Bile acid sulfotransferase I from rat liver sulfates bile acids and 3-hydroxy steroids: purification, N-terminal amino acid sequence, and kinetic properties

Nuclear Receptor Pathways Mediating the Development of Boar Taint

The nuclear receptors PXR, CAR, and FXR are activated by various ligands and function as transcription factors to control the expression of genes that regulate the synthesis and metabolism of androstenone and skatole. These compounds are produced in entire male pigs and accumulate in the fat to cause the development of a meat quality issue known as boar taint. The extent of this accumulation is influenced by the synthesis and hepatic clearance of androstenone and skatole. For this reason, PXR, CAR, and FXR-mediated signaling pathways have garnered interest as potential targets for specialized treatments designed to reduce the development of boar taint. Recent research has also identified several metabolites produced by gut microbes that act as ligands for these nuclear receptors (e.g., tryptophan metabolites, short-chain fatty acids, bile acids); however, the connection between the gut microbiome and boar taint development is not clear. In this review, we describe the nuclear receptor signaling pathways that regulate the synthesis and metabolism of boar taint compounds and outline the genes involved. We also discuss several microbial-derived metabolites and dietary additives that are known or suspected nuclear receptor ligands and suggest how these compounds could be used to develop novel treatments for boar taint.

The Relative Importance of the Small Intestine and the Liver in Phase II Metabolic Transformations and Elimination of p-Nitrophenol Administered in Different Doses in the Rat

Intestinal and hepatic function have been investigated in phase II metabolic reactions and elimination of p-nitrophenol (PNP) in the rat. A jejunal loop was cannulated and recirculated with isotonic solutions containing PNP in different concentrations (0, 20, 100, 500, 1000 µM). Samples were obtained from the perfusate at given intervals. To investigate the metabolic and excretory functions of the liver, the bile duct was cannulated, and the bile was collected. Metabolites of PNP were determined by validated HPLC (high pressure liquid chromatography) methods. The results demonstrated the relative importance of the small intestine and the liver in phase II metabolic transformations and elimination of PNP. There were significant differences between the luminal and biliary appearances of p-nitrophenol-glucuronide (PNP-G) and p-nitrophenol–sulfate (PNP-S). The PNP-G appeared in the intestinal lumen at the lower PNP concentrations (20 µM and 100 µM) at higher rate than in the bile. No significant difference was found between the intestinal and the biliary excretion of PNP-G when PNP was administered at a concentration of 500 µM. However, a reverse ratio of these parameters was observed at the administration of 1000 µM PNP. The results indicated that both the small intestine and the liver might play an important role in phase II metabolic reactions and elimination of PNP. However, the relative importance of the small intestine and the liver can be dependent on the dose of drugs.

Identification and characterization of a novel PPARα-regulated and 7α-hydroxyl bile acid-preferring cytosolic sulfotransferase mL-STL (Sult2a8)

PPARα has been known to play a pivotal role in orchestrating lipid, glucose, and amino acid metabolism via transcriptional regulation of its target gene expression during energy deprivation. Recent evidence has also suggested that PPARα is involved in bile acid metabolism, but how PPARα modulates the homeostasis of bile acids during fasting is still not clear. In a mechanistic study aiming to dissect the spectrum of PPARα target genes involved in metabolic response to fasting, we identified a novel mouse gene (herein named mL-STL for mouse liver-sulfotransferase-like) that shared extensive homology with the Sult2a subfamily of a superfamily of cytosolic sulfotransferases, implying its potential function in sulfonation. The mL-STL gene expressed predominantly in liver in fed state, but PPARα was required to sustain its expression during fasting, suggesting a critical role of PPARα in regulating the mL-STL-mediated sulfonation during fasting. Functional studies using recombinant His-tagged mL-STL protein revealed its narrow sulfonating activities toward 7α-hydroxyl primary bile acids, including cholic acid, chenodeoxycholic acid, and α-muricholic acid, and thus suggesting that mL-STL may be the major hepatic bile acid sulfonating enzyme in mice. Together, these studies identified a novel PPARα-dependent gene and uncovered a new role of PPARα as being an essential regulator in bile acid biotransformation via sulfonation during fasting.

Finding sequences for over 270 orphan enzymes

Despite advances in sequencing technology, there are still significant numbers of well-characterized enzymatic activities for which there are no known associated sequences. These 'orphan enzymes' represent glaring holes in our biological understanding, and it is a top priority to reunite them with their coding sequences. Here we report a methodology for resolving orphan enzymes through a combination of database search and literature review. Using this method we were able to reconnect over 270 orphan enzymes with their corresponding sequence. This success points toward how we can systematically eliminate the remaining orphan enzymes and prevent the introduction of future orphan enzymes.

Regulation of the cytosolic sulfotransferases by nuclear receptors

The cytosolic sulfotransferases (SULTs) are a multigene family of enzymes that catalyze the transfer of a sulfonate group from the physiologic sulfate donor, 3'-phosphoadenosine-5'-phosphosulfate, to a nucleophilic substrate to generate a polar product that is more amenable to elimination from the body. As catalysts of both xenobiotic and endogenous metabolism, the SULTs are major points of contact between the external and physiological environments, and modulation of SULT-catalyzed metabolism can not only affect xenobiotic disposition, but it can also alter endogenous metabolic processes. Therefore, it is not surprising that SULT expression is regulated by numerous members of the nuclear receptor (NR) superfamily that function as sensors of xenobiotics as well as endogenous molecules, such as fatty acids, bile acids, and oxysterols. These NRs include the peroxisome proliferator-activated receptors, pregnane X receptor, constitutive androstane receptor, vitamin D receptor, liver X receptors, farnesoid X receptor, retinoid-related orphan receptors, and estrogen-related receptors. This review summarizes current information about NR regulation of SULT expression. Because species differences in SULT subfamily composition and tissue-, sex-, development-, and inducer-dependent regulation are prominent, these differences will be emphasized throughout the review. In addition, because of the central role of the SULTs in cellular physiology, the effect of NR-mediated SULT regulation on physiological and pathophysiological processes will be discussed. Gaps in current knowledge that require further investigation are also highlighted.

