Isolation and expression of a cDNA encoding a male-specific rat sulfotransferase that catalyzes activation of N-hydroxy-2-acetylaminofluorene

Sulfotransferases (SULTs), enzymatic and genetic variation in Carnivora: Limited sulfation capacity in pinnipeds

Wild carnivorans are one of the most important species due to their high positions in the food chain. They are also highly affected by numerous environmental contaminants through bioaccumulation and biomagnification. Xenobiotic metabolism is a significant chemical defense system from xenobiotics because it degrades the activity of a wide range of chemicals, generally into less active forms, resulting in their deactivation. Sulfotransferases (SULTs) are one of the most important xenobiotic metabolic enzymes, which catalyze the sulfonation of a variety of endogenous and exogenous chemicals, such as hormones, neurotransmitters, and a wide range of xenobiotic compounds. Although SULTs are of such high importance, little research has focused on these enzymes in wild carnivorans. In this study, we clarified the genetic properties of SULTs in a wide range of mammals, focusing on carnivorans, using in silico genetic analyses. We found genetic deficiencies of SULT1E1 and SULT1D1 isoforms in all pinnipeds analyzed and nonsense mutations in SULT1Cs in several carnivorans including pinnipeds. We further investigated the enzymatic activity of SULT1E1 in vitro using liver cytosols from pinnipeds. Using a SULT1E1 probe substrate, we found highly limited estradiol sulfonation in pinnipeds, whereas other mammals had relatively high sulfation. These results suggest that pinnipeds have severely or completely absent SULT1E1 activity, which importantly catalyzes the metabolism of estrogens, drugs, and environmental toxins. This further implies a high susceptibility to a wide range of xenobiotics in these carnivorans, which are constantly exposed to environmental chemicals throughout their lifetime.

Molecular cloning and characterization of common marmoset SULT1C subfamily members that catalyze the sulfation of thyroid hormones

Cytosolic sulfotransferase SULT1C subfamily is one of the most flexible gene subfamily during mammalian evolution. The physiological functions of SULT1C enzymes still remain to be fully understood. In this study, common marmoset (Callithrix jacchus), a promising primate animal model, was used to investigate the functional relevance of the SULT1C subfamily. Gene database search revealed three intact SULT1C genes and a pseudogene in its genome. These four genes were named SULT1C1, SULT1C2, SULT1C3P, and SULT1C5, according to the sequence homology and gene location. Since SULT1C5 is the orthologous gene for human SULT1C2P, we propose, here, to revisit the designation of human SULT1C2P to SULT1C5P. Purified recombinant SULT1C enzymes showed sulfating activities toward a variety of xenobiotic compounds and thyroid hormones. Kinetic analysis revealed high catalytic activities of SULT1C1 and SULT1C5 for 3,3'-T2. It appears therefore that SULT1C isoforms may play a role in the thyroid hormone metabolism in common marmoset.

Function and organization of the human cytosolic sulfotransferase (SULT) family

The sulfuryl transfer reaction is of fundamental biological importance. One of the most important manifestations of this process are the reactions catalyzed by members of the cytosolic sulfotransferase (SULT) superfamily. These enzymes transfer the sulfuryl moiety from the universal donor PAPS (3'-phosphoadenosine 5'-phosphosulfate) to a wide variety of substrates with hydroxyl- or amino-groups. Normally a detoxification reaction this facilitates the elimination of a multitude of xenobiotics, although for some molecules sulfation is a bioactivation step. In addition, sulfation plays a key role in endocrine and other signalling pathways since many steroids, sterols, thyroid hormones and catecholamines exist primarily as sulfate conjugates in humans. This article summarizes much of our current knowledge of the organization and function of the human cytosolic sulfotransferases and highlights some of the important interspecies differences that have implications for, among other things, drug development and chemical safety analysis.

Expression of the orphan cytosolic sulfotransferase SULT1C3 in human intestine: characterization of the transcript variant and implications for function

The cystolic sulfotransferse 1C3 (SULT1C3) gene was identified by computational analysis of the human genome and suggested to contain duplications of its last two exons (7a/b and 8a/b). Although the SULT1C3 isoform containing the more downstream exons 7b and 8b (SULT1C3d) has been expressed in Escherichia coli, crystallized, and characterized for activity, there is currently no evidence that SULT1C3 is expressed in any human tissue. Using reverse-transcription polymerase chain reaction, we detected SULT1C3 mRNA in the colorectal adenocarcinoma cell line (LS180), colon, and small intestine, but the amplified fragment contained the more upstream exons 7a and 8a. 3'-Rapid amplification of cDNA ends (RACE) confirmed that the SULT1C3 transcript expressed in LS180 cells contained exons 7a/8a, whereas 5'-RACE identified a noncoding exon 1. Full-length SULT1C3 transcript containing exons 7a/8a was amplified from LS180 and intestinal RNA, and in vitro transcription-translation of the cloned cDNA indicated that translation primarily began at the first of three in-frame ATG codons. Since SULT1C3 containing exons 7a/8a (SULT1C3a) would differ by 30 amino acids from SULT1C3d containing exons 7b/8b, we considered the functional implications of expressing one or the other isoform by generating structural models based on the reported crystal structure for SULT1C3d. Comparison of the structures indicated that five of the residues forming the substrate-binding pocket differed between the two isoforms, resulting in a change in both electron density and charge distribution along the inner wall of the substrate-binding pocket. These data indicate that SULT1C3 is expressed in human intestine but suggest that the expressed isoform is likely to differ functionally from the isoform that has been previously characterized.

Expression of the sulfotransferase 1C family: implications for xenobiotic toxicity

The SULT1C enzymes are a relatively under-studied branch of the cytosolic sulfotransferase (SULT) multigene family. Concrete information about SULT1C tissue-specific expression, substrate preference, role in physiology and regulation is just emerging in the literature. The role of SULT1Cs in normal physiology is uncertain, but SULT1C-catalyzed sulfonation of thyroid hormones may be a mechanism to titrate the pre-receptor levels of biologically active thyroid hormone in target tissues. Both rat and human cytosolic SULT1Cs are most noted for their ability to bioactivate potent procarcinogens such as N-hydroxy-2-acetylaminofluorene. This implicates a possible role for the SULT1Cs as modulators of environmental carcinogen exposure and determinants of neoplastic transformation. In humans, the SULT1Cs are likely to function physiologically in cell proliferation and organogenesis pathways during development, as SULT1Cs appear to be preferentially expressed during fetal life. In recent years, the SULT1C nomenclature as presented in the literature has undergone major changes in response to updated genomic information. The purpose of this review is to summarize the current literature on the SULT1Cs and to clarify perspectives on SULT1C species differences, tissue-specific expression, nomenclature and role in pathophysiology. The ultimate goal is to understand the undiscovered impact of SULT1C expression on hormone homeostasis and xenobiotic toxicity during human development and as a prelude to disease development later in life.

Focused DNA microarray analysis for sex-dependent gene expression of drug metabolizing enzymes, transporters and nuclear receptors in rat livers and kidneys

Cytochrome P450(CYP)s are known to show a sexual dimorphic expression in rat livers. However, the comprehensive analysis for the sex-dependent gene expressions of drug metabolizing enzymes except for CYPs, transporters and nuclear receptors in rat livers and kidneys has not been investigated yet. The purpose of the present study was to identify the novel drug metabolizing and pharmacokinetics (DMPK)-related gene(s) which show the sex difference in the mRNA expressions in rat livers and kidneys. Total RNAs were prepared from livers and kidneys in both male and female rats (Crl:CD(SD) and Crlj:WI). A DNA microarray analysis using a "GeneSQUARE Multiple Assay DNA Microarray Drug Metabolism Gene Expression for Rat" was performed. DMPK-related genes which showed sex differences in the mRNA expression were identified in rat livers or kidneys. Especially, the female dominant expressions of UDP glucuronosyltransferase (UGT) s were seen in rat livers and kidneys. The sex difference of UGT expressions in rats might be one of the causal factors of the sex difference of the biological response to UGT substrates.

