Sulfotransferase catalyzing sulfation of heterocyclic amines

Structural and Dynamic Characterizations Highlight the Deleterious Role of SULT1A1 R213H Polymorphism in Substrate Binding

Sulfotransferase 1A1 (SULT1A1) is responsible for catalyzing various types of endogenous and exogenous compounds. Accumulating data indicates that the polymorphism rs9282861 (R213H) is responsible for inefficient enzymatic activity and associated with cancer progression. To characterize the detailed functional consequences of this mutation behind the loss-of-function of SULT1A1, the present study deployed molecular dynamics simulation to get insights into changes in the conformation and binding energy. The dynamics scenario of SULT1A1 in both wild and mutated types as well as with and without ligand showed that R213H induced local conformational changes, especially in the substrate-binding loop rather than impairing overall stability of the protein structure. The higher conformational changes were observed in the loop3 (residues, 235-263), turning loop conformation to A-helix and B-bridge, which ultimately disrupted the plasticity of the active site. This alteration reduced the binding site volume and hydrophobicity to decrease the binding affinity of the enzyme to substrates, which was highlighted by the MM-PBSA binding energy analysis. These findings highlight the key insights of structural consequences caused by R213H mutation, which would enrich the understanding regarding the role of SULT1A1 mutation in cancer development and also xenobiotics management to individuals in the different treatment stages.

Proteomic analysis of chronic restraint stress-induced Gan (肝)-stagnancy syndrome in rats

OBJECTIVE

To analyze the proteomic characteristics of Gan (肝)-stagnancy syndrome (GSS) by seeking the differential protein in blood and tissues of GSS model rats.

METHODS

GSS model rats were established by chronic restraint stress, keeping rats in restrain chamber for 6 h every day for 21 successive days. Their blood and liver samples were collected at the end of experiment for differential protein detection with methods of isoelectrofocusing and polyacrylamide SDS-PAGE, silver staining, and scanning. The gel images were analyzed with Imagemaster 2D Elite software, and the excavated differential protein spots were identified with matrix assistant laser resolving TOF mass spectrometry, Western blot, ELISA, and RT-PCR, respectively.

RESULTS

A method for isolating the protein in blood serum and tissues by two-dimensional gel electrophoresis was established and optimized. Six serum proteins and three liver proteins that differentially expressed were identified. The down-regulated differential proteins in serum of GSS model rats were serum albumin precursor, beta 1 globin, antibody against muscle acetylcholine receptor, Ig lambda-2 C region, and transthyretin (TTR), and those in liver tissue were aryl sulfotransferase, enoyl-CoA hydratase, and TTR. TTR down-regulation was found in both serum and liver. Preliminary biological information analysis showed that these differential proteins involved in immune, neuroendocrine, nutrition, and substance metabolism.

CONCLUSION

Proteomic analysis of differential proteins showed that TTR, aryl sulfotransferase, and enoyl-CoA hydratase expressions are downregulated in the GSS model rats, suggesting that the susceptibility of cancer could be enhanced by chronic stress.

Chemopreventive compounds--view from the other side

Increasing attention is being paid to the possibility of applying chemopreventive agents for the protection of individuals from cancer risk. The beneficial potential of chemoprotective compounds is usually well documented by extensive experimental data. To assure the desired effect, these compounds are frequently concentrated to produce dietary supplements for human use. The additive and synergistic effects of other food constituents are, however, frequently ignored. Even natural chemopreventive compounds have to be considered as xenobiotics. Thus, as much attention has to be paid to their testing prior to their wide application as is usual in drug development for human treatment. Unfortunately, much of the research in this area is solely based on simplified in vitro systems that cannot take into account the complexity of biotransformation processes, e.g. chemopreventive compound-drug interaction, effect on metabolism of endogenic compounds. Hence, the predicted chemopreventive potential is not attained in respect of cancer prevention; moreover, the administration of high doses of chemopreventive compounds might be even detrimental for the human health.

