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Unexpected Transformation of Dissolved Phenols to Toxic Dicarbonyls by Hydroxyl Radicals and UV Light

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Unexpected Transformation of Dissolved Phenols to Toxic Dicarbonyls by Hydroxyl Radicals and UV Light Unexpected transformation of dissolved phenols to toxic dicarbonyls by hydroxyl radicals and UV light Carsten Prassea,b, Breanna Fordc, Daniel K. Nomurac,d,e, and David L. Sedlaka,1 aDepartment of Civil and Environmental Engineering, University of California, Berkeley, CA 94720; bDepartment of Environmental Health and Engineering, Johns Hopkins University, Baltimore, MD 21218; cDepartment of Nutritional Sciences and Toxicology, University of California, Berkeley, CA 94720; dDepartment of Chemistry, University of California, Berkeley, CA 94720; and eDepartment of Molecular and Cell Biology, University of California, Berkeley, CA 94720 Edited by Thomas M. Young, University of California, Davis, CA, and accepted by Editorial Board Member David W. Schindler January 22, 2018 (received for review September 8, 2017) Water treatment systems frequently use strong oxidants or UV Despite the growing recognition that toxic transformation light to degrade chemicals that pose human health risks. Un- products may be formed during oxidative water treatment (9), fortunately, these treatments can result in the unintended trans- the potential health effects of this practice are uncertain. Be- formation of organic contaminants into toxic products. We report cause thousands of anthropogenic compounds may be present in an unexpected reaction through which exposure of phenolic com- drinking water sources, assessment of the effects of every com- pounds to hydroxyl radicals (•OH) or UV light results in the forma- pound that might be present is not feasible. Rather, novel ap- tion of toxic α,β-unsaturated enedials and oxoenals. We show that proaches are needed to prioritize further investigations of these transformation products damage proteins by reacting with compounds that are inherently toxic or that might be trans- lysine and cysteine moieties. We demonstrate that phenolic com- formed to toxic transformation products during water treatment. pounds react with •OH produced by the increasingly popular UV/ In toxicology, recognition of the importance of molecular in- hydrogen peroxide (H2O2) water treatment process or UV light to teractions of chemicals with biomolecules has led to the devel- form toxic enedials and oxoenals. In addition to raising concerns opment of the adverse outcome pathway concept (10). As a key about potential health risks of oxidative water treatment, our feature, molecular initiating events (e.g., the formation of co- SCIENCES findings suggest the potential for formation of these toxic com- valent adducts by reaction of both endogenous and exogenous ENVIRONMENTAL pounds in sunlit surface waters, atmospheric water, and living electrophiles with proteins and DNA) have been recognized as cells. For the latter, our findings may be particularly relevant to an important mechanism involved in a variety of adverse health efforts to understand cellular damage caused by in vivo produc- outcomes, including cancer and cardiovascular diseases (11, 12). tion of reactive oxygen species. In particular, we demonstrate that This has also led to the development of screening tools that allow exposure of the amino acid tyrosine to •OH yields an electrophilic for the assessment of reactive candidate pharmaceuticals and enedial product that undergoes cross-linking reaction with both their metabolites by investigating the formation of covalent ad- lysine and cysteine residues. ducts formed when test compounds react with amino acids and proteins (13, 14). To assess the potential for toxic products of reactive transformation products | water treatment | advanced oxidation processes | chemoproteomics | exposome oxidative water treatment, we adapted this approach to iden- tify reactive electrophiles that are formed during oxidative y 2050, two-thirds of the world’s population will be living in Significance Bcities that are increasingly reliant on drinking water sources affected by agricultural runoff and industrial and municipal wastewater discharges (1, 2). These drinking water sources often Phenols are common anthropogenic and natural chemicals that contain trace concentrations of phenolic compounds that are contaminate drinking water sources. To reduce exposure to widely used in dyes, surfactants, pharmaceuticals, and pesticides, these compounds, hydroxyl radicals are often used as chemical including bisphenol A, triclosan, and nonylphenol-ethoxylates (3, oxidants during water treatment. Although this treatment 4). As a result of concerns about adverse health effects from process removes phenols, we have found that it unexpectedly chronic exposure to phenolic compounds, the Environmental