Solubilization of the alternative oxidase of cuckoo-pint (Arum maculatum) mitochondria. Stimulation by high concentrations of ions and effects of specific inhibitors.

Selective solubilization of cyanide- and antimycin-insensitive duroquinol oxidase activity from cuckoo-pint (Arum maculatum) mitochondria was achieved using taurocholate. Inhibitor-sensitivities and water-forming DQH2 (tetramethyl-p-hydroquinone, reduced form): O2 stoichiometry were the same for the alternative oxidase of intact Arum mitochondria. Cyanide-insensitive oxidation of DQH2 by intact and solubilized mitochondria was stimulated by up to four-fold by high concentrations of anions high in the Hofmeister series, such as phosphate, sulphate or citrate. Optimal (0.7 M) sodium citrate increased Vmax. for DQH2 oxidation by the solubilized preparation from 450 to 2400 nmol of O2 X min-1 X mg of protein-1 and decreased the apparent Km for DQH2 from 0.53 to 0.38 mM. Inhibition of solubilized DQH2 oxidase activity by CLAM (m-chlorobenzhydroxamic acid) and SHAM (salicylhydroxamic acid) was mixed competitive/non-competitive, with apparent inhibition constants for CLAM of 25 microM (Ki) and 81 microM (KI) and for SHAM of 53 microM (Ki) and 490 microM (KI). Propyl gallate and UHDBT were non-competitive inhibitors with respect to DQH2 (apparent Ki = 0.3 microM and 12 nM respectively). Low concentrations of C18 fatty acids selectively inhibited cyanide-insensitive oxidation by intact and solubilized mitochondria, and inhibition was reversed by 1% (w/v) bovine serum albumin. Inhibition was competitive with DQH2, suggesting that fatty acids interfere reversably with the binding of DQH2 to the oxidase. These results tend to support the view that quinol oxidation by the alternative pathway of Arum maculatum mitochondria is catalysed by a quinol oxidase protein, rather than by a non-enzymic mechanism involving fatty acid peroxidative reaction. [Rustin, Dupont & Lance (1983) Trends Biochem. Sci. 8, 155-157; (1983) Arch. Biochem. Biophys. 225, 630-639].     

AMP-dependence of the cyanide-insensitive pathway in the respiratory chain of Paramecium tetraurelia

The AMP-dependent stimulation of the cyanide-insensitive respiration of Paramecium mitochondria was investigated. The nucleotides exhibiting a stimulatory effect on the cyanide-insensitive oxidation of pyruvate (+ malate) in a medium supplemented with EDTA or carboxyatractyloside were, in decreasing order of efficiency, AMP, GMP, IMP, UMP and TMP. On the other hand, ADP, ATP and cyclic AMP were ineffective. In the presence of carboxyatractyloside, addition of AMP to Paramecium mitochondria incubated with pyruvate (+malate) led to an increase in membrane potential. In the absence of light, the photoactivable derivative of AMP, 3'-[4-[N-(4-azido-2-nitrophenyl)amino]butyryl]-AMP (NAP4-AMP) added to Paramecium mitochondria opposed the stimulatory effect of AMP on the cyanide-insensitive respiration; the Ki for NAP4-AMP was much lower than the Km for AMP, 0.2 microM compared with 120 microM. The ADP-stimulated respiration was not affected. Photoirradiation of Paramecium mitochondria in the presence of NAP4-AMP resulted in irreversible inhibition of the AMP-stimulated cyanide-insensitive respiration. No effect on the ADP-stimulated respiration was observed. A heatlabile cyanide-insensitive ubiquinol oxidase was extracted from Paramecium mitochondria with the detergent NN-dimethyl-N-(3-laurylamidopropyl)amine oxide. The quinol oxidase activity was slightly stimulated by AMP.     

