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

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dc.contributor.advisor Ronald Hines, Committee Member en_US
dc.contributor.advisor Yoshiaki Tsuji, Committee Member en_US
dc.contributor.advisor Andrew Wallace, Committee Member en_US
dc.contributor.advisor Ernest Hodgson, Committee Chair en_US
dc.contributor.advisor Jun Ninomiya-Tsuji, Committee Member en_US Croom, Edward Lee en_US 2010-08-19T18:13:58Z 2010-08-19T18:13:58Z 2010-04-13 en_US
dc.identifier.other etd-01092009-141447 en_US
dc.description.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. en_US
dc.rights I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dis sertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to NC State University or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. en_US
dc.subject bioactivation en_US
dc.subject chlorpyrifos en_US
dc.subject CYP2B6 en_US
dc.subject CYP3A4 en_US
dc.subject developmental expression en_US
dc.title Human Hepatic Expression of CYP2B6: Developmental Pattern and In Vitro Bioactivation of Chlorpyrifos. en_US PhD en_US dissertation en_US Toxicology en_US

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