Biosynthesis and molecular genetics of polyketides in marine dinoflagellates

Marine dinoflagellates are the single most important group of algae that produce toxins, which have a global impact on human activities. The toxins are chemically diverse, and include macrolides, cyclic polyethers, spirolides and purine alkaloids. Whereas there is a multitude of studies describing the pharmacology of these toxins, there is limited or no knowledge regarding the biochemistry and molecular genetics involved in their biosynthesis. Recently, however, exciting advances have been made. Expressed sequence tag sequencing studies have revealed important insights into the transcriptomes of dinoflagellates, whereas other studies have implicated polyketide synthase genes in the biosynthesis of cyclic polyether toxins, and the molecular genetic basis for the biosynthesis of paralytic shellfish toxins has been elucidated in cyanobacteria. This review summarises the recent progress that has been made regarding the unusual genomes of dinoflagellates, the biosynthesis and molecular genetics of dinoflagellate toxins. In addition, the evolution of these metabolic pathways will be discussed, and an outlook for future research and possible applications is provided.

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.

The farnesoid X receptor: a novel drug target?

Bile acids are end products of cholesterol metabolism. They are exclusively synthesised by the liver and subsequently secreted via the bile duct into the intestine to facilitate the absorption of dietary fat and fat-soluble vitamins. Nuclear receptors are ligand-activated transcription factors. The farnesoid X receptor (FXR) has recently been identified as a bile acid-activated nuclear receptor. FXR controls bile-acid synthesis, conjugation and transport, as well as lipid metabolism. Recent advances in FXR biology demonstrate that FXR may represent a valuable target for the identification of novel drugs to treat dyslipidaemia and cholestasis. However, for therapeutic purposes the development of selective FXR modulators, which only activate or inhibit specific FXR target genes and as such induce specific responses, will be required.

The Farnesoid X receptor: a molecular link between bile acid and lipid and glucose metabolism

Bile acids are the end products of cholesterol metabolism. They are synthesized in the liver and secreted via bile into the intestine, where they aid in the absorption of fat-soluble vitamins and dietary fat. Subsequently, bile acids return to the liver to complete their enterohepatic circulation. The Farnesoid X receptor (FXR) is a member of the nuclear receptor superfamily and has emerged as a key player in the control of multiple metabolic pathways. On its activation by bile acids, FXR regulates bile acid synthesis, conjugation, and transport, as well as various aspects of lipid and glucose metabolism. This review summarizes recent advances in deciphering the role of FXR in the context of hepatic lipid and glucose homeostasis and discusses the potential of FXR as a pharmacological target for therapeutic applications.

Regulation of glucocorticoid-inducible hydroxysteroid sulfotransferase (SULT2A-40/41) gene transcription in primary cultured rat hepatocytes: role of CCAAT/enhancer-binding protein liver-enriched transcription factors

The mechanism responsible for glucocorticoid receptor (GR)-mediated induction of rat hepatic hydroxysteroid sulfotransferase (SULT2A-40/41) gene transcription was investigated. We previously reported that the region of the SULT2A-40/41 5'-flanking region delimited by -158 to -77 nucleotides relative to the transcription start site was sufficient to support GR-inducible expression. This region of the SULT2A-40/41 gene does not contain a consensus glucocorticoid receptor-responsive element, but does contain two consensus sites for liver-enriched CCAAT/enhancer-binding protein (C/EBP) transcription factors. In the present study, incubation of primary cultured rat hepatocytes with a GR-activating concentration (10(-7) M) of a potent glucocorticoid, dexamethasone or triamcinolone acetonide (TA), rapidly produced increases in C/EBPalpha and C/EBPbeta nuclear protein contents, as measured by Western blot or in vitro DNA-binding activity analysis, that preceded increases in SULT2A-40/41 mRNA and protein levels. Transient cotransfection of SULT2A-40/41 reporter plasmids with a dominant negative C/EBP expression plasmid completely blocked TA-inducible SULT2A-40/41 reporter gene expression. Linker scanning and site-directed mutagenesis of the proximal SULT2A-40/41 5'-flanking region, complemented by in vitro DNA-binding analyses, indicated that the more distal C/EBP site was important for controlling SULT2A-40/41 promoter activity. These data support a role for GR-inducible C/EBPalpha and C/EBPbeta expression in the transactivation of hepatic SULT2A-40/41 expression.

