Primary biliary cirrhosis (PBC) is a chronic cholestatic liver disease characterized by the presence of antimitochondrial antibodies in the serum. It is possible that the PBC-specific immunoreactive trypsin-sensitive antigens on the inner mitochondrial membrane, termed M2, are important in the pathogenesis of this autoimmune disease. We have previously shown that a major M2"a" antigen is the E2 component of the pyruvate dehydrogenase multienzyme complex located within mitochondria. Analysis of the primary structure of the E2 components of all three 2-oxo acid dehydrogenase complexes reveals a high degree of homology with a similar highly segmented structure including lipoyl domains, E3-binding domains, C-terminal catalytic domains, and interdomain linker sequences. Immunoblotting of PBC patients' sera against purified E2 protein from 2-oxoglutarate dehydrogenase complex and branched-chain 2-oxo acid dehydrogenase complex reveals that these polypeptides are also autoantigens in this disease. Sera from 29 of 40 (72.5%) PBC patients gave a positive response against bovine 2-oxoglutarate dehydrogenase complex E2 and from 25 of 40 (62.5%) PBC patients gave a positive response against bovine branched-chain 2-oxo acid dehydrogenase complex E2. All 40 PBC patients (100%) have autoantibodies directed against at least one of the E2 components of the family of 2-oxo acid dehydrogenase complexes.
Primary bilary cirrhosis (PBC) is a chronic cholestatic liver disease characterized by the presence of antimitochondrial autoantibodies in the serum. The major antigens recognized by the antibodies are the E2 components of the 2-oxo acid dehydrogenase complexes, all of which possess covalently attached lipoic acid cofactors. A bacterial etiology has been proposed for the disease, and patients' antibodies are known to recognize the E2 subunits (E2p) of both mammalian and bacterial pyruvate dehydrogenase complexes. Immunoblotting and ELISA inhibition techniques using extracts of Escherichia coli deletion strains, genetically restructured E2 polypeptides, and isolated lipoyl domains demonstrate that (i) the E2o subunit of the E. coli 2-oxoglutarate dehydrogenase complex is recognized by patients' antibodies; (ii) the main immunogenic region of E2p lies within the lipoyl domains; (ii) the presence of a lipoyl residue within the domain is crucial for effective recognition by the antibodies; and (iv) octanoylated E2p, octanoylated E2o, and octanoylated lipoyl domain, produced by a mutant deficient in lipoate biosynthesis, are recognized by patients' antibodies but not as effectively as their lipoylated counterparts. These rmdings indicate that antibodies in PBC patients' sera bind to a unique peptide-cofactor conformation within the lipoyl domains of the E2 polypeptides and that this epitope is partially mimicked by substituting the lipoyl cofactor with an octanoyl group.
We have previously identified four of the M2 antigens in primary biliary cirrhosis as the E2 components (dihydrolipoamide acyltransferases) of pyruvate dehydrogenase complex, branched-chain 2-oxo acid dehydrogenase complex and 2-oxoglutarate dehydrogenase complex and the protein X component of pyruvate dehydrogenase complex (approximate molecular masses: 74, 50, 50 and 52 kD, respectively). In the present study, we have examined by immunoblotting the frequency of IgG and IgM autoantibodies to these four proteins in 129 patients with primary biliary cirrhosis (36 histological Stage I, 42 Stage II/III, 51 Stage IV) and 77 controls (49 non-primary biliary cirrhosis chronic liver disease, 16 primary Sjögren's syndrome, 12 healthy normal women). One hundred twenty-seven of 129 (98%) primary biliary cirrhosis patients had antibodies against at least one of the four M2 polypeptides, compared to 2/77 controls (both had autoimmune chronic active hepatitis and were antimitochondrial antibody positive by indirect immunofluorescence). One hundred twenty-one of 129 (94%) primary biliary cirrhosis sera reacted with the E2 component and protein X of pyruvate dehydrogenase complex, 69/129 (53%) primary biliary cirrhosis sera reacted with E2 of branched-chain 2-oxo acid dehydrogenase complex and 113/129 (88%) reacted with E2 of 2-oxoglutarate dehydrogenase complex. Primary biliary cirrhosis patients with histological Stage I disease had a lower incidence of autoantibodies to each M2 protein, compared to more advanced disease (IgG, p less than 0.05) but only 2/36 Stage I patients had no anti-M2 antibodies. There was no correlation between the presence of IgG or IgM antibodies to the M2 polypeptides and established prognostic markers in primary biliary cirrhosis (serum bilirubin and albumin levels).(ABSTRACT TRUNCATED AT 250 WORDS)
Autoantibodies in the sera of patients with primary biliary cirrhosis, shown previously to recognise the E2 polypeptide of the mammalian pyruvate dehydrogenase complex (PDC), have been demonstrated to react with the E2 component of PDC from bacteria (E. coli) and yeast (S. cerevisiae). Limited tryptic digestion, which cleaves E2 into well-characterised domains, followed by Western blotting indicates that the main immunodominant region of PDC E2 lies within the lipoic acid-containing domains of the polypeptide.
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