Characterization and ontogenic study of novel steroid-sulfating SULT3 sulfotransferases from zebrafish

In vertebrates, sulfation as catalyzed by members of the cytosolic sulfotransferase (SULT) family has been suggested to be involved in the homeostasis of steroids. To establish the zebrafish as a model for investigating how sulfation functions to regulate steroid metabolism during the developmental process, we have embarked on the identification of steroid-sulfating SULTs in zebrafish. By searching the GenBank database, we identified two putative cytosolic SULT sequences from zebrafish, designated SULT3 ST1 and ST2. The recombinant proteins of these two zebrafish SULT3 STs were expressed in and purified from BL21 (DE3) cells transformed with the pGEX-2TK expression vector harboring SULT3 ST1 or ST2 cDNA. Upon enzymatic characterization, purified SULT3 ST1 displayed the strongest sulfating activity toward 17beta-estradiol among the endogenous substrates tested, while SULT3 ST2 exhibited substrate specificity toward hydroxysteroids, particularly dehydroepiandrosterone (DHEA). The pH-dependence and kinetic constants of these two enzymes with 17beta-estradiol and DHEA were determined. A developmental expression study revealed distinct patterns of the expression of SULT3 ST1 and ST2 during embryonic development and throughout the larval stage onto maturity. Collectively, these results imply that these two steroid-sulfating SULT3 STs may play differential roles in the metabolism and regulation of steroids during zebrafish development and in adulthood.

Dietary flavonoids: effects on xenobiotic and carcinogen metabolism

Flavonoids are present in fruits, vegetables and beverages derived from plants (tea, red wine), and in many dietary supplements or herbal remedies including Ginkgo Biloba, Soy Isoflavones, and Milk Thistle. Flavonoids have been described as health-promoting, disease-preventing dietary supplements, and have activity as cancer preventive agents. Additionally, they are extremely safe and associated with low toxicity, making them excellent candidates for chemopreventive agents. The cancer protective effects of flavonoids have been attributed to a wide variety of mechanisms, including modulating enzyme activities resulting in the decreased carcinogenicity of xenobiotics. This review focuses on the flavonoid effects on cytochrome P450 (CYP) enzymes involved in the activation of procarcinogens and phase II enzymes, largely responsible for the detoxification of carcinogens. A number of naturally occurring flavonoids have been shown to modulate the CYP450 system, including the induction of specific CYP isozymes, and the activation or inhibition of these enzymes. Some flavonoids alter CYPs through binding to the aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor, acting as either AhR agonists or antagonists. Inhibition of CYP enzymes, including CYP 1A1, 1A2, 2E1 and 3A4 by competitive or mechanism-based mechanisms also occurs. Flavones (chrysin, baicalein, and galangin), flavanones (naringenin) and isoflavones (genistein, biochanin A) inhibit the activity of aromatase (CYP19), thus decreasing estrogen biosynthesis and producing antiestrogenic effects, important in breast and prostate cancers. Activation of phase II detoxifying enzymes, such as UDP-glucuronyl transferase, glutathione S-transferase, and quinone reductase by flavonoids results in the detoxification of carcinogens and represents one mechanism of their anticarcinogenic effects. A number of flavonoids including fisetin, galangin, quercetin, kaempferol, and genistein represent potent non-competitive inhibitors of sulfotransferase 1A1 (or P-PST); this may represent an important mechanism for the chemoprevention of sulfation-induced carcinogenesis. Importantly, the effects of flavonoids on enzymes are generally dependent on the concentrations of flavonoids present, and the different flavonoids ingested. Due to the low oral bioavailability of many flavonoids, the concentrations achieved in vivo following dietary administration tend to be low, and may not reflect the concentrations tested under in vitro conditions; however, this may not be true following the ingestion of herbal preparations when much higher plasma concentrations may be obtained. Effects will also vary with the tissue distribution of enzymes, and with the species used in testing since differences between species in enzyme activities also can be substantial. Additionally, in humans, marked interindividual variability in drug-metabolizing enzymes occurs as a result of genetic and environmental factors. This variability in xenobiotic metabolizing enzymes and the effect of flavonoid ingestion on enzyme expression and activity can contribute to the varying susceptibility different individuals have to diseases such as cancer. As well, flavonoids may also interact with chemotherapeutic drugs used in cancer treatment through the induction or inhibition of their metabolism.

