the various toxin actions are mediated via interaction of the toxin with one target site, most probably a 13 kilodalton polypeptide unique to T mitochondria. N,N'-dicyclohexylcarbodiimide may confer protection by modifying an amino acid residue in a hydrophobic portion of the target site.

Respiration of Mitochondria Isolated from Leaves and Protoplasts of Avena sativa

Mitochondria isolated from mesophyll protoplasts differed from mitochondria isolated directly from leaves of Avena sativa in that protoplast mitochondria (a) had a lower overall respiratory capacity, (b) were less able to use low concentrations of exogenous NADH, (c) did not respond rapidly or strongly to added NAD, (d) appeared to accumulate more oxaloacetate, and (e) oxidized both succinate and tetramethyl-p-phenylene-diamine (an electron donor for cytochrome oxidase) more slowly than did leaf mitochondria. It is concluded that cytochrome oxidase activity was inhibited, the external NADH dehydrogenase had a reduced affinity for NADH, succinate oxidation was inhibited, NAD and oxaloacetate porters were probably inhibited, and accessibility to respiratory paths may have been reduced in protoplast mitochondria. The results also suggest that there was a reduced affinity of a succinate porter for this substrate in oat mitochondria. In addition, all oat mitochondria required salicylhydroxamic acid (SHAM) as well as cyanide to block malate and succinate oxidation. Malate oxidation that did not appear to saturate the cytochrome pathway was sensitive to SHAM in the absence of cyanide, suggesting that the oat mitochondria studied had concomitant alternative and subsaturating cytochrome oxidase pathway activity.

Photosynthesis, Leaf Anatomy, and Cellular Constituents in the Polyploid C(4) Grass Panicum virgatum

Photosynthetic gas exchange, activities of six key C(4) cycle enzymes, amounts of soluble protein, chlorophyll, and DNA, and various leaf anatomical and structural features were measured in naturally occurring tetraploid and octaploid plants of the NAD-malic enzyme type C(4) grass Panicum virgatum L. On a leaf area basis, the photosynthetic rate and concentrations of DNA, soluble protein, and chlorophyll were 40 to 50% higher, and enzyme activities 20 to 70% higher in the octaploid than in the tetraploid. Photosynthetic cells in the octaploid were only 17 to 19% larger in volume, yet contained 33 to 38% more chloroplasts than cells in the tetraploid. On a per cell basis the contents of DNA, soluble protein, and chlorophyll, activities of carboxylating photosynthetic enzymes, and carbon assimilation rate were all doubled in octaploid compared with tetraploid cells. Since cellular volume did not double with genome doubling, cellular constituents were more concentrated in the cells of the octaploid. The influences of polyploidy were balanced between mesophyll and bundle sheath cells since the changes in physical and biochemical parameters with ploidy level were similar in both cell types. We conclude that photosynthetic activity in these two polyploid genotypes of P. virgatum is determined by enzyme activities and concentrations of biochemical constituents, and that selection for smaller cell volume has led to higher photosynthetic rates per unit leaf area in the octaploid. The ratio of DNA content to cellular volume is a major factor determining the concentrations of gene products in cells. The number of chloroplasts, however, is controlled more by cellular volume than by the number of nuclear chromosomes.

The responses of isolated plant mitochondria to external nicotinamide adenine dinucleotide

The effects of added NAD on substrate oxidation by turnip (Brassica rapa L.) and beetroot (Beta vulgaris L.) mitochondria were investigated. State 3 malate and 2-oxoglutarate oxidation rates with turnip mitochondria were stimulated 25 to 40% by external NAD. Following NAD-depletion this stimulation by NAD was increased to 70 to 80%. With purified beetroot mitochondria, state 3 malate and 2-oxoglutarate oxidation rates were only marginally increased (10-15%) by the addition of NAD but after NAD-depletion treatments this stimulation increased to 55%. The effect of added NAD on oxidation rates could be reduced by preloading mitochondria with NAD in the presence of succinate. Oxidation rates were found to be most sensitive to the addition of external NAD when rotenone was present. The uptake of external NAD into beetroot mitochondria appeared to be composed of both an active and a diffusive component. The active component displayed saturation kinetics with an approximate K(m) of 0.105 +/- 0.046 millimolar. These results provide further evidence, reported previously with potato mitochondria, that NAD can move across the inner membrane of plant mitochondria. They are particularly significant with respect to beetroot mitochondria which in contrast to other plant mitochondria, have not demonstrated any response to added NAD.