Matrix NADH dehydrogenases of plant mitochondria and sites of quinone reduction by complex I

Assessment of respiration in isolated plant mitochondria using Clark-type electrodes

Mitochondrial respiration involves two key gas exchanges, the consumption of oxygen and the release of carbon dioxide. The ability to measure the consumption of oxygen via Clark-type electrodes has been one of the key techniques for advancing our knowledge of mitochondrial function in whole organisms, tissue samples, cells, and isolated subcellular fractions. In plants, oxygen electrode analyses provided the first evidence for some of the unique respiratory properties of plant mitochondria. This chapter briefs the principles of respiration and oxidative phosphorylation, how oxygen consumption measurements can be used to assess the quality of isolated mitochondrial preparations, and how these measurements can answer important questions in plant biochemistry and physiology. Finally, it presents instructions on assembling the oxygen electrode apparatus and how to conduct various assays.

The Cytotoxic lipid peroxidation product 4-hydroxy-2-nonenal covalently modifies a selective range of proteins linked to respiratory function in plant mitochondria

Plants encounter a variety of environmental stresses that affect their cellular machinery and that they adapt to on a daily basis. Lipid peroxidation is one consequence, at the cellular level, of such stresses and yields cytotoxic lipid aldehydes, including 4-hydroxy-2-nonenal (HNE), that react with specific sites in proteins, leading to diverse changes in protein function and/or stability. We have assessed the sensitivity of plant mitochondrial proteins to HNE modification, using one-dimensional and two-dimensional denaturing PAGE and blue native-PAGE coupled to immunological detection and tandem mass spectrometry identification. A select range of proteins was modified by exogenous application of HNE to mitochondria isolated from Arabidopsis cell cultures. These included a number of proteins that directly interact with the ubiquinone pool, as well as a number of soluble matrix proteins. Mitochondria isolated from cell cultures following hydrogen peroxide, antimycin A, or menadione treatment had significantly reduced respiratory capacity and elevated levels of HNE adduction to specific subsets of proteins. Targets identified included the proteins affected by direct application of HNE but also some new proteins, including a number of matrix dehydrogenases, the inner membrane adenine nucleotide translocator, and the outer membrane voltage-dependent anion channel. Degradation products of some proteins were also found to be HNE adducted, suggesting a link between HNE adduction and protein turnover. Some of the major enzyme complexes that were HNE adducted did not show demonstrable changes in their maximal activity measured with artificial acceptors, but changes did occur in associations between respiratory chain complexes following stress treatments.

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.

The sites of interaction of triphenyltetrazolium chloride with mitochondrial respiratory chains

The inability of cells and microorganisms to reduce the colourless electron acceptor triphenyltetrazolium chloride (TTC) to a red formazan precipitate is commonly used as a means of screening for cells that have a dysfunctional respiratory chain. The site of reduction of TTC is often stated to be at the level of cytochrome c oxidase where it is assumed to compete with oxygen for reducing equivalents. However, we show here that TTC is reduced not by cytochrome c oxidase but instead by dehydrogenases, particularly complex I, probably by accepting electrons directly from low potential cofactors. The reduction rate is fastest in coupled membranes because of accumulation in the matrix of the positively charged TTC+ cation. However, the initial product of TTC reduction is rapidly reoxidised by molecular oxygen, so that generation of the stable red formazan product from this intermediate occurs only under strictly anaerobic conditions. Colonies of mutants defective in cytochrome oxidase do not generate sufficiently anaerobic conditions to allow the intermediate to form the stable red formazan. This revision of the mode of interaction of TTC with respiratory chains has implications for the types of respiratory-defective mutants that might be detected by TTC screening.

Physiological, biochemical and molecular aspects of mitochondrial complex I in plants

Respiratory complex I of plant mitochondria has to date been investigated with respect to physiological function, biochemical properties and molecular structure. In the respiratory chain complex I is the major entry gate for low potential electrons from matrix NADH, reducing ubiquinone and utilizing the released energy to pump protons across the inner membrane. Plant complex I is active against a background of several other NAD(P)H dehydrogenases, which do not contribute in proton pumping, but permit and establish several different routes of shuttling electrons from NAD(P)H to ubiquinone. Identification of the corresponding molecular structures, that is the proteins and genes of the different NADH dehydrogenases, will allow more detailed studies of this interactive regulatory network in plant mitochondria. Present knowledge of the structure of complex I and the respective mitochondrial and nuclear genes encoding various subunits of this complex in plants is summarized here. Copyright 1998 Elsevier Science B.V.

Purification and characterization of a 43-kDa rotenone-insensitive NADH dehydrogenase from plant mitochondria

A 43-kDa NAD(P)H dehydrogenase was purified from red beetroot mitochondria. An antibody against this dehydrogenase was used in conjunction with the membrane-impermeable protein cross-linker 3,3'-dithiobis(sulfosuccinimidylpropionate) to localize the dehydrogenase on the matrix side of the inner membrane. Immunoblotting showed that the dehydrogenase was found in mitochondria isolated from several plant species but not from rat livers. Antibodies against the purified dehydrogenase partially inhibited rotenoneinsensitive internal NADH oxidation by inside-out submitochondrial particles. The level of rotenone-insensitive respiration with NAD-linked substrates correlated with the amount of 43-kDa NAD(P)H dehydrogenase present in mitochondria isolated from different soybean tissues. Based on these results, we conclude that the 43-kDa NAD(P)H dehydrogenase is responsible for rotenone-insensitive internal NADH oxidation in plant mitochondria.

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.

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.

MPP+ and MPDP+ induced oxygen radical formation with mitochondrial enzymes

MPP+ has been reported to inhibit reduced nicotinamide adenine dinucleotide (NADH) dehydrogenase in mitochondria, which results in the formation of O2(.-). The current report demonstrates that H2O2 and HO. are also products of MPP+ interaction with NADH dehydrogenase. It is possible that MPP. formation precedes the formation of some of these active oxygen species. Reducing equivalents for radical formation come from NADH. MPP+ may be capable of interacting with submitochondrial particles at a site other than the rotenone site, which results in some formation of oxygen radicals. Plasma amine oxidase incubations with MPDP+ resulted in O2.- H2O2, and perhaps HO. formation. This is probably due to MPP. formation from the oxidation of MPDP+. This study presents new findings that indicate the potential importance of oxygen radical formation in mitochondria during MPTP toxicity.

Cloning of ndhK from soybean chloroplasts using antibodies raised to mitochondrial complex I

A soybean shoot cDNA expression library was screened with polyclonal antibodies raised against red beet complex I and several clones were identified. One clone, consisting of a 1 kb insert, was fully sequenced. The sequence of 1025 bp was found to contain two extended open reading frames and the proteins encoded were identified as the ndhK and ndhJ products of the chloroplast genome. Nuclear, mitochondrial and chloroplast DNA was isolated and probed with a ndhK-specific probe. The chloroplast DNA contained a single copy of the cloned insert. With nuclear DNA, positively hybridising bands of 1.2, 2.7 and 3.2 kb were observed indicating that at least one gene homologous to ndhK of the chloroplast genome, is also present in the nucleus. The mitochondrial DNA did not hybridise with the ndhK probe. Western analysis of thylakoid proteins with the mitochondrial complex I antibodies revealed several bands. It is suggested that soybean contains two copies of the ndhK gene, one, on the plastid genome, coding for a subunit of a chloroplast NAD(P)H dehydrogenase, and the other, in the nucleus, coding for a subunit of mitochondrial complex I.