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Isoforms of Mammalian Cytochrome C Oxidase: Correlation with Human Cytochrome C Oxidase Deficiency

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Isoforms of Mammalian Cytochrome C Oxidase: Correlation with Human Cytochrome C Oxidase Deficiency 003 1-3998/90 /2 805-0529$0 2.00/0 PEDIATRI C RESEARCH Vol. 28. No.5. 1990 Copyright © 1990 Internat ional Pediatric Research Foundation. Inc. Prill/ I'd ill U.S.A. Isoforms of Mammalian Cytochrome c Oxidase: Correlation with Human Cytochrome c Oxidase Deficiency NANCY G . KENNAWAY. ROQUE D. CARRERO -VAL ENZUELA. GA RY EWART. VIJAY K. BALAN. ROB ERT L1GHTOWLERS. YU -ZHONG ZH ANG . BERKLEY R. POW ELL RODERICK A. CAPA LDI. AN D NEIL R. M. BUIST Departmen ts ofMedica! Generics [NG.K., R.D.C.- v., V.K.B.. N R.M.B.j and Pediatrics [B.R.P., N R.M. B.j, Oregon Health Sciences Univcrsitv. Portla nd. Oregon 9720i and Institute ofMolecular Biology [G.E.. R.L., Y-z.z., R.A.Cj. University ofOregon, Eugene. Oregon 97403 ABSTRACf. We have reviewed the structure, function, IV (cytochro me c oxidase) (5) have all been described. Th e and biogenesis of mammalian cytochrome c oxida se, ex­ severity of these disorders is very variable, and the spectrum of amined the tissue-specific expression of isoforms of cyto­ clinical disease is impressive, ran ging from isolated myopath y or chrome c oxidase subunits in different mammals, and at­ cardiomyopathy in some patients to a multisystem disease such tempted to correlate the data with our knowledge of cyto­ as Leigh's syndrome in others. Thi s variability presum ably results chrome c oxidase deficiency, illustrated by one particular in part from the complexity ofthe respirato ry chain proteins, the patient. Cytochrome c oxidase was isolated from bovine smallest of which (complex II) comprises four or five individual tissues, and individual subunits examined by SDS-PAGE, subunits, and the largest of which (complex I) comprises at least N-terminal peptide sequencing, and antibody binding. Iso­ 26 indi vidual subunits (6). Some of these subunits appear as forms of subunits VIa, VIla, and VIII were identified, tissue-specific isoforms in mammals. manifesting one pattern of expression in heart and skeletal The ident ification of patients with respiratory chai n deficien­ muscle, and another in liver, kidney, and brain . In rat heart cies limited to on e or a few tissues suggests that isoforms of some and liver, only one form of subunit VIla was identified. of these proteins may also exist in humans and provide an Northern analysis of bovine and rat tissues suggested that explan ation for the tissue-specific expression of the disease in the tissue-specific expression of subunits VIa and VIII is such patients. Until very recentl y, attempts to define the tissue regulated transcriptionally in liver, kidne y, and brain, and specificity of the respiratory chain have focused mai nly on beef posttranscriptionally in heart and skeletal muscle. In hu­ and rat. not only because of the ready avai lability of tissue mans, antibody binding documented isoforms of subunits material. but also because of the assumptio n that the pattern of VIa and VIla, with the pattern of expression in heart and expression ofthese isoforms would be conserved in all mammals, skeletal muscle differing from that in liver, kidney, and includ ing humans. There is preliminary evidence of isoforms of brain; our data suggested that both isoforms of subunit VIa some subunits of complex I (7); however, the mo st comprehen­ may be expressed in human heart. In a patient with cyto­ sive data have been obtained on cytochrome c oxidase. In this chrom e c oxidase deficiency, the clinical, morphologic, and report we will: 1) review the stru cture, function , and biogenesis biochemical manifestations were much more severe in heart of mammalian cytochro me c oxidase ; 2) discuss the tissue spec­ than in skeletal muscle. Antibod y binding suggested partial ificity and expression of isoforms of cytochrome c oxidase in assembly of the enzyme in heart. These and other data mammals, including humans; and 3) relate thi s briefly to the suggest considerably more variability in the tissue-specific pheno types of cytochrome c oxidase deficiency, and to on e expression of isoforms of cytochrome c oxidase subunits patient that we have studi ed recently in parti cular. than previously recognized . (Pediatr Res 28: 529-535, 1990) CYT OCHROME c OX IDASE: TE RMINA L OXIDASE IN RESPIRATI ON Abbreviations Structure.funaion, and biogenesis. Cytochrome c oxidase, the mtDNA, mitochondrial DNA terminal component of the respiratory chain, spans the mito­ chond rial inner membrane and tran sfers electrons from reduced cytochrome c to molecular oxygen with generatio n of a trans­ mem brane proton gradient. In mammals, the enzyme is a mul ­ Over the past few years, there has been increasing recognit ion ticomponent complex containing 13 different polypeptides (8). of the importance of inherited disorders of the mit ochondrial The three largest