Regulation of Alternative Pathway Activity in Plant Mitochondria : Deviations from Q-Pool Behavior during Oxidation of NADH and Quinols

External NADH and succinate were oxidized at similar rates by soybean (Glycine max) cotyledon and leaf mitochondria when the cytochrome chain was operating, but the rate of NADH oxidation via the alternative oxidase was only half that of succinate. However, measurements of the redox poise of the endogenous quinone pool and reduction of added quinones revealed that external NADH reduced them to the same, or greater, extent than did succinate. A kinetic analysis of the relationship between alternative oxidase activity and the redox state of ubiquinone indicated that the degree of ubiquinone reduction during external NADH oxidation was sufficient to fully engage the alternative oxidase. Measurements of NADH oxidation in the presence of succinate showed that the two substrates competed for cytochrome chain activity but not for alternative oxidase activity. Both reduced Q-1 and duroquinone were readily oxidized by the cytochrome oxidase pathway but only slowly by the alternative oxidase pathway in soybean mitochondria. In mitochondria isolated from the thermogenic spadix of Philodendron selloum, on the other hand, quinol oxidation via the alternative oxidase was relatively rapid; in these mitochondria, external NADH was also oxidized readily by the alternative oxidase. Antibodies raised against alternative oxidase proteins from Sauromatum guttatum cross-reacted with proteins of similar molecular size from soybean mitochondria, indicating similarities between the two alternative oxidases. However, it appears that the organization of the respiratory chain in soybean is different, and we suggest that some segregation of electron transport chain components may exist in mitochondria from nonthermogenic plant tissues.     

Activation of the alternative oxidase of Yarrowia lipolytica by adenosine 5'-monophosphate

The study of the effect of nucleoside phosphates on the activity of cyanide-resistant oxidase in the mitochondria and the submitochondrial particles of Yarrowia lipolytica showed that adenosine monophosphate (5'-AMP, AMP) did not stimulate the respiration of the intact mitochondria. The incubation of the mitochondria at room temperature (25 degrees C) for 3-5 h or their treatment with ultrasound, phospholipase A, and detergent Triton X-100 at a low temperature inactivated the cyanide-resistant alternative oxidase. The inactivated alternative oxidase could be reactivated by AMP. The reactivating effect of AMP was enhanced by azolectin. Some other nucleoside phosphates also showed reactivating ability in the following descending order. AMP = GMP > GDP > GTP > XMP > IMP. The apparent reaction rate constant Km for AMP upon the reactivation of the alternative oxidase of mitochondria treated with Triton X-100 or incubated at 25 degrees C was 12.5 and 20 microM, respectively. The Km for AMP upon the reactivation of the alternative oxidase of submitochondrial particles was 15 microM. During the incubation of yeast cells under conditions promoting the development of alternative oxidase, the content of adenine nucleotides (AMP, ADP, and ATP) in the cells and their respiration tended to decrease. The subsequent addition of cyanide to the cells activated their respiration, diminished the intracellular content of ATP three times, and augmented the content of AMP five times. These data suggest that the stimulation of cell respiration by cyanide may be due to the activation of alternative oxidase by AMP.     

The Content of Alternative Oxidase of Euglena Mitochondria is Increased by Growth in the Presence of Cyanide and is not Cytochrome o.

ABSTRACT A study of the effect of respiratory inhibitors on O₂ uptake of Euglena gracilis mitochondria, isolated from cells grown in the presence of cyanide or with ethanol as carbon source, was undertaken. The contents of cytochrome c oxidase and alternative oxidase were also determined. Inhibition of respiration by antimycin and cyanide was only partial and it was dependent on the oxidizable substrate used. Succinate oxidation was the most sensitive to cyanide whereas lactate oxidation was the most resistant. Cell growth in the presence of cyanide or with ethanol as carbon source brought about an enhanced content of alternative oxidase without a concomitant increase in cytochrome aa₃ content. However, a correlation between cyanide-resistant respiration and alternative oxidase content was not found. Analysis of heme types in mitochondrial membranes revealed the absence of heme O. The data suggest the presence of an inducible alternative oxidase in Euglena mitochondria which has high resistance to cyanide and contains heme B. A close relationship between Euglena alternative oxidase and bacterial quinol oxidases containing B-type heme is proposed.     