The structures of the unique sulfotransferase retinol dehydratase with product and inhibitors provide insight into enzyme mechanism and inhibition

The structure of retinol dehydratase (DHR) from Spodoptera frugiperda, a member of the sulfotransferase superfamily, in complexes with the inactive form of the cofactor PAP 3'-phosphoadenosine 5'-phosphate (PAP) and (1) the product of the reaction with retinol anhydroretinol (AR), (2) the retinoid inhibitor all-trans-4-oxoretinol (OR), and (3) the potent steroid inhibitor androsterone (AND) have been determined and compared to the enzyme complex with PAP and retinol. The structures show that the geometry of the active-site amino acids is largely preserved in the various complexes. However, the beta-ionone rings of the retinoids are oriented differently with respect to side chains that have been shown to be important for the enzymatic reaction. In addition, the DHR:PAP:AND complex reveals a novel mode for steroid binding that contrasts significantly with that for steroid binding in other sulfotransferases. The molecule is displaced and rotated approximately 180 degrees along its length so that there is no acceptor hydroxyl in close proximity to the site of sulfate transfer. This observation explains why steroids are potent inhibitors of retinol dehydratase activity, rather than substrates for sulfonation. Most of the steroid-protein contacts are provided by the alpha-helical cap that distinguishes this member of the superfamily. This observation suggests that in addition to providing a chemical environment that promotes the dehydration of a sulfonated intermediate, the cap may also serve to minimize a promiscuous sulfotransferases activity.

Enzyme polymorphisms influencing the metabolism of heterocyclic aromatic amines

Heterocyclic aromatic amines are dietary carcinogens possibly involved in human carcinogenesis, DNA-adduct formation being an obligatory step in this multistage process. Heterocyclic amine binding to DNA largely depends on the balance between metabolic activation and detoxification pathways and DNA repair efficiency. Several genes coding for metabolic enzymes are polymorphic, which affects gene expression and/or enzyme activity. This paper briefly reviews the effect of polymorphisms of activating/detoxifying enzymes on the metabolism of heterocyclic amines. Despite some epidemiological evidence of an association between genetic polymorphisms and susceptibility to cancer possibly resulting from dietary exposure to heterocyclic aromatic amines (HA), the genetic polymorphisms had only slight effects on biomarker levels, suggesting the existence of further unknown factors.

Active site mutations and substrate inhibition in human sulfotransferase 1A1 and 1A3

Human SULT1A1 is primarily responsible for sulfonation of xenobiotics, including the activation of promutagens, and it has been implicated in several forms of cancer. Human SULT1A3 has been shown to be the major sulfotransferase that sulfonates dopamine. These two enzymes shares 93% amino acid sequence identity and have distinct but overlapping substrate preferences. The resolution of the crystal structures of these two enzymes has enabled us to elucidate the mechanisms controlling their substrate preferences and inhibition. The presence of two p-nitrophenol (pNP) molecules in the crystal structure of SULT1A1 was postulated to explain cooperativity at low and inhibition at high substrate concentrations, respectively. In SULT1A1, substrate inhibition occurs with pNP as the substrate but not with dopamine. For SULT1A3, substrate inhibition is found for dopamine but not with pNP. We investigated how substrate inhibition occurs in these two enzymes using molecular modeling, site-directed mutagenesis, and kinetic analysis. The results show that residue Phe-247 of SULT1A1, which interacts with both p-nitrophenol molecules in the active site, is important for substrate inhibition. Mutation of phenylalanine to leucine at this position in SULT1A1 results in substrate inhibition by dopamine. We also propose, based on modeling and kinetic studies, that substrate inhibition by dopamine in SULT1A3 is caused by binding of two dopamine molecules in the active site.

