Tissue-specificity overrides species-specificity in cytoplasmic cytochrome c oxidase polypeptides

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.

Two-locus mitochondrial and nuclear gene models for mitochondrial disorders

Stimulated by a large pedigree with a cochlear form of deafness, for which we considered a two-locus mitochondrial and nuclear gene model, we have extended the classic methods of segregation analysis to these classes of two-locus disorders. Based on the unique maternal transmission pattern of the mitochondria, we demonstrate that utilization of the maternal line pedigree allows us to simplify the various two-locus mitochondrial models to "one nuclear locus" models. Classifying the nuclear families into different independent groups by the mother's phenotypes allows us to estimate the nuclear gene frequency in one group and to use this estimate as the expected value to test the fitness of the model on the other group. In addition, if we restrict the analysis to specific subsets of the mating type(s), we can also test the model on specific groups of nuclear families without estimating the gene frequency. Goodness-of-fit tests can be performed on pooled sibship data as well as individual sibship data. These methods of analysis should assume increasing importance as more disorders with features of mitochondrial inheritance are identified.

Co-ordinate expression of cytochrome c oxidase subunit III and VIc mRNAs in rat tissues

Cytochrome c oxidase (EC 1.9.3.1) is an enzyme which is composed of subunits derived from both the mitochondrial and the nuclear genomes. To determine whether or not the expression of these two genomes is co-ordinated at the mRNA level, we have examined the steady-state levels of mRNAs coding for cytochrome c oxidase subunit III (mitochondrially encoded) and subunit VIc (nuclear-encoded) in rat tissues. This was compared with the tissue concentration of the holoenzyme, which was estimated by measuring cytochrome c oxidase enzyme activity. The tissues (heart, brain, liver, kidney, soleus muscle and superficial white vastus muscle) possessed a 13-fold range of enzyme activity, which was highest in heart and lowest in the superficial vastus muscle. Specific subunit mRNA levels were quantified by using slot-blot hybridization of cDNA probes to total tissue RNA. The highest values for subunit III and Vlc mRNA tissue contents were found in kidney, followed by liver and heart (40-60% of that of kidney). The white vastus muscle contained the lowest subunit mRNA level (15% of that of kidney). Although some variability was apparent within each tissue, a parallel pattern of mRNA expression of the nuclear- and mitochondrially encoded subunits was observed. Differences between muscle (heart, vastus and soleus) and non-muscle tissues were noted in the relationship between mRNA and protein levels of expression. Thus, although this suggests that tissue-specific regulatory processes operate, the steady-state expression of subunit III and subunit Vlc mRNAs appears to be co-ordinately regulated.

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.

Evidence for the existence of tissue specific isoenzymes of mitochondrial NADH dehydrogenase

Mitochondrial NADH dehydrogenase from a variety of rat tissues was isolated by immunoprecipitation with an antiserum to the bovine heart enzyme. The subunit composition of the enzyme from liver, kidney and lung differed from that present in heart, brain and skeletal muscle by the absence of an Mr 18,500 protein and the absence of an Mr 17,000 protein, suggesting the existence of tissue-specific isoenzymes.

Human heart cytochrome c oxidase subunit VIII. Purification and determination of the complete amino acid sequence

Subunit VIII was purified from a preparation of the human heart cytochrome c oxidase and its complete amino acid sequence was determined. The sequence proved to be much more related to that of the bovine liver oxidase subunit VIII than to that found in bovine heart. Our finding of a 'liver-type' subunit VIII in the human heart enzyme suggests that either there are no isoforms of human subunit VIII or the human oxidase does not show the type of tissue specificity that has been reported for the oxidase in other mammals.

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.

