Integration of metabolomics and in vitro metabolism assays for investigating the stereoselective transformation of triadimefon in rainbow trout

Using in vitro derived enzymatic reaction rates of metabolism to inform pesticide body burdens in amphibians

Understanding how pesticide exposure to non-target species influences toxicity is necessary to accurately assess the ecological risks these compounds pose. To assess the potential metabolic activation of broad use pesticides in amphibians, in vitro and in vivo metabolic rate constants were derived from toad (Anaxyrus terrestris) livers in experiments measuring the depletion of atrazine (ATZ), triadimefon (TDN), and fipronil (FIP) as well as formation of their metabolites. To determine the predictability of these in vitro derived rate constants, Fowler's toads (Anaxyrus fowleri) were exposed to soil contaminated with each of the pesticides at maximum application rate. Desethyl atrazine (DEA) and deisopropyl atrazine (DIA), both metabolites of ATZ, exhibited similar velocities (V_max) while the K_M constant for DIA was two times higher than DEA. TDN was metabolized into two diastereomers of triadimenol (TDL A and TDL B), where TDL B had a V_max around two times higher than TDL A. The metabolite fipronil sulfone's V_max and K_M were 150 pmol min^-1 mg^-1 and 29 μM, respectively. While intrinsic clearance rates for the pesticides ranged from 0.54 to 38.31 mL min^-1 kg^-1. Thus, gaining knowledge on differences in metabolism of pesticides within amphibians is important in estimating risk to these non-target species since the inherent toxicity of metabolites can differ from the parent compound.

Mechanistic approach to understanding the toxicity of the azole fungicide triadimefon to a nontarget aquatic insect and implications for exposure assessment

Mechanistic and stereoselective based in vitro metabolism assays were utlilized to gain insight into the toxic mode of action of the 1,2,4-triazole fungicide, triadimefon, with black fly (Diptera: Simuliidae) larvae. Based on results from enzyme inhibitor studies, the metabolism of triadimefon in black fly larvae microsomes was found to occur predominantly via an oxidative P450-mediated pathway; triadimenol was formed via the stereoselective reduction of the prochiral carbonyl group of triadimefon. The relatively minor contribution of carbonyl reduction suggests that triadimefon may inhibit ecdysone 20-monooxygenase and disrupt insect molting hormone biosynthesis. 48-h LC50 tests for triadimefon and triadimenol with black fly larvae yielded median values (with 95% confidence intervals) of 6.1 (5.8-6.4) and 22.3 (20.3-24.1) mg/L respectively. The exposure of black fly larvae to sublethal concentrations of triadimefon resulted in increased microsomal P450 activity and affected the microsomal rates of both triadimefon depletion and triadimenol formation. In contrast to trout, black fly larvae produced a higher fraction of the more toxic triadimenol stereoisomers, which may explain in part why triadimefon exhibited a significantly greater toxicity with black fly larvae than trout. These results illustrate that while LC50 tests conducted with commercial triadimenol would presumably expose each organism to the same relative abundance of the four triadimenol stereoisomers, LC50 tests with triadimefon ultimately expose each organism to a unique set of triadimenol stereoisomers depending upon the organism's stereoselective metabolism.