Role of Conformational Dynamics of Sulfotransferases SULT1A1 and SULT1A3 in Substrate Specificity.

Sulfotransferases (SULTs) are phase II metabolizing enzymes catalyzing the sulfoconjugation from the co-factor 3'-Phosphoadenosine 5'-Phosphosulfate (PAPS) to a wide variety of endogenous compounds, drugs and natural products. Although SULT1A1 and SULT1A3 share 93% identity, SULT1A1, the most abundant SULT isoform in humans, exhibits a broad substrate range with specificity for small phenolic compounds, while SULT1A3 displays a high affinity toward monoamine neurotransmitters like dopamine. To elucidate the factors determining the substrate specificity of the SULT1 isoenzymes, we studied the dynamic behavior and structural specificities of SULT1A1 and SULT1A3 by using molecular dynamics (MD) simulations and ensemble docking of common and specific substrates of the two isoforms. Our results demonstrated that while SULT1A1 exhibits a relatively rigid structure by showing lower conformational flexibility except for the lip (loop L1), the loop L2 and the cap (L3) of SULT1A3 are extremely flexible. We identified protein residues strongly involved in the recognition of different substrates for the two isoforms. Our analyses indicated that being more specific and highly flexible, the structure of SULT1A3 has particularities in the binding site, which are crucial for its substrate selectivity.

A reappraisal of the 6-O-desmethylnaproxen-sulfating activity of the human cytosolic sulfotransferases.

In this study, we aimed to obtain a comprehensive account of the human cytosolic sulfotransferases (SULTs) that are capable of sulfating 6-O-desmethylnaproxen (O-DMN), a major metabolite of naproxen. Of the 13 known human SULTs tested, 7 (SULT1A1, SULT1A2, SULT1A3, SULT1B1, SULT1C2, SULT1C4, and SULT1E1) displayed O-DMN-sulfating activity, when analyzed using an elevated substrate concentration (500 µmol·L-1) together with 14 µmol·L-1 of the sulfate donor, 3'-phosphoadenosine-5'-phosphosulfate (PAPS). At 10 µmol·L-1 O-DMN concentration, however, only SULT1A1 and SULT1A3 displayed detectable activity, with the former being nearly 2 orders of magnitude more active than the latter. A pH-dependence study indicated that SULT1A1 exhibited a broad pH optimum spanning pH 5.5-7. Kinetic parameters of the sulfation of O-DMN by SULT1A1 were determined. The production and release of sulfated O-DMN was demonstrated using cultured human HepG2 hepatoma cells and Caco-2 colon carcinoma cells. Moreover, assays using human organ specimens revealed that the O-DMN-sulfating activities present in the cytosols of liver and small intestine (at 502.5 and 497.2 pmol·min-1·(mg protein)-1, respectively) were much higher than those detected for the cytosols of lung and kidney. Taken together, these results provided relevant information concerning the sulfation of O-DMN both in vitro and in vivo.

RNA Adduction Resulting from the Metabolic Activation of Myristicin by P450 Enzymes and Sulfotransferases.

Myristicin (MYR) mainly occurs in nutmeg and belongs to alkoxy-substituted allylbenzenes, a class of potentially toxic natural chemicals. RNA interaction with MYR metabolites in vitro and in vivo has been investigated in order to gain a better understanding of MYR toxicities. We detected two guanosine adducts (GA1 and GA2), two adenosine adducts (AA1 and AA2), and two cytosine adducts (CA1 and CA2) by LC-MS/MS analysis of total RNA extracts from cultured primary mouse hepatocytes and liver tissues of mice after exposure to MYR. An order of nucleoside adductions was found to be GAs > AAs > CAs, and the result of density functional theory calculations was in agreement with that detected by the LC-MS/MS-based approach. In vitro and in vivo studies have shown that MYR was oxidized by cytochrome P450 enzymes to 1'-hydroxyl and 3'-hydroxyl metabolites, which were then sulfated by sulfotransferases (SULTs) to form sulfate esters. The resulting sulfates would react with the nucleosides by SN1 and/or SN2 reactions, resulting in RNA adduction. The modification may alter the biochemical properties of RNA and disrupt RNA functions, perhaps partially contributing to the toxicities of MYR.

