Sera from patients with primary biliary cirrhosis inhibit the activity of the mitochondrial pyruvate dehydrogenase complex. We utilized this effect to develop a simple, miniaturized, semiautomated spectrophotometric assay as a diagnostic aid. The sera studied were from 71 patients with primary biliary cirrhosis and 62 other subjects. The assays included enzyme inhibition, immunofluorescence on HEp‐2 cells, enzyme‐linked immunosorbent assay using recombinant pyruvate dehydrogenase complex‐E2 and immunoblotting on bovine heart mitochondria. With the 71 primary biliary cirrhosis sera, on which M2 antibody was detected by immunofluorescence in 64 (90%), antibodies against pyruvate dehydrogenase complex were detected in 53 (83%) by means of enzyme inhibition, in 57 (89%) by means of enzyme‐linked immunosorbent assay and in 60 (94%) by means of immunoblotting. Of the 64 sera positive by immunofluorescence, 60 reacted with pyruvate dehydrogenase complex‐E2 on immunoblotting, and the miniaturized enzyme inhibition assay was positive in 53 of these. The enzyme inhibition assay and enzyme‐linked immunosorbent assay were calibrated to give a specificity of 100%. At this level, the sensitivities for detection of pyruvate dehydrogenase complex antibody were 83% and 87%, respectively. We found no significant changes in levels of reactivity with the enzyme inhibition assay or enzyme‐linked immunosorbent assay according to disease stage. Treatment with cyclosporine was accompanied by a significant decrease in levels of antibody to pyruvate dehydrogenase complex‐E2 that matched improved indexes of biochemical liver function. The semiautomated enzyme inhibition assay as described provides an additional diagnostic procedure in pyruvate dehydrogenase complex; this type of assay involving specific enzyme inhibition could also be generically applied to any autoantibody antigen system. (Hepatology 1994;20:1220–1224).
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