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.     

Glyceollin: a site-specific inhibitor of electron transport in isolated soybean mitochondria

The glyceollin inhibition of electron transport by isolated soybean and corn mitochondria was similar to that of rotenone, acting at site I between the internal NADH dehydrogenase and coenzyme Q. Coupled state 3 malate oxidation was inhibited by glyceollin and rotenone with apparent K_i values of about 15 and 5 micromolar, respectively. Carbonylcyanide m-chlorophenyl hydrazone uncoupled state 4 malate oxidation was also inhibited by glyceollin and rotenone, but uncoupled succinate and exogenous NADH state 4 oxidation was only slightly inhibited by both compounds. Glyceollin also inhibited ferricyanide reduction with malate as the electron donor, with an apparent K_i of 5.4 micromolar, but failed to inhibit such reduction with succinate or externally added NADH as electron donors. Glyceollin did not inhibit state 4 oxidation of malate, succinate, or exogenous NADH. Glyceollin did not act as a classical uncoupler or as an inhibitor of oxidative phosphorylation.     

Cooperative Substrate Oxidation by Mitochondria from Castor Bean Hypocotyls

Cooperative oxidation of succinate and exogenous NADH was followed in the mitochondria from five- to six-day-old castor bean (Ricinus communisL.) seedlings. Although succinate was oxidized at a much higher rate than NADH, the former inconsiderably (less than 15%) inhibited the oxidation of the latter substrate in state 4, while, in state 3 (in the presence of ATP), the two substrates did not compete and were jointly oxidized. When two substrates were oxidized by the mitochondria with the alternative CN-resistant oxidase (AO) inhibited with salicylhydroxamic acid, the rate of NADH oxidation in state 4 dropped by over 40% as compared to the initial rate. Meanwhile, the rate of succinate oxidation was not considerably affected by AO inhibition. We believe that one of the AO functions in the mitochondria is to provide for noncompeting oxidation of two (or more) substrates by employing two (or several) dehydrogenases of the respiratory chain.     

Effects of flavone on the oxidative properties of intact plant mitochondria

The effects of flavone on the oxidative and phosphorylative properties of plant mitochondria from potato tubers and etiolated mung bean hypocotyls were investigated. Flavone inhibited the state 3 oxidation rates of malate, NADH and, to a lesser extent, succinate but was without effect on the ascorbate-TMPD oxidation rate. The inhibition was the same whether the mitochondria were in state 3 or in an uncoupled state 3. When 100 μM flavone was added during the state 4, the tight coupling of succinate or NADH oxidation was not released. In the electron transfer chain, flavone inhibition appeared to be located in the flavoprotein region. All forms of NADH dehydrogenases seemed to be affected but the greatest inhibition appeared when exogenous NADH was used.     

Effect of methyl methacrylate on mitochondrial function and structure

• 1.1. Treatment of isolated rat liver mitochondria with methyl methacrylate (MM) produced membrane disruption as evidenced by the release of citrate synthase, and changes in the ultrastructure of mitochondria.
• 2.2. At concentration 0.1%, MM uncoupled oxidative phosphorylation as evidenced by stimulation of state 4 respiration supported either by pyruvate plus malate or succinate (+rotenone) and ATP-ase activity in intact mitochondria.
• 3.3. At concentration 1% MM stimulated ATP-ase activity in intact mitochondria and succinate (+rotenone) oxidation at state 4 and was without effect on this substrate oxidation at state 3.
• 4.4. MM inhibited pyruvate plus malate oxidation either at state 3 or in the presence of uncoupling agents.
• 5.5. MM inhibited the NADH oxidase of electron transport particles at a concentration which failed to inhibit either succinic oxidase or the NADH-ferricyanide reductase activity.
• 6.6. The data presented suggest that in the isolated mitochondria MM inhibits NADH oxidation in the vicinity of the rotenone sensitive site of complex I.
• 7.7. The general conclusion is that MM may block an electron transport and to uncouple oxidative phosphorylation in rat liver mitochondria. The overall in vitro effect would be to prevent ATP synthesis which could result in cell death under in vivo conditions.

     

Oxidation processes and ubiquinone localization in the branched respiratory system of mi-1 mutant of Neurospora crassa.

