Unconjugated bilirubin inhibits C1 esterase activity.

To evaluate if unconjugated bilirubin (UB) inhibits C1 esterase activity. Esterase activity was evaluated by C1-mediated hydrolysis of N-acetyl-L-tyrosine ethyl ester, and binding of UB to C1r and C1s was assessed by dot-blot analysis. UB inhibited C1 enzymatic activity. C1r, C1s and human serum albumin bound [(14)C]-UB to a similar extent. UB inhibits C1 esterase activity, apparently due to a direct pigment-protein interaction. This could explain the inhibitory action of UB on complement activation.