Purification and kinetic characterization of a phenol-sulfating form of phenol sulfotransferase from human brain

Bioinformatic Analysis of Sulfotransferases from an Unexplored Gut Microbe, Sutterella wadsworthensis 3_1_45B: Possible Roles towards Detoxification via Sulfonation by Members of the Human Gut Microbiome

Sulfonation, primarily facilitated by sulfotransferases, plays a crucial role in the detoxification pathways of endogenous substances and xenobiotics, promoting metabolism and elimination. Traditionally, this bioconversion has been attributed to a family of human cytosolic sulfotransferases (hSULTs) known for their high sequence similarity and dependence on 3'-phosphoadenosine 5'-phosphosulfate (PAPS) as a sulfo donor. However, recent studies have revealed the presence of PAPS-dependent sulfotransferases within gut commensals, indicating that the gut microbiome may harbor a diverse array of sulfotransferase enzymes and contribute to detoxification processes via sulfation. In this study, we investigated the prevalence of sulfotransferases in members of the human gut microbiome. Interestingly, we stumbled upon PAPS-independent sulfotransferases, known as aryl-sulfate sulfotransferases (ASSTs). Our bioinformatics analyses revealed that members of the gut microbial genus Sutterella harbor multiple asst genes, possibly encoding multiple ASST enzymes within its members. Fluctuations in the microbes of the genus Sutterella have been associated with various health conditions. For this reason, we characterized 17 different ASSTs from Sutterella wadsworthensis 3_1_45B. Our findings reveal that SwASSTs share similarities with E. coli ASST but also exhibit significant structural variations and sequence diversity. These differences might drive potential functional diversification and likely reflect an evolutionary divergence from their PAPS-dependent counterparts.

Computational Analysis of Chemical Space of Natural Compounds Interacting with Sulfotransferases

The aim of this study was to investigate the chemical space and interactions of natural compounds with sulfotransferases (SULTs) using ligand- and structure-based in silico methods. An in-house library of natural ligands (hormones, neurotransmitters, plant-derived compounds and their metabolites) reported to interact with SULTs was created. Their chemical structures and properties were compared to those of compounds of non-natural (synthetic) origin, known to interact with SULTs. The natural ligands interacting with SULTs were further compared to other natural products for which interactions with SULTs were not known. Various descriptors of the molecular structures were calculated and analyzed. Statistical methods (ANOVA, PCA, and clustering) were used to explore the chemical space of the studied compounds. Similarity search between the compounds in the different groups was performed with the ROCS software. The interactions with SULTs were additionally analyzed by docking into different experimental and modeled conformations of SULT1A1. Natural products with potentially strong interactions with SULTs were outlined. Our results contribute to a better understanding of chemical space and interactions of natural compounds with SULT enzymes and help to outline new potential ligands of these enzymes.

Evaluation of a conserved tryptophanyl residue in donor substrate binding and catalysis by a phenol sulfotransferase (SULT1A1)

Structural determinations of members of the sulfotransferase (SULT) family suggest a direct interaction between a conserved tryptophanyl side chain and bound 3'-phosphoadenosine-5'-phosphate (PAP). We have prepared and purified mutants of the bovine SULT1A1, a very conserved homolog to the human SULT1A1, in which tryptophanyl-53 was sequentially trimmed to tyrosine, leucine, and alanine. Differential scanning fluorimetry indicated structural stabilities of the mutant proteins comparable to the wild type SULT1A1; however, less thermal stabilizations by PAP plus pentachlorophenol were observed with the mutants, suggesting weakened ligand binding. Protein fluorescence of the wild type enzyme decreased 6.5% upon binding PAP, whereas no changes occurred with the mutant enzymes. This reveals that W53, or its positional counterpart, has been the source of emission intensity changes used in previous investigations of other SULTs. Fluorescence resonance energy transfer from excited tryptophans to bound 7-hydroxycoumarin, as induced by PAP, indicated weakened binding of ligands to the mutant SULTs. This was also encountered and quantified in initial rate kinetic analyses. Ablation of the PAPS adenine-to-W53 ring interaction, shown by the W53A mutant enzyme, resulted in a 6.4-fold increase in K_PAPS and a 92% decrease in k_cat/K_PAPS. Measured K_PAPS values reveal the W53 indole ring contribution to PAPS binding to be 1.1 kcal/mol (4.6 kJ/mol). These results verify the structurally-inferred role for the π-π stacking interaction between PAP(S) and the conserved tryptophanyl residue in SULT1A1 and other members of the SULT family.

Sulfotransferase 1A1 Substrate Selectivity: A Molecular Clamp Mechanism

The human cytosolic sulfotransferases (SULTs) regulate hundreds, perhaps thousands, of small molecule metabolites and xenobiotics via transfer of a sulfuryl moiety (-SO3) from PAPS (3'-phosphoadenosine 5'-phosphosulfate) to the hydroxyls and primary amines of the recipients. In liver, where it is abundant, SULT1A1 engages in modifying metabolites and neutralizing toxins. The specificity of 1A1 is the broadest of any SULT, and understanding its selectivity is fundamental to understanding its biology. Here, for the first time, we show that SULT1A1 substrates separate naturally into two classes: those whose affinities are either enhanced ∼20-fold (positive synergy) or unaffected (neutral synergy) by the presence of a saturating nucleotide. kcat for the positive-synergy substrates is shown to be ∼100-fold greater than that of neutral-synergy compounds; consequently, the catalytic efficiency (kcat/Km) is approximately 3 orders of magnitude greater for the positive-synergy species. All-atom dynamics modeling suggests a molecular mechanism for these observations in which the binding of only positive-synergy compounds causes two phenylalanine residues (F81 and 84) to reposition and "sandwich" the phenolic moiety of the substrates, thus enhancing substrate affinity and positioning the nucleophilic oxygen for attack. Molecular dynamics movies reveal that the neutral-synergy compounds "wander" about the active site, infrequently achieving a reactive position. In-depth analysis of select point mutants strongly supports the model and provides an intimate view of the interdependent catalytic functions of subsections of the active site.

