Sulphotransferase-mediated activation of the carcinogen 5-hydroxymethyl-chrysene. Species and sex differences in tissue distribution of the enzyme activity and a possible participation of hydroxysteroid sulphotransferases

[Indoor air guide values for 2-butanone oxime. Communication from the Ad-hoc Working Group on Indoor Guide Values of the Indoor Air Hygiene Commission and the States' Supreme Health Authorities]

The German Working Group on Indoor Guidelines of the Indoor Air Hygiene Committee and of the Supreme State Health Authorities is issuing indoor air guide values to protect public health. No reliable human studies are available for health evaluation of 2-butanone oxime in indoor air. In a well documented chronic inhalation animal study with rats and mice assessed as reliable, degenerative changes in the olfactory epithelium were observed, which led to a dose related increased incidence and severity, especially in mice. Using a benchmark approach the Working Group assessed a BMD10 of 13.8 mg 2-butanone oxime/m(3) for continuous exposure for the endpoint degeneration of the olfactory epithelium. For interspecies differences a reduced factor of 1 was applied due to the same susceptibility of rodents than human for this endpoint. By applying a factor of 10 for interindividual variability, and a factor of 2 to account for the higher respiratory rate of children compared to adults, a health hazard guide value (RW II) of 0.06 mg 2-butanone oxime/m(3) indoor air is obtained. A precautionary guide value of 0.02 mg 2-butanone oxime/m(3) indoor air is recommended.

Age- and sex-dependent expression of multiple murine hepatic hydroxysteroid sulfotransferase (SULT2A) genes

Hydroxysteroid sulfotransferase (SULT2A) enzymes play important roles in hepatic steroid and xenobiotic metabolism. Unlike humans, which express one SULT2A, inspection of mouse genome information indicated the presence of seven SULT2A genes within a cluster on chromosome 7. The age- and sex-dependent expressions of the seven murine SULT2A family members were characterized in the livers of C57BL/6 mice using real-time RT-PCR. The transcripts for three of the SULT2A forms (NCBI reference/model sequences XM_001471624, NM_009286 and NM_001111296) were abundant in pre-pubertal male and female mouse liver but were essentially silenced in the livers of adult male mice. The mRNAs of three other SULT2A forms (NM_001101534, XM_894052 and NM_001081325) were also expressed in pre-pubertal male and female mouse liver, but at markedly reduced levels relative to those of the abundant forms. The mRNA levels of these lower-abundance forms were further suppressed in adult animals. A seventh SULT2A mRNA (XM_983034) was expressed in adult male and female mouse liver, but was not detected in pre-pubertal mouse liver of either sex. Full-length amplifications with primers targeting untranslated regions confirmed that all SULT2A forms were expressed. However, while the XM_001471624, NM_001111296, NM_001101534, XM_894052 and NM_001081325 transcripts were detected at their predicted sizes, the NM_009286 and XM_983034 transcripts each lacked two predicted exons. These results demonstrate that seven murine SULT2As display different profiles of age- and sex-dependent hepatic expression.

Pentachlorophenol and other chlorinated phenols are substrates for human hydroxysteroid sulfotransferase hSULT2A1

Pentachlorophenol (PCP) is a persistent chemical contaminant that has been extensively investigated in terms of its toxicology and metabolism. Similar to PCP, other chlorinated phenol derivatives are also widely present in the environment from various sources. Even though some of the chlorine-substituted phenols, and particularly PCP, are well-known inhibitors of phenol sulfotransferases (SULTs), these compounds have been shown to undergo sulfation in humans. To investigate the enzymatic basis for sulfation of PCP in humans, we have studied the potential for PCP as well as the mono-, di-, tri-, and tetra-chlorinated phenols to serve as substrates for human hydroxysteroid sulfotransferase, hSULT2A1. Our results show that all of these compounds are substrates for this isoform of sulfotransferase, and the highest rates of sulfation are obtained with PCP, trichlorophenols, and tetrachlorophenols. Much lower rates of sulfation were obtained with isomers of monochlorophenol and dichlorophenol as substrates for hSULT2A1. Thus, the sulfation of polychlorinated phenols catalyzed by hSULT2A1 may be a significant component of their metabolism in humans.

