Enzymatic aspects of the phenol (aryl) sulfotransferases

Engineering sulfonate group donor regeneration systems to boost biosynthesis of sulfated compounds

Sulfonation as one of the most important modification reactions in nature is essential for many biological macromolecules to function. Development of green sulfonate group donor regeneration systems to efficiently sulfonate compounds of interest is always attractive. Here, we design and engineer two different sulfonate group donor regeneration systems to boost the biosynthesis of sulfated compounds. First, we assemble three modules to construct a 3'-phosphoadenosine-5'-phosphosulfate (PAPS) regeneration system and demonstrate its applicability for living cells. After discovering adenosine 5'-phosphosulfate (APS) as another active sulfonate group donor, we engineer a more simplified APS regeneration system that couples specific sulfotransferase. Next, we develop a rapid indicating system for characterizing the activity of APS-mediated sulfotransferase to rapidly screen sulfotransferase variants with increased activity towards APS. Eventually, the active sulfonate group equivalent values of the APS regeneration systems towards trehalose and p-coumaric acid reach 3.26 and 4.03, respectively. The present PAPS and APS regeneration systems are environmentally friendly and applicable for scaling up the biomanufacturing of sulfated products.

The Roles of microRNAs in Cancer Multidrug Resistance

Simple Summary

The resistance of neoplastic cells to multiple drugs is a serious problem in cancer chemotherapy. The molecular causes of multidrug resistance in cancer are largely known, but less is known about the mechanisms by which cells deliver phenotypic changes that resist the attack of anticancer drugs. The findings of RNA interference based on microRNAs represented a breakthrough in biology and pointed to the possibility of sensitive and targeted regulation of gene expression at the post-transcriptional level. Such regulation is also involved in the development of multidrug resistance in cancer. The aim of the current paper is to summarize the available knowledge on the role of microRNAs in resistance to multiple cancer drugs.

Abstract

Cancer chemotherapy may induce a multidrug resistance (MDR) phenotype. The development of MDR is based on various molecular causes, of which the following are very common: induction of ABC transporter expression; induction/activation of drug-metabolizing enzymes; alteration of the expression/function of apoptosis-related proteins; changes in cell cycle checkpoints; elevated DNA repair mechanisms. Although these mechanisms of MDR are well described, information on their molecular interaction in overall multidrug resistance is still lacking. MicroRNA (miRNA) expression and subsequent RNA interference are candidates that could be important players in the interplay of MDR mechanisms. The regulation of post-transcriptional processes in the proteosynthetic pathway is considered to be a major function of miRNAs. Due to their complementarity, they are able to bind to target mRNAs, which prevents the mRNAs from interacting effectively with the ribosome, and subsequent degradation of the mRNAs can occur. The aim of this paper is to provide an overview of the possible role of miRNAs in the molecular mechanisms that lead to MDR. The possibility of considering miRNAs as either specific effectors or interesting targets for cancer therapy is also analyzed.

Transformation Products of Emerging Pollutants Explored Using Non-Target Screening: Perspective in the Transformation Pathway and Toxicity Mechanism—A Review

The scientific community has increasingly focused on forming transformation products (TPs) from environmental organic pollutants. However, there is still a lot of discussion over how these TPs are generated and how harmful they are to living terrestrial or aquatic organisms. Potential transformation pathways, TP toxicity, and their mechanisms require more investigation. Non-target screening (NTS) via high-resolution mass spectrometry (HRMS) in model organisms to identify TPs and the formation mechanism on various organisms is the focus of this review. Furthermore, uptake, accumulation process, and potential toxicity with their detrimental consequences are summarized in various organisms. Finally, challenges and future research initiatives, such as performing NTS in a model organism, characterizing and quantifying TPs, and evaluating future toxicity studies on TPs, are also included in this review.

Enzyme Kinetics of PAPS-Sulfotransferase

The cytosolic sulfotransferase (SULT) enzymes are found in human liver, kidney, intestine, and other tissues. These enzymes catalyze the transfer of the -SO₃ group from 3'-phospho-adenosyl-5'-phosphosulfate (PAPS) to a nucleophilic hydroxyl or amine group in a drug substrate. SULTs are stable as dimers, with a highly conserved dimerization domain near the C-terminus of the protein. Crystal structures have revealed flexible loop regions in the native proteins, one of which, located near the dimerization domain, is thought to form a gate that changes position once PAPS is bound to the PAPS-binding site and modulates substrate access and enzyme properties. There is also evidence that oxidation and reduction of certain cysteine residues reversibly regulate the binding of the substrate and PAPS or PAP to the enzyme thus modulating sulfonation. Because SULT enzymes have two substrates, the drug and PAPS, it is common to report apparent kinetic constants with either the drug or the PAPS varied while the other is kept at a constant concentration. The kinetics of product formation can follow classic Michaelis-Menten kinetics, typically over a narrow range of substrate concentrations. Over a wide range of substrate concentrations, it is common to observe partial or complete substrate inhibition with SULT enzymes. This chapter describes the function, tissue distribution, structural features, and properties of the human SULT enzymes and presents examples of enzyme kinetics with different substrates.

Tocopherol Moderately Induces the Expressions of Some Human Sulfotransferases, which are Activated by Oxidative Stress

Oxidative stress is generated in biological system by several endogenous/exogenous factors like environmental-pollution/toxicity/diseases and by daily-life-stress. We previously showed that oxidative-stress impaired the activities/expressions of phase-II drug-metabolizing enzyme, sulfotransferases (SULTs). The SULT catalyzes sulfation of endogenous/exogenous compounds. Vitamin E is globally consumed by a large number of individuals for the cellular protection from oxidative stress and aging. Here, vitamin E (tocopherol; α/γ and tocotrienol; α/γ; 0, 1, 10, or 100 μM) was tested in human carcinoma cell line, HepG2 for their influences on SULTs expression/(western blotting). The effects of oxidant (glutathione-oxidized/GSSG) or reductant (glutathione-reduced/GSH, Dithiothreitol/DTT) on SULT activities were studied in rat-liver/human intestinal tissues. Results suggest, tocopherol is more inductive to monoamine-SULT (MPST) and Dehydroepiandrosterone-SULT (DHEAST) compared to that of tocotrienol (inconsistent change in PPST, phenol sulfotransferase/MPST/EST, estrogen sulfotransferase). The nuclear-factor constitutive androstane receptor (CAR) was found to be induced moderately. This study overall describes that vitamin E moderately influences SULTs expression. The induction ability of tocopherol should be judged taking into account its long-term consummation. Oxidative stress activates rat and human SULTs activities and expressions. Further studies are necessary in this regard.

