title: Evidence for induced microsomal bilirubin degradation by cytochrome P450 2A5.
abstract: Oxidative metabolism of bilirubin (BR) -- a breakdown product of haem with cytoprotective and toxic properties -- is an important route of detoxification in addition to glucuronidation. The major enzyme(s) involved in this oxidative degradation are not known. In this paper, we present evidence for a major role of the hepatic cytochrome P450 2A5 (Cyp2a5) in BR degradation during cadmium intoxication, where the BR levels are elevated following induction of haem oxygenase-1 (HO-1). Treatment of DBA/2J mice with CdCl(2) induced both the Cyp2a5 and HO-1, and increased the microsomal BR degradation activity. By contrast, the total cytochrome P450 (CYP) content and the expression of Cyp1a2 were down-regulated by the treatment. The induction of the HO-1 and Cyp2a5 was substantial at the mRNA, protein and enzyme activity levels. In each case, the up-regulation of HO-1 preceded that of Cyp2a5 with a 5-10h interval. BR totally inhibited the microsomal Cyp2a5-dependent coumarin hydroxylase activity, with an IC(50) approximately equal to the substrate concentration. The 7-methoxyresorufin 7-O-demethylase (MROD) activity, catalyzed mainly by the Cyp1a2, was inhibited up to 36% by BR. The microsomal BR degradation was inhibited by coumarin and a monoclonal antibody against the Cyp2a5 by about 90%. Furthermore, 7-methoxyresorufin, a substrate for the Cyp1a2, inhibited BR degradation activity by approximately 20%. In sum, the results strongly suggest a major role for Cyp2a5 in the oxidative degradation of BR. Secondly, the coordinated up-regulation of the HO-1 and Cyp2a5 during Cd-mediated injury implicates a network of enzyme systems in the maintenance of balancing BR production and elimination.
authors.affiliation: National Research Centre for Environmental Toxicology, University of Queensland, 39 Kessels Road, Coopers Plains, 4108 Brisbane, Qld., Australia. s4020308@student.uq.edu.au
authors.forename: A'edah
authors.initials: A
authors.lastname: Abu-Bakar
authors.forename: Michael R
authors.initials: MR
authors.lastname: Moore
authors.forename: Matti A
authors.initials: MA
authors.lastname: Lang
mesh_terms: D000818:Animals; D001189:Aryl Hydrocarbon Hydroxylases / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism / Q000494:pharmacology*; D001663:Bilirubin / Q000737:chemistry / Q000378:metabolism*; D019256:Cadmium Chloride / Q000008:administration & dosage / Q000009:adverse effects / Q000378:metabolism; D065637:Cytochrome P-450 CYP2A6; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D000072467:Cytochrome P450 Family 2; D051547:Heme Oxygenase-1 / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism; D008658:Inactivation, Metabolic / Q000235:genetics / Q000502:physiology; D007274:Injections, Intraperitoneal; D008099:Liver / Q000737:chemistry / Q000187:drug effects / Q000378:metabolism; D008297:Male; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism; D051379:Mice; D008811:Mice, Inbred DBA; D008862:Microsomes, Liver / Q000737:chemistry / Q000187:drug effects / Q000378:metabolism*; D006899:Mixed Function Oxygenases / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism / Q000494:pharmacology*; D010078:Oxazines / Q000037:antagonists & inhibitors / Q000378:metabolism; D018384:Oxidative Stress / Q000187:drug effects; D010088:Oxidoreductases / Q000037:antagonists & inhibitors / Q000378:metabolism; D012333:RNA, Messenger; D013997:Time Factors; D015854:Up-Regulation / Q000187:drug effects
chemical_list: D065607:Cytochrome P-450 Enzyme Inhibitors; D008565:Membrane Proteins; D010078:Oxazines; D012333:RNA, Messenger; C007649:ethoxyresorufin; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; D010088:Oxidoreductases; C076307:methoxyresorufin-O-demethylase; D001189:Aryl Hydrocarbon Hydroxylases; C585520:Cyp2a5 protein, mouse; D065637:Cytochrome P-450 CYP2A6; D000072467:Cytochrome P450 Family 2; D051547:Heme Oxygenase-1; C496542:Hmox1 protein, mouse; D019256:Cadmium Chloride; D001663:Bilirubin