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

2',3' cyclic nucleotide 3' phosphodiesterase 1 functional isoform antagonizes HIV-1 particle assembly

IFN-stimulated gene 2',3' cyclic nucleotide 3' phosphodiesterase (CNP) comprises two isoforms: the short CNP1 and the long CNP2, featuring an additional N-terminal segment of 20 amino acids (N20aa) proposed as a mitochondrial targeting sequence. Notably, CNP1 can be produced by cleaving the N20aa segment from CNP2. Although previous investigations have recognized the HIV-1 particle assembly impairment capability of CNP2, the antiviral activity of CNP1 remains ambiguous. Our study clarifies that CNP1, as opposed to CNP2, serves as the primary isoform exerting anti-HIV-1 activity. Both CNP1 and CNP2 can localize to the cell membrane, but the N20aa segment of CNP2 impedes CNP2-HIV-1 Gag interaction. Cleavage of the N20aa segment from CNP2 results in the formation of a functional, truncated form known as CNP1. Intriguingly, this posttranslational processing of CNP2 N20aa occurs within the cytoplasmic matrix rather than the mitochondria. Regulated by CTII motif prenylation, CNP1 proteins translocate to the cell membrane and engage with HIV-1 Gag. In conclusion, our findings underscore the pivotal role of posttranslational modification in governing the inhibitory potential of CNP in HIV-1 replication.

Cholinergic Receptor Muscarinic 1 Co-Localized with Mitochondria in Cultured Dorsal Root Ganglion Neurons, and Its Deletion Disrupted Mitochondrial Ultrastructure in Peripheral Neurons: Implications in Alzheimer's Disease

Background

Loss of Cholinergic Receptor Muscarinic 1 (CHRM1) has been linked to the pathogenesis of Alzheimer's disease (AD). Our recent study found significantly lower CHRM1 protein levels in AD patient cortices, linked to reduced survival. Furthermore, using knockout mice (Chrm1-/-) we demonstrated that deletion of Chrm1 alters cortical mitochondrial structure and function, directly establishing a connection between its loss and mitochondrial dysfunction in the context of AD. While CHRM1's role in the brain has been extensively investigated, its impact on peripheral neurons in AD remains a crucial area of research, especially considering reported declines in peripheral nerve conduction among AD patients.

Objective

The objective was to characterize Chrm1 localization and mitochondrial deficits in Chrm1-/- dorsal root ganglion (DRG) neurons.

Methods

Recombinant proteins tagged with Green or Red Fluorescent Protein (GFP/RFP) were transiently expressed to investigate the localization of Chrm1 and mitochondria, as well as mitochondrial movement in the neurites of cultured primary mouse DRG neurons, using confocal time-lapse live cell imaging. Transmission electron microscopy was performed to examine the ultrastructure of mitochondria in both wild-type and Chrm1-/- DRGs.

Results

Fluorescence imaging revealed colocalization and comigration of N-terminal GFP-tagged Chrm1 and mitochondrial localization signal peptide-tagged RFP-labelled mitochondria in the DRGs neurons. A spectrum of mitochondrial structural abnormalities, including disruption and loss of cristae was observed in 87% neurons in Chrm1-/- DRGs.

Conclusions

This study suggests that Chrm1 may be localized in the neuronal mitochondria and loss of Chrm1 in peripheral neurons causes sever mitochondrial structural aberrations resembling AD pathology.

Role of Mitochondrial Protein Import in Age-Related Neurodegenerative and Cardiovascular Diseases

Mitochondria play a critical role in providing energy, maintaining cellular metabolism, and regulating cell survival and death. To carry out these crucial functions, mitochondria employ more than 1500 proteins, distributed between two membranes and two aqueous compartments. An extensive network of dedicated proteins is engaged in importing and sorting these nuclear-encoded proteins into their designated mitochondrial compartments. Defects in this fundamental system are related to a variety of pathologies, particularly engaging the most energy-demanding tissues. In this review, we summarize the state-of-the-art knowledge about the mitochondrial protein import machinery and describe the known interrelation of its failure with age-related neurodegenerative and cardiovascular diseases.

Muscarinic Acetylcholine Type 1 Receptor Activity Constrains Neurite Outgrowth by Inhibiting Microtubule Polymerization and Mitochondrial Trafficking in Adult Sensory Neurons

The muscarinic acetylcholine type 1 receptor (M₁R) is a metabotropic G protein-coupled receptor. Knockout of M₁R or exposure to selective or specific receptor antagonists elevates neurite outgrowth in adult sensory neurons and is therapeutic in diverse models of peripheral neuropathy. We tested the hypothesis that endogenous M₁R activation constrained neurite outgrowth via a negative impact on the cytoskeleton and subsequent mitochondrial trafficking. We overexpressed M₁R in primary cultures of adult rat sensory neurons and cell lines and studied the physiological and molecular consequences related to regulation of cytoskeletal/mitochondrial dynamics and neurite outgrowth. In adult primary neurons, overexpression of M₁R caused disruption of the tubulin, but not actin, cytoskeleton and significantly reduced neurite outgrowth. Over-expression of a M₁R-DREADD mutant comparatively increased neurite outgrowth suggesting that acetylcholine released from cultured neurons interacts with M₁R to suppress neurite outgrowth. M₁R-dependent constraint on neurite outgrowth was removed by selective (pirenzepine) or specific (muscarinic toxin 7) M₁R antagonists. M₁R-dependent disruption of the cytoskeleton also diminished mitochondrial abundance and trafficking in distal neurites, a disorder that was also rescued by pirenzepine or muscarinic toxin 7. M₁R activation modulated cytoskeletal dynamics through activation of the G protein (Gα13) that inhibited tubulin polymerization and thus reduced neurite outgrowth. Our study provides a novel mechanism of M₁R control of Gα13 protein-dependent modulation of the tubulin cytoskeleton, mitochondrial trafficking and neurite outgrowth in axons of adult sensory neurons. This novel pathway could be harnessed to treat dying-back neuropathies since anti-muscarinic drugs are currently utilized for other clinical conditions.

