Reactivity of anti-mitochondrial autoantibodies in primary biliary cirrhosis: definition of two novel mitochondrial polypeptide autoantigens.

Sera that contained autoantibodies to mitochondria (AMA) by immunofluorescence were examined by immunoblotting for reactivity with mitochondrial polypeptides from various mammalian species, yeast, and E. coli. Mitochondrial polypeptides were separated by polyacrylamide gel electrophoresis, were immobilized on nitrocellulose, and were exposed to sera. The sera tested included 18 AMA-positive sera from patients with primary biliary cirrhosis (PBC), two AMA-positive sera from patients without PBC, and 53 AMA-negative sera. All AMA-positive sera reacted with either one or the other, or usually both of two human mitochondrial polypeptides of 70 kilodalton (kD) and 45 kD. The 53 AMA-negative sera were not reactive with the 70 kD polypeptide, but six reacted with the 45 kD polypeptide. The reactivity of the 70 kD and the 45 kD polypeptide was destroyed by brief exposure to trypsin. The counterpart of the 70 kD reactive polypeptide in human mitochondria was a 65 to 70 kD polypeptide in rat and mouse mitochondria, and a 55 kD polypeptide in yeast and in E. coli. The apparent 45 kD polypeptide was similar in all mitochondrial preparations tested, but no counterpart could be identified in E. coli. Beef heart mitochondria were used to show that the reactive polypeptides were present in a semipurified preparation of the F1 portion of mitochondrial H+ ATPase; however, sera did not react with the beta subunit of ATPase, proposed as a candidate mitochondrial autoantigen. The present molecular characterization of two particular antigens should lead to the more precise identification of these antigens, and also to a clearer insight into the pathogenesis of PBC.