Molecular cloning, expression, and functional characterization of novel mouse sulfotransferases

Analysis of evolutionary and functional features of the bullfrog SULT1 family

We identified the bullfrog Rana catesbeiana sulfotransferase 1 (SULT1) family from the BLAST search tool of the public databases based on the SULT1 families of Nanorana parkeri, Xenopus laevis, and Xenopus tropicalis as queries, revealing the characteristics of the anuran SULT1 family. The results showed that the anuran SULT1 family comprises six subfamilies, four of which were related to the mammalian SULT1 subfamily. Additionally, the bullfrog has two SULT1Cc subfamily members that are consistent with the characteristics of the expanded Xenopus SULT1C subfamily. Several members of the bullfrog SULT1 family were suggested to play important roles in sulfation during metamorphosis. Among these, cDNAs encoding SULT1Cc1 and SULT1Y1 were cloned, and the sulfation activity was analyzed using recombinant proteins. The affinity for 2-naphthol and 3'-phosphoadenosine 5'-phosphosulfate (PAPS) and the enzymatic reaction rate were higher in SULT1Cc1 than in SULT1Y1. Both the enzymes showed inhibitory effect of many thyroid hormones (THs) analogs on the sulfation of 2-naphthol. The potency of sulfation activities of SULT1Cc1 and SULT1Y1 against T4 indicated their possible role in the intracellular T4 clearance during metamorphosis.

Sex steroid hormone synthesis, metabolism, and the effects on the mammalian olfactory system

Sex steroid hormones influence olfactory-mediated social behaviors, and it is generally hypothesized that these effects result from circulating hormones and/or neurosteroids synthesized in the brain. However, it is unclear whether sex steroid hormones are synthesized in the olfactory epithelium or the olfactory bulb, and if they can modulate the activity of the olfactory sensory neurons. Here, we review important discoveries related to the metabolism of sex steroids in the mouse olfactory epithelium and olfactory bulb, along with potential areas of future research. We summarize current knowledge regarding the expression, neuroanatomical distribution, and biological activity of the steroidogenic enzymes, sex steroid receptors, and proteins that are important to the metabolism of these hormones and reflect on their potential to influence early olfactory processing. We also review evidence related to the effects of sex steroid hormones on the development and activity of olfactory sensory neurons. By better understanding how these hormones are metabolized and how they act both at the periphery and olfactory bulb level, we can better appreciate the complexity of the olfactory system and discover potential similarities and differences in early olfactory processing between sexes.

Gene expression profile data of the developing small intestine of Id2-deficient mice

This article contains data related to the research article entitled “Id2 determines intestinal identity through repression of the foregut transcription factor, Irx5” [1]. Id2 deficient (Id2^−/−) mice developed gastric tumors and heterotopic squamous epithelium in the small intestine. These tumors and heterotopic tissues were derived from ectopic gastric cells and squamous cells formed in the small intestine respectively during development. In this study, microarray data of the developing small intestine of Id2^−/− mice was analyzed.

SULFATION PATHWAYS: Expression of SULT2A1, SULT2B1 and HSD3B1 in the porcine testis and epididymis

In the porcine testis, in addition to estrogen sulfates, the formation of numerous sulfonated neutral hydroxysteroids has been observed. However, their functions and the underlying synthetic pathways are still widely unclear. To obtain further information on their formation in postpubertal boars, the expression of sulfotransferases considered relevant for neutral hydroxysteroids (SULT2A1, SULT2B1) was investigated in the testis and defined segments of the epididymis applying real-time RT-qPCR, Western blot and immunohistochemistry (IHC). Sulfotransferase activities were assessed in tissue homogenates or cytosolic preparations applying dehydroepiandrosterone and pregnenolone as substrates. A high SULT2A1 expression was confirmed in the testis and localized in Leydig cells by IHC. In the epididymis, SULT2A1 expression was virtually confined to the body. SULT2B1 expression was absent or low in the testis but increased significantly along the epididymis. Immunohistochemical observations indicate that both enzymes are secreted into the ductal lumen via an apocrine mechanism. The results from the characterization of expression patterns and activity measurements suggest that SULT2A1 is the prevailing enzyme for the sulfonation of hydroxysteroids in the testis, whereas SULT2B1 may catalyze the formation of sterol sulfates in the epididymis. In order to obtain information on the overall steroidogenic capacity of the porcine epididymis, the expression of important steroidogenic enzymes (CYP11A1, CYP17A1, CYP19, HSD3B1, HSD17B3, SRD5A2) was monitored in the defined epididymal segments applying real-time RT-qPCR. Surprisingly, in addition to a high expression of SRD5A2 in the epididymal head, a substantial expression of HSD3B1 was detected, which increased along the organ.