Design and Interpretation of Human Sulfotransferase 1A1 Assays

The human sulfotransferases (SULTs) regulate the activities of hundreds, if not thousands, of small molecule metabolites via transfer of the sulfuryl-moiety (-SO3) from the nucleotide donor, 3'-phosphoadenosine 5'-phosphosulfate (PAPS) to the hydroxyls and amines of the recipients. Our understanding of the molecular basis of SULT catalysis has expanded considerably in recent years. The basic kinetic mechanism of these enzymes, previously thought to be ordered, has been redefined as random for SULT2A1, a representative member of the superfamily. An active-site cap whose structure and dynamics are highly responsive to nucleotides was discovered and shown to be critical in determining SULT selectivity, a topic of longstanding interest to the field. We now realize that a given SULT can operate in two specificity modes-broad and narrow-depending on the disposition of the cap. More recent work has revealed that the caps of the SULT1A1 are controlled by homotropic allosteric interactions between PAPS molecules bound at the dimer's active sites. These interactions cause the catalytic efficiency of SULT1A1 to vary in a substrate-dependent fashion by as much as two orders of magnitude over a range of PAPS concentrations that spans those found in human tissues. SULT catalysis is further complicated by the fact that these enzymes are frequently inhibited by their substrates. This review provides an overview of the mechanistic features of SULT1A1 that are important for the design and interpretation of SULT1A1 assays.

3'-Phosphoadenosine 5'-phosphosulfate allosterically regulates sulfotransferase turnover

Human cytosolic sulfotransferases (SULTs) regulate the activities of thousands of small molecules—metabolites, drugs, and other xenobiotics—via the transfer of the sulfuryl moiety (-SO₃) from 3′-phosphoadenosine 5′-phosphosulfate (PAPS) to the hydroxyls and primary amines of acceptors. SULT1A1 is the most abundant SULT in liver and has the broadest substrate spectrum of any SULT. Here we present the discovery of a new form of SULT1A1 allosteric regulation that modulates the catalytic efficiency of the enzyme over a 130-fold dynamic range. The molecular basis of the regulation is explored in detail and is shown to be rooted in an energetic coupling between the active-site caps of adjacent subunits in the SULT1A1 dimer. The first nucleotide to bind causes closure of the cap to which it is bound and at the same time stabilizes the cap in the adjacent subunit in the open position. Binding of the second nucleotide causes both caps to open. Cap closure sterically controls active-site access of the nucleotide and acceptor; consequently, the structural changes in the cap that occur as a function of nucleotide occupancy lead to changes in the substrate affinities and turnover of the enzyme. PAPS levels in tissues from a variety of organs suggest that the catalytic efficiency of the enzyme varies across tissues over the full 130-fold range and that efficiency is greatest in those tissues that experience the greatest xenobiotic “load”.

Human sulfotransferase 1A1-dependent mutagenicity of 12-hydroxy-nevirapine: the missing link?

Nevirapine (NVP) is a frequently used anti-HIV drug. Despite its efficacy, NVP has been associated with serious skin and liver injuries in exposed patients and with increased incidences of hepatoneoplasias in rodents. Current evidence supports the involvement of reactive metabolites in the skin and liver toxicities of NVP, formed by cytochrome P450-mediated oxidations and/or subsequent phase II sulfonation. However, to date, standard in vitro genotoxicity tests have provided no evidence that NVP is either mutagenic or clastogenic. The human sulfotransferase 1A1-dependent mutagenicity of 12-hydroxy-NVP, one of the major metabolites of NVP, is demonstrated here.

Paradigms of sulfotransferase catalysis: the mechanism of SULT2A1

Human cytosolic sulfotransferases (SULTs) regulate the activities of thousands of signaling small molecules via transfer of the sulfuryl moiety (-SO3) from 3'-phosphoadenosine 5'-phosphosulfate (PAPS) to the hydroxyls and primary amines of acceptors. Sulfonation controls the affinities of ligands for their targets, and thereby regulates numerous receptors, which, in turn, regulate complex cellular responses. Despite their biological and medical relevance, basic SULT mechanism issues remain unresolved. To settle these issues, and to create an in-depth model of SULT catalysis, the complete kinetic mechanism of a representative member of the human SULT family, SULT2A1, was determined. The mechanism is composed of eight enzyme forms that interconvert via 22 rate constants, each of which was determined independently. The result is a complete quantitative description of the mechanism that accurately predicts complex enzymatic behavior. This is the first description of a SULT mechanism at this resolution, and it reveals numerous principles of SULT catalysis and resolves previously ambiguous issues. The structures and catalytic behaviors SULTs are highly conserved; hence, the mechanism presented here should prove paradigmatic for the family.

The gate that governs sulfotransferase selectivity

Human cytosolic sulfotransferases (SULTs) transfer the sulfuryl moiety (-SO(3)) from activated sulfate [3'-phosphoadenosine 5'-phosphosulfate (PAPS)] to the hydroxyls and primary amines of numerous metabolites, drugs, and xenobiotics. Receipt of the sulfuryl group often radically alters acceptor-target interactions. How these enzymes select particular substrates from the hundreds of candidates in a complex cytosol remains an important question. Recent work reveals PAPS binding causes SULT2A1 to undergo an isomerization that controls selectivity by constricting the opening through which acceptors must pass to enter the active site. The enzyme maintains an affinity for large substrates by isomerizing between the open and closed states with nucleotide bound. Here, the molecular basis of the nucleotide-induced closure is explored in equilibrium and nonequilibrium molecular dynamics simulations. The simulations predict that the active-site "cap," which covers both the nucleotide and acceptor binding sites, opens and closes in response to nucleotide. The cap subdivides into nucleotide and acceptor halves whose motions, while coupled, exhibit an independence that can explain the isomerization. In silico weakening of electrostatic interactions between the cap and base of the active site causes the acceptor half of the cap to open and close while the nucleotide lid remains shut. Simulations predict that SULT1A1, the most abundant SULT in human liver, will utilize a similar selection mechanism. This prediction is tested using fulvestrant, an anti-estrogen too large to pass through the closed pore, and estradiol, which is not restricted by closure. Equilibrium and pre-steady-state binding studies confirm that SULT1A1 undergoes a nucleotide-induced isomerzation that controls substrate selection.

Methamphetamine regulation of sulfotransferase 1A1 and 2A1 expression in rat brain sections

Sulfotransferase catalyzed sulfation regulates the biological activities of various neurotransmitters/hormones and detoxifies xenobiotics. Rat sulfotransferase rSULT1A1 catalyzes the sulfation of neurotransmitters and xenobiotic phenolic compounds. rSULT2A1 catalyzes the sulfation of hydroxysteroids and xenobiotic alcoholic compounds. In this work, Western blot and real-time RT-PCR were used to investigate the effect of methamphetamine on rSULT1A1 and rSULT2A1 protein and mRNA expression in rat cerebellum, frontal cortex, hippocampus, and striatum. After 1-day treatment, significant induction of rSULT1A1 was observed only in the cerebellum; rSULT2A1 was induced significantly in the cerebellum, frontal cortex, and hippocampus. After 7 days of exposure, rSULT1A1 was induced in the cerebellum, frontal cortex, and hippocampus, while rSULT2A1 was induced significantly in all four regions. Western blot results agreed with the real-time RT-PCR results, suggesting that the induction occurred at the gene transcriptional level. Results indicate that rSULT1A1 and rSULT2A1 are expressed in rat frontal cortex, cerebellum, striatum, and hippocampus. rSULT1A1 and rSULT2A1are inducible by methamphetamine in rat brain sections in a time dependable manner. rSULT2A1 is more inducible than rSULT1A1 by methamphetamine in rat brain sections. Induction activity of methamphetamine is in the order of cerebellum>frontal cortex, hippocampus>striatum. These results suggest that the physiological functions of rSULT1A1 and rSULT2A1 in different brain regions can be affected by methamphetamine.

