Mixture Toxicity of SN2-Reactive Soft Electrophiles: 3. Evaluation of Ethyl α-Halogenated Acetates with α-Halogenated Acetonitriles

Abstract

Mixture toxicity for each of four ethyl α-halogenated acetates (ExACs) with each of three α-halogenated acetonitriles (xANs) was assessed. Inhibition of bioluminescence in Vibrio fischeri was measured after 15, 30 and 45-min of exposure. Concentration-response curves were developed for each chemical at each exposure duration and used to develop predicted concentration-response curves for the dose-addition and independence models of combined effect. Concentration-response curves for each mixture and each exposure duration were then evaluated against the predicted curves, using three metrics per model: 1) EC50-based additivity quotient (AQ) or independence quotient (IQ) values, 2) mean AQ (mAQ) or mean IQ (mIQ) values, calculated by averaging the EC25, EC50 and EC75 AQ or IQ values, and 3) deviation values from additivity (DV-A) or independence (DV-I). Mixture toxicity for ethyl iodoacetate (EIAC) was dose-additive with each of the xANs at all exposure durations and was often consistent with independence as well. The same was true for mixture toxicity of ethyl bromoacetate (EBAC) with each xAN. However, for the two more slowly reactive chemicals ethyl chloroacetate (ECAC) and ethyl fluoroacetate (EFAC) mixture toxicity with each xAN only became consistent with dose-addition upon increasing exposure duration. Consistency with independence for both ECAC and EFAC with the xANs was essentially limited to the EC50-IQ metric; thereby demonstrating the utility of calculating the mean quotient (mAQ, mIQ) and deviation value (DV-A, DV-I) metrics. Upon review of these findings with those from the first two papers in the series, the results suggest that instances in which mixture toxicity was not consistent with dose-addition relate: 1) to differences in the capability of the chemicals to form strong H-bonds with water and 2) to differences in relative reactivity and time-dependent toxicity levels of the chemicals.

Energy of the Lowest Unoccupied Molecular Orbital, Thiol Reactivity, and Toxicity of Three Monobrominated Water Disinfection Byproducts

“…These data support the hypothesis that the soft electrophiles in this DBP mixture have a cumulative toxicity and support previous findings of dose additivity in S N 2-reactive electrophiles. 12–14 …”

“…9–11 Dawson et al investigated cumulative toxicity of S N 2-reactive haloacetonitriles (HANs) and ethyl- α -halogenated acetates and reported some instances of dose additive toxicity within and among these chemical classes. 12–14 The electrophiles within these classes reacted with glutathione (GSH) in chemico ; the reported dose additive toxicity may be attributed to collective GSH depletion as a shared mechanism or molecular initiating event (MIE). 12–14 Investigating S N 2-reactive compounds or GSH-depleting agents as a CMG may aid in evaluating additive toxicity within complex DBP mixtures.…”

“…12–14 The electrophiles within these classes reacted with glutathione (GSH) in chemico ; the reported dose additive toxicity may be attributed to collective GSH depletion as a shared mechanism or molecular initiating event (MIE). 12–14 Investigating S N 2-reactive compounds or GSH-depleting agents as a CMG may aid in evaluating additive toxicity within complex DBP mixtures. Many of the genotoxic DBP chemical classes, including the HANs, 10 haloacetamides (HAMs), 11 and haloacetic acids (HAAs), 15 expressed correlations between genotoxicity and S N 2 reactivity; furthermore, many DBPs, including these chemical classes, are known to react with or deplete GSH and generate oxidative stress.…”

Monohalogenated acetamide-induced cellular stress and genotoxicity are related to electrophilic softness and thiol/thiolate reactivity

“…In fact, experiments with α,β-unsaturated carbonyl derivatives, showed additive toxicity in binary and ternary mixtures in vitro and in an in vivo model (Zhang et al, 2016). Work by Dawson and colleagues showed that HANs and halogenated ethyl acetates (α-halogenated carbonyl containing compounds) generated, in some cases, dose additive toxicity within and among these chemical classes (Dawson et al, 2010; Dawson et al, 2011; Dawson et al, 2014). Additive genotoxicity was observed in binary mixtures of BAM and BAN (Pals et al, 2016).…”

Evaluation of consistency for multiple experiments of a single combination in the time-dependence mixture toxicity assay

“…These studies have examined toxicity of selected mixtures of industrial organic chemicals (e.g., Gagan et al 2007, Dawson et al 2008), developed and evaluated the 5PL-1P function for concentration-response curve-fitting (Dawson et al 2012, 2016) and examined the viability of using time-dependent toxicity (TDT) of the individual chemicals for predicting TDT of the mixtures (Dawson et al 2014a). The design of these studies has included multiple tests of individual chemicals (Dawson et al 2010, 2011, 2014b, 2016) thereby allowing for assessment of test-to-test consistency of those data, but none of these studies had evaluated such consistency over several tests of the same mixture. For this system to be a reliable approach for evaluating mixture toxicity of commonly used industrial organics such an assessment was needed.…”