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.

Mouse cytosolic sulfotransferase SULT2B1b interacts with cytoskeletal proteins via a proline/serine-rich C-terminus

Cytosolic sulfotransferase (SULT) SULT2B1b had previously been characterized as a cholesterol sulfotransferase. Like human SULT2B1, mouse SULT2B1b contains a unique, 31 amino acid C-terminal sequence with a proline/serine-rich region, which is not found in members of other SULT families. To gain insight into the functional relevance of this proline/serine-rich region, we constructed a truncated mouse SULT2B1b lacking the 31 C-terminal amino acids, and compared it with the wild-type enzyme. Enzymatic characterization indicated that the catalytic activity was not significantly affected by the absence of those C-terminal residues. Glutathione S-transferase pulldown assays showed that several proteins interacted with mouse SULT2B1b specifically through this C-terminal proline/serine-rich region. Peptide mass fingerprinting revealed that of the five SULT2B1b-binding proteins analyzed, three were cytoskeletal proteins and two were cytoskeleton-binding molecular chaperones. Furthermore, wild-type mouse SULT2B1b, but not the truncated enzyme, was associated with the cytoskeleton in experiments with a cytoskeleton-stabilizing buffer. Collectively, these results suggested that the unique, extended proline/serine-rich C-terminus of mouse SULT2B1b is important for its interaction with cytoskeletal proteins. Such an interaction may allow the enzyme to move along microfilaments such as actin filaments, and catalyze the sulfation of hydroxysteroids, such as cholesterol and pregnenolone, at specific intracellular locations.

Glucocorticoids stimulate hepatic and renal catecholamine inactivation by direct rapid induction of the dopamine sulfotransferase Sult1d1

During the stress response and metabolic fasting, glucocorticoids acting via the glucocorticoid receptor (GR) stimulate hepatic glucose production by activating specific gluconeogenic enzyme target genes. To characterize novel direct GR-regulated hepatic target genes under glucocorticoid control, we performed a whole genome gene expression microarray using dexamethasone-treated GR-null mice. Strongly induced previously characterized genes included phosphoenolpyruvate carboxykinase, serine dehydratase, tyrosine oxygenase, lipin 1, metallothionine, and cdkn1A. Novel induced genes included Ddit4, Fkbp5, Megf9, Sult1e1, and Sult1d1, and all were verified by real-time PCR. Sult1d1, a sulfotransferase, is a member of a large superfamily of detoxification enzymes and has an important role in the inactivation of endogenous dopamine-derived compounds, including the catecholamines. Treatment of primary mouse hepatocytes with dexamethasone for 6 h dramatically increased Sult1d1 mRNA levels, whereas cotreatment with RU-486, a GR antagonist, blocked induction by dexamethasone. Sult1d1 mRNA levels were also increased by dexamethasone in the kidney, a major site of Sult1d1 synthesis. Sult1d1 mRNA was localized by in situ hybridization to renal collecting ducts and was rapidly induced by glucocorticoids in renal inner medullary collecting duct (IMCD3) cells. Hepatic and renal Sult1d1 enzymatic activity was significantly induced in vivo in wild-type mice 6 h after dexamethasone treatment. Chromatin immunoprecipitation assay analysis upstream of the Sult1d1 gene promoter identified a glucocorticoid response element close to the neighboring glucocorticoid-responsive estrogen sulfotransferase Sult1e1 gene, indicating that both genes potentially share a common glucocorticoid response element. These results suggest that Sult1d1 in mice is directly induced by glucocorticoids and may attenuate elevated catecholamine activity during the stress response.