Effect of Dietary Constituents With Chemopreventive Potential on Adduct Formation of a Low Dose of the Heterocyclic Amines PhIP and IQ and Phase II Hepatic Enzymes

We conducted a study to evaluate dietary chemopreventive strategies to reduce genotoxic effects of the carcinogens 2-amino-1-methyl-6-phenyl-imidazo[4,5-b]pyridine (PhIP) and 2-amino-3-methylimidazo[4,5-f]quinoline (IQ). PhIP and IQ are heterocyclic amines (HCAs) that are found in cooked meat and may be risk factors for cancer. Typical chemoprevention studies have used carcinogen doses many thousand-fold higher than usual human daily intake. Therefore, we administered a low dose of [14C]PhIP and [3H]IQ and utilized accelerator mass spectrometry to quantify PhIP adducts in the liver, colon, prostate, and blood plasma and IQ adducts in the liver and blood plasma with high sensitivity. Diets supplemented with phenethylisothiocyanate (PEITC), genistein, chlorophyllin, or lycopene were evaluated for their ability to decrease adduct formation of [14C]PhIP and [3H]IQ in rats. We also examined the effect of treatments on the activity of the phase II detoxification enzymes glutathione S-transferase (GST), UDP-glucuronyltransferase (UGT), phenol sulfotransferase (SULT) and quinone reductase (QR). PEITC and chlorophyllin significantly decreased PhIP-DNA adduct levels in all tissues examined, which was reflected by similar changes in PhIP binding to albumin in the blood. In contrast, genistein and lycopene tended to increase PhIP adduct levels. The treatments did not significantly alter the level of IQ-DNA or -protein adducts in the liver. With the exception of lycopene, the treatments had some effect on the activity of one or more hepatic phase II detoxification enzymes. We conclude that PEITC and chlorophyllin are protective of PhIP-induced genotoxicity after a low exposure dose of carcinogen, possibly through modification of HCA metabolism.

Enantioselectivity of human hydroxysteroid sulfotransferase ST2A3 with naphthyl-1-ethanols

Hydroxysteroid (alcohol) sulfotransferases catalyze the sulfation of several endogenous steroids and many hydrophobic xenobiotic alcohols. The substrate stereoselectivities of sulfotransferases may be critically important in determining their overall roles in metabolism of drugs, carcinogens, and other xenobiotics. In the present work, stereoselectivity of the human hydroxysteroid sulfotransferase ST2A3 (also variously named as SULT2A1 or human DHEA-ST) was examined through analysis of its catalytic activities with the enantiomers of 1-naphthyl-1-ethanol and 2-naphthyl-1-ethanol. The kcat/Km value for sulfation of the R-(+)-enantiomer of 1-naphthyl-1-ethanol catalyzed by ST2A3 was 3.3 min-1mM-1, whereas the S-(-)-enantiomer was not a substrate for the enzyme. S-(-)-1-naphthyl-1-ethanol did however interact with ST2A3 as an inhibitor of the sulfation of dehydroepiandrosterone. This substrate stereospecificity was not present with the enantiomers of 2-naphthyl-1-ethanol, since both were substrates for the enzyme. Such differences between the sulfation of 1- and 2-naphthyl-1-ethanol are consistent with the importance of steric interactions between the ethanol group and a hydrogen atom at the peri-position (C8) on the naphthyl ring in 1-naphthyl-1-ethanol that combine with the topology of the enzyme's active site to determine stereospecificity.

Identification of rat targets of anti-soluble liver antigen autoantibodies by serologic proteome analysis

BACKGROUND

Anti-soluble liver antigen (SLA) autoantibodies are specific for autoimmune hepatitis type 1 and are the only immunologic marker found in 15-20% of hepatitis cases previously considered cryptogenic. Anti-SLA antibodies react with the 100 000g supernatant from rat liver homogenate, but the molecular targets remain controversial.