Mechanisms of gender-specific regulation of mouse sulfotransferases (Sults)

1. Marked gender differences in the expression of sulfotransferases (Sults) are known to exist in several species including rats, mice and hamsters. However, the mechanism for this gender difference is not known. Therefore, in the present study, it was determined whether sex and/or growth hormone (GH) are responsible for the gender difference in the expression of Sults using gonadectomized (GNX), hypophysectomized (HX) and GH-releasing hormone receptor-deficient little (lit/lit) mouse models. 2. Sult1a1 and Papss2 in liver and kidney, and Sult1d1 in liver are female-predominant in mice because of suppressive effects of both androgens and male-pattern GH secretion. Sult2a1/a2 is the most markedly female-predominant Sult in mouse liver due to suppressive effects of androgens and male-pattern GH secretion, as well as stimulatory effects by estrogens and female-pattern GH secretion. Sult3a1 is female-predominant in mouse liver due to suppressive effects of androgens as well as stimulatory effects of estrogens and female-pattern GH secretion. Sult1c1 expression is male-predominant in mouse liver and kidney because of stimulatory effects of androgens in males. Sult4a1 expression is female-predominant in mouse brain due to stimulatory effects of estrogens. 3. In conclusion, gender-divergent Sults are mostly female-predominant and Sult1c1 is the only male-dominant Sult. The gender differences in expression of various mouse Sults are influenced by various mechanisms involving sex and/or GHs.

Molecular cloning, expression and characterization of a novel mouse SULT6 cytosolic sulfotransferase

By searching the mouse EST database, we identified a novel mouse cytosolic sulfotransferase (SULT) cDNA (RIKEN cDNA 2410078J06). Sequence analysis revealed that this new SULT belongs to the cytosolic SULT6 gene family. The recombinant form of this newly identified SULT, designated SULT6B1, was expressed using the pGEX-4T-1 glutathione S-transferase fusion system and purified from transformed BL21 Escherichia coli cells. Purified mouse SULT6B1 exhibited sulfonating activity toward thyroxine and bithionol among a variety of endogenous and xenobiotic compounds tested as substrates. pH optimum of purified mouse SULT6B1 was determined to be 8.0. Tissue-specific expression of mouse and human SULT6B1 was examined by RT-PCR. While human SULT6B1 was specifically expressed in kidney and testis, mouse SULT6B1 was detected in brain, heart, kidney, thymus, lung, liver and testis. Further studies are needed in order to clarify the role of SULT6B1 in the metabolism of thyroxine and possibly some xenobiotics in mouse.

Strategy for genotoxicity testing—Metabolic considerations

The report from the 2002 International Workshop on Genotoxicity Tests (IWGT) Strategy Expert Group emphasized metabolic considerations as an important area to address in developing a common strategy for genotoxicity testing. A working group convened at the 2005 4th IWGT to discuss this area further and propose practical strategy recommendations. To propose a strategy, the working group reviewed: (1) the current status and deficiencies, including examples of carcinogens "missed" in genotoxicity testing, established shortcomings of the standard in vitro induced S9 activation system and drug metabolite case examples; (2) the current status of possible remedies, including alternative S9 sources, other external metabolism systems or genetically engineered test systems; (3) any existing positions or guidance. The working group established consensus principles to guide strategy development. Thus, a human metabolite of interest should be represented in genotoxicity and carcinogenicity testing, including evaluation of alternative genotoxicity in vitro metabolic activation or test systems, and the selection of a carcinogenicity test species showing appropriate biotransformation. Appropriate action triggers need to be defined based on the extent of human exposure, considering any structural knowledge of the metabolite, and when genotoxicity is observed upon in vitro testing in the presence of metabolic activation. These triggers also need to be considered in defining the timing of human pharmaceutical ADME assessments. The working group proposed two strategies to consider; a more proactive approach, which emphasizes early metabolism predictions to drive appropriate hazard assessment; and a retroactive approach to manage safety risks of a unique or "major" metabolite once identified and quantitated from human clinical ADME studies. In both strategies, the assessment of the genotoxic potential of a metabolite could include the use of an alternative or optimized in vitro metabolic activation system, or direct testing of an isolated or synthesized metabolite. The working group also identified specific areas where more data or experiences need to be gained to reach consensus. These included defining a discrete exposure action trigger for safety assessment and when direct testing of a metabolite of interest is warranted versus the use of an alternative in vitro activation system, a universal recommendation for the timing of human ADME studies for drug candidates and the positioning of metabolite structural knowledge (through in silico systems, literature, expert analysis) in supporting metabolite safety qualification. Lastly, the working group outlined future considerations for refining the initially proposed strategies. These included the need for further evaluation of the current in vitro genotoxicity testing protocols that can potentially perturb or reduce the level of metabolic activity (potential alterations in metabolism associated with both the use of some solvents to solubilize test chemicals and testing to the guidance limit dose), and proposing broader evaluations of alternative metabolic activation sources or engineered test systems to further challenge the suitability of (or replace) the current induced liver S9 activation source.

Tissue Distribution and Ontogeny of Sulfotransferase Enzymes in Mice

Sulfotransferases (Sults) are phase-II conjugation enzymes that catalyze the transfer of a sulfonate group from 3'-phosphoadenosine 5'-phosphosulfate (PAPS) to target endo and xenobiotics. PAPS is formed from inorganic sulfate by the action of the enzyme PAPS synthase (PAPSs). In the present study, the tissue distribution and developmental changes in the mRNA expression of 11 Sult isozymes and 2 PAPSs isoforms in mice were quantified. Sult1a1, 1b1, 1c1, 1c2, 1d1, 1e1, 2a1/2, 2b1, 3a1, 4a1, 5a1, PAPSs1, and PAPSs2 mRNA expression was quantified in 14 tissues from male and female mice using the branched DNA signal amplification assay. Sult2a1/2 and 3a1 expression were highest in liver; Sult1b1, 2b1, and PAPSs2 in small intestine; Sult1a1 in large intestine; Sult1c2 in stomach; Sult1d1 in kidney; Sult1e1 in placenta; and Sult4a1 in brain. Sult1c1, 5a1, and PAPSs1 were ubiquitously expressed in most tissues. These enzymes demonstrated three different ontogenic expression patterns in liver. Sult1a1, 1c2, 1d1, 2a1/2, and PAPSs2 hepatic expression gradually increased from birth until about 3 weeks of age and then declined somewhat thereafter, Sult1c1 expression was highest before birth and declined after that, and Sult3a1 mRNA expression was very low in fetal livers and remained low until 30 days of age, when expression in females dramatically increased, whereas it never increased in males. The organ-specific distribution of Sults as well as the different expression of the Sults in young animals may affect the pharmacokinetic behavior and organ-specific toxicity of xenobiotics.

Integration of hepatic drug transporters and phase II metabolizing enzymes: Mechanisms of hepatic excretion of sulfate, glucuronide, and glutathione metabolites

The liver is the primary site of drug metabolism in the body. Typically, metabolic conversion of a drug results in inactivation, detoxification, and enhanced likelihood for excretion in urine or feces. Sulfation, glucuronidation, and glutathione conjugation represent the three most prevalent classes of phase II metabolism, which may occur directly on the parent compounds that contain appropriate structural motifs, or, as is usually the case, on functional groups added or exposed by phase I oxidation. These three conjugation reactions increase the molecular weight and water solubility of the compound, in addition to adding a negative charge to the molecule. As a result of these changes in the physicochemical properties, phase II conjugates tend to have very poor membrane permeability, and necessitate carrier-mediated transport for biliary or hepatic basolateral excretion into sinusoidal blood for eventual excretion into urine. This review summarizes sulfation, glucuronidation, and glutathione conjugation reactions, as well as recent progress in elucidating the hepatic transport mechanisms responsible for the excretion of these conjugates from the liver. The discussion focuses on alterations of metabolism and transport by chemical modulators, and disease states, as well as pharmacodynamic and toxicological implications of hepatic metabolism and/or transport modulation for certain active phase II conjugates. A brief discussion of issues that must be considered in the design and interpretation of phase II metabolite transport studies follows.