Regulation of sulfotransferase and UDP-glucuronosyltransferase gene expression by the PPARs

During phase II metabolism, a substrate is rendered more hydrophilic through the covalent attachment of an endogenous molecule. The cytosolic sulfotransferase (SULT) and UDP-glucuronosyltransferase (UGT) families of enzymes account for the majority of phase II metabolism in humans and animals. In general, phase II metabolism is considered to be a detoxication process, as sulfate and glucuronide conjugates are more amenable to excretion and elimination than are the parent substrates. However, certain products of phase II metabolism (e.g., unstable sulfate conjugates) are genotoxic. Members of the nuclear receptor superfamily are particularly important regulators of SULT and UGT gene transcription. In metabolically active tissues, increasing evidence supports a major role for lipid-sensing transcription factors, such as peroxisome proliferator-activated receptors (PPARs), in the regulation of rodent and human SULT and UGT gene expression. This review summarizes current information regarding the regulation of these two major classes of phase II metabolizing enzyme by PPARs.

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.

Snapshot of a Michaelis complex in a sulfuryl transfer reaction: Crystal structure of a mouse sulfotransferase, mSULT1D1, complexed with donor substrate and accepter substrate

We report the crystal structure of mouse sulfotransferase, mSULT1D1, complexed with donor substrate 3'-phosphoadenosine 5'-phosphosulfate and accepter substrate p-nitrophenol. The structure is the first report of the native Michaelis complex of sulfotransferase. In the structure, three proposed catalytic residues (Lys48, Lys106, and His108) were in proper positions for engaging in the sulfuryl transfer reaction. The data strongly support that the sulfuryl transfer reaction proceeds through an S(N)2-like in-line displacement mechanism.

Bile Acid sulfation: a pathway of bile acid elimination and detoxification

Sulfotransferase-2A1 catalyzes the formation of bile acid-sulfates (BA-sulfates). Sulfation of BAs increases their solubility, decreases their intestinal absorption, and enhances their fecal and urinary excretion. BA-sulfates are also less toxic than their unsulfated counterparts. Therefore, sulfation is an important detoxification pathway of BAs. Major species differences in BA sulfation exist. In humans, only a small proportion of BAs in bile and serum are sulfated, whereas more than 70% of BAs in urine are sulfated, indicating their efficient elimination in urine. The formation of BA-sulfates increases during cholestatic diseases. Therefore, sulfation may play an important role in maintaining BA homeostasis under pathologic conditions. Farnesoid X receptor, pregnane X receptor, constitutive androstane receptor, and vitamin D receptor are potential nuclear receptors that may be involved in the regulation of BA sulfation. This review highlights current knowledge about the enzymes and transporters involved in the formation and elimination of BA-sulfates, the effect of sulfation on the pharmacologic and toxicologic properties of BAs, the role of BA sulfation in cholestatic diseases, and the regulation of BA sulfation.

Mining the mouse transcriptome of receptive endometrium reveals distinct molecular signatures for the luminal and glandular epithelium

Epithelia coat most tissues where they sense and respond to the environment and participate in innate immune responses. In the adult mouse uterus, columnar epithelium lines the central lumen and the glands that penetrate the underlying stroma. A nidatory surge of estrogen causes differentiation of the luminal epithelium to the receptive state that permits blastocyst attachment and allows subsequent implantation. Here, using laser-capture microdissection to isolate the luminal and glandular epithelia separately, we have profiled gene expression 2 h before embryo attachment to determine whether there are unique roles for these two epithelial structures in this process. Although most genes were expressed in both compartments, there was greater expression of 153 and 118 genes in the lumen and glands, respectively. In the luminal epithelium, there is enrichment in lipid, metal-ion binding, and carbohydrate-metabolizing enzymes, whereas in the glands, immune response genes are emphasized. In situ hybridization to uterine sections obtained from mice during the preimplantation period validated these data and indicated an array of previously undocumented genes expressed with unique patterns in these epithelia. The data show that each epithelial compartment has a distinct molecular signature and that they act differentially and synergistically to permit blastocyst implantation.

