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IN VITRO DETERMINATION OF KINETIC CONSTANTS FOR 1,3-DICHLOROPROPANE, 2,2-DICHLOROPROPANE, AND 1,1-DICHLOROPROPENE IN RAT LIVER MICROSOMES AND CYTOSOL
Citation:
TORNERO-VELEZ, R., J. Laskey, AND M V. Evans. IN VITRO DETERMINATION OF KINETIC CONSTANTS FOR 1,3-DICHLOROPROPANE, 2,2-DICHLOROPROPANE, AND 1,1-DICHLOROPROPENE IN RAT LIVER MICROSOMES AND CYTOSOL. Presented at Society of Toxicology, Salt Lake City, UT, March 9-13, 2003.
Impact/Purpose:
Research will be conducted to develop and apply integrated microenvironmental, and physiologically-based pharmacokinetic (PBPK) exposure-dose models and methods (that account for all media, routes, pathways and endpoints). Specific efforts will focus on the following areas:
1) Develop the Exposure Related Dose Estimating Model (ERDEM) System.
Includes: Updating the subsystems and compartments of the ERDEM models with those features needed for modeling chemicals of interest to risk assessors;
Designing and implementing the graphical user interface for added features.
Refining the exposure interface to handle various sources of exposure information;
Providing tools for post processing as well as for uncertainty and variability analyses;
Research on numerical and symbolic mathematical/statistical solution methods and computational algorithms/software for deterministic and stochastic systems analysis.
2) Apply ERDEM and other quantitative models to understand pharmacokinetics (PK) and significantly reduce the uncertainty in the dosimetry of specific compounds of regulatory interest.
Examples of the applications are:
exposure of children to pesticides
study design
route-to-route extrapolation
species extrapolation
experimental data analysis
relationship between parametric uncertainty and the distribution of model results
validity of scaling methods within species
validity of scaling methods from one species to another species
reduction of uncertainty factors for risk assessment
Description:
The Safe Drinking Water Act requires the U.S. EPA to establish a list of contaminants to aid in priority setting for the Agency's drinking water program. The 1998 Contaminant Candidate List (CCL) designated 1,3-Dichloropropane (1,3-DCP), 2,2-dichloropropane (2,2-DCP), and 1,1-dichloropropene (1,1-DCP) for health effects research. We determined kinetic constants for these compounds for cytochrome P450 and glutathione (GSH)-dependent conjugation because these pathways play an important role in the toxicity of halogenated hydrocarbons. Hepatic microsomal (M) and cytosolic (C) fractions were prepared from male Fisher 344 rats and an automated equilibrium headspace technique was employed to assess substrate loss. Headspace concentrations were determined by gas-chromatography and concentration time-courses were modeled with a two-compartmental gas-medium model and analyzed with Matlab?. We observed no metabolism for 2,2-DCP via the glutathione-dependent conjugation pathway and only a minor level of clearance via the P450 pathway (7x10-4 L/h/mg M-protein). The clearance of 1,3-DCP via the P450 pathway was substantially greater than that of 2,2-DCP (4.1x10-2 L/h/mg M-protein; Vmax=641 pmol/min/mg M-protein, Km=0.95 uM). The clearance of 1,3-DCP via the GSH conjugation pathway was minor relative to the P450 pathway (2.4x10-4 L/h/mg C-protein; 0.6% of microsomal rate), and was consistent with the results of other a,w-dihaloalkanes that show increased rates of GSH conjugation with increasing chain length. The clearance of 1,1-DCP via the P450 pathway (2.26x10-2 L/h/mg M-protein; Vmax=3430 pmol/min/mg M-protein, Km=2.5 uM ) was substantially greater than for the GSH conjugation pathway (3.9x10-4 L/h/mg C-protein; 1.72% of microsomal rate). In summary, this in vitro approach has proven useful in evaluating the metabolism of volatile halogenated hydrocarbons. These rate constants may be incorporated in pharmacokinetic (PK) models to predict in vivo kinetics.
This is an abstract of a proposed presentation and does not necessarily reflect United States Environmental Protection Agency (EPA) policy. The actual presentation has not been peer reviewed by EPA. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.