Antimitochondrial
autoantibodies recognizing 68 to 74 and 50 to 52 kD inner membrane mitochondrial
antigens are characteristically present in sera of patients with
primary biliary cirrhosis. The biochemical identification of the
antigens, however, has remained elusive. We report herein that the 52 kD
antigen is the
dihydrolipoamide acyltransferase of the branched-chain
alpha-keto acid dehydrogenase complex. This was demonstrated by three experiments through the use of
recombinant fusion protein expressed in Escherichia coli from a
cDNA insert encoding the human
autoantigen. First, 33 of 37
primary biliary cirrhosis patients exhibiting reactivity toward the 50 to 52 kD mitochondrial
antigen by immunoblotting also showed reactivity toward the
recombinant fusion protein. Second, absorption of
primary biliary cirrhosis sera with
recombinant fusion protein, but not with an irrelevant recombinant clone, the F-specific rat liver
antigen, was effective in absorbing out reactivity against the 50 to 52 kD mitochondrial
antigen but not the 68 to 74 kD
antigen. Third, complete removal of reactivity toward all four different isoelectric point
polypeptides at 50 to 52 kD was observed in two-dimensional gel analysis. Furthermore,
primary biliary cirrhosis sera were analyzed with mitochondria from three sources, rat liver, human placenta and bovine heart, in order to compare reactivity patterns and to determine precisely the comparative molecular weights of the
autoantigens in the three species. The availability of recombinant
autoantigens will provide improved diagnostic tests and, more importantly, will allow definite issues in
primary biliary cirrhosis to be studied, including identification of
immunodominant epitopes, the significance of
autoantigen recognition and the establishment of autoreactive T cell clones.