Testicular sulfoconjugation of the 16-androstene steroids by hydroxysteroid sulfotransferase: Its effect on the concentrations of 5α-androstenone in plasma and fat of the mature domestic boar1

This study examined the relationship between sulfoconjugation and the degree to which 5alpha-androstenone can accumulate in fat. Analysis of the unconjugated and sulfoconjugated fractions of peripheral plasma from 25 mature Yorkshire boars and testicular vein plasma from an additional 20 mature Yorkshire boars revealed that the majority of 5alpha-androstenone is present as a sulfoconjugate, reaching levels up to 69 +/- 4.3 and 72 +/- 6.2%, respectively, relative to its unconjugated form. The presence of this steroid in the sulfoconjugate fraction was confirmed by gas chromatography-mass spectrometry. Plasma concentrations of 5alpha-androstenone in the sulfoconjugate fraction were negatively correlated (r = -0.36; P < 0.01) with the concentrations of 5alpha-androstenone in fat. High concentrations of 5alpha-androstenone in the sulfate fraction were only associated with animals that had fat androstenone concentrations < 0.5 microg/g. In addition, there was a positive correlation (r = 0.31; P < 0.01) between the concentrations of unconjugated 5alpha-androstenone in plasma and 5alpha-androstenone in fat. These findings indicate that the levels of the sulfoconjugated form present in the peripheral plasma influence the accumulation of 5alpha-androstenone in fat. The specific sulfotransferase enzyme involved in sulfoconjugating these steroids was identified by incubating Leydig cells with specific sulfotransferase inhibitors for 8 h. It was discovered that the enzyme responsible for the sulfoconjugation of the 16-androstene steroids is hydroxysteroid sulfotransferase. Hydroxysteroid sulfotransferase may play a significant role in determining the levels of sulfated 16-androstene steroids present in plasma. The results of this study indicate that sulfoconjugation may serve to regulate the quantity of unconjugated 5alpha-androstenone present in the circulation and thus available for accumulation. Animals with a decreased ability to sulfoconjugate 5alpha-androstenone would have a subsequent increase in the levels of unconjugated 5alpha-androstenone in circulation, allowing for the accumulation of high levels in fat and thereby potentially leading to the development of boar taint.

Nephrotoxicity induced by C- and N-arylsuccinimides

The succinimide ring is incorporated into hundreds of compounds that are widely used as agricultural, industrial, and pharmaceutical agents. Some succinimide derivatives that contain an aryl group on the ethylene bridge of the succinimide ring (C-arylsuccinimides) or on the nitrogen atom (N-arylsuccinimides) induce nephrotoxicity in humans and/or laboratory animals. Acute toxicity induced by this general class of compounds is typically characterized as polyuric renal failure, while chronic nephrotoxicity is seen as chronic interstitial nephritis. In this review, the structure-nephrotoxicity relationships, biotransformation, and mechanisms of nephrotoxicity for the C- and N-arylsuccinimides are examined.

In vitro nephrotoxicity induced by N-(3,5-dichlorophenyl)succinimide (NDPS) metabolites in isolated renal cortical cells from male and female Fischer 344 rats: evidence for a nephrotoxic sulfate conjugate metabolite

The agricultural fungicide N-(3,5-dichlorophenyl)succinimide (NDPS) induces nephrotoxicity in vivo that is characterized as acute polyuric renal failure and proximal tubular necrosis. However, earlier in vitro studies have failed to reproduce the in vivo nephrotoxicity seen with NDPS or its nephrotoxic metabolites N-(3,5-dichlorophenyl)-2-hydroxysuccinimide (NDHS) and N-(3,5-dichlorophenyl)-2-hydroxysuccinamic acid (2-NDHSA). The purpose of this study was to examine the nephrotoxic potential of NDPS, its known non-conjugated metabolites, the O-sulfate conjugate of NDHS (NSC), and the putative metabolite N-(3,5-dichlorophenyl)maleimide (NDPM) and its hydrolysis product N-(3,5-dichlorophenyl)maleamic acid (NDPMA) using freshly isolated renal cortical cells (IRCC). IRCC were obtained from untreated male or female Fischer 344 rats following collagenase perfusion of the kidneys. Cells (approximately 4 million per ml) (N=4) were incubated with up to 1.0 mM NDPS or an NDPS metabolite or vehicle for up to 120 min. Cytotoxicity was determined by measuring lactate dehydrogenase (LDH) release into the medium. Only NSC (>0.5 mM) and NDPM (> or =0.5 mM) exposure increased LDH release from IRCC. NSC 1.0 mM or NDPM 0.5 mM increased LDH release from IRCC within 15--30 min of exposure. NDPS or the remaining NDPS metabolites did not increase LDH release at bath concentrations of 1.0 mM for exposures of 120 min. IRCC from male and female rats responded similarly to the toxic effects of NDPS and its metabolites. These results demonstrate that sulfate conjugates of NDPS metabolites can be fast acting nephrotoxicants and could contribute to NDPS nephrotoxicity in vivo. These results also suggest that the kidney probably accumulates toxic sulfate conjugates of NDPS metabolites rather than forming the conjugates. In addition, mechanisms responsible for gender differences in nephrotoxicity seen with NDPS and NDPS metabolites in vivo either occur prior to renal accumulation of sulfate conjugates and/or represent biochemical/physiological differences between the genders.