Roles of human sulfotransferases in genotoxicity of carcinogens using genetically engineered umu test strains

Human sulfotransferase (SULT) 1A1, 1A2, and 1A3 cDNA genes were subcloned separately into the pTrc99A(KM) vector. The generated plasmids were introduced into the Salmonella typhimurium O-acetyltransferase-deficient strain NM6000 (TA1538/1,8-DNP/pSK1002), resulting in the new strains NM7001, NM7002, and NM7003. We compared the sensitivities of these three strains with the parental strain NM7000 against 51 chemicals including aromatic amines, nitroarenes, alkenylbenzenes, estrogens-like chemicals, and other compounds with and without S9 mix by making use of the umu test system that is based on the bacterial SOS induction. 2-Amino-6-methyl-dipyrido[1,2-α:3',2'-d]imidazole, 3-methoxy-4-aminoazobenzene, 3-nitrobenzanthrone, 5-nitroacenaphthene, and 3,9-dinitrofluoranthene caused high genotoxicity in the NM7001 strain. The genotoxic effects of 2-aminofluorene, 2-acetylaminofluorene, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine, 2-nitrofluorene, 1-nitropyrene, and 2-nitropropane were stronger in the NM7002 strain compared with the NM7001 and NM7003 strains. Among the tested benzylic and allylic compounds, 1-hydroxymethylpyrene was detected in the NM7001 strain with the highest sensitivity. Estragole and 1'-hydroxysafrole exhibited strong genotoxicity in the NM7003 strain. The estrogen-like chemicals such as bisphenol A, genistein, p,n-nonylphenol, and 4-hydroxytamoxifen were not detected as genotoxins in any strain used. Collectively, the present results suggest that the generated test strains are valuable tools in order to elucidate the role of SULT enzymes in the bioactivation of chemicals to environmental carcinogens.

Catalytic mechanism of Golgi-resident human tyrosylprotein sulfotransferase-2: a mass spectrometry approach

Human tyrosylprotein sulfotransferases catalyze the transfer of a sulfuryl moiety from the universal sulfate donor PAPS to the hydroxyl substituent of tyrosine residues in proteins and peptides to yield tyrosine sulfated products and PAP. Tyrosine sulfation occurs in the trans-Golgi network, affecting an estimated 1% of the tyrosine residues in all secreted and membrane-bound proteins in higher order eukaryotes. In this study, an effective LC-MS-based TPST kinetics assay was developed and utilized to measure the kinetic properties of human TPST-2 and investigate its catalytic mechanism when G protein-coupled CC-chemokine receptor 8 (CCR8) peptides were used as acceptor substrates. Through initial rate kinetics, product inhibition studies, and radioactive-labeling experiments, our data strongly suggest a two-site ping-pong model for TPST-2 action. In this mechanistic model, the enzyme allows independent binding of substrates to two distinct sites, and involves the formation of a sulfated enzyme covalent intermediate. Some insights on the important amino acid residues at the catalytic site of TPST-2 and its covalent intermediate are also presented. To our knowledge, this is the first detailed study of the reaction kinetics and mechanism reported for human TPST-2 or any other Golgi-resident sulfotransferase.

Expression and localization of cytosolic sulfotransferase (SULT) 1A1 and SULT1A3 in normal human brain

Cytosolic sulfotransferases (SULTs) are a family of Phase II drug-metabolizing enzymes that catalyze the transfer of a sulfonate group from 3'-phosphoadenosine 5'-phosphosulfate to endogenous and xenobiotic compounds. Several SULT isoform messages have been detected in the human brain; however, protein expression patterns have not been characterized. Immunoblot analysis of the SULT1A1 and 1A3 isoforms was carried out with cytosolic fractions isolated from superior temporal gyrus, hippocampus, cerebellum, occipital pole, frontal pole, and temporal pole regions of normal adult human brains. SULT1A1 expression was highest in cytosolic fractions isolated from cerebellum, occipital, and frontal lobes, whereas, SULT1A3 expression was highest in cytosol from superior temporal gyrus, hippocampus, and temporal lobe. SULT1A1 and SULT1A3 immunoreactivities were found in both neurons and glial cells by immunohistochemical analysis in all brain regions studied. SULT1A1 is known to catalyze the metabolism of small phenols, whereas SULT1A3 sulfates catecholamine neurotransmitters. Because SULT1A1 and 1A3 have distinct substrate specificities, the differences in expression pattern and cellular localization of the SULT1A isoforms are probably associated with the distribution and function of their selective substrates in the different brain regions.

Isotope exchange at equilibrium indicates a steady state ordered kinetic mechanism for human sulfotransferase

Cytosolic sulfotransferase (SULT)-catalyzed sulfation regulates biosignaling molecular biological activities and detoxifies hydroxyl-containing xenobiotics. The universal sulfuryl group donor for SULTcatalyzed sulfation is adenosine 3'-phosphate 5'-phosphosulfate (PAPS). The reaction products are a sulfated product and adenosine 3',5'-diphosphate (PAP). Although the kinetics has been reported since the 1980s,SULT-catalyzed reaction mechanisms remain unclear. Human SULT1A1 catalyzes the sulfation of xenobiotic phenols and has very broad substrate specificity. It has been recognized as one of the most important phase II drug-metabolizing enzymes. Understanding the kinetic mechanism of this isoform is important in understanding drug metabolism and xenobiotic detoxification. In this report, we investigated the SULT1A1-catalyzed phenol sulfation mechanism. The SULT1A1-catalyzed reaction was brought to equilibrium by varying substrate (1-naphthol) and PAPS initial concentrations. Equilibrium constants were determined. Two isotopic exchanges at equilibrium ([14C]1-naphthol <=>[14C]1-naphthyl sulfate and[35S]PAPS<=>[35S]1-naphthyl sulfate) were conducted. First-order kinetics, observed for all the is otopic exchange reactions studied over the entire time scale that was monitored, indicates that the system was truly at equilibrium prior to addition of an isotopic pulse. Complete suppression of the 35S isotopic exchange rate was observed with an increase in the levels of 1-naphthol and 1-naphthyl sulfate in a constant ratio,while no suppression of the 14C exchange rate was observed with an increase in the levels of PAPS and PAP in a constant ratio. Data are consistent with a steady state ordered kinetic mechanism with PAPS and PAP binding to the free enzyme.