Evaluation of a conserved tryptophanyl residue in donor substrate binding and catalysis by a phenol sulfotransferase (SULT1A1)

Structural determinations of members of the sulfotransferase (SULT) family suggest a direct interaction between a conserved tryptophanyl side chain and bound 3'-phosphoadenosine-5'-phosphate (PAP). We have prepared and purified mutants of the bovine SULT1A1, a very conserved homolog to the human SULT1A1, in which tryptophanyl-53 was sequentially trimmed to tyrosine, leucine, and alanine. Differential scanning fluorimetry indicated structural stabilities of the mutant proteins comparable to the wild type SULT1A1; however, less thermal stabilizations by PAP plus pentachlorophenol were observed with the mutants, suggesting weakened ligand binding. Protein fluorescence of the wild type enzyme decreased 6.5% upon binding PAP, whereas no changes occurred with the mutant enzymes. This reveals that W53, or its positional counterpart, has been the source of emission intensity changes used in previous investigations of other SULTs. Fluorescence resonance energy transfer from excited tryptophans to bound 7-hydroxycoumarin, as induced by PAP, indicated weakened binding of ligands to the mutant SULTs. This was also encountered and quantified in initial rate kinetic analyses. Ablation of the PAPS adenine-to-W53 ring interaction, shown by the W53A mutant enzyme, resulted in a 6.4-fold increase in K_PAPS and a 92% decrease in k_cat/K_PAPS. Measured K_PAPS values reveal the W53 indole ring contribution to PAPS binding to be 1.1 kcal/mol (4.6 kJ/mol). These results verify the structurally-inferred role for the π-π stacking interaction between PAP(S) and the conserved tryptophanyl residue in SULT1A1 and other members of the SULT family.

Combinatorial Biosynthesis of Sulfated Benzenediol Lactones with a Phenolic Sulfotransferase from Fusarium graminearum PH-1

Total biosynthesis or whole-cell biocatalytic production of sulfated small molecules relies on the discovery and implementation of appropriate sulfotransferase enzymes. Although fungi are prominent biocatalysts and have been used to sulfate drug-like phenolics, no gene encoding a sulfotransferase enzyme has been functionally characterized from these organisms. Here, we identify a phenolic sulfotransferase, FgSULT1, by genome mining from the plant-pathogenic fungus Fusarium graminearum PH-1. We expressed FgSULT1 in a Saccharomyces cerevisiae chassis to modify a broad range of benzenediol lactones and their nonmacrocyclic congeners, together with an anthraquinone, with the resulting unnatural natural product (uNP) sulfates displaying increased solubility. FgSULT1 shares low similarity with known animal and plant sulfotransferases. Instead, it forms a sulfotransferase family with putative bacterial and fungal enzymes for phase II detoxification of xenobiotics and allelochemicals. Among fungi, putative FgSULT1 homologues are encoded in the genomes of Fusarium spp. and a few other genera in nonsyntenic regions, some of which may be related to catabolic sulfur recycling. Computational structure modeling combined with site-directed mutagenesis revealed that FgSULT1 retains the key catalytic residues and the typical fold of characterized animal and plant sulfotransferases. Our work opens the way for the discovery of hitherto unknown fungal sulfotransferases and provides a synthetic biological and enzymatic platform that can be adapted to produce bioactive sulfates, together with sulfate ester standards and probes for masked mycotoxins, precarcinogenic toxins, and xenobiotics.IMPORTANCE Sulfation is an expedient strategy to increase the solubility, bioavailability, and bioactivity of nutraceuticals and clinically important drugs. However, chemical or biological synthesis of sulfoconjugates is challenging. Genome mining, heterologous expression, homology structural modeling, and site-directed mutagenesis identified FgSULT1 of Fusarium graminearum PH-1 as a cytosolic sulfotransferase with the typical fold and active site architecture of characterized animal and plant sulfotransferases, despite low sequence similarity. FgSULT1 homologues are sparse in fungi but form a distinct clade with bacterial sulfotransferases. This study extends the functionally characterized sulfotransferase superfamily to the kingdom Fungi and demonstrates total biosynthetic and biocatalytic synthetic biological platforms to produce unnatural natural product (uNP) sulfoconjugates. Such uNP sulfates may be utilized for drug discovery in human and veterinary medicine and crop protection. Our synthetic biological methods may also be adapted to generate masked mycotoxin standards for food safety and environmental monitoring applications and to expose precarcinogenic xenobiotics.

Mechanisms of promiscuity among drug metabolizing enzymes and drug transporters

Detoxication, or 'drug-metabolizing', enzymes and drug transporters exhibit remarkable substrate promiscuity and catalytic promiscuity. In contrast to substrate-specific enzymes that participate in defined metabolic pathways, individual detoxication enzymes must cope with substrates of vast structural diversity, including previously unencountered environmental toxins. Presumably, evolution selects for a balance of 'adequate' kcat /KM values for a wide range of substrates, rather than optimizing kcat /KM for any individual substrate. However, the structural, energetic, and metabolic properties that achieve this balance, and hence optimize detoxication, are not well understood. Two features of detoxication enzymes that are frequently cited as contributions to promiscuity include the exploitation of highly reactive versatile cofactors, or cosubstrates, and a high degree of flexibility within the protein structure. This review examines these intuitive mechanisms in detail and clarifies the contributions of the classic ligand binding models 'induced fit' (IF) and 'conformational selection' (CS) to substrate promiscuity. The available literature data for drug metabolizing enzymes and transporters suggest that IF is exploited by these promiscuous detoxication enzymes, as it is with substrate-specific enzymes, but the detoxication enzymes uniquely exploit 'IFs' to retain a wide range of substrates at their active sites. In contrast, whereas CS provides no catalytic advantage to substrate-specific enzymes, promiscuous enzymes may uniquely exploit it to recruit a wide range of substrates. The combination of CS and IF, for recruitment and retention of substrates, can potentially optimize the promiscuity of drug metabolizing enzymes and drug transporters.

Unambiguous Characterization of p-Cresyl Sulfate, a Protein-Bound Uremic Toxin, as Biomarker of Heart and Kidney Disease

p-Cresyl sulfate is one of the bound uremic toxins whose level increases in the sera of patients with the severity of chronic kidney disease and is therefore used as a standard for clinical investigations. Our first attempts to obtain p-cresyl sulfate led exclusively to the product of sulfonation of the aromatic ring instead of sulfation on the OH moiety. Nevertheless, this initial discouraging result allowed us to handle both p-cresyl sulfate and 2-hydroxy-5-methylbenzenesulfonic acid obtained by different synthetic pathways. Interestingly, the comparison between the two isomers pointed out that the two molecules show the same fragmentation pattern and are indistinguishable by mass spectrometry. They cannot be separated on several commercially available columns. The only difference between the two compounds is a 10-fold higher ionization yield under negative ion electrospray ionization. NMR spectral studies definitely confirmed the different molecular structures. We present here an unambiguous biomimetic synthetic route for p-cresyl sulfate and the spectroscopic characterization of both the compounds by nuclear magnetic resonance and mass spectrometry.