Bacterial expression, purification, and characterization of rat hydroxysteroid sulfotransferase STa

Hydroxysteroid (alcohol) sulfotransferase catalyzes numerous reactions that are important to our understanding of the metabolism of both endogenous steroids and exogenous alcohols. Here we report a method for prokaryotic expression and rapid purification of the recombinant hydroxysteroid sulfotransferase STa, a major isoform of hydroxysteroid sulfotransferase in the rat. The cDNA encoding STa was cloned into a pET-3c vector and expressed in Escherichia coli BL21 cells. After disruption of the cells by sonication, the enzyme was purified in one step by affinity chromatography on adenosine 3',5'-diphosphate-agarose. The purified recombinant STa had a relative molecular mass on SDS-PAGE that was identical with the native hepatic STa in rat liver. The expressed enzyme displayed similar substrate inhibition characteristics with dehydroepiandrosterone as have been noted previously with the native enzyme purified from rat liver. Furthermore, the catalytic efficiency in sulfation of 7-hydroxymethyl-12-methylbenz[a]anthracene, as well as the stereoselectivity of sulfation of the enantiomers of 1-phenyl-1-heptanol and 1-naphthyl-1-ethanol, catalyzed by the recombinant STa were consistent with characteristics of the STa isolated from rat liver.

Effects of molybdate and pentachlorophenol on the sulfation of acetaminophen

Pentachlorophenol (PCP) is an inhibitor of phenol-sulfotransferases and has been used to ascertain the role of sulfation in toxicology. Recently, molybdate has been shown to inhibit the sulfation of various chemicals by decreasing hepatic concentrations of the cosubstrate, 3'-phosphoadenosine 5'-phosphosulfate (PAPS). The purpose of this study was to compare the effectiveness of these two chemicals in inhibiting the sulfation of various doses of acetaminophen (AA) in the rat. PCP (40 micromol/kg) decreased the 2-h combined biliary and urinary excretion of AA-sulfate by 78, 83, 84, and 47% of the 0.1, 0.3, 1, and 3 mmol/kg doses of AA, respectively. Molybdate (7.5 mmol/kg) decreased the sulfation of these same doses of AA by 50, 65, 62, and 81%, respectively. These data indicate that PCP is more effective in decreasing the sulfation of low than high doses of AA, which may result from less AA, at lower doses, to compete with PCP for sulfotransferases. Conversely, molybdate is more effective in decreasing sulfation of high rather than low doses of AA because molybdate decreases sulfate availability and decreases PAPS synthesis. More PAPS is required for the sulfation of high than low doses of AA. Therefore, PCP inhibits sulfation more effectively at low doses of AA when sulfation is limited by sulfotransferases, and molybdate inhibits sulfation more effectively at high doses of AA when sulfation is limited by PAPS.