Sequence analysis of cDNAs for the human and bovine ATP synthase beta subunit: mitochondrial DNA genes sustain seventeen times more mutations

We have cloned and sequenced human and bovine cDNAs for the beta subunit of the ATP synthase (ATP-syn beta), a nuclear DNA (nDNA) encoded oxidative phosphorylation (OXPHOS) gene. The two cDNAs were found to share 99% amino acid homology and 94% nucleotide homology. The evolutionary rate of ATPsyn beta was then compared with that of two mitochondrial DNA (mtDNA) ATP synthase genes (ATPase 6 and 8), seven other mtDNA OXPHOS genes, and a number of nuclear genes. The synonymous substitution rate for ATPsyn beta proved to be 1.9 x 10(-9) substitutions per site per year (substitutions x site-1 x year-1) (SSY). This is less than 1/2 that of the average nDNA gene, 1/12 the rate of ATPase 6 and 8, and 1/17 the rate of the average mtDNA gene. The synonymous and replacement substitution rates were used to calculate a new parameter, the "selective constraint ratio". This revealed that even the most variable mtDNA protein was more constrained than the average nDNA protein. Thus, the high substitution mutation rate and strong selective constraints of mammalian mtDNA proteins suggest that mtDNA mutations may result in a disproportionately large number of human hereditary diseases of OXPHOS.

Human cytochrome c oxidase isoenzymes from heart and skeletal muscle; purification and properties

Human cytochrome c oxidase was isolated in an active form from heart and from skeletal muscle by a fast, small-scale isolation method. The procedure involves differential solubilisation of the oxidase from mitochondrial fragments by laurylmaltoside and KCl, followed by size-exclusion high-performance liquid chromatography. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate showed differences between the subunit VI region of cytochrome c oxidases from human heart and skeletal muscle, suggesting different isoenzyme forms in the two organs. This finding might be of importance in explaining mitochondrial myopathy which shows a deficiency of cytochrome c oxidase in skeletal muscle only. In SDS polyacrylamide gel electrophoresis most human cytochrome c oxidase subunits migrated differently from their bovine counterparts. However, the position of subunits III and IV was the same in the human and in the bovine enzymes. The much higher mobility of human cytochrome c oxidase subunit II is explained by a greater hydrophobicity of this polypeptide than of that of the subunit II of the bovine enzyme.

Cytochrome c oxidase deficiency in subacute necrotizing encephalomyelopathy

Two new patients with Leigh's syndrome (subacute necrotizing encephalomyelopathy) due to deficiency of cytochrome c oxidase are presented and their data are compared with those of the four Leigh's syndrome patients previously reported with this deficiency. It is not possible to distinguish between the various biochemical aetiologies of Leigh's syndrome on clinical grounds. Investigation of pyruvate metabolism and of the respiratory chain will reveal the enzymatic defect in some of the patients. It has now been firmly established that a relationship exists between Leigh's syndrome and deficiency of cytochrome c oxidase. There are, however, other syndromes which are also associated with a deficiency of this enzyme. In Leigh's syndrome, the enzyme deficiency has been reported in many organ systems and in cultured fibroblasts. In the liver, however, decreased, intermediate or normal values of cytochrome c oxidase activity have been found. Selective or more widespread involvement of organ systems, due to mutations of either the nuclear or the mitochondrial DNA encoding for different subunits of the enzyme molecule (some of which may be organ- or tissue-specific), could explain the clinical and biochemical heterogeneity of syndromes associated with a cytochrome c oxidase deficiency.

Fatal mitochondrial cardiomyopathy in Kearns-Sayre syndrome with deficiency of cytochrome-c-oxidase in cardiac and skeletal muscle. An enzymehistochemical--ultra-immunocytochemical--fine structural study in longterm frozen autopsy tissue

Morphological studies in a 26-year-old man with long-standing Kearns-Sayre syndrome, with cardiac arrhythmias and a fatal congestive cardiomyopathy, revealed a mitochondrial myopathy of both skeletal and myocardial muscle (Hübner et al. 1986). Histochemical investigation of cytochrome-c-oxidase showed multiple enzyme defects of both cardiac and skeletal muscle present in myocytes with normal and abnormal numbers of mitochondria demonstrated by ultracytochemistry. Immunohistochemical studies with antibodies against the holoenzyme and various subunits revealed that in the heart the enzyme defect affected both contractile and conductive fibres and was characterized by a severe reduction but not a complete loss of nuclear and mitochondrially coded immunoreactive enzyme protein. In skeletal muscle, however, where up to 30% of the fibres lacked enzyme activity, immunoreactivity was reduced only very occasionally. These results are most consistent with a defective enzyme assembly in the inner mitochondrial membrane and probably indicate heterogeneity of mitochondria, i.e. organ-specific pathological reaction patterns.

