Pentachlorophenol and other chlorinated phenols are substrates for human hydroxysteroid sulfotransferase hSULT2A1

Cytosolic sulfotransferases in endocrine disruption

The mammalian cytosolic sulfotransferases (SULTs) catalyze the sulfation of endocrine hormones as well as a broad array of drugs, environmental chemicals, and other xenobiotics. Many endocrine-disrupting chemicals (EDCs) interact with these SULTs as substrates and inhibitors, and thereby alter sulfation reactions responsible for metabolism and regulation of endocrine hormones such as estrogens and thyroid hormones. EDCs or their metabolites may also regulate expression of SULTs through direct interaction with nuclear receptors and other transcription factors. Moreover, some sulfate esters derived from EDCs (EDC-sulfates) may serve as ligands for endocrine hormone receptors. While the sulfation of an EDC can lead to its excretion in the urine or bile, it may also result in retention of the EDC-sulfate through its reversible binding to serum proteins and thereby enable transport to other tissues for intracellular hydrolysis and subsequent endocrine disruption. This mini-review outlines the potential roles of SULTs and sulfation in the effects of EDCs and our evolving understanding of these processes.

Co-exposure health risk of benzo[a]pyrene with aromatic VOCs: Monoaromatic hydrocarbons inhibit the glucuronidation of benzo[a]pyrene

Occupational workers and residents near petrochemical industry facilities are exposed to multiple contaminants on a daily basis. However, little is known about the co-exposure effects of different pollutants based on biotransformation. The study examined benzo[a]pyrene (BaP), a representative polycyclic aromatic hydrocarbon related to the petrochemical industry, to investigate changes in toxicity and co-exposure mechanism associated with different monoaromatic hydrocarbons (MAHs). A central composite design method was used to simulate site co-exposure scenarios to reveal biotransformation of BaP when co-exposed with benzene, toluene, chlorobenzene, or nitrobenzene in microsome systems. BaP metabolism depended on MAH concentration, and association of MAH with microsome concentration/incubation time. Particularly, MAH co-exposure negatively affected BaP glucuronidation, an important phase Ⅱ detoxification process. BaP metabolite intensities decreased to 43%-80% for OH-BaP-G, and 32%-71% for diOH-BaP-G in co-exposure system with MAHs, compared with control group. Furthermore, glucuronidation was affected by competitive and time-dependent inhibition. Co-exposure significantly decreased gene expression of UGT 1A10 and BCRP/ABCG2 in HepG2 cells, which are involved in BaP detoxification through metabolism and transmembrane transportation. Therefore, human co-exposure to multiple contaminants may deteriorate toxic effects of these chemicals by disturbing metabolic pathways. This study provides a reference for assessing toxic effects and co-exposure risks of pollutants.

Suspect and non-target screening of contaminants of emerging concern in streams in agricultural watersheds

Surface water runoff is an important source of water contamination affecting nearby rivers and streams. Many rural creeks are documented habitats for important aquatic species and the focus of restoration activities. In this study, we collected creek water samples in watersheds with a range of commercial-to-agricultural land use during rain events, and applied suspect and non-target screening with high-resolution mass spectrometry (HRMS) to characterize the occurrence of contaminants of emerging concern (CECs). In total, 58 CECs were identified, and 36 of them were confirmed and semi-quantified with reference standards. Pesticides were detected in all land use, including urban/commercial areas. Some pesticides were observed at concentrations of >10,000 ng/L demonstrating the strong contamination input during rain events. Five pesticides (azoxystrobin, fludioxonil, 4-hydroxy-chlorothalonil, imidacloprid, 2-methyl-4-chlorophenoxyacetic acid) were prioritized based on their risk quotients. HRMS chemical profiles demonstrated the wide range of chemical exposures in a given stream system and that compounds associated with specific land uses occur across land uses. Temporal trends suggested that some CECs remain present in creek water for months, resulting in chronic exposures across the life stages of aquatic species. These findings highlight the potential for contamination from agricultural runoff and the associated ecological risk to aquatic species. SYNOPSIS: Suspect and non-target screening revealed the chronic occurrence of emerging contaminants in streams in agricultural catchments during rain events.

