Oxidation and reduction of exogenous cytochrome c by the activity of the respiratory chain.

Oxidation of exogenous NADH by isolated rat liver mitochondria is generally accepted to be mediated by endogenous cytochrome c which shuttles electrons from the outer to the inner mitochondrial membrane. More recently it has been suggested that, in the presence of added cytochrome c, NADH oxidation is carried out exclusively by the cytochrome oxidase of broken or damaged mitochondria. Here we show that electrons can be transferred in and out of intact mitochondria. It is proposed that at the contact sites between the inner and the outer membrane, a "bi-trans-membrane" electron transport chain is present. The pathway, consisting of Complex III, NADH-b5 reductase, exogenous cytochrome c and cytochrome oxidase, can channel electrons from the external face of the outer membrane to the matrix face of the inner membrane and viceversa. The activity of the pathway is strictly dependent on both the activity of the respiratory chain and mitochondrion integrity.     

Effects of adriamycin on respiratory chain activities in mitochondria from rat liver, rat heart and bovine heart. Evidence for a preferential inhibition of complex III and IV

The inhibition of respiratory chain activities in rat liver, rat heart and bovine heart mitochondria by the anthracycline antibiotic adriamycin was measured in order to determine the adriamycin-sensitive sites. It appeared that complex III and IV are efficiently affected such that their activities were reduced to 50% of control values at 175 +/- 25 microM adriamycin. Complex I displayed a minor sensitivity to the drug. Of the complex-I-related activities tested, only duroquinone oxidation appeared sensitive (50% inhibition at approx. 450 microM adriamycin). Electron-transfer activities catalyzed by complex II remained essentially unaltered up to high drug concentrations. Of the activities measured for this complex, only duroquinone oxidation was significantly affected. However, the adriamycin concentration required to reduce this activity to 50% exceeded 1 mM. Mitochondria isolated from rat liver, rat heart and bovine heart behaved essentially identical in their response to adriamycin. These data support the conclusion that, in these three mitochondrial systems, the major drug-sensitive sites lie in complex III and IV. Cytochrome c oxidase and succinate oxidase activity in whole mitochondria exhibited a similar sensitivity towards adriamycin, as inner membrane ghosts, suggesting that the drug has direct access to its inner membrane target sites irrespective of the presence of the outer membrane. By measuring NADH and succinate oxidase activities in the presence of exogenously added cytochrome c, it appeared that adriamycin was less inhibitory under these conditions. This suggests that adriamycin competes with cytochrome c for binding to the same site on the inner membrane, presumably cardiolipin.     

Cytochrome c as an electron shuttle between the outer and inner mitochondrial membranes

Addition of exogenous NADH to rotenone- and antimycin A-treated mitochondria, in 125 mM KCl, results in rates of oxygen uptake of 0.5-1 and 10-12 nanoatoms of oxygen X mg protein-1 X min-1 in the absence and presence of cytochrome c, respectively. During oxidation of exogenous NADH there is a fast and complete reduction of cytochrome b5 while endogenous or added exogenous cytochrome c become 10-15% and 100% reduced, respectively. The reoxidation of cytochrome b5, after exhaustion of NADH, precedes that of cytochrome c. NADH oxidation is blocked by mersalyl, an inhibitor of NADH-cytochrome b5 reductase. These observations support the view of an electron transfer from the outer to the inner membrane of intact mitochondria. Both the rate of exogenous NADH oxidation and the steady state level of cytochrome c reduction increase with the increase of ionic strength, while the rate of succinate oxidation undergoes a parallel depression. These observations suggest that the functions of cytochrome c as an electron carrier in the inner membrane and as an electron shuttle in the intermembrane space are alternative. It is concluded that aerobic oxidation of exogenous NADH involves the following pathway: NADH leads to NADH-cytochrome b5 reductase leads to cytochrome b5 leads to intermembrane cytochrome c leads to cytochrome oxidase leads to oxygen. It is suggested that the communication between the outer and inner membranes mediated by cytochrome c may affect the oxidation-reduction level of cytosolic NADH and the related oxidation-reduction reactions.     