Mutagenicity of N-nitrosodiethanolamine in a V79-derived cell line expressing two human biotransformation enzymes

N-nitrosodiethanolamine (NDELA) has demonstrated carcinogenic activity in various rodent models. However, it is negative or only weakly active in standard in vitro genotoxicity assays. This poor response might be due to the requirement of specific enzymes for its activation. Previous work indicated that cytochrome P450 (CYP) 2E1, alcohol dehydrogenases and sulphotransferases (SULTs) can convert NDELA into reactive metabolites. We report here that NDELA induces concentration-dependent gene mutations (at the hprt locus) in V79-hCYP2E1-hSULT1A1 cells, engineered for expression of human CYP2E1 and human SULT1A1, but is inactive in parental V79 cells. Mutagenicity of NDELA in V79-hCYP2E1-hSULT1A1 cells was abolished by the CYP2E1 inhibitor 1-aminobenzotriazole, but unaffected by the SULT1A1 inhibitor pentachlorophenol. The efficiency and specificity of these inhibitors was demonstrated in gene mutation assays using SULT- and CYP2E1-dependent reference mutagens, 2-nitropropane and N-nitrosodimethylamine, respectively. In this study, it is documented for the first time that NDELA can induce gene mutations in mammalian cells. Whereas human CYP2E1 was required for its activation, human SULT1A1 was not involved either in its activation or its inactivation in our cell model.

Reversible covalent inhibition of a phenol sulfotransferase by coenzyme A

Phenol sulfotransferases (SULTs), which normally bind 3'-phosphoadenosine-5'-phosphosulfate as the donor substrate, are inhibited by CoA and its thioesters. Here, we report that inhibition of bovine SULT1A1 by CoA is time-dependent at neutral pH under non-reducing conditions. The rates of inactivation by CoA indicate an initial reversible SULT:CoA complex with a dissociation constant of 5.7 microM and an inactivation rate constant of 0.07 min(-1). Titrations with CoA and prolonged incubations reveal that inactivation of the dimeric enzyme is stoichiometric, consistent with the observation of complete conversion of the protein to a slightly decreased electrophoretic mobility. Both activity and normal electrophoretic migration are restored by 2-mercaptoethanol. Mutagenesis demonstrated that Cys168 is the site of CoA adduction, and a consistent model was constructed that reveals a new SULT molecular dynamic. Cysteine reaction kinetics with Ellman's reagent revealed a PAPS-induced structural change consistent with the model that accounts for binding of CoA.

Phenol sulfotransferase 1A1 activity in human liver: kinetic properties, interindividual variation and re-evaluation of the suitability of 4-nitrophenol as a probe substrate

Sulfation is an important metabolic pathway in humans for xenobiotics, hormones and neurotransmitters, and is catalysed by the cytosolic sulfotransferase (SULT) enzymes. Phenol SULTs, especially SULT1A1, are particularly important in xenobiotic and drug metabolism because of their broad substrate specificity and extensive tissue distribution. A common variant SULT1A1 allozyme (SULT1A1*2) exists in the population, and is less stable than the wild-type SULT1A1*1. 4-Nitrophenol is widely used as a substrate for quantifying SULT1A1 activity. However, our kinetic experiments suggest that 4-nitrophenol is not an ideal substrate when determining SULT1A1 activity in human liver. Assays with a bank of 68 human liver cytosols revealed three distinct kinetic profiles for 4-nitrophenol sulfation in the population: linear, biphasic and inhibition. Sulfation of 4-nitrophenol by purified, recombinant SULT1A1*1 and SULT1A1*2 shows marked substrate inhibition, with inhibition at 4-nitrophenol concentrations greater than 4 and 10 microM, respectively. Furthermore, sulfation of 4-nitrophenol by purified recombinant SULT1B1 was significant at concentrations of 4-nitrophenol less than 10 microM. Western blots showed that the SULT1A1 levels in liver are highly variable between liver samples and that no correlation was observed between SULT1A1 activity and protein level in liver cytosols. However, a correlation between SULT1A1 activity and protein level was observed in human placental cytosols, where SULT1B1 is not expressed. We believe that in human liver other SULT isoforms (particularly SULT1B1) contribute to the sulfation of 4-nitrophenol. Therefore, 4-nitrophenol is not an ideal substrate with which to quantitate SULT1A1 activity in human liver tissue.

Spectrofluorimetric analysis of 7-hydroxycoumarin binding to bovine phenol sulfotransferase

Phenol sulfotransferases (SULT1s, EC 2.8.2.1) catalyze sulfuryl group transfer from 3'-phosphoadenosine-5'-phosphosulfate (PAPS) to the hydroxyl oxygen of aromatic acceptor substrates. Previous work with the bovine SULT1A1 has utilized the highly fluorescent substrate 7-hydroxycoumarin (7-HC, umbelliferone) as an acceptor substrate [Biochem. Biophys. Res. Commun. 261 (1999) 815]. Here we report that adenosine-3',5'-bisphosphate (PAP)-dependent binding of 7-HC to bSULT1A1 can be observed due to the appearance of a 400-420-nm shoulder in the emission spectrum, using an excitation wavelength of 280 nm. This emission was observed by placing 7-HC in ethanol, which is consistent with bSULT1A1 phenol binding site hydrophobicity. Titrations with 7-HC indicate a K(d) for 7-HC of 0.58 microM and substoichiometric binding to the homodimeric enzyme. The bSULT1A1:PAP:7-HC complex could be disrupted with pentachlorophenol (PCP), titrations with which indicated 0.5 equivalents per enzyme subunit. Titrations of enzyme plus 7-HC with PAP also indicated 0.5 equivalents per enzyme subunit. These results suggest a model of homodimeric bSULT1A1 in which subunit interactions favor half-site reactivity in the formation of a dead end complex.