Occurrence and dietary exposure assessment of pentachlorophenol in livestock, poultry, and aquatic foods marketed in Guangdong Province, China: Based on food monitoring data from 2015 to 2018

As a persistent organic pollutant, pentachlorophenol (PCP) has serious impacts on human health. However, its presence in animal source food products sold in the Guangdong Province (GD) of China, and the resultant dietary exposure have not been elucidated. To address this gap, 3,100 samples from seven food categories, including beef, pork, mutton, offals, broilers, hen eggs, and farmed freshwater fish, marketed throughout four geographical regions of GD, were collected from 2015 to 2018. Gas chromatography coupled with mass spectrometry was employed to detect PCP levels in these food matrices. PCP was found in all food categories, but the average contamination levels were low, ranging from 0.40 µg/kg wet weight (ww) (hen eggs) to 5.85 µg/kg ww (offals). However, higher concentrations of PCP were detected (P < 0.05) in animal source food from the North region. Additionally, a temporal declining trend was observed in this four-year consecutive survey. The estimated human dietary exposure of PCP to population groups, including the general population and subgroups (male and female, children, and adults), was found to be far below the permissible daily intake (3 µg/kg body weight). Therefore, the health impacts of PCP should be correspondingly low for local residents, based on current toxicological knowledge. Regional exposure patterns varied due to different extents of contamination in the four areas, and pork, broilers, and freshwater fish were the major sources of dietary PCP exposure. PRACTICAL APPLICATION: As a persistent organic pollutant, pentachlorophenol (PCP) has serious impacts on human health. However, its presence in animal source food products sold in Guangdong Province of China, and the resultant dietary exposure have not been elucidated. In this study, we conducted an in-depth investigation on the occurrence of PCP in major foodstuff categories, including beef, pork, mutton, broilers, offals, hen eggs, and farmed freshwater fish, marketed in all 21 prefecture-level divisions of Guangdong Province, in order to provide integral insights for regulatory authorities.

A peroxidase coordinating to Zn (II) preventing heme bleaching and resistant to the interference of H2 O2

Dehaloperoxidase (DHP) catalyzes detoxifying halophenols. It is a heme-containing enzyme using H₂ O₂ as the oxidant. Heme bleaching from the active site is of great concern. In addition, the interference of DHP by H₂ O₂ leads to the inactivation of the enzyme. To solve these two problems, DHP is coordinated to Zn (II) in PBS buffer to form a biomineralized composite (DHP&Zn-CP). DHP&Zn-CP was characterized by measuring SEM and confocal images, as well as energy dispersive X-ray spectrometry mapping. Fluorescence spectra demonstrated that DHP&Zn-CP can prevent heme bleaching. Two-dimensional FTIR spectra were measured, dynamically providing insight into the structural change of DHP along the coordination process. Raman spectra were performed to analyze the structural change. The optical spectra confirmed that the forming of DHP&Zn-CP had a little effect on the structures of DHP. For the dehalogenation of 2,4,6-trichlorophenol, DHP&Zn-CP can tolerate the presence of H₂ O₂ and is resistant to the interference by H₂ O₂ . The catalytic efficiency of DHP&Zn-CP is much higher than that of free DHP.

Ascorbic acid induced activation of persulfate for pentachlorophenol degradation

In the present study, ascorbic acid (AA) induced persulfate activation was investigated for the further exploration of organic pollutants oxidation by persulfate. We interestingly found that AA showed a significant catalytic activity to persulfate. Under neutral pH and room temperature condition, about 71.3% of pentachlorophenol (PCP, 10 mg L^-1) was decomposed in 180 min with 40 mmol L^-1 persulfate and 1.0 mmol L^-1 AA, while only 15.4% and 3.2% of PCP was removed by alone persulfate and AA respectively. The result of EPR spectra identified sulfate radical (SO₄^•-) and hydroxyl radical (OH) were generated during the reaction between persulfate and AA. Quenching experiments confirmed that both SO₄^- and OH contributed to the decomposition of PCP. With the addition of AA augmented from 0 to 1 mmol L^-1, the PCP degradation ratio continuously increased. However, excess AA could consume the generated reactive oxygen species (ROSs) that led to the inhibition of PCP degradation. Meanwhile, the PCP degradation by persulfate-AA was strongly pH dependent. The PCP degradation rate was declined as the initial pH increased from 3.5 to 10.5. At pH above 12.5, the base activation began to predominate over AA activation of persulfate. Furthermore, it was observed that the AA inducing persulfate activation was related to the extent of AA ionization, while C₆H₈O₆ promoted the highest persulfate activation for the PCP degradation, and C₆H₆O₆^2- induced the lowest persulfate activation. This study indicates the high potential of AA induced persulfate activation for treatment of organochlorine contaminated water.

