M2 autoantigen is a key mark of antimitochondrial autoantibodies（AMA)， a typical serological feature in patients suffering from primary biliary cirrhosis（PBC) which is a serious autoimmune liver disease accompanied by damage to intrahepatic bile ducts. Molecular definition of the M2 antigen has displayed it as no less than 3 separate target proteins. The M2 role is like the so-called E2 subunits（or dihydrolipoamide transferases) of different mitochondrial dehydrogenase complexes:
*pyruvate dehydrogenase complex.
*branched chain 2-oxo-acid dehydrogenase complex.
*2-oxoglutarate dehydrogenase complex.
Biochemically， these complexes catalyze the oxidative decarboxylation of various alpha-keto-acid substrates and systematically engage with a prosthetic lipoamide group； they are situated in the mitochondrial matrix in association with the inner membrane. The most well-known reactivity of AMA positive PBC sera is against PDC-E2. Some patients have AMA which reacts with PDC-E2 alone（95%)， but most patients also show reactivity against OGDC-E2（39-88%) and/or BCOADC-E2（53-55%). Actually， patients can be found with reactivity only against OGDC-E2 and/or BCOADC-E2 and no PDC-E2 autoantibodies. These patients will be overlooked in assays based on natural source-derived， predominantly PDC-E2-containing M2 antigen preparations.