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.

Human Sulfotransferases and Their Role in Chemical Metabolism

Sulfonation is an important reaction in the metabolism of numerous xenobiotics, drugs, and endogenous compounds. A supergene family of enzymes called sulfotransferases (SULTs) catalyze this reaction. In most cases, the addition of a sulfonate moiety to a compound increases its water solubility and decreases its biological activity. However, many of these enzymes are also capable of bioactivating procarcinogens to reactive electrophiles. In humans three SULT families, SULT1, SULT2, and SULT4, have been identified that contain at least thirteen distinct members. SULTs have a wide tissue distribution and act as a major detoxification enzyme system in adult and the developing human fetus. Nine crystal structures of human cytosolic SULTs have now been determined, and together with site-directed mutagenesis experiments and molecular modeling, we are now beginning to understand the factors that govern distinct but overlapping substrate specificities. These studies have also provided insight into the enzyme kinetics and inhibition characteristics of these enzymes. The regulation of human SULTs remains as one of the least explored areas of research in the field, though there have been some recent advances on the molecular transcription mechanism controlling the individual SULT promoters. Interindividual variation in sulfonation capacity may be important in determining an individual's response to xenobiotics, and recent studies have begun to suggest roles for SULT polymorphism in disease susceptibility. This review aims to provide a summary of our present understanding of the function of human cytosolic sulfotransferases.

Molecular cloning and functional analysis of porcine SULT1A1 gene and its variant: a single mutation SULT1A1 causes a significant decrease in sulfation activity

The characterization of the SULT1A1 gene and its variants should have an important impact on the efforts to develop genetic markers to select for low skatole in pigs. Raising intact male pigs would have a significant economic impact on the pork industry; however, the presence of skatole (a major cause of boar taint) in meat from male pigs would be highly objectionable to consumers. It has been shown that the phase II metabolism of skatole metabolites by phenol sulfotransferase is related to the clearance of skatole in the liver. The aim of this study was to isolate and characterize the SULT1A1 gene from pig liver, examine its expression, identify genetic polymorphisms, and study how a genetic variation in this enzyme translates into interindividual variation in skatole levels. We show here that a substitution of A-->G at base 546 of SULT1A1 causes a significant decrease in its sulfation activity and thus may be at least partially responsible for a higher level of skatole in pigs. Our findings provide an important basis toward the goal of making it possible to predict the sulfation status in pigs and the development of genetic markers for low skatole.

Unique properties of a renal sulfotransferase, St1d1, in dopamine metabolism

Although catecholamine sulfation is higher in the kidney than in the liver of mice, no detectable amounts of previously reported sulfotransferases (STs) such as St1a, St1b, St1c, and St1e were expressed in mouse kidney cytosols. A new sulfotransferase (St1d1) cDNA was isolated from kidney cDNA library of BALB/c strain by reverse transcription-polymerase chain reaction (RTPCR) using information from expressed sequence tags (EST) database. The cDNA sequence resembled that of cDNA reported previously (AA238910) (Sakakibara et al., 1998) but differed in two amino acids, (206)Q/K and (216)Y/F, in the deduced amino acid sequence. The St1d1 expressed had unique substrate specificities for catecholamine derivatives, which preferred their deaminated metabolites rather than their parent amines. St1d1 showed the highest activity toward 3,4-dihydroxyphenylacetic acid (230.2 +/- 2.69 nmol/mg/min) among the examined substrate. St1d1 protein was abundant in kidney, followed by liver, lung, and uterus. Furthermore, an addition of anti-St1d1 serum in the cytosolic reaction mixture resulted in complete inhibition of the sulfotransferase activity suggesting a major role of St1d1 on catecholamine sulfations. No human ST1D ortholog was detected at both mRNA and protein levels, although ST1A5 selectively catalyzing parent amine sulfation was detected in human kidney. These results indicate the functional basis of sulfation and the clear species difference on renal catecholamine metabolisms in mice and humans.

