Mixture Toxicity of SN2-Reactive Soft Electrophiles: 1. Evaluation of Mixtures Containing α-Halogenated Acetonitriles

Abstract

The concept of multiple modes of toxic action denotes that an individual chemical can induce two or more toxic effects within the same series of concentrations, for example, reactive toxicity and narcosis. It appears that such toxicity confounds the ability to develop precise predictions of mixture toxicity and makes it more difficult to clearly link a dose-additive combined effect to agents in the mixture having a single common mechanism of toxic action. This initial study of a three-part series begins to examine this issue in greater detail by testing three α-halogenated acetonitriles: (1) in sham combinations, (2) in true combinations, and (3) with a nonreactive nonpolar narcotic. Iodo-, bromo-, and chloro-derivatives of acetonitrile were selected for testing based on their electro(nucleo)philic reactivity, via the SN2 mechanism, and their time-dependent toxicity individually. Reactivity of each agent was assessed in tests with the model nucleophile glutathione (GSH). Each acetonitrile was reactive with GSH, but the nonpolar narcotic 3-methyl-2-butanone was not. In addition, toxicity of the agents alone and in mixtures was assessed using the Microtox® acute toxicity test at three time points: 15, 30, and 45 min of exposure. Each of the three agents alone had time-dependent toxicity values of about 100%, making it likely that most of the toxicity of these agents, at these times, was due to reactivity. In contrast, the nonpolar narcotic agent lacked time-dependent toxicity. In mixture testing, sham combinations of each acetonitrile showed a combined effect consistent with predicted effects for dose-addition at each time point, as did the sham combination of the nonpolar narcotic. Mixture toxicity results for true acetonitrile combinations were also consistent with dose-addition, but the acetonitrile–nonpolar narcotic combinations were generally not consistent with either the dose-addition or independence models of combined effect. Based on current understanding of mixture toxicity, these results were expected and provide a foundation for the second and third studies in the series.

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

“…The RC 50 values, concentrations that eliminated 50% of the initial free thiol, were 0.559 mM and 0.973 mM for IAM and BAM respectively (Table 1). These data were consistent with the I > Br > Cl pattern of reactivity in a larger group of α-halogenated carbonyl compounds (Dawson et al, 2010; Dawson et al, 2011; Schultz et al, 2007). …”

“…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).…”

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.…”