The oxidation of external NADH by an intermembrane electron transfer in mitochondria from the ubiquinone-deficient mutant E3-24 of Saccharomyces cerevisiae

Cells of the E3-24 mutant of the strain D273-10B of Saccharomyces cerevisiae, grown in a fermentable substrate not showing catabolite repression of respiration (2% galactose), are able to respire, in spite of their ubiquinone deficiency in mitochondrial membranes. Mitochondria isolated from these mutant cells oxidize exogenous NADH through a pathway insensitive to antimycin A but inhibited by cyanide. Addition of methanolic solutions of ubiquinone homologs stimulates the oxidation rate and restores antimycin A sensitivity in both isolated mitochondria and whole cells. Mersalyl preincubation of isolated mitochondria inhibits both NADH oxidation and NADH-cytochrome c oxido-reductase activity (assayed in the presence of cyanide) with the same pattern. Electrons resulting from the oxidation of exogenous NADH reduce both cytochrome b5 and endogenous cytochrome c. The increase in ionic strength stimulates NADH oxidation, which is also coupled to the ATP synthesis with an ATP/O ratio similar to that obtained with ascorbate plus N,N,N',N'-tetramethyl-p-phenylendiamine (TMPD) as substrate. The effect of cyanide on these activities and on NADH-induced endogenous cytochrome c reduction is also comparable. These results support the existence in vivo and in isolated mitochondria of a energy-conserving pathway for the oxidation of cytoplasmatic NADH not related to the dehydrogenases of the inner membrane, the ubiquinone, and the b-c1 complex, but involving a cytochrome c shuttle between the NADH-cytochrome c reductase of the outer membrane and cytochrome oxidase in the inner membrane.     

Avicins, natural anticancer saponins, permeabilize mitochondrial membranes

Avicins are a class of natural saponins with selective pro-apoptotic activity in cancer cells. In this work, we studied the influence of avicins on metabolic state of rat liver mitochondria. Avicin-treated mitochondria underwent a significant decrease in oxygen consumption rate that was completely restored by addition of exogenous cytochrome c. On the other hand, avicins increased the rotenone-insensitive oxidation of external NADH in the presence of exogenous cytochrome c, long before high amplitude swelling of mitochondria was observed. The increase in external NADH oxidation was cyclosporin A-insensitive. Avicin G significantly accelerated hydroperoxide-induced oxidation of mitochondrial endogenous NAD(P)H, the drop of the inner membrane potential and the high amplitude swelling of mitochondria. The obtained data might explain selective induction of apoptosis in tumor cells by avicins. Based on other studies showing that tumor cells generate hydroperoxides with a very high rate, avicins could provide a new strategy of anticancer therapy by sensitizing cells with high levels of reactive oxygen species to apoptosis.     

Cytochrome c sorption-desorption effects on the external NADH oxidation by mitochondria: experimental and computational study

The rupture of the outer mitochondrial membrane is known to be critical for cell death, but the mechanism, specifically its redox-signaling aspects, still needs to be studied in more detail. In this work, the external NADH oxidation by rat liver mitochondria was studied under the outer membrane rupture induced by the mitochondria hypotonic treatment or the inner membrane permeability transition. The saturation of the oxidation rate was observed as a function of mitochondrial protein concentration. This effect was shown to result from cytochrome c binding to the mitochondrial membranes. At a relatively high concentration of mitochondria, the oxidation rate was strongly activated by 4 mm Mg(2+) due to cytochrome c desorption from the membranes. A minimal kinetic model was developed to explain the main phenomena of the external NADH oxidation modulated by cytochrome c and Mg(2+) in mitochondria with the ruptured outer membrane. The computational behavior of the model closely agreed with the experimental data. We suggest that the redox state of the released cytochrome c, considered by other authors to be important for apoptosis, may strongly depend on its oxidation by the fraction of mitochondria with the ruptured outer membrane and on the cytoplasmic cytochrome c reductase activity.     