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.

Human cytosolic sulfotransferase database mining: identification of seven novel genes and pseudogenes

A total of 10 SULT genes are presently known to be expressed in human tissues. We performed a comprehensive genome-wide search for novel SULT genes using two different but complementary approaches, and developed a novel graphical display to aid in the annotation of the hits. Seven novel human SULT genes were identified, five of which were predicted to be pseudogenes, including two processed pseudogenes and three pseudogenes that contained introns. Those five pseudogenes represent the first unambiguous SULT pseudogenes described in any species. Expression-profiling studies were conducted for one novel gene, SULT6B1, and a series of alternatively spliced transcripts were identified in the human testis. SULT6B1 was also present in chimpanzee and gorilla, differing at only seven encoded amino-acid residues among the three species. The results of these database mining studies will aid in studies of the regulation of these SULT genes, provide insights into the evolution of this gene family in humans, and serve as a starting point for comparative genomic studies of SULT genes.

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.

Regulation of a xenobiotic sulfonation cascade by nuclear pregnane X receptor (PXR)

The nuclear receptor PXR (pregnane X receptor) protects the body from hepatotoxicity of secondary bile acids such as lithocholic acid (LCA) by inducing expression of the hydroxylating cytochrome P450 enzyme CYP3A and promoting detoxification. We found that activation of PXR also increases the activity and gene expression of the phase II conjugating enzyme dehydroepiandrosterone sulfotransferase (STD) known to sulfate LCA to facilitate its elimination. This activation is direct and appears to extend to other xenobiotic sulfotransferases as well as to 3'-phosphoadenosine 5'-phosphosulfate synthetase 2 (PAPSS2), an enzyme that generates the donor cofactor for the reaction. Because sulfation plays an important role in the metabolism of many xenobiotics, prescription drugs, and toxins, we propose that PXR serves as a master regulator of the phase I and II responses to facilitate rapid and efficient detoxification and elimination of foreign chemicals.

Sulfonation and molecular action

The sulfonation of endogenous molecules is a pervasive biological phenomenon that is not always easily understood, and although it is increasingly recognized as a function of fundamental importance, there remain areas in which significant cognizance is still lacking or at most minimal. This is particularly true in the field of endocrinology, in which the sulfoconjugation of hormones is a widespread occurrence that is only partially, if at all, appreciated. In the realm of steroid/sterol sulfoconjugation, the discovery of a novel gene that utilizes an alternative exon 1 to encode for two sulfotransferase isoforms, one of which sulfonates cholesterol and the other pregnenolone, has been an important advance. This is significant because cholesterol sulfate plays a crucial role in physiological systems such as keratinocyte differentiation and development of the skin barrier, and pregnenolone sulfate is now acknowledged as an important neurosteroid. The sulfonation of thyroglobulin and thyroid hormones has been extensively investigated and, although this transformation is better understood, there remain areas of incomplete comprehension. The sulfonation of catecholamines is a prevalent modification that has been extensively studied but, unfortunately, remains poorly understood. The sulfonation of pituitary glycoprotein hormones, especially LH and TSH, does not affect binding to their cognate receptors; however, sulfonation does play an important role in their plasma clearance, which indirectly has a significant effect on biological activity. On the other hand, the sulfonation of distinct neuroendocrine peptides does have a profound influence on receptor binding and, thus, a direct effect on biological activity. The sulfonation of specific extracellular structures plays an essential role in the binding and signaling of a large family of extracellular growth factors. In summary, sulfonation is a ubiquitous posttranslational modification of hormones and extracellular components that can lead to dramatic structural changes in affected molecules, the biological significance of which is now beginning to be appreciated.

