Mitochondrial antibodies in primary biliary cirrhosis. VI. Association of the complement fixing antigen with a component of the mitochondrial F1-ATPase complex.

The complement fixing antigen of the inner mitochondrial membrane previously shown to be associated with the mitochondrial ATPase could be further purified by subjecting the ATPase extracted from beef heart and brown fat mitochondria to ion exchange and gel filtration chromatography. Although the ATPase activity could be clearly dissociated from the complement fixing activity, subunits of the F1-ATPase complex were always found in the purified fractions. The alpha, gamma, delta and epsilon subunits of the complex could be excluded with high probability as target antigens in contrast to the beta band which was always found in association with the antigen activity. These findings imply that the active centre of the ATPase enzyme is not involved in the antibody reaction but molecules of the ATPase complex may have antigen binding capacity. Treatment of ATPase associated antigen with trypsin did not markedly affect the complement binding, while SMP's treated in the same way lost their antigen activity indicating that sera from patients with primary biliary cirrhosis (PBC) may have mitochondrial antibodies of different specificities reacting with trypsin sensitive as well as trypsin insensitive components of the inner membrane. The purified antigen reacted exclusively with sera from patients with PBC and may be therefore used as a marker antigen.