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THE IMPORTANCE OF OBTAINING INFORMATION ON THE SPECIFIC CONTENT OF TISSUE ENZYMES METABOLIZING ORGANOPHOSPHORUS PESTICIDES, PRIOR TO DETERMINE VMAX, KM VALUES FOR USE IN PBPK MODELS
Citation:
Knaak, J. B., F W. Power, J N. Blancato, AND C C. Dary. THE IMPORTANCE OF OBTAINING INFORMATION ON THE SPECIFIC CONTENT OF TISSUE ENZYMES METABOLIZING ORGANOPHOSPHORUS PESTICIDES, PRIOR TO DETERMINE VMAX, KM VALUES FOR USE IN PBPK MODELS. Presented at Science Forum 2003, Washington, DC, May 5-7, 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:
Physiological pharmacokinetic/pharmacodynamic models require Vmax, Km values for the metabolism of OPs by tissue enzymes. Current literature values cannot be easily used in OP PBPK models (i.e., parathion and chlorpyrifos) because standard methodologies were not used in their determination. In practically all cases, the investigators failed to determine the specific content of the enzymes of interest in tissues or tissue fractions (PON1 and P450 isozymes) under investigation prior to use, by either spectral or immunochemical methods. The concentration of plasma PON1 (paraoxonase) may vary as much as 60 to 600 ug/ml making it difficult to select proper enzyme-substrate concentrations for the determination of Vmax, Km. The same problem exists measuring P-450 activity using microsomal protein, when the specific content (pmoles of P-450, total or individual CYPs) in the microsomes is unknown. Rendic and Guengerich recently recommended individual CYPs (i.e, 1A2, 2B6, 2D6 and 3A4) be used to obtain Vmax, Km values in place of human whole liver microsomes. Mathematical equations are available for expressing individual CYP data (specific content and activity) in terms of native liver microsomes and using the data in PBPK models. Methods will be presented for obtaining Vmax, Km values for parathion and chlorpyrifos based on the specific content of PON1 and P-450 in plasma and liver microsomes.