Secretory expression of the rat aryl sulfotransferases IV with improved catalytic efficiency by molecular engineering

The rat aryl sulfotransferases IV (AST IV) has been used to catalyze 3'-phosphoadenosine-5'-phosphate (PAP) into the sulfuryl group donor 3'-phosphoadenosine-5'-phosphosulfate (PAPS) in biotechnological production of glycosaminoglycans. The performance of native AST IV is not satisfying due to the lower catalytic activity with substrate PAP. In the present study, we achieved secretory expression of the AST IV and improved its catalytic efficiency by molecular engineering. Fusion with signal peptides Cex, YebF and PelB allow for secretory expression of AST IV and the secreted AST IV yield reached 4.21 ± 0.23 U/mL, 8.67 ± 0.34 U/mL and 21.35 ± 0.87 U/mL, respectively. Modification of PelB further increased the secretory expression by more than fourfold, to 89.67 ± 1.34 U/mL. On this basis, molecular evolution of the predicted PAP-binding pocket gate loop was performed and a positive mutant L89S/E90L with higher activity was identified. Considering the importance of the sites Leu⁸⁹ and Glu⁹⁰, we performed site-saturation mutagenesis and found the mutant L89M/E90Q with much higher PAP affinity (K _m= 0.46 ± 0.02 mM) and catalytic efficiency (k _cat/K _m = 1816.32 ± 12.72/s/M). The secretory expression of the AST IV variant L89M/E90Q with higher catalytic efficiency should facilitate the studies on biosynthesis of sulfated polysaccharides.

Repeated restraint stress upregulates rat sulfotransferase 1A1

BackgroundSulfotransferases (SULTs) are phase II drug-metabolizing enzymes. SULTs also regulate the biological activities of biological signaling molecules, such as various hormones, bile acids, and monoamine neurotransmitters; therefore, they play critical roles in the endocrine and nervous systems. People are subject to various kinds of physical, chemical, toxicological, physiological, and psychological stresses at one time or another. The study of the effects produced by stress may lead to finding novel remedies for many disease conditions. The effect of repeated restraint stress on rat SULT expression has not been studied. MethodsThis study involves the effect of repeated restraint stress on SULT1A1 expressions. Male Sprague-Dawley rats (n=4) were subjected to repeated restraint stress 2 h/day for 7 days. Protein and RNA expression of SULT1A1 were analyzed by western blot and quantitative real time reverse transcription polymerase chain reaction, respectively, in important tissues. ResultsWe observed that repeated restraint stress increased the expression of SULT1A1 in the liver, adrenal glands, cerebellum, hypothalamus, and cerebral cortex in male rats. Patterns of enhanced expression were observed at both mRNA and protein level, indicating that repeated restraint stress stimulates enzyme expression at the transcriptional level. ConclusionsChanges of SULT1A1 expression in important tissues caused by repeated restraint stress will have a significant effect on drug metabolism and xenobiotics detoxification. The significant changes in endocrine glands and brain sections may also cause disturbances in hormone homeostasis, therefore leading to disease conditions. This report provides clues for the understanding of the effect of stresses on health.

Implications of sulfotransferase activity in interindividual variability in drug response: clinical perspective on current knowledge

The interindividual variability in drug response is a major issue in clinical practice and in drug development. Sulfoconjugation is an important Phase II reaction catalyzed by cytosolic sulfotransferases (SULTs), playing a major role in homeostatic functions, xenobiotic detoxification, and carcinogen bioactivation. SULT display wide interindividual variability, explained only partially by genetic variation, suggesting that other non-genetic, epigenetic, and environmental influences could be major determinants of variability in SULT activity. This review focuses on the factors known to influence SULT variability in expression and activity and the available evidence regarding the impact of SULT variability on drug response.

Age-related change of hepatic uridine diphosphate glucuronosyltransferase and sulfotransferase activities in male chickens and pigs

The hepatic activities of uridine diphosphate glucuronosyltransferase (UGT) and sulfotransferase (SULT) of male Ross 708 broiler chickens at the age of 1, 7, 14, 28, and 56 days and male Camborough-29 pigs at the age of 1 day and 2, 5, 10, and 20 weeks were investigated. Glucuronidation and sulfation of 4-nitrophenol were used to evaluate the activities. Porcine hepatic UGT and SULT activities were low at birth, peaked at around 5-10 weeks, and then declined. Both hepatic UGT and SULT activities of chickens were high at hatch and declined. Chicken hepatic UGT activity had a peak at the age of 28 days. Affinity of hepatic SULT to 4-nitrophenol is similar in chickens and pigs, but the affinity of hepatic UGT in pigs was about 10 times higher than that in chickens. 4-nitrophenol was predominantly conjugated by SULT instead of UGT in chicken livers from hatch to day 56. Conversely, hepatic UGT contributed predominantly in 4-nitrophenol conjugation than the SULT in pigs from birth to 20 weeks. Therefore, age has significant impact on hepatic activities of UGT and SULT, and the importance of UGT and SULT on conjugation is different in chickens and pigs.

MicroRNAs and Cancer Drug Resistance

The discovery of small regulatory noncoding RNAs revolutionized our thinking on gene regulation. The class of microRNAs (miRs), a group of small noncoding RNAs (20-22 nt in length) that bind imperfectly to the 3'-untranslated region of target mRNA, has been insistently implicated in several pathological conditions including cancer. Indeed, major hallmarks of cancer, such as cell differentiation, cell proliferation, cell cycle, cell survival, and cell invasion, has been described as being regulated by miRs. Recent studies have also implicated miRs in cancer drug resistance. Regardless of the several studies done until now, drug resistance still is a burden for cancer therapy and patients' outcome, often resulting in more aggressive tumors that tend to metastasize to distant organs. Hence, with this review, we aim to summarize the miRs that influence molecular pathways that are involved in cancer drug resistance, such as drug metabolism, drug influx/efflux, DNA damage response (DDR), epithelial-to-mesenchymal transition (EMT), and cancer stem cells.

Enzyme kinetics of conjugating enzymes: PAPS sulfotransferase

The sulfotransferase (SULT) enzymes catalyze the formation of sulfate esters or sulfamates from substrates that contain hydroxy or amine groups, utilizing 3'-phosphoadenosyl-5'-phosphosulfate (PAPS) as the donor of the sulfonic group. The rate of product formation depends on the concentrations of PAPS and substrate as well as the sulfotransferase enzyme; thus, if PAPS is held constant while varying substrate concentration (or vice versa), the kinetic constants derived are apparent constants. When studied over a narrow range of substrate concentrations, classic Michaelis-Menten kinetics can be observed with many SULT enzymes and most substrates. Some SULT enzymes exhibit positive or negative cooperativity during conversion of substrate to product, and the kinetics fit the Hill plot. A characteristic feature of most sulfotransferase-catalyzed reactions is that, when studied over a wide range of substrate concentrations, the rate of product formation initially increases as substrate concentration increases, then decreases at high substrate concentrations, i.e., they exhibit substrate inhibition or partial substrate inhibition. This chapter gives an introduction to sulfotransferases, including a historical note, the nomenclature, a description of the function of SULTs with different types of substrates, presentation of examples of enzyme kinetics with SULTs, and a discussion of what is known about mechanisms of substrate inhibition in the sulfotransferases.