Sex-specific induction of apoptosis by cyproterone acetate in primary rat hepatocytes

The synthetic steroid cyproterone acetate (CPA) has been reported to be hepatogenotoxic in female rats depending on sex-specific expression of a hydroxysteroid sulfotransferase (HST) which is involved in the bioactivation of CPA to reactive metabolites. In the present study the ability of CPA to initiate apoptosis in rat hepatocytes in vitro was investigated with respect to sex-specific effects and dependency on HST activity. Incubation of primary hepatocytes of female rats with CPA (0.1-30 microM) caused a strong increase in percent of cells undergoing apoptosis. The lowest concentration leading to apoptosis was 0.3 microM. In contrast, hepatocytes isolated from male rats showed a very weak response at high exposure to CPA (30 microM) only. Treatment with transforming growth factor-beta1 induced high levels of apoptotis in hepatocytes of both genders. Megestrol acetate and chlormadinone acetate, two structural analogues of CPA with a much lower genotoxic potency, did not induce apoptosis. Pre-addition of 10 or 50 microM dehydroepiandrosterone (DHEA), a known inhibitor of hepatic HST, almost completely inhibited CPA-induced apoptosis in hepatocytes of female rats. Using similar test concentrations, DHEA also reduced CPA-induced DNA excision repair as measured in the unscheduled DNA synthesis test. The results suggest that apoptosis induction is directly related to DNA damage induced by HST-dependent CPA metabolites.

Effects of molybdate and pentachlorophenol on the sulfation of dehydroepiandrosterone

Pentachlorophenol (PCP) and molybdate have been shown to inhibit the sulfoconjugation of various chemicals in rats and therefore are useful to examine the role of sulfoconjugation on the toxicity of a chemical. PCP inhibits sulfation by competing with substrates for phenol-sulfotransferases, but not hydroxysteroid-sulfotransferases. In contrast, molybdate decreases sulfation by limiting sulfate availability and thereby decreasing the synthesis of 3'-phosphoadenosine 5'-phosphosulfate (PAPS), which is the obligate cosubstrate for sulfation. Therefore, it was of interest to determine whether PCP or molybdate is effective in decreasing the in vivo sulfation of dehydroepiandrosterone (DHEA), which is a substrate for hydroxysteroid-sulfotransferases. PCP (40 micromol/kg ip) or molybdate (7.5 mmol/kg po) was given 45 min and 4 h, respectively, prior to the start of DHEA infusion. The effects of these two sulfation inhibitors on DHEA sulfation were dependent on the rate of DHEA infusion in rats. PCP had different effects on the sulfation of various infusion rates of DHEA in rats. PCP had little effect on the sulfation after the two lowest infusion rates of DHEA (12.5 and 25 mg/kg) and actually increased (233%) DHEA-sulfate serum concentrations with the highest DHEA infusion rate (50 mg/kg). Although molybdate had little affect on the sulfation of the lowest DHEA infusion rate, it significantly decreased (50-85%) DHEA-sulfate serum concentrations with the two higher DHEA infusion rates. These data indicate that molybdate, unlike PCP, decreases the sulfation of DHEA and may be a useful tool to decrease the sulfation of other substrates of hydroxysteroid-sulfotransferases.

Regulation of sulfotransferase gene expression by glucocorticoid hormones and xenobiotics in primary rat hepatocyte culture

