Cytochrome c interactions with membranes. A photoaffinity-labeled cytochrome c.

The aryl azide, 2,4-dinitro-5-fluorophenylazide, was reacted with horse heart cytochrome c to give a photoaffinity-labeled derivative of this heme protein. The modified cytochrome c, with one to two dinitroazidophenyl groups per mole of the enzyme, has a half-reduction potential the same (± 10 mV) as native cytochrome c. The dissociation constant for the modified cytochrome c from cytochrome c-depleted mitochondrial membranes and the apparent K_m for the reaction with cytochrome c oxidase were each five to six times greater than the values for native cytochrome c. Irradiation of cytochrome c-depleted mitochondrial membranes supplemented with an excess of photoaffinity-labeled cytochrome c resulted in covalent binding of the derivative to the mitochondrial membranes. Fractionation of the irradiated mitochondria in the presence of detergents and salts followed by chromatography on agarose, Bio-Gel A, showed that labeled cytochrome c was bound covalently to cytochrome c oxidase in a 1:1 molar complex. The covalently linked cytochrome c-cytochrome c oxidase complex was active in mediating the electron transfer between N,N,N′,N′-tetramethyl-p-phenylenediamine/ascorbate and the oxidase.     

Steady-state redox behavior of cytochrome c, cytochrome a, and CuA of cytochrome c oxidase in intact rat liver mitochondria.

We have examined the steady-state redox behavior of cytochrome c (Fec), Fea, and CuA of cytochrome c oxidase during steady-state turnover in intact rat liver mitochondria under coupled and uncoupled conditions. Ascorbate was used as the reductant and TMPD (N,N,N',N'-tetramethyl-1,4-phenylenediamine) as the redox mediator. After elimination of spectroscopic interference from the oxidized form of TMPD, we found that Fea remains significantly more oxidized than previously thought. During coupled turnover, CuA always appears to be close to redox equilibrium with Fec. By increasing the amount of TMPD, both centers can be driven to fairly high levels of reduction while Fea remains relatively oxidized. The reduction level at Fea is close to a linear function of the enzyme turnover rate, but the levels at Fec and CuA do not keep pace with enzyme turnover. This behavior can be explained in terms of a redox equilibrium among Fec, CuA, and Fea, where Fea is the electron donor to the oxygen reduction site, but only if Fea has an effective Em (redox midpoint potential) of 195 mV. This is too low to be accounted for on the basis of nonturnover measurements and the effects of the membrane potential. However, if there is no equilibrium, the internal CuA----Fea electron-transfer rate constant must be slow in the time average (about 200 s-1). Other factors which might contribute to such a low Em are discussed. In the presence of uncoupler, this situation changes dramatically. Both Fec and CuA are much less reduced; within the resolution of our measurements (about 10%), we were unable to measure any reduction of CuA. Fea and CuA remain too oxidized to be in redox equilibrium with Fec during steady-state turnover. Furthermore, our results indicate that, in the uncoupled system, the (time-averaged) internal electron-transfer rate constants in cytochrome oxidase must be of the order of 2500 s-1 or higher. When turnover is slowed by azide, the relative redox levels at Fea and Fec are much closer to those predicted from nonturnover measurements. In presence of uncouplers, Fea is always more reduced than Fec, but in the absence of uncouplers, the two centers track together. Unlike the uninhibited, coupled system, the redox behavior here is consistent with the known effect of the electrical membrane potential on electron distribution in the enzyme. Interestingly, in these circumstances (azide and uncoupler present), Fea behaves as if it were no longer the kinetically controlling electron donor to the bimetallic center.     

Interactions of cytochrome c with mitochondrial membranes. Binding to succinate-cytochrome c reductase

Methyl-4-azidobenzoimidate was reacted with horse heart cytochrome c to give a photoaffinity-labeled derivative of this heme protein. The modified cytochrome c bound to cytochrome c-depleted mitochondria with the same Kd as native cytochrome c and restored oxygen uptake to the same extent. Irradiation of cytochrome c-depleted mitochondrial membranes with 3- to 4-fold excess of photoaffinity-labeled cytochrome c over cytochrome c oxidase resulted in covalent binding of the derivative to the membranes. Fractionation of the irradiated mitochondria in the presence of detergents and salts followed by chromatography on an agarose Bio-Gel-A-5m showed that the labeled cytochrome c was bound covalently to succinate-cytochrome c reductase. The covalently bound cytochrome c was active in mediating electron transfer between its reductase and oxidase. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of the succinate-cytochrome c reductase containing photoaffinity-labeled 125I-cytochrome c showed that the reductase contained a protein binding site for cytochrome c. It is suggested that cytochrome c1 is the most likely site for the cytochrome c binding in mitochondria in situ.     