Fibronectin is synthesized as an acute phase reactant in rat hepatocytes

Elevated concentrations of fibronectin were found in plasma of rats under different acute phase conditions. Untreated animals showed a plasma fibronectin concentration of 150 +/- 50 mg/l, which increased to 412 +/- 59 mg/l 24 h after subcutaneous injection of turpentine. The time course of the changes in plasma fibronectin concentration showed a peak at 24 h and a decline to normal concentrations 72 h after turpentine treatment. Additional stimulation by dexamethasone resulted in plasma fibronectin concentrations of 661 +/- 49 mg/l. No or only slight elevations of fibronectin concentrations were observed after treatment with adrenaline, thyroxine and triiodothyronine as compared with saline-injected animals. The common identity of plasma fibronectin in controls, turpentine and turpentine-dexamethasone-treated animals was shown by slab gel electrophoresis under nonreducing conditions, followed by western blot and immunofluorescence staining. One dimensional immunoelectrophoresis performed with polyclonal antibodies to human fibronectin cross-reacting with rat fibronectin (shown by Ouchterlony gel diffusion) revealed identical precipitation lines for the plasma of control and acute phase animals. Hepatocytes of turpentine-pretreated rats show a threefold increase of [14C]valine incorporation into total protein and a fourfold increase of immunoreactive radioactively labeled fibronectin in the culture medium, compared with control hepatocyte cultures. These results point to the role of hepatocytes in the synthesis of plasma fibronectin, which behaves in rats as an acute phase reactant.

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.

Structure and function of cytochrome-c oxidase

Recent works on the structure and the function of cytochrome-c oxidase are reviewed. The subunit composition of the mitochondrial enzyme depends on the species and is comprised of between 5 and 13 subunits. It is reduced to 1 to 3 subunits in prokaryotes. The complete amino acid composition has been derived from protein sequencing. Gene sequences are partially known in several eukaryote species. Metal centers are only located in subunits I and II. The mitochondrial cytochrome-c oxidase is Y-shaped; the arms of the Y cross the inner membrane, the stalk protrudes into the intermembrane space. The bacterial enzyme has a simpler, elongated shape. A number of data have been accumulated on the subunit topology and on their location within the protein. All available spectrometric techniques have been used to investigate the environment of the metal centers as well as their interactions. From the literature, attention must be paid to what may be considered or not as an active form. The steady improvement of the instrumentation has yielded evidence for different kinds of heterogeneities which could reflect the in vivo situation. The 'pulsed' and 'resting' conformers have been well characterized. The 'oxygenated' form has been identified as a peroxide derivative of the fully oxidized cytochrome-c oxidase. The mammalian enzyme has been isolated in fully active monomeric form which does not preclude the initially suggested dimeric behavior in situ. The role of the lipids is still largely investigated, mainly through reconstitution experiments. Kinetic studies of electron transfer between cytochrome c and cytochrome-c oxidase lead to a single catalytic site model to account for the multiphasic kinetics. Results related to the low temperature investigation of the intermediate steps in the reaction between oxygen and cytochrome-c oxidase received a sound confirmation by the resolution of compound A at room temperature. It is also pointed out that the so-called mixed valence state might not be a transient state in the catalytic reduction of oxygen. The functioning of cytochrome-c oxidase as a proton pump has been supported by a number of experimental results. Subunit III would be involved in this process. The redox link to the proton pump has been suggested to be at the Fea-CuA site. The molecular mechanism responsible for the proton pumping is still unknown.

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.

Mitochondrial myopathies

Mitochondrial myopathies are clinically heterogeneous disorders that can affect multiple systems besides skeletal muscle (mitochondrial encephalomyopathies or cytopathies) and are usually defined by morphological abnormalities of muscle mitochondria. There are a few distinctive syndromes, such as the Kearns-Sayre syndrome; myoclonus epilepsy with ragged-red fibers; and mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes. Biochemically, mitochondrial myopathies can be divided into defects of substrate utilization, oxidation-phosphorylation coupling, and the respiratory chain. Because mitochondria have their own DNA and their own translation and transcription apparatuses, mitochondrial myopathies can be due to defects of either a nuclear or mitochondrial genome and can be transmitted by mendelian or maternal inheritance.