In the rat liver, hydroxysteroid sulfotransferase-a (HST-a) and aryl sulfotransferase IV (ASTIV) represent two major rat hepatic sulfotransferases that are important to xenobiotic metabolism. Prototypic CYP1A1 and CYP2B/3A inducers regulate rat hepatic sulfotransferase gene expression although not necessarily in a coordinate direction. It has been previously reported that in vivo treatment with CYP1A1 inducer 3-methylcholanthrene (3-MC) suppresses rat hepatic HST-a mRNA expression in a dose-dependent manner. Similarly, HST-a and ASTIV mRNA levels become suppressed or induced, respectively, following in vivo treatment with phenobarbital (PB)-like CYP2B/3A inducers or prototypic CYP3A inducers such as glucocorticoid hormones. In the whole animal, sulfotransferase gene expression is modulated by members of the hypothalamic/pituitary-adrenal gonadal hormone axis. However, studies in primary rat hepatocyte culture suggest that prototypic P450 inducers regulate HST-a and ASTIV gene expression directly at the level of the hepatocyte. Glucocorticoid-mediated sulfotransferase expression was compared with the regulation of tyrosine amino transferase (TAT), a gene that is transcriptionally regulated by ligand bound glucocorticoid receptor. It was found that lower doses of dexamethasone (DEX, 10(-7) M) produced concomitant increases in ASTIV and TAT mRNA expression, whereas HST-a mRNA expression continued to rise as the DEX dose was increased through 10(-5) M. When hepatocytes were co-incubated with DEX and antiglucocorticoid/antiprogestin RU-486, DEX-stimulated HST-a mRNA expression was not significantly inhibited by RU-486, but ASTIV and TAT mRNA expression were inhibited to a similar extent. The results suggested that ASTIV, like TAT, is likely regulated by a classical glucocorticoid receptor mediated mechanism, whereas HST-a is probably regulated by glucocorticoids via an alternative mechanism. In contrast to the positive effects of glucocorticoid hormones, HST-a and ASTIV mRNA expression was negatively regulated by xenobiotics such as PB-like CYP2B/3A inducers or aryl hydrocarbon receptor (AhR) agonist CYP1A1 inducers. Incubation of primary cultured rat hepatocytes with PB or structurally dissimilar PB-like inducers clotrimazole, diphenylhydantoin, heptachlor, gamma-hexachlorocyclohexane or 2,2',4,4',5,5'-hexachlorobiphenyl suppressed HST-a and ASTIV mRNA levels. Also, incubation of primary cultured rat hepatocytes with CYP1A1 inducer beta-naphthoflavone or with archetypic AhR agonist, 2,3,7,8-tetrachloro-p-dioxin (TCDD) markedly suppressed HST-a and ASTIV mRNA expression. These data suggest that the rat HST-a and ASTIV genes are positively regulated by glucocorticoid hormones and negatively regulated by xenobiotics as a result of molecular and cellular mechanisms that act directly on the hepatocyte.

On the nature of rat hepatic and mouse olfactory sulfotransferases

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

Effects of sodium sulfate on acute N-(3,5-dichlorophenyl)succinimide (NDPS) nephrotoxicity in the Fischer 344 rat

The agricultural fungicide N-(3,5-dichlorophenyl)succinimide (NDPS) induces acute polyuric renal failure in rats. Results of previous studies have suggested that NDPS may induce nephrotoxicity via conjugates of NDPS metabolites. Thus, the purpose of this study was to examine if administered sodium sulfate could alter NDPS nephrotoxicity. Male Fischer 344 rats (four rats per group) were administered a single intraperitoneal (i.p.) injection of sodium sulfate (0.035, 0.07, 0.35 or 3.5 mmol/kg) or sodium chloride (7.0 mmol/kg) 20 min before NDPS (0.2, 0.4 or 0.8 mmol/kg) or NDPS vehicle (sesame oil, 2.5 ml/kg) and renal function monitored at 24 and 48 h. High dose sodium sulfate (3.5 mmol/kg) markedly attenuated NDPS nephrotoxicity, while sodium chloride had no effect on NDPS-induced renal effects. NDPS nephrotoxicity was also attenuated by a pretreatment dose of 0.35 mmol/kg sodium sulfate, while 0.07 mmol/kg sodium sulfate pretreatment potentiated NDPS 0.2 mmol/kg to produce nephrotoxicity without markedly attenuating NDPS 0.4 mmol/kg to induce renal effects. A dose of 0.035 mmol/kg sodium sulfate did not potentiate NDPS 0.2 mmol/kg to induce nephrotoxicity. These results suggest that sulfate conjugates of NDPS metabolites might contribute to NDPS nephrotoxicity.

High sulfotransferase activity for phenolic aromatic odorants present in the mouse olfactory organ

Mouse nasal cytosols show high sulfotransferase (ST) activities toward phenolic aromatic odorants, but have little activities for most alcoholic aromatic odorants. Most ST activities toward the phenolic odorants preferred slightly acidic pH (6.4) and were sensitive to 2,6-dichloro-4-nitrophenol, a specific inhibitor for phenol ST (P-ST) but were not inhibited by triethylamine and tetra-n-butylammonium chloride, which are specific inhibitors for hydroxysteroid ST (HS-ST). These results suggested that P-ST activities are responsible for sulfation of the phenolic odorants. The spectra of the ST activities for these odorants were similar in mouse nasal and liver cytosols, however, nasal cytosols showed much higher ST activity toward cinnamyl alcohol than liver cytosols. This activity preferred higher pH (7.4) compared to the phenolic odorant-ST activities and was inhibited by both types of inhibitors, specific for P-ST and HS-ST. These results appear to indicate the participation of multiple ST isoforms for the sulfation of odorants in mouse nasal cytosols. The existence of P-ST(s) active for the phenolic odorants in olfactory cytosols suggests a role in odorant perception, in particular, in the signal termination process.