produces toxic transformation products. We identify these Protection Agency and other regulatory agencies require the products and simultaneously assess their toxicity with a tech- removal of certain phenolic compounds during water treatment. nique that detects products formed when the transformation One approach for treating phenol-containing water involves the products react with amino acids and peptides. Our results oxidation of these compounds with hydroxyl radicals (•OH). highlight the potential risks of using oxidative treatment on •OH-based treatment technologies, such as the use of UV light alternative drinking water sources, such as contaminated groundwater and recycled municipal wastewater. They also to photolyze hydrogen peroxide (i.e., the UV/H2O2 process), are becoming increasingly common in drinking water treatment, suggest that these reactions produce these toxic trans- potable water reuse, and remediation of contaminated ground- formation products in other situations, including in clouds and water at hazardous waste sites. In these processes, •OH trans- sunlit surface waters and within living cells. form phenolic compounds and other organic contaminants Author contributions: C.P. and D.L.S. designed research; C.P. and B.F. performed research; through a series of reactions that result in addition of oxygen- D.K.N. contributed new reagents/analytic tools; C.P. and B.F. analyzed data; and C.P., B.F., containing functional groups to the compounds (5, 6). Although D.K.N., and D.L.S. wrote the paper. the transformation products formed in these reactions are fre- The authors declare no conflict of interest. quently less toxic then the parent compounds and often can be This article is a PNAS Direct Submission. T.M.Y. is a guest editor invited by the Editorial more easily removed in subsequent water treatment processes, Board. oxidation of phenols can also lead to the formation of toxic This open access article is distributed under Creative Commons Attribution-NonCommercial- transformation products such as p- and o-benzoquinone (7). NoDerivatives License 4.0 (CC BY-NC-ND). Exposure to quinones is a toxicological concern because their 1To whom correspondence should be addressed. Email: [email protected]. electrophilic character leads to cellular damage through reac- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. tions with nucleophilic groups in proteins and DNA (8). 1073/pnas.1715821115/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1715821115 PNAS Latest Articles | 1of6 water treatment. To ensure the sensitive detection of oxidation the degradation of phenol or the formation of 2-butene-1,4-dial. products of toxicological relevance, we targeted adducts pro- The formation of the transformation product was also observed duced when the oxidation products reacted with nucleophilic when a low-pressure mercury lamp was used as light source. moieties in biomolecules, including primary amine moieties in Decades of research have established a pathway for phenol N-α-acetyl-lysine (NAL) and thiol functional groups in glutathi- oxidation by •OH or UV light, consisting of initial hydroxylation one (GSH) and N-acetyl-cysteine (NAC). of the aromatic ring to form hydroquinone, benzoquinone, and catechol, followed by subsequent ring hydroxylation and cleav- Results and Discussion age to produce short-chain organic acids, such as maleic acid, To assess a representative treatment system, we oxidized phenol formic acid, and oxalic acid (7). Direct cleavage of aromatic • with OH produced by the UV/H2O2 process. After exposure, we rings without sequential hydroxylation has been postulated added NAL and detected the formation of an adduct, R-2- (19), but our findings represent experimental evidence that (acetylamino)-6-(2,5-dihydro-2-oxo-1H-pyrrol-1-yl)-1-hexanoic α,β-unsaturated aldehydes are formed as an initial product of acid (m/z 255), which increased in concentration as the phenol aqueous phase phenol oxidation by exposure to •OH or UV concentration decreased (Fig. 1A and SI Appendix, Fig. S1). light. The formation of α,β-unsaturated aldehydes, including 2- Similarly, addition of either GSH or an equimolar mixture of butene-1,4-dial and 4-oxo-2-pentenal, from the reaction of •OH NAC and NAL after oxidation of phenol in the UV/H2O2 pro- with benzene and various methylated benzenes has been ob- cess yielded the adducts N-[4-carboxy-4-(3-mercapto-1H-pyrrol- served in the gas phase (20–24). The initial step in the gas phase 1-yl)-1-oxobutyl]-L-cysteinylglycine cyclic sulfide (m/z 356) and reaction mechanism yields carbon-centered radicals, either by N-acetyl-S-[1-[5-(acetylamino)-5-carboxypentyl]-1H-pyrrol-3-yl]- H-abstraction or OH-addition, which subsequently react with L-cysteine adducts (m/z 400), respectively. These data indicate dioxygen to produce peroxides. These unstable intermediates that the oxidation of phenol by
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