The mitochondrial oxidation of quinol monophosphates

1. Mitochondria from ox heart and rat liver catalysed a slow cyanide-sensitive oxidation of 2,3-dimethylnaphthaquinol monophosphate, duroquinol monophosphate, menadiol 1-phosphate and menadiol 4-phosphate. 2. The release of P(i) was concomitant with oxygen uptake. 3. The oxidation was somewhat stimulated by Ca(2+) and P(i), and weakly inhibited by 2,4-dinitrophenol. 4. The quinol monophosphates effected a rapid reduction of free cytochrome c, and consequently addition of cytochrome c greatly increased the rate of the mitochondrial oxidation of 2,3-dimethylnaphthaquinol monophosphate. 5. This quinol phosphate interacts with the electron-transport chain at the level of cytochrome c. 6. Polylysine promoted an interaction between 2,3-dimethylnaphthaquinol monophosphate and cytochrome oxidase. Thus, although polylysine blocks mitochondrial oxidations via reduced cytochrome c, the oxidation of the quinol phosphate was strongly stimulated. 7. This stimulation was most effective in the most intact mitochondrial preparations and was inhibited by ADP and by P(i). 8. The implications of these results for factors limiting the rate of quinol phosphate oxidation, the mode of action of stimulators and the mechanism of P(i) formation are discussed.     

Reactivation of the alternative oxidase of Yarrowia lipolytica by nucleoside monophosphates

The study of the effect of nucleoside phosphates on the activity of cyanide-resistant oxidase in the mitochondria and submitochondrial particles of Yarrowia lipolytica showed that adenosine monophosphate (5'-AMP, AMP) did not stimulate the respiration of intact mitochondria. The incubation of mitochondria at room temperature (25 degrees C) for 3-5 h or their treatment with ultrasound, phospholipase A, and the detergent Triton X-100 at a low temperature inactivated the cyanide-resistant alternative oxidase. The inactivated alternative oxidase could be reactivated with AMP. The reactivating effect of AMP was enhanced by azolectin. Some other nucleoside phosphates also showed reactivating ability in the following descending order: AMP = GMP > GDP > GTP > MP > IMP. The apparent K(m) values for AMP in reactivation of the alternative oxidase of submitochondrial particles or mitochondria treated with Triton X-100 and incubated at 25 degrees C were calculated. Physiological aspects of activation of the alternative oxidase are discussed in connection with the impairment of electron transfer through the cytochrome pathway.     

Antibiotics LL-Z1272 identified as novel inhibitors discriminating bacterial and mitochondrial quinol oxidases

To counter antibiotic-resistant bacteria, we screened the Kitasato Institute for Life Sciences Chemical Library with bacterial quinol oxidase, which does not exist in the mitochondrial respiratory chain. We identified five prenylphenols, LL-Z1272β, γ, δ, ? and ζ, as new inhibitors for the Escherichia coli cytochrome bd. We found that these compounds also inhibited the E. coli bo-type ubiquinol oxidase and trypanosome alternative oxidase, although these three oxidases are structurally unrelated. LL-Z1272β and ? (dechlorinated derivatives) were more active against cytochrome bd while LL-Z1272γ, δ, and ζ (chlorinated derivatives) were potent inhibitors of cytochrome bo and trypanosome alternative oxidase. Thus prenylphenols are useful for the selective inhibition of quinol oxidases and for understanding the molecular mechanisms of respiratory quinol oxidases as a probe for the quinol oxidation site. Since quinol oxidases are absent from mammalian mitochondria, LL-Z1272β and δ, which are less toxic to human cells, could be used as lead compounds for development of novel chemotherapeutic agents against pathogenic bacteria and African trypanosomiasis.     