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 proposed nomenclature system for the cytosolic sulfotransferase (SULT) superfamily

A nomenclature system for the cytosolic sulfotransferase (SULT) superfamily has been developed. The nomenclature guidelines were applied to 65 SULT cDNAs and 18 SULT genes that were characterized from eukaryotic organisms. SULT cDNA and gene sequences were identified by querying the GenBank databases and from published reports of their identification and characterization. These sequences were evaluated and named on the basis of encoded amino acid sequence identity and, in a few cases, a necessity to maintain historical naming convention. Family members share at least 45% amino acid sequence identity whereas subfamily members are at least 60% identical. cDNAs which encode amino acid sequences of at least 97% identity to each other were assigned identical isoform names. We also attempted to categorize orthologous enzymes between various species, where these have been identified, and the nomenclature includes a species descriptor. We present recommendations for the naming of allelic variants of SULT genes and their derived allozymes arising from single nucleotide polymorphisms and other genetic variation. The superfamily currently comprises 47 mammalian SULT isoforms, one insect isoform and eight plant enzymes, and collectively these sequences represent nine separate SULT families and 14 subfamilies. It is hoped that this nomenclature system will be widely adopted and that, as novel SULTs are identified and characterized, investigators will name their discoveries according to these guidelines.

Characterization of rat iodothyronine sulfotransferases

Sulfation appears to be an important pathway for the reversible inactivation of thyroid hormone during fetal development. The rat is an often used animal model to study the regulation of fetal thyroid hormone status. The present study was done to determine which sulfotransferases (SULTs) are important for iodothyronine sulfation in the rat, using radioactive T4, T3, rT3, and 3,3'-T2 as substrates, 3'-phosphoadenosine-5'-phosphosulfate (PAPS) as cofactor, and rat liver, kidney and brain cytosol, and recombinant rat SULT1A1, -1B1, -1C1, -1E1, -2A1, -2A2, and -2A3 as enzymes. Recombinant rat SULT1A1, -1E1, -2A1, -2A2, and -2A3 failed to catalyze iodothyronine sulfation. For all tissue SULTs and for rSULT1B1 and rSULT1C1, 3,3'-T2 was by far the preferred substrate. Apparent Km values for 3,3'-T2 amounted to 1.9 microM in male liver, 4.4 microM in female liver, 0.76 microM in male kidney, 0.23 microM in male brain, 7.7 microM for SULT1B1, and 0.62 microM for SULT1C1, whereas apparent Km values for PAPS showed less variation (2.0-6.9 microM). Sulfation of 3,3'-T2 was inhibited dose dependently by other iodothyronines, with similar structure-activity relationships for most enzymes except for the SULT activity in rat brain. The apparent Km values of 3,3'-T2 in liver cytosol were between those determined for SULT1B1 and -1C1, supporting the importance of these enzymes for the sulfation of iodothyronines in rat liver, with a greater contribution of SULT1C1 in male than in female rat liver. The results further suggest that rSULT1C1 also contributes to iodothyronine sulfation in rat kidney, whereas other, yet-unidentified forms appear more important for the sulfation of thyroid hormone in rat brain.

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.

Differential activation of promutagens by alloenzymes of human sulfotransferase 1A2 expressed in Salmonella typhimurium

Various enzymatically formed sulfuric acid esters are chemically reactive and mutagenic. This metabolic activation pathway is not detected in standard in-vitro mutagenicity test systems. We describe the construction of Salmonella typhimurium TA1538-derived strains expressing alloenzymes *1, *2, *3, *5, *6 of human sulfotransferase 1A2 (SULT1A2). The reference compounds, 1-hydroxymethylpyrene (1-HMP), N-hydroxy-2-acetylaminofluorene (OH-AAF) and 2-hydroxylamino-5-phenylpyridine (OH-APP), were activated to mutagens in these strains. Their activity differed 7- to 16-fold between strains expressing various alloenzymes. It was strongest and weakest in the strains expressing the common alloenzymes, *1 and *2, respectively. The SULT1A2 protein expression levels, and the V(max) and K(m) values with the reference substrate 4-nitrophenol, varied 2.5-, 4-, and 110-fold, respectively, in cytosolic preparations from strains TA1538-SULT1A2*1 and *2. Strains with varying protein levels were constructed via insertion of silent mutations in the 5'-part of the cDNA. TA1538-SULT1A2*1Z and TA1538-SULT1A2*2Y showed equal expression levels of alloenzymes *1 and *2, respectively, which were 3 times above those of TA1538-SULT1A2*1. The mutagenicity of OH-AAF and OH-APP was unchanged in strain TA1538-SULT1A2*1Z versus *1, and moderately increased in TA1538-SULT1A2*2Y versus *2. The influence of the protein level was stronger with 1-HMP. Nevertheless, mutagenic activity of 1-HMP was still 11 times higher in TA1538-SULT1A2*1Z than in TA1538-SULT1A2*2Y. Thus, differences in the properties between alloenzymes can lead to differences in the activation of promutagens. The model compounds were also tested in strains expressing the other ten human SULTs identified. Whereas OH-AAF and OH-APP showed the highest mutagenic activities in strains expressing SULT1A2, 1-HMP was more potent in strains expressing other SULT forms. With the limitation that little is known about the tissue distribution and regulation of SULT1A2, the findings suggest that its polymorphism may affect the individual susceptibility towards procarcinogens, in particular certain aromatic amines and amides.

Protection by beverages, fruits, vegetables, herbs, and flavonoids against genotoxicity of 2-acetylaminofluorene and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in metabolically competent V79 cells

Chinese hamster lung fibroblasts, genetically engineered for the expression of rat cytochrome P450 dependent monooxygenase 1A2 and rat sulfotransferase 1C1 (V79-rCYP1A2-rSULT1C1 cells), were utilized to check for possible protective effects of beverages of plant origin, fruits, vegetables, and spices against genotoxicity induced by 2-acetylaminofluorene (AAF) or 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). Antigenotoxic activities of juices from spinach and red beets against AAF could be monitored with similar effectivity by the HPRT-mutagenicity test (IC(50)=0.64%; 2.57%) and alkaline single cell gel electrophoresis (comet assay; IC(50)=0.12%; 0.89%) which detects DNA strand breaks and abasic sites. Applying the comet assay, genotoxicity of PhIP could, however, be demonstrated only in the presence of hydroxyurea and 1-[beta-D-arabinofuranosyl]cytosine, known inhibitors of DNA repair synthesis. As expected, AAF and PhIP were unable to induce any genotoxic effects in the parent V79 cells. Genotoxic activity of PhIP was strongly reduced in a dose-related manner by green tea and red wine, by blueberries, blackberries, red grapes, kiwi, watermelon, parsley, and spinach, while two brands of beer, coffee, black tea, rooibos tea, morellos, black-currants, plums, red beets, broccoli (raw and cooked), and chives were somewhat less active. One brand of beer was only moderately active while white wine, bananas, white grapes, and strawberries were inactive. Similarly, genotoxicity of AAF was strongly reduced by green, black, and rooibos tea, red wine, morellos, black-currants, kiwi, watermelon, and spinach while plums, red beets, and broccoli (raw) were less potent. Broccoli cooked exerted only moderate and white wine weak antigenotoxic activity. With respect to the possible mechanism(s) of inhibition of genotoxicity, benzo[a]pyrene-7,8-dihydrodiol (BaP-7,8-OH) and N-OH-PhIP were applied as substrates for the CYP1A family and for rSULT 1C1, respectively. Morellos, black-currants, and black tea strongly reduced the genotoxicity of BaP-7,8-OH, onions, rooibos tea, and red wine were less potent while red beets and spinach were inactive. On the other hand, red beets and spinach strongly inhibited the genotoxicity of N-OH-PhIP, rooibos tea was weakly active while all other items were inactive. These results are suggestive for enzyme inhibition as mechanism of protection by complex mixtures of plant origin. Taken together, our results demonstrate that protection by beverages, fruits, and vegetables against genotoxicity of heterocyclic aromatic amines may take place within metabolically competent mammalian cells as well as under the conditions of the Salmonella/reversion assay.