1. Stimulation of succinate oxidation in mi-1 mitochondria by Mg2+ and Pi is abolished on uncoupling, which points to the energy-linked activation of succinate oxidation. 2. Mitochondria exhibited respiratory control with succinate and NADH when the cyanide-insensitive oxidation was inhibited by salicylhydroxamic acid (SHAM). SHAM lowered the oxidation rate with NADH and succinate to the same level, though the NADH oxidation rate was 2.5 times as high as with succinate. 3. Despite the high stimulation of succinate oxidation via the SHAM-sensitive pathway in the active and controlled state of mitochondria, the redox state of UQ in all metabolic states remains unchanged. On inhibition of the cyanide-insensitive pathway, UQ reduction is greatly increased only in the controlled and active state. With NADH as a substrate, UQ does not respond to the metabolic states of mitochondria. 4. The redox changes of cytochrome c parallel those of UQ. 5. Branching of the respiratory chain in mi-1 mitochondria is discussed.     

Impaired substrate utilization in mitochondria from strain 129 dystrophic mice

Mitochondria from skeletal muscle, heart and liver of strain 129/ReJ-dy dystrophic mice and their littermate controls were characterized with respect to their respiratory and phosphorylating activities. Skeletal muscle mitochondria from dystrophic mice showed significantly lower state 3 respiratory rates than controls with both pyruvate + malate and succinate as substrates (P < 0.01). ADP/O and Ca²⁺/O ratios were found to be normal. A decreased rate of NADH oxidation (0.01 <P < 0.05) by sonicated mitochondrial suspensions from dystrophic mice was also seen. High respiratory rates with ascorbate + phenazine methosulfate as substrates indicated that cytochrome oxidase was not rate limiting in the oxidation of either pyruvate + malate or succinate. Skeletal muscle mitochondria from dystrophic mice showed no deficiency in any of the cytochromes or coenzyme Q. Mg²⁺-stimulated ATPase activity was higher in dystrophic muscle mitochondria than in controls, but basal and oligomycin-insensitive activities were virtually identical to those of controls. A significant reduction in the intramitochondrial NAD⁺ content (0.01 <P < 0.02) was seen in dystrophic skeletal muscle as compared to controls. Heart mitochondria from dystrophic mice showed similar, though less extensive abnormalities while liver mitochondria were essentially normal. We concluded from these results that skeletal muscle mitochondria from strain 129 dystrophic mice possess impairments in substrate utilization which may result from (1) an abnormality in the transfer of electrons on the substrate side of coenzyme Q in the case of succinate oxidation; (2) a defect on the path of electron flow from NADH to cytochrome c, and (3) a deficiency of NAD⁺ in the case of NAD⁺-linked substrates.     

Cytokinin modification of mitochondrial function

6-Benzylaminopurine, 6-(Δ²-isopentenylamino)purine, 6-furfurylaminopurine, rotenone, and antimycin A inhibited oxidation of NADH by mitochondrial sonicates or submitochondrial particles (but not by intact mitochondria) from pea (Pisum sativum L., cult. Alaska) stems and mung bean (Vigna radiata L. Wilczak) hypocotyls. The above purine cytokinins can interfere with electron transport from NADH to the cytochrome system in the inner mitochondrial membrane. Adenine did not inhibit oxidation by sonicated mitochondria, and zeatin was almost ineffective. Zeatin scarcely inhibited state 3 malate respiration by intact mitochondria, but the O-formyl and O-n-propionyl esters of zeatin and the O-acetyl ester of 2-chlorozeatin were more active. Perhaps zeatin is ineffective because it does not get into the inner membranes of the isolated mitochondria, whereas the esters and other cytokinins mentioned above do. N-4-(2-chloropyridyl)-N′-Phenylurea, which has cytokinin-like effects on plant growth and development, inhibited NADH oxidation by sonicated mitochondria. It also inhibited malate, succinate, and NADH oxidation by intact mitochondria; in contrast, the latter two oxidations were not decreased by purine cytokinins.     