Hydroxylated and sulfated metabolites of commonly occurring airborne polychlorinated biphenyls inhibit human steroid sulfotransferases SULT1E1 and SULT2A1

Polychlorinated biphenyls (PCBs) are ubiquitous environmental contaminants that are associated with varied adverse health effects. Lower chlorinated PCBs are prevalent in indoor and outdoor air and can be metabolized to their hydroxylated derivatives (OH-PCBs) followed by sulfation to form PCB sulfates. Sulfation is also a means of signal termination for steroid hormones. The human estrogen sulfotransferase (SULT1E1) and alcohol/hydroxysteroid sulfotransferase (SULT2A1) catalyze the formation of steroid sulfates that are inactive at steroid hormone receptors. We investigated the inhibition of SULT1E1 (IC50s ranging from 7.2 nM to greater than 10 μM) and SULT2A1 (IC50s from 1.3 μM to over 100 μM) by five lower-chlorinated OH-PCBs and their corresponding PCB sulfates relevant to airborne PCB-exposure. Several congeners of lower chlorinated OH-PCBs relevant to airborne PCB exposures were potent inhibitors of SULT1E1 and SULT2A1 and thus have the potential to disrupt regulation of intracellular concentrations of the receptor-active steroid substrates for these enzymes.

The European water-based environmental quality standard for pentachlorophenol is NOT protective of benthic organisms

Risk management of toxic substances is often based on Environmental Quality Standards (EQS) set for the water compartment, assuming they will also protect benthic organisms. In the absence of experimental data, EQS for sediments can be estimated by the equilibrium partitioning approach. The present study investigates whether this approach is protective of benthic organisms against pentachlorophenol (PCP), a legacy contaminant and EU priority substance still used in some parts of the world. Three freshwater species of invertebrates with different life cycles and feeding behaviors (the oligochaetes Lumbriculus variegatus, Tubifex tubifex and the dipteran insect Chironomus riparius) were exposed to PCP spiked sediments (2.10-46.03mgPCP/kg d.w. plus controls) in laboratory standard tests. Exposure duration was 28days for T. tubifex and L. variegatus and 10 and 28days for C. riparius; according to the corresponding OECD guidelines. For each investigated end-point, dose-response data were normalized to the mean control and fitted to a four-parameter log-logistic model for calculating the corresponding EC₅₀ and EC₁₀. The ranges for EC₅₀ and EC₁₀ estimates were 4.39 (Chironomus riparius-emergence)-27.50 (Tubifex tubifex-cocoon) and 0.30 (T. tubifex-young worms) -16.70 (T. tubifex-cocoon) mg/kg d.w., respectively. The EC₅₀ and the EC₁₀ values of L. variegatus were within these ranges. Following the EU Technical Guidance for deriving EQS, the lowest EC₁₀ value of 0.30mg/kg (T. tubifex-young worms) resulted in a PCP quality standard (QS) for sediments of 30ng/g, about one fourth of the tentative QS of 119ng/g estimated by the equilibrium partitioning (EqP) approach. The response of benthic biota to PCP varied across organisms and across end-points for the same organism, so that the use of sediment PCP-QS calculated using the EqP-approach may be under-protective of the most sensitive organisms. Information on the possible effects of PCP on resident organisms must therefore be collected for appropriately managing aquatic systems.

Mechanistic insights into the specificity of human cytosolic sulfotransferase 2A1 (hSULT2A1) for hydroxylated polychlorinated biphenyls through the use of fluoro-tagged probes

Determining the relationships between the structures of substrates and inhibitors and their interactions with drug-metabolizing enzymes is of prime importance in predicting the toxic potential of new and legacy xenobiotics. Traditionally, quantitative structure activity relationship (QSAR) studies are performed with many distinct compounds. Based on the chemical properties of the tested compounds, complex relationships can be established so that models can be developed to predict toxicity of novel compounds. In this study, the use of fluorinated analogues as supplemental QSAR compounds was investigated. Substituting fluorine induces changes in electronic and steric properties of the substrate without substantially changing the chemical backbone of the substrate. In vitro assays were performed using purified human cytosolic sulfotransferase hSULT2A1 as a model enzyme. A mono-hydroxylated polychlorinated biphenyl (4-OH PCB 14) and its four possible mono-fluoro analogues were used as test compounds. Remarkable similarities were found between this approach and previously published QSAR studies for hSULT2A1. Both studies implicate the importance of dipole moment and dihedral angle as being important to PCB structure in respect to being substrates for hSULT2A1. We conclude that mono-fluorinated analogues of a target substrate can be a useful tool to study the structure activity relationships for enzyme specificity.