Involvement of CYP2J2 on the intestinal first-pass metabolism of antihistamine drug, astemizole

Orally administered astemizole is well absorbed but undergoes an extensive first-pass metabolism to O-desmethylastemizole. Desmethylastemizole is formed in the human microsomal systems of the small intestine as well as the liver, which suggests the role of cytochromes P450 (P450s) in the first-pass metabolism of astemizole. Human P450s involved in the O-demethylation of astemizole have, however, not been identified, and the involvement of twelve known drug-metabolizing P450s were denied. During the course of the P450 identification study, higher activities of the astemizole O-demethylation in the rabbit small intestine than in the liver (about 3-fold) were found. These data suggest the possible involvement of CYP2J, since P450 included in this subfamily is dominantly expressed in the small intestine of rabbits. Therefore, CYP2J2 cDNA has been isolated from the human cDNA library and expressed in COS-1 cells. A clear activity of astemizole O-demethylation was detected in recombinant CYP2J2 with K(m) = 0.65 microM and V(max) = 1129 pmol/nmol P450/min. Expression of the immunoreactive protein with CYP2J2 antibody was detected in the small intestine and liver. Expression levels of the immunoreactive protein with the CYP2J2 antibody in the small intestine were well correlated with the activities of the astemizole O-demethylation (r = 0.901, n = 5, p < 0.05). The CYP2J2 substrates, arachidonic acid and ebastine, strongly inhibited the microsomal astemizole O-demethylation in the human small intestines and recombinant CYP2J2. These results indicate the involvement of CYP2J2 in the presystemic elimination of astemizole in the human small intestine.

Isolation and characterisation of a novel rabbit sulfotransferase isoform belonging to the SULT1A subfamily

Sulfotransferases (SULTs) catalyse the sulfonation of both endogenous and exogenous compounds including hormones, catecholamines, drugs and xenobiotics. While in most occasions, sulfonation is a detoxication pathway, in the case of certain drugs and carcinogens, it leads to metabolic activation. Since, the rabbit has been extensively used for both pharmacological and toxicological studies, the purpose of this study was to further characterise the sulfotransferase system of this animal. In the present study, a novel sulfotransferase isoform (GenBank Accession no. AF360872) was isolated from a rabbit liver cDNA lambdaZAP II library. The full-length sequence of the clone was 1138 bp long and contained a coding region of 888 bp encoding a cytosolic protein of 295 amino acids (deduced molecular weight 34,193 Da). The amino acid sequence of this novel SULT isoform showed >70% identity with members of the SULT1A subfamily of sulfotransferases from other species. Upon expression of the encoded rabbit sulfotransferase in Escherchia coli (E. coli), it was shown that the enzyme was capable of sulfonating both p-nitrophenol (K(m) and Vmax values of 0.15 microM and 897.5 nmol/min/mg protein, respectively) and dopamine (K(m) and V(max) values of 175.3 microM and 151.1 nmol/min/mg protein, respectively). Based on the sequence data obtained and substrate specificity, this new rabbit sulfotransferase was named rabSULT1A1. Immunoblotting was used to demonstrate that rabSULT1A1 protein is expressed in liver, duodenum, jejunum, ileum, colon and rectum.

Down-regulation of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx)-induced CYP1A2 expression is associated with bovine lactoferrin inhibition of MeIQx-induced liver and colon carcinogenesis in rats

The inhibitory influence of bovine lactoferrin (bLF) on induction of preneoplastic hepatic glutathione S-transferase placental form-positive (GST-P( +)) cell foci and colon aberrant crypt foci (ACF) by diethylnitrosamine (DEN) and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) was investigated in F344 rats. Rats were initially treated with DEN, then placed on basal diet containing MeIQx (200 ppm) alone, MeIQx plus 2% bLF, or MeIQx plus 0.2% bLF from week 2 to week 8, with partial hepatectomy performed at week 3. Concomitant administration of 2% or 0.2% bLF with MeIQx caused significant dose-dependent decreases in both number and unit area of GST-P(+) cell foci (2% bLF, P < 0.001; 0.2% bLF, P < 0.01). Similar results were observed for MeIQx-induced colon ACF in the groups without DEN treatment (2% and 0.2% bLF, P < 0.05). To investigate the underlying mechanisms, we analyzed the influence of bLF on levels of cytochrome P4501A2 (CYP1A2), a metabolically activating enzyme of MeIQx in the liver. The results demonstrated that combined administration of 2% bLF significantly reduced levels of MeIQx-induced CYP1A2 mRNA (P < 0.05) and protein (P < 0.05) to the normal levels, in association with reduced values for MeIQx-DNA adducts (P < 0.05), liver GST-P(+) cell foci and colon ACF. These results suggest that bLF is a chemopreventive agent for DEN alone or DEN plus MeIQx-induced liver, and MeIQx-induced colon carcinogenesis in rats. One possible mechanism is a normalizing down-regulation of CYP1A2 expression by bLF, with consequent reduction of carcinogen activation and adduct formation.

Human sulfotransferase SULT1C1 pharmacogenetics: gene resequencing and functional genomic studies

Sulfotransferase (SULT) enzymes catalyze an important phase II reaction in the biotransformation of many drugs and other xenobiotics. We previously cloned the human SULT1C1 cDNA and gene as steps toward pharmacogenetic studies. We have now 'resequenced' the exons, portions of introns flanking exons and approximately 315 bp of the 5' flanking region of SULT1C1 in 89 DNA samples from Caucasian subjects to identify common genetic polymorphisms. Nineteen separate polymorphisms were observed, including four nonsynonymous coding region single nucleotide polymorphisms (cSNPs) and five insertions/deletions. These data were also used to determine and/or infer common SULT1C1 haplotypes. Three of the four nonsynonymous cSNPs had allele frequencies greater than 1%, including one with a frequency of 6.7%. Expression constructs were created for all of the nonsynonymous cSNPs observed, and those constructs were used to transfect COS-1 cells. Three of the four SULT1C1 variant allozymes had significantly reduced enzyme activity when compared with the wild-type enzyme. Among the variant allozymes, apparent Km values for 3'-phosphoadenosine 5'-phosphosulfate (PAPS), the sulfate donor for the reaction, varied 7-fold, and quantitative Western blot analysis showed variable levels of immunoreactive protein when compared to the wild-type enzyme. Therefore, mechanisms responsible for decreased activity involved both alterations in levels of enzyme protein and alterations in substrate kinetics. In summary, application of a 'genotype to phenotype' strategy has resulted in the identification of a series of functionally significant common genetic polymorphisms for SULT1C1. It will now be possible to evaluate the possible contribution of these polymorphisms to variation in the sulfate conjugation of drugs, other xenobiotics and/or disease pathophysiology.

Enzymatic aspects of the phenol (aryl) sulfotransferases

The sulfotransferases that are active in the metabolism of xenobiotics represent a large family of enzymes that catalyze the transfer of the sulfuryl group from 3'-phosphoadenosine 5'-phosphosulfate to phenols, to primary and secondary alcohols, to several additional oxygen-containing functional groups, and to amines. Restriction of this review to the catalytic processes of phenol or aryl sulfotransferases does not really narrow the field, because these enzymes have overlapping specificity, not only for specific compounds, but also for multiple functional groups. The presentation aims to provide an overview of the wealth of phenol sulfotransferases that are available for study but concentrates on the enzymology of rat and human enzymes, particularly on the predominant phenol sulfotransferase from rat liver. The kinetics and catalytic mechanism of the rat enzyme is extensively reviewed and is compared with observations from other sulfotransferases.