Inhibition of Human Sulfotransferases by Phthalate Monoesters.

Objective: This study aimed to investigate the inhibition of human important phase II metabolic enzyme sulfotransferases (SULTs) by phthalate monoesters, which are important metabolites of phthalate esters (PAEs). Method: Recombinant SULT-catalyzed metabolism of p-nitrophenol (PNP) was employed as the probe reactions of SULTs to investigate the inhibition of 8 kinds of phthalate monoesters towards SULT isoforms. An in vitro incubation system was utilized for preliminary screening, and 100 µM of phthalate monoesters was used. Inhibition kinetics were carried out to determine the inhibition of SULTs by phthalate monoesters. Result: Multiple phthalate monoesters have been demonstrated to exert strong inhibition potential towards SULT1A1, SULT1B1, and SULT1E1, and no significant inhibition of phthalate monoesters towards SULT1A3 was found. The activity of SULT1A1 was strongly inhibited by mono-hexyl phthalate (MHP), mono-octyl phthalate (MOP), mono-benzyl phthalate (MBZP), and mono-ethylhexyl phthalate (MEHP). Monobutyl phthalate (MBP), MHP, MOP, mono-cyclohexyl phthalate (MCHP), and MEHP significantly inhibited the activity of SULT1B1. MHP, MOP, and MEHP significantly inhibited the activity of SULT1E1. MOP was chosen as the representative phthalate monoester to determine the inhibition kinetic parameters (Ki) towards SULT1B1 and SULT1E1. The inhibition kinetic parameters (Ki) were calculated to be 2.23 µM for MOP-SULT1B1 and 5.54 µM for MOP-SULT1E1. In silico docking method was utilized to understand the inhibition mechanism of SULT1B1 by phthalate monoesters. Conclusions: All these information will be beneficial for understanding the risk of phthalate monoester exposure from a new perspective.

Inhibition of hydroxylated polychlorinated biphenyls (OH-PCBs) on sulfotransferases (SULTs).

Polychlorinated biphenyls (PCBs) have been demonstrated as a kind of the persistent organic pollutants (POPs) that could exert complicated influences towards metabolism in human bodies. Since hydroxylated polychlorinated biphenyls (OH-PCBs) are important metabolites of PCBs, our study focuses on investigating the potential inhibitory capability of OH-PCBs on four human sulfotransferase (SULT) isoforms. P-nitrophenol (PNP) was utilized as nonselective probe substrate for this study, and recombinant SULT isoforms were utilized as the enzyme resources. Ultra-performance liquid chromatography (UPLC)-UV detecting system was used to analyze PNP and its metabolite PNP-sulfate. As result, 100 µM of most tested OH-PCBs significantly inhibited the activity of four SULT isoforms. Concentration-dependent inhibition of OH-PCBs towards SULTs was found, and half inhibition concentration values (IC50) of some inhibition processes were determined. Inhibition kinetics (inhibition kinetic type and parameters) were determined using 4'-OH-PCB106 as the representative OH-PCB, SULT1B1 and SULT1E1 as representative SULT isoforms. The inhibition kinetic parameters (Ki) were 1.73 µM and 1.81 µM for the inhibition of 4'-OH-PCB106 towards SULT1B1 and SULT1E1, respectively. In silico docking simulation was utilized to analyze the inhibition capability of 2'-OH-PCB5, 4'-OH-PCB9, 2'-OH-PCB12 towards SULT1A3.All these results obtained in this study are helpful for further understanding the toxicity of PCBs.

Co-culture with mouse embryonic fibroblasts improves maintenance of metabolic function of human small hepatocyte progenitor cells.

Derivation and culture of small hepatocyte progenitor cells (SHPCs) capable of proliferating in vitro has been described in rodents and recently in humans. These cells are capable of engrafting in injured livers, however, they display de-differentiated morphology and reduced xenobiotic metabolism activity in culture over passages. Here we report that SHPCs derived from adult primary human hepatocytes (PHHs) and cultured on mouse embryonic fibroblasts (MEFs) not only display differentiated morphology and exhibit gene expression profiles similar to adult PHHs, but importantly, they retain their phenotype over several passages. Further, unlike previous reports, where extensive manipulations of culture conditions are required to convert SHPCs to metabolically functional hepatocytes, SHPCs in our co-culture system maintain expression of xenobiotic metabolism-associated genes. We show that SHPCs in co-culture are able to perform xenobiotic metabolism at rates equal to their parent PHHs as evidenced by the metabolism of acetaminophen to all of its major metabolites. In summary, we present an improved co-culture system that allows generation of SHPCs from adult PHHs that maintain their differentiated phenotype over multiple passages. Our findings would be useful for expansion of limited PHHs for use in studies of drug metabolism and toxicity testing.