ClickIn: a flexible protocol for quantifying mitochondrial uptake of nucleobase derivatives

There is an increasing interest in targeting molecules to the mitochondrial matrix. Many proteins are naturally imported through the translocase complexes found in the outer and inner mitochondrial membranes. One possible means for importing molecules is therefore to use a mitochondrial pre-protein as a vector and assess what forms of molecules can be attached to the pre-protein as cargo. A major difficulty with this approach is to ensure that any chimaeric molecule does indeed access the mitochondrial matrix and does not merely associate with the mitochondrial membranes. We have recently demonstrated that click chemistry can be used both to demonstrate convincingly mitochondrial import of a peptide-peptide nucleic acid conjugate and also to quantify the mitochondrial uptake for specific synthetic conjugates. We now report an adaptation of the synthesis to facilitate simple quantification of multiple molecules and hence to calculate the efficiency of their mitochondrial import.

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.

Unfolding-resistant translocase targeting: a novel mechanism for outer mitochondrial membrane localization exemplified by the Bbeta2 regulatory subunit of protein phosphatase 2A

Heterotrimeric serine/threonine protein phosphatase 2A (PP2A) consists of scaffolding (A), catalytic (C), and variable (B, B', and B'') subunits. Variable subunits dictate subcellular localization and substrate specificity of the PP2A holoenzyme. The Bbeta regulatory subunit gene is mutated in spinocerebellar ataxia type 12, and one of its splice variants, Bbeta2, targets PP2A to mitochondria to promote apoptosis in PC12 cells (Dagda, R. K., Zaucha, J. A., Wadzinski, B. E., and Strack, S. (2003) J. Biol. Chem. 278, 24976-24985). Here, we report that Bbeta2 is localized to the outer mitochondrial membrane by a novel mechanism, combining a cryptic mitochondrial import signal with a structural arrest domain. Scanning mutagenesis demonstrates that basic and hydrophobic residues mediate mitochondrial association and the proapoptotic activity of Bbeta2. When fused to green fluorescent protein, the N terminus of Bbeta2 acts as a cleavable mitochondrial import signal. Surprisingly, full-length Bbeta2 is not detectably cleaved and is retained at the outer mitochondrial membrane, even though it interacts with the TOM22 import receptor, as shown by luciferase complementation in intact cells. Mutations that open the C-terminal beta-propeller of Bbeta2 facilitate mitochondrial import, indicating that this rigid fold acts as a stop-transfer domain by resisting the partial unfolding step prerequisite for matrix translocation. Because hybrids of prototypical import and beta-propeller domains recapitulate this behavior, we predict the existence of other similarly localized proteins and a selection against highly stable protein folds in the mitochondrial matrix. This unfolding-resistant targeting to the mitochondrial translocase is necessary but not sufficient for the proapoptotic activity of Bbeta2, which also requires association with the rest of the PP2A holoenzyme.

Adaptive evolution of cytochrome c oxidase subunit VIII in anthropoid primates

Cytochrome c oxidase (COX) is a 13-subunit protein complex that catalyzes the last step in mitochondrial electron transfer in mammals. Of the 10 subunits encoded by nuclear DNA (three are mtDNA products), some are expressed as tissue- and/or development-specific isoforms. For COX subunit VIII, previous work showed that expression of the contractile muscle-specific isoform gene, COX8H, is absent in humans and Old World monkeys, and the other isoform gene, COX8L, is expressed ubiquitously. Here, we show that COX8H is transcribed in most primate clades, but its expression is absent in catarrhines, that is, in Old World monkeys and hominids (apes, including humans), having become a pseudogene in the stem of the catarrhines. The ubiquitously expressed isoform, COX8L, underwent nonsynonymous rate acceleration and elevation in the ratio of nonsynonymous/synonymous changes in the stem of anthropoid primates (New World monkeys and catarrhines), possibly setting the stage for loss of the heart-type (H) isoform. The most rapidly evolving region of VIII-L is one that interacts with COX I, suggesting that the changes are functionally coadaptive. Because accelerated rates of nonsynonymous substitutions in anthropoids such as observed for COX8L are also shown by genes for at least 13 other electron transport chain components, these encoded amino acid replacements may be viewed as part of a series of coadaptive changes that optimized the anthropoid biochemical machinery for aerobic energy metabolism. We argue that these changes were linked to the evolution of an expanded neocortex in anthropoid primates.

Amino acid replacement is rapid in primates for the mature polypeptides of COX subunits, but not for their targeting presequences

We examined inferred amino acid replacements for 16 genes that encode the proteins of the cytochrome c oxidase (COX) holoenzyme in eight vertebrate species. Phylogeny-based analysis revealed that the human lineage (primates) has had an unusually large, statistically significant, number of amino acid replacements in the mature protein coding region of these genes. This finding is similar to earlier observations of an accelerated non-synonymous substitution rate for some lineages of primates for COX1, COX2, COX4, and COX7AH. In contrast, the mitochondrial targeting presequences of these same proteins have not undergone a concomitant rate change. This more comprehensive analysis suggests that COX5A, COX6B, COX6C, COX7C, and COX8L have also undergone an acceleration in amino acid replacement rates in anthropoid primates. Some of these rate accelerations (e.g. in COX5A and COX7C) are so pronounced that non-human mammalian sequences are more similar to sequences from Xenopus or zebrafish than they are to human. Since the functions of the targeting and mature proteins of these polypeptides are different, the mature portions of these genes are likely to have undergone a functionally significant change that is adaptive in nature.