Direct interaction between the internal NADH: ubiquinone oxidoreductase and ubiquinol:cytochrome c oxidoreductase in the reduction of exogenous quinones by yeast mitochondria.

The reduction of duroquinone (DQ) and 2,3-dimethoxy-5-methyl-6-decyl-1,4-benzoquinone (DB) by NADH and ethanol was investigated in intact yeast mitochondria with good respiratory control ratios. In these mitochondria, exogenous NADH is oxidized by the NADH dehydrogenase localized on the outer surface of the inner membrane, whereas the NADH produced by ethanol oxidation in the mitochondrial matrix is oxidized by the NADH dehydrogenase localized on the inner surface of the inner membrane. The reduction of DQ by ethanol was inhibited 86% by myxothiazol; however, the reduction of DQ by NADH was inhibited 18% by myxothiazol, suggesting that protein-protein interactions between the internal (but not the external) NADH: ubiquinone oxidoreductase and ubiquinol:cytochrome c oxidoreductase (the cytochrome bc1 complex) are involved in the reduction of DQ by NADH. The reduction of DQ and DB by NADH and ethanol was also investigated in mutants of yeast lacking cytochrome b, the iron-sulfur protein, and ubiquinone. The reduction of both quinone analogues by exogenous NADH was reduced to levels that were 10 to 20% of those observed in wild-type mitochondria; however, the rate of their reduction by ethanol in the mutants was equal to or greater than that observed in the wild-type mitochondria. Furthermore, the reduction of DQ in the cytochrome b and iron-sulfur protein lacking mitochondria was myxothiazol sensitive, suggesting that neither of these proteins is an essential binding site for myxothiazol. The mitochondria from the three mutants also contained significant amounts of antimycin- and myxothiazol-insensitive NADH:cytochrome c reductase activity, but had no detectable succinate:cytochrome c reductase activity. These results suggest that the mutants lacking a functional cytochrome bc1 complex have adapted to oxidize NADH.     

Generation of transmembrane electrical potential during NADH oxidation via the external pathway and the fatty acid uncoupling effect after transient opening of the Ca2+-dependent cyclosporin A-sensitive pore in liver mitochondria

The effects of transient pore opening on generation of the transmembrane gradient of electrical potential across the inner mitochondrial membrane (DeltaPsi) induced by NADH oxidation through the external pathway as well as on the uncoupling effect of fatty acids were studied. The pore opening was monitored by changes in the DeltaPsi value. The cycle of pore opening/closing was found to have only an insignificant effect on the sensitivity of DeltaPsi to fatty acid uncoupling. Once this cycle is over, NADH oxidation in the presence of exogenous cytochrome c results in generation of DeltaPsi. In the absence of cytochrome c, the generation of DeltaPsi induced by oxidation of exogenous NADH is observed if the incubation medium pH has been decreased from 7.4 to 7.0. The generation of DeltaPsi was inhibited by cyclosporin A. In isotonic salt medium containing 125 mM KCl, the maximum level of DeltaPsi generated by exogenous NADH after the cycle of pore opening/closing was significantly lower than the maximum level of DeltaPsi generated in hypotonic incubation medium. The data obtained in this work suggest that the cycle of pore opening/closing has little if any effect on the energy coupling in liver mitochondria, whereas the external pathway of NADH oxidation activated by this cycle may support the energy-dependent functions of liver mitochondria.     