Involvement of UDP-Glucuronosyltransferases and Sulfotransferases in the Excretion and Tissue Distribution of Resveratrol in Mice.

Resveratrol is a naturally occurring polyphenolic compound with various pharmacological activities. It is unknown whether the expression of metabolizing enzymes correlates with resveratrol levels in organs and tissues. Therefore, we investigated the metabolism and tissue distribution of resveratrol in mice and assessed its association with the expression of UDP-glucuronosyltransferase (Ugt) and sulfotransferase (Sult) genes. Plasma, urine, feces, and various organs were analyzed using high-performance liquid chromatography at up to 8 h after intragastric resveratrol administration. The metabolism of resveratrol was pronounced, leading to the formation of resveratrol glucuronides and sulfates. Concentrations of resveratrol and its metabolites were high in the gastrointestinal organs, urine, and feces, but low in the liver and kidneys. In lung, heart, thymus, and brain tissues, parent resveratrol levels exceeded the sulfate and glucuronide concentrations. The formation of resveratrol conjugates correlated with the expression of certain Ugt and Sult genes. Reverse transcription quantitative PCR (RT-qPCR) analysis revealed high mRNA expression of Ugt1a1 and Ugt1a6a in the liver, duodenum, jejunum, ileum, and colon, leading to high concentrations of resveratrol-3-O-glucuronide in these organs. Strong correlations of resveratrol-3-O-sulfate and resveratrol-3-O-4'-O-disulfate formation with Sult1a1 mRNA expression were also observed, particularly in the liver and colon. In summary, our data revealed organ-specific expression of Sults and Ugts in mice that strongly affects resveratrol concentrations; this may also be predictive in humans following oral uptake of dietary resveratrol.

Hepatoprotective effect of isoquercitrin against acetaminophen-induced liver injury.

AIMS: Acetaminophen (APAP) overdose leads to severe hepatotoxicity. Isoquercitrin exhibited potential hepatoprotective effect in our previous study. The present investigation aimed to evaluate the effect of isoquercitrin against APAP induced liver injury and to explore its possible mechanism. MAIN METHODS: Mice were treated intragastrically with isoquercitrin (10, 20, or 50mg/kg) for 3days before APAP (300mg/kg) injection. After 24h from APAP treatment, the levels of serum aminotransferase, hepatic oxidative stress and nitrosative stress biomarkers were determined by commercial kits or western bolt. Activities of UDP-glucuronosyltransferases (UGTs), sulfotransferases (SULTs) and cytochrome 2E1 (CYP2E1) were evaluated using ELISA methods and standard biochemical procedures. Subsequently, the protein and mRNA levels of inflammatory factors including TNF-α, IL-1ß, IL-6 and iNOS were determined using ELISA methods, western blot or real-time PCR. The effect of isoquercitrin on APAP activated NFκB/MAPK pathway was assessed by western bolt. KEY FINDINGS: Isoquercitrin pretreatments markedly attenuated APAP induced hepatic oxidative stress, nitrosative stress and centrilobular necrosis. In addition to potent antioxidant activity, isoquercitrin was able to regulate the activities of SULTs and CYP2E1, therefore promoted APAP hepatic detoxification. The anti-inflammatory activity of isoquercitrin which involved in the amelioration of iNOS, TNF-α, IL-1ß and IL-6 production via the blockade of NF-κB and MAPK pathways also responsible for its hepatoprotective effect. SIGNIFICANCE: Our data evidenced that isoquercitrin protected liver from APAP induced injury though inhibition of oxidative stress, nitrosative stress and inflammation, as well as regulation of APAP metabolism, suggesting that isoquercitrin could be a potential hepatoprotective agent.