Adenylate control of respiration in plants: the contribution of rotenone-insensitive electron transport to ADP-limited oxygen consumption by soybean mitochondria

The aim was to test the hypothesis that rotenone-insensive electron transport (bypass of complex I) may underlie rapid state 4 (ADP-limited) mitochondrial respiration. A comparison of mitochondria from soybean ( Glycine max L. cv. Bragg) cotyledons and nodules showed that ADP-sufficient (state 3) malate plus pyruvate oxidation by mitochondria from 7-day-old cotyledons was inhibited 50% by rotenone and state 4 rates were rapid, whereas nodule mitochondria were 80% inhibited by rotenone and had slower state 4 rates of malate plus pyruvate oxidation. Respiration of malate alone (pH 7.6) by cotyledon mitochondria was slow, especially in the absence of ADP; subsequent addition of pyruvate dramatically increased state 4 oxygen uptake concomitant with a rapid rise in mitochondrial NADH (determined by fluorimetry). Rotenone had no effect on this increased rate of state 4 respiration. The rate of malate oxidation by nodule mitochondria was relatively rapid compared with cotyledon mitochondria. The addition of pyruvate in state 4 caused a slow increase in matrix NADH and only a slight stimulation of oxygen uptake. Rotenone inhibited state 4 malate plus pyruvate oxidation by 50% in these mitochondria. From a large number of cotyledon and nodule mitochondrial preparations, a close correlation was found between the rate of state 4 oxygen uptake and rotenone-resistance. During cotyledon development increased rotenone-resistance was associated with an increase in the alternative oxidase. Addition of pyruvate to cotyledon mitochondria, during state 4 oxidation of malate in the presence of antimycin A, significantly stimulated O₂ uptake and also almost eliminated respiratory control. Such combined operation of the rotenone-insensitive bypass and the alternative oxidase in vivo will significantly affect the extent to which adenylates control the rate of electron transport.     

L-lactate metabolism in potato tuber mitochondria

We investigated the metabolism of L-lactate in mitochondria isolated from potato tubers grown and saved after harvest in the absence of any chemical agents. Immunologic analysis by western blot using goat polyclonal anti-lactate dehydrogenase showed the existence of a mitochondrial lactate dehydrogenase, the activity of which could be measured photometrically only in mitochondria solubilized with Triton X-100. The addition of L-lactate to potato tuber mitochondria caused: (a) a minor reduction of intramitochondrial pyridine nucleotides, whose measured rate of change increased in the presence of the inhibitor of the alternative oxidase salicyl hydroxamic acid; (b) oxygen consumption not stimulated by ADP, but inhibited by salicyl hydroxamic acid; and (c) activation of the alternative oxidase as polarographically monitored in a manner prevented by oxamate, an L-lactate dehydrogenase inhibitor. Potato tuber mitochondria were shown to swell in isosmotic solutions of ammonium L-lactate in a stereospecific manner, thus showing that L-lactate enters mitochondria by a proton-compensated process. Externally added L-lactate caused the appearance of pyruvate outside mitochondria, thus contributing to the oxidation of extramitochondrial NADH. The rate of pyruvate efflux showed a sigmoidal dependence on L-lactate concentration and was inhibited by phenylsuccinate. Hence, potato tuber mitochondria possess a non-energy-competent L-lactate/pyruvate shuttle. We maintain, therefore, that mitochondrial metabolism of L-lactate plays a previously unsuspected role in the response of potato to hypoxic stress.     

Myxothiazol resistance in human mitochondria

We have investigated electron transfer activities of respiratory chain complexes in platelet mitochondria of a patient with intermittent ataxia and lactic acidosis who was previously reported to be deficient in the E1 (decarboxylase) component of the pyruvate dehydrogenase complex. Electron transfer from succinate to cytochrome c was normal, but the mitochondria exhibited moderately decreased (63% of control) quinol: cytochrome-c oxidoreductase activity, suggesting a defect in complex III. Consistent with some perturbation in complex III, electron flux through complex III was resistant to inhibition by myxothiazol compared to normal controls. In contrast, titration with antimycin revealed a less abnormal pattern of inhibition. The extreme specificity of myxothiazol binding at or near the quinol oxidase domain of mitochondrial cytochrome b, i.e., b-566, suggests a defect in this region of complex III which may perturb the kinetics or thermodynamics of quinol oxidation in the complex. These data suggest that the patient's illness results from a mutation in the quinol oxidase domain of mitochondrial cytochrome b (b-566).     