THE OXIDATION OF EXOGENOUS AND ENDOGENOUS CYTOCHROME C IN MITOCHONDRIA : A Biochemical and Ultrastructural Study

The effect of the nonionic detergent Lubrol on the oxidation of endogenous and exogenous cytochrome c by cytochrome oxidase in intact and fragmented mitochondria was studied. Mitochondria and mitochondrial fragments from liver, kidney, heart, and skeletal muscle have been used. Negatively stained preparations of intact mitochondria showed the particles of Fernández-Morán on the matrix side of their inner membrane system: under these conditions, the oxidation rate of externally added cytochrome c was very high, and it was stimulated very poorly by Lubrol. Mechanical fragmentation of liver mitochondria yielded vesicles with a smooth external profile: also under these conditions, the oxidation of externally added cytochrome c was very high, and poorly stimulated by Lubrol. The oxidation of endogenous cytochrome c was also unaffected by Lubrol. On the other hand, fragmentation of heart and skeletal muscle mitochondria yielded vesicles having numerous particles of Fernández-Morán on their external profiles. Under these conditions, the oxidation of exogenous cytochrome c was low and was markedly stimulated by Lubrol. On the contrary, no activation of the oxidation of endogenous cytochrome c was induced by the detergent. The results indicate a difference in the permeability properties of the two faces of the inner mitochondrial membrane: a permeability barrier for cytochrome c is suggested to exist at the inner face.     

ATP synthesis during exogenous NADH oxidation. A reappraisal

This paper reports a reinvestigation on the pathway for mitochondrial oxidation of exogenous NADH and on the related ATP synthesis, first reported 30 years ago (Lehninger, A.L. (1951) J. Biol. Chem. 190, 345-359). NADH oxidation, both in intact and in water-treated mitochondria, is 90% inhibited by mersalyl, an inhibitor of the outer membrane NADH-cytochrome b5 reductase, and 10% inhibited by rotenone. The mersalyl-sensitive, but not the rotenone-sensitive, portion of NADH oxidation is stimulated by exogenous cytochrome c. Part of ATP synthesis is independent of exogenous NADH and cytochrome c, and is inhibited by rotenone and antimycin A, and is therefore due to oxidation of endogenous substrates. Another part of ATP synthesis is dependent on exogenous NADH and cytochrome c, is insensitive to rotenone and antimycin A, and is due to operation of cytochrome oxidase. It is concluded that (i) oxidation of exogenous NADH in the presence of cytochrome c proceeds mostly through NADH-cytochrome b5 reductase and cytochrome b5 on the outer membrane and then through cytochrome oxidase via the cytochrome c shuttle, and (ii) ATP synthesis during oxidation of exogenous NADH is partly due to oxidation of endogenous substrates and partly to operation of cytochrome oxidase receiving electrons from the outer membrane via cytochrome c.     

The pro-apoptotic proteins, Bid and Bax, cause a limited permeabilization of the mitochondrial outer membrane that is enhanced by cytosol

During apoptosis, an important pathway leading to caspase activation involves the release of cytochrome c from the intermembrane space of mitochondria. Using a cell-free system based on Xenopus egg extracts, we examined changes in the outer mitochondrial membrane accompanying cytochrome c efflux. The pro-apoptotic proteins, Bid and Bax, as well as factors present in Xenopus egg cytosol, each induced cytochrome c release when incubated with isolated mitochondria. These factors caused a permeabilization of the outer membrane that allowed the corelease of multiple intermembrane space proteins: cytochrome c, adenylate kinase and sulfite oxidase. The efflux process is thus nonspecific. None of the cytochrome c-releasing factors caused detectable mitochondrial swelling, arguing that matrix swelling is not required for outer membrane permeability in this system. Bid and Bax caused complete release of cytochrome c but only a limited permeabilization of the outer membrane, as measured by the accessibility of inner membrane-associated respiratory complexes III and IV to exogenously added cytochrome c. However, outer membrane permeability was strikingly increased by a macromolecular cytosolic factor, termed PEF (permeability enhancing factor). We hypothesize that PEF activity could help determine whether cells can recover from mitochondrial cytochrome c release.     