Effects of dexamethasone on aryl (SULT1A1)- and hydroxysteroid (SULT2A1)-sulfotransferase gene expression in primary cultured human hepatocytes

To determine whether the dexamethasone (DEX)-inducible hepatic sulfotransferase gene expression that has been described in the rat is conserved in humans, the effects of DEX treatment on hydroxysteroid sulfotransferase (SULT2A1) and aryl sulfotransferase (SULT1A1) gene expression were investigated in primary cultured human hepatocytes. Hepatocytes were prepared from nontransplantable human livers by collagenase perfusion of the left hepatic lobe, and cultured in Williams' medium E that was supplemented with 0.25 U/ml insulin. As reported in the rat, DEX treatment produced concentration-dependent increases in SULT2A1 mRNA and protein expression, with maximum increases observed at concentrations of DEX that would be expected to activate the pregnane X receptor (PXR) transcription factor. In contrast to the rat, in which DEX-inducible SULT1A1 expression has been demonstrated, SULT1A1 expression in primary cultured human hepatocytes was not measurably increased by DEX. In transient transfections conducted in primary cultured rat hepatocytes, the PXR ligands DEX and pregnenolone-16 alpha-carbonitrile significantly induced transcription of human and rat SULT2A reporter gene constructs. Cotransfection of either the human or rat SULT2A reporter gene with a PXR dominant negative construct significantly reduced DEX-inducible transcription. These results underscore that while certain features of rat hepatic sulfotransferase gene regulation are conserved in humans, important differences exist across species. The findings also implicate a role for the PXR transcription factor in DEX-inducible rat and human SULT2A gene expression.

Dehydroepiandrosterone sulfotransferase gene induction by bile acid activated farnesoid X receptor

Dehydroepiandrosterone sulfotransferase (STD) is a hydroxysteroid sulfo-conjugating enzyme with preferential substrate specificity for C-19 androgenic steroids and C-24 bile acids. STD is primarily expressed in the liver, intestine and adrenal cortex. Earlier studies have shown that androgens inhibit the rat Std promoter function through a negative androgen response region located between -235 and -310 base pair positions (Song, C. S., Jung, M. H., Kim, S. C., Hassan, T., Roy, A. K., and Chatterjee, B. (1998) J. Biol. Chem. 273, 21856-21866). Here we report that the primary bile acid chenodeoxycholic acid (CDCA) also acts as an important regulator of the Std gene promoter. CDCA is a potent inducer of the Std gene, and its inducing effect is mediated through the bile acid-activated farnesoid X receptor (FXR), a recently characterized member of the nuclear receptor superfamily. The ligand-activated FXR acts as a heterodimer with the 9-cis-retinoic acid receptor (RXR) and regulates the Std gene by binding to an upstream region at base pair positions -169 to -193. This specific binding region was initially identified by bile acid responsiveness of the progressively deleted forms of the Std promoter in transfected HepG2 hepatoma and enterocyte-like Caco-2 cells. Subsequently, the precise RXR/FXR binding position was established by protein-DNA interaction using in vitro footprinting and electrophoretic mobility shift analyses. Unlike all other previously characterized FXR target genes, which contain an inverted repeat (IR) of the consensus hexanucleotide half-site (A/G)G(G/T)TCA with a single nucleotide spacer (IR-1), the bile acid response element of the Std promoter does not contain any spacer between the two hexanucleotide repeats (IR-0). A promoter-reporter construct carrying three tandem copies of the IR-0 containing -169/-193 element, linked to a minimal thymidine kinase promoter, can be stimulated more than 70-fold in transfected Caco-2 cells upon CDCA treatment. Autoregulation of the STD gene by its bile acid substrate may provide an important contributing role in the enterohepatic bile acid metabolism and cholesterol homeostasis.

Human dehydroepiandrosterone sulfotransferase: purification and characterization of a recombinant protein

Dehydroepiandrosterone sulfate is the most abundant sulfated steroid transformed in human tissues and serves as a precursor for steroid hormones. Recombinant human dehydroepiandrosterone sulfotransferase (DHEA-ST) expressed in glutathione sulfotransferase fusion form in E. coli was purified using glutathione sepharose 4B affinity adsorption chromatography, a Factor Xa cleavage step, and Q-sepharose fast flow column chromatography. The homogeneous preparation had an activity toward dehydroepiandrosterone (DHEA) of 150+/-40 nmol/min per mg of protein under the assay conditions at an overall yield of 38.4%. The recombinant human DHEA-ST was shown to have a subunit mass of 34 kDa by sodium dodecyl sulfate polyacrylamide gel electrophoresis, while having a molecular mass of 67.2 kDa by Superose-12 gel filtration. Our results indicate that the active recombinant enzyme expressed in E. coli is a homodimer.Biochemical properties for purified DHEA-ST were studied using DHEA as a substrate. The optimum pH ranged from pH 7 to 8, and the optimum temperature 40-45 degrees C. Ninety percent of basal DHEA-ST activity remained even after the enzyme was treated at 45 degrees C for 15 min. The 50% inactivation concentration of NaCl for DHEA-ST activity was determined to be around 500 mM. The K(m) value for DHEA was 1.9+/-0.3 microM and V(max)=190+/-18 nmol/min per mg of protein at 37 degrees C, pH 7.5.