Human APE2 protein is mostly localized in the nuclei and to some extent in the mitochondria, while nuclear APE2 is partly associated with proliferating cell nuclear antigen

In human cells APE1 is the major AP endonuclease and it has been reported to have no functional mitochondrial targeting sequence (MTS). We found that APE2 protein possesses a putative MTS. When its N-terminal 15 amino acid residues were fused to the N-terminus of green fluorescent protein and transiently expressed in HeLa cells the fusion protein was localized in the mitochondria. By electron microscopic immunocytochemistry we detected authentic APE2 protein in mitochondria from HeLa cells. Western blotting of the subcellular fraction of HeLa cells revealed most of the APE2 protein to be localized in the nuclei. We found a putative proliferating cell nuclear antigen (PCNA)-binding motif in the C-terminal region of APE2 and showed this motif to be functional by immunoprecipitation and in vitro pull-down binding assays. Laser scanning immunofluorescence microscopy of HeLa cells demonstrated both APE2 and PCNA to form foci in the nucleus and also to be co-localized in some of the foci. The incubation of HeLa cells in HAT medium containing deoxyuridine significantly increased the number of foci in which both molecules were co-localized. Our results suggest that APE2 participates in both nuclear and mitochondrial BER and also that nuclear APE2 functions in the PCNA-dependent BER pathway.

Evolution of nuclear- and mitochondrial-encoded subunit interaction in cytochrome c oxidase

Mitochondrial DNA (mtDNA)-encoded proteins function in eukaryotes as subunits of respiratory complexes that also contain nuclear DNA (nDNA)-encoded subunits. The importance of functional interactions between mtDNA- and nDNA-encoded proteins was previously demonstrated by testing the survivability of cybrid cells or individuals containing nDNA and mtDNA from different populations or species. This report focuses on the multisubunit respiratory complex cytochrome c oxidase (COX), made up of both mtDNA-encoded and nDNA-encoded subunits. A combination of evolutionary and crystallographic data is employed to determine whether rates of nonsynonymous substitutions have been higher, the same, or lower for residues in close proximity that are encoded by a different genome (nDNA or mtDNA). This determination is performed by simply taking the ratio, called the interaction ratio i, of the nonsynonymous substitution rate of the close-contact residues to the nonsynonymous substitution rate of the noncontact residues. We assume that the close-contact residues (which are more likely to interact) are functionally important and that, therefore, amino acid replacements among these residues cannot escape the scrutiny of natural selection. i = 1 indicates that the close-contact residues have been under neither greater purifying selection nor greater positive selection than the noncontact residues as a specific consequence of their being encoded by separate genomes. i < 1 indicates that the close-contact residues have been under greater purifying selection but less positive selection than have the noncontact residues. Conversely, i > 1 indicates that the close-contact residues have been under less purifying but greater positive selection than have the noncontact residues. i < 1 may be referred to as a constraining interaction; i.e., the close-contact residues compared with the noncontact residues appear to be under greater structural-functional constraints. On the other hand, i > 1 may be referred to as an optimizing interaction; i.e., apparently many different amino acid replacements are required to optimize this subunit's interaction with the other subunit. A major finding is that the nDNA-encoded residues in close physical proximity to mtDNA-encoded residues evolve more slowly than the other nuclear-encoded residues (and thus display a constraining interaction), whereas the mtDNA-encoded residues in close physical proximity to nDNA-encoded residues evolve more rapidly than the other mitochondrial-encoded residues (and thus display an optimizing interaction). A possible reason for this striking difference between the nuclear- and mitochondrial-encoded COX subunits in how their functional interaction evolves is discussed.

Molecular evolution of cytochrome c oxidase subunit I in primates: is there coevolution between mitochondrial and nuclear genomes?

Phylogenetic analyses carried out on cytochrome c oxidase (COX) subunit I mitochondrial genes from 14 primates representing the major branches of the order and four outgroup nonprimate eutherians revealed that transversions and amino acid replacements (i.e., the more slowly occurring sequence changes) contained lower levels of homoplasy and thus provided more accurate information on cladistic relationships than transitions (i.e., the more rapidly occurring sequence changes). Several amino acids, each with a high likelihood of functionality involving the binding of cytochrome c or interaction with COX VIII, have changed in Anthropoidea, the primate suborder grouping New World monkey, Old World monkey, ape, and human lineages. They are conserved in other mammalian lineages and in nonanthropoid primates. Maximum-likelihood ancestral COX I nucleotide sequences were determined utilizing a near most parsimonious branching arrangement for the primate sequences that was consistent with previously hypothesized primate cladistic relationships based on larger and more diverse data sets. Relative rate tests of COX I mitochondrial sequences showed an elevated nonsynonymous (N) substitution rate for anthropoid-nonanthropoid comparisons. This finding for the largest mitochondrial (mt) DNA-encoded subunit is consistent with previous observations of elevated nonsynonymous substitution/synonymous substitution (S) rates in primates for mt-encoded COX II and for the nuclear-encoded COX IV and COX VIIa-H. Other COX-related proteins, including cytochrome c and cytochrome b, also show elevated amino acid replacement rates or N/S during similar time frames, suggesting that this group of interacting genes is likely to have coevolved during primate evolution.