Surface-enhanced Raman scattering substrate based on cysteamine-modified gold nanoparticle aggregation for highly sensitive pentachlorophenol detection

A new surface-enhanced Raman scattering (SERS) substrate based on cysteamine-modified gold nanoparticles (AuNPs) on the glass surface has been developed for the monitoring of pentachlorophenol (PCP) in water samples.

Endoxifen and other metabolites of tamoxifen inhibit human hydroxysteroid sulfotransferase 2A1 (hSULT2A1)

Although tamoxifen is a successful agent for treatment and prevention of estrogen-dependent breast cancer, its use has been limited by the low incidence of endometrial cancer. Human hydroxysteroid sulfotransferase 2A1 (hSULT2A1) catalyzes the formation of an α-sulfooxy metabolite of tamoxifen that is reactive toward DNA, and this has been implicated in its carcinogenicity. Also, hSULT2A1 functions in the metabolism of steroid hormones such as dehydroepiandrosterone (DHEA) and pregnenolone (PREG). These roles of hSULT2A1 in steroid hormone metabolism and in generating a reactive metabolite of tamoxifen led us to examine its interactions with tamoxifen and several of its major metabolites. We hypothesized that metabolites of tamoxifen may regulate the catalytic activity of hSULT2A1, either through direct inhibition or through serving as alternate substrates for the enzyme. We found that 4-hydroxy-N-desmethyltamoxifen (endoxifen) is a potent inhibitor of hSULT2A1-catalyzed sulfation of PREG and DHEA, with Ki values of 3.5 and 2.8 μM, respectively. In the hSULT2A1-catalyzed sulfation of PREG, 4-hydroxytamoxifen (4-OHTAM) and N-desmethyltamoxifen (N-desTAM) exhibited Ki values of 12.7 and 9.8 μM, respectively, whereas corresponding Ki values of 19.4 and 17.2 μM were observed with DHEA as substrate. A Ki value of 9.1 μM was observed for tamoxifen-N-oxide with DHEA as substrate, and this increased to 16.9 μM for the hSULT2A1-catalyzed sulfation of PREG. Three metabolites were substrates for hSULT2A1, with relative sulfation rates of 4-OHTAM > N-desTAM > > endoxifen. These results may be useful in interpreting ongoing clinical trials of endoxifen and in improving the design of related molecules.

Luminescent threat: toxicity of light stick attractors used in pelagic fishery

Light sticks (LS) are sources of chemiluminescence commonly used in pelagic fishery, where hundreds are discarded and reach the shores. Residents from fishing villages report an improper use of LS contents on the skin. Given the scarce information regarding LS toxicity, the effects of LS solutions in cell cultures were evaluated herein. Loss of viability, cell cycle changes and DNA fragmentation were observed in HepG2 cell line and skin fibroblasts. A non-cytotoxic LS concentration increased the occurrence of the mutagenic lesion 1,N(6)-εdAdo in HepG2 DNA by three-fold. Additionally, in vitro incubations of spent LS contents with DNA generated dGuo-LS adducts, whose structure elucidation revealed the presence of a reactive chlorinated product. In conclusion, the LS contents were found to be highly cyto- and genotoxic. Our data indicate an urgent need for LS waste management guidelines and for adequate information regarding toxic outcomes that may arise from human exposure.

Binding interactions of hydroxylated polychlorinated biphenyls (OHPCBs) with human hydroxysteroid sulfotransferase hSULT2A1