Sulfation and glucuronidation of benzo[a]pyrene-7,8-dihydrodiol in intestinal mucosa of channel catfish (Ictalurus punctatus)

Intestinal metabolism plays a significant role in the bioavailability of ingested environmental toxicants. In this study, the potential for first pass, phase 2 biotransformation of benzo[a]pyrene-7,8-dihydrodiol (BaP-7,8-diol) in intestinal mucosa was examined. Sulfotransferase and Uridine 5'-Diphospho-Glucuronyl-transferase activity were measured in cytosol, and microsomes respectively. Radiolabeled conjugation products were analyzed by TLC and high-performance liquid chromatography (HPLC). The results indicated that BaP-7,8-diol was a poor substrate for intestinal sulfotransferase. Vmax for the sulfation of BaP-7,8-diol was 0.002 nmol mg-1 min-1, which is at least three orders of magnitudes lower than the Vmax for phenolic BaP metabolites. Studies with 3'phosphoadenosine-5' phosphosulfate (PAP)-35S as co-substrate showed that an unidentified compound in the reaction mixture was sulfated, dependent on the BaP-7,8-diol concentration. This could indicate that BaP-7,8-diol was interacting with a regulatory site on the enzyme and stimulated sulfation of an endogenous molecule in cytosol. Kinetic analysis of microsomal glucuronidation resulted in a Vmax of 0.30 nmol mg-1 min-1 (+/- 0.06 S.D., n = 4), with a Km of 23.39 microM (+/- 2.66 S.D.). The Km for the co-substrate UDP-glucuronic acid was approximately 43 microM. The slow rates for sulfation and glucuronidation of BaP-7,8-diol may explain its relatively high systemic availability when ingested or produced by intestinal phase 1 enzymes.

3'-Phosphoadenosine 5'-phosphosulfate binding site of flavonol 3-sulfotransferase studied by affinity chromatography and 31P NMR

The function of Lys-59, Arg-141, and Arg-277 in PAPS binding and catalysis of the flavonol 3-sulfotransferase was investigated. Affinity chromatography of conservative mutants with PAPS analogues allowed us to determine that Lys-59 interacts with the 5' portion of the nucleotide, while Arg-141 interacts with the 3' portion, confirming assignments deduced from the crystal structure of mouse estrogen sulfotransferase [Kakuta, Y., Pedersen, L. G., Carter, C. W. , Negishi, M., and Pedersen, L. C. (1997) Nat. Struct. Biol. 4, 904-908]. The affinity chromatography method could be used to characterize site-directed mutants for other types of enzymes that bind nucleoside 3',5'- or 2',5'-diphosphates. 31P NMR spectra of enzyme-PAP complexes were recorded for the wild-type enzyme and K59R and K59A mutants. The results of these experiments suggest that Lys-59 is involved in the determination of the proper orientation of the phosphosulfate group for catalysis.

Site-directed mutagenesis of rat hepatic hydroxysteroid sulfotransferases

Two cDNA clones of rat hepatic hydroxysteroid sulfotransferase (ST) (ST-40 and ST-20) were isolated and expressed in Escherichia coli cells. Several histidine residues in their coding regions are highly conserved in the ST superfamily, and histidine mutants were constructed by site-directed mutagenesis. The substitution of alanine or lysine for the histidine at position 98 in the ST-40 enzyme resulted in a loss of ST activities toward dehydroepiandrosterone (DHEA), androsterone (AD) and cortisol (CS). The mutation of histidine 98 into alanine abolished the specific binding to 3'-phosphoadenosine 5'-phosphate agarose, suggesting that the residue is located at a critical position in the 3'-phosphoadenosine 5'-phosphosulfate (PAPS) binding site. In the ST-20 enzyme, the replacement of histidine 98 with alanine also resulted in the loss of ST activity toward its preferential substrate, CS. In the ST-40 enzyme, the mutation at histidine 256 into alanine markedly reduced CS-ST activity, but DHEA-ST activity was not changed. Furthermore, selective decrease in CS-ST activity was also observed in the alanine mutant at lysine 254 or at asparagine 255 of the ST-40 enzyme. Kinetic analysis on the ST-40 and its mutant at asparagine 255 indicated that the Km value for CS was significantly increased in the mutant without any change in the Km values for 3'-phosphoadenosine 5'-phosphosulfate and DHEA. Inhibition studies demonstrated that DHEA-ST activity was competitively inhibited by AD, but not by CS in the ST-40 enzyme, whereas triethylamine, a noncompetitive inhibitor of hydroxysteroid ST, inhibited DHEA-ST activity in the ST-40 enzyme but did not inhibit CS-ST activity in either ST-40 or ST-20 enzymes. These data provide evidence that DHEA and CS bind to different sites, which probably function in a different manner in the ST-40 enzyme.

Structural relationships among members of the mammalian sulfotransferase gene family

Sulfotransferases constitute a superfamily of related enzymes that play critical roles in the regulation of steroid hormone action, neurotransmitter function, detoxification, and carcinogenesis. Understanding the functional relationships among these enzymes has so far been difficult due to their overlapping substrate specificities. To help clarify these relationships, we conducted a thorough and comprehensive molecular phylogenetic analysis of 25 different mammalian sulfotransferase cDNA and gene (St) sequences using maximum parsimony and distance matrix methods. This analysis suggested five distinct gene families: an alcohol/androgen/hydroxysteroid/dehydroepiandrosterone (Std) family, an aryl/minoxidil/phenol (Stp) family, an estrone/estrogen (Ste) family, a thyroid hormone family (St1b1), and a family (St1c1) defined so far only on the basis of its specificity for the carcinogen N-hydroxy-2-acetylaminofluorene. New insights obtained through this study include (1) a bootstrap analysis supporting the reliability of family subgroupings, (2) identification of an insertion that appears to be characteristic of the St1b1 and Stlc1 families, (3) identification of sequences likely to represent paralogs of multigene families, and (4) identification of species likely to contain, or not contain, orthologous multigene families and thus their specialized functions.