Two separate pathways underlie NADH and succinate oxidation in swine heart mitochondria: Kinetic evidence on the mobile electron carriers

We have investigated NADH and succinate aerobic oxidation in frozen and thawed swine heart mitochondria. Simultaneous oxidation of NADH and succinate showed complete additivity under a variety of experimental conditions, suggesting that the electron fluxes originating from NADH and succinate are completely independent and do not mix at the level of the so-called mobile diffusible components. We ascribe the results to mixing of the fluxes at the level of cytochrome c in bovine mitochondria: the Complex IV flux control coefficient in NADH oxidation was high in swine mitochondria but very low in bovine mitochondria, suggesting a stronger interaction of cytochrome c with the supercomplex in the former. This was not the case in succinate oxidation, in which Complex IV exerted little control also in swine mitochondria. We interpret the data in swine mitochondria as restriction of the NADH flux by channelling within the I-III₂-IV supercomplex, whereas the flux from succinate shows pool mixing for both Coenzyme Q and probably cytochrome c. The difference between the two types of mitochondria may be ascribed to different lipid composition affecting the cytochrome c binding properties, as suggested by breaks in Arrhenius plots of Complex IV activity occurring at higher temperatures in bovine mitochondria.     

Effects of chronic ethanol treatment of mitochondrial functions damage to coupling site I

Chronic ethanol feeding to rats produces changes in hepatic mitochondria which persist in the absence of ethanol metabolism. The integrity of isolated mitochondria is well preserved, as evidenced by unchanged activities of latent, Mg²⁺- and dinitrophenol-stimulated ATPase activity, and unaltered permeability to NADH. With succinate or ascorbate as substrates, oxygen uptake by mitochondria from ethanol-fed rats was decreased compared to pair-fed controls. The decrease was comparable under state 4 or state 3 conditions, or in the presence of an uncoupler. However, with the NAD⁺-dependent substrates, ADP-stimulated oxygen consumption (state 3) was decreased to a greater extent than state 4 or uncoupler-stimulated oxygen consumption in mitochondria from ethanol-fed rats. This suggests that the decrease in energy-dependent oxygen consumption at site I may be superimposed upon damage to the respiratory chain. Using NAD⁺-dependent substrates (glutamate, α-ketoglutarate or β-hydroxybutyrate) the respiratory control ratio and the $\text{P}\text{O}$ ratio of oxidative phosphorylation were significantly decreased in mitochondria isolated from the livers of rats fed ethanol. By contrast, when succinate or ascorbate served as the electron donor these functions were unchanged. The rate of phosphorylation is decreased 70% with the NAD⁺-dependent substrates because of a decreased flux of electrons, as well as a lower efficiency of oxidative phosphorylation. With succinate and ascorbate as substrates, the rate of phosphorylation is decreased 20–30%, owing to a decreased flux of electrons. These data suggest the possibility that, in addition to effects on the respiratory chain, energy-coupling site I may be damaged by ethanol feeding. Energy-dependent Ca²⁺ uptake, supported by either substrate oxidation or ATP hydrolysis, was inhibited by chronic ethanol feeding.Concentrations of acetaldehyde (1–3 mm) which inhibited phosphorylation associated with the oxidation of NAD⁺-dependent substrates had no effect on that of succinate or ascorbate. Many of the effects of chronic ethanol feeding on mitochondrial functions are similar to those produced by acetaldehyde in vitro.     

Some Reactions of Isolated Corn Mitochondria Influenced by Juglone

The effects of juglone on the uptake of O₂ by excised corn roots (Zea mays L., Wf9 cms- T × M14) and isolated corn mitochondria arc reported. The O₂ uptake by excised corn roots, as measured by an O₂ electrode, was inhibited more than 90% after a one-hour treatment of 500 μM juglone. Lesser inhibitions were observed with 50 μM and 250 μM juglone. In a KC1 reaction medium in the absence of inorganic phosphate (Pi), juglone stimulated the rate of O₂ uptake by isolated mitochondria oxidizing NADH, succinate, or malate + pyruvate. In the presence of Pi, juglone concentrations of 3 μM and greater inhibited the state 3 oxidation rates of succinate and malate + pyruvate, lowered respiratory control and ADP/O ratios obtained from the oxidation of NADH, malate + pyruvate, or succinate, and reduced the coupled deposition of calcium phosphate within isolated mitochondria driven, by the oxidation of malate + pyruvate. The inhibition of state 3 O₂ uptake by isolated mitochondria, an oxidative state in which electron transfer is coupled to ATP production, is seen to correlate with the inhibition affected by juglone when applied to tissues in vivo.     