Targeted overexpression of androgen receptor with a liver-specific promoter in transgenic mice

The rodent liver displays marked age- and sex-dependent changes in androgen sensitivity due to the sexually dimorphic and temporally programmed expression of the androgen receptor (AR) gene. We have altered this normal phenotype by constitutive overexpression of the rat AR transgene in the mouse liver by targeting it via the human phenylalanine hydroxylase (hPAH) gene promoter. These transgenic animals in their heterozygous state produce an approximately 30-fold higher level of the AR in the liver as compared with the nontransgenic control. Androgen inactivation via sulfonation of the hormone by dehydroepiandrosterone sulfotransferase (DST), an androgen-repressible enzyme, also contributes to the age- and sex-dependent regulation of hepatic androgen sensitivity. DST has a broad range of substrate specificity and is responsible for the age- and sex-specific activation of certain polycyclic aromatic hepatocarcinogens as well, by converting them to electrophilic sulfonated derivatives. In the transgenic female, the hepatic expression of DST was approximately 4-fold lower than in normal females, a level comparable to that in normal males. The hPAH-AR mice will serve as a valuable model for studying the sex- and age-invariant expression of liver-specific genes, particularly those involved in the activation of environmental hepatocarcinogens such as the aromatic hydrocarbons.

Stable heterologous expression of hydroxysteroid sulphotransferase in Chinese hamster V79 cells and their use for toxicological investigations

Various benzylic alcohols are metabolically activated to electrophilic, potentially mutagenic and carcinogenic sulphuric acid esters. The involved sulphotransferases are not expressed in the cell lines in culture which are commonly used for mutagenicity testing. The liver of adult female rats is very efficient in the bioactivation of 1-hydroxymethylpyrene. The major enzyme involved was purified and identified as hydroxysteroid sulphotransferase a. Its cDNA was stably expressed in Chinese hamster V79 cells, which are particularly suited for the quantitative detection of various types of mutations and other genotoxic and cytotoxic effects. The mRNA, protein and enzyme activity levels in the constructed cell lines (V79rSTa-1 and V79rSTa-2) were measured, and the cells were also used in mutagenicity and cytotoxicity investigations with benzylic alcohols. 1-Hydroxymethylpyrene, 9-hydroxymethylanthracene and 6-hydroxymethylbenzo[a]pyrene showed enhanced cytotoxicity in V79rSTa-1 and V79rSTa-2 cells, as compared with sulphotransferase-deficient control cells. In addition, 1-hydroxymethylpyrene induced sister chromatid exchanges, and 6-hydroxymethylbenzo[a]pyrene induced gene mutations in V79rSTa-1 cells. We intend carrying out more investigations with other chemicals on these cell lines. Their advantages, as compared with systems with external metabolising systems, include the formation of the active metabolites within the target cell, as in ST-proficient cells in vivo, eliminating the problems which may result from restricted intercellular transport of reactive and ionized sulphuric acid conjugates. Furthermore, cells expressing other sulphotransferases, including human enzymes, may be constructed and used for comparative investigations.

Sulfotransferase gene expression in rat hepatic and extrahepatic tissues

Enzymatic sulfation has been implicated to play a key role in a number of essential biological pathways including xenobiotic detoxication, carcinogen activation, and the regulation of intra-tissue hormone activity. In order to increase our understanding of the critical determinants governing the regulation of sulfotransferase gene expression, we investigated age-, gender-, and xenobiotic-related alterations in hydroxysteroid sulfotransferase-a or aryl sulfotransferase-IV gene expression. Northern blot and slot blot analyses showed that rat hepatic hydroxysteroid sulfotransferase-a mRNA expression was responsive to age- and gender-related signals. The results also suggested that the rat hepatic aryl sulfotransferase-IV and hydroxysteroid sulfotransferase-a genes are differentially regulated. Northern blot and reverse transcriptase polymerase chain reaction analyses demonstrated that hydroxysteroid sulfotransferase-a mRNA was expressed to a greater extent in female rat liver than in lung or kidney tissue. In addition, rat hepatic hydroxysteroid sulfotransferase-a gene expression in mature female rats, although not substantially altered in response to short-term fasting or high-dose dexamethasone treatment, was suppressed after treatment with the polycyclic aromatic hydrocarbon, 3-methylcholanthrene.

