Endosulfan-alpha Induces CYP26 and CYP3A4 by Activating the Pregnane X Receptor But Not the Constitutive Androstane Receptor
Abstract
The purpose of this research was to establish the metabolic pathway of endosulfan in humans and to elucidate a potential mechanism for endosulfan's endocrine disruptive effects. We hypothesized that endosulfan may exert its endocrine disrupting effects by activating the pregnane X receptor (PXR) and/or the constitutive androstane receptor (CAR) and inducing the expression levels of cytochrome P450 (CYP) enzymes, thereby increasing metabolic rates and the biotransformation of testosterone. In these studies, we utilized endosulfan-cx, the more predominant isomer in technical-grade endosulfan. In Chapter 1, we determined that endosulfan-a is metabolized to a single metabolite, endosulfan sulfate, in pooled human liver microsomes (K(sub m) = 9.8 micronmeter, V(sub max) = 178.5 pmol/mg/min). With the use of recombinant cytochrome P450 (rCYP) isoforms expressed in baculovirus-infected cells (supersomes), we identified CYP2B6 (K(sub m) = 16.2 micronmeter, V(sub max) = 11.4 umol/nmol CYP/min) and CYP3A4 (K(sub m) = 14.4 micronmeter, V(sub max) = 1.3 nmol/nmol CYP/min) as the primary enzymes catalyzing the metabolism of endosulfan-alpha, albeit CYP2B6 had an 8-fold higher intrinsic clearance rate (CL(sub int) = 0.70 micronL/min/pmol CYP) than CYP3A4 (CL(sub int) = 0.09 micronL/min/pmol CYP). Using commercially available individual human liver microsomes (FILM), a strong correlation was observed with endosulfan sulfate formation and S- 2 mephenytoin N-demethylase activity of CYP2B6 (r(exp 2) = 0.79) and a moderate correlation with testosterone 6-beta-hyroxylase activity of CYP3A4 (r(exp 2) = 0.54).
Document Details
- Document Type
- Technical Report
- Publication Date
- Jan 01, 2006
- Accession Number
- ADA450047
Entities
People
- Richard C. Casabar
Organizations
- North Carolina State University