Oxidation of External NAD(P)H by Purified Mitochondria from Fresh and Aged Red Beetroots (Beta vulgaris L.)

Mitochondria were isolated from fresh beetroots (Beta vulgaris L. cvs Rubria and Nina) by differential centrifugation followed by Percoll gradient centrifugation. These purified mitochondria oxidized external NADH, although relatively slowly (20-40 versus 100-120 nanomoles oxygen per minute times milligram protein for NADH and succinate oxidation, respectively), with respiratory control ratios of two to three and ADP/O ratios of 1.2 to 1.6. NADPH was also oxidized, but even more slowly and with little or no coupling. The optimum for both NADH and NADPH oxidation by fresh beetroot mitochondria was pH 6. The rate of external NADH oxidation by isolated mitochondria was enhanced threefold during storage of the intact tubers at 10°C for 12 weeks. The optimum of the induced NADH oxidation was approximately pH 6.8. Succinate and malate oxidation only increased by 30% during the same period and NADPH oxidation was constant. This is strong evidence that NADH and NADPH oxidation are catalyzed by different enzymes at least in beetroots. Activity staining of nondenaturing polyacrylamide gels with NADH and Nitro Blue Tetrazolium did not show differences in banding pattern between mitochondria isolated from fresh and stored beetroots. The induction is discussed in relation to physiological aging processes.     

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.     

Effects of short-term ozone exposure on lung mitochondrial oxidative and energy metabolism

Ozone effects on lung mitochondrial oxidative metabolism were examined after short-term exposure of rats and monkeys to O₃. Exposure of animals to 2 ppm O₃ for 8 hr or to 4 ppm O₃ for 4 hr caused a 15–27% (P < 0.05) depression of lung mitochondrial O₂ consumption, using 2-oxoglutarate, succinate, and glycerol-1-phosphate. but not ascorbate plus Wurster's blue as substrates. Under these exposure conditions (4 ppm 4 hr) the ADP:O ratios dropped 25–36% (P < 0.05) and the respiratory control indices decreased 27–33% (P < 0.02) for oxidation of all substrates examined. Lung mitochondria from control animals were relatively impermeable to added NADH, but those from O₃-exposed animals showed an increased permeability as judged from NADH oxidation at a rate 3-fold higher than the control. Likewise, added cytochrome c caused a 22% (P < 0.01) stimulation of succinate oxidation in exposed lung mitochondria as against 5% (nonsignificant) in controls. Ozone exposure also caused a 20% (P < 0.01) oxidation of thiol groups in lung mitochondria, but no lipid peroxidation products were detectable in O₃-exposed lung tissue. The depression of substrate utilization, coupled phosphorylation and respiratory control observed in lung mitochondria of O₃-exposed animals might be related to alteration of membrane permeability, and inhibition of respiratory enzymes (dehydrogenases) due to oxidation of functional thiol groups.     

Reduction of superoxide production by mitochondria oxidizing NADH in the presence of organic acids

Effects of multiple substrates on oxygen uptake and superoxide production by mitochondria isolated from the pericarp tissue of green bell pepper (Capsicum annuum L.) were studied. Mitochondria isolated from peppers stored at 4 °C for 5 and 6 days had higher rates of oxygen uptake and were less sensitive to cyanide than mitochondria isolated from freshly harvested peppers. Succinate enhanced state 2 and state 4 rates of oxygen uptake with exogenous NADH in the absence of cytochrome path inhibitors, but not state 3 rates by mitochondria isolated from either freshly harvested or cold-stored bell peppers. The sensitivity of NADH oxidation to cyanide was reduced by both malate and succinate in mitochondria from cold-stored bell peppers, whereas only succinate was effective in mitochondria from freshly harvested peppers.Mitochondria isolated from both freshly harvested peppers and those stored at 4 °C for 5 and 6 days produced superoxide in the absence of exogenous substrates. Superoxide production by mitochondria from freshly harvested bell peppers increased when the mitochondria were supplied with malate, succinate or NADH, but only NADH enhanced superoxide production by mitochondria from cold-stored peppers. Both succinate and malate reduced the production of superoxide by mitochondria isolated from cold-stored bell peppers. Succinate and malate as second substrates also reduced the production of superoxide with NADH by mitochondria from both freshly harvested and cold-stored bell peppers. Malonate, a competitive inhibitor of succinate dehydrogenase, was inhibitory to oxygen uptake and to superoxide production.Mitochondria isolated from cold-stored bell peppers converted succinate to pyruvate at 25 °C at considerably higher rates than those of mitochondria from freshly harvested bell peppers. Since pyruvate has been shown to activate the alternative oxidase and the presence of pyruvate is essential for continued alternative oxidase activity, we suggest that pyruvate limits superoxide production by enhancing the flow of electrons through the alternative path. A direct scavenging of superoxide by succinate, malate and pyruvate, however, cannot be ruled out.     