Inhibition of bovine phenol sulfotransferase (bSULT1A1) by CoA thioesters. Evidence for positive cooperativity and inhibition by interaction with both the nucleotide and phenol binding sites

Previous work with the bovine phenol sulfotransferase (bSULT1A1, EC ) demonstrated inhibition by CoA that was competitive with respect to the sulfuryl donor substrate, 3'-phosphoadenosine-5'-phosphosulfate (PAPS) (Leach, M., Cameron, E., Fite, N., Stassinopoulos, J., Palmreuter, N., and Beckmann, J. D. (1999) Biochem. Biophys. Res. Commun. 261, 815-819). Here we report that long chain acyl-CoAs are more potent inhibitors of bSULT1A1 and also of human dopamine sulfotransferase (SULT1A3) when compared with unesterified CoA and short chain-length acyl-CoAs. A complex pattern of inhibition was revealed by systematic variation of palmitoyl-CoA, PAPS, and 7-hydroxycoumarin, the acceptor substrate. Convex plots of apparent K(m)/V(max) versus [palmitoyl-CoA] were adequately modeled using an ordered rapid equilibrium scheme with PAPS as the leading substrate and by accounting for the possible binding of two equivalents of inhibitor to the dimeric enzyme. Interestingly, the first K(i) of 2-3 microm was followed by a second K(i) of only 0.01-0.05 microm, suggesting that positive subunit cooperativity enhances binding of long chain acyl-CoAs to this sulfotransferase. Simultaneous interaction of palmitoyl-CoA with both the nucleotide and phenol binding sites is suggested by two experiments. First, the acyl-CoA displaced 7-hydroxycoumarin from the highly fluorescent bSULT1A1.PAP.7-HC complex in a cooperative manner. Second, palmitoyl-CoA prevented the quenching of bSULT1A1 fluorescence observed with pentachlorophenol. Finally, titrations of bSULT1A1-pentachlorophenol complex with palmitoyl-CoA caused the return of protein fluorescence, and the binding of palmitoyl-CoA was highly cooperative (Hill constant of 1.9). Overall, these results suggest a model of sulfotransferase inhibition in which the 3'-phosphoadenosine-5'-diphosphate moiety of CoA docks to the PAPS domain, and the acyl-pantetheine group docks to the hydrophobic phenol binding domain.

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.

Crystal structure-based studies of cytosolic sulfotransferase

Sulfation is a widely observed biological reaction conserved from bacterium to human that plays a key role in various biological processes such as growth, development, and defense against adversities. Deficiencies due to the lack of the ubiquitous sulfate donor 3'-phosphoadenosine-5'-phosphosulfate (PAPS) are lethal in humans. A large group of enzymes called sulfotransferases catalyze the transfer reaction of sulfuryl group of PAPS to the acceptor group of numerous biochemical and xenochemical substrates. Four X-ray crystal structures of sulfotransferases have now been determined: cytosolic estrogen, hydroxysteroid, aryl sulfotransferases, and a sulfotransferase domain of the Golgi-membrane heparan sulfate N-deacetylase/N-sulfotransferase 1. These have revealed the conserved core structure of the PAPS binding site, a common reaction mechanism, and some information concerning the substrate specificity. These crystal structures introduce a new era of the study of the sulfotransferases.

Inhibition and binding studies of coenzyme A and bovine phenol sulfotransferase

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

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

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

Cooperative ligand binding by bovine phenol sulfotransferase

Although several phenol sulfotransferases (PSTs) that metabolize hormones and xenobiotics have been purified and examined by steady state kinetic methods, little is known about ligand binding and subunit interactions in these enzymes. Inhibition of a purified recombinant homodimeric bovine PST by 2,6-dichloro-4-nitrophenol (DCNP) and pentachlorophenol (PCP) displayed very sharp titration curves that required modeling with Hill equations with slope factors of 2 and 3, respectively. This observation suggested positive cooperative ligand binding during catalytic turnover. The binding of PCP was also monitored by intrinsic protein fluorescence, which was quenched up to 36% upon saturation with the inhibitor. In the absence of 3'-phosphoadenosine-5'-phosphosulfate (PAPS), quenching curves were fit with the Hill equation and an interaction factor of 1. In contrast, inclusion of PAPS increased the association of PCP and resulted in positive cooperative binding with an interaction factor of 1.6-1.9. Whereas adenosine-3',5'-diphosphate (PAP) also facilitated cooperative binding of PCP, adenosine-5'-monophosphate (AMP) was not effective. This correlated to inhibition of PST by PAP, whereas AMP was not inhibitory up to 1 mM. Therefore, occupation of the PST nucleotide binding site(s) facilitates a subunit interaction that can promote subsequent binding of phenolic ligands.

Characterization of an N-acetylglucosamine-6-O-sulfotransferase from human respiratory mucosa active on mucin carbohydrate chains

A microsomal GlcNAc-6-O-sulfotransferase activity from human bronchial mucosa, able to transfer a sulfate group from adenosine 3'-phosphate 5'-phosphosulfate onto methyl-N-acetylglucosaminides or terminal N-acetylglucosamine residues of carbohydrate chains from human respiratory mucins, has been characterized. The reaction products containing a terminal HO3S-6GlcNAc were identified by high performance anion-exchange chromatography. Using methyl-beta-N-acetylglucosaminide as a substrate, the optimal activity was obtained with 0.1% Triton X-100, 30 mM NaF, 20 mM Mn2+, 5 mM AMP in a 30 mM MOPS (3-(N-morpholino) propanesulfonic acid) buffer at pH 6.7. The apparent Km values for adenosine 3'-phosphate 5'-phosphosulfate and methyl-beta-N-acetylglucosaminide were observed at 9.1 x 10(-6) M and 0.54 x 10(-3) M, respectively. The enzyme had more affinity for carbohydrate chains with a terminal GlcNAc residue than for methyl-beta-N-acetylglucosaminide; it was unable to catalyze the transfer of sulfate to position 6 of the GlcNAc residue contained in a terminal Galbeta1-4GlcNAc sequence. However, oligosaccharides with a nonreducing terminal HO3S-6GlcNAc were substrates for a beta1-4 galactosyltransferase from human bronchial mucosa. These data point out that GlcNAc-6-O-sulfotransferase must act before beta1-4 galactosylation in mucin-type oligosaccharide biosynthesis.