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.

Two nonidentical forms of subunit V are functional in yeast cytochrome c oxidase

In Saccharomyces cerevisiae, the inner mitochondrial membrane protein cytochrome c oxidase is composed of nine polypeptide subunits. Six of these subunits (IV, V, VI, VII, VIIa, VIII) are encoded by the nuclear genome, and the remaining three (I, II, III) are encoded by mitochondrial DNA. We report here the existence of two nonidentical subunit V polypeptides, which are encoded by separate genes within the yeast genome. One gene, COX5a, encodes the polypeptide Va, normally found in preparations of holocytochrome c oxidase. The other gene, COX5b, encodes the polypeptide Vb, which cross-reacts with anti-subunit Va antiserum and restores respiratory competency and cytochrome oxidase activity in transformants of cox5a structural gene mutants. This polypeptide also copurifies with the holoenzyme prepared from these transformants. We have found that COX5b is expressed in vegetatively growing yeast cells, and that the Vb polypeptide can be detected in mitochondria from strain JM28, a cox5a mutant. This mutant has 15%-20% residual cytochrome oxidase activity, and it respires at 10%-15% the wild-type rate. By disrupting the COX5b gene in this strain, we show that this residual activity is directly attributable to the presence of a chromosomal copy of the COX5b gene. Taken together, these results suggest that Va or Vb can function as cytochrome oxidase subunits in yeast and that Vb may be used under some specific, as yet undefined, physiological conditions.

Fatal infantile mitochondrial myopathy and renal dysfunction caused by cytochrome c oxidase deficiency: immunological studies in a new patient

A 3-month-old female infant had profound generalized weakness, de Toni-Fanconi-Debre syndrome, and lactic acidosis. She required assisted ventilation and died at the age of 8 months. Muscle biopsy showed accumulation of mitochondria, glycogen, and lipid droplets. Histochemical reaction and immunocytochemical stain for cytochrome c oxidase showed very weak results, but both reactions were normal in intrafusal fibers of the muscle spindle. In crude extracts of the patient's muscle, cytochrome c oxidase activity was undetectable and enzyme-linked immunosorbent assay showed decreased reaction at all dilutions of antiserum. These data indicate that the amount of immunoreactive enzyme protein is markedly decreased in muscle of patients with fatal infantile cytochrome c oxidase deficiency and renal dysfunction.

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.

Progesterone dependent uptake of uteroglobin by rabbit endometrium

Unlabeled or 3H-labeled UGL, isolated from rabbit lungs, was injected intraluminally into the uteri of ovariectomized low dose progesterone substituted and unsubstituted animals as well as into mated animals on the fifth day of pregnancy. It was offered to the endometrium, in this last case, in competition with endogenously synthesized UGL. 3H-labeled IgG served as a control. The animals were killed 24 h after the experiment. Immunohistograms and autoradiograms showed that UGL was taken up by the whole endometrium of the ovariectomized animals but only when progesterone was present. In the preimplantation uterus, uptake was limited to clusters of smaller cells accumulated in the lumen adjacent to the epithelium, whereas IgG remained within the uterine lumen, forming a dense layer on the endometrial surface.

Interactions in cytochrome oxidase: functions and structure

Mitochondrial cytochrome c oxidase is an exceedingly complex multistructural and multifunctional membranous enzyme. In this review, we will provide an overview of the many interactions of cytochrome oxidase, stressing developments not covered by the excellent monograph of Wikström, Krab, and Saraste (1981), and continuing into early 1983. First we describe its functions (both in the nominal sense, as a transporter of electrons between cytochrome c and oxygen, and in its role in energy transduction). Then we describe its structure, emphasizing the protein (its structure as a whole, the number and stoichiometry of its subunits, their biosynthetic origin, and their interactions with each other, with other components of the enzyme complex, and with the membrane as a whole). Finally, we present a model in which the protein conformation serves as the focus for the dynamic interaction of its two major functions.

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.