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.

Roles of electrophilic sulfuric acid ester metabolites in mutagenesis and carcinogenesis by some polynuclear aromatic hydrocarbons

Hydroxylation of meso-methyl groups with subsequent formation of reactive benzylic esters bearing a good leaving group (e.g. sulfate) was proposed as a possible biochemical mechanism of activation and tumorigenicity of methyl-substituted polycyclic aromatic hydrocarbons (PAHs). In support of this postulation, recent studies have demonstrated the formation by rodent hepatic sulfotransferase activity of electrophilic, mutagenic, and carcinogenic sulfuric acid esters of several hydroxymethyl aromatic hydrocarbons including hydroxymethyl derivatives of benz[a]anthracene, 6-hydroxymethylbenzo[a]pyrene, 5-hydroxymethylchrysene, 9-hydroxymethyl-10-methylanthracene, and 1-hydroxymethylpyrene. Besides these hydroxymethyl PAHs containing a primary benzylic alcoholic group, some aromatic hydrocarbons with secondary benzylic hydroxyl functional group(s) are also metabolically activated through sulfuric acid esterification.

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.

Carcinogen activation by sulfate conjugate formation

The foregoing pages presented a substantial body of data that established that sulfotransferase conjugation can transform many xenobiotics into agents that can modify cellular macromolecules. However, activation by sulfation is rarely the only metabolic pathway that is open to these compounds; other pathways can become more important in response to a variety of factors. This metabolic switching can be produced by substrate concentration, cofactor availability, kinetic factors that dictate the velocity of the various possible conjugation reactions, and, in some cases, competition between Phase-I and Phase-II metabolism. Also, it is important to realize that demonstration of activation by sulfate ester formation in vitro does not necessarily mean that a similar activation process will occur in vivo. Experience also teaches that argument by analogy can be very misleading in the case of sulfate activation. Small structural differences can upset the delicate balance between sulfate activation and the various other competing pathways. Nevertheless, sulfation is an important mechanism by which a number of chemicals are transformed to their activated forms.

Characterization of a human class-Theta glutathione S-transferase with activity towards 1-menaphthyl sulphate

A purification scheme is described for a glutathione S-transferase (GST) from human liver that catalyses the conjugation of 1-menaphthyl sulphate (MS) with GSH; the method devised results in an approx. 500-fold increase in specific activity towards MS. The human enzyme which metabolizes MS is a homodimer comprising subunits of M(r) 25,100, and immunochemical experiments have shown it to be a member of the class-Theta GSTs. Automated Edman degradation of this enzyme has confirmed that it is a Theta-class GST bu the amino acid sequence obtained differs from that of GST theta described previously [Meyer, Coles, Pemble, Gilmore, Fraser & Ketterer (1991) Biochem. J. 274, 409-414]. We have therefore designated the enzyme that catalyses the conjugation of MS with GSH GST T2-2* (in the absence of complete amino acid sequence data, the T1 and T2 subunits are provisionally designated T1* and T2*); the evidence which indicates that GST theta (which should possibly now be called GST T1-1*) and GST T2-2* represent distinct isoenzymes is discussed.

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.

Purification and immunochemical characterization of a rat liver sulphotransferase conjugating paracetamol

Paracetamol sulphotransferase (ST) was purified 250-fold from male rat liver, and the pure enzyme used to elicit antibodies in rabbit. The enzyme was active towards paracetamol at pH 9.0, as well as towards several commonly used drugs, and formed sulphates at both O- and N-atoms. Comparison of the substrate specificity of paracetamol ST with that of aryl sulphotransferases isolated by other workers suggested that we have purified a previously unknown isoenzyme of rat liver ST, although the difficulties of characterization of STs based on their substrate specificities is noted. The antibody preparation recognized only one polypeptide (Mr = 35,000) on immunoblot analysis of rabbit liver cytosol, corresponding to purified paracetamