title: Autoantibodies to BCOADC-E2 in patients with primary biliary cirrhosis recognize a conformational epitope.
abstract: Primary biliary cirrhosis (PBC) is an autoimmune disease of liver associated with a unique serologic response to mitochondrial autoantigens. Many of the autoantigens recognized by autoantibodies in PBC are members of the 2-oxo-acid dehydrogenase complex. The two major autoantigens are the E2 component of the pyruvate dehydrogenase complex (PDC-E2) and the E2 component of the branched chain 2-oxo-acid dehydrogenase complex (BCOADC-E2). The autoantibody response to PDC-E2 has been mapped to one immunodominant epitope, which consists of both linear and conformational components. The presence of a single immunodominant epitope in PDC-E2 is unusual when contrasted to the immune response to autoantigens in other human autoimmune diseases. We have mapped the epitope recognized by antimitochondrial autoantibodies (AMA) specific to BCOADC-E2 in patients with PBC by taking advantage of the full-length bovine BCOADC-E2 complementary DNA (cDNA) and a series of expression clones spanning the entire molecule. Reactivity to the various expression clones was studied by immunoblotting, enzyme-linked immunosorbent assay (ELISA), as well as selective absorption of patient sera by expressed protein fragments. Autoantibodies to BCOADC-E2 map within peptides spanning amino acid residues 1 to 227 of the mature protein; our data demonstrate that the epitope is dependent on conformation and includes the lipoic acid binding region. However, only the full-length clone (amino acid residue 1 to 421) is sufficient to remove all detectable BCOADC-E2 reactivity. Moreover, the absence of lipoic acid on the recombinant polypeptides used in this study indicates that antibody binding to BCOADC-E2 is not dependent on the presence of lipoic acid.
authors.affiliation: Division of Rheumatology, Allergy and Clinical Immunology, University of California, Davis 95616, USA.
authors.forename: P S
authors.initials: PS
authors.lastname: Leung
authors.forename: D T
authors.initials: DT
authors.lastname: Chuang
authors.forename: R M
authors.initials: RM
authors.lastname: Wynn
authors.forename: S
authors.initials: S
authors.lastname: Cha
authors.forename: D J
authors.initials: DJ
authors.lastname: Danner
authors.forename: A
authors.initials: A
authors.lastname: Ansari
authors.forename: R L
authors.initials: RL
authors.lastname: Coppel
authors.forename: M E
authors.initials: ME
authors.lastname: Gershwin
mesh_terms: D042942:3-Methyl-2-Oxobutanoate Dehydrogenase (Lipoamide); D000595:Amino Acid Sequence; D000918:Antibody Specificity; D001323:Autoantibodies / Q000097:blood*; D001324:Autoantigens / Q000737:chemistry / Q000276:immunology; D001665:Binding Sites; D004797:Enzyme-Linked Immunosorbent Assay; D000939:Epitopes / Q000737:chemistry* / Q000276:immunology; D006801:Humans; D007163:Immunosorbent Techniques; D007658:Ketone Oxidoreductases / Q000276:immunology*; D008105:Liver Cirrhosis, Biliary / Q000276:immunology*; D008969:Molecular Sequence Data; D009097:Multienzyme Complexes / Q000276:immunology*; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000032:analysis / Q000737:chemistry / Q000276:immunology; D017421:Sequence Analysis; D017385:Sequence Homology; D008063:Thioctic Acid / Q000032:analysis / Q000378:metabolism
chemical_list: D001323:Autoantibodies; D001324:Autoantigens; D000939:Epitopes; D009097:Multienzyme Complexes; D011994:Recombinant Proteins; D008063:Thioctic Acid; D007658:Ketone Oxidoreductases; D042942:3-Methyl-2-Oxobutanoate Dehydrogenase (Lipoamide)