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