Estrogen-related receptor ERRα regulation of human hydroxysteroid sulfotransferase (SULT2A1) gene expression in human Caco-2 cells

Human hydroxysteroid sulfotransferase, SULT2A1, is important for xenobiotic detoxification and the maintenance of hydroxysteroid homeostasis. Our published report suggested that estrogen-related receptor ERRα downregulates SULT2A1 in Hep G2 cells. The results shown in this study suggest that ERRα upregulates SULT2A1 transcription in Caco-2 cells. The deletion analysis suggested that SULT2A1 promoter region between -65 and -44 is important for this upregulation. Our further investigation suggested that ERRα binding element, ERRE51, mediates ERRα activation of SULT2A1 promoter transcription in Caco-2 cells. The interaction of ERRE51 with ERRα was confirmed by electrophoretic mobility shift assay and chromatin immunoprecipitation analysis. Results also suggest that the difference of constitutive androstane receptor transcription levels in Hep G2 and Caco-2 cells at least partially contribute to the cell type dependent ERRα modulation of SULT2A1 promoter transcription. ERRα regulates human SULT2A1 transcription by competing with other nuclear receptors binding to the DNA-promoter region.

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.

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.

Liver X receptor alpha mediated genistein induction of human dehydroepiandrosterone sulfotransferase (hSULT2A1) in Hep G2 cells

Cytosolic sulfotransferases are one of the major families of phase II drug metabolizing enzymes. Sulfotransferase-catalyzed sulfonation regulates hormone activities, metabolizes drugs, detoxifies xenobiotics, and bioactivates carcinogens. Human dehydroepiandrosterone sulfotransferase (hSULT2A1) plays important biological roles by sulfating endogenous hydroxysteroids and exogenous xenobiotics. Genistein, mainly existing in soy food products, is a naturally occurring phytoestrogen with both chemopreventive and chemotherapeutic potential. Our previous studies have shown that genistein significantly induces hSULT2A1 in Hep G2 and Caco-2 cells. In this study, we investigated the roles of liver X receptor (LXRα) in the genistein induction of hSULT2A1. LXRs have been shown to induce expression of mouse Sult2a9 and hSULT2A1 gene. Our results demonstrate that LXRα mediates the genistein induction of hSULT2A1, supported by Western blot analysis results, hSULT2A1 promoter driven luciferase reporter gene assay results, and mRNA interference results. Chromatin immunoprecipitation (ChIP) assay results demonstrate that genistein increase the recruitment of hLXRα binding to the hSULT2A1 promoter. These results suggest that hLXRα plays an important role in the hSULT2A1 gene regulation. The biological functions of phytoestrogens may partially relate to their induction activity toward hydroxysteroid SULT.

Oxidative modification of rat sulfotransferase 1A1 activity in hepatic tissue slices correlates with effects on the purified enzyme

Mammalian cytosolic sulfotransferases (SULTs) catalyze the sulfation of xenobiotics as well as numerous endogenous molecules. The major aryl (phenol) SULT in rat liver, rSULT1A1, has been used extensively as a model enzyme for understanding the catalytic function of SULTs. Previous studies showed that purified rSULT1A1 displays significant catalytic changes in the presence of GSSG and other oxidants. In the present study, the effects of diamide [1,1'-azobis(N,N-dimethylformamide)] and tert-butyl hydroperoxide (TBHP) on the activity of rSULT1A1 in rat hepatic slices were compared with the effects of these oxidants on a homogeneous preparation of the enzyme. Precision-cut hepatic slices were incubated with 10 μM 7-hydroxycoumarin (7-HC) in the presence of varied concentrations of either diamide or TBHP. Analysis of the 7-hydroxycoumarin sulfate released into the incubation medium indicated that both oxidants significantly increased the sulfation of 7-HC, and this occurred at optimal concentrations of 5 and 10 μM, respectively. Cellular GSH and GSSG levels in the hepatic slices were not significantly altered from control values at these concentrations of diamide and TBHP. Exposure of homogeneous rSULT1A1 to diamide or TBHP also increased the rate of sulfation of 7-HC, although the optimal concentrations of diamide and TBHP were lower (50- and 100-fold, respectively) than those required for effects with the hepatic slices. These results indicate that both diamide and TBHP may modify the rSULT1A1 in intact cells in a manner similar to that observed with the homogeneous purified enzyme.

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.

Physicochemical properties of hydroxylated polychlorinated biphenyls aid in predicting their interactions with rat sulfotransferase 1A1 (rSULT1A1)

Hydroxylated metabolites of polychlorinated biphenyls (OHPCBs) interact with rat sulfotransferase 1A1 (rSULT1A1) as substrates and inhibitors. Previous studies have shown that there are complex and incompletely understood structure-activity relationships governing the interaction of rSULT1A1 with these molecules. Furthermore, modification of the enzyme with glutathione disulfide (GSSG) results in the conversion of some OHPCBs from inhibitors to substrates. We have now examined estimated values for the acid-dissociation constant (K(a)) and the octanol-water distribution coefficient (D), as well as experimentally determined dissociation constants for enzyme complexes, to assist in the prediction of interactions of OHPCBs with rSULT1A1. Under reducing conditions, initial velocities for rSULT1A1-catalyzed sulfation exhibited a positive correlation with pK(a) and a negative correlation with logD of the OHPCBs. IC(50) values of inhibitory OHPCBs decreased with decreasing pK(a) values for both the glutathione (GSH)-pretreated and GSSG-pretreated forms of rSULT1A1. Comparison of GSH- and GSSG-pretreated forms of rSULT1A1 with respect to binding of OHPCB in the presence and absence of adenosine 3',5'-diphosphate (PAP) revealed that the dissociation constants with the two redox states of the enzyme were similar for each OHPCB. Thus, pK(a) and logD values are useful in predicting the binding of OHPCBs to the two redox forms of rSULT1A1 as well as the rates of sulfation of those OHPCBs that are substrates. However, the differences in substrate specificity for OHPCBs that are seen with changes in redox status of the enzyme are not directly related to specific structural effects of individual OHPCBs within inhibitory enzyme-PAP-OHPCB complexes.