Protective effect of hyperoside against acetaminophen (APAP) induced liver injury through enhancement of APAP clearance.

Acetaminphen (APAP) overdose leads to severe hepatotoxicity. Apocynum venetum L. (A. venetum) possess potent hepatoprotective effect. Hyperoside is one of the major compounds exist in Apocynum venetum L. and might be a potential agent to protect against APAP-induce liver injury. In this study, we investigated the effect of hyperoside on APAP hepatotoxicity in mice. Mice were treated intragastrically with hyperoside (10, 50 or 100 mg/kg) for 3 days before APAP (300 mg/kg) injection. APAP alone caused severe liver injury characterized by significantly increased serum aminotransferase levels, hepatic malondialdehyde (MDA) and 3-nitrotyrosine (3-NT) formation, as well as liver superoxide dismutase (SOD), glutathione peroxidase (GPx) and glutathione (GSH) depletions. Hyperoside significantly attenuated APAP-induced liver damages in a dose dependent manner, and 100 mg/kg was the most effective dose. Further study confirmed that hyperoside was able to increase activities and mRNA expressions of uridine diphoshate glucuronosyltransferases (UGTs) and sulfotransferases (SULTs), as well as to inhibit CYP2E1 activities, and thereby suppressed toxic intermediate formation and promoted APAP hepatic detoxification. Nrf-2 activation might be involved in hyperoside induced up-regulation of phase II enzymes. Collectively, our data provide evidence that hyperoside protected the liver against APAP induced injury mainly by accelerating APAP harmless metabolism, implying that hyperoside can be considered as a potential natural hepatoprotective agent.

Sulfonation of curcuminoids: characterization and contribution of individual SULT enzymes.

SCOPE: Poor oral bioavailability of curcuminoids limited their various applications, and one of the main reasons is their rapid metabolism in vivo. Sulfonation via sulfotransferases (SULTs) is an important metabolic pathway for such compounds. The objective of this study is to determine the SULT-isoform-specific metabolic fingerprint, tissue-specific rate, and reaction kinetic profiles to describe the characterization and contribution of curcuminoids sulfonation. METHODS AND RESULTS: Sulfonation of curcuminoids was investigated by using nine expressed SULT isoforms and four pooled human tissue S9 fractions. The results showed that human small intestine is the predominant tissue responsible for sulfonation of curcuminoids. SULT1A3 is a major isoform catalyzing sulfonation of curcumin and demethoxycurcumin, but not for bisdemethoxycurcumin. SULT1B1 is only responsible for sulfonation of curcumin. Although SULT1C4 and 1E1 could highly catalyze the sulfate conjugations toward all the three compounds, the correlativities with human small intestine S9 fractions were much weaker (R(2) = 0.100-0.482). Almost all the kinetic profiles of the SULT isoforms for curcuminoids exhibited substrate inhibition kinetics. CONCLUSION: This investigation contributed to elucidate the SULT-mediated metabolism and detoxication of curcuminoids at molecular levels and in different organs.

Human hydroxylated metabolites of BDE-47 and BDE-99 are glucuronidated and sulfated in vitro.

Polybrominated diphenyl ethers (PBDEs) were used worldwide as additive flame retardants and are classified as persistent, bioaccumulable and toxic environmental pollutants. In humans, the hydroxylated metabolites of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) and 2,2',4,4',5-pentabromodiphenyl ether (BDE-99) formed in vitro have also been detected in vivo. To further characterize the metabolism of BDE-47 and BDE-99 and to identify candidate markers for monitoring the human exposure to PBDEs using non-invasive approaches, glucuronidation and sulfation of hydroxylated metabolites of BDE-47 and BDE-99 were investigated using human liver microsomes and cytoplasm, respectively. The formed Phase II metabolites were analyzed by liquid chromatography-tandem mass spectrometry using a novel approach to develop analytical methods in absence of authentic standards. All available standards for hydroxylated metabolites of BDE-47 and BDE-99 were glucuronidated and sulfated, showing that glucuronidation and sulfation are part of the metabolism pathway of BDE-47 and BDE-99 in vitro. The major glucuronidated and sulfated analogs of hydroxylated metabolites of BDE-47 were (a) 2,4-DBP-Gluc and 5-Gluc-BDE-47, and (b) 2'-Sulf-BDE-28, 4-Sulf-BDE-42 and 3-Sulf-BDE-47, respectively. The major glucuronidated and sulfated analogs of hydroxylated metabolites of BDE-99 were (a) 2,4,5-TBP-Gluc and 6'-Gluc-BDE-99, and (b) 3'-Sulf-BDE-99 and 5'-Sulf-BDE-99, respectively. Apparent Km values associated with the formation of sulfated metabolites of BDE-47 and BDE-99 were ten times lower than those of the corresponding glucuronidated metabolites, suggesting that sulfated rather than glucuronidated metabolites of OH-PBDEs might be used as markers of human exposure to PBDEs using a non-invasive approach based on urine sample collection.