Effect of growth at low temperature on the alternative pathway respiration in Acanthamoeba castellanii mitochondria

Mitochondria of amoeba Acanthamoeba castellanii in addition to the conventional cytochrome pathway possess, like plant mitochondria, a cyanide-resistant alternative quinol oxidase. In mitochondria isolated from amoeba batch culture grown temporarily at low temperature (6 degrees C), higher respiration was accompanied by lower coupling parameters as compared to control culture (grown at 28 degrees C). In the presence of benzohydroxamate, respiratory rates and coupling parameters were similar in both types of mitochondria indicating that growth in cold conditions did not disturb the cytochrome pathway. Increased contribution of alternative oxidase in total mitochondrial respiration in low-temperature-grown amoeba cells was confirmed by calculation of its contribution using ADP/O measurements. Furthermore, in mitochondria from low-temperature- grown cells the content of the alternative oxidase was increased and correlated with the increase in the unstimulated and GMP-stimulated cyanide-resistant respiratory activity. A possible physiological role of higher activity of alternative oxidase as response to growth at a low temperature in unicellular organisms, such as amoeba, is discussed.     

Partial Purification and Characterization of the Quinol Oxidase Activity of Arum maculatum Mitochondria

The menadiol oxidase activity of Arum maculatum mitochondria has been solubilized and fractionated. A preparation has been obtained which has an increased specific activity and a greatly decreased polypeptide composition when compared to the mitochondria. This preparation retains normal inhibitor sensitivities in that the oxidation of menadiol remains insensitive to cyanide and is inhibited by aromatic hydroxamates. Metal analyses of the preparation showed that only iron was closely correlated with the oxidase activity. No unusual lipid components were detected in the preparation. The results are discussed in relation to chemical quinol oxidation mechanisms and to several recent hypotheses concerning the nature of the higher plant alternative oxidase.     

Covalent and Noncovalent Dimers of the Cyanide-Resistant Alternative Oxidase Protein in Higher Plant Mitochondria and Their Relationship to Enzyme Activity

Evidence for a mixed population of covalently and noncovalently associated dimers of the cyanide-resistant alternative oxidase protein in plant mitochondria is presented. High molecular mass (oxidized) species of the alternative oxidase protein, having masses predicted for homodimers, appeared on immunoblots when the sulfhydryl reductant, dithiothreitol (DTT), was omitted from sodium dodecyl sulfate-polyacrylamide gel sample buffer. These oxidized species were observed in mitochondria from soybean (Glycine max [L.] Merr. cv Ransom), Sauromatum guttatum Schott, and mung bean (Vigna radiata [L.] R. Wilcz). Reduced species of the alternative oxidase were also present in the same mitochondrial samples. The reduced and oxidized species in isolated soybean cotyledon mitochondria could be interconverted by incubation with the sulfhydryl reagents DTT and azodicarboxylic acid bis(dimethylamide) (diamide). Treatment with chemical cross-linkers resulted in cross-linking of the reduced species, indicating a noncovalent dimeric association among the reduced alternative oxidase molecules. Alternative pathway activity of soybean mitochondria increased following reduction of the alternative oxidase protein with DTT and decreased following oxidation with diamide, indicating that electron flow through the alternative pathway is sensitive to the sulfhydryl/disulfide redox poise. In mitochondria from S. guttatum floral appendix tissue, the proportion of the reduced species increased as development progressed through thermogenesis.     