Reaction product affinity regulates activation of human sulfotransferase 1A1 PAP sulfation

Cytosolic sulfotransferase (SULT)-catalyzed sulfation regulates the activity of bio-signaling molecules and aids in metabolizing hydroxyl-containing xenobiotics. The sulfuryl donor for the SULT reaction is adenosine 3'-phosphate 5'-phosphosulfate (PAPS), while products are adenosine 3',5'-diphosphate (PAP) and a sulfated alcohol. Human phenol sulfotransferase (SULT1A1) is one of the major detoxifying enzymes for phenolic xenobiotics. The mechanism of SULT1A1-catalyzed sulfation of PAP by pNPS was investigated. PAP was sulfated by para-nitrophenyl sulfate (pNPS) in a concentration-dependent manner. 2-Naphthol inhibited sulfation of PAP, competing with pNPS, while phenol activated the sulfation reaction. At saturating PAP, a ping pong kinetic mechanism is observed with pNPS and phenol as substrates, consistent with phenol intercepting the E-PAPS complex prior to dissociation of PAPS. At high concentrations, phenol competes with pNPS, consistent with formation of the E-PAP-phenol dead-end complex. Data are consistent with the previously reported mechanism for sulfation of 2-naphthol by PAPS, and its activation by pNPS. Overall, data are consistent with release of PAP from E-PAP and PAPS from E-PAPS contributing to rate-limitation in both reaction directions.

para-Nitrophenyl sulfate activation of human sulfotransferase 1A1 is consistent with intercepting the E[middle dot]PAP complex and reformation of E[middle dot]PAPS

Cytosolic sulfotransferase (SULT)-catalyzed sulfation regulates biological activities of various biosignaling molecules and metabolizes hydroxyl-containing drugs and xenobiotics. The universal sulfuryl group donor for SULT-catalyzed sulfation is adenosine 3'-phosphate 5'-phosphosulfate (PAPS), whereas the reaction products are a sulfated product and adenosine 3',5'-diphosphate (PAP). Although SULT-catalyzed kinetic mechanisms have been studied since the 1980s, they remain unclear. Human SULT1A1 is an important phase II drug-metabolizing enzyme. Previously, isotope exchange at equilibrium indicated steady-state ordered mechanism with PAPS and PAP binding to the free SULT1A1 (Tyapochkin, E., Cook, P. F., and Chen, G. (2008) Biochemistry 47, 11894-11899). On the basis of activation of SULT1A1 by para-nitrophenyl sulfate (pNPS), an ordered bypass mechanism has been proposed where pNPS sulfates PAP prior to its release from the E.PAP complex regenerating E.PAPS. Data are consistent with uncompetitive substrate inhibition by naphthol as a result of formation of the E.PAP.naphthol dead-end complex; formation of the complex is corroborated by naphthol/PAP double inhibition experiments. pNPS activation data demonstrate an apparent ping-pong behavior with pNPS adding to E.PAP, and competitive inhibition by naphthol consistent with formation of the E.PAP.naphthol complex. Exchange against forward reaction flux (PAPS plus naphthol) beginning with [35S]PAPS and generating [35S]naphthyl sulfate is also consistent with pNPS intercepting the E.PAP complex. Overall, data are consistent with the proposed ordered bypass mechanism.

Structure-activity relationships for hydroxylated polychlorinated biphenyls as substrates and inhibitors of rat sulfotransferases and modification of these relationships by changes in thiol status

Hydroxylated metabolites of polychlorinated biphenyls (OH-PCBs) are inhibitors and substrates for various human sulfotransferases (SULTs). Although the rat is often used in toxicological studies on PCBs, the interactions of OH-PCBs with rat SULTs are less well understood. In the present study, 15 OH-PCBs were investigated as potential substrates or inhibitors of purified recombinant rSULT1A1 and rSULT2A3, the major family 1 and family 2 SULTs present in rat liver, respectively. None of these OH-PCBs were substrates for rSULT2A3, 11 weakly inhibited rSULT2A3-catalyzed sulfation of dehydroepiandrosterone, and 4 had no effect on the reaction. With rSULT1A1, 4-OH-PCB 8, 4'-OH-PCB 3, 9, 12, 35, and 6'-OH-PCB 35 were substrates, whereas 4'-OH-PCB 6, 4-OH-PCB 14, 4'-OH-PCB 25, 4'-OH-PCB 33, 4-OH-PCB 34, 4-OH-PCB36, 4'-OH-PCB 36, 4'-OH-PCB 68, and 4-OH-PCB 78 inhibited the sulfation of 2-naphthol catalyzed by this enzyme. OH-PCBs with a 3,5-dichloro-4-hydroxy substitution were the most potent inhibitors of rSULT1A1, and the placement of chlorine atoms in the ortho- and meta-positions on either ring of para-OH-PCBs resulted in significant differences in activity as substrates and inhibitors. The specificity of rSULT1A1 for several inhibitory OH-PCBs was altered by pretreatment of the enzyme with oxidized glutathione (GSSG). Four OH-PCBs that were inhibitors of rSULT1A1 under reducing conditions became substrates after pretreatment of the enzyme with GSSG. This alteration in specificity of rSULT1A1 for certain OH-PCBs suggests that conditions of oxidative stress may significantly alter the sulfation of some OH-PCBs in the rat.

Effect of folic acid on methotrexate induction of sulfotransferases in rats

Our earlier investigation showed that MTX is an inducer of rat and human sulfotransferases. Here we report that folic acid treatment inhibited MTX induction of aryl sulfotransferase (AST-IV) in female rat liver and hydroxysteroid sulfotransferase (STa) in male rat liver. This is important for understanding the clinical mechanisms of MTX.

Genistein induction of human sulfotransferases in HepG2 and Caco-2 cells

Sulfotransferases are phase II drug-metabolizing enzymes. While the induction of sulfotransferases by hormones and endogenous molecules is relatively well known, induction by xenobiotics is not well studied. Isoflavones are naturally occurring phyto-oestrogens, mainly existing in soy food products. They have been described as health-promoting, disease-preventing dietary supplements and as agents with cancer-preventive activities. Recently, isoflavones have been reported to interact with nuclear receptors, including those that are known to mediate the induction of drug-metabolizing enzymes. In the present investigation, the isoflavone genistein was shown to be a xenobiotic inducer of human sulfotransferases in transformed human liver cells (HepG2) and colon carcinoma cells (Caco-2). Enzymatic activity assay, Western blot, and real-time reverse transcription-polymerase chain reaction (RT-PCR) results demonstrated that genistein significantly induced protein and mRNA expression of human simple phenol sulfotransferase (hSULT1A1) and human dehydroepiandrosterone sulfotransferase (hSULT2A1) in HepG2 and Caco-2 cells. The induction was time-dependent and dose-dependent. Western blot results agreed well with real-time RT-PCR results, suggesting that induction occurred at the gene transcription level. This isoflavone is the first nutritionally related phyto-oestrogen shown to induce human sulfotransferases in HepG2 and Caco-2 cells.

The role of biotransformation and bioactivation in toxicity

Biotransformation is essential to convert lipophilic chemicals to water-soluble and readily excretable metabolites. Formally, biotransformation reactions are classified into phase I and phase II reactions. Phase I reactions represent the introduction of functional groups, whereas phase II reactions are conjugations of such functional groups with endogenous, polar products. Biotransformation also plays an essential role in the toxicity of many chemicals due to the metabolic formation of toxic metabolites. These may be classified as stable but toxic products, reactive electrophiles, radicals, and reactive oxygen metabolites. The interaction of toxic products formed by biotransformation reactions with cellular macromolecules initiates the sequences resulting in cellular damage, cell death and toxicity.