METHODS

We characterized anti-SLA targets by one- and two-dimensional immunoblotting analysis. The recognized proteins were identified by peptide mass fingerprint analysis after matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry.

RESULTS

Three proteins of 35 kDa and pI 6.0, 50 kDa and pI between 6.0 and 6.5, and 58 kDa and pI between 6.5 and 7.0 were stained more intensely by anti-SLA positive-sera than by control sera. After in-gel tryptic digestion, MALDI-TOF analysis of the generated peptides enabled the clear identification of N-hydroxyarylamine sulfotransferase, isoforms of alpha-enolase, and isoforms of catalase.

CONCLUSIONS

Possible antigens for anti-SLA antibodies include a sulfotransferase, alpha-enolase(s), and catalase(s). Two-dimensional electrophoresis combined with mass spectrometry offers a versatile tool to identify molecular targets of autoantibodies and thus to improve diagnostic tools and the understanding of the immune process.

Sulfation of flavonoids and other phenolic dietary compounds by the human cytosolic sulfotransferases

The protective effects of diet, especially soya products, tea, and many fruits, against a variety of human cancers, as suggested by epidemiological studies, has focused attention on flavonoids, isoflavonoids, and other phenolic dietary compounds as chemoprotectants. Among the mechanisms suggested for their chemoprotective action, their ability to inhibit the bioactivation of carcinogens by the human cytosolic sulfotransferases (STs) and the direct effects of their sulfoconjugates are being increasingly studied. We report here a systematic study on the sulfation of representative flavonoids, isoflavonoids, anti-oxidants, and other phenolic dietary compounds by all ten known human cytosolic STs. All ten recombinant human cytosolic STs were prepared in a pure form and tested for their sulfating activities with a variety of these compounds. P-form (SULT1A1) phenol ST (PST) showed high sulfating activity with most of these compounds. M-form (SULT1A3) PST showed high activity with the flavonoids but not with the isoflavonoids. SULT1C ST #2 showed high activity with the isoflavonoids and also sulfated most of the other compounds. Possible relevance of these results to the chemoprotective effects of these dietary compounds is discussed.

N-Acetyltransferases, sulfotransferases and heterocyclic amine activation in the breast

Heterocyclic amines are mammary carcinogens in rats and their N-hydroxy metabolites are substrates for subsequent metabolic activation by N-acetyltransferases (NAT) and sulfotransferases (SULT) in man. We investigated the expression of these enzymes in human breast tissue and the relationship between NAT genotype and NAT mRNA expression or enzyme activity. Immunohistochemical staining of sections of breast tissue identified expression of NAT1 and NAT2 protein in human mammary epithelial cells, but not in the stroma. We also measured the formation of DNA adducts of the heterocyclic amines 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine in calf thymus DNA after incubation of their promutagenic N-hydroxy metabolites with mammary cytosols prepared from reduction mammoplasty tissue. Experimental observations gained from use of enzyme cofactors and NAT and/or SULT inhibitors on cytosolic enzyme activity, recombinant NAT1 activity and heterocyclic amine-DNA adduct formation suggest that both NAT1 and SULT1A enzymes contribute significantly to the activation of N-hydroxylated heterocyclic amines in mammary tissue. NAT1 mRNA transcript levels were found to be two- to three-fold higher than mRNA transcripts of the NAT2 gene in reduction mammoplasty tissue and mammary epithelial cells. NAT1-specific p-aminobenzoic acid acetylation activity, but not NAT2-specific sulfamethazine acetylation activity, was detectable in mammary cytosols. There was no association apparent between NAT genotype and the levels of NAT mRNA or NAT enzyme activity, or between NAT1 genotype and IQ-DNA adduct formation mediated by mammary cytosols. Western blot analysis of mammary cytosolic protein showed detectable levels of SULT1A1 and SULT1A3.