Turkey cytochrome c oxidase contains subunit VIa of the liver type associated with low efficiency of energy transduction

Cytochrome c oxidase was isolated from turkey liver, heart and breast skeletal muscle and separated by SDS/PAGE. The N-terminal amino-acid sequence of subunit VIa from all tissues and internal sequences from the skeletal muscle enzyme show homology to the mammalian liver-type subunit VIaL, which was verified by isolation and sequencing of the cDNA of turkey subunit VIa. No cDNA corresponding to subunit VIaH (mammalian heart-type) could be found by RACE-PCR with mRNA from all turkey tissues. Measurement of proton translocation with the reconstituted enzymes from turkey liver and heart revealed H+/e- ratios below 0.5 that were independent of the intraliposomal ATP/ADP ratio, as previously found with the bovine liver enzyme. Under identical conditions, the bovine heart enzyme revealed H+/e- ratios of 0.85 at low and 0.48 at high intraliposomal ATP/ADP ratios. The results suggest that in birds the lower H+/e-ratio of cytochrome c oxidase participates in elevated resting metabolic rate and thermogenesis.

Tissue-specific expression and mapping of the Cox7ah gene in mouse

We have isolated and examined the gene for the heart isoform of cytochrome c oxidase subunit VIIa (COX VIIa-H) in mouse, an isoform gene previously thought to be lacking in rodents. Interspecies amino acid comparisons indicate that mouse COX VIIa-H protein displays 82.5 and 70.9% identity with the bovine and human heart isoforms of COX VIIa, but only 53.7% identity with the paralogous mouse liver isoform (COX VIIa-L). Expression in adult mouse tissues is limited to heart and skeletal muscle, as found in other species. In the early mouse embryo, Cox7al was the exclusive isoform expressed and Cox7ah mRNA was not detectable until day 17 postcoitum. That the mouse Cox7ah gene characterized in this study is orthologous to the human COX7AH gene was also suggested by its mapping to mouse chromosome 7, to a conserved region syntenic with the human chromosome location of COX7AH, 19q13.1. As a result, all three COX heart isoform genes in mouse group to chromosome 7. Interestingly, mapping of the mouse Cox7al to chromosome 9 suggests a new syntenic region between the mouse and the human genomes.

Influence of thyroid hormone on the tissue-specific expression of cytochrome c oxidase isoforms during cardiac development

In mammals, cytochrome c oxidase (COX) is composed of 13 different protein subunits. In the rat, two nuclear-encoded subunits, COX VIa and VIII, exist as tissue-specific isoforms: heart and liver. Using Northern-blot analysis, the levels of transcripts for the heart and liver isoforms of VIa and VIII were examined in developing rat hearts. The liver isoform was found to be the predominant form of subunit VIa and the exclusive form of VIII in the 18-day fetal hearts. The mRNA levels of the heart isoform of both subunits increased dramatically to reach adult levels by 14 days. Although the levels of the VIa- and VIII-liver isoform mRNAs remained stable throughout early development, their levels decreased by 40 and 36% respectively between the 18-day fetal stage and 18-day neonatal stage. Therefore the up-regulation of the heart isoforms and down-regulation of the liver isoforms appear to be regulated in a co-ordinated manner during development. To determine if thyroid hormone influences the expression of these developmentally regulated isoforms, the RNA was also extracted from the hearts of 2-week-old hypothyroid rats. The results showed that the levels of VIII-heart and VIa-liver COX mRNAs were approx. 40% lower in the hypothyroid hearts, while VIII-liver and VIa-heart COX isoform expression remained unchanged. These data demonstrate that the isoforms of COX subunits VIa and VIII are not co-ordinately regulated by changes in thyroid hormone levels. Therefore we conclude that, although thyroid hormone influences the expression of isoforms, it appears to do so via a different mechanism from that which regulates the developmental transition.

Purification and characterization of cytochrome c oxidase from the insect trypanosomatid Crithidia fasciculata

Cytochrome c oxidase was purified from the mitochondrial lysate of the insect trypanosomatid Crithidia fasciculata with the aid of a methyl hydrophobic interaction column in a rapid one-step procedure. The purified complex displayed all characteristics expected from a eukaryotic cytochrome c oxidase: the presence of CuA in electron paramagnetic resonance analysis, a characteristic 605 nm peak in reduced-minus-oxidized optical spectroscopy, and the capacity to efficiently oxidize homologous, but not heterologous, cytochrome c. Two-dimensional PAGE showed that C. fasciculata cytochrome c oxidase consists of at least 10 different subunits. N-terminal sequences were obtained from the six smallest subunits of the complex, one of them showing significant similarity to Neurospora crassa cytochrome c oxidase subunit V. The N-terminus of each of the four largest subunits was found to be blocked.

Structural characterization and regulatory element analysis of the heart isoform of cytochrome c oxidase VIa

In order to investigate the mechanism(s) governing the striated muscle-specific expression of cytochrome c oxidase VIaH we have characterized the murine gene and analyzed its transcriptional regulatory elements in skeletal myogenic cell lines. The gene is single copy, spans 689 base pairs (bp), and is comprised of three exons. The 5'-ends of transcripts from the gene are heterogeneous, but the most abundant transcript includes a 5'-untranslated region of 30 nucleotides. When fused to the luciferase reporter gene, the 3.5-kilobase 5'-flanking region of the gene directed the expression of the heterologous protein selectively in differentiated Sol8 cells and transgenic mice, recapitulating the pattern of expression of the endogenous gene. Deletion analysis identified a 300-bp fragment sufficient to direct the myotube-specific expression of luciferase in Sol8 cells. The region lacks an apparent TATA element, and sequence motifs predicted to bind NRF-1, NRF-2, ox-box, or PPAR factors known to regulate other nuclear genes encoding mitochondrial proteins are not evident. Mutational analysis, however, identified two cis-elements necessary for the high level expression of the reporter protein: a MEF2 consensus element at -90 to -81 bp and an E-box element at -147 to -142 bp. Additional E-box motifs at closely located positions were mutated without loss of transcriptional activity. The dependence of transcriptional activation of cytochrome c oxidase VIaH on cis-elements similar to those found in contractile protein genes suggests that the striated muscle-specific expression is coregulated by mechanisms that control the lineage-specific expression of several contractile and cytosolic proteins.

