Human Hepatic Expression of CYP2B6: Developmental Pattern and In Vitro Bioactivation of Chlorpyrifos.

Show full item record

Title: Human Hepatic Expression of CYP2B6: Developmental Pattern and In Vitro Bioactivation of Chlorpyrifos.
Author: Croom, Edward Lee
Advisors: Ronald Hines, Committee Member
Yoshiaki Tsuji, Committee Member
Andrew Wallace, Committee Member
Ernest Hodgson, Committee Chair
Jun Ninomiya-Tsuji, Committee Member
Abstract: CROOM, EDWARD LEE. Human Hepatic Expression of CYP2B6: Developmental Pattern and In Vitro Bioactivation of Chlorpyrifos. (Under the direction of Dr. Ernest Hodgson and the late Dr. Randy L. Rose). Chlorpyrifos is a widely used organophosphorothioate insecticide. Known human exposures to chlorpyrifos range from low dietary levels to the intentional ingestion of concentrated chlorpyrifos solutions as a means of suicide. Chlorpyrifos derives its in vivo toxicity through bioactivation by the cytochromes P450 (CYPs) to the neurotoxic metabolite, chlorpyrifos-oxon (CPO). Chlorpyrifos-induced toxicity occurs when the level of CPO produced exceeds the capacity to detoxify CPO before acetylcholinesterase inhibition occurs. Several human esterases detoxify CPO, the most efficient being the serum esterase, paraoxonase 1 (PON1). The capacity of PON1 to detoxify CPO is understood to depend on genotype, age and diet. PON1 expression can be developmentally delayed not plateauing until at least six months of age. The ability to produce CPO has not been as well studied in humans and there are questions regarding the impact of age, genetic variation on chlorpyrifos bioactivation. To assess CPO production variability, a series of in vitro metabolism studies were conducted with individual human liver microsomes incubated with chlorpyrifos. CPO production varied over 14-fold and was predicted by CYP2B6 activity but no relationship with CP2B6 genotype was observed. CYP expression can change substantially during development. However, CYP2B6 ontogeny has been poorly characterized. Earlier reports suggested a general lack of CYP2B6 expression in neonatal and fetal liver samples. CYP2B6 levels in 220 individual human liver microsomal samples ranging from 10 weeks gestation to 17 years were semi-quantitatively measured by western blot. CYP2B6 protein expression was determined to be significantly higher after the neonatal period and the percent of samples with detectable CYP2B6 protein increased from a low of 60% detectable in the first-trimester to over 90% detectable in samples from donors over 11 years of age. CYP2B6 is highly efficient at producing CPO, with a Km <1µM, and high Vmax >15 nmoles, which suggests that low levels of CYP2B6 could produce significant amounts of CPO. The increase in CYP2B6 expression after the neonatal period precedes the reported increase in PON1 levels before two years of age and suggests that some children could be at greater risk of CPS poisoning than older subjects.
Date: 2010-04-13
Degree: PhD
Discipline: Toxicology
URI: http://www.lib.ncsu.edu/resolver/1840.16/6156


Files in this item

Files Size Format View
etd.pdf 474.8Kb PDF View/Open

This item appears in the following Collection(s)

Show full item record