Molecular cloning, expression, and enzymatic characterization of rabbit hydroxysteroid sulfotransferase AST-RB2

Inhibitory effects of kynurenic acid, a tryptophan metabolite, and its derivatives on cytosolic sulfotransferases

KYNA (kynurenic acid) is an endogenous metabolite of tryptophan in the kynurenine pathway and has been characterized as an antagonist of ionotropic glutamate receptors. In addition, we have reported this endogenous compound as a potent inhibitor of SULTs (cytosolic sulfotransferases). In the present study we characterized the inhibitory effects of KYNA on several human (h) and mouse (m) recombinant SULTs. No sulfate metabolite of KYNA was detected with mouse and human SULTs examined under the conditions used, suggesting that it is a bona fide inhibitor of SULTs. Among the mouse enzymes examined, KYNA exhibited selective inhibitory effects on Sult1b1-mediated sulfation of various compounds with IC50 values in the low micromolar range (2.9–4.9 μM). KYNA also exerted an inhibitory activity towards hSULT1A1 and hSULT1B1. The inhibitory potency of KYNA for mSult1b1 was stronger than that of 2,6-dichloro-4-nitrophenol, a known non-specific SULT inhibitor, whereas the potencies of these two inhibitors for hSULT1B1 were comparable. The inhibitory characteristics of KYNA were clearly distinct from those of mefenamic acid, a selective inhibitor of SULT1A enzymes. The KYNA derivatives 5,7-dichlorokynurenic acid and L689,560 exhibited preferential inhibitory effects on hSULT1A1 and hSULT1B1 respectively. Interestingly, gavestinel, another KYNA derivative, was found to be an extremely potent inhibitor of hSULT1B1. Finally, we have demonstrated that the mechanism underlying the KYNA inhibition varied depending on the enzyme and substrate involved. Taken together, the present results unveil another distinct aspect of KYNA and its derivatives as an inhibitor of SULTs.

Chenodeoxycholic acid-mediated activation of the farnesoid X receptor negatively regulates hydroxysteroid sulfotransferase

Hydroxysteroid sulfotransferase catalyzing bile acid sulfation plays an essential role in protection against lithocholic acid (LCA)-induced liver toxicity. Hepatic levels of Sult2a is up to 8-fold higher in farnesoid X receptor-null mice than in the wild-type mice. Thus, the influence of FXR ligand (chenodeoxycholic acid (CDCA) and LCA) feeding on hepatic Sult2a expression was examined in FXR-null and wild-type mice. Hepatic Sult2a protein content was elevated in FXR-null and wild-type mice fed a LCA (1% and 0.5%) diet. Treatment with 0.5% CDCA diet decreased hepatic Sult2a to 20% of the control in wild-type mice, but increased the content in FXR-null mice. Liver Sult2a1 (St2a4) mRNA levels were reduced to 26% in wild-type mice after feeding of a CDCA diet, while no decrease was observed on Sult2a1 mRNA levels in FXR-null mice after CDCA feeding. A significant inverse relationship (r(2)=0.523) was found between hepatic Sult2a protein content and small heterodimer partner (SHP) mRNA level. PCN-mediated increase in Sult2a protein levels were attenuated by CDCA feeding in wild-type mice, but not in FXR-null mice. Human SULT2A1 protein and mRNA levels were decreased in HepG2 cells treated with the FXR agonists, CDCA or GW4064 in dose-dependent manners, although SHP mRNA levels were increased. These results suggest that SULT2A is negatively regulated through CDCA-mediated FXR activation in mice and humans.

A proposed nomenclature system for the cytosolic sulfotransferase (SULT) superfamily

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

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

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

Identification of ST2A1 as a rat brain neurosteroid sulfotransferase mRNA

A hydroxysteroid sulfotransferase (ST2A1) was identified as a form mediating neurosteroid sulfation in rat brain. The sole expression among known rat ST2A forms was indicated by brain RT-PCR. All nucleotide sequences of seven ST2A cDNA clones isolated from brain matched completely with that of hepatic ST2A1. The recombinant ST2A1 protein mediated neurosteroid sulfation. These data strongly suggest a functional role of ST2A1 as a neurosteroid sulfotransferase in rat brain.

Pharmacogenetics of sulfotransferase

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

Sulfotransferase catalyzing sulfation of heterocyclic amines

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

Testosterone sulfotransferase: evidence in the guinea pig that this reaction is carried out by 3 alpha-hydroxysteroid sulfotransferase

During the course of isolating, characterizing, and cloning estrogen and 3-hydroxysteroid sulfotransferases from the guinea pig adrenal gland, it was noted that cytosolic preparations from this tissue would also sulfonate testosterone. Therefore, we set out to isolate and clone the enzyme that performs this reaction. Testosterone sulfotransferase (TST) was isolated from the guinea pig adrenal by using the standard procedures of ion exchange, affinity, and high-performance liquid chromatography. When purified, TST was examined by liquid-phase nondenaturing isoelectric focusing, it was found that the TST activity profile completely overlapped with the activity profile of the 3alpha-hydroxysteroid sulfotransferase (3alphaHST) isoform, but not the 3beta-hydroxysteroid sulfotransferase (3betaHST) isoform. This finding was further investigated by overexpressing the cDNAs for 3alphaHST and 3betaHST in Escherichia coli and examining the expressed proteins for TST activity. This experiment confirmed that 3alphaHST does indeed function as a TST. In addition, 3alphaHST was also found to sulfonate estradiol but not estrone, a finding that further suggested that 3alphaHST may function as a general 17beta-hydroxysteroid sulfotransferase.

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

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

Purification and characterization of two amine N-sulfotransferases, AST-RB1 (ST3A1) and AST-RB2 (ST2A8), from liver cytosols of male rabbits

Two sulfotransferases (STs), designated as AST-RB1 (ST3A1) and AST-RB2 (ST2A8), with high a amine N-sulfonating activity, were purified from male rabbit liver cytosols. AST-RB1 and AST-RB2 were purified to homogeneity by the anion-exchange, affinity, and hydroxyapatite chromatography. The N-terminus of both enzymes were blocked. The subunit molecular mass of both enzymes was estimated to be 34 kDa on SDS-PAGE. AST-RB1 efficiently catalyzed N-sulfonation of alicyclic, alkyl, and arylamines such as 4-phenyl-1,2,3, 6-tetrahydropyridine, 1-[(5-chloro-2-oxo-3(2H)-benzothiazolyl)acetyl]-piperazine, desipramine, and aniline, whereas its catalytic activities toward 2-naphthol and dehydroepiandrosterone (DHEA) were very low. On the other hand, AST-RB2 efficiently catalyzed sulfonation of desipramine and DHEA, but had no activity toward 2-naphthol. Amino acid sequences of peptide fragments derived from the purified AST-RB1 showed no significant homology with previously reported STs, but those from the purified AST-RB2 shared a high similarity with those of the ST2 family. Both enzymes were expressed specifically in the liver. The present results strongly suggest that the purified AST-RB1 is a novel enzyme in terms of structure and catalytic properties showing high selectivity for amine substrates, and AST-RB2 is a quite unique from among ST2A enzymes of other species in its substrate specificity.