Metabolism of calcium and effect of divalent cations on respiratory activity of yeast mitochondria

The adsorption of Ca²⁺ to the mitochondria ofSaccharomyces cerevisiae was investigated and it was found that, in contrast with animal mitochondria, Ca²⁺ is not accumulated through an energydependent process but is more probably adsorbed to mitochondrial membranes. The adsorption magnitude depends both on the amount of added calcium and on the ionic composition of the medium. It was found by study of the effect of divalent cations on the respiratory activity of yeast mitochondria that (a) Ca²⁺ and Mg²⁺ inhibit their oxidation competitively with succinate or citrate, the oxidation of NADH not being affected; (b) stimulation of oxidation of NADH and inhibition of oxidation of citrate and succinate may be observed with Ca²⁺ in the mitochondria ofTorulopsis utilis and with Co²⁺ in the mitochondria ofSaccharomyces cerevisiae; (c) Zn²⁺ inhibits the oxidation of NADH and of citrate; (d) the rate of oxidation of NADH in the presence of Cd²⁺ is several-fold greater than State 3 activity—on the other hand, oxidation of suceinate and citrate is inhibited by cadmium. In comparison with animal mitochondria, the fate of Ca²⁺ as well as the effects of other divalent cations on the respiratory activity of yeast mitochondria are different.     

Metabolism of N6,O2'-[3H]dibutyryl cyclic adenosine 3',5'-monophosphate and macromolecular interactions of the products perfused rat liver.

Mitochondria from liver, kidney, brain, and skeletal muscle metabolized acetaldehyde. Acetaldehyde oxidation by liver and kidney mitochondria was maximal at low levels of acetaldehyde and was sensitive to rotenone, suggesting the involvement of a NAD⁺-dependent aldehyde dehydrogenase with a high affinity for acetaldehyde. Acetaldehyde oxidation was stimulated 50% by ADP, suggesting that, in state 4, reoxidation of NADH is rate limiting for acetaldehyde oxidation. In state 4, acetaldehyde oxidation was decreased by NAD⁺-dependent substrates, as well as by succinate and ascorbate. The inhibition by the latter two substrates was prevented by ADP, dinitrophenol, valinomycin, and gramicidin, but not by oligomycin. Since these compounds are linked to energy transduction and utilization, the data suggest that the inhibition is mediated via energy-dependent reversed electron transport. In state 3, all of these substrates caused considerably less inhibition of acetaldehyde oxidation, suggesting that the activity of aldehyde dehydrogenase, and not of NADH reoxidation, is probably rate limiting for acetaldehyde oxidation. The ionophores valinomycin and gramicidin stimulated acetaldehyde oxidation to a greater extent than ADP. These ionophores also stimulated acetaldehyde oxidation in the presence of ADP. Stimulation by valinomycin occurred in the presence of monovalent cations transported by this ionophore, e.g., K⁺, Rb⁺, Cs⁺. Stimulation by gramicidin also occurred in the presence of these cations, but did not occur with Na⁺ or Li⁺. Na⁺ prevents the stimulation of acetaldehyde oxidation, which occurs in the presence of gramicidin and K⁺. The stimulation by valinomycin and gramicidin was energy dependent and required the presence of a permeant anion. In the absence of an ionophore, potassium phosphate had no effect on acetaldehyde oxidation. These data suggest that the oxidation of acetaldehyde by rat liver and kidney mitochondria is influenced by the oxidation-reduction state of the mitochondria and by the cationic environment. With brain and muscle mitochondria, the rate of acetaldehyde oxidation increased two- to threefold as the concentration of acetaldehyde was raised from 0.167 to 0.50 mm. Acetaldehyde oxidation in these mitochondria was also sensitive; to rotenone, indicating dependence on NAD⁺. ADP, valinomycin, gramicidin, and succinate, compounds which either increased or decreased the rate of acetaldehyde oxidation by liver and kidney mitochondria, had no effect on acetaldehyde oxidation by muscle or brain mitochondria. In state 4, mitochondria from Becker-transplantable hepatocellular carcinoma HC-252 oxidized acetaldehyde at the same rate as liver mitochondria. However, in the presence of ADP, dinitrophenol, valinomycin and gramicidin, the rate of acetaldehyde oxidation by the tumor mitochondria was two to three times greater than that of liver mitochondria, suggesting the presence of a more active; acetaldehyde-oxidizing system in tumor than in liver mitochondria.     