Mutational analysis of human hydroxysteroid sulfotransferase SULT2B1 isoforms reveals that exon 1B of the SULT2B1 gene produces cholesterol sulfotransferase, whereas exon 1A yields pregnenolone sulfotransferase

As a result of an alternative exon 1, the gene for human hydroxysteroid sulfotransferase (SULTB1) encodes for two peptides differing only at their amino termini. The SULT2B1b isoform preferentially sulfonates cholesterol. Conversely, the SULT2B1a isoform avidly sulfonates pregnenolone but not cholesterol. The outstanding structural feature that distinguishes the SULT2B1 isoforms from the prototypical SULT2A1 isozyme is the presence of extended amino- and carboxyl-terminal ends in the former. Investigating the functional significance of this unique characteristic reveals that removal of 53 amino acids from the relatively long carboxyl-terminal end that is common to both SULT2B1 isoforms has no effect on the catalytic activity of either isoform. On the other hand, removal of 23 amino acids from the amino-terminal end that is unique to SULT2B1b results in loss of cholesterol sulfotransferase activity, whereas removal of 8 amino acids from the amino-terminal end that is unique to SULT2B1a has no effect on pregnenolone sulfotransferase activity. Deletion analysis along with site-directed mutagenesis of SULT2B1b reveal that the amino acid segment 19-23 residues from the amino terminus and particularly isoleucines at positions 21 and 23 are crucial for cholesterol catalysis. In the gene for SULT2B1, exon 1B encodes for only the unique amino-terminal region of SULT2B1b; however, exon 1A encodes for the unique amino-terminal end of SULT2B1a plus an additional 48 amino acids. Thus, if the gene for SULT2B1 employs exon 1B, cholesterol sulfotransferase is synthesized, whereas if exon 1A is used, pregnenolone sulfotransferase is produced.

Highly conserved mouse and human brain sulfotransferases: molecular cloning, expression, and functional characterization

By employing reverse transcription-polymerase chain reaction (RT-PCR) in conjunction with 5'-rapid amplification of cDNA ends technique, we have cloned a novel mouse sulfotransferase cDNA. Database search led to the identification of a human gene encoding the homologue of this newly discovered mouse sulfotransferase. RT-PCR technique was employed to clone the cDNA encoding the human enzyme. Sequence analysis revealed that the novel mouse and human sulfotransferases display nearly 98% identity in their amino acid sequences. Their amino acid sequence identity to other known cytosolic sulfotransferases, however, was found to be below 36%. These two highly conserved sulfotransferases therefore appear to belong to a family different from the two major mammalian cytosolic sulfotransferase gene families. Northern blot analysis revealed the neuronal tissue-specific expression of these two novel sulfotransferases. Recombinant mouse and human brain sulfotransferases, expressed using the pGEX-2TK prokaryotic expression system and purified from transformed Escherichia coli cells, migrated as 33 kD proteins upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Purified mouse and human brain sulfotransferases displayed enzymatic activities toward endogenous and xenobiotic compounds, including L-triiodothyronine, thyroxine, estrone, p-nitrophenol, 2-naphthylamine, and 2-naphthol. Using mouse brain filtrate as substrate, both brain sulfotransferases were shown to catalyze specifically the sulfation of only a few compounds.

Human cytosolic sulphotransferases: genetics, characteristics, toxicological aspects

Cytosolic sulphotransferases transfer the sulpho moiety from the cofactor 5'-phosphoadenosine-3'-phosphosulphate (PAPS) to nucleophilic groups of xenobiotics and small endogenous compounds (such as hormones and neurotransmitters). This reaction often leads to products that can be excreted readily. However, other sulpho conjugates are strong electrophiles and may covalently bind with DNA and proteins. All known cytosolic sulphotransferases are members of an enzyme/gene superfamily termed SULT. In humans, 10 SULT genes are known. One of these genes encodes two different enzyme forms due to the use of alternative first exons. Different SULT forms substantially differ in their substrate specificity and tissue distribution. Genetic polymorphisms have been described for three human SULTs. Several allelic variants differ in functional properties, including the activation of promutagens. Only initial results are available from the analysis of SULT allele frequencies in different population groups, e.g. subjects suffering from specific diseases and corresponding controls.

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.