Single nucleotide polymorphisms, metabolic activation and environmental carcinogenesis: why molecular epidemiologists should think about enzyme expression

This commentary was written to stimulate thoughts on, and consideration of, enzyme expression data in target organs when investigating possible associations between polymorphisms in carcinogen activation enzymes, lifestyle/dietary factors and cancer risk. The lung and breast are taken as examples. There is overwhelming evidence for a genotoxic mechanism in lung cancer development, and compelling evidence for the contribution of genotoxins to breast cancer aetiology. A consistent association has been shown where lung cancer risk is decreased by a G-->A polymorphism in the myeloperoxidase (MPO) gene, which is expressed in neutrophils recruited to the lung after chemical or immunological insults. In the breast, a consistent lack of association has been observed for women who are fast N:-acetyltransferase type 2 (NAT2) acetylators consuming cooked meat. This could be explained by the lack of detectable NAT2-associated sulfamethazine acetylation activity in cytosols prepared from mammary tissue, suggesting a minor contribution to carcinogen activation. The recent identification in mammary cytosols of detectable sulfotransferase isoforms (SULT1A1 and SULT1A3), which have high catalytic efficiency for activating N:-hydroxylated heterocyclic amines (HCAs, mutagens in cooked meat), offers a more important role for these enzymes in the metabolic activation of genotoxins in the breast. The possible contribution of MPO and lactoperoxidase enzymes to carcinogen activation in mammary tissue is also considered. Sulfotransferases and peroxidases have wide substrate specificity in terms of carcinogen activation (HCAs, aromatic amines and polycyclic aromatic hydrocarbons-all present in cooked meat and tobacco smoke) compared with NATs (HCAs and aromatic amines only). For gene-environment interactions, investigations into functional polymorphisms in SULT and peroxidase genes may, therefore, offer new evidence for the involvement of genotoxins in the initiation of carcinogenesis. Identification of the isoforms (if any) of carcinogen activation enzymes that are expressed in the organs of interest will help to determine which genes to investigate in these studies.

Metabolic activation of carcinogens and expression of various cytochromes P450 in human prostate tissue

Epidemiological evidence suggests a link between meat consumption and prostate cancer. In this study, benign prostatic hyperplasia tissues, obtained by transurethral resection or radical retropubic prostatectomy from UK-resident individuals (n = 18), were examined for CYP1 expression and for their ability, in short-term organ culture, to metabolically activate carcinogens found in cooked meat. Semi-quantitative RT-PCR analysis of CYP1 expression detected CYP1A2 mRNA transcripts in the prostates of four individuals, as well as mRNA transcripts from CYP1A1 and CYP1B1. The compounds tested for metabolic activation were 2-amino-1-methyl-6-phenylimidazo[4, 5-b]pyridine (PhIP; 500 microM, n = 9) and its metabolite N:-hydroxy PhIP (20 microM, n = 8), 2-amino-3-methylimidazo[4,5-f]quinoline (IQ; 500 microM, n = 6) and benzo[a]pyrene (B[a]P; 50 microM, n = 5). After incubation (PFMR medium, 22 h, 37 degrees C), DNA was isolated from tissue fragments and DNA adducts were detected and quantified by (32)P-postlabelling analysis. DNA adduct formation was detected in all samples incubated with PhIP (mean, adducts per 10(8) nucleotides), N:-hydroxy-PhIP (2736/10(8)) or B[a]P (1/10(8)). IQ-DNA adducts were detected in 5/6 tissues (mean, 1/10(8)). The CYP1 inhibitor alpha-naphthoflavone (10 microM) reduced B[a]P-DNA adduct formation in tissues from two individuals by 96 and 64%, respectively. This pilot study shows that human prostate tissue can metabolically activate 'cooked meat' carcinogens, a process that could contribute to prostate cancer development.