The Interaction between Exogenous NADH Oxidase and Succinate Oxidase in Jerusalem Artichoke (Helianthus tuberosus) Mitochondria

Most isolated plant mitochondria oxidize exogenous NADH viaan electron transport pathway which is resistant to piericidinA and coupled to the synthesis of two molecules of ATP. Resultspresented show that succinate can inhibit this oxidation ofadded NADH. The inhibition was most marked in the absence ofADP (state 4), less obvious in the presence of added ADP (state3), and absent in the presence of a weak acid uncoupling agent.The presence of malonate prevented the inhibition. The degreeof inhibition was dependent on the concentration of succinateand appeared to be non-competitive in nature. The inhibitionwas shown not to be the result of the reversed flow of electronsfrom succinate to NAD^$. The presence of external NADH appearednot to alter the rate of oxidation of succinate.     

Effects of pH, NADH, succinate and malate on the oxidation of glycine in spinach leaf mitochondria

The effect of external pH on several reactions catalyzed by glycine decarboxylase in spinach leaf mitochondria was investigated. Glycine-dependent oxygen consumption showed a pH optimum at 7.6, whereas the release of CO₂ and NH3 from glycine in the presence of oxaloacetate both showed pH maxima at 8.1. Glycine-dependent reduction of 2,6-dichlorophenolindophenol. on the other hand showed a pH optimum at 8.4. It is concluded that these three reactions have different rate-limiting steps. The rate of the glycine-bicarbonate exchange reaction catalyzed by glycine decarboxylase showed no optimum in the pH range investigated, pH 7–9, but increased with decreasing pH. This suggests that CO₂ may be the true substrate in this reaction.
The oxidation of glycine inhibited the oxidation of both malate, succinate and external NADH since the addition of malate, succinate or NADH to mitochondria oxidizing glycine in state 3 resulted in a rate of oxygen consumption which was lower than the sum of the rates when the substrates were oxidized individually. The addition of malate, succinate or NADH did not, however, decrease the rate of CO₂ or NH, release from glycine. It is suggested that the preferred oxidation of glycine by-spinach leaf mitochondria may constitute an important regulatory mechanism for the function of leaf mitochondria during photosynthesis.     

Loss of Sensitivity to Helminthosporium maydis Race T Toxin during Aging of Mitochondria Isolated from Texas Cytoplasm Corn

Helminthosporium maydis Race T toxin caused the expected changes in freshly isolated mitochondria from T cytoplasm corn, namely complete uncoupling of oxidative phosphorylation, pronounced stimulation of succinate and NADH respiration, complete inhibition of malate respiration, and increased mitochondrial swelling. In contrast, identical toxin treatments of the mitochondria after 12 hours aging on ice resulted in partial uncoupling, much lower stimulation of succinate and NADH respiration, no inhibition of malate respiration, and no mitochondrial swelling. Almost all of the toxin sensitivity was lost by 6 hours aging. At this stage, the mitochondria were 208× and 66× less sensitive to toxin-induced changes in coupling of malate respiration and state 4 malate respiration rates, respectively. Loss of toxin sensitivity did not occur when the mitochondria were aged under nitrogen or in the presence of 5 millimolar dithiothreitol. This suggested that the aging effect was due to oxidation, possibly of sulfhydryl groups in one or more mitochondrial membrane proteins.