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The metabolic rate constants and specific activity of human and rat hepatic cytochrome P-450 2E1 toward toluene and chloroform.

Authors
Lipscomb-JC; Barton-HA; Tornero-Velez-R; Evans-MV; Alcasey-S; Snawder-JE; Laskey-J
Source
J Toxicol Environ Health, A 2004 Apr; 67(7):537-553
NIOSHTIC No.
20024735
Abstract
Chloroform (CHCl3) is a near-ubiquitous environmental contaminant, a by-product of the disinfection of drinking water sources and a commercially important compound. Standards for safe exposure have been established based on information defining its toxicity, which is mediated by metabolites. The metabolism of CHCl3 is via cytochrome P-450 2E1 (CYP2E1)-mediated oxidation to phosgene, which is known to obey a saturable mechanism. CYP2E1 is a highly conserved form, expressed in all mammalian systems studied, and is responsible for the metabolism of a great many low-molecular-weight (halogenated) compounds. However, the Michaelis-Menten rate constants for CHCl3 oxidation have not been derived in vitro, and the specific activity of CYP2E1 toward CHCl3 has not been reported. In this investigation with microsomal protein (MSP), apparent V(max) values of 27.6 and 28.3 nmol/h/mg MSP and apparent K(m) values of 1 and 0.15 microM in rats and human organ donors, respectively, were demonstrated. The specific activity of CYP2E1 toward CHCl3 in rats and humans was 5.29 and 5.24 pmol/min/pmol CYP2E1, respectively. Toluene metabolism to benzyl alcohol (BA), another CYP2E1-dependent reaction, was also highly dependent on CYP2E1 content in humans, and was more efficient than was CHCl3 metabolism. The specific activity of human CYP2E1 toward toluene metabolism in human MSP was 23 pmol/min/pmol CYP2E1. These results demonstrate that differences in CYP2E1 content of MSP among individuals and between species are largely responsible for observed differences in toluene and CHCl3 metabolism in vitro.
Keywords
Chloromethanes; Toluenes; Metabolic-rate; Metabolism; Enzymes; Microsomal-enzymes; Humans; Animals; In-vitro-study; Hepatic-microsomal-enzymes; Liver-cells; Liver-function; Genes; Oxidation; Solvents; Environmental-contamination
Contact
U.S. Environmental Protection Agency, Office of Research and Development, National Center for Environmental Assessment, Cincinnati, OH 45268
CODEN
JTEHD6
CAS No.
67-66-3; 108-88-3; 75-44-5; 100-51-6
Publication Date
20040409
Document Type
Journal Article
Email Address
lipscomb.john@epa.gov
Funding Type
Construction
Fiscal Year
2004
NTIS Accession No.
NTIS Price
Issue of Publication
7
ISSN
1528-7394
NIOSH Division
DART
Priority Area
Research Tools and Approaches: Exposure Assessment Methods
Source Name
Journal of Toxicology and Environmental Health, Part A: Current Issues
State
NC; OH
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