Isotope exchange at equilibrium indicates a steady state ordered kinetic mechanism for human sulfotransferase

Cytosolic sulfotransferase (SULT)-catalyzed sulfation regulates biosignaling molecular biological activities and detoxifies hydroxyl-containing xenobiotics. The universal sulfuryl group donor for SULTcatalyzed sulfation is adenosine 3'-phosphate 5'-phosphosulfate (PAPS). The reaction products are a sulfated product and adenosine 3',5'-diphosphate (PAP). Although the kinetics has been reported since the 1980s,SULT-catalyzed reaction mechanisms remain unclear. Human SULT1A1 catalyzes the sulfation of xenobiotic phenols and has very broad substrate specificity. It has been recognized as one of the most important phase II drug-metabolizing enzymes. Understanding the kinetic mechanism of this isoform is important in understanding drug metabolism and xenobiotic detoxification. In this report, we investigated the SULT1A1-catalyzed phenol sulfation mechanism. The SULT1A1-catalyzed reaction was brought to equilibrium by varying substrate (1-naphthol) and PAPS initial concentrations. Equilibrium constants were determined. Two isotopic exchanges at equilibrium ([14C]1-naphthol <=>[14C]1-naphthyl sulfate and[35S]PAPS<=>[35S]1-naphthyl sulfate) were conducted. First-order kinetics, observed for all the is otopic exchange reactions studied over the entire time scale that was monitored, indicates that the system was truly at equilibrium prior to addition of an isotopic pulse. Complete suppression of the 35S isotopic exchange rate was observed with an increase in the levels of 1-naphthol and 1-naphthyl sulfate in a constant ratio,while no suppression of the 14C exchange rate was observed with an increase in the levels of PAPS and PAP in a constant ratio. Data are consistent with a steady state ordered kinetic mechanism with PAPS and PAP binding to the free enzyme.

Redox regulation of human estrogen sulfotransferase (hSULT1E1)

Sulfotransferases (SULTs) are enzymes that catalyze the sulfation of hydroxyl-containing compounds. Sulfation regulates hormone activities and detoxifies xenobiotics. Human estrogen sulfotransferase (hSULT1E1) catalyzes the sulfation of estrogens and regulates estrogen bioactivities. Oxidative regulation provides a biological mechanism for regulating enzyme activities in vivo. The oxidative regulation of human SULTs has not been reported. In this study, we used amino acid modification, manipulation of intracellular redox state, and site-directed mutagenesis to study the redox regulation of human SULTs and specifically the mechanism of hSULT1E1 inhibitory regulation by oxidized glutathione (GSSG). Of the four major human SULTs, hSULT1A1, hSULT1A3, and hSULT2A1 do not undergo redox regulation; hSULT1E1, on the other hand, can be redox regulated. GSSG inactivated hSULT1E1 activity in an efficient, time- and concentration-dependant manner. The co-enzyme adenosine 3'-phosphate 5'-phosphosulfate protected hSULT1E1 from GSSG-associated inactivation. A reduced glutathione (GSH) inducer (N-acetyl cysteine) significantly increased while a GSH depletor (buthionine sulfoxamine) significantly decreased hSULT1E1 activity, but both failed to affect the amount of hSULT1E1 protein in human hepatocyte carcinoma Hep G2 cells. Crystal structure suggested that no Cys residues exist near the active sites of hSULT1A1, hSULT1A3, and hSULT2A1, but Cys residues do exist within the active site of hSULT1E1. Site-directed mutagenesis demonstrated that Cys83 is critical for the redox regulation of hSULT1E1. This first report on the redox regulation of human SULTs suggests that the redox regulation of hSULT1E1 may interrupt the regulation and function of estrogens under various physiological and pathological conditions.

Human constitutive androstane receptor mediated methotrexate induction of human dehydroepiandrosterone sulfotransferase (hSULT2A1)

Sulfotransferases (SULTs) catalyzed sulfation is important in the regulation of biological activities of hormones and neurotransmitters, the metabolism of drugs, and the detoxification of xenobiotic toxicants. Sulfation also leads to the bioactivation of procarcinogens. Human dehydroepiandrosterone sulfotransferase (hSULT2A1) is a major SULT catalyzing the sulfation of hydroxysteroids and xenobiotic alcohols. Our previous studies had shown that the anti-folate drug methotrexate (MTX) can up-regulate several major isoforms of human SULTs. To determine the mechanisms controlling the regulation of hSULT2A1, the 5'-flanking region of hSULT2A1 was constructed into the pGL3-Basic luciferase reporter vector. The transcriptional regulation mechanism of hSULT2A1 promoter was studied using Caco-2 cell line based on the reporter gene assay. Nuclear receptor co-transfection results indicated that human constitutive androstane receptor (hCAR) and human retinoid X receptor alpha (hRXRalpha) were involved in the transcriptional regulation of hSULT2A1. RNA interference experiments further proved the role of hCAR in hSULT2A1 regulation. Progressive promoter deletion, DNA sequence alignment, and site directed promoter mutation results suggested that an imperfect inverted repeat DNA motif, IR2 (-186AGCTCAGATGACCC-173), within the hSULT2A1 promoter region mediated the hSULT2A1 induction by MTX. Furthermore, electrophoretic mobility shift assay and super shift assay were employed to characterize the interactions of hCAR and hRXRalpha with the IR2 element. In summary, we identified an IR2 DNA cis-element located at -186/-173 of hSULT2A1 promoter region; the IR2 element mediates the MTX induction of hSULT2A1 through interacting with hCAR and hRXRalpha.

Reversible covalent inhibition of a phenol sulfotransferase by coenzyme A

Phenol sulfotransferases (SULTs), which normally bind 3'-phosphoadenosine-5'-phosphosulfate as the donor substrate, are inhibited by CoA and its thioesters. Here, we report that inhibition of bovine SULT1A1 by CoA is time-dependent at neutral pH under non-reducing conditions. The rates of inactivation by CoA indicate an initial reversible SULT:CoA complex with a dissociation constant of 5.7 microM and an inactivation rate constant of 0.07 min(-1). Titrations with CoA and prolonged incubations reveal that inactivation of the dimeric enzyme is stoichiometric, consistent with the observation of complete conversion of the protein to a slightly decreased electrophoretic mobility. Both activity and normal electrophoretic migration are restored by 2-mercaptoethanol. Mutagenesis demonstrated that Cys168 is the site of CoA adduction, and a consistent model was constructed that reveals a new SULT molecular dynamic. Cysteine reaction kinetics with Ellman's reagent revealed a PAPS-induced structural change consistent with the model that accounts for binding of CoA.