subunits are coded on mtDNA and synthesized respiratory chain in a wide variety of human diseases (I ). Dis­ in the mit ochond rion. The se mitochond rially-coded subunits orders of complex I (NADH:ubiquino ne oxidoreductase) (2), appear to be the functional core of the enzyme, but the func­ complex II (succinate:ubiquinone oxidoredu ctase) (3), complex tion(s) of the nuclear coded subunits is presentl y uncle ar. Figure III (ubiquinol:cytochro me c oxidoreductase) (4), and complex 1 shows the resolut ion of all the subunits of beef heart cyto­ chrom e c oxidase on highly resolving SDS-PAGE and presents Corre spondence and reprint requests: Dr. Nan cy G, Kcn naway, Department of two nomenclatures currently in use for the different subunits; Medical Genetics/ L473. Oregon Health Sciences Un iversity. 3181 S.W. Sam Jack­ the nomenclature of Kadenbach et al. (9) is used in this review . son Park Rd.. Portland. OR 9710 1-3098. Supported in part by NI H G rant 22050. a gran t from the Musc ular Dystrophy The biosynthesis of cytochrome c oxidase is complicated, Association. and a grant from the Medical Research Foundation of Oregon. involving components coded on two geno mes and synthesized 529 530 KENNAWAY ET AL. KADEN BACH CAPALDI CYTOPLASM et al1983 at al1987 I II Mtll III Mtm IV CI y Va Cy Vb C ---- YI VIa ASA VIb AED VIc STA "- VII a ____ CYII . VII b IHQ Fig. 2. Schematic representatio n of the synthesis and assembly ofthe respirator y chain complexes in mammals. Reproduced from Takamiya :======: VII c Gym VIII 1'1al. (12), with permi ssion . CIX Fig. I. Subunit str ucture ofbeef heart cytochro me c oxidase, showing Rat . two nome nclature s cur rentl y in use (9, 10). The enzyme was purified In their original experiments , Merle and Kadenbach (14) from mit ochondrial preparatio ns essentially as described (11), sub units found an altered migration of subunit VIa when the composition separated on an 18.5% polyacrylamide gel containing 6 M urea (9), and of the heart and liver enzyme was compared by SDS-PAGE (14). proteins stained with Coo massie Brilliant Blue. The presence of isoforms of subunit VIII as well as VIa was indicated by differences in labeling of these two subunits in different tissues by sulfhydryl reagents (15). Characterization of in two compart ments, namely the mitochondrial matrix and the two cDNA, one isolated from a rat liver, the other from a rat cytoplasm. This overall process is shown schematically in Figure heart cDNA library, offers definitive evidence that there are two 2. Briefly, nuclear coded subunits, synthesized on free ribosomes, forms of subunit VIa (16). The two forms of subunit VIa encoded are released in the cytoplasm as precursor proteins, frequently by these cDNA show only 50% homology. The expression of the containing an N-terminal extension or leader peptide that targets isoforms of subunit VIa was explored by Schlerf et al. (16) using the protein to the mitochondrion and that may help direct the Northern analysis; the heart form of this subunit was found to protein to the correct location within the organelle, i.e. intracris­ be transcribed in heart and skeletal muscle only but, surprisingly, tal space, inner membrane, or matrix. Mitochondrially synthe­ the liver form was transcribed in all tissues examined (16). sized proteins are transcribed as a polycistronic message that is More recently, Kuhn- Nentwig and Kadenbach (17) have used processed to give mRNA for individual subunits. The three quantitative immunoassays to compare subunits from a variety subunits of cytochrome c oxidase are synthesized as mature of adult and fetal rat tissues. Based on these studies, they have polypeptides in mammals and may be cotran slationally inserted concluded that all of the nuclear coded subunits of cytochrome into the mitochondrial inner membrane. Steps involved in the c oxidase may occur in different tissue-specific forms in mam­ addition of heme and coppers to subunits I and II have not been mals, that more than three different isoforms are present for identified yet, but may occur at the inner membran e concurrent many subunits, and that there are major differences between with the assembly of the mitochondrially and nuclear coded fetal and adult tissues. However, these results are open to ques­ subunits into the complex. tion, because some of the observed antigenic differences may have resulted from acquired conformatio nal differences such as ISOFORMS OF MAMMALIAN CYTOCHROME c OXIDASE the extent of denat uration of the same subunit from different tissues. The occurrence of tissue-specific isoforms of mammalian cy­ The early data of Kadenbach et al. (13) showed no difference tochrome c oxidase was first suggested by Kadenbach et al. (13), in the migration of cytochrome c oxidase subunit VIla in rat based on slight differences in the apparent molecular weight of heart and liver by SDS-PAGE, in contrast to what was seen in some subunits in different tissues from each of several species some other mamm als (13).
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