Paraoxon equivalence

Data Reduction and Analysis For inhibition profiles, the means of triplicate data points (at individual inhibitor concentrations) were plotted. IC50 values were determined using either a log-logit or four parameter fit. (Correlation coefficients were determined from the best fit of these data.) Error bars representing standard deviations (SD) are only presented in selected inhibition profiles. Paraoxon equivalence (%), PE, was determined using the following relationship ... 

IC50 Values Because of its stability, strong inhibitory effect and commercial availability, paraoxon has often been used as a reference compound for cholinesterase inhibition (7). Asa result of the widespread use of paraoxon as a reference inhibitor, we elected to compare the relative potency of compounds assayed to this inhibitor in the form of paraoxon equivalence. IC50 values and % inhibition relative to paraoxon (paraoxon equivalence) were determined for a wide range of cholinesterase inhibitors (Table III). The extent to which the data fit a four parameter curve fit or log-logit fit are also included as correlation coefficients. The compounds are placed in ascending order of calculated IC50 values. 

IC50 Values for Oxidized OP Compounds Table IV shows IC50 values and paraoxon equivalence values for selected phosphorothioate OP insecticides that were assayed with and without prior bromine oxidation. In addition, where commercially available, the oxon derivatives are also compared. The bromine oxidation protocol significantly increased the sensitivity of this assay to many of these insecticides. In about half of the cases the IC50 values were decreased between 20 and 300 times, however, several of the compounds showed only... 

Fig. 21 A plot of the observed pseudo-first-order rate constants (kabs) for methanolysis of 2.5 x 10 SM paraoxon vs. [35 2Zn(II)] in the presence of 1 equivalent of added CH30 per complex, pH = 9.5, T = 25 + 0.1 °C. Dotted line is presented as a visual aid directed through all actual data (O) solid line is a linear fit through the data ( ) corrected for inhibition by triflate counterions. Reproduced with permission from ref. 95.

The procedure involves converting oxon to thion toxicity equivalents by multiplying the oxon value by its relative toxicity (ED of thion r ED,.q of oxon) in Table I. The ED. value is the aermal dose in ug/cnr of total body surface which produces 50% inhibition of red cell ChE activity 72 hours after application. The total thion and oxon level is then divided by the thion toxicity equivalents and the factor is multiplied by the safe level established for thion in Table I. This procedure was conducted for the dislodgeable residues of parathion-paraoxon, methidathion-methidathion oxon, and azinphosmethyl-azinphosmethyl oxon. The safe levels for the total disloggeable residues were determined to be 0.06, 0.2 and 1.6 ug/cm, respectively, for... 

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