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COMPARISON BETWEEN ELLMAN AND RADIOMETRIC METHODS FOR ASSESSING CHOLINESTERASE (CHE) INHIBITION IN RATS TREATED WITH N-METHYL CARBAMATE INSECTICIDES.
Citation:
Padilla, S J., R. Marshall, D L. Hunter, V C. Moser, R. W. Setzer, D. Chen, D. Chen, AND A. Lowit. COMPARISON BETWEEN ELLMAN AND RADIOMETRIC METHODS FOR ASSESSING CHOLINESTERASE (CHE) INHIBITION IN RATS TREATED WITH N-METHYL CARBAMATE INSECTICIDES. Presented at Society of Toxicology, New Orleans, LA, March 06 - 10, 2005.
Description:
Carbamylated ChE is unstable and readily reactivates. This reactivation, promoted by increasing temperature and dilution, could have an impact on ex vivo ChE assays by decreasing apparent ChE inhibition. To assess the best method for measuring ChE inhibition in brain and RBCs from animals treated with one of 5 different carbamate insecticides, ChE was analyzed using both the Ellman method (encourages reactivation through tissue dilution and higher temperature) and a radiometric method (limits reactivation with lower tissue dilution and lower temperature). All tissues were prepared in exactly the same manner, and only diluted immediately before assay. Each tissue was assessed using both assays on the same day. Adult male rats were dosed orally with carbaryl, methomyl, methiocarb, oxamyl, or propoxur. At the time of peak effect, following a single dose of these compounds, the Ellman method underestimated the level of ChE inhibition; the degree of reactivation depended on the carbamate and ranged from 10-40% for the brain and 10-45% for the RBCs. Comparison of the dose-response curves showed an even greater difference between the methods: at the extreme, the Ellman method showed no brain ChE inhibition in any propoxur dose group, whereas radiometric data showed a significant, predictable dose-response relationship. In addition, the variability in the data was increased using the Ellman method. The combined coefficients of variation (CV; (StDev/mean) x 100) for all dosage groups for all compounds were 11% (brain) and 25% (RBC) for the radiometric method, but for the Ellman method, CVs were increased: 21% (brain) and 38% (RBC). These data indicate that the radiometric method is far superior to the Ellman method for assessing ChE inhibition in animals treated with N-methyl carbamate insecticides. This is an abstract of a proposed presentation and does not reflect Agency policy.