Cholesterol and hydroxycholesterol sulfotransferases: identification, distinction from dehydroepiandrosterone sulfotransferase, and differential tissue expression

In humans, the biotransformation of cholesterol and its hydroxylated metabolites (oxysterols) by sulfonation is a fundamental process of great importance. Nevertheless, the sulfotransferase enzyme(s) that carries out this function has never been clearly identified. Cholesterol is a relatively poor substrate for the previously cloned hydroxysteroid sulfotransferase (HST), i.e. dehydroepiandrosterone (DHEA) sulfotransferase (HST1). Recently, cloning of a single human gene that encodes for two proteins related to HST1 was reported. These newly cloned sulfotransferases (HST2a and HST2b), while exhibiting sequence similarity to other members of the soluble sulfotransferase superfamily, also contain unique structural features. This latter aspect prompted an examination of their substrate specificity for comparison with HST1. Thus, HST1, HST2a, and HST2b were overexpressed as fusion proteins and purified. Furthermore, a novel procedure for the isolation of cholesterol and oxysterol sulfonates was developed that was used in association with HPLC to resolve specific sterol sulfonates. HST1 preferentially sulfonated DHEA and, to a lesser extent, oxysterols; whereas cholesterol was a negligible substrate. The reverse, however, was the case for the HST2 isoforms, particularly HST2b, which preferentially sulfonated cholesterol and oxysterols, in contrast to DHEA, which served as a poor substrate for this enzyme. RT-PCR analysis revealed distinct patterns of HST1, HST2a, and HST2b expression. It was particularly notable that both HST2 isoforms, but not HST1, were expressed in skin, a tissue where cholesterol sulfonation plays an important role in normal development of the skin barrier. In conclusion, substrate specificity and tissue distribution studies strongly suggest that HST2a and HST2b, in contrast to HST1, represent normal human cholesterol and oxysterol sulfotransferases. Furthermore, this study represents the first example of the sulfonation of oxysterols by a specific human HST.

Identification of a new subfamily of sulphotransferases: cloning and characterization of canine SULT1D1

Sulphation is an important conjugation pathway in drug metabolism that has been studied in several species including humans. However, few studies have been performed using the dog as a subject. In this report we describe the cloning and characterization of a canine cytosolic sulphotransferase (SULT). The overall primary structure of this enzyme is very similar to that of a rat phenol-sulphating enzyme found in the EMBL Database and to a mouse SULT termed amine-N-sulphotransferase (81% identity). The expressed canine SULT conjugates small phenols and aromatic amines such as dopamine, minoxidil, p-nitrophenol and 5-hydroxytryptamine, but not dehydroepiandrosterone or beta-oestradiol. These results are in agreement with the results reported for the mouse SULT. In contrast with the mouse enzyme, the canine SULT does not conjugate eicosanoid compounds, i.e. prostaglandins, thromboxane B(2) or leukotriene E(4). The canine SULT is expressed at high levels in the colon of both genders; it is also expressed in the small intestine, kidney and liver. Furthermore, because the canine, mouse and rat SULT forms exhibit significant sequence identity (more than 80%), they seem to represent a distinct group in the SULT family tree. This suggestion is strengthened by the low identity with other SULTs. The subfamily that is most similar to this new group is SULT1A, with approx. 60% similarity. However, the mouse and canine enzymes are not characterized by the efficient sulphation of p-nitrophenol, dopamine, beta-oestradiol or oestrone. Thus these results seem to exclude them from the SULT1A subfamily. We therefore propose a new subfamily in the phenol SULT family, designated SULT1D, and consequently the canine enzyme is termed SULT1D1.