Carvedilol inhibits the exogenous NADH dehydrogenase in rat heart mitochondria

There are several reports on the oxidation of external NADH by an exogenous NADH dehydrogenase in the outer leaflet of the inner membrane of rat heart mitochondria. Until now, however, little was known about its physiological role in cellular metabolism. The present work shows that carvedilol (?1-[carbazolyl-(4)-oxy]-3-[2-methoxyphenoxyethyl)amino]-pro - panol-(2)?) is a specific inhibitor of an exogenous NADH dehydrogenase in rat heart mitochondria. Carvedilol does not affect oxygen consumption linked to the oxidation of succinate and internal NADH. It is also demonstrated that the inhibition of exogenous NADH dehydrogenase by carvedilol is accompanied by the inhibition of alkalinization of the external medium. In contrast to the addition of glutamate/malate or succinate, exogenous NADH does not generate a membrane potential in rat heart mitochondria, as observed with a TPP(+) electrode. It is also demonstrated that the oxygen consumption linked to NADH oxidation is not due to permeabilized mitochondria, but to actual oxidase activity in the inner membrane. The enzyme has a K(m) for NADH of 13 microM. Carvedilol is a noncompetitive inhibitor of this external NADH dehydrogenase with a K(i) of 15 microM. Carvedilol is the first inhibitor described to this organospecific enzyme. Since this enzyme was demonstrated to play a key role in the cardiotoxicity of anticancer drugs of the anthracycline family (e.g., adriamycin), we may suggest that the administration of carvedilol to tumor patients treated with adriamycin might be of great help in the prevention of the cardioselective toxicity of this antibiotic.     

The mitochondrial respiratory chain is partially organized in a supercomplex assembly: kinetic evidence using flux control analysis

The model of the respiratory chain in which the enzyme complexes are independently embedded in the lipid bilayer of the inner mitochondrial membrane and connected by randomly diffusing coenzyme Q and cytochrome c is mostly favored. However, multicomplex units can be isolated from mammalian mitochondria, suggesting a model based on direct electron channeling between complexes. Kinetic testing using metabolic flux control analysis can discriminate between the two models: the former model implies that each enzyme may be rate-controlling to a different extent, whereas in the latter, the whole metabolic pathway would behave as a single supercomplex and inhibition of any one of its components would elicit the same flux control. In particular, in the absence of other components of the oxidative phosphorylation apparatus (i.e. ATP synthase, membrane potential, carriers), the existence of a supercomplex would elicit a flux control coefficient near unity for each respiratory complex, and the sum of all coefficients would be well above unity. Using bovine heart mitochondria and submitochondrial particles devoid of substrate permeability barriers, we investigated the flux control coefficients of the complexes involved in aerobic NADH oxidation (I, III, IV) and in succinate oxidation (II, III, IV). Both Complexes I and III were found to be highly rate-controlling over NADH oxidation, a strong kinetic evidence suggesting the existence of functionally relevant association between the two complexes, whereas Complex IV appears randomly distributed. Moreover, we show that Complex II is fully rate-limiting for succinate oxidation, clearly indicating the absence of substrate channeling toward Complexes III and IV.     

Porin and cytochrome oxidase containing contact sites involved in the oxidation of cytosolic NADH

Cytochrome c (cyto-c) added to isolated mitochondria promotes the oxidation of extra-mitochondrial NADH and the reduction of molecular oxygen associated to the generation of an electrochemical membrane potential available for ATP synthesis. The electron transport pathway activated by exogenous cyto-c molecules is completely distinct from the one catalyzed by the respiratory chain. Dextran sulfate (500 kDa), known to interact with porin (the voltage-dependent anion channel), other than to inhibit the release of ATP synthesized inside the mitochondria, greatly decreases the activity of exogenous NADH/cyto-c system of intact mitochondria but has no effect on the reconstituted system made of mitoplasts and external membrane preparations. The results obtained are consistent with the existence of specific contact sites containing cytochrome oxidase and porin, as components of the inner and the outer membrane respectively, involved in the oxidation of cytosolic NADH. The proposal is put forward that the bi-trans-membrane electron transport chain activated by cytosolic cyto-c becomes, in physio-pathological conditions: (i) functional in removing the excess of cytosolic NADH; (ii) essential for cell survival in the presence of an impairment of the first three respiratory complexes; and (iii) an additional source of energy at the beginning of apoptosis.     