High level expression and characterization of recombinant human hippocampus phenol sulfotransferase: a novel phenol-sulfating form of phenol sulfotransferase

Phenol sulfotransferases (PSTs) represent a family of sulfotransferase enzymes that modify the biologic activities and excretion of phenolic compounds and monoamines. A novel human hippocampal PST (H-PST) cDNA with homology to phenol (P) and monoamine (M) forms of PST was previously isolated from brain. To compare the biochemical properties of H-PST with that of phenol (P-PST) and monoamine (M-PST) sulfotransferases, high level expression of recombinant H-PST was achieved in this study with the pET3c vector in BL21(DE3) Escherichia coli cells. Expression was demonstrated by isopropyl beta-D-thiogalactopyranoside induction of 34-kDa H-PST that represented 5-10% of total E. coli proteins. Purification by ion-exchange chromatography on DEAE-Sepharose yielded more than 2 mg of H-PST. Characterization showed that H-PST exists as a homodimer of 60-65 kDa by gel filtration chromatography. H-PST prefers p-nitrophenol as substrate and does not sulfate dopamine or neuropeptide substrates. Kinetic studies showed that H-PST possessed K(m(app)) and Vmax(app) values of 3 microM p-nitrophenol and 160 nmol/min/mg, respectively. H-PST was sensitive to inhibition by DCNP (2,6-dichloro-4-nitrophenol). H-PST is thermolabile since its activity was reduced upon preincubation at 37 degrees C. These results indicate that H-PST shows similarities and differences compared to P-PST and M-PST sulfotransferases. P-PST prefers p-nitrophenol as substrate, is sensitive to inhibition by DCNP, and is thermostable; in contrast, M-PST prefers monoamines as substrate, is not sensitive to DCNP, and is thermolabile. The distinct profile of biochemical properties of H-PST, and its primary sequence homology to P-PST and M-PST, suggests that H-PST represents a novel allelic variant of human phenol sulfotransferases. Importantly, this study demonstrates that high level expression of H-PST allows determination of distinguishing characteristics of variant forms of PSTs.

Retinoic acid inhibits hydrocortisone-stimulated expression of phenol sulfotransferase in bovine bronchial epithelial cells

The airway epithelium, which is commonly exposed to xenobiotics, contains the conjugative enzyme phenol sulfotransferase (PST). We have previously reported that hydrocortisone (HC) stimulates the expression of PST severalfold in cultured bovine bronchial epithelial cells (Beckmann et al., 1994, J. Cell. Physiol. 160:603-610). Here we report that this stimulation is attenuated by retinoic acid (RA). Dose-response measurements of both enzyme activities and mRNA levels indicated a 50% inhibition of HC-stimulated PST expression with 0.05 nM RA. Varied concentrations of RA had a general repressive effect on HC-stimulated PST expression, with no change in the half-maximal HC stimulatory concentration of 12.5 nM. Steady state kinetic measurements indicated no significant changes in apparent Km values of 3-5 microM for the acceptor substrate, 2-naphthol; only HC- and RA-dependent changes in Vmax were observed. These changes were likely due to altered enzyme expression, as evidenced by immunoblot and Northern blot hybridization analyses. Thus, the expression of PST within bronchial epithelial cells is not merely constitutive, but is subject to both positive and negative controls.

N-sulphation of desipramine in the rat brain

1. Amine N-sulphotransferase (NST) activity with desipramine (DMI) as substrate was assayed in vitro in various areas of the rat brain. Biosynthesis of 3'-phosphoadenosine-5'-phospho35sulphate (PAPS) from sodium 35sulphate and ATP was also measured by coupling it to the sulphation of minoxidil by minoxidil sulphotransferase (MST). 2. For the DMI-NST reaction, an apparent Km = 0.5 mM was obtained for DMI and two apparent Kms = 0.3 and 1.7 microM for PAPS, whereas in the PAPS-generating reactions, Km for sodium 35sulphate = 20 microM. 3. Both the enzyme activities were widely distributed in rat brain. The rate of NST activity was 2-3 orders of magnitude lower than that of PAPS generation. N-sulphoconjugation of DMI, which is proposed as a possible biotransformation pathway of DMI in the rat brain, could conceivably be supported adequately by the 'active sulphate' generated within the same areas of the brain.

Characterization of a sulfotransferase from human airways responsible for the 3-O-sulfation of terminal galactose in N-acetyllactosamine-containing mucin carbohydrate chains

A galactose 3-O-sulfotransferase activity able to transfer a sulfate group from adenosine 3'-phosphate 5'-phosphosulfate to methyl galactosides or terminal N-acetyllactosamine-containing carbohydrate chains from human respiratory mucins was characterized in microsomal fractions prepared from human respiratory mucosa. The reaction products, methyl alpha- or beta-galactose 3-sulfate and three oligosaccharide alditols containing the sequence HSO3-3Gal beta 1-4GlcNAc beta 1-6GalNAc-itol were identified by high performance anion-exchange chromatography. Using methyl beta-galactoside as a substrate, the optimum activity was obtained with 0.1% Triton X-100, 30 mM NaF, 20 mM Mn2+, and 10 mM AMP in a 30 mM 2-(N-morpholino)ethanesulfonic acid buffer at pH 6.1. The apparent Km for methyl beta-galactoside and for adenosine 3'-phosphate 5'-phosphosulfate were observed at 0.69 x 10(-3) M and at 4 x 10(-6) M respectively. This sulfotransferase is different from that responsible for sulfatide synthesis.

Quercetin, a potent and specific inhibitor of the human P-form phenosulfotransferase

The natural product quercetin was a potent inhibitor of the human P-form phenolsulfo-transferase with an IC50 value of 0.10 +/- 0.03 microM (mean +/- SEM; N = 5), which was three to four orders of magnitude more potent than its inhibition of other human sulfotransferases. The inhibition was noncompetitive with a Ki value of 0.10 microM. The potency and mechanism of this inhibition appear similar to those of the current standard P-form inhibitor, 2,6-dichloro-4-nitrophenol. Among other flavonoids examined, kaempferol was found to have an IC50 value of 0.39 +/- 0.07 microM, naringenin 10.6 +/- 1.6 microM and naringin 265 +/- 90 microM (N = 3). These observations suggest the potential for clinically important pharmacologic and toxicologic interactions by flavonoid-containing foods and beverages.

Regulation of arylsulfate sulfotransferase from a human intestinal bacterium by nucleotides and magnesium ion

Arylsulfate sulfotransferase (ASST) from a human intestinal bacterium stoichiometrically catalyzed the transfer of a sulfate group from phenylsulfate esters to phenolic compounds. Pentachlorophenol, one of the selective inhibitors of phenol sulfoconjugation in mammalian tissues, inhibited both phenol and tyramine sulfation by ASST. Nucleotide triphosphates such as ATP, GTP, UTP and CTP, and pyrophosphate inhibited the ASST activity, whereas Mg2+ and Mn2+ activated the enzyme and prevented its inhibition by ATP and pyrophosphate. Equimolar binding of [alpha-] and [gamma-32P]ATP to the enzyme showed that the enzyme protein was not phosphorylated, but bound ATP. These results suggest that nucleotide triphosphates and divalent cations are important modulators in the control of ASST activity.