Specificity of the Organic Acid Activation of Alternative Oxidase in Plant Mitochondria

The claim that succinate and malate can directly stimulate the activity of the alternative oxidase in plant mitochondria (A.M. Wagner, C.W.M. van den Bergen, H. Wincencjusz [1995] Plant Physiol 108: 1035-1042) was reinvestigated using sweet potato (Ipomoea batatas L.) mitochondria. In whole mitochondria, succinate (in the presence of malonate) and both L- and D-malate stimulated respiration via alternative oxidase in a pH- (and NAD+)-dependent manner. Solubilized malic enzyme catalyzed the oxidation of both L- and D-malate, although the latter at only a low rate and only at acid pH. In submitochondrial particle preparations with negligible malic enzyme activity, neither L- nor D-malate stimulated alternative oxidase activity. However, even in the presence of high malonate concentrations, some succinate oxidation was observed via the alternative oxidase, giving the impression of stimulation of the oxidase. Neither L-malate nor succinate (in the presence of malonate) changed the dependence of alternative oxidase activity on ubiquinone reduction state in submitochondrial particles. In contrast, a large change in this dependence was observed upon addition of pyruvate. Half-maximal stimulation of alternative oxidase by pyruvate occurred at less than 5 [mu]M in submitochondrial particles, one-twentieth of that reported for whole mitochondria, suggesting that pyruvate acts on the inside of the mitochondrion. We suggest that malate and succinate do not directly stimulate alternative oxidase, and that reports to the contrary reflect intra-mitochondrial generation of pyruvate via malic enzyme.     

Mobilization of mitochondrial Ca2+ by hydroperoxy-eicosatetraenoic acid

Hydroperoxy-eicosatetraenoic acids (HPETEs) and less effectively, also hydroxy-eicosatetraenoic acids (HETEs) stimulated Ca2+ release from rat liver mitochondria. Ca2+ release is accompanied by intramitochondrial pyridine nucleotide oxidation and hydrolysis. Both Ca2+ release and pyridine nucleotide oxidation are impeded when the flow of electrons between pyridine nucleotides and HPETE is impaired. Measurements of the mitochondrial membrane potential indicate that HPETE-stimulated Ca2+ release is not due to uncoupling of mitochondria. It is suggested that HPETEs and HETEs may act as mobilizers of mitochondrial Ca2+ during signal transduction related to proliferation and tumor promotion.     

The cyanide-resistant alternative oxidases from the fungi Pichia stipitis and Neurospora crassa are monomeric and lack regulatory features of the plant enzyme

Both plant and fungal mitochondria have cyanide-resistant alternative oxidases that use reductant from the mitochondrial ubiquinone pool to reduce oxygen to water in a reaction that conserves no energy for ATP synthesis. The dimeric plant alternative oxidase is relatively inactive when its subunits are linked by a disulfide bond. When this bond is reduced, the enzyme can then be stimulated by its activators, alpha-keto acids. A Cys in the N-terminal section of the protein is responsible for both of these features. We examined the alternative oxidases in mitochondria isolated from two fungi Neurospora crassa and Pichia stipitis for dimeric structure, ability to form an intermolecular disulfide, and sensitivity to alpha-keto acids. Neither of the two fungal alternative oxidases could be covalently linked by diamide, which induces disulfide bond formation between nearby Cys residues, nor could they be cross-linked by a Lys-specific reagent or glutaraldehyde at concentrations which cross-link the plant alternative oxidase dimer completely. Alternative oxidase activity in fungal mitochondria was not stimulated by the alpha-keto acids pyruvate and glyoxylate. Pyruvate did stimulate activity when succinate was the respiratory substrate, but this was not a direct effect on the alternative oxidase. In contrast, added GMP was a strong activator of fungal alternative oxidase activity. Analysis of plant and fungal alternative oxidase protein sequences revealed a unique domain of about 40 amino acids surrounding the regulatory Cys in the plant sequences that is not present in the fungal sequences. This domain may be where dimerization of the plant enzymes occurs. In contrast to plant enzymes, the fungal alternative oxidases studied here are monomeric and their activities are independent of alpha-keto acids.     