Regulation of mitochondrial energy generation in health and disease

In mammalian cytochrome c oxidase (COX) three of the ten nuclear coded subunits (VIa, VIIa, VIII) occur in tissue-specific isoforms. The isoform distribution, however, varies in liver and heart of different species. Subunit VIII is different in liver and heart of bovine, dog, rat and chicken, but identical in human (liver-type) on one hand, and sheep, rabbit and rainbow trout (heart-type) on the other hand, as determined by N-terminal sequencing. Two moles of trinitrophenyl-ATP bind to monomeric COX from bovine heart and one to COX from bovine liver with dissociation equilibrium constant (Kd) values of about 3 microM. One binding site at the heart enzyme is blocked by a monoclonal antibody to subunit VIa-H. ATP (and/or ADP) interact with COX at two or three high-affinity binding sites, as shown by titration of the spectral changes of COX. Isolated COX from bovine heart was reconstituted with variable intraliposomal ATP/ADP ratios. By measuring the RCR (respiratory control ratio) and RCRVal (related to the valinomycin-respiration), which is a direct measure of the H+/e(-)-stoichiometry (Wilson and Prochaska, Arch. Biochem. Biophys. 282 (1990) 413-420), almost complete inhibition of the proton pump activity of COX by high intraliposomal ATP concentrations was found. The vectorial of protons for the formation of water, however, appears to be unaffected by nucleotides. This regulatory mechanism is assumed to have physiological significance for thermogenesis in muscle at rest. COX of fibroblasts from patients suffering from Leigh's syndrome, which is associated with a decreased COX activity, are suggested to have an incompletely assembled enzyme complex. This suggestion is further corroborated by the higher temperature-sensitivity of the enzyme when compared with COX from normal control fibroblasts. Defective regulation of COX via nuclear coded subunits is also proposed to cause mitochondrial diseases.

Sequence of the cDNA for the heart/muscle isoform of mouse cytochrome c oxidase subunit VIII

We have isolated and sequenced cDNAs for the heart/muscle (H) isoform of mouse cytochrome c oxidase subunit VIII (COX VIII-H). The deduced protein sequence enables us to compare the heart/muscle COX VIII isoforms from several species and to determine that the most highly conserved region of this subunit is the C-terminal domain.

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.

The tissue-specific RNA-binding protein COLBP is differentially regulated during myogenesis

Cytochrome c oxidase L-form transcript-binding protein (COLBP) activity parallels the tissue-specific mRNA expression of bovine cytochrome c oxidase liver isopeptides. A similar RNA-binding activity is found in human myoblast and liver Hep G2 cell homogenates. Human COLBP activity, however, is not present in myotubes or adult skeletal muscle. It is proposed that COLBP down-regulation during muscle cell differentiation may underlie oxidase isoform switching during myogenesis.

Structural organization and evolution of the liver isoform gene for bovine cytochrome c oxidase subunit VIIa

Subunit VIIa of mammalian cytochrome c oxidase is one of three nuclear-encoded subunits that exhibit isoforms, existing predominantly as an H-form in cardiac and skeletal muscle tissues and as an L-form in others. We have isolated and characterized the L-isoform gene (COX7aL). It is 5.4 kb long, consists of four exons, and is located at a CpG island. Sp1 sites and an NRF1 site are located in an approximately 100-bp region immediately upstream of the gene. Comparison of the sequence and organization with the previously described H-isoform gene shows identical intron-exon organizations, with the first intron of both isoform genes splitting the presequence coding region almost identically. These results suggest that the isoform genes arose by duplication from a common ancestor prior to the mammalian radiation and that the ancestor already contained the presequences. In addition, four processed pseudogenes of the L-type have been isolated and characterized, one of which (COX7aLP1) contains no deletions, insertions, or frame-shifts and can encode a precursor protein of 83 amino acids. Construction of a phylogenetic tree employing extant COX7aL cDNA and bovine pseudogene sequences suggests that the expressed bovine gene and COX7aLP1 arose from a gene duplication event 4.6-6.8 Mya.

Tissue distribution of cytochrome c oxidase isoforms in mammals. Characterization with monoclonal and polyclonal antibodies

Monoclonal and polyclonal antibodies specific to the two isoforms of subunit VIa of bovine cytochrome c oxidase were generated and used to study the tissue distribution of this subunit pair in beef, human and rat. The so-called H-(heart) form was found exclusively in heart and skeletal muscle, whereas the so-called L-(liver) form was the only isoform present in brain, kidney, liver and smooth muscle. Little or no L-form was detected in skeletal muscle. In bovine heart no subunit VIa-L was detected, while in human heart the subunit VIa-H and VIa-L isoforms were present in roughly equal proportions. These results imply that, in humans, the deficiency of a subunit VIa isoform may have a different effect on the physiology of heart then on the physiology of skeletal muscle.