Binding of arylazidocytochrome c to yeast cytochrome c peroxidase

Cytochrome c derivatives modified with a photoactivatable arylazido group in selected lysine residues were irradiated in the presence of cytochrome c peroxidase (EC 1.11.1.5). A derivative modified at lysine 13 was able to cross-link to the enzyme and inhibit electron transfer activity. Complete inhibition of cytochrome c peroxidase activity was obtained when 1 mol of cytochrome c was covalently bound per mol of cytochrome c peroxidase. Chemical cleavage of the covalent complex has been used for a preliminary characterization of the site of cross-linking of cytochrome c to cytochrome c peroxidase. This linkage site was localized to the NH2 terminal part of cytochrome c peroxidase including residues 1-51.     

Import of cytochrome c into mitochondria. Cytochrome c heme lyase

The import of cytochrome c into mitochondria can be resolved into a number of discrete steps. Here we report on the covalent attachment of heme to apocytochrome c by the enzyme cytochrome c heme lyase in mitochondria from Neurospora crassa. A new method was developed to measure directly the linkage of heme to apocytochrome c. This method is independent of conformational changes in the protein accompanying heme attachment. Tryptic peptides of [35S]cysteine-labelled apocytochrome c, and of enzymatically formed holocytochrome c, were resolved by reverse-phase HPLC. The cysteine-containing peptide to which heme was attached eluted later than the corresponding peptide from apocytochrome c and could be quantified by counting 35S radioactivity as a measure of holocytochrome c formation. Using this procedure, the covalent attachment of heme to apocytochrome c, which is dependent on the enzyme cytochrome c heme lyase, could be measured. Activity required heme (as hemin) and could be reversibly inhibited by the analogue deuterohemin. Holocytochrome c formation was stimulated 5--10-fold by NADH greater than NADPH greater than glutathione and was independent of a potential across the inner mitochondrial membrane. NADH was not required for the binding of apocytochrome c to mitochondria and was not involved in the reduction of the cysteine thiols prior to heme attachment. Holocytochrome c formation was also dependent on a cytosolic factor that was necessary for the heme attaching step of cytochrome c import. The factor was a heat-stable, protease-insensitive, low-molecular-mass component of unknown function. Cytochrome c heme lyase appeared to be a soluble protein located in the mitochondrial intermembrane space and was distinct from the previously identified apocytochrome c binding protein having a similar location. A model is presented in which the covalent attachment of heme by cytochrome c heme lyase also plays an essential role in the import pathway of cytochrome c.     

Pinocytic loading of cytochrome c into intact cells specifically induces caspase-dependent permeabilization of mitochondria: evidence for a cytochrome c feedback loop

Previous studies introduced cytochrome c into intact cells via the disruptive techniques of microinjection or electroporation to provide support for the hypothesis that, in whole cells, cytochrome c release from mitochondria triggers caspase activation and other degradative changes. However, the types of measurements that could be undertaken with these techniques was limited. We used the simple and relatively gentle technique of pinocytic loading to demonstrate that, in intact cells, cytosolic cytochrome c specifically induced activation of caspase-3- and -9-like enzymes, and a loss of mitochondrial polarization coincident with an increase in mitochondrial permeability. Our results support the prediction from in vitro studies that activation of caspases-3 and -9 is downstream of cytochrome c release and provide the first direct evidence that, in whole cells, cytochrome c-dependent caspase-activation can exert a feedback effect to elicit mitochondrial permeabilization and collapse of the mitochondrial trans-membrane potential.     

Coupling of heme attachment to import of cytochrome c into yeast mitochondria. Studies with heme lyase-deficient mitochondria and altered apocytochromes c

Cytochrome c is synthesized in the cytoplasm as apocytochrome c, lacking heme, and then imported into mitochondria. The relationship between attachment of heme to the apoprotein and its import into mitochondria was examined using an in vitro system. Apocytochrome c transcribed and translated in vitro could be imported with high efficiency into mitochondria isolated from normal yeast strains. However, no import of apocytochrome c occurred with mitochondria isolated from cyc3- strains, which lack cytochrome c heme lyase, the enzyme catalyzing covalent attachment of heme to apocytochrome c. In addition, amino acid substitutions in apocytochrome c at either of the 2 cysteine residues that are the sites of the thioether linkages to heme, or at an immediately adjacent histidine that serves as a ligand of the heme iron, resulted in a substantial reduction in the ability of the precursor to be translocated into mitochondria. Replacement of the methionine serving as the other iron ligand, on the other hand, had no detectable effect on import of apocytochrome c in this system. Thus, covalent heme attachment is a required step for import of cytochrome c into mitochondria. Heme attachment, however, can occur in the absence of mitochondrial import since we have detected CYC3-encoded heme lyase activity in solubilized yeast extracts and in an Escherichia coli expression system. These results suggest that protein folding triggered by heme attachment to apocytochrome c is required for import into mitochondria.     