Substrate kinetics of the Acanthamoeba castellanii alternative oxidase and the effects of GMP

In Acanthamoeba castellanii mitochondria, the apparent affinity values of alternative oxidase for oxygen were much lower than those for cytochrome c oxidase. For unstimulated alternative oxidase, the K(Mox) values were around 4-5 microM both in mitochondria oxidizing 1 mM external NADH or 10 mM succinate. For alternative oxidase fully stimulated by 1 mM GMP, the KK(Mox) values were markedly different when compared to those in the absence of GMP and they varied when different respiratory substrates were oxidized (K(Mox) was around 1.2 microM for succinate and around 11 microM for NADH). Thus, with succinate as a reducing substrate, the activation of alternative oxidase (with GMP) resulted in the oxidation of the ubiquinone pool, and a corresponding decrease in K(Mox). However, when external NADH was oxidized, the ubiquinone pool was further reduced (albeit slightly) with alternative oxidase activation, and the K(Mox) increased dramatically. Thus, the apparent affinity of alternative oxidase for oxygen decreased when the ubiquinone reduction level increased either by changing the activator or the respiratory substrate availability.     

The inhibition by valproic acid of the mitochondrial oxidation of monocarboxylic and omega-hydroxymonocarboxylic acids: possible implications for the metabolism of gamma-aminobutyric acid

The interactions of 1-5 mM valproic acid with the hepatic fatty acid oxidation are here described. Valproic acid was not substrate for hepatic peroxisomal fatty acid oxidation. Its activation outside the mitochondrial matrix compartment was poor when compared to that of octanoic acid, a fatty acid containing the same number of carbones. Valproic acid did not inhibit the fatty acyl-CoA oxidase nor the cyanide-insensitive acyl-CoA oxidation. Valproic acid inhibited the mitochondrial oxidations of both long-chain monocarboxylyl-CoAs and omega-hydroxymonocarboxylyl-CoAs. Valproic acid prevented the oxidation by coupled mitochondria of decanoic and 10-hydroxydecanoic acids. Both butyric and 4-hydroxybutyric acids were oxidized by coupled mitochondria. These activities were abolished by preincubating the enzyme source with valproic acid. Administration to rats of 0.5% (w/w)- or 1% (w/w)-valproate containing diets were efficient in producing increased liver peroxisomal population and beta-oxidation. Preliminary investigations on the effects of valproic acid on mitochondrial fatty acid oxidation as a function of the animal used for the experiments pointed out an association of the protection of the mitochondrial process against the toxicity of the drug with enhanced carnitine acyltransferase and acyl-CoA hydrolase activities.     

Involvement of the energy-dissipating systems in modulating the energetic efficiency of respiration in mitochondria from etiolated winter wheat seedlings

Effects of cyanide-resistant alternative oxidase (AOX) and modulators of plant uncoupling mitochondrial proteins (PUMP) on respiration rate and generation of transmembrane electric potential (ΔΨ) were investigated during oxidation of various substrates by isolated mitochondria from etiolated coleoptiles of winter wheat (Triticum aestivum L.). Oxidative phosphorylation in wheat mitochondria during malate and succinate oxidation was quite effective (it was characterized by high respiratory control ratio as defined by Chance, high ADP/O ratio, and rapid ATP synthesis). Nevertheless, the effectiveness of oxidative phosphorylation was substantially modulated by operation of energy-dissipating systems. The application of safranin dye revealed the partial dissipation of ΔΨ during inhibition of cytochrome-mediated malate oxidation by cyanide and antimycin A and demonstrated the operation of AOX-dependent compensatory mechanism for ΔΨ generation. The complex I of mitochondrial electron transport chain was shown to play the dominant role in ΔΨ generation and ATP synthesis during AOX functioning upon inhibition of electron transport through the cytochrome pathway. Effects of linoleic acid (PUMP activator) at physiologically low concentrations (4–10 μM) on respiration and ΔΨ generation in mitochondria were examined. The uncoupling effect of linoleic acid was shown in activation of the State 4 respiration, as well as in ΔΨ dissipation; this effect was eliminated in the presence of BSA but was insensitive to purine nucleotides. The uncoupling effect of linoleic acid was accompanied by reversible inhibition of AOX activity. The results are discussed with regard to possible physiological role of mitochondrial energy-dissipating systems in regulation of energy transduction in plant cells under stress conditions.