Epigenetic regulation of drug metabolism and transport.

The drug metabolism is a biochemical process on modification of pharmaceutical substances through specialized enzymatic systems. Changes in the expression of drug-metabolizing enzyme genes can affect drug metabolism. Recently, epigenetic regulation of drug-metabolizing enzyme genes has emerged as an important mechanism. Epigenetic regulation refers to heritable factors of genomic modifications that do not involve changes in DNA sequence. Examples of such modifications include DNA methylation, histone modifications, and non-coding RNAs. This review examines the widespread effect of epigenetic regulations on genes involved in drug metabolism, and also suggests a network perspective of epigenetic regulation. The epigenetic mechanisms have important clinical implications and may provide insights into effective drug development and improve safety of drug therapy.

Effect on metabolic enzymes and thyroid receptors induced by BDE-47 by activation the pregnane X receptor in HepG2, a human hepatoma cell line.

2,2',4,4'-Tetra-bromodiphenyl ether (BDE-47), an important congener among polybrominated diphenyl ether (PBDE) compounds, has been predominantly in environmental samples and human tissue. Thyroid disruption is the most sensitive endpoint effect among a number of health effects of exposure to BDE-47 in animals and humans. However, the detailed underlying mechanisms in humans are not well understood. In the present study, human pregnane X receptor (hPXR)-overexpressing HepG2 cell model and a dual-luciferase reporter assay system were constructed to investigate the role of hPXR in BDE-47-induced alterations of expression of metabolic enzymes and TR in vitro. The results showed that hPXR was significantly activated by BDE-47, and expression levels of both mRNA and protein of the thyroid receptor (TR) isoforms TRα1 and TRß1 were decreased in hPXR-overexpressing HepG2 cells after BDE-47 treatment. However, the increased expression of hepatic microsomal phase I enzyme CYP3A4 and phase II enzymes, UGT1A3 and SULT2A1 were also found. Taken together, the results indicated that BDE-47 was a strong hPXR activator, activation of hPXR played an important role in BDE-47-induced down-regulation of TR, and up-regulations of CYP3A4, UGT1A3, and SULT2A1 participated in the process, which may provide more toxicological evidence on mechanisms of disruption of thyroid hormone induced by BDE-47.

Estrogen associated gene polymorphisms and their interactions in the progress of Alzheimer's disease.

The extensive neuroprotective effects of estrogen against Alzheimer's disease (AD) have been proven in numerous laboratory studies. However, in clinical studies, the exact role of estrogen in AD is still ambiguous. Some evidences even suggested the high levels of estrogen or estrogen replacement treatment increased the risk of AD. Thus, there must be other factors affecting the neuroprotective effects of estrogen. Multiple enzymes and receptor proteins are involved in the biosynthesis, metabolism and signaling pathways of estrogen, and mediate the beneficial effects of estrogen on AD. Previous studies have suggested some polymorphisms of genes encoding these enzymes and proteins are associated with the risk of AD. In addition to the genes associated with estrogen biosynthesis and metabolism and the genes encoding estrogen receptor proteins, some other genes also modulate the effects of estrogen on AD, or interact with other estrogen-associated genes on the progress of AD. The gene-hormone and gene-gene interactions may be key to unraveling the conflicting results regarding the effect of estrogen on AD. In this paper, we will review and discuss the associations between polymorphisms of these genes and their interactions and the susceptibility to AD. A better understanding of these estrogen-associated genes is significant to explore the pathogenesis of AD.