Immunological and chemical characterization of rat liver cytochrome c oxidase

NDUFA4 (Renamed COXFA4) Is a Cytochrome-c Oxidase Subunit

Groundbreaking work by Kadenbach and colleagues in the 1980s revealed the presence of 13 subunits in the mammalian mitochondrial cytochrome-c oxidase (COX; Complex IV). This observation stood the test of time until 2012 when it was demonstrated that NDUFA4, a polypeptide previously attributed to mitochondrial Complex I, was a 14th subunit of COX. In his recent opinion article, Kadenbach argued that NDUFA4 is not a subunit of COX. However, based on the findings that NDUFA4 deficiency results in a severe loss of COX activity and that NDUFA4 represents a stoichiometric component of the individual COX complex, we reason that NDUFA4 is a bona fide COX subunit and propose renaming it as COX subunit FA4 (COXFA4).

Regulation of Mammalian 13-Subunit Cytochrome c Oxidase and Binding of other Proteins: Role of NDUFA4

Cytochrome c oxidase (CcO) is the final oxygen accepting enzyme complex (complex IV) of the mitochondrial respiratory chain. In contrast to the other complexes (I, II, and III), CcO is highly regulated via isoforms for six of its ten nuclear-coded subunits, which are differentially expressed in species, tissues, developmental stages, and cellular oxygen concentrations. Recent publications have claimed that NADH dehydrogenase (ubiquinone) 1 alpha subcomplex 4 (NDUFA4), originally identified as subunit of complex I, represents a 14th subunit of CcO. Results on CcO composition in tissues from adult animals and the review of data from recent literature strongly suggest that NDUFA4 is not a 14th subunit of CcO but may represent an assembly factor for CcO or supercomplexes (respirasomes) in mitochondria of growing cells and cancer tissues.

The subunit composition and function of mammalian cytochrome c oxidase

Cytochrome c oxidase (COX) from mammals and birds is composed of 13 subunits. The three catalytic subunits I-III are encoded by mitochondrial DNA, the ten nuclear-coded subunits (IV, Va, Vb, VIa, VIb, VIc, VIIa, VIIb, VIIc, VIII) by nuclear DNA. The nuclear-coded subunits are essentially involved in the regulation of oxygen consumption and proton translocation by COX, since their removal or modification changes the activity and their mutation causes mitochondrial diseases. Respiration, the basis for ATP synthesis in mitochondria, is differently regulated in organs and species by expression of tissue-, developmental-, and species-specific isoforms for COX subunits IV, VIa, VIb, VIIa, VIIb, and VIII, but the holoenzyme in mammals is always composed of 13 subunits. Various proteins and enzymes were shown, e.g., by co-immunoprecipitation, to bind to specific COX subunits and modify its activity, but these interactions are reversible, in contrast to the tightly bound 13 subunits. In addition, the formation of supercomplexes with other oxidative phosphorylation complexes has been shown to be largely variable. The regulatory complexity of COX is increased by protein phosphorylation. Up to now 18 phosphorylation sites have been identified under in vivo conditions in mammals. However, only for a few phosphorylation sites and four nuclear-coded subunits could a specific function be identified. Research on the signaling pathways leading to specific COX phosphorylations remains a great challenge for understanding the regulation of respiration and ATP synthesis in mammalian organisms. This article reviews the function of the individual COX subunits and their isoforms, as well as proteins and small molecules interacting and regulating the enzyme.

Diagnosis of cyanide intoxication by measurement of cytochrome c oxidase activity

Cytochrome c oxidase (CCO), a mitochondrial enzyme, is inactivated by cyanide or carbon monoxide (CO) intoxication. We measured CCO activity, in the major organs of the rat at various times after death caused by cyanide intoxication. Tissue samples were homogenized, and the CCO activity in the mitochondrial fraction was measured using ferrous cytochrome c as the substrate. The CCO activity inhibition was highest in the brain, although the cyanide concentration was lowest level. As a result of this and the clinical symptoms displayed, we consider the brain to be the primary organ of cyanide intoxication. As cyanide is highly toxic to humans, in small amounts and many patients and victims have already had some medical care, it is difficult to detect cyanide in criminal investigations. The CCO activities in various organs remained significantly low for 2 days after the cyanide intoxication, suggesting that the diagnosis may be possible by measuring not only the cyanide concentration but also the CCO activity.

Postmortem changes in cytochrome c oxidase activity in various organs of the rat and in human heart

Cytochrome c oxidase (COX), a mitochondrial enzyme, is inactivated by cyanide or carbon monoxide (CO) intoxication. To test whether cytochrome c has potential as an indicator of these toxins in cadavers, we measured COX activity in the main organs of the rat, and in the human heart, at various times after death. Each tissue sample or organ was homogenized and the COX activity in the mitochondrial fraction was measured using ferrous cytochrome c as the substrate. COX activity was significantly higher in rat brain, heart and kidney than in lung and liver from 0 to 4 days after death. The loss of COX activity was significantly slower in the brain and heart than in the lung, liver and kidney. Most importantly, COX activity correlated with the time-since-death for each of the rat organs we tested (r2=0.70-0.95), but for the human heart (r2=0.47). It may be possible that COX activity is likely to be a useful indicator of the time-since-death, and is worth pursuing as an indicator of the tissue cyanide and CO content.

Matrix-assisted laser desorption/ionization mass spectrometry analysis and thiol-group determination of isoforms of bovine cytochrome c oxidase, a hydrophobic multisubunit membrane protein

Matrix-assisted laser desorption/ionization-mass spectra (MALDI-MS) are obtained from entire bovine heart (H) and liver (L) cytochrome c oxidase membrane protein complexes. Molecular masses of most of the subunits are in excellent agreement with the published sequences. Some corrections are necessary for the nuclear coded subunit IX, which is N-acetylated, and X, with a corrected C-terminal peptide sequence. The mass values of two of the three tissue-specific subunits (VIII-L and X-L) are not in agreement with the DNA-deduced sequences and have been corrected by protein sequencing. For the investigation of the cysteine status 7-diethyl-amino-3-(4'-maleimidylphenyl)-4-methylcoumarin proved to be an excellent site-specific reagent. MALDI-MS with the SH-reacted enzyme indicates disulfide bridges only in subunit VII and a distorted tetrahedral S coordination of the zinc in subunit VI.

The subunit structure of cytochrome-c oxidase from tuna heart and liver

Cytochrome-c oxidase was isolated from tuna liver and heart, and the subunit composition was analysed by SDS/PAGE by two separation systems. Two additional subunits of the enzyme complex were immunoprecipitated from solubilized mitochondria with an antibody against bovine subunit IV. The N-terminal and internal amino acid sequences of all nuclear-coded subunits were determined after blotting onto poly(vinylidene difluoride) membranes or by tryptic hydrolysis of gel bands and HPLC separation of peptides, respectively. 13 subunits were identified with isoforms for subunits Va, VIc, VIIb and VIII. The isoforms for subunits Va and VIIb are found in liver and heart, isoforms for subunit VIc only in heart, and isoforms for subunit VIII only in liver. Isoforms for subunits Va, VIc and VIIb have not been described in other species. The postulated mechanism of thermogenesis in mammals, based on decreased H+/e- stoichiometry at high ATP/ADP ratios due to binding of ATP to the heart-type subunit VIa [Frank, V. & Kadenbach, B. (1996) FEBS Lett. 382, 121-124], appears not to occur in tuna, because no isoforms of subunit VIa were found.

Disproportionate regulation of nuclear- and mitochondrial-encoded cytochrome oxidase subunit proteins by functional activity in neurons

Cytochrome oxidase is the terminal enzyme in the mitochondrial respiratory chain engaged in oxidative metabolism and energy production. In mammals, the holoenzyme is composed of 13 subunits encoded by both nuclear and mitochondrial genomes. The goal of the present study was to compare the effect of afferent impulse blockade on the expression of these two genomes at the subunit protein level. It also aimed to determine the correlation between the level of cytochrome oxidase activity and the relative amount of subunit proteins. Relative enzyme activity was analysed histochemically, and relative amounts of subunits IV (nuclear-encoded) and II/III (mitochondrial-derived) proteins were obtained immunohistochemically by anti-subunit IV and anti-subunit II/III antibodies in the lateral geniculate nucleus and the primary visual cortex of adult monkeys. In the normal visual centers, similar staining patterns were found for all three markers. After three and seven days of tetrodotoxin treatment, levels of enzyme activity and subunit proteins declined disproportionately in the deprived laminae of the visual center. Densitometric analysis indicates that changes in enzyme activity and subunit IV proteins were significantly greater than those of subunit II/III proteins (P < 0.01). The finding that nuclear and mitochondrial genomes are disproportionately regulated at subunit protein levels by neuronal activity implies that the two genomes operate under different regulatory mechanisms. Changes in subunit IV paralleled most closely those of cytochrome oxidase activity (coefficient of determination r2 = 0.95). This suggests that nuclear-derived subunit IV protein may play a pivotal role in controlling cytochrome oxidase holoenzyme activity.

Immunohistochemical analysis of muscle cytochrome c oxidase deficiency in children

Despite the demonstration of a clear biochemical defect, the genetic alterations causing childhood forms of cytochrome c oxidase (COX) deficiency remain unknown. The double genetic origin (nuclear and mitochondrial DNA), and the complexity of COX enzyme structure and regulation, indicate the need for genetic investigations of the molecular structure of individual COX subunits. In the present study a new monoclonal antibody, which reacts exclusively with heart-type human COX subunit VIIa (VIIa-H), and other monoclonal antibodies against human COX subunits, were used in the immunohistochemical analysis of skeletal muscle from children with different forms of mitochondrial myopathy with COX deficiency. By immunohistochemical investigation a normal reaction was seen with antibodies to COX subunits IV, Va+Vb, and VIa+VIc in all four cases, and in two cases with antibodies to COX VIIa-H and VIIa+VIIb. In muscle from a fatal infantile case with cardiac and skeletal muscle involvement, no immunohistochemical reaction was seen with the monoclonal antibody against the tissue-specific subunit VIIa-H. In muscle from an 11-year-old boy with exclusive muscular symptoms and signs, immunohistological reactions were absent with COX subunit VIIa-H and COX subunits VIIa+VIIb, and slightly decreased with COX subunit II, thus demonstrating a different molecular mechanism in each case. It is concluded that the molecular basis of COX deficiency in childhood may vary greatly between patients.

Identification of tissue-specific isoforms for subunits Vb and VIIa of cytochrome c oxidase isolated from rainbow trout

Cytochrome c oxidase was isolated from heart and liver of rainbow trout (Salmo gairdnerii). SDS/PAGE analysis showed the presence of 11 different polypeptide subunits in the fish enzyme. The nuclear-coded subunits IV, Va, Vb, VIc, VIIa, VIIc and VIII could be identified by their N-terminal amino acid sequences. The mammalian subunits VIa and VIIb appear to be absent (or blocked at the N-terminal) in cytochrome c oxidase from trout. For subunit Vb, two polypeptides of different electrophoretic mobilities were found which differed in their N-terminal sequences, and represent a new pair of cytochrome-c-oxidase subunit isoforms, not found in mammalia. Both isoforms of subunit Vb were found in cytochrome c oxidase from heart and liver, but at different ratios. Subunit VIIa also seemed to occur in different isoforms, whereas subunit VIII had the same N-terminal amino acid sequence in cytochrome c oxidase of liver and heart, similar to the human-type subunit but different from rat, bovine and chicken.

Subunit IV of human cytochrome c oxidase, polymorphism and a putative isoform

As part of our study of isoenzyme forms of human cytochrome c oxidase, we purified subunit IV from human heart and skeletal muscle with reversed-phase HPLC and determined the N-terminal amino acid sequences and the electrophoretic mobility. The N-terminus of human heart subunit IV proved to be ragged with 30% of the protein lacking the first three residues. Also a Tyr/Phe polymorphism was observed at residue 16. No differences in N-terminal sequence and electrophoretic mobility were observed between subunit IV of cytochrome c oxidase from human heart and skeletal muscle. Therefore, our results suggest that identical subunits IV are present in cytochrome c oxidase from human heart and skeletal muscle. A putative isoform of subunit IV with a blocked N-terminus was purified from human heart cytochrome c oxidase, which proved to have a different retention time on a reversed-phase column and also a slightly higher electrophoretic mobility on an SDS-polyacrylamide gel compared to the native subunit IV. We could not demonstrate the existence of isoforms of subunit IV in human skeletal muscle.

Isoforms of human cytochrome-c oxidase. Subunit composition and steady-state kinetic properties

The subunit pattern and the steady-state kinetics of cytochrome-c oxidase from human heart, muscle, kidney and liver were investigated. Polyacrylamide gel electrophoresis of immunopurified cytochrome-c oxidase preparations suggest that isoforms of subunit VIa exist, which show differences in staining intensity and electrophoretic mobility. No differences in subunit pattern were observed between the other nucleus-encoded subunits of the various cytochrome-c oxidase preparations. Tissue homogenates, in which cytochrome-c oxidase was solubilised with laurylmaltoside, were directly used in the assays to study the cytochrome-c oxidase steady-state kinetics. Cytochrome-c oxidase concentrations were determined by immunopurification followed by separation and densitometric analysis of subunit IV. When studied in a medium of low ionic strength, the biphasic kinetics of the steady-state reaction between human ferrocytochrome c and the four human cytochrome-c oxidase preparations revealed large differences for the low-affinity TNmax (maximal turnover number) value, ranging from 77 s-1 for kidney to 273 s-1 for liver cytochrome-c oxidase at pH 7.4, I = 18 mM. It is proposed that the low-affinity kinetic phase reflects an internal electron-transfer step. For the steady-state reaction of human heart cytochrome-c oxidase with human cytochrome c, Km and TNmax values of 9 microM and 114 s-1 were found, respectively, at high ionic strength (I = 200 mM, pH 7.4). Only minor differences were observed in the steady-state activity of the various human cytochrome-c oxidases. The interaction between human cytochrome-c oxidase and human cytochrome-c proved to be highly specific. At high ionic strength, a large decrease in steady-state activity was observed when reduced horse, rat or bovine cytochrome c was used as substrate. Both the steady-state TNmax and Km parameters were strongly affected by the type of cytochrome c used. Our findings emphasize the importance of using human cytochrome c in kinetic assays performed with tissues from patients with a suspected cytochrome-c oxidase deficiency.

Fatal infantile mitochondrial cardiomyopathy and myopathy with heterogeneous tissue expression of combined respiratory chain deficiencies

A 5-month-old boy died of progressive heart failure that started at the age of 3 months. Autopsy revealed a mitochondrial cardiomyopathy and a mitochondrial myopathy of the limb muscle and diaphragm. Cytochemically random defects of cytochrome c oxidase were visualized by light and electron microscopy in the diaphragm and especially the heart muscle, the limb muscle showing a diffuse attenuation whereas the liver and kidneys reacted normally. The activities of NADH-dehydrogenase (complex I) and cytochrome c oxidase (complex IV) were severely diminished (20% residual activity of controls) in the skeletal and heart muscle. In the heart, succinate cytochrome c reductase (complex II/III) was additionally decreased to the same degree. Loss of cytochrome c oxidase activity was based on a reduction of both mitochondrial and nuclear derived subunits in the heart and diaphragm as revealed by immunohistochemical analysis, whereas the limb muscle showed a normal immunoreactive protein content. The results illustrate heterogeneous tissue expression of respiratory chain enzyme defects and demonstrate that a cardiomyopathy may be the leading presentation of a mitochondrial disorder in early infancy.

Isolation of cDNAs encoding subunit VIb of cytochrome c oxidase and steady-state levels of coxVIb mRNA in different tissues

A full-length cDNA clone specifying the nuclear-encoded subunit VIb of human cytochrome c oxidase (COX) was isolated from a human skeletal muscle cDNA expression library. This was done with antiserum directed against the group of subunits VIa, b and c of bovine heart COX. A potential ribosome-binding site was located immediately upstream from the initiation codon. The predicted amino acid sequence revealed 85% similarity with the corresponding subunit of bovine heart COX. Subunit VIb lacks a cleavable presequence for mitochondrial addressing. We assume that there are no tissue-specific isoforms of subunit VIb, since (i) in a Northern blot experiment a single hybridizing band of approx. 500 nucleotides was demonstrated in RNA from liver, skeletal muscle, MOLT-4 cells and fibroblasts and (ii) a full-length cDNA clone with an identical sequence was isolated from a human liver cDNA library. Steady-state levels of the coxVIb transcript were different in the tissues examined.

Cytochrome oxidase from thermophilic bacterium PS3 contains a fourth protein subunit

Monoclonal antibodies prepared against subunits II and IV of beef heart cytochrome oxidase were found to cross-react with thermophilic bacterial PS3 oxidase. Each individual antibody affects the enzymatic activity. "Western" blot analyses showed that subunit II antibodies of beef heart recognized subunit II of PS3 and subunit IV antibody likewise recognized a fourth protein subunit on slab gels. This fourth subunit previously thought to be a contaminant or a degradation product has a molecular weight of about 10,500 on SDS-gels, and appears to exist in stoichiometric amount. We have extracted this subunit from slab gels and compared its amino acid composition with that of subunit III.

Immunohistochemical demonstration of fibre type-specific isozymes of cytochrome c oxidase in human skeletal muscle

The immunohistochemical reaction of monoclonal as well as polyclonal antibodies against cytochrome c oxidase (COX) subunits with serial sections of normal human skeletal muscle was investigated. The stronger reactivity of polyclonal antibodies to COX subunits II-III and VIIbc with type I as compared to type II fibres, correlated well with the higher histochemical reactivity of NADH dehydrogenase, succinate dehydrogenase and cytochrome c oxidase in type I fibres. In contrast an almost exclusive reaction of a monoclonal antibody against subunit IV with type I fibre and a preponderant reaction of a polyclonal antibody against subunits Vab with type II fibres was obtained. Antibodies against subunits I, Vb and VIc did not reveal a fibre-type-specific reactivity. The data indicate in human muscle the occurrence of fibre type-specific isozymes of cytochrome c oxidase differing in subunits IV and Va or Vb.

Novel use of a chimpanzee pseudogene for chromosomal mapping of human cytochrome c oxidase subunit IV

We have isolated a chimpanzee processed pseudogene for subunit IV of cytochrome c oxidase (COX; EC 1.9.3.1) by screening a chimpanzee genomic library in lambda Charon 32 with a bovine liver cDNA encoding COX subunit IV (COX IV), and localized it to a 1.9-kb HindIII fragment. Southern-blot analysis of genomic DNA from five primates showed that DNAs from human, gorilla, and chimpanzee each contained the 1.9-kb pseudogene fragment, whereas orangutan and pigtail macaque monkey DNA did not. This result clearly indicates that the pseudogene arose before the divergence of the chimpanzee and gorilla from the primate lineage. By screening Chinese hamster x human hybrid panels with the human COX4 cDNA, we have mapped COX4 genes to two human chromosomes, 14 and 16. The 1.9-kb HindIII fragment containing the pseudogene, COX4P1, can be assigned to chromosome 14, and by means of rearranged chromosomes in somatic cell hybrids, to 14q21-qter. Similarly, the functional gene, COX4, has been mapped to 16q22-qter.

Modulation of cytochrome oxidase kinetics by indirect antibody action

Polyclonal antibodies raised against isolated subunit V from beef heart cytochrome oxidase or against the intact enzyme increase its apparent affinity for the substrate cytochrome c at the high-affinity site while diminishing the turnover at that site. At the low-affinity site the major action of both types of antibody is to reduce the apparent affinity for cytochrome c. At high ionic strengths the kinetic effect of anti-subunit V is very small although it still binds to the enzyme. The results are interpreted in terms of a model for the enzyme in which antibodies can modulate cytochrome oxidase kinetics by affecting the binding of cytochrome c, even if the antibody-binding site is on a subunit not directly involved in substrate binding.

Fatal mitochondrial myopathy with cytochrome-c-oxidase deficiency and subunit-restricted reduction of enzyme protein in two siblings: an autopsy-immunocytochemical study

Lack of cytochrome-c oxidase activity and of cytochromes aa3 + b has been reported previously in the skeletal muscle of one of two siblings (Müller-Höcker et al, 1983). The present study reports a deficiency of immunoreactive enzyme protein in the skeletal muscle of both siblings, who had an identical fatal clinical course. In all specimens the defect did not involve the whole enzyme protein, but was selectively expressed in the mitochondrially derived subunits II/III and nuclear coded subunits VIIbc. Neither the specific fibers of the muscle spindles nor the mitochondria of the heart, liver, kidneys, vessel walls and/or gastrointestinal tract were affected. These results are most consistent with a primary nuclear defect being responsible for the organ specific and subunit selective expression of the enzyme defect.

Cytochrome oxidase deficiency: immunological studies of skeletal muscle mitochondrial fractions

We report a 2-year-old girl who presented with delayed development, weakness and persistent vomiting. She had a demyelinating peripheral neuropathy. The activity of cytochrome oxidase in skeletal muscle from the patient was 10% of controls. Immunochemical studies using antibodies to holo-cytochrome oxidase and the individual subunits showed a low concentration of all detectable subunits.

Bovine cytochrome c oxidases, purified from heart, skeletal muscle, liver and kidney, differ in the small subunits but show the same reaction kinetics with cytochrome c

(1) Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate of purified cytochrome c oxidase preparations revealed that bovine kidney, skeletal muscle and heart contain different cytochrome c oxidase isoenzymes, which show differences in mobility of the subunits encoded by the nuclear genome. No differences in subunit pattern were observed between the oxidase preparations isolated from kidney and liver. (2) The kinetics of the steady-state reactions between bovine ferrocytochrome c and the four types of bovine cytochrome c oxidase preparation were compared under conditions of both high- and low-ionic strength. Also the pre-steady-state kinetics were studied. Only minor differences were observed in the electron-transfer activity of the isoenzymes. Thus, our experiments do not support the notion that the subunits encoded by the nuclear genome act as modulators conferring different activities to the isoenzymes of cytochrome c oxidase. (3) The cytochrome c oxidase preparation from bovine skeletal muscle was found to consist mainly of dimers, whereas the enzymes isolated from bovine kidney, liver and heart were monomeric.

Isolation and characterization of human heart cytochrome c oxidase

Cytochrome c oxidase was isolated from human hearts and separated by SDS gel electrophoresis. The identity of polypeptide bands with known subunits was demonstrated by immunoblotting with monospecific antisera to rat liver cytochrome c oxidase subunits. The polarographically determined kinetics of cytochrome c oxidation were similar to those reported for the bovine heart enzyme.

Immunochemical evidence for an inactive form of cytochrome oxidase in mitochondrial membranes of ethanol-fed rats

Previous studies have established that rats fed ethanol chronically exhibit a 50% decrease in hepatic mitochondrial cytochrome oxidase compared to pair-fed controls, based on both heme aa3 content and specific activity. To determine whether the 'missing' 50% of cytochrome oxidase is present in the membrane but catalytically inactive, or entirely absent, we used immunochemical techniques to determine the content of cytochrome oxidase protein in hepatic submitochondrial particles. Rabbit antiserum against purified rat liver cytochrome oxidase precipitated cytochrome oxidase from detergent-solubilized submitochondrial particles. Immunoinhibition titrations of a fixed amount of anti-oxidase serum with increasing amounts of submitochondrial particle protein showed that similar percentages of added oxidase activity were recovered in supernatants after immunoprecipitation with preparations from both alcoholic and control rats. Similarly, titrations of a fixed amount of submitochondrial particle protein with increasing amounts of antiserum showed comparable decreases in oxidase activity. Equivalent amounts of protein were obtained in immunoprecipitates from both preparations. Immunoprecipitates demonstrated comparable oxidase subunit profiles by electrophoresis, except that one additional band, migrating in the region of oxidase subunit IV, was present in samples from alcoholic rats. The data indicate that cytochrome oxidase immunologic reactivity is quantitatively similar in both types of membranes. The results suggest that the 'missing' cytochrome oxidase is actually present within the membranes of alcoholic animals in an inactive form, apparently devoid of heme aa3.

Tissue-specific and species-specific distribution of -SH groups in cytochrome c oxidase subunits

Cytochrome c oxidase was isolated from pig, bovine, rat and human tissues including liver, heart, diaphragm and kidney. The native and the sodium-dodecyl-sulfate (SDS)-dissociated enzymes were labelled under optimal conditions with N-ethyl-[2,3-14C]maleimide before and after reduction with dithiothreitol, separated into 13 subunits by SDS gel electrophoresis and the radioactive bands were visualized by fluorography. In some cases the radioactive bands were cut out and counted. All isozymes were labelled in subunits I, III, Va and VIIb, and in subunit II after reduction. Labelling of subunit Vb was equivocal, and in no case were subunits IV and VIc labelled. All other subunits were labelled tissue-specifically and/or species-specifically. No differences were found between labelling of the native and SDS-dissociated enzyme. By relating the molar amount of bound N-ethylmaleimide to the known amount of cysteines in subunits of bovine heart cytochrome c oxidase, the percentage of -SH group reactivity was calculated. Only the cysteine of subunit Va was found to be 100% reactive. The distinct and different reactivity of subunit VIIb as compared to subunits VIIa and VIIc clearly establishes this polypeptide as an independent subunit of mammalian cytochrome c oxidase.

Regulation of respiration and ATP synthesis in higher organisms: hypothesis

The present view on the regulation of respiration and ATP synthesis in higher organisms implies only Michaelis-Menten type kinetics and respiratory control as regulatory principles. Recent experimental observations, suggesting further regulatory mechanisms at respiratory chain complexes, are reviewed. A new hypothesis is presented implying regulation of respiration and ATP synthesis in higher organisms mainly via allosteric modification of respiratory chain complexes, in particular of cytochrome c oxidase. The allosteric effectors, e.g., metabolites, cofactors, ions, hormones, and the membrane potential are suggested to change the activity and the coupling degree of cytochrome c oxidase by binding to specific sites at nuclear coded subunits. Recent results on the structure and activity of cytochrome c oxidase, supporting the hypothesis, are reviewed.

Effect of trypsin on the kinetic properties of reconstituted beef heart cytochrome c oxidase

Isolated beef heart cytochrome c oxidase was reconstituted in liposomes by the cholate dialysis method with 85% of the binding site for cytochrome c oriented to the outside. Trypsin cleaved specifically subunit VIa and half of subunit IV from the reconstituted enzyme. The kinetic properties of the reconstituted enzyme were changed by trypsin treatment if measured by the spectrophotometric assay but not by the polarographic assay. It is concluded that subunit VIa and/or subunit IV participate in the electron transport activity of cytochrome c oxidase.

Characterization of electron transfer and proton translocation activities in trypsin-treated bovine heart mitochondrial cytochrome c oxidase

Bovine heart mitochondrial cytochrome c oxidase has been treated with trypsin in order to investigate the role of components a, b, and c (nomenclature of Capaldi) in cytochrome c binding, electron transfer, and proton-pumping activities. Cytochrome c oxidase was dispersed in nondenaturing detergent solution (B. Ludwig, N. W. Downer, and R. A. Capaldi (1979) Biochemistry 18, 1401) and treated with trypsin. This treatment inhibited electron transfer activity by 9% when compared to a similarly treated control in a polarographic assay (493 s-1) and had no large effect on the high affinity (Km = 6.1 X 10(-8) M) or low affinity (Km = 2.2 X 10(-6) M) sites of cytochrome c interaction with cytochrome c oxidase. Direct thermodynamic binding experiments with cytochrome c showed that neither the high affinity (1.04 +/- 0.06 mol cytochrome c/mol cytochrome c oxidase) nor the high-plus-low affinity (2.21 +/- 0.15 mol cytochrome c/mol cytochrome c oxidase) binding sites of cytochrome c on the enzyme were perturbed by the trypsin treatment. Control and trypsin-treated enzyme incorporated into phospholipid vesicles (prepared by the cholate dialysis method) exhibited respiratory control ratios of 6.5 +/- 0.7 and 6.3 +/- 0.6, respectively. The vectorial proton translocation activity in the phospholipid vesicles was unaffected by trypsin treatment with proton translocated to electron transferred ratios being equivalent to the control. NaDodSO4-PAGE showed that components a, b, and c were completely removed by the trypsin treatment. [14C]Iodoacetamide labeling experiments showed that the content of component c in the enzyme was depleted by 85% and that greater than 50% of component a was cleaved upon the trypsin treatment. These results suggest that components a, b, and c are not required for maximum electron transfer and proton translocation activities in the isolated enzyme.

Orientation of rat liver cytochrome c oxidase subunits investigated with subunit-specific antisera

The orientation of rat liver cytochrome c oxidase subunits in the inner mitochondrial membrane was investigated with monospecific antisera against subunit II and nine nuclear-coded subunits. Mitoplasts were incubated with the antisera and the amount of bound antibodies was determined either directly with fluorescein-conjugated protein A or indirectly by back-titration of unbound antibodies with a nitrocellulose immunoassay. All subunits were found oriented to the cytosolic side, except subunits VIb and VIIc which did not react with their corresponding antisera. Antisera against subunits I, III and Vb were not available.

Isolation and properties of cytochrome c oxidase from rat liver and quantification of immunological differences between isozymes from various rat tissues with subunit-specific antisera

Cytochrome c oxidase was isolated from rat liver either by affinity chromatography on cytochrome-c--Sepharose 4B or by chromatography on DEAE-Sepharose. Dodecyl sulfate gel electrophoresis of both preparations showed the same subunit pattern consisting of 13 different polypeptides. Kinetic analysis of the two preparations gave a higher Vmax for the enzyme isolated by chromatography on DEAE-Sephacel. Specific antisera were raised in rabbits against nine of the ten nuclear endoded subunits. A monospecific reaction of each antiserum with its corresponding subunit was obtained by Western blot analysis, thus excluding artificial bands in the gel electrophoretic pattern of the isolated enzyme due to proteolysis, aggregation or conformational modification of subunits. With an antiserum against rat liver holocytochrome c oxidase a different reactivity was found by Western blot analysis for subunits VIa and VIII between isolated cytochrome c oxidases from pig liver or kidney and heart or skeletal muscle. For a quantitative analysis of immunological differences a nitrocellulose enzyme-linked immunosorbent assay was developed. Monospecific antisera against 12 of the 13 subunits of rat liver cytochrome c oxidase were titrated with increasing amounts of total mitochondrial proteins from different rat tissues dissolved in dodecyl sulfate and dotted on nitrocellulose. The absorbance of a soluble dye developed by the second peroxidase-conjugated antibody was measured. From the data the following conclusions were obtained: (a) The mitochondrial encoded catalytic subunits I-III of cytochrome c oxidase are probably identical in all rat tissues. (b) All nine investigated nuclear encoded subunits of cytochrome c oxidase showed immunological differences between two or more tissues. Large immunological differences were found between liver, kidney or brain and heart or skeletal muscle. Minor but significant differences were observed for some subunits between heart and skeletal muscle and between liver, kidney and brain. (c) Between corresponding nuclear encoded subunits of cytochrome c oxidase from fetal and adult tissues of liver, heart and skeletal muscle apparent immunological differences were observed. The data could explain cases of fatal infantile myopathy due to cytochrome c oxidase deficiency.

Structure of the cytochrome c oxidase complex of rat liver. 1. Studies on nearest-neighbour relationship of polypeptides with cross-linking reagents

Isolated rat liver cytochrome c oxidase was cross-linked with the cleavable reagents dimethyl-3,3'-dithiobispropionimidate (DTBP), 3,3'-dithiobis(succinimidyl)propionate (DSP) and cupric di(1,10-phenanthroline) (CuP). The cross-linked products were separated by high-resolving two-dimensional dodecyl sulfate gel electrophoresis, which separates all thirteen polypeptides of the mammalian enzyme. With cupric di(1,10-phenanthroline) seven polypeptides (I-III, Va, Vb, VIIb and VIII) were cross-linked with each other and with itself, indicating the occurrence of free -SH groups in these polypeptides and a rearrangement of the native structure of the complex by cupric di(1,10-phenanthroline). With dimethyl-3,3'-dithiobispropionimidate or 3,3'-dithiobis(succinimidyl)propionate all nuclear-coded polypeptides, with the exception of polypeptide VIIa, formed cross-linked products with the three 'catalytic' polypeptides I-III, which are coded on mitochondrial DNA. Five additional cross-linked pairs were found between nuclear coded polypeptides. The close arrangement of nuclear coded polypeptides with the catalytic polypeptides suggests a regulatory function of these polypeptides.

Structure of the cytochrome c oxidase complex of rat liver. 2. Topological orientation of polypeptides in the membrane as studied by proteolytic digestion and immunoblotting

The orientation of the thirteen polypeptides of rat-liver cytochrome c oxidase in the inner mitochondrial membrane was studied by proteolytic digestion of mitoplasts and sonicated particles. After separation by sodium dodecylsulfate gel electrophoresis proteins were transferred on nitrocellulose, and individual polypeptides were identified by incubation with polypeptide-specific antisera, followed by fluorescein-isothiocyanate-conjugated protein A. The three catalytic polypeptides I-III and seven nuclear coded polypeptides (IV, Vb, VIa, VIc, VIIa, VIIb and VIII) were found accessible to proteases from the cytoplasmic phase. Polypeptides II, IV, Va, Vb and VIa were accessible from the matrix phase, indicating a transmembraneous orientation of polypeptides II, IV, Vb and VIa. Together with data on cross-linking and on cytochrome-c-protected labeling of polypeptides, a model of the cytochrome c oxidase complex was developed. It is suggested that the cytochrome c binding site on polypeptide II is surrounded by several nuclear-coded polypeptides, which may modulate the affinity of the enzyme towards cytochrome c.

Immunological identification of four different polypeptides in 'subunit VII' of mammalian cytochrome c oxidase

Rat liver cytochrome c oxidase was separated by SDS-gel electrophoresis into 13 polypeptide bands. Monospecific antisera against the isolated polypeptides VIIa, VIIb and VIIc were raised in rabbits. Cytochrome c oxidase was blotted on nitrocellulose and incubated with the antisera. The antisera reacted only with their corresponding polypeptides, indicating no immunological relationship between polypeptides VIIa, VIIb and VIIc. The data also exclude that these polypeptides are proteolytic breakdown products of larger subunits.

Resolution of bovine heart cytochrome c oxidase into smaller complexes by controlled subunit denaturation

Bovine heart cytochrome c oxidase has been partially denaturated under mild conditions with 0.1-0.25% lithium dodecyl sulfate and 0.05% Triton X-100. From its reactivity towards CO and CN-, an unmasking of the heme a was inferred in this enzyme. The catalytic activity was lost during the denaturation and small spectral differences became visible. Spectra and ligand binding properties of the denatured enzyme were reversed by dilution in 2% Triton X-100. This suggests that during the denaturation procedure the hemes were not displaced from their original sites. By gel filtration of the partially denatured enzyme the following complexes of subunits were obtained: I-III, I-II-III, II-IV-V-VI-VII and IV-V-VI-VII. The first three complexes retained almost all the heme, and their spectral characteristics were very similar to those of the partially denatured cytochrome c oxidase. The data, in combination with the information that subunit III does not contain heme [Saraste et al. (1980) FEBS Lett. 114, 35-38], suggest that the hemes are attached to subunit I and II. After denaturation of cytochrome c oxidase under more drastic conditions some of the heme was also found to be associated with the smaller subunits, but its spectral characteristics were radically altered, becoming almost identical to those of free heme.

Separation of mammalian cytochrome c oxidase into 13 polypeptides by a sodium dodecyl sulfate-gel electrophoretic procedure

A sodium dodecyl sulfate-gel electrophoretic procedure which allows the separation of isolated cytochrome c oxidase from different mammalian sources into 13 different polypeptides is described. Application of the silver-staining procedure results in the same protein pattern as obtained by Coomassie blue staining. From the correlation of the gel bands with 12 isolated polypeptides from which the complete amino acid sequence is known, it is concluded that mammalian cytochrome c oxidase consists of 13 different polypeptides which can all be separated by the described procedure.

The protein component(s) of the isolated phosphate-transport system of mitochondria

The ethylmaleimide-sensitive phosphate-transport system of heart mitochondria was isolated and the activity reconstituted in liposomes. The identification of the phosphate-carrier protein was complicated by several factors. 1. The phosphate-carrier fraction, isolated through different procedures in different laboratories, contains 4-5 protein components of very similar apparent molecular weight, as shown by gradient dodecylsulfate gel electrophoresis. 2. The amino acid composition of protein 1, 2 and 3(apparent Mr = 34 500, 34 000 and 33 000, respectively) is quite similar although not identical. 3. Including several protease inhibitors during isolation of the phosphate-carrier fraction does not influence the protein pattern. 4. Labeled N-ethylmaleimide binds only protein 1 and 3, labeled N,N'-dicyclohexylcarbodiimide bonds only to protein 1 and 2. 5. N,N'-Dicyclohexylcarbodiimide had no effect on the reconstituted phosphate-exchange activity after incubation with intact mitochondria or proteoliposomes. 6. In the presence of protecting mersalyl concentrations N-ethylmaleimide inhibits the phosphate transport in intact mitochondria. Under these conditions no label ethylmaleimide was bound to the phosphate-carrier fraction. 7. Comparison of the reconstituted transport activity with the protein pattern of fractions, isolated with or without cardiolipin, shows correlation of transport with the amount of protein 2 but not with that of protein 1, which binds ethylmaleimide. A model of the mitochondrial phosphate carrier is presented which suggests a proteolytic degradation of the phosphate-carrier protein during its isolation. The model explains the contradictory results of this investigation.

Kinetic and structural differences between cytochrome c oxidases from beef liver and heart

1. The cytochrome content of beef liver mitochondria differs from that of beef heart mitochondria by an eightfold lower cytochrome aa3 and a twofold lower cytochrome b and c + c1 content. 2. The kinetic properties of cytochrome c oxidases from beef liver and heart were measured with intact cytochrome c-depleted membranes, deoxycholate-dissolved membranes, and with the isolated enzymes at various cytochrome c concentrations with an oxygen electrode. Under all conditions a higher V was found for the liver enzyme, both for the low-affinity and for the high-affinity binding site for cytochrome c. Differences were also found for the Km of the two enzymes. 3. Isolated beef heart mitochondria contained about twice as much cardiolipin than beef liver mitochondria. The isolated enzymes contained one mole cardiolipin per mole of the heart enzyme, but 2 moles cardiolipin per mole of the liver enzyme. 4. By application of a high performance sodium dodecylsulfate gel electrophoretic system the two isolated enzymes could be separated into 13 different protein components, three of which (polypeptides VIa, VIIa and VIII) were found to differ in their apparent molecular weights. The functional meaning of cytochrome c oxidase isoenzymes in liver and heart is discussed.