Evidence for induced microsomal bilirubin degradation by cytochrome P450 2A5

Biochem Pharmacol. 2005 Nov 15;70(10):1527-35. doi: 10.1016/j.bcp.2005.08.009. Epub 2005 Sep 23.

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.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Aryl Hydrocarbon Hydroxylases / antagonists & inhibitors
  • Aryl Hydrocarbon Hydroxylases / genetics
  • Aryl Hydrocarbon Hydroxylases / metabolism
  • Aryl Hydrocarbon Hydroxylases / pharmacology*
  • Bilirubin / chemistry
  • Bilirubin / metabolism*
  • Cadmium Chloride / administration & dosage
  • Cadmium Chloride / adverse effects
  • Cadmium Chloride / metabolism
  • Cytochrome P-450 CYP2A6
  • Cytochrome P-450 Enzyme Inhibitors
  • Cytochrome P-450 Enzyme System / metabolism
  • Cytochrome P450 Family 2
  • Heme Oxygenase-1 / biosynthesis
  • Heme Oxygenase-1 / genetics
  • Heme Oxygenase-1 / metabolism
  • Inactivation, Metabolic / genetics
  • Inactivation, Metabolic / physiology
  • Injections, Intraperitoneal
  • Liver / chemistry
  • Liver / drug effects
  • Liver / metabolism
  • Male
  • Membrane Proteins / genetics
  • Membrane Proteins / metabolism
  • Mice
  • Mice, Inbred DBA
  • Microsomes, Liver / chemistry
  • Microsomes, Liver / drug effects
  • Microsomes, Liver / metabolism*
  • Mixed Function Oxygenases / antagonists & inhibitors
  • Mixed Function Oxygenases / genetics
  • Mixed Function Oxygenases / metabolism
  • Mixed Function Oxygenases / pharmacology*
  • Oxazines / antagonists & inhibitors
  • Oxazines / metabolism
  • Oxidative Stress / drug effects
  • Oxidoreductases / antagonists & inhibitors
  • Oxidoreductases / metabolism
  • RNA, Messenger
  • Time Factors
  • Up-Regulation / drug effects

Substances

  • Cytochrome P-450 Enzyme Inhibitors
  • Membrane Proteins
  • Oxazines
  • RNA, Messenger
  • ethoxyresorufin
  • Cytochrome P-450 Enzyme System
  • Mixed Function Oxygenases
  • Oxidoreductases
  • methoxyresorufin-O-demethylase
  • Aryl Hydrocarbon Hydroxylases
  • Cyp2a5 protein, mouse
  • Cytochrome P-450 CYP2A6
  • Cytochrome P450 Family 2
  • Heme Oxygenase-1
  • Hmox1 protein, mouse
  • Cadmium Chloride
  • Bilirubin