Identification of amino acid residues critical for catalysis and cosubstrate binding in the flavonol 3-sulfotransferase

The comparison of the deduced amino acid sequences of plant and animal sulfotransferases (ST) has allowed the identification of four well conserved regions, and previous experimental evidence suggested that regions I and IV might be involved in the binding of the cosubstrate, 3'-phosphoadenosine 5'-phosphosulfate (PAPS). Moreover, region IV is homologous to the glycine-rich phosphate binding loop (P-loop) motif known to be involved in nucleotide phosphate binding in several protein families. In this study, the function of amino acid residues within these two regions was investigated by site-directed mutagenesis of the plant flavonol 3-ST. In region I, our results identify Lys59 as critical for catalysis, since replacement of this residue with alanine resulted in a 300-fold decrease in specific activity, while a 15-fold reduction was observed after the conservative replacement with arginine. Photoaffinity labeling of K59R and K59A with [35S]PAPS revealed that Lys59 is not required for cosubstrate binding. However, the K59A mutant had a reduced affinity for 3'-phosphoadenosine 5'-phosphate (PAP)-agarose, suggesting that Lys59 may participate in the stabilization of an intermediate during the reaction. In region IV, all substitutions of Arg276 resulted in a marked decrease in specific activity. Conservative and unconservative replacements of Arg276 resulted in weak photoaffinity labeling with [35S]PAPS and the R276A/T73A and R276E enzymes displayed reduced affinities for PAP-agarose, suggesting that the Arg276 side chain is required to bind the cosubstrate. The analysis of the kinetic constants of mutant enzymes at residues Lys277, Gly281, and Lys284 allowed to confirm that region IV is involved in cosubstrate binding.

Structure-activity relationships of alkylamines that inhibit rat liver hydroxysteroid sulfotransferase activities in vitro

Tetraalkylammonium salts having n-propyl to n-amyl side chains inhibited rat liver sulfotransferase (ST) activities toward dehydroepiandrosterone and cortisol, but not ST activity toward 2-naphthol, whereas trialkylamines having ethyl to n-amyl side chains inhibited ST activity toward dehydroepiandrosterone, but not ST activities toward cortisol and 2-naphthol. A comparison of I50 values, which represent inhibitor concentration resulting in 50% inhibition of dehydroepiandrosterone ST activity, revealed that the values for the tetraalkylammonium salts were 0.015 to 0.017 mM, whereas the values for the trialkylamines were 0.20 to 0.33 mM. Introduction of hydrophilic groups such as hydroxyl, thiol, nitrile and acetamide groups or substitution by methyl and allyl groups in the alkyl side chains markedly diminished the inhibitory effect of triethylamine. These data indicate that ethyl to n-amyl side chains are a prerequisite for the alkylamine-type inhibitor. Tertiary amine drugs such as imipramine, dimenhydrinate, cyclizine, chlorpromazine and promethazine inhibited ST activities toward dehydroepiandrosterone and cortisol similar to the tetraalkylammonium salts, although the drugs were weaker inhibitors of hydroxysteroid ST activities. These results imply that in addition to trialkylamine side chains, the other portion of the drugs may participate in the inhibition of hydroxysteroid ST activities.

Biochemistry and molecular biology of drug-metabolizing sulfotransferase

Sulfation is an important conjugation reaction in the metabolism of various xenobiotics and endogenous compounds and is catalyzed by sulfotransferase (ST) present in cytosols. The cloning studies on STs have provided the basis for the understanding of the ST multigene family. STs are classified into hydroxysteroid (or alcohol), aryl (or phenol), estrogen, flavonol and polysaccharide STs and recent developments in the molecular characterization of these isoforms are reviewed. Regulation and localization of ST isoforms in various tissues are characterized at the molecular level by virtue of the specific antibodies and the corresponding cDNA probes. The recent developments are summarized. ST inhibitors are potent tools for the study on ST multiplicity and for the characterization of the enzyme structure. It also appears to be important to understand exogenous and endogenous ST inhibitors in clinical environment. The recent developments are reviewed.

Major hydroxysteroid sulfotransferase STa in rat liver cytosol may consist of two microheterogeneous subunits

The possible existence of two microheterogeneous subunits, designated ST-40P and ST-41P, of hydroxysteroid sulfotransferases in female Sprague-Dawley rat liver cytosol was demonstrated by cloning and sequencing of cDNAs, both isolated from two rat liver cDNA libraries. These subunits consisted of an equal number of amino acid residues with only one amino acid substitution. ST-40P and ST-41P expressed as homodimers from the ST-40 and ST-41 cDNAs in Escherichia coli had enzyme activities toward all of the examined 20 hydroxysteroids, 13 bile acids, and the carcinogen 5-hydroxymethylchrysene (5-HCR), with formation of the reactive metabolite 5-HCR sulfate, at rates very similar to those by STa, the major hydroxysteroid sulfotransferase in rat liver cytosol. This strongly suggested that they are essential components of STa. The present study carried out by using the recombinant enzymes provides the first direct evidence for the identity of sulfotransferases catalysing the sulfation of hydroxysteroids and bile acids and proposes that the current nomenclature system used for distinguishing hydroxysteroid sulfotransferases from bile acid sulfotransferases should be improved.

