Relationship between the target antigen of liver-kidney microsomal (LKM) autoantibodies and rat isoenzymes of cytochrome P-450

Chronic active hepatitis (CAH) is a clinical syndrome of different etiologies. Liver-kidney microsomal (LKM) autoantibodies characterize a subgroup of HBsAg negative CAH, which is considered to be an autoimmune liver disease. By immunoblotting analysis (IB) LKM positive sera have been shown to react strongly with a poly-peptide band at 50 kD. Therefore we investigated various rat microsomal enzymes with a molecular weight around 50 kD as potential candidate target antigens. These included epoxide hydrolase, cytochrome P-450 reductase, and phenobarbital-inducible isoenzymes of cytochrome P-450 (PB1, PB2, PB3a, PB3b). By radioimmunoassay (RIA) and IB LKM positive sera were shown to react with the phenobarbital-inducible isoenzymes of cytochrome P-450 but not with epoxide hydrolase or with cytochrome P-450 reductase. Cytochrome P-450 PB3a and PB3b were both able to absorb exhaustively the reactivity of LKM sera with liver tissue in immunofluorescence (IF) and with purified rat liver microsomes in RIA. It is concluded that LKM antibodies are directed against an epitope present in preparations of various isoenzymes of cytochrome P-450.