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

Abstract

Disinfection of drinking water protects public health against waterborne pathogens. However, during disinfection, toxic disinfection byproducts (DBPs) are formed. Exposure to DBPs was associated with increased risk of bladder cancer in humans. DBPs are generated at concentrations below their carcinogenic potencies; it is unclear how exposure leads to adverse health outcomes. We used computational estimates of the energy of the lowest unoccupied molecular orbital (ELUMO) to predict thiol reactivity and additive toxicity among soft electrophile DBPs. Bromoacetic acid (BAA) was identified as non-thiol-reactive, which was supported by in chemico and in vitro data. Bromoacetonitrile (BAN) and bromoacetamide (BAM) were thiol-reactive. Genotoxicity induced by these compounds was reduced by increasing the thiol pool with N-acetyl l-cysteine (NAC), while NAC had little effect on BAA. BAN and BAM shared depletion of glutathione (GSH) or cellular thiols as a molecular initiating event (MIE), whereas BAA induces toxicity through another pathway. Binary mixtures of BAM and BAN expressed a potentiating effect in genotoxicity. We found that soft electrophile DBPs could be an important predictor of common mechanism groups that demonstrated additive toxicity. In silico estimates of ELUMO could be used to identify the most relevant DBPs that are the forcing factors of the toxicity of finished drinking waters.

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

“…identified an IAM > BAM > CAM rank order of electrophilic softness. This pattern matched the measured in chemico reactivity; a similar trend was previously observed where the low E LUMO compounds BAN and BAM were more thiol reactive than bromoacetate, which had a higher LUMO energy (Pals et al, 2016). These observations suggest electrophilic softness is a useful indicator of thiol/thiolate reactivity in chemico for α-halogenated carbonyl/nitrile containing DBPs.…”

“…After exposure to DBPs (30 min), the remaining free thiol was quantified with Ellman’s reagent (Ellman, 1959) with minor modifications of the previously published protocol (Pals et al, 2016). For these experiments, Ellman’s reagent was prepared from stock solutions of 100 mM DTNB in dimethyl sulfoxide (DMSO), and 200 mM EDTA in deionized water diluted to 1 mM and 0.1 mM, respectively, in 200 mM Tris buffered deionized water (pH 8.0).…”

“…haloacetonitriles (HANs), haloacetamides (HAMs), and haloacetic acids (HAAs) correlated with predicted S N 2 reactivity (Muellner et al, 2007; Plewa et al, 2008; Plewa et al, 2010). Genotoxicity induced by the α-halogenated carbonyl (or nitrile) containing DBPs, bromoacetamide (BAM), and bromoacetonitrile (BAN) were significantly reduced by co-treating with N-acetylcysteine (NAC) (Pals et al, 2016). Therefore, reactivity seems to play a significant role in the toxicity of DBPs directly perturbing the intracellular thiol pool through alkylation of cellular thiol/thiolates leading to a toxic response by disrupting various cellular processes (Schultz et al, 2006).…”

“…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). …”

“…Estimations of E LUMO , therefore, provide a theoretical method for identifying soft electrophiles among identified DBPs. Our previous work showed that E LUMO was a useful predictor of thiol reactivity and genotoxicity in a set of mono-brominated DBPs (bromoacetic acid, BAM, and BAN) (Pals et al, 2016). However, because reactivity of alkyl halides is dependent on halogen leaving efficiency, the effect of variable halogen substitution requires additional investigation.…”