title: Inducible bilirubin oxidase: a novel function for the mouse cytochrome P450 2A5.
abstract: We have previously shown that bilirubin (BR), a breakdown product of haem, is a strong inhibitor and a high affinity substrate of the mouse cytochrome P450 2A5 (CYP2A5). The antioxidant BR, which is cytotoxic at high concentrations, is potentially useful in cellular protection against oxygen radicals if its intracellular levels can be strictly controlled. The mechanisms that regulate cellular BR levels are still obscure. In this paper we provide preliminary evidence for a novel function of CYP2A5 as hepatic "BR oxidase". A high-performance liquid chromatography/electrospray ionisation mass spectrometry screening showed that recombinant yeast microsomes expressing the CYP2A5 oxidise BR to biliverdin, as the main metabolite, and to three other smaller products with m/z values of 301, 315 and 333. The metabolic profile is significantly different from that of chemical oxidation of BR. In chemical oxidation the smaller products were the main metabolites. This suggests that the enzymatic reaction is selective, towards biliverdin production. Bilirubin treatment of primary hepatocytes increased the CYP2A5 protein and activity levels with no effect on the corresponding mRNA. Co-treatment with cycloheximide (CHX), a protein synthesis inhibitor, resulted in increased half-life of the CYP2A5 compared to cells treated only with CHX. Collectively, the observations suggest that the CYP2A5 is potentially an inducible "BR oxidase" where BR may accelerate its own metabolism through stabilization of the CYP2A5 protein. It is possible that this metabolic pathway is potentially part of the machinery controlling intracellular BR levels in transient oxidative stress situations, in which high amounts of BR are produced.
authors.affiliation: The University of Queensland, National Research Centre for Environmental Toxicology (Entox), 4072 Brisbane, Queensland, Australia. a.abubakar@uq.edu.au
authors.forename: A'edah
authors.initials: A
authors.lastname: Abu-Bakar
authors.forename: Dionne Maioha
authors.initials: DM
authors.lastname: Arthur
authors.forename: Simona
authors.initials: S
authors.lastname: Aganovic
authors.forename: Jack C
authors.initials: JC
authors.lastname: Ng
authors.forename: Matti A
authors.initials: MA
authors.lastname: Lang
mesh_terms: D000818:Animals; D001189:Aryl Hydrocarbon Hydroxylases / Q000096:biosynthesis / Q000378:metabolism / Q000502:physiology*; D001663:Bilirubin / Q000378:metabolism / Q000502:physiology; D001664:Biliverdine / Q000096:biosynthesis; D003513:Cycloheximide / Q000494:pharmacology; D065637:Cytochrome P-450 CYP2A6; D000072467:Cytochrome P450 Family 2; D004790:Enzyme Induction; D022781:Hepatocytes / Q000378:metabolism; D008297:Male; D051379:Mice; D008811:Mice, Inbred DBA; D008861:Microsomes / Q000378:metabolism; D010084:Oxidation-Reduction; D044925:Oxidoreductases Acting on CH-CH Group Donors / Q000096:biosynthesis / Q000378:metabolism / Q000502:physiology*; D012441:Saccharomyces cerevisiae / Q000378:metabolism
chemical_list: D003513:Cycloheximide; D001189:Aryl Hydrocarbon Hydroxylases; C585520:Cyp2a5 protein, mouse; D065637:Cytochrome P-450 CYP2A6; D000072467:Cytochrome P450 Family 2; D044925:Oxidoreductases Acting on CH-CH Group Donors; C042401:bilirubin oxidase; D001664:Biliverdine; D001663:Bilirubin