Phenolsulphotransferase: localization in kidney during human embryonic and fetal development

The aim of our study was to localize phenolsulphotransferase (PST) in the developing mesonephric and metanephric kidneys of the human embryo and fetus using immunohistochemical methods with an antibody preparation recognizing members of the human phenolsulphotransferase enzyme family. In embryonic and early fetal development of the metanephric kidney, PST is located primarily in derivatives of the ureteric bud such as the ureter, pelvis, calyces and collecting ducts. This predominance declines by mid-fetal life: first, as nephrons evolve and develop they become increasingly PST-immunoreactive such that in mature metanephric kidney, the proximal tubules are highly PST-reactive, with other elements of the nephron also immunopositive (albeit at lower reactivities) and secondly, with the formation of an immunonegative transitional epithelium in ureter, pelvis and calyces, the reactivity retained in collecting ducts is only a small proportion of the total. The distribution of PST immunoreactivity is relatively uniform in proximal tubular cells throughout development, in contrast to collecting ducts, where, in fetal life, this reactivity is displaced to apices and bases by intracellular glycogen deposits. Mesonephric kidney tubules and the mesonephric duct are PST-immunoreactive and although mesonephric immunopositivity overlaps with that in the developing metanephric kidney the renal contribution to sulphation is absent or low at a time when the developing conceptus is most vulnerable to the potential toxic effects of teratogens.

Functional characterization of two human sulphotransferase cDNAs that encode monoamine- and phenol-sulphating forms of phenol sulphotransferase: substrate kinetics, thermal-stability and inhibitor-sensitivity studies

The present paper describes the functional characterization of two human aryl sulphotransferase (HAST) cDNAs, HAST1 and HAST3, previously isolated by us from liver and brain, respectively [Zhu, Veronese, Sansom, and McManus (1993) Biochem. Biophys. Res. Commun. 192, 671-676; Zhu, Veronese, Bernard, Sansom and McManus (1993) Biochem. Biophys. Res. Commun. 195, 120-127]. These appear to encode the two major forms of phenol sulphotransferase (PST) characterized in a number of human tissue cytosols, these being the phenolsulphating (P-PST) and monoamine-sulphating (M-PST) forms of phenol sulphotransferase. HAST1 and HAST3 cDNAs were functionally expressed in COS-7 cells and kinetically characterized using the model substrates for P-PST and M-PST, p-nitrophenol and dopamine (3,4-dihydroxyphenethylamine) respectively. COS-expressed HAST1 was shown to be enzymatically active in sulphating p-nitrophenol with high affinity (Km 0.6 microM), whereas dopamine was the preferred substrate for HAST3 (Km 9.7 microM). HAST1 could also sulphate dopamine, as could HAST3 sulphate p-nitrophenol, but the Km for these reactions were at least two orders of magnitude greater than for the preferred substrates. COS-expressed HAST1 and HAST3 displayed inhibition profiles with the ST inhibitor 2,6-dichloro-4-nitrophenol (DCNP), identical with human liver cytosolic P-PST and M-PST activities respectively. Thermal-stability studies with the expressed enzymes showed that HAST1 was considerably more thermostable (TS) than HAST3, which is consistent with P-PST being termed the TS PST and M-PST being termed the thermolabile (TL) PST. Western immunoblot analyses of the expressed PST proteins using an antibody generated to a bacterially expressed rat liver aryl/phenol ST showed that HAST1 and HAST3 migrated as single proteins with different electrophoretic mobilities (32 versus 34 kDa). This is consistent with the differences in electrophoretic mobilities observed for P-PST and M-PST in a variety of tissues reported by other workers. This report on the functional characterization of P-PST and M-PST cDNAs provides important information on the structural as well as functional relationships of human PSTs, which sulphate a vast array of exogenous and endogenous compounds.

Regulation of phenol sulfotransferase expression in cultured bovine bronchial epithelial cells by hydrocortisone

One conjugative pathway for the inactivation of endogenous and exogenous hydroxylated aromatic compounds is catalyzed by phenol (aryl) sulfotransferases (PSTs), which esterify phenolic acceptors with sulfate. The tracheobronchial epithelium is commonly exposed to phenolic drugs and pollutants, and metabolic sulfation and PST activity in this tissue have been previously demonstrated. To determine what factors may control PST expression, extracts of serum-free, growth factor-supplemented cultures of bovine bronchial epithelial cells were assayed for PST activity and PST antigen. The most significant finding was dose-dependent, apparent stimulated expression by hydrocortisone (EC50 = 4 nM, maximal stimulation at 20 nM). Time-course experiments, however, revealed progressive loss of PST in the absence of corticosteroid. After decay of extant PST in steroid-free medium, hydrocortisone reinduced the expression of PST three to fivefold. Western blots using mouse anti-bovine PST revealed corresponding increases in 32 kDa PST protein levels in response to hydrocortisone. Steady state kinetic analyses indicated apparent Km values of 1-3 microM for 2-naphthol regardless of culture conditions. These results suggest that detoxification of phenolic compounds by sulfation may be regulated by corticosteroids.

Sulphation of N-hydroxy-4-aminobiphenyl and N-hydroxy-4-acetylaminobiphenyl by human foetal and neonatal sulphotransferase

Sulphation of the genotoxic compounds N-hydroxy-4-aminobiphenyl (N-OH-4ABP) and N-hydroxy-4-acetylaminobiphenyl (N-OH-4AABP) was determined in cytosolic preparations of human foetal, neonatal and adult liver and foetal and neonatal adrenal gland. Sulphotransferase (ST) activity capable of sulphating these compounds was present in foetal liver and adrenal gland by 14 weeks of gestation. Sulphation of N-OH-4ABP was higher in foetal and neonatal adrenal cytosol than was sulphation of N-OH-4AABP and in general, N-OH-4ABP ST activity was also greater than that towards 1-naphthol. In foetal and neonatal liver cytosol the sulphation of N-OH-4ABP was also higher than that of N-OH-4AABP (approximately 2-fold). In adult liver cytosols, however, N-OH-4AABP ST activity was higher than that for N-OH-4ABP and 1-naphthol sulphation. Aromatic hydroxylamines and hydroxamic acids are known to be converted by sulphotransferase into reactive, electrophilic compounds capable of reacting with DNA. Our data show that the human foetus and neonate have the capacity to sulphate these compounds and thus is able to produce the reactive mutagenic metabolites. Therefore, this class of genotoxic compounds may be bioactivated by humans during development--a time when they are most vulnerable to the effects of genotoxins.