Studies with a reconstituted muscle glycolytic system. The rate and extent of glycolysis in simulated post-mortem conditions

The stimulation of succinate-cytochrome c reductase in Jerusalem artichoke mitochondria by lowering osmolarity was found to be associated with conformational changes in the inner membrane rather than with rupture of the outer membrane. This conclusion is based on the following evidence. (1) When the activation of succinate dehydrogenase was measured by using either K(3)Fe(CN)(6) or exogenous cytochrome c as an electron acceptor, electron flow to cytochrome c was always 7% of that to K(3)Fe(CN)(6) throughout the activation process. (2) The rate of exogenous cytochrome c reduction by succinate and NADH was directly related to the maximum rate of electron flow as determined by oxygen utilization. These two observations are not consistent with the low rate of succinate-cytochrome c reductase being limited by a permeability barrier at the outer membrane. (3) In addition to stimulating the succinate-cytochrome c reductase, lowering the osmolarity caused simultaneous changes in the permeability of the inner membrane to ferricyanide and NADH. The data show that lowering the osmolarity results in progressive changes in the permeability of the inner membrane. The first change detected was an increased permeability to K(3)Fe(CN)(6), then a simultaneous increase in accessibility of the respiratory chain to exogenous cytochrome c and an increased permeability to NADH, followed finally by rupture as measured by the release of malate dehydrogenase.     

Chromium(VI) interaction with plant and animal mitochondrial bioenergetics: a comparative study

The mechanism of Cr(VI)-induced toxicity in plants and animals has been assessed for mitochondrial bioenergetics and membrane damage in turnip root and rat liver mitochondria. By using succinate as the respiratory substrate, ADP/O and respiratory control ratio (RCR) were depressed as a function of Cr(VI) concentration. State 3 and uncoupled respiration were also depressed by Cr(VI). Rat mitochondria revealed a higher sensitivity to Cr(VI), as compared to turnip mitochondria. Rat mitochondrial state 4 respiration rate triplicated in contrast to negligible stimulation of turnip state 4 respiration. Chromium(VI) inhibited the activity of the NADH-ubiquinone oxidoreductase (complex I) from rat liver mitochondria and succinate-dehydrogenases (complex II) from plant and animal mitochondria. In rat liver mitochondria, complex I was more sensitive to Cr(VI) than complex II. The activity of cytochrome c oxidase (complex IV) was not sensitive to Cr(VI). Unique for plant mitochondria, exogenous NADH uncoupled respiration was unaffected by Cr(VI), indicating that the NADH dehydrogenase of the outer leaflet of the plant inner membrane, in addition to complexes III and IV, were insensitive to Cr(VI). The ATPase activity (complex V) was stimulated in rat liver mitochondria, but inhibited in turnip root mitochondria. In both, turnip and rat mitochondria, Cr(VI) depressed mitochondrial succinate-dependent transmembrane potential (Deltapsi) and phosphorylation efficiency, but it neither affected mitochondrial membrane permeabilization to protons (H+) nor induced membrane lipid peroxidation. However, Cr(VI) induced mitochondrial membrane permeabilization to K+, an effect that was more pronounced in turnip root than in rat liver mitochondria. In conclusion, Cr(VI)-induced perturbations of mitochondrial bioenergetics compromises energy-dependent biochemical processes and, therefore, may contribute to the basal mechanism underlying its toxic effects in plant and animal cells.     