Expression of human cytochrome c oxidase subunits during fetal development

Expression of human cytochrome c oxidase (COX) subunits was examined at fetal (20-28 weeks) and adult state by Northern blot hybridization with mRNA from liver, heart, skeletal muscle, and intestine. The data were related to COX and citrate synthase activities and to immunodetected COX subunits (II/III, IV, VIIaH). In liver little changes of COX transcripts are observed from fetal to adult state. In contrast, in heart and skeletal muscle all transcripts of COX subunits increase between 2-20-fold, when related to the amount of 28S rRNA. In fetal heart and skeletal muscle the relative amounts of the liver-type transcripts of subunit VIa were 30% and 25% from total VIa transcripts (VIaL+VIaH), respectively, but decrease to only 2-5% at adult state. The liver-type transcripts of subunit VIIa occur to 50% in fetal heart and skeletal muscle, which remained unchanged in adult heart and decrease to 5-8% in adult skeletal muscle. The results clearly indicate a switch of gene expression in heart and skeletal muscle during development, from the liver type to the heart/muscle type of subunit VIa (and partly VIIa).

Nucleotide sequence of cDNA encoding subunit VIII of cytochrome c oxidase from rat heart

In this study, the complete cDNA of subunit VIII-h of rat cytochrome c oxidase is presented. A rat skeletal muscle cDNA library was screened with a 132 bp fragment of the cDNA of rat COX subunit VIII-h. Four positive clones were sequenced in both directions.

Rapid evolution of the human gene for cytochrome c oxidase subunit IV

We have compared the DNA sequences of nine mammalian genes for cytochrome c oxidase subunit IV (COX4 genes)--four expressed genes (human, bovine, rat, and mouse) and five pseudogenes (human, chimpanzee, orangutan, squirrel monkey, and bovine)--and constructed the sequence of the ancestral mammalian COX4 gene. By analyzing these sequences to determine the pattern and rate of nucleotide substitution in each branch of the evolutionary tree, we deduced that the human gene has evolved rapidly since the origin of the primate pseudogene approximately 41 million years ago, and we discuss the suggestion that this results from coevolution of nuclear and mitochondrial genes for cytochrome c oxidase.

Steady-state transcript levels of cytochrome c oxidase genes during human myogenesis indicate subunit switching of subunit VIa and co-expression of subunit VIIa isoforms

Steady-state levels of the mitochondrial rRNAs, of mRNAs for mitochondrially and nuclear-encoded subunits of cytochrome c oxidase and for the beta subunit of ATP synthase were assessed by Northern blot hybridizations during the in vitro differentiation of human myoblasts. Transcript levels of the so-called liver-type form of subunit VIa of cytochrome c oxidase diminished during the course of differentiation, while transcription of the so-called heart-type form was induced. Transcripts for the liver-type form and for the heart-type form of subunit VIIa of cytochrome c oxidase were detected in all myogenic cultures; the levels of the heart-type form progressively increased during the course of differentiation. The levels of the other transcripts studied did not change substantially. The results suggest subunit switching of subunit VIa and co-expression of subunit VIIa isoforms during myogenesis. The differential changes in mRNA levels of the heart-type subunits VIa and VIIa and the differential changes in mRNA levels of the liver-type subunits VIa and VIIa demonstrate that different transcriptional regulation mechanisms are present for both heart-type genes as well as for both liver-type genes.

Presteady-state and steady-state kinetic properties of human cytochrome c oxidase. Identification of rate-limiting steps in mammalian cytochrome c oxidase

Human cytochrome c oxidase was purified in a fully active form from heart and skeletal muscle. The enzyme was selectively solubilised with octylglucoside and KCl from submitochondrial particles followed by ammonium sulphate fractionation. The presteady-state and steady-state kinetic properties of the human cytochrome c oxidase preparations with either human cytochrome c or horse cytochrome c were studied spectrophotometrically and compared with those of bovine heart cytochrome c oxidase. The interaction between human cytochrome c and human cytochrome c oxidase proved to be highly specific. It is proposed that for efficient electron transfer to occur, a conformational change in the complex is required, thereby shifting the initially unfavourable redox equilibrium. The very slow presteady-state reaction between human cytochrome c oxidase and horse cytochrome c suggests that, in this case, the conformational change does not occur. The proposed model was also used to explain the steady-state kinetic parameters under various conditions. At high ionic strength (I = 200 mM, pH 7.4), the kcat was highly dependent on the type of oxidase and it is proposed that the internal electron transfer is the rate-limiting step. The kcat value of the 'high-affinity' phase, observed at low ionic strength (I = 18 mM, pH 7.4), was determined by the cytochrome c/cytochrome c oxidase combination applied, whereas the Km was highly dependent only on the type of cytochrome c used. Our results suggest that, depending on the cytochrome c/cytochrome c oxidase combination, either the dissociation of ferricytochrome c or the internal electron transfer is the rate-limiting step in the 'high-affinity' phase at low ionic strength. The 'low-affinity' kcat value was not only determined by the type of oxidase used, but also by the type of cytochrome c. It is proposed that the internal electron-transfer rate of the 'low-affinity' reaction is enhanced by the binding of a second molecule of cytochrome c.

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.