Biogenesis of cytochrome c in Neurospora crassa. Synthesis of apocytochrome c, transfer to mitochondria and conversion to Holocytochrome c.

1. Precipitating antibodies specific for apocytochrome c and holocytochrome c, respectively, were employed to study synthesis and intracellular transport of cytochrome c in Neurospora in vitro. 2. Apocytochrome c as well as holocytochrome c were found to be synthesized in a cell-free homogenate. A precursor product relationship between the two components is suggested by kinetic experiments. 3. Apocytochrome c synthesized in vitro was found in the post-ribosomal fraction and not in the mitochondrial fraction, whereas holocytochrome c synthesized in vitro was mainly detected in the mitochondrial fraction. A precursor product relationship between postribosomal apocytochrome c and mitochondrial holocytochrome c is indicated by the labelling data. In the microsomal fraction both apocytochrome c and holocytochrome c were found in low amounts. Their labeling kinetics do not subbest a precursor role of microsomal apocytochrome c or holocytochrome c. 4. Formation of holocytochrome c from apocytochrome c was observed when postribosomal supernatant containing apocytochrome c synthesized in vitro was incubated with isolated mitochondria, but not when incubated in the absence of mitochondria. The cytochrome c formed under these conditions was detected in the mitochondria. 5. Conversion of labelled apocytochrome c synthesized in vitro to holocytochrome c during incubation of a postribosomal supernatant with isolated mitochondria was inhibited when excess isolated apocytochrome c, but not when holocytochrome c was added. 6. The data presented are interpreted to show that apocytochrome c is synthesized on cytoplasmic ribosomes and released into the supernatant. It is suggested that apocytochrome c migrates to the inner mitochondrial membrane, where the heme group is covalently linked to the apoprotein. The hypothesis is put forward that the concomitant change in conformation leads to trapping of holocytochrome c in the membrane. The problems of permeability of the outer mitochondrial membrane to apocytochrome c and the site and nature of the reaction by which the heme group is linked to the apoprotein are discussed.     

Optimum concentration and pH of phosphate buffer for assay of cytochrome c oxidase in intact plant mitochondria

For measurement of cytochrome c oxidase activity in intact plant mitochondria the optimum concentration of K-Pi buffer and pH in the reaction was found to be 75 mM and 7.4 respectively. The suitable concentration of K-Pi buffer for suspending and storing mitochondria, however, was found to be 20 mM or lower. These requirements applied equally well for mitochondria from wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), maize (Zea mays L.), and snap bean (Phaseolus vulgaris L.).     

Role of cytochrome c heme lyase in the import of cytochrome c into mitochondria

The import of cytochrome c into Neurospora crassa mitochondria was examined at distinct stages in vitro. The precursor protein, apocytochrome c, binds to mitochondria with high affinity and specificity but is not transported completely across the outer membrane in the absence of conversion to holocytochrome c. The bound apocytochrome c is accessible to externally added proteases but at the same time penetrates far enough through the outer membrane to interact with cytochrome c heme lyase. Formation of a complex in which apocytochrome c and cytochrome c heme lyase participate represents the rate-limiting step of cytochrome c import. Conversion from the bound state to holocytochrome c, on the other hand, occurs 10-30-fold faster. Association of apocytochrome c with cytochrome c heme lyase also takes place after solubilizing mitochondria with detergent. We conclude that the bound apocytochrome c, spanning the outer membrane, forms a complex with cytochrome c heme lyase from which it can react further to be converted to holocytochrome c and be translocated completely into the intermembrane space.     

Loading rat liver mitochondria with apocytochrome c provokes exit of cytochrome c

Rat liver mitochondria were loaded with cytochrome c by incubation with large amounts of [14C]apocytochrome c. After being washed they were incubated with either more apocytochrome c or cytochrome c. There was no release of labeled proteins from the mitochondria when incubated with cytochrome c. However, there was when incubated with apocytochrome c. The material released showed only one radioactive band which migrated as cytochrome c. Also no release of proteins other than cytochrome c was detected when liver mitochondria isolated from rats injected with [35S]methionine were incubated with apocytochrome c. These results suggest that the level and possibly the turnover of cytochrome c in rat liver mitochondria is regulated by the entry of apocytochrome c into mitochondria.     

A near-infrared investigation of cytochrome c oxidase in higher plant mitochondria

Optical features of cytochrome c oxidase in potato mitochondria have been characterized in the near-ir region. In order to discriminate the respective properties of the various redox centers, the redox state was monitored from free and inhibited, bound species. Appropriate comparisons singled out difference spectra which can be attributed specifically to CuA and CuB. The CuA difference spectrum (red-ox) exhibits a negative band centered at 812 nm and, analogous to its mammalian counterpart, the so-called 830-nm band (delta epsilon red/ox = -2.0 mM-1 cm-1). The unusual difference spectrum (red-ox) assigned to CuB is characterized by a broad positive band also centered at 812 nm with an extinction coefficient of delta epsilon red/ox = 4.3 mM-1 cm-1.     

A mutant of Neurospora crassa deficient in cytochrome c heme lyase activity cannot import cytochrome c into mitochondria

The nuclear cyt-2-1 mutant of Neurospora crassa is characterized by a gross deficiency of cytochrome c (Bertrand, H., and Collins, R. A. (1978) Mol. Gen. Genet. 166, 1-13). The mutant produces mRNA that can be translated into apocytochrome c in vitro. Apocytochrome c is also synthesized in vivo in cyt-2-1, but it is rapidly degraded and thus does not accumulate in the cytosol. Mitochondria from wild-type cells bind apocytochrome c made in vitro from either wild-type or cyt-2-1 mRNA and convert it to holocytochrome c. This conversion depends on the addition of heme by cytochrome c heme lyase and is coupled to translocation of cytochrome c into the intermembrane space. Mitochondria from the cyt-2-1 strain are deficient in the ability to bind apocytochrome c. They are also completely devoid of cytochrome c heme lyase activity. These defects explain the inability of the cyt-2-1 mutant to convert apocytochrome c to the holo form and to import it into mitochondria.     

Mechanisms of cytochrome c release from mitochondria

In healthy cells, cytochrome c (Cyt c) is located in the mitochondrial intermembrane/intercristae spaces, where it functions as an electron shuttle in the respiratory chain and interacts with cardiolipin (CL). Several proapoptotic stimuli induce the permeabilization of the outer membrane, facilitate the communication between intermembrane and intercristae spaces and promote the mobilization of Cyt c from CL, allowing for Cyt c release. In the cytosol, Cyt c mediates the allosteric activation of apoptosis-protease activating factor 1, which is required for the proteolytic maturation of caspase-9 and caspase-3. Activated caspases ultimately lead to apoptotic cell dismantling. Nevertheless, cytosolic Cyt c has been associated also to vital cell functions (i.e. differentiation), suggesting that its release not always occurs in an all-or-nothing fashion and that mitochondrial outer membrane permeabilization may not invariably lead to cell death. This review deals with the events involved in Cyt c release from mitochondria, with special attention to its regulation and final consequences.     

Affinity purification of cytochrome c reductase from potato mitochondria.

Ubiquinol-cytochrome-c oxidoreductase has been isolated from potato (Solanum tuberosum L.) mitochondria by cytochrome-c affinity chromatography and gel-filtration chromatography. The procedure, which up to now only proved applicable to Neurospora, yields a highly pure and active protein complex in monodisperse state. The molecular mass of the purified complex is about 650 kDa, indicating that potato cytochrome c reductase occurs as a dimer. Upon reconstitution into phospholipid membranes, the dimeric enzyme catalyzes electron transfer from a synthetic ubiquinol to equine cytochrome c with a turnover number of 50 s-1. The activity is inhibited by antimycin A and myxothiazol. A myxothiazol-insensitive and antimycin-sensitive transhydrogenation reaction, with a turnover number of 16 s-1, can be demonstrated as well. The protein complex consists of ten subunits, most of which have molecular masses similar to those of the nine-subunit fungal enzyme. Individual subunits were identified immunologically and spectral properties of b and c cytochromes were monitored. Interestingly, an additional 'core' polypeptide which is not present in other cytochrome bc1 complexes forms part of the enzyme from potato. Antibodies raised against individual polypeptides reveal that the core proteins are clearly immuno-distinguishable. The additional subunit may perform a specific function and contribute to the high molecular mass which exceeds those reported for other cytochrome-c-reductase dimers.