Valinomycin induced energy-dependent mitochondrial swelling,cytochrome <Emphasis Type="Italic">c</Emphasis> release,cytosolic NADH/cytochrome <Emphasis Type="Italic">c</Emphasis> oxidation and apoptosis

In valinomycin induced stimulation of mitochondrial energy dependent reversible swelling, supported by succinate oxidation, cytochrome c (cyto-c) and sulfite oxidase (Sox) [both present in the mitochondrial intermembrane space (MIS)] are released outside. This effect can be observed at a valinomycin concentration as low as 1 nM. The rate of cytosolic NADH/cyto-c electron transport pathway is also greatly stimulated. The test on the permeability of mitochondrial outer membrane to exogenous cyto-c rules out the possibility that the increased rate of exogenous NADH oxidation could be ascribed either to extensively damaged or broken mitochondria. Accumulation of potassium inside the mitochondria, mediated by the highly specific ionophore valinomycin, promotes an increase in the volume of matrix (evidenced by swelling) and the interaction points between the two mitochondrial membranes are expected to increase. The data reported and those previously published are consistent with the view that “respiratory contact sites” are involved in the transfer of reducing equivalents from cytosol to inside the mitochondria both in the absence and the presence of valinomycin. Magnesium ions prevent at least in part the valinomycin effects. Rather than to the dissipation of membrane potential, the pro-apoptotic property of valinomycin can be ascribed to both the release of cyto-c from mitochondria to cytosol and the increased rate of cytosolic NADH coupled with an increased availability of energy in the form of glycolytic ATP, useful for the correct execution of apoptotic program.     

Arrangement of the subunits in solubilized and membrane-bound cytochrome c oxidase from bovine heart.

The arrangement of the six cytochrome c oxidase subunits in the inner membrane of bovine heart mitochondria was investigated. The experiments were carried out in three steps. In the first step, exposed subunits were coupled to the membrane-impermeant reagent p-diazonium benzene [32S]sulfonate. In the second step, the membranes were lysed with cholate anc cytochrome c oxidase was isolated by immunoprecipitation. In the third step, the six cytochrome c oxidase subunits were separated from each other by dodecyl sulfate-acrylamide gel electrophoresis and scanned for radioactivity. Exposed subunits on the outer side of the mitochondrial inner membrane were identified by labeling intact mitochondria. Exposed subunits on the matrix side of the inner membrane were identified by labeling sonically prepared submitochondrial particles in which the matrix side of the inner membrane is exposed to the suspending medium. Since sonic irradiation leads to a rearrangement of cytochrome c oxidase in a large fraction of the resulting submitochondrial particles, an immunochemical procedure was developed for isolating particles with a low content of displaced cytochrome c oxidase. With mitochondria, subunits II, V, and VI were labeled, whereas in purified submitochondrial particles most of the label was in subunit III. The arrangement of cytochrome c oxidase in the mitochondrial inner membrane is thus transmembraneous and asymmetric; subunits II, V, and VI are situated on the outer side, subunit III is situated on the matrix side, and subunits I and IV are buried in the interior of the membrane. In a study of purified cytochrome c oxidase labeled with p-diazonium benzene [32S]sulfonate, the results were similar to those obtained with the membrane-bound enzyme. Subunits I and IV were inaccessible to the reagent, whereas the other four subunits were accessible. In contrast, all six subunits became labeled if the enzyme was dissociated with dodecyl sulfate before being exposed to the labeling reagent.     

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.     

Characteristics of External NADH Oxidation by Beetroot Mitochondria

Mitochondria isolated from fresh red beetroot (Beta vulgaris L.) tissue do not oxidize external NADH with O₂ as the electron acceptor. These mitochondria have a rotenone- and antimycin-insensitive pathway of NADH oxidation associated with the outer membrane and are capable of reducing cytochrome c or potassium ferricyanide. They are also capable of oxidizing internal NADH via the inner membrane electron transport chain with normal rotenone and antimycin sensitivity and ADP/O ratios. They differ from other plant mitochondria in the apparent lack of the NADH dehydrogenase located on the outer surface of the inner membrane. It is shown that this activity develops during the aging of red beetroot slices in aerated dilute CaSO₄ solutions, and is present in the mitochondria isolated from aged tissue.