Subunits VIIa,b,c of human cytochrome c oxidase. Identification of both 'heart-type' and 'liver-type' isoforms of subunit VIIa in human heart

The N-terminal amino acid sequences and the electrophoretic mobilities of the subunits VIIa, VIIb and VIIc of cytochrome c oxidase purified from human heart were investigated and compared with those from human skeletal muscle and from bovine heart. In purified human heart cytochrome c oxidase, both so-called 'heart-type' and 'liver-type' isoforms of subunit VIIa were found. The first 30 residues of the N-terminal amino acid sequences of these 'heart-type' and 'liver-type' subunits VIIa showed nine differences. The two isoforms of subunit VIIa in human heart were present in almost equal amounts, in contrast to the situation in skeletal muscle, where the 'heart-type' subunit VIIa was predominant. Therefore, our results imply that in human heart a cytochrome c oxidase isoform pattern is present that differs from that found in skeletal muscle. Subunits VIIb and VIIc purified from human heart oxidase proved to be very similar to their bovine heart counterparts. Our direct demonstration of the presence of subunit VIIb, the sequence of which has only recently been identified in the bovine heart enzyme, suggests that human cytochrome c oxidase also contains 13 subunits. We found no evidence for the presence of different isoforms of subunit VIIc in cytochrome c oxidase from human heart and skeletal muscle. We observed clear differences in the electrophoretic mobility of the subunits VIIa,b,c between bovine and human cytochrome c oxidase. On Tricine/glycerol/SDS/polyacrylamide gels the 'heart-type' and 'liver-type' subunits VIIa present in human heart cytochrome c oxidase migrated with almost the same electrophoretic mobility. Subunit VIIb migrated only slightly faster than subunit VIIa, whereas VIIc proved to have the highest electrophoretic mobility on Tricine/SDS/glycerol/polyacrylamide gels. Our findings may have implications for the elucidation of certain tissue-specific cytochrome c oxidase deficiencies in man.

Switching of bovine cytochrome c oxidase subunit VIa isoforms in skeletal muscle during development

A cDNA encoding the liver isoform of bovine cytochrome c oxidase subunit VIa (VIaL) was cloned from bovine liver RNA by reverse transcription and the polymerase chain reaction. The nucleotide and deduced amino acid sequences show high conservation with the corresponding rat and human liver subunits. The sequence similarity between beef heart and beef liver VIa is 60%. Northern analyses of the steady-state levels of the VIa-heart (VIaH) and VIa-liver (VIaL) transcripts showed that adult liver and brain contained only VIaL transcripts, the VIaH transcript predominated in heart with a small amount of VIaL also present, while in adult skeletal muscle VIaH was present exclusively. The VIaL transcript was found in heart with a small amount of VIaL also present, while in adult skeletal muscle VIaH was present exclusively. The VIaL transcript was found in fetal heart and skeletal muscle from 104-215-day-old fetuses, in as much as 25% of the amount of VIaH transcript. The down-regulation of VIaL transcript in skeletal muscle at or close to birth may be correlated with a change in amount of cytochrome c oxidase relative to the bc1 complex (complex III) observed spectrally when fetal and adult muscle samples were compared.

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.

Human cytochrome c oxidase subunit VIb: characterization and mapping of a multigene family

We report the isolation and sequence of a human heart cDNA coding for cytochrome c oxidase (COX) subunit VIb (COX VIb). This cDNA extends 50 bp upstream from the region coding for the mature peptide. By Northern analysis, a single transcript of approx. 550 nucleotides (nt) has been identified in six human tissues. Southern analysis of human genomic DNA demonstrates the presence of multiple loci that show high homology to the cDNA. These loci cosegregate with either five or six different human chromosomes in human-rodent somatic cell hybrids. Using the COX6b cDNA, genomic sequences representing two of these loci have been isolated and characterized. The nt sequence analysis suggests that both loci represent COX6b pseudogenes.

Evolutionary aspects of cytochrome c oxidase

The presence of additional subunits in cytochrome oxidase distinguish the multicellular eukaryotic enzyme from that of a simple unicellular bacterial enzyme. The number of these additional subunits increases with increasing evolutionary stage of the organism. Subunits I-III of the eukaryotic enzyme are related to the three bacterial subunits, and they are encoded on mitochondrial DNA. The additional subunits are nuclear encoded. Experimental evidences are presented here to indicate that the lower enzymatic activity of the mammalian enzyme is due to the presence of nuclear-coded subunits. Dissociation of some of the nuclear-coded subunits (e.g. VIa) by laurylmaltoside and anions increased the activity of the rat liver enzyme to a value similar to that of the bacterial enzyme. Further, it is shown that the intraliposomal nucleotides influence the kinetics of ferrocytochrome c oxidation by the reconstituted enzyme from bovine heart but not from P. denitrificans. The regulatory function attributed to the nuclear-coded subunits of mammalian cytochrome c oxidase is also demonstrated by the tissue-specific response of the reconstituted enzyme from bovine heart but not from bovine liver to intraliposomal ADP. These enzymes from bovine heart and liver differ in the amino acid sequences of subunits VIa, VIIa, and VIII. The results presented here are taken to indicate a regulation of cytochrome c oxidase activity by nuclear-coded subunits which act like receptors for allosteric effectors and influence the catalytic activity of the core enzyme via conformational changes.

Evolutionary analysis of the nucleus-encoded subunits of mammalian cytochrome c oxidase

The cytochrome c oxidase enzyme complex of eukaryotes is made up of three mitochondrial-coded subunits and a variable number of nuclear-coded subunits. Some nuclear-coded subunits are present in multiple forms and probably perform a tissue- or development-specific function. A detailed evolutionary analysis of the cytochrome c oxidase subunits that have been sequenced to date is reported here. We have found that gene duplication events from which the liver and heart isoforms of rat subunits VIa and subunit VIII originated can both be dated at about 240 +/- 90 million years ago, long before the radiation of mammalian lineages. Sequence divergence between the processed-type pseudogenes for the subunits IV, VIc and VIII have been estimated. Our results indicate that they arose fairly recently, thus suggesting that retroposition is a continuing process. We show that the rate of silent substitution in mitochondrial-coded subunits is 5-10 times higher than in nuclear-coded subunits; on the other hand replacement rates, although differing from gene to gene, are roughly of the same order of magnitude in both nuclear and mitochondrial genes. In the case of most of the nuclear-coded proteins we observed a slightly greater similarity between rats and cow, which agrees with the data obtained for mitochondrial-coded subunits.