Sex-dependent regulation by dexamethasone of murine hydroxysteroid sulfotransferase gene expression

To determine whether glucocorticoid-inducible expression of hepatic hydroxysteroid sulfotransferase is conserved in mouse, the effects of dexamethasone (DEX) on hydroxysteroid sulfotransferase (mSULT2A) gene expression were investigated in primary cultured hepatocytes prepared from C57BL/6J mice. In female mouse hepatocytes, DEX (10(-7) and 10(-5) M, respectively) produced 8.2- and 17.8-fold increases, respectively, in the amounts of mSULT2A mRNA relative to control. By contrast, mSULT2A mRNA levels were undetectable in male mouse hepatocytes. Female-predominant mSULT2A mRNA expression was also observed in liver samples from C57BL/6J and three other mouse strains. Treatment of primary cultured female mouse hepatocytes with dihydrotestosterone in the presence of DEX suppressed mSULT2 expression. Transfection of primary cultured male or female mouse hepatocytes with a rat SULT2-40/41 reporter construct revealed that hepatocytes of both sexes have sufficient machinery to achieve DEX-inducible SULT2 transcription. However, treatment with the potent histone deacetylase inhibitor trichostatin A failed to elicit mSULT2A expression in male mouse hepatocytes.

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.

Identification of phenobarbitone-modulated genes in mouse liver by differential display

The molecular basis of how rodent nongenotoxic hepatocarcinogens such as phenobarbitone cause liver-tumor formation is poorly understood. An early effect of phenobarbitone exposure is to induce hepatocyte proliferation transiently, and there is evidence that this may be important for subsequent tumor development. In this investigation, we have used the differential display reverse transcriptase polymerase chain reaction technique to analyze differential gene expression in male C57B1/10J mouse liver during the mitogenic phase of the phenobarbitone response. Seventy-seven putative differentially expressed cDNAs were isolated by differential display, and 13 of them were subsequently confirmed as being differentially expressed (both increased and decreased by phenobarbitone). Seven of the cDNAs were homologous to known mouse or human genes (carboxylesterase, coagulation factor X, amine N-sulphotransferase, human protein disulphide isomerase-related protein, cytochrome c oxidase subunit IV, golgin-245, thioredoxin reductase, betaine-homocysteine methyl transferase) and the remainder were novel. The expression pattern of the sulphotransferase was further characterized, and in mouse liver it was found to be significantly induced by phenobarbitone and not by five other rodent nongenotoxic hepatocarcinogens. In summary, the technique has enabled the identification of previously uncharacterized genes whose expression patterns are differentially altered by phenobarbitone in the mouse liver.

Photoaffinity labeling probe for the substrate binding site of human phenol sulfotransferase (SULT1A1): 7-azido-4-methylcoumarin

A novel fluorescent photoactive probe 7-azido-4-methylcoumarin (AzMC) has been characterized for use in photoaffinity labeling of the substrate binding site of human phenol sulfotransferase (SULT1A1 or P-PST-1). For the photoaffinity labeling experiments, SULT1A1 cDNA was expressed in Escherichia coli as a fusion protein to maltose binding protein (MBP) and purified to apparent homogeneity over an amylose column. The maltose moiety was removed by Factor Xa cleavage. Both MBSULT1A1 and SULT1A1 were efficiently photolabeled with AzMC. This labeling was concentration dependent. In the absence of light, AzMC competitively inhibited the sulfation of 4MU catalyzed by SULT1A1 (Ki = 0.47 +/- 0.05 mM). Moreover, enzyme activity toward 2-naphthol was inactivated in a time- and concentration-dependent manner. SULT1A1 inactivation by AzMC was protected by substrate but was not protected by cosubstrate. These results indicate that photoaffinity labeling with AzMC is highly suitable for the identification of the substrate binding site of SULT1A1. Further studies are aimed at identifying which amino acids modified by AzMC are localized in the binding site.

Bacterial expression, purification, and characterization of a novel mouse sulfotransferase that catalyzes the sulfation of eicosanoids

Analysis of the nucleotide sequence of a recently cloned mouse sulfotransferase cDNA (clone 679153) revealed the presence in its 3'-untranslated sequence of an AT-rich region which contains four ATTTA motifs and an TTATTTAT-like sequence, commonly found among those encoding inflammation-related proteins. The recombinant enzyme expressed in Escherichia coli and purified to near electrophoretic homogeneity displayed strong sulfotransferase activities toward various prostaglandins, thromboxane B2, and leukotriene E4. These results mark the first discovery of the sulfation of eicosanoids catalyzed by a distinct sulfotransferase.