Structure-function modeling of the interactions of N-alkyl-N-hydroxyanilines with rat hepatic aryl sulfotransferase IV

Although previous investigations have clearly shown that N-hydroxy arylamines and N-hydroxy heterocyclic amines are substrates for sulfotransferases, relatively little is known about which structural features of the N-hydroxy arylamines are important for sulfation to occur. The purpose of this investigation was to determine the extent to which secondary N-alkyl-N-hydroxy arylamines interact with aryl sulfotransferase (AST) IV (also known as tyrosine-ester sulfotransferase or ST1A1) and to evaluate these interactions using molecular modeling techniques. AST IV is a major cytosolic sulfotransferase in the rat, and it catalyzes the sulfation of various phenols, benzylic alcohols, arylhydroxamic acids, oximes, and primary N-hydroxy arylamines. In this study, three secondary N-hydroxy arylamines, N-hydroxy-N-methylaniline, N-ethyl-N-hydroxyaniline, and N-hydroxy-N-n-propylaniline, were found to be substrates for the purified rat hepatic AST IV. However, when the N-alkyl substituent was an n-butyl group (i.e., N-n-butyl-N-hydroxyaniline), the interaction with the enzyme changed from that of a substrate to competitive inhibition. This change in specificity was further explored through the construction and use of a model for AST IV based on mouse estrogen sulfotransferase, an enzyme whose crystal structure has been previously determined to high resolution. Molecular modeling techniques were used to dock each of the above N-hydroxy arylamines into the active site of the homology model of AST IV and determine optimum ligand geometries. The results of these experiments indicated that steric constraints on the orientation of binding of secondary N-alkyl-N-hydroxy arylamines at the active site of AST IV play a significant role in determining the nature of the interaction of the enzyme with these compounds.

Sulfotransferases in the bioactivation of xenobiotics

Conjugation of xenobiotics is often associated with detoxification. However, this traditional view is one-sided. In particular, numerous compounds are known that are metabolized to chemically reactive metabolites via sulfation (O-sulfonation). This can be rationalized by the fact that the sulfate group is electron-withdrawing and may be cleaved off heterolytically in appropriate molecules, thus leading to the formation of a strongly electrophilic cation. The heterologous expression of sulfotransferases in indicator cells of standard mutagenicity tests has substantially improved the accessibility of this activation pathway. The use of this technology is important, since many reactive sulfate conjugates only show strong toxicological effects if they are generated directly within the indicator cell, due to their insufficient penetration of cell membranes. Xenobiotic-metabolizing sulfotransferases are cytosolic enzymes, which form a superfamily (SULT). Eleven distinct human SULT forms are known, which strongly differ in their tissue distribution and their substrate specificity. Common functionally relevant genetic polymorphisms of the transcribed region are known for two of the forms, SULT1A1 and 1A2. Studies using recombinant test systems demonstrate that many promutagens are activated with high selectivity by an individual SULT form. Pronounced differences in promutagen activation were detected between the different human forms, including their allelic variants, and also between orthologous SULTs from different species. Therefore, SULTs may be involved in the individual genetic disposition, species differences, and organotropisms for toxicological effects of chemicals. Activation by SULTs differs from other activation pathway in its cyclic nature: reaction of a sulfuric acid ester with water usually regenerates the hydroxylated compound, which becomes available for a new cycle of activation. SULT-mediated reactivation may even occur if another initial reactive species, e.g. an epoxide, has reacted with water.

Pharmacogenetics of sulfotransferase

Cytosolic sulfotransferase catalyzes sulfoconjugation of relatively small lipophilic endobiotics and xenobiotics. At least 44 cytosolic sulfotransferases have been identified from mammals, and based on their amino acid sequences, these forms are shown to constitute five different families. In humans, 10 sulfotransferase genes have been identified and shown to localize on at least five different chromosomes. The enzymatic properties characterized in the recombinant forms indicate the association of their substrate specificity with metabolisms of such nonpeptide hormones as estrogen, corticoid, and thyroxine, although most forms are also active on the sulfation of various xenobiotics. Genetic polymorphisms are observed on such human sulfotransferases as ST1A2, ST1A3, and ST2A3.

Enzymatic properties, tissue-specific expression, and lysosomal location of two highly homologous rat SULT1C2 sulfotransferases

We have isolated two highly homologous but distinct rat sulfotransferase cDNAs termed ratSULT1C2 and ratSULT1C2A encoding polypeptides of 297 amino acids each. The amino acid sequence of ratSULT1C2 is 84% identical to the human SULT1C2 and 81% identical to a rabbit SULT1C2 sulfotransferase. ratSULT1C2 and ratSULT1C2A are 92% identical but differ in 22 amino acids. The majority of these amino acid substitutions in ratSULT1C2A is not found in the human and rabbit SULT1C2, which identifies ratSULT1C2 as the orthologue of these sulfotransferases, whereas SULT1C2A is a closely related but distinct enzyme. ratSULT1C2 and 2A sulfotransferases do not sulfonate steroids, dopamine, acetaminophen, or alpha-naphthol, but only p-nitrophenol. Prokaryotically expressed ratSULT1C2A is less active than ratSULT1C2. ratSULT1C2/2A mRNAs are abundant in kidney and less abundant in stomach and liver. The enzymes are expressed as 34-kDa polypeptides in rat kidney, liver, and stomach. In addition, a 28-kDa cross-reacting polypeptide is found in kidney only. Immunohistochemistry revealed expression of ratSULT1C2/2A in the epithelial cells of the proximal tubules of the kidney, bile duct epithelia, hepatocytes, and the epithelium of the gastric mucosal glands. Although the cDNA predicted amino acid sequence identifies both sulfotransferases as cytosolic enzymes, in tissue sections, in the kidney cell line NRK 52, and in transiently transfected BHK cells a considerable fraction of the enzyme was found in a granular perinuclear compartment. Costaining with a lysosomal marker in gastric mucosa tissue sections and cultured cells identifies these structures as lysosomes.

Induction of rat hepatic aryl sulfotransferase (SULT1A1) gene expression by triamcinolone acetonide: impact on minoxidil-mediated hypotension

The hypotensive agent minoxidil (6-imino-1, 2-dihydro-1-hydroxy-2-imino-4-piperidinopyrimidine) depends upon aryl sulfotransferase (SULT1)-catalyzed sulfation for its bioactivation. Previous reports suggest that glucocorticoids induce class-specific SULT1 and isoform-specific SULT1A1 gene expression in rat liver. In the present study, rats were treated with the glucocorticoid triamcinolone acetonide (TA, 5 mg/kg/day i.p. x 3 days) or its vehicle, 2% Tween-20, prior to minoxidil, and subsequent effects on mean arterial pressure (MAP), heart rate (HR), and hepatic SULT1 gene expression were characterized. Minoxidil treatment (1.5 mg/kg) resulted in a steady decline in MAP values of 16.3 to 18.6% relative to basal control levels at 35 to 60 min following minoxidil injection. Pentachlorophenol (PCP, 40 micromol/kg i.p.), an inhibitor of SULT1 enzyme activity, effectively ablated the hypotensive effects of minoxidil. By contrast, pretreatment with TA significantly enhanced minoxidil-induced hypotension. Relative to vehicle-treated controls, TA-treated rats displayed a steeper rate of decline in MAP and more profound levels of hypotension with decreases in MAP following minoxidil administration of 27.8%. TA also produced significant increases in hepatic SULT1 mRNA expression (of 271%) and SULT1A1 immunoreactive protein levels (of 273%), relative to vehicle-treated controls. These results provide physiological evidence to support the biological relevance of SULT1A1 induction by glucocorticoids. The data indicate that steroid treatment induces SULT1A1 gene expression and, as a consequence, accentuates the hypotensive effects of minoxidil.

