title: Unconjugated bilirubin inhibits C1 esterase activity.
abstract: OBJECTIVE
  To evaluate if unconjugated bilirubin (UB) inhibits C1 esterase activity.
  
  
  METHODS
  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.
  
  
  RESULTS
  UB inhibited C1 enzymatic activity. C1r, C1s and human serum albumin bound [(14)C]-UB to a similar extent.
  
  
  CONCLUSIONS
  UB inhibits C1 esterase activity, apparently due to a direct pigment-protein interaction. This could explain the inhibitory action of UB on complement activation.
authors.affiliation: Area Bioquímica Clínica y Fisiopatología, Facultad de Ciencias Bioquímicas y Farmacéuticas, Universidad Nacional de Rosario, Rosario, Argentina. sarriaga@fbioyf.unr.edu.ar
authors.forename: Sandra M
authors.initials: SM
authors.lastname: Arriaga
authors.forename: Cecilia L
authors.initials: CL
authors.lastname: Basiglio
authors.forename: Aldo D
authors.initials: AD
authors.lastname: Mottino
authors.forename: Adriana M
authors.initials: AM
authors.lastname: Almará
mesh_terms: D001663:Bilirubin / Q000737:chemistry* / Q000494:pharmacology*; D003173:Complement C1s / Q000378:metabolism*; D004789:Enzyme Activation / Q000187:drug effects; D006801:Humans; D015151:Immunoblotting
chemical_list: D003173:Complement C1s; D001663:Bilirubin