Polychlorinated biphenyls (PCBs) are persistent environmental contaminants, and exposure to PCBs and their hydroxylated metabolites (OHPCBs) has been associated with various adverse health effects. The mammalian cytosolic sulfotransferases (SULTs) catalyze the sulfation of OHPCBs, and the interaction of OHPCBs with both the SULT1 and SULT2 families of these enzymes has received attention both with respect to metabolic disposition of these molecules and the potential mechanisms for their roles in endocrine disruption. We have previously shown that OHPCBs interact with human hydroxysteroid sulfotransferase hSULT2A1, an enzyme that catalyzes the sulfation of dehydroepiandrosterone (DHEA), other alcohol-containing steroids, bile acids, and many xenobiotics. The objective of our current studies is to investigate the mechanism of inhibition of hSULT2A1 by OHPCBs by combining inhibition kinetics with determination of equilibrium binding constants and molecular modeling of potential interactions. Examination of the effects of fifteen OHPCBs on the sulfation of DHEA catalyzed by hSULT2A1 showed predominantly noncompetitive inhibition patterns. This was observed for OHPCBs that were substrates for sulfation reactions catalyzed by the enzyme as well as those that solely inhibited the sulfation of DHEA. Equilibrium binding experiments and molecular modeling studies indicated that the OHPCBs bind at the binding site for DHEA on the enzyme, and that the observed noncompetitive patterns of inhibition are consistent with binding in more than one orientation to more than one enzyme complex. These results have implications for the roles of SULTs in the toxicology of OHPCBs, while also providing molecular probes of the complexity of substrate/inhibitor interactions with hSULT2A1.

Effective synthesis of sulfate metabolites of chlorinated phenols

Chlorophenols are an important class of persistent environmental contaminants and have been implicated in a range of adverse health effects, including cancer. They are readily conjugated and excreted as the corresponding glucuronides and sulfates in the urine of humans and other species. Here we report the synthesis and characterization of a series of ten chlorophenol sulfates by sulfation of the corresponding chlorophenols with 2,2,2-trichloroethyl (TCE) chlorosulfate using N,N-dimethylaminopyridine (DMAP) as base. Deprotection of the chlorophenol diesters with zinc powder/ammonium formate yielded the respective chlorophenol sulfate ammonium salts in good yield. The molecular structure of three TCE-protected chlorophenol sulfate diesters and one chlorophenol sulfate monoester were confirmed by X-ray crystal structure analysis. The chlorophenol sulfates were stable for several months if stored at -20 °C and, thus, are useful for future toxicological, environmental and human biomonitoring studies.

Chlorinated biphenyl quinones and phenyl-2,5-benzoquinone differentially modify the catalytic activity of human hydroxysteroid sulfotransferase hSULT2A1

Human hydroxysteroid sulfotransferase (hSULT2A1) catalyzes the sulfation of a broad range of environmental chemicals, drugs, and other xenobiotics in addition to endogenous compounds that include hydroxysteroids and bile acids. Polychlorinated biphenyls (PCBs) are persistent environmental contaminants, and oxidized metabolites of PCBs may play significant roles in the etiology of their adverse health effects. Quinones derived from the oxidative metabolism of PCBs (PCB-quinones) react with nucleophilic sites in proteins and also undergo redox cycling to generate reactive oxygen species. This, along with the sensitivity of hSULT2A1 to oxidative modification at cysteine residues, led us to hypothesize that electrophilic PCB-quinones react with hSULT2A1 to alter its catalytic function. Thus, we examined the effects of four phenylbenzoquinones on the ability of hSULT2A1 to catalyze the sulfation of the endogenous substrate, dehydroepiandrosterone (DHEA). The quinones studied were 2'-chlorophenyl-2,5-benzoquinone (2'-Cl-BQ), 4'-chlorophenyl-2,5-benzoquinone (4'-Cl-BQ), 4'-chlorophenyl-3,6-dichloro-2,5-benzoquinone (3,6,4'-triCl-BQ), and phenyl-2,5-benzoquinone (PBQ). At all concentrations examined, pretreatment of hSULT2A1 with the PCB-quinones decreased the catalytic activity of hSULT2A1. Pretreatment with low concentrations of PBQ, however, increased the catalytic activity of the enzyme, while higher concentrations inhibited catalysis. A decrease in substrate inhibition with DHEA was seen following preincubation of hSULT2A1 with all of the quinones. Proteolytic digestion of the enzyme followed by LC/MS analysis indicated PCB-quinone- and PBQ-adducts at Cys55 and Cys199, as well as oxidation products at methionines in the protein. Equilibrium binding experiments and molecular modeling suggested that changes due to these modifications may affect the nucleotide binding site and the entrance to the sulfuryl acceptor binding site of hSULT2A1.