Purification and characterization of hepatic and intestinal phenol sulfotransferase with high affinity for benzo[a]pyrene phenols from channel catfish, Ictalurus punctatus

Cytosol from channel catfish liver and intestinal mucosa has high sulfotransferase activity with low concentrations of 3-, 7-, or 9-hydroxybenzo[a]pyrene. To further investigate this conjugation pathway, sulfotransferase activity toward 9-hydroxybenzo[a]pyrene was isolated from catfish intestinal and hepatic cytosol by chromatography on anion exchange and PAP-agarose affinity columns. SDS-PAGE of the active fractions showed that one major band with molecular size of about 41,000 Da was isolated from intestine, while two bands of about 41,000 and 31,000 Da were obtained from liver. Antibodies against human phenol-sulfating sulfotransferase cross-reacted strongly with the 41,000-Da bands from liver and intestine, but weakly with the hepatic 31,000-Da protein. N-Terminal sequence information could not be obtained from the pure proteins. Following digestion, an internal sequence of 20 amino acid residues was obtained from the hepatic 41,000-Da protein, which matched a sequence found in several mammalian sulfotransferases. No fish sulfotransferase sequences were available for comparison. The identity of the hepatic 31,000-Da protein was not established. The purified 41,000-Da proteins had very high activities with 3-, 7-, or 9-hydroxybenzo[a]pyrene, with K(m) values in the 40-100 nM range and V(max) 125-300 nmol/min/mg of protein. Substrate inhibition was observed when the concentrations of hydroxylated benzo[a]pyrenes were above 0.5 microM. As well as benzo[a]pyrene phenols, the purified 41,000-Da sulfotransferases catalyzed sulfation of 2-naphthol, 4-nitrophenol, 4-methylumbelliferone, 7-(hydroxymethyl)-12-methylbenz[a]anthracene, dehydroepiandrosterone, estrone, and 17beta-estradiol. Phenolic compounds were the preferred substrates for the purified enzymes.

Human sulfotransferases SULT1C1 and SULT1C2: cDNA characterization, gene cloning, and chromosomal localization

Sulfate conjugation catalyzed by sulfotransferase (SULT) enzymes is an important pathway in the biotransformation of many drugs, other xenobiotics, neurotransmitters, and hormones. We previously described a human cDNA, SULT1C1, that encoded a protein similar in sequence to that of rat ST1C1. Subsequently, a related human cDNA, SULT1C2, was reported. In the present study, we set out to characterize further the human SULT1C1 cDNA and then to clone, structurally characterize, and map its gene. As an initial step, we performed 5'- and 3'-RACE with SULT1C1 cDNA. Those experiments demonstrated that a small number of SULT1C1 transcripts contained an "insert," which we later showed resulted from alternative splicing that involved an Alu sequence in intron 3 of SULT1C1. We then cloned and structurally characterized the SULT1C1 gene from a human genomic BAC library. Because the sequence of SULT1C2 was closely related to that of SULT1C1 and because the genes for other human SULT paralogues occur in clusters, we screened the BAC clones that had been positive for SULT1C1 to search for SULT1C2 and discovered a clone that contained both genes. That BAC was used to sequence and structurally characterize SULT1C2. SULT1C1 and SULT1C2 were approximately 21 and 10 kb in length, respectively. Both genes contained seven exons that encoded protein, and both had structures that were similar to those of other genes that encode members of the SULT1 family. Finally, human SULT1C1 and SULT1C2 mapped to 2q11.2 by fluorescence in situ hybridization. The cloning and structural characterization of SULT1C1 and SULT1C2 will now make it possible to perform molecular genetic and pharmacogenomic studies of these sulfate-conjugating enzymes in humans.

Thyroid hormone modulation of rat sulphotransferase mRNA expression

1. Thyroid hormones modulate sulphotransferase (SULT) enzyme expression. Specific substrates are not available for the study of the SULT isoforms, so the regulation of hepatic SULT mRNA expression by thyroid hormones was examined by Northern blot analysis with oligonucleotide probes specific for each SULT mRNA, including male-dominant phenol SULT (1A1, 1C1, 1E2), female-dominant hydroxysteroid SULT (20/21, 40/41, 60), and a non-sex-dependent SULT, 1B1. The male and female rat were either untreated, thyroidectomized (TX), or TX and given thyroid hormones (thyroxine [T4, 20 microg kg(-1) day(-1)] and 3,5,3'-triiodothyronine [T3, 5 microg kg(-1) day(-1)]). 2. With regard to phenol SULTs, expression of SULT1A1 or SULT1B1 mRNA was not altered in either sex by TX. TX increased SULT1E2 mRNA expression 3-fold in the male and 2.5-fold in the female rat. The increase in SULT1E2 mRNA was partially reversed by infusion of T3/T4 in the male, and was not reversed in the female. 3. With regard to hydroxysteroid SULTs, TX decreased expression of SULT20/21 mRNA in the male rat by 70 and 60% in the female, and these decreases were reversed by T3/T4 infusion. SULT40/41 mRNA expression increased in the male rat 3-fold and decreased in the female TX rat by 25%. SULT60 mRNA expression increased 3-fold by TX in the female rats. The effects of TX on SULT40/41 and SULT60 mRNA expression were reversed by infusion of T2/T4. 5. Thus, phenol sulphotransferases were not markedly affected by thyroid hormones except for SULT1E2, but each hydroxysteroid sulphotransferase isoform was affected by thyroidectomy. Therefore, thyroid hormones regulate SULT gene expression in an isoform-specific manner.

Sulfotransferase catalyzing sulfation of heterocyclic amines

Cytosolic sulfation of arylamines to form sulfamates is found to be mediated by sulfotransferases of three gene families (SULT1 to 3). Among them, a SULT3 form (ST3A1) showed a high selectivity for N-sulfation of N-substituted aryl and alicyclic compounds. SULT1 (phenol) and SULT2 (hydroxysteroid) sulfotransferases showed N-sulfating activities of carcinogenic heterocyclic amines. For N-hydroxyarylamine O-sulfation, SULT1 forms showed high activity. In rats, ST1C1 mediated the metabolic activation of N-hydroxyarylamines. However, the related form (ST1C2) in humans showed the negligible activity. Instead, ST1A3 showed high metabolic activating abilities among human sulfotransferases.

Inhibition and binding studies of coenzyme A and bovine phenol sulfotransferase

Phenol sulfotransferases (PSTs, EC 2.8.2.1) catalyze sulfonyl group transfer from 3'-phosphoadenosine-5'-phosphosulfate (PAPS) to the hydroxyl oxygen of aromatic acceptor substrates. The structural overlap between PAPS and coenzyme A (CoA) suggested a possible role of this common acyl carrier in modulating PST activity. To test this hypothesis, purified recombinant bovine PST was examined by kinetic and affinity chromatographic approaches. After demonstrating PST enzyme inhibition by CoA, systematic variation of CoA and PAPS concentrations indicated simple competitive inhibition with K(i) = 1. 3 microM. PST bound to CoA-agarose, attached via the pantetheinyl thiol group, was eluted with PAP but not by 2-naphthol. This observation was consistent with the pattern of inhibition. Additional members of the sulfotransferase superfamily, as well as acylated CoAs, should be further investigated.

Alteration of hepatic sulfation by endotoxin

Administration of an endotoxin, lipopolysaccharide (1 mg/kg body weight), caused decreases in hepatic sulfations of xenobiotics in male rats through changes in the amounts of at least three forms of sulfotransferases, ST1A1, ST1B1 and ST1C1. The time-dependent decrease was first observed with ST1B1, followed by ST1A1 and ST1C1; and the decrease was more profound in the levels of mRNAs than those of proteins. The decreases in their mRNAs were, however, prevented by the pretreatment with dexamethasone. These results suggest that the endotoxin suppresses the hepatic sulfation through a cytokine-mediated mechanism to change individually hepatic levels of ST1A1, ST1B1 and ST1C1.

Molecular cloning, expression, localisation and functional characterisation of a rabbit SULT1C2 sulfotransferase

The importance of sulfotransferases in xenobiotic metabolism is gaining recognition. The gastrointestinal (GI) tract is a major portal of entry for many xenobiotics, yet little is known about the contribution of sulfotransferases to detoxication or bioactivation metabolism in these tissues. To this end, isolation and characterisation of sulfotransferases expressed in the stomach of rabbits was undertaken. A unique sulfotransferase cDNA (GenBank Accession No. AF026304) was isolated from a rabbit stomach cDNA library. This cDNA was 1439 base pairs (bp) long and has an open reading frame of 888 bp. On expression of the cDNA in both COS cells and E. coli, a protein molecular weight of 34 kDa was detected on SDS-PAGE. Immunoblotting using an antibody raised in goats against the bacterially expressed protein detected expression of the protein in GI tract tissues. The 34 kDa immunoreactive band was detected in rabbit GI tract tissues (stomach, duodenum, jejunum, ileum, colon, caecum and rectum), liver and kidneys, but not in the lungs (n = 3). The human ortholog (GenBank Accession No AF026303) of the rabbit enzyme was cloned from a human stomach cDNA library. These two enzymes share 84% amino acid sequence identity and have been termed 1C2 sulfotransferases. When functional and kinetic characterisation of the recombinant rabbit and human proteins was carried out using 16 known ST substrates, detectable sulfonation activity was observed only with p-nitrophenol (with Km values of 2.2 mM and 13.3 mM, respectively). In conclusion, we have identified a rabbit GI tract sulfotransferase belonging to a newly defined sulfotransferase subfamily.

Altered patterns of gene expression in Arabidopsis elicited by cauliflower mosaic virus (CaMV) infection and by a CaMV gene VI transgene

Cauliflower mosaic virus (CaMV) gene VI protein (P6) is an important determinant of symptom expression. Differential display polymerase chain reaction (PCR) was used to identify changes in gene expression in Arabidopsis elicited by a P6 transgene that causes a symptomatic phenotype. We used slot blot hybridization to measure the abundance of mRNAs complementary to 66 candidate PCR products in transgenic, CaMV-infected, and uninfected Arabidopsis plants. CaMV-infected and P6 transgenic plants showed broadly similar changes in abundance of mRNA species. In P6 transgenic plants we detected 18 PCR products that showed unambiguous changes in abundance plus another 15 that showed more limited changes (approximately twofold). CaMV-infected plants showed 17 unambiguous and 13 limited changes. Down-regulated species include those encoding a novel, phenol-like sulfotransferase, and a glycine-rich, RNA-binding protein. Up-regulated species included ones encoding an myb protein, glycine-rich and stress-inducible proteins, and a member of a previously unreported gene family. CaMV infection causes alterations in expression of many Arabidopsis genes. Transgene-mediated expression of P6 mimics virus infection in its effect on host gene expression, providing a potential mechanism for this process.

Bacterial expression and functional characterization of a rat thyroid hormone sulfotransferase, ST1B1

At least three forms of phenol sulfotransferase (ST) ST1B1, ST1A1 and ST1C1 are contained in rat livers. To identify the form contributing to the metabolism of 3,3',5-triiodothyronine (T3), functional characterization of these forms was performed by expression in Escherichia coli. ST1B1 and ST1C1 were shown to be active on sulfation towards T3 with high affinity (Km: 44.4 and 25.8 microM, respectively), whereas ST1A1 had low affinity. In Western blotting using antibodies raised against the individual ST, hepatic contents of each ST were quantitatively determined. ST1B1 showed no clear sex-difference, whereas the level of ST1C1 was higher in adult males than adult females. The content of ST1B1 was 1.4, 6.8 and 10 times higher than that of ST1C1 in adult males, adult females and both sexes of immature rats, respectively. The developmental pattern of ST1B1 was similar to that of ST1A1, but differed from that of ST1C1. These results indicate that ST1B1 and ST1C1 are involved in T3 metabolism in rats and ST1B1 is the constitutive form across sexes and ages.

Activation of propane 2-nitronate to a genotoxicant in V79-derived cell lines engineered for the expression of rat hepatic sulfotransferases

2-Nitropropane (2-NP) is a genotoxic hepatocarcinogen in rats. The genotoxicity of the compound has been attributed to a sulfotransferase-mediated formation of DNA-reactive species from the anionic form of 2-NP, propane 2-nitronate (P2N). Several observations have suggested that sulfotransferases (SULTs) 1A1 and/or 1C1 may be important in the activation of P2N to a genotoxicant in rat liver, but a definite proof is lacking. In order to identify the sulfotransferase(s) of rat liver that are capable of activating P2N, we have investigated the genotoxicity of P2N in various V79-derived cell lines engineered for expression of individual forms of rat hepatic sulfotransferases. Genotoxicity was assessed by measuring the induction of DNA repair synthesis. 1-Hydroxymethylpyrene (HMP), which is metabolically activated by most sulfotransferases, served as a positive control. Neither P2N nor HMP induced DNA repair in the parental V79-MZ cells, which do not show any sulfotransferase activity. P2N was also inactive in V79-rHSTa and V79-rHST20 cells, which express specific hydroxysteroid sulfotransferases. By contrast, a clear and concentration-dependent induction of repair synthesis by P2N was observed in V79-rPST-IV and V79-rST1C1 cells, which express rat SULT1A1 and SULT1C1, respectively. HMP was genotoxic in all sulfotransferase-expressing cell lines. Acetone oxime (AO), the tautomeric form of the first reduction product of 2-NP, 2-nitrosopropane, was inactive in all cell lines. The results corroborate the essential role of sulfotransferases in the metabolic activation of P2N to genotoxic products and identify two rat sulfotransferases which are capable of catalyzing the activation step.

Substrate specificity and kinetic mechanism of the insect sulfotransferase, retinol dehydratase

Spodoptera frugiperda retinol dehydratase catalyzes the conversion of retinol to the retro-retinoid anhydroretinol. It shares sequence homology with the family of mammalian cytosolic sulfotransferases and provides the first link between sulfotransferases and retinol metabolism. In this study the enzymatic properties of retinol dehydratase were examined using bacterially expressed protein. We show that retinol dehydratase can catalyze the transfer of the sulfonate moiety to small phenolic compounds and exhibits many functional similarities to the mammalian cytosolic sulfotransferases. The bisubstrate reaction that it catalyzes between retinol and the universal sulfonate donor 3'-phosphoadenosine 5'-phosphosulfate seems to involve ternary complex formation and to proceed via a Random Bi Bi mechanism. In addition to the low nanomolar Km value for free retinol, retinol dehydratase is strongly inhibited by retinol metabolites, suggesting a preference for retinoids. Conversely, a number of tested mammalian cytosolic sulfotransferases do not utilize retinol, indicating that retinol is not a general substrate for sulfotransferases.

Molecular cloning, expression, and characterization of novel human SULT1C sulfotransferases that catalyze the sulfonation of N-hydroxy-2-acetylaminofluorene

Upon sulfonation, carcinogenic hydroxyarylamines such as N-hydroxy-2-acetylaminofluorene (N-OH-2AAF) can be further activated to form ultimate carcinogens in vivo. Previous studies have shown that a SULT1C1 sulfotransferase is primarily responsible for the sulfonation of N-OH-2AAF in rat liver. In the present study, two novel human sulfotransferases shown to be members of the SULT1C sulfotransferase subfamily based on sequence analysis have been cloned, expressed, and characterized. Comparisons of the deduced amino acid sequence encoded by the human SULT1C sulfotransferase cDNA 1 reveal 63.7, 61.6, and 85.1% identity to the amino acid sequences of rat SULT1C1 sulfotransferase, mouse SULT1C1 sulfotransferase, and rabbit SULT1C sulfotransferase. In contrast, the deduced amino acid sequence of the human SULT1C sulfotransferase 2 cDNA displays 62.9, 63.1, 63.1, and 62.5% identity to the amino acid sequences of the human SULT1C sulfotransferase 1, rat SULT1C1 sulfotransferase, mouse SULT1C1 sulfotransferase, and rabbit SULT1C sulfotransferase. Recombinant human SULT1C sulfotransferases 1 and 2, expressed in Escherichia coli and purified to near electrophoretic homogeneity, were shown to cross-react with the antiserum against the rat liver SULT1C1 sulfotransferase and exhibited sulfonating activities with N-OH-2AAF as substrate. Tissue-specific expression of these novel human SULT1C sulfotransferases were examined by employing the Northern blotting technique. The results provide a foundation for the investigation into the functional relevance of these new SULT1C sulfotransferases in different human tissues/organs.