Inhibition of phenol sulfotransferase by pyridoxal phosphate

The biologically abundant cofactor, pyridoxal-5-phosphate (PLP), is a potent inhibitor of bovine phenol (aryl) sulfotransferase (PST). Preincubation of purified enzyme with as little as 1 microM PLP decreased PST activity by 50%. Excess 2-naphthol protected PST from inactivation by PLP, whereas 2-naphthyl sulfate and PAPS were not protective. Although PLP inhibition was apparently competitive with 2-naphthol, a steady-state kinetic Ki value could not be measured due to non-linear Lineweaver-Burk plots in the presence of the inhibitor. Kinetic progress curves revealed that this was due to progressive loss of activity during catalysis. The kinetics of inactivation of PST by PLP were pseudo-first-order and exhibited saturation. The derived KI value for the binding of PLP to PST in the initial reversible step was 23 microM, with a maximal rate of inactivation of 0.077 min(-1). Absorbance spectra of the PST/PLP complex indicated the formation of a Schiff base conjugate, and this is consistent with decreased electrophoretic mobility of the protein-PLP adduct in the presence of dodecyl sulfate only after reduction with borohydride. These results point to the possible regulation of an important detoxification enzyme by a ubiquitous cofactor.

Estrogen and phenol sulfotransferase activities in human fetal lung

The sulfation of steroid hormones and xenobiotics by human fetal lung cytosol was examined. 1-Naphthol and estrone were extensively sulfated, whereas paracetamol and dehydroepiandrosterone were not good substrates for the pulmonary enzyme. Investigation of the thermostability and inhibition by 2,6-dichloro-4-nitrophenol (DCNP) of the 1-naphthol and estrone sulfotransferase (ST) activities revealed that the estrone ST activity was more thermolabile and more readily inhibited by DCNP than was the 1-naphthol ST activity. Anion exchange chromatography by FPLC resulted in the resolution of two 1-naphthol ST activities, with the estrone ST activity co-eluting with the more basic 1-naphthol ST activity. When human fetal lung cytosol was subjected to gel filtration FPLC, both the 1-naphthol and estrone ST activities had the same native molecular weight of 63,000 Da. this is the first demonstration of estrogen ST activity in human fetal lung. These results suggest that there are at least two forms of sulfotransferase in human fetal lung and that this tissue is capable of sulfating both xenobiotics and endogenous compounds.

Immunohistochemical detection of phenol sulfotransferase-containing neurons in human brain

Using antibodies raised against human platelet phenol sulfotransferase (PST), immunohistochemical studies were performed to determine the cellular localization of PST in several areas of human brain. In the hippocampus PST immunoreactivity was localized in both the pyramidal and nonpyramidal neurons and was in greatest abundance in the CA2 and CA3 areas. In the striatum the immunoreactivity was most predominant in the large neurons of the globus pallidus and in the medulla the staining was scattered throughout the neurons of the raphe nucleus and the reticular formation. The selective presence of PST in the neurons of the CNS raises the issue as to the role of this enzyme in sulfating neurotransmitters because PST has been shown to be capable of conjugating a variety of neurotransmitters including the catecholamines as well as the tyrosine moiety of a number of small peptides such as enkephalin and cholecystokinin.

Purification of a rat liver cytosolic sulfotransferase responsible for the conjugation of digitoxigenin

Previous investigations on the digitoxin metabolism hardly considered the role of the sulfate ester conjugation. Therefore, this study examined whether digitoxin (dt-3) or one of its cleavage products might be sulfated in vitro. It was proven that digitoxigenin (dt-0) is by far the best substrate for the cytosolic sulfotransferases (ST). Digitoxigenin-monodigitoxoside (dt-1) and digitoxigenin-bisdigitoxoside (dt-2) are sulfated in trace amounts whereas dt-3 is not sulfated at all. The purification of the responsible enzyme was performed by liquid chromatography on Q-Sepharose and hydroxyapatite. During the purification procedure this enzymatic activity corresponded exactly to that towards dehydroepiandrosterone (DHEA). The 134-fold purified and gel electrophoretically homogeneous enzyme protein (Mr 33,000) showed a Vmax of 12.5 nmoles dt-0 sulfate/min mg protein and a KM of 37 mumol/L. The purified enzyme conjugated dt-1 and dt-2 in trace amounts only and was inhibited competitively by DHEA. It can be concluded that in the rat a 3 beta-hydroxy-steroid sulfotransferase is responsible for the sulfation of dt-0. The purified enzyme reacts with dt-1, dt-2 and digoxigenin (dg-0) in traces only, a sulfation of dt-3 is not detectable.

Sulfation in male reproductive organs. Bull and boar testis phenol sulfotransferases

Phenol sulfotransferases (PST) from bull and boar testis were partially purified and characterized. A single form of PST adsorbed on DEAE-cellulose was found in the bull testis, whereas from boar testis two different peaks of PST activity were separated. The bull testis PST and both boar testis enzymes were active with p-nitrophenol and adrenalin. They all showed higher affinity to pNP than to adrenalin and were inhibited by these substrates at higher concentrations. Their optimal pH was at 8.5. Bull testis PST and boar PST II which were adsorbed on DEAE-cellulose were thermostable, whereas boar PST I was thermolabile. Those three PST forms differed in sensitivity to 2,6-dichloro-4-nitrophenol (DCNP), N-ethyl maleimide (NEM), iodoacetamide (IAA) and phenylglyoxal (PG). Bull and boar PST II were more rapidly inactivated in the presence of DCNP than boar PST I. In the presence of NEM, the--SH groups reagent, the bull phenol sulfotransferase and boar PST I lost their activity, whereas the activity of boar PST increased. Also iodoacetamide, another--SH group modificator, raised boar PST II activity and decreased boar PST I activity. DTT, which protects thiol groups, had an opposite effect on the enzymes studied than NEM. Phenylglyoxal, a reagent specific for arginine residues inhibited bull testis PST and both boar phenol sulfotransferases. Substrate protection experiments were also performed to determine the localization of reactive groups in bull and boar testis phenol sulfotransferases.

Inhibition of M and P phenol sulfotransferase by analogues of 3'-phosphoadenosine-5'-phosphosulfate

Structural analogues of the sulfate donor 3'-phosphoadenosine-5'-phosphosulfate (3',5'-PAPS) were examined for their ability to inhibit dopamine and phenol sulfation by the M and P forms of phenol sulfotransferase (PST), respectively. The Ki values for each of the adenosine derivatives were calculated from the rate equation for PST. For both M and P PST, the naturally occurring product 3'-phosphoadenosine-5'-phosphate, (3',5'-PAP), was shown to be the most effective inhibitor. The weakest inhibitors of the two sulfotransferases were 5'-adenosine phosphosulfate and the three AMP derivatives, which were less than 1,000 times as effective as 3',5'-PAP. 5'-ATP, 2',5'-PAPS, 2',5'-PAP, and 5'-ADP were similar in their inhibition of M and P PST and were all approximately 100 times less effective than the natural end product. These data reveal that there is a rigid structural requirement for binding of the ribose portion of adenosine to both M and P PST that involves the groups on both the 3' and 5' positions. The effectiveness of binding to the two enzymes may depend on both steric factors as well as the distribution of negative charges on the ribose ring.