Cysteamine-modified silver nanoparticle aggregates for quantitative SERS sensing of pentachlorophenol with a portable Raman spectrometer

Cysteamine-modified silver nanoparticle aggregates has been fabricated for pentachlorophenol (PCP) sensing by surface-enhanced Raman spectroscopy (SERS) using a portable Raman spectrometer. The cysteamine monolayers could preconcentrate PCP close to the substrate surface through the electrostatic interaction, which makes the SERS detection of PCP possible. Moreover, the Raman bands of cysteamine could be used as the internal spectral reference in the quantitative analysis. Qualitative detection of PCP was carried out by SERS without any sample pretreatment. Quantitative analysis of PCP was further realized based on the prepared substrate, as the log-log plot of normalized SERS intensity of PCP versus its concentrations exhibits a good linear relationship. The SERS signals collected on 20 randomly selected points show that the relative standard deviation of the normalized Raman intensity is 5.8%, which indicates the substrate had good uniformity. The PCP sensor also shows good long-term stability in the analyte solution. The substrate was cyclic immersed into PCP and methanol solution; after several cycles, the sensor still had good adsorption to PCP, which revealed the sensor has good reusability. Coupling with a portable Raman spectrometer, the cysteamine-modified silver nanoparticle aggregates have the potential to be used for in situ and routine SERS analysis of PCP in environmental samples.

Modification of the catalytic function of human hydroxysteroid sulfotransferase hSULT2A1 by formation of disulfide bonds

The human cytosolic sulfotransferase hSULT2A1 catalyzes the sulfation of a broad range of xenobiotics, as well as endogenous hydroxysteroids and bile acids. Reversible modulation of the catalytic activity of this enzyme could play important roles in its physiologic functions. Whereas other mammalian sulfotransferases are known to be reversibly altered by changes in their redox environment, this has not been previously shown for hSULT2A1. We have examined the hypothesis that the formation of disulfide bonds in hSULT2A1 can reversibly regulate the catalytic function of the enzyme. Three thiol oxidants were used as model compounds to investigate their effects on homogeneous preparations of hSULT2A1: glutathione disulfide, 5,5'-dithiobis(2-nitrobenzoic acid), and 1,1'-azobis(N,N-dimethylformamide) (diamide). Examination of the effects of disulfide bond formation with these agents indicated that the activity of the enzyme is reversibly altered. Studies on the kinetics of the hSULT2A1-catalyzed sulfation of dehydroepiandrosterone (DHEA) showed the effects of disulfide bond formation on the substrate inhibition characteristics of the enzyme. The effects of these agents on the binding of substrates and products, liquid chromatography-mass spectrometry identification of the disulfides formed, and structural modeling of the modified enzyme were examined. Our results indicate that conformational changes at cysteines near the nucleotide binding site affect the binding of both the nucleotide and DHEA to the enzyme, with the specific effects dependent on the structure of the resulting disulfide. Thus, the formation of disulfide bonds in hSULT2A1 is a potentially important reversible mechanism for alterations in the rates of sulfation of both endogenous and xenobiotic substrates.

Induction of DNA double-strand breaks by monochlorophenol isomers and ChKM in human gingival fibroblasts

Phenol has been traditionally used in dental treatment as a sedative for the pulp or as disinfectant for carious cavity and root canal. However, phenol is regarded as a mutagenic and carcinogenic agent and its use in dental practice is now therefore restricted. Monochlorophenols are derivatives of phenol, which are still used clinically as root canal disinfectants, they are even more active antiseptics/disinfectants than phenol, and the so-called Walkhoff (ChKM) solution makes use of monochlorophenol for root canal disinfection. Ingredients in the ChKM solution are the monochlorophenol compound 4-chlorophenol (4-CP), camphor, and menthol. In literature, the use of the ChKM solution is controversial because of a possible DNA toxicity of the ingredient 4-CP. However, it is unknown whether ChKM can really induce DNA damage in human oral cells. In this study, the induction of DNA double-strand breaks (DSBs) by ChKM and monochlorophenol compounds (2-chlorophenol, 2-CP; 3-chlorophenol, 3-CP; and 4-chlorophenol, 4-CP) was tested in human gingival fibroblasts (HGFs). DNA DSBs (foci) induced in HGFs unexposed and exposed to monochlorophenols or ChKM solution were investigated using the γ-H2AX DNA focus assay, which is a direct marker for DSBs. DSBs result in the ATM-dependent phosphorylation of the histone H2AX. When cells were exposed to medium or medium + DMSO (1 %) (negative controls), an average of 3 foci per cell were found. In positive control cells (H₂O₂ + medium, or H₂O₂ + medium + DMSO (1 %), an average of 35 foci each were found. About 20 DSB foci per cell were found, when HGFs were exposed to 2-CP (4 mM), 3-CP (2.3 mM), 4-CP (2.1 mM), or ChKM (corresponding to 1.5 mM 4-CP). Our results show increasing DNA toxicities in the order of 2-CP < 3-CP < 4-CP < ChKM solution. An additive DNA toxicity was found for 4-CP in combination with camphor in the ChKM solution, compared to the 4-CP alone. No significant differences regarding multi-foci cells (cells that contain more than 40 foci) were found when HGFs were exposed to the EC₅₀ concentrations (given in parenthesis) of ChKM (1.5 mM), 4-CP (2.1 mM), or 2-CP (4 mM). Significantly fewer multi-foci cells were found when HGFs were exposed to the EC₅₀ concentration (given in parenthesis) of 3-CP (2.3 mM), compared to the EC₅₀ concentrations of ChKM, 4-CP, or 2-CP. Monochlorophenol compounds and/or ChKM solution can induce DSBs in primary human oral (cavity) cells, which underscores their genotoxic capacity.