In vitro inhibition of thyroid hormone sulfation by hydroxylated metabolites of halogenated aromatic hydrocarbons

Earlier studies in our laboratory showed that hydroxylated metabolites of polychlorinated biphenyls (PCBs), dibenzo-p-dioxins (PCDDs), and dibenzofurans (PCDFs) competitively inhibit thyroxine (T4) binding to transthyretin (TTR) and type I deiodinase (D1) activity. In this study, we investigated the possible inhibitory effects of hydroxylated metabolites of polyhalogenated aromatic hydrocarbons (PHAHs) on iodothyronine sulfotransferase activity. Rat liver cytosol was used as a source of sulfotransferase enzyme in an in vitro assay with 125I-labeled 3,3'-diiodothyronine (T2) as a model substrate. Increasing amounts of hydroxylated PCBs, PCDDs, or PCDFs or extracts from incubation mixtures of PHAHs and induced liver microsomes were added as potential inhibitors of T2 sulfotransferase activity. Hydroxylated metabolites of PCBs, PCDDs, and PCDFs were found to be potent inhibitors of T2 sulfotransferase activity in vitro with IC50 values in the low micromolar range (0.2-3.8 microM). The most potent inhibitor of T2 sulfotransferase activity in our experiments was the PCB metabolite 3-hydroxy-2,3',4, 4',5-pentachlorobiphenyl with an IC50 value of 0.2 microM. A hydroxyl group in the para or meta position appeared to be an important structural requirement for T2 sulfotransferase inhibition by PCB metabolites. Ortho hydroxy PCBs were much less potent, and none of the parent PHAHs was capable of inhibiting T2 sulfotransferase activity. In addition, the formation of T2 sulfotransferase-inhibiting metabolites of individual brominated diphenyl ethers and nitrofen as well as from some commercial PHAH mixtures (e.g., Bromkal, Clophen A50, and Aroclor 1254) was also demonstrated. These results indicate that hydroxylated PHAHs are potent inhibitors of thyroid hormone sulfation. Since thyroid hormone sulfation may play an important role in regulating free hormone levels in the fetus, and PCB metabolites are known to accumulate in fetal tissues after maternal exposure to PCBs, these observations may have implications for fetal thyroid hormone homeostasis and development.

Biology and function of the reversible sulfation pathway catalysed by human sulfotransferases and sulfatases

Sulfation and sulfate conjugate hydrolysis play an important role in metabolism, and are catalysed by members of the sulfotransferase and sulfatase enzyme super-families. In general, sulfation is a deactivating, detoxication pathway, but for some chemicals the sulfate conjugates are much more reactive than the parent compound. The range of compounds which are sulfated is enormous, yet we still understand relatively little of the function of this pathway. This review summarises current knowledge of the sulfation system and the enzymes involved, and illustrates how heterologous expression of sulfotransferases (SULTs) and sulfatases is aiding our appreciation of the properties of these important proteins. The role of sulfation in the bioactivation of procarcinogens and promutagens is discussed, and new data on the inhibition of the sulfotransferase(s) involved by common dietary components such as tea and coffee are presented. The genetic and environmental factors which are known to influence the activity and expression of human SULTs and sulfatases are also reviewed.

On the nature of rat hepatic and mouse olfactory sulfotransferases

Rat hydroxysteroid sulfotransferase (HS-SULT) cDNAs, ST-40 and ST-20 are 90% identical in amino acid sequences and show different substrate specificities toward dehydroepiandrosterone (DHEA), androsterone (AD) and cortisol (CS). ST-40 enzyme is active toward the three substrates, whereas ST-20 enzyme is preferentially active for CS. First we prepared mutants of well conserved histidine, lysine and asparagine by site-directed mutagenesis. Secondly we constructed 20 chimeric HS-SULTs by reciprocal exchange of five protein domains between ST-20 and ST-40 enzymes. The studies on the expressed mutant and chimeric enzymes indicate the importance of the C-terminal region for the substrate specificity and the involvement of multiple regions for the enzyme activities. Next we determined the genetic loci of ST-40 and ST-20 by fluorescence in situ hybridization. Biotinylated ST-20 and ST-40 probes gave a pair of fluorescent spots on the same region of rat chromosome 1 and the loci of these genes were localized to the same chromosomal region of 1q21.3 --> q22.1. Finally we studied mouse olfactory phenol SULT (P-SULT). It was immunolocalized in the cytoplasm of mouse olfactory sustentacular cells and mouse nasal cytosols show high SULT activities toward phenolic aromatic odorants. We subsequently isolated a mouse P-SULT cDNA from mouse olfactory cDNA library. It encodes 304 amino acid polypeptide and is 94% identical with rat ST1C1 in amino acid sequences.

Characterization of thyroid hormone sulfotransferases

Sulfation is an intriguing pathway of thyroid hormone metabolism since it facilitates the degradation of the hormone by the type I deiodinase (D1). This study reports the preliminary characterization of iodothyronine sulfotransferase activities of rat and human liver cytosol and recombinant rSULT1C1 and hSULT1A1 isoenzymes. All these enzyme preparations catalyzed the sulfation of--in decreasing order of efficiency--3,3'-diiodothyronine (3,3'-T2) > 3,3',5-triiodothyronine (T3) approximately 3,3',5'-triiodothyronine (rT3) > thyroxine (T4). 3,3'-T2 sulfotransferase activity was found to be higher in male than in female rat liver, which has also been shown by others for the expression of rSULT1A1 and rSULT1C1. No sulfation of iodothyronines was observed with rSULT1A1. Different phenol derivatives were found to be potent inhibitors of the sulfation of 3,3'-T2 by native and recombinant sulfotransferases, with pentachlorophenol and 2,4,6-tribromophenol being the most potent. The inhibitions exerted by the different phenols on 3,3'-T2 sulfation by rSULT1C1 correlated better with the effects observed in male than with those in female liver. A strong correlation was also observed between the inhibition profiles of human liver cytosol and hSUL1T1A1. These results suggest that: (1) rSULT1C1 is an important isoenzyme for the sulfation of thyroid hormone in male rat liver; (2) another isoenzyme with similar properties, perhaps rSULT1B1, is responsible for thyroid hormone sulfation in female rat liver and may also contribute to this process in male rat liver; and (3) hSULT1A1 is an important isoenzyme for thyroid hormone sulfation in human liver.

Sulfotransferase-mediated activation of mutagens studied using heterologous expression systems

Sulfation is a common final step in the biotransformation of xenobiotics and is traditionally associated with inactivation. However, the sulfate group is electron-withdrawing and may be cleaved off heterolytically in some molecules leading to electrophilic cations which may form adducts with DNA and other important cellular structures. Since endogenous sulfotransferases do not appear to be expressed in indicator cells of standard mutagenicity tests, rat and human sulfotransferases have been stably expressed in his- Salmonella typhimurium strain TA1538 and Chinese hamster V79 cells. Using these recombinant indicator cells, sulfotransferase-dependent genotoxic activities were detected with N-hydroxy-2-acetylaminofluorene, 2-acetylaminofluorene (in the presence of co-expressed rat cytochrome P450 1A2), hycanthone, 1'-hydroxysafrole, alpha-hydroxytamoxifen and various benzylic alcohols derived from polycyclic aromatic hydrocarbons. In several cases, it was critical that the reactive sulfuric acid conjugates were formed directly within the indicator cells, owing to the inefficient penetration of cell membranes. In other cases, spontaneous benzylic substitution reactions with medium components, such as halogenide ions or amino acids, led to secondary, membrane-penetrating reactive species. Different sulfotransferases, including related forms from rat and human, substantially differed in their substrate specificity towards the investigated promutagens. It is known that some sulfotransferases are expressed with high tissue and cell type specificities. This site-dependent expression together with the limitations in the distribution of reactive sulfuric acid conjugates may explain organotropic effects of compounds activated by this metabolic pathway.