Nuclear receptor interactions in methotrexate induction of human dehydroepiandrosterone sulfotransferase (hSULT2A1)

Cytosolic sulfotransferases (SULTs) are a major family of phase II drug-metabolizing enzymes. SULT-catalyzed sulfonation regulates hormone activities metabolizes drugs detoxifies xenobiotic toxicants bioactivates carcinogens. Human dehydroepiandrosterone sulfotransferase (hSULT2A1 DHEA-ST) plays a very important role in sulfating endogenous hydroxysteroids and exogenousxenobiotics. Our recent studies have shown that methotrexate can induce hSULT2A1 expression. To investigate the molecular mechanism involved in hSULT2A1 induction we generated the promoter sequence of hSULT2A1 by PCR and constructed a reporter gene vector. Both reporter gene assay and endogenous induction results suggested that human constitutive active receptor (hCAR) mediates the methotrexate induction of hSULT2A1 in both Caco-2 and Hep G2 cells. Human vitamin D receptor (hVDR) also upregulated hSULT2A1 gene expression while human pregnane X receptor (hPXR) downregulated it. Human pregnane X receptor suppressed hCAR-mediated methotrexate induction of hSULT2A1 in both Caco-2 and Hep G2 cells. hVDR competed with hCAR for the hSULT2A1 promoter in Caco-2 cells. hCAR inhibited hVDR-mediated vitamin D3 induction of hSULT2A1 but not methotrexate induction of hSULT2A1. These results strongly support the hypothesis that cross-talk occurs among nuclear receptors in the signal transduction pathway of hSULT2A1 and that interactions among nuclear receptors also depend on ligands (inducers) in the system.

In vivo and in vitro oxidative regulation of rat aryl sulfotransferase IV (AST IV)

Sulfotransferase catalyzed sulfation is important in the regulation of different hormones and the metabolism of hydroxyl containing xenobiotics. In the present investigation, we examined the effects of hyperoxia on aryl sulfotransferase IV in rat lungs in vivo. The enzyme activity of aryl sulfotransferase IV increased 3- to 8-fold in >95% O2 treated rat lungs. However, hyperoxic exposure did not change the mRNA and protein levels of aryl sulfotransferase IV in lungs as revealed by Western blot and RT-PCR. This suggests that oxidative regulation occurs at the level of protein modification. The increase of nonprotein soluble thiol and reduced glutathione (GSH)/oxidized glutathione (GSSG) ratios in treated lung cytosols correlated well with the aryl sulfotransferase IV activity increase. In vitro, rat liver cytosol 2-naphthol sulfation activity was activated by GSH and inactivated by GSSG. Our results suggest that Cys residue chemical modification is responsible for the in vivo and in vitro oxidative regulation. The molecular modeling structure of aryl sulfotransferase IV supports this conclusion. Our gel filtration chromatography results demonstrated that neither GSH nor GSSG treatment changed the existing aryl sulfotransferase IV dimer status in cytosol, suggesting that oxidative regulation of aryl sulfotransferase IV is not caused by dimer-monomer status change.

Sulfonation in pharmacology and toxicology

Sulfonation has a major function in modulating the biological activities of a wide number of endogenous and foreign chemicals, including: drugs, toxic chemicals, hormones, and neurotransmitters. The activation as well as inactivation of many xenobiotics and endogenous compounds occurs via sulfonation. The process is catalyzed by members of the cytosolic sulfotransferase (SULT) superfamily consisting of at least ten functional genes in humans. The reaction in intact cells may be reversed by arylsulafatase present in the endoplasmic reticulum. Under physiological conditions, sulfonation is regulated, in part, by the supply of the co-substrate/donor molecule 3'-phosphadensoine-5-phosphosulfate (PAPS), and transport mechanisms by which sulfonated conjugates enter and leave cells. Variation in the response of individuals to certain drugs and toxic chemicals may be related to genetic polymorphisms documented to occur in each of the above pathways. Sulfonation has a major function in regulating the endocrine status of an individual by modulating the receptor activity of estrogens and androgens, steroid biosynthesis, and the metabolism of catecholamines and iodothyronines Sulfonation is a key reaction in the body's defense against injurious chemicals and may have a major function during early development since SULTs are highly expressed in the human fetus. As with many Phase I and Phase II reactions, sulfonation may also serve as the terminal step in activating certain dietary and environmental agents to very reactive toxic intermediates implicated in carcinogenesis.

A Comparative Molecular Field Analysis‐Based Approach to Prediction of Sulfotransferase Catalytic Specificity

Understanding the catalytic function and substrate specificity of cytosolic sulfotransferases (SULTs) involved in drug metabolism is essential for predicting the metabolic outcomes of many xenobiotics. Although multiple isoforms of cytosolic SULTs have been identified and characterized in humans and other species, relatively little is known about the specific molecular interactions that govern their selectivity for substrates. The use of three-dimensional quantitative structure-activity relationship (3D-QSAR) techniques has emerged as a powerful tool for understanding the relationships among protein structure, catalytic function, and substrate specificity. We have found that a specific adaptation of a ligand-based 3D-QSAR method, comparative molecular field analysis (CoMFA), is particularly useful for prediction of the catalytic efficiencies of SULTs. This approach has been used to study the function of a prototypical rat hepatic phenol SULT and has now been extended to a member of the hydroxysteroid SULT family. Key aspects of this methodology incorporate strategies for finding the most meaningful bioactive conformation with respect to the protein structure, use of a model of an enzyme-substrate complex incorporating the mechanism of sulfuryl transfer, and the utilization of log(k(cat)/K(m)) as the parameter for correlation analysis. The success of this approach with members of two different families of cytosolic SULTs suggests that it may be of more general use in the study of other SULTs.

Sulfotransferases and Acetyltransferases in Mutagenicity Testing: Technical Aspects

Sulfotransferases (SULTs) and N-acetyltransferases (NATs) mediate the terminal activation step of various mutagens and carcinogens. Target cells of standard in vitro mutagenicity tests do not express any endogenous SULTs. NATs are expressed in some cells, but may not reflect the substrate specificity of human NATs. External activating systems usually lack the cofactors for these enzymes. Upon addition of the cofactor, the ultimate mutagen may be formed, but especially sulfo conjugates--anions--may not reliably penetrate into the target cells. This chapter presents methods used to incorporate these enzyme systems into in vitro mutagenicity test systems and to identify the critical human forms. The method of choice is direct expression of the enzymes in target cells. We present procedures on how this can be reached in bacteria and in mammalian cell lines in culture. Furthermore, genetically manipulated mouse models are a very promising perspective for answering open questions.