Molecular cloning and functions of rat liver hydroxysteroid sulfotransferases catalysing covalent binding of carcinogenic polycyclic arylmethanols to DNA

Three sulfotransferases (STs) catalysing the metabolic activation of potent carcinogenic polycyclic arylmethanols were purified from female Sprague-Dawley (SD) rat liver cytosol without loss of their enzyme activities in the presence of Tween 20 used for preventing the enzymes from aggregation during purification and identified as hydroxysteroid sulfotransferases (HSTs). All the purified HSTs, STa, STb, and STc, with different electric charges had an apparently equal size of subunit (30.5 kDa) and cross-reacted with polyclonal antibody raised against STa. Our study on molecular cloning of cDNA libraries from two female SD rat livers indicated that both contained cDNA inserts coding for 5 different HST subunits, consisting of 284-285 amino acid residues (M(r), 33,084-33,535) and sharing strong amino acid sequence identity (> 83%). Of the 5 HST subunits, two had an identical amino acid sequence except for only one amino acid residue, and the other two contained only 6 amino acid substitutions in their sequences.

Biochemistry of cytosolic sulfotransferases involved in bioactivation

Numerous studies have indicated that two classes of cytosolic STs are involved in the bioactivation of procarcinogens and drugs to reactive electrophiles, especially in rodent tissues. These two classes of STs are the hydroxysteroid STs, which are involved in the conjugation of hydroxymethyl PAHs, and the phenol STs involved in the sulfation of alkenylbenzenes and N-hydroxyarylamines. Purification studies of rat liver STs have clearly indicated that specific isoforms of hydroxysteroid and phenol STs are capable of sulfating procarcinogens in vitro. Rat liver STa and BAST I are structurally similar hydroxysteroid STs, which have been shown to sulfate and bioactive HMBA. Molecular cloning studies of the rat hydroxysteroid STs indicate that these enzymes are probably part of a family of closely related genes. The single human hydroxysteroid ST that has been characterized is very similar to the rat enzymes, but its role in the bioactivation of hydroxymethyl PAHs has not been established. Phenol STs have been demonstrated to have an important role in the bioactivation of alkenylbenzenes and N-hydroxyarylamines. Purification of rat phenol STs has identified several different forms, but only some appear to be involved in bioactivation of procarcinogens. Four isoforms (HAST I and II, AST III and IV) are apparently responsible for the majority of N-hydroxyarylamine sulfation. The relationship between these enzymes has not been established but they may represent similar enzymes. Different isoforms of rat phenol ST are also involved in the bioactivation of procarcinogens and drugs. However, the role of these phenol STs, PST-1, Mx-ST, and paracetamol ST, in carcinogenesis requires further study. In human tissues, only two phenol STs, P-PST and M-PST, have been identified. The role of these enzymes or unidentified STs in the sulfation of N-hydroxyarylamine procarcinogens has not yet been established. Initial reports of the molecular cloning and expression of the rat and human phenol ST genes will provide a valuable mechanism for the characterization of roles of the individual enzymes in bioactivation.

Immunochemical characterisation of a dehydroepiandrosterone sulfotransferase in rats and humans

A member of the rat liver hydroxysteroid sulfotransferase (ST) enzyme family metabolising dehydroepiandrosterone (DHEA) was purified from female rats and used to raise rabbit polyclonal antibodies. Characterisation of this antibody preparation demonstrated that it was specific for DHEA ST, and recognised a single 30-kDa protein on immunoblot analysis of rat liver cytosol which was expressed preferentially in female rat liver, and immunohistochemical localisation of the protein in female rat liver determined that DHEA ST was distributed homogeneously in the cytoplasm of hepatocytes. Examination of the extrahepatic expression of this protein showed it to be located predominantly in the liver, although a small amount of enzyme activity was found in the kidney which was not apparently subject to the same sex difference as the hepatic activity. Immunological analysis suggested that this activity was not due to the action of DHEA ST, but to another, unidentified ST isozyme. The antibody cross-reacted strongly with adult human liver DHEA ST, recognising a protein of 35 kDa on immunoblotting. Using this antibody preparation, the distribution of DHEA ST in mid-trimester human fetal tissues was examined, and it was shown that the enzyme is expressed in the adrenal and liver, but not to any significant extent in the kidney or lung. This antibody therefore provides a powerful tool for investigating the function of DHEA ST.

Immunochemical characterization of developmental changes in rat hepatic hydroxysteroid sulfotransferase

A major isoenzyme of hepatic androsterone-sulfating sulfotransferase (AD-ST) was purified from adult female rats. The activity was purified 122-fold over that found in the cytosol and showed a single protein band with a subunit molecular mass of 30 kDa after sodium dodecyl sulfate polyacrylamide gel electrophoresis. The purified enzyme exhibited four isoelectric variants of subunits on denaturing isoelectrofocusing gels (pI = 5.8, 6.1, 6.7 and 7.2). Rabbit antiserum raised against the enzyme specifically detected AD-ST polypeptide in rat liver cytosol. Immunoblot analysis of liver cytosol from female and male rats at various ages showed good correlation between the levels of AD-ST activity and AD-ST polypeptide. Significant levels of AD-ST activity and polypeptide were detected in senescent male rats, though normal adult male rats have very low levels of AD-ST activity and protein. The relative content of the isoelectric variants of AD-ST were different in liver cytosol of weanling and adult females, indicating that age- and gender-related alterations of hepatic AD-ST activity are primarily determined by the levels of AD-ST polypeptide and the relative amounts of the four isoelectric variants of the enzyme.