Structure-activity relationships for hydroxylated polychlorinated biphenyls as inhibitors of the sulfation of dehydroepiandrosterone catalyzed by human hydroxysteroid sulfotransferase SULT2A1

Polychlorinated biphenyls (PCBs) are persistent worldwide pollutants that are of concern due to their bioaccumulation and health effects. Metabolic oxidation of PCBs results in the formation of hydroxylated metabolites (OHPCBs). Among their biological effects, OHPCBs have been shown to alter the metabolism of endocrine hormones, including inhibition of mammalian cytosolic sulfotransferases (SULTs) that are responsible for the inactivation of thyroid hormones and phenolic steroids (i.e., hSULT1A1, hSULT1B1, and hSULT1E1). OHPCBs also interact with a human hydroxysteroid sulfotransferase that plays a role in the sulfation of endogenous alcohol-containing steroid hormones and bile acids (i.e., hSULT2A1). The objectives of our current study were to examine the effects of a series of OHPCB congeners on the activity of hSULT2A1 and to develop a three-dimensional quantitative structure-activity relationship (3D-QSAR) model for OHPCBs as inhibitors of the enzyme. A total of 15 OHPCBs were examined, and the sulfation of 1 μM [(3)H] dehydroepiandrosterone (DHEA) was utilized as a model reaction catalyzed by the enzyme. All 15 OHPCBs inhibited the sulfation of DHEA, with IC(50) values ranging from 0.6 μM to 96 μM, and eight of these OHPCBs were also substrates for the enzyme. Comparative molecular field analysis (CoMFA) provided a predictive 3D-QSAR model with a q(2) value of 0.697 and an r(2) value of 0.949. The OHPCBs that had the highest potency as inhibitors of DHEA sulfation were those with a 3, 5-dichloro-4-hydroxy substitution pattern on the biphenyl ring system, and these congeners were also substrates for sulfation catalyzed by hSULT2A1.

Substrate inhibition in human hydroxysteroid sulfotransferase SULT2A1: studies on the formation of catalytically non-productive enzyme complexes

The cytosolic sulfotransferase hSULT2A1 is the major hydroxysteroid (alcohol) sulfotransferase in human liver, and it catalyzes the 3'-phosphoadenosine-5'-phosphosulfate (PAPS)-dependent sulfation of various endogenous hydroxysteroids as well as many xenobiotics that contain alcohol and phenol functional groups. The hSULT2A1 often displays substrate inhibition, and we have hypothesized that a key element in this response to increasing substrate concentration is the formation of non-productive ternary dead-end enzyme complexes involving the nucleotide product, adenosine 3',5'-diphosphate (PAP). One of these substrates for hSULT2A1 is dehydroepiandrosterone (DHEA), a major circulating steroid hormone in humans that serves as precursor to both androgens and estrogens. We have utilized DHEA in both initial velocity studies and equilibrium binding experiments in order to evaluate the potential role of ternary complexes in substrate inhibition of the enzyme. Our results indicate that hSULT2A1 forms non-productive ternary complexes that involve either DHEA or dehydroepiandrosterone sulfate, and the formation of these ternary complexes displays negative cooperativity in the binding of DHEA.