Different isozymes of cytochrome c oxidase are expressed in bovine smooth muscle and skeletal or heart muscle

Cytochrome c oxidase (COX) was isolated from bovine smooth muscle (rumen), and compared with the enzyme from bovine liver, heart and skeletal muscle. A new isozyme of COX was found to be expressed in smooth muscle, which differs from the isozyme in liver and heart or skeletal muscle. SDS-PAGE as well as N-terminal amino acid sequencing of separated subunits from gel bands revealed the expression of the liver isoforms for subunits VIa and VIII and of the heart isoform for subunits VIIa in COX from smooth muscle.

Beta-turns as structural motifs for the proteolytic processing of seed proteins

Fifteen NH2- and COOH-terminal ends from both small and large chains of the most abundant 2 S albumins from Brassica napus seeds have been sequenced. This allows the determination of the exact proteolytic maturation sites of these proteins. Each one of these proteins arises from a polypeptide precursor which is cleaved during the post-translational processing at four sites, giving two different chains linked by disulphide bridges on the mature 2 S albumin. The hydrolyzed bonds involved in the processing are located in proline and glycine-rich regions, forming tetra-peptides with a very high beta-turn probability. Similar results have been found through the analysis of the 2 S albumin precursors from other seeds. These facts are interpreted in terms of the existence of a beta-turn specific endoprotease activity involved in the maturation process of 2 S albumins.

Demonstration of two isoforms of subunit VIIa of cytochrome c oxidase from human skeletal muscle. Implications for mitochondrial myopathies

Two different isoforms of subunit VIIa have been found in cytochrome c oxidase isolated from human skeletal muscle. The first 22 residues of the N-terminal amino acid sequences showed 5 differences. Our results provide the first conclusive evidence for the existence of cytochrome c oxidase isoenzymes in man. Since the two cytochrome c oxidase isoforms were both present in skeletal muscle tissue, though not necessarily in the same cell type, this suggests that human cytochrome c oxidase isoforms are not strictly tissue-specific. These findings may have important implications for the elucidation of genetic diseases in man in which a deficiency of cytochrome c oxidase is restricted to certain tissues.

Differential expression of nuclear genes for cytochrome c oxidase during myogenesis

Recent studies of patients with mitochondrial myopathies suggest the existence of both muscle-specific and developmentally regulated isoforms of cytochrome c oxidase (COX), the terminal enzyme complex of the electron transport chain. To investigate the temporal pattern of gene expression of nuclear genes for COX in developing muscle, the steady-state levels of COX mRNA in total RNA from a satellite cell-derived mouse muscle cell line, C2C12, were analyzed and compared with COX mRNA levels in mature rat skeletal muscle. Undifferentiated myoblasts, myotubes just after fusion (early myotubes), and fully differentiated, contractile, striated myotubes (late myotubes) were analyzed for mRNA levels for four of the 10 different nuclear-encoded COX subunits: IV, Vb, Vlc and VIII-liver. Of these, IV, Vb and Vlc are identical in both bovine heart and liver, whereas subunit VIII has heart and liver isoforms. In C2C12 myoblasts, the level of mRNA for subunits IV, Vb, and VIII-liver is equal to or greater than the level in tissues such as brain, skeletal muscle, and liver. As myoblasts fuse and differentiate into myotubes, the levels of mRNA for these subunits undergo radically different changes. Transcripts for subunits IV and Vb accumulate to higher levels during myogenesis. The level of subunit VIII transcripts decreases during myogenesis, providing additional evidence that subunit VIII has tissue-specific isoforms in the rat. Little mRNA for COX Vlc was detected in either the C2C12 cell line or in primary embryonic rat myoblasts or myotubes in culture in spite of high levels in adult skeletal muscles, suggesting that subunit Vlc may have both fetal and adult isoforms in rodents.

The two oxygen-regulated subunits of cytochrome c oxidase in Dictyostelium discoideum derive from a common ancestor

The two smallest polypeptide components of D. discoideum cytochrome c oxidase, whose alternative expression depends on oxygen concentration [Schiavo, G. and Bisson, R. (1989) J. Biol. Chem. 264, 7129-7134], have been partially sequenced. They show 45% homology and are isoforms of the same subunit, which must be encoded on two different genes.

Tissue- and species-specific expression of cytochrome c oxidase isozymes in vertebrates

Cytochrome c oxidase was isolated from brown fat tissue of the rat and compared with the isozymes from rat liver and heart, which differ at least in subunits VIa and VIII. ELISA titrations of COX from the three tissues with monospecific antisera to all 13 subunits of the rat liver enzyme showed differences between the three enzymes. The N-terminal amino-acid sequence analysis of subunits VIa and VIII from SDS-PAGE gel bands of the three enzymes indicates the occurrence of three different isozymes in the rat. N-terminal amino-acid sequence analysis of subunits VIa and VIII from cytochrome c oxidase of bovine and human heart demonstrates also species-specific differences in the expression of the 'liver-type' and 'heart-type' of subunits VIa and VIII.

Tissue-specific genes for respiratory proteins

All but one of the mitochondrial respiratory complexes are composed of products of both the mitochondrial and the nuclear genomes. The recent isolation of cDNAs for several nuclear-encoded respiratory proteins reveals that some of them are present in at least two forms. Although some of these forms are traditional in differing somewhat in amino acid sequence, a new class, termed silent isoforms, differs in the presequence but contains identical processed proteins. What are the roles of tissue isoforms in oxidative metabolism?