Bioactivation of 7-hydroxymethyl-12-methylbenz[a]anthracene by rat liver bile acid sulfotransferase I

The bioactivation of 7-hydroxymethyl-12-methylbenz[a]anthracene (HMBA) to an electrophilic sulfuric acid ester metabolite has been shown to be catalyzed by rat liver bile acid sulfotransferase I (BAST I). The sulfation and activation of HMBA by BAST I was determined by the ability of sulfated HMBA to form DNA adducts. The BAST I was also shown to react with rabbit anti-human dehydroepiandrosterone sulfotransferase antisera and to represent a major form of hydroxysteroid/bile acid sulfotransferase in female rat liver cytosol. Higher levels of BAST I activity and immunoreactivity as well as HMBA-DNA adduct formation were detected in female rat liver cytosol than in male rat liver cytosol. The bioactivation of HMBA by pure BAST I was dependent on the presence of 3'-phosphoadenosine 5'-phosphosulfate (PAPS) in the reaction and was inhibited by dehydroepiandrosterone, a physiological substrate for BAST I. Glutathione, a cellular nucleophile with important protective properties, decreased DNA adduct formation in the HMBA sulfation reaction in the absence of glutathione S-transferase activity. These results indicate the usefulness of BAST I to investigate the sulfation and activation of HMBA and probably other hydroxymethylated polyaromatic hydrocarbons to electrophilic and mutagenic metabolites under defined reaction conditions.

Molecular enzymology of human liver cytosolic sulfotransferases

Sulfoconjugation of drugs and small xenobiotic and endogenous compounds in human tissues is primarily catalysed by cytosolic forms of sulfotransferase. In most instances, sulfate conjugation is associated with a decrease in the biological activity and an increase in the excretion of the sulfated compounds. However, sulfation may also be involved in the bioactivation of both therapeutic drugs and carcinogens. Two isoenzymes of phenol sulfotransferase and one steroid/bile acid sulfotransferase have been isolated from human liver cytosol. Biochemical and immunological characterization of these proteins has provided insights into their structural similarities, tissue localization and possible biological functions.

Enzymic synthesis of steroid sulphates. XVII. On the structure of bovine estrogen sulphotransferase

Estrogen sulphotransferase plays a major role in controlling intracellular levels of 17 beta-estradiol in human mammary cancer cells and human endometrium. Bovine estrogen sulphotransferase c-DNA has recently been cloned; the encoded protein having a maximum Mr of 35,000 (Nash, A.R. et al. (1988) Aust. J. Biol. Sci. 41, 507-516). Enzyme of Mr 35,000 by SDS-PAGE has now been isolated and cyanogen bromide-cleaved peptides sequenced. The latter were identified in the c-DNA-predicted amino acid sequence which confirms that the active enzyme (Mr approximately 70,000) exists as a dimer of identical subunits. Sequence data on similar peptides isolated from an enzyme preparation containing a protein of Mr 74,000 as the major species on SDS-PAGE, which was previously thought to represent the enzyme, suggested that this protein was transferrin. This was confirmed by PAGE, SDS-PAGE, susceptibility to neuraminidase and reaction with bovine transferrin antibody. Isoelectric focusing experiments show that active enzyme exists in two or three polymorphic forms (pI values 5.3, 5.7 and possibly 5.9) having similar physicochemical properties of polymorphic forms of transferrin so that they overlap on ion-exchange chromatography and PAGE. The enzyme shows some homology to the amino acid sequence close to the Fe-binding site in lactoferrin and the question is raised as to the possible presence of a tightly bound metal in estrogen sulphotransferase involved in the binding of adenosine 3'-phosphate 5'-phosphosulphate.

Age- and sex-related differences in activation of the carcinogen 7-hydroxymethyl-12-methylbenz[a]anthracene to an electrophilic sulfuric acid ester metabolite in rats. Possible involvement of hydroxysteroid sulfotransferase activity

Metabolic activation of 7-hydroxymethyl-12-methylbenz[a]anthracene (HMBA) and related hydroxymethyl polycyclic aromatic hydrocarbons to electrophilic and mutagenic sulfuric acid esters has been demonstrated previously (Watabe et al., In: Xenobiotic Metabolism and Disposition (Eds. Kato R, Estabrook RW and Cayen MN), pp. 393-400. Taylor & Francis, London, 1989). In the present study, the rat hepatic sulfotransferase activity catalyzing the formation of such reactive sulfuric acid esters was inhibited strongly by dehydroepiandrosterone, a typical substrate hydroxysteroid sulfotransferases (HSSTs). Pentachlorophenol, a potent phenol sulfotransferase inhibitor, had little effect in this regard. A marked sex difference was observed for the hepatic cytosolic sulfotransferase activity for HMBA in rats. This sex difference was age-related; no significant difference was observed in preweanling rats, whereas in adult rats female rat liver showed a much higher enzyme activity. These age- and sex-related differences in the sulfonation of HMBA reflect the regulation of HMBA sulfotransferase activity by gonadal hormones as previously demonstrated with HSSTs. Thus, pretreatment with estradiol benzoate significantly enhanced the sulfotransferase activity for HMBA in both male and female rats, (P less than 0.01 and P less than 0.05 respectively), whereas testosterone propionate pretreatment decreased this activity. Castration of male rats increased the HMBA sulfotransferase activity 2- to 3-fold compared with that in control animals. By contrast, ovariectomy reduced the enzyme activity 38% in females. These results imply that rat liver HSST activity is responsible for the sulfonation of HMBA. Intraperitoneal injection of HMBA (0.25 mumol/g body wt) into infant rats produced benzylic DNA adducts in the liver which were chromatographically identical with those obtained from incubations of HMBA with deoxyguanosine and deoxyadenosine in the presence of hepatic cytosolic sulfotransferase activity. Intraperitoneal administration of sodium 7-sulfooxymethyl-12-methylbenz[a]anthracene resulted in much higher levels of these adducts and the deoxycytidine adduct in the liver DNA than did an equimolar amount of the parent hydroxymethyl hydrocarbon. The levels of hepatic benzylic DNA adducts formed from HMBA were reduced markedly by pretreatment of rats with dehydroepiandrosterone, a strong inhibitor of hepatic sulfotransferase activity for this hydrocarbon.