Influence of phenylalanines 77 and 138 on the stereospecificity of aryl sulfotransferase IV

Aryl sulfotransferase (AST) IV (also named tyrosine-ester sulfotransferase and ST1A1) is a major phenol sulfotransferase in the rat, and it catalyzes the sulfation of many drugs, carcinogens, and other xenobiotics that contain phenol, benzylic alcohol, N-hydroxy arylamine, and oxime functional groups. Previous work discovered a stereospecificity of AST IV toward the enantiomers of 1,2,3,4-tetrahydro-1-naphthol and varying degrees of stereoselectivity with other chiral benzylic alcohols. The studies described here were directed toward understanding the roles of specific amino acid residues at the substrate binding site in determining the stereoselectivity of this sulfotransferase isoform. Docking experiments with a homology model of AST IV revealed three amino acid residues, Phe77, Phe138, and Tyr236, that may potentially be important for interactions with substituents on the chiral carbon of a benzylic alcohol serving as a sulfuryl acceptor, thereby imparting stereoselectivity. To test this hypothesis, mutants were constructed wherein each of the above residues was substituted with alanine. Kinetic studies on the sulfation of the enantiomers of 1,2,3,4-tetrahydro-1-naphthol indicated that the stereospecificity of the sulfotransferase was altered by the substitutions of alanine for either Phe77 or Phe138, but stereospecificity was maintained by alanine substitution at Tyr236. Molecular models of the mutant enzymes interacting with enantiomers of 1,2,3,4-tetrahydro-1-naphthols and with 2-naphthol indicate that Phe77 and Phe138 provide significant steric interactions at the active site that both enhance catalytic efficiency and impart stereospecificity in molecular recognition of substrates and inhibitors.

Gene expression profiles and genetic damage in benzo(a)pyrene diol epoxide-exposed TK6 cells

Microarray analysis is a powerful tool to identify the biological effects of drugs or chemicals on cellular gene expression. In this study, we compare the relationships between traditional measures of genetic toxicology and mutagen-induced alterations in gene expression profiles. TK6 cells were incubated with 0.01, 0.1, or 1.0 microM +/-anti-benzo(a)pyrene-trans-7,8-dihydrodiol-9,10-epoxide (BPDE) for 4 h and then cultured for an additional 20 h. Aliquots of the exposed cells were removed at 4 and 24 h in order to quantify DNA adduct levels by 32P post-labeling and measure cell viability by cloning efficiency and flow cytometry. Gene expression profiles were developed by extracting total RNA from the control and exposed cells at 4 and 24 h, labeling with Cy3 or Cy5 and hybridizing to a human 350 gene array. Mutant frequencies in the Thymidine Kinase and Hypoxanthine Phosphoribosyl Transferase genes were also determined. The 10alpha-(deoxyguanosin-N(2)-yl)-7alpha,8beta,9beta-trihydroxy-7,8,9,10-tetrahydrobenzo(a)pyrene (dG-N(2)-BPDE) adduct increased as a function of dose and was the only adduct identified. A dose-related decrease in cell viability was evident at 24 h, but not at 4 h. Cell death occurred by apoptosis. At 4 h, analysis of the gene expression profiles revealed that Glutathione Peroxidase and Gadd45 were consistently upregulated (greater than 1.5-fold and significantly (P < 0.001) greater than the control in two experiments) in response to 1.0 microM BPDE exposure. Fifteen genes were consistently down-regulated (less than 0.67-fold and significantly (P < 0.001) lower than the control in two experiments) at 4 h in cultures exposed to 1.0 microM BPDE. Genes with altered expression at 4 h included genes important in the progression of the cell-cycle and those that inhibit apoptosis. At 24 h post-exposure, 16 genes, involved in cell-cycle control, detoxification, and apoptosis were consistently upregulated; 10 genes were repressed in cultures exposed to the high dose of BPDE. Real-time quantitative PCR confirmed the differential expression of selected genes. These data suggest that changes in gene expression will help to identify effects of drugs and chemicals on molecular pathways in cells, and will provide useful information about the molecular responses associated with DNA damage. Of the endpoints evaluated, DNA adduct formation was the most sensitive indicator of DNA damage. DNA adduct formation was clearly evident at low doses, but the number of genes with significantly altered expression (P < 0.001) was minimal. Alterations in gene expression were more robust at doses associated with cellular toxicity and induction of mutations.

Protein profile of aging and its retardation by caloric restriction in neural retina

Aging is a slow, gradual deterioration process of an organism. The only experimental intervention, which can reliably retard aging and age-related degenerative diseases, is dietary caloric restriction (CR). To gain insight into the mechanism of CR intervention, we have investigated the protein profile of aging and its retardation by CR in the neural retina of Brown Norway (BN) rats using the comprehensive proteomic approach. We found that the intensities of 18 proteins decreased significantly with age. CR intervention can completely prevent seven of them, and partially protect eight of them, from such age-related declines. The major protein targets protected by CR intervention appear to be glycolytic enzymes and molecular chaperones. These data are the first to suggest that CR may retard the age-related degeneration of retina by maintaining sufficient glucose metabolism, by ensuring proper protein folding, and/or by preventing protein denaturation in the neural retina.

Predicting undesirable drug interactions with promiscuous proteins in silico

Although computational tools have been used to predict toxic responses resulting from molecules binding either as substrates or inhibitors to proteins, there are complications to be resolved. Some proteins appear promiscuous in their ability to bind a diverse array of hydrophobic molecules. This promiscuity arises from the binding site simultaneously accommodating more than one molecule, multiple separate binding sites, protein flexibility, or a combination of all these properties. With the availability of more crystal structures for these non-target proteins, we should be able to predict binding in silico with a greater accuracy, thus avoiding or managing toxic side effects, therefore ultimately improving the success of drug discovery.

TRANSACTIVATION OF GLUCOCORTICOID-INDUCIBLE RAT ARYL SULFOTRANSFERASE (SULT1A1) GENE TRANSCRIPTION

The purpose of the current study was to establish the role of the glucocorticoid receptor (GR) and androgen receptor (AR) transcription factors in the transactivation of rat aryl sulfotransferase (SULT1A1) gene transcription and to identify the functional hormone-responsive element(s) in the SULT1A1 gene. A cis-acting inverted repeat with three intervening bases (IR3) was identified in the 5'-flanking of the SULT1A1 gene that mediates the transactivation of SULT1A1 gene transcription by both the GR and AR. CV-1 cells were cotransfected with SULT1A1-luciferase reporter plasmids and either wild-type or mutant GR or AR expression constructs. In cotransfectants expressing the wild-type GR, treatment with triamcinolone acetonide produced an approximately 4- to 6-fold induction of luciferase activity in IR3-containing SULT1A1 reporter plasmids. IR3-containing SULT1A1 reporter constructs were also activated by treatment with the synthetic androgen R1881 in cells cotransfected with wild-type but not mutant AR. In primary cultured rat hepatocytes, androgen-inducible expression of IR3-containing SULT1A1 reporter plasmids required cotransfection with AR expression plasmid. Targeted disruption of the SULT1A1 IR3 by mutation of a conserved GT sequence in the 3' half-site of the element ablated GR and AR responsiveness. These results indicate that a proximal IR3 element in the 5'-flanking region of the rat SULT1A1 gene is sufficient for the transactivation of SULT1A1 gene transcription by the GR and AR, and that relative to the GR, functional AR activity is reduced in primary cultured rat hepatocytes.