The strong hepatocarcinogenicity of the electrophilic and mutagenic metabolite 6-sulfooxymethylbenzo[a]pyrene and its formation of benzylic DNA adducts in the livers of infant male B6C3F1 mice

6-Hydroxymethylbenzo[a]pyrene was activated to an electrophilic and mutagenic sulfuric acid ester metabolite by rat and mouse liver sulfotransferase activity. The intrinsic mutagenicity of this reactive ester, 6-sulfooxymethylbenzo[a]pyrene, was inhibited by glutathione and glutathione S-transferase. A single i.p. dose of 2.5 nmol/g body wt of 6-sulfooxymethylbenzo[a]pyrene in infant male B6C3F1 mice induced liver tumors in 35 of 36 mice at 10 months with an average multiplicity of 4.4. A comparable dose of the parent hydrocarbon, 6-hydroxymethylbenzo[a]pyrene, was only a tenth as active. The electrophilic sulfuric acid ester produced high levels of benzylic DNA adducts in the livers of these mice that accounted for about 80% of the total DNA adducts. These results strongly suggest that this sulfuric acid ester is an important ultimate electrophilic and carcinogenic metabolite in carcinogenesis by 6-hydroxymethylbenzo[a]pyrene and possibly even by 6-methylbenzo[a]pyrene and benzo[a]pyrene in mouse liver.

Properties of estrogen and hydroxysteroid sulphotransferases in human mammary cancer

Partial purification (approximately x 140-fold) of estrogen sulphotransferase (EC 2.8.2.4) in human mammary estrogen receptor positive cancer tissue was achieved by affinity chromatography on adenosine-3',5'-diphosphate-agarose. It had a Mr of approximately 70,000 by gel filtration and upon electrophoresis on concave gradient polyacrylamide gels, showed a major (Mr 70,000) and a minor (Mr 200,000) peak of activity. Kinetics of this preparation (estradiol-17 beta and estrone as substrates), and also that of hydroxysteroid sulphotransferase (EC 2.8.2.2) contained in the cytosol of human mammary cancer MCF-7 cells (5-androstene-3 beta,17 beta-diol and dehydroepiandrosterone as substrates), were compared. The enzymes showed very similar behaviour, characterized by high affinity for their steroid substrates (low nM range) and co-operativity in their binding. For hydroxysteroid sulphotransferase, the adrenal-derived estrogen 5-androstene-3 beta,17 beta-diol was the preferred substrate compared to dehydro-epiandrosterone in the 0-40 nM concentration range. Such properties of the enzymes might be designed to limit the exposure of nuclear receptor to free ligand. Alternatively, a defined subcellular location would perhaps involve the enzymes in the elimination of estrogen after processing of the ligand-bound receptor.

cDNA cloning of the hydroxysteroid sulfotransferase STa sharing a strong homology in amino acid sequence with the senescence marker protein SMP-2 in rat livers

A cDNA encoding hydroxysteroid sulfotransferase a (STa), which catalyzes activation of carcinogenic polycyclic hydroxymethyl-arenes, was isolated from a lambda gtll cDNA expression library constructed from poly(A)+RNA of a female Sprague-Dawley (SD) rat liver. The cDNA, designated as ST-40, consisted of 1,015 base pairs which had an open reading frame of 852 base pairs encoding the entire rat STa subunit of 284 amino acids. The nucleotide base sequence of the ST-40 cDNA shared a strong homology of 94.4% with that of ST-20 cDNA encoding a hydroxysteroid ST which had been reported by us. The deduced amino acid sequence of STa had a homology of 73.7% with that of an SD rat liver senescence marker protein (SMP-2) consisting of 282 amino acid residues. However, STa was found to share a much stronger homology of 92% on the average with SMP-2 in their four specific regions corresponding to about 60% of the total sequences, indicating SMP-2 to be an isozyme of hydroxysteroid ST.

Cloning and sequence analysis of a rat liver cDNA encoding hydroxysteroid sulfotransferase

Nothing has been known of the cDNAs encoding sulfotransferases (STs) that catalyze sulfation of steroids and xenobiotics. In the present study, a female Sprague-Dawley (SD) rat liver cDNA library was screened with rabbit anti-serum raised against hydroxysteroid ST a (STa) purified from female SD rat liver cytosol. The cDNA isolated from the library consisted of 1,028 base pairs which had an open reading frame of 852 base pairs encoding the entire rat ST subunit of 284 amino acids. The N-terminal amino acid sequences of STa and the rat liver hydroxysteroid ST, bile acid ST I, both elucidated previously by the chemical method, had a strong homology with that deduced from the cDNA. Northern blot analysis of total RNAs from female and male rat livers showed a marked sex difference (female much greater than male) in the expressed level of the mRNA for the predicted ST subunit protein. A remarkable sex difference (female much greater than male) was also observed by immuno-blot analysis in the level of the hydroxysteroid ST protein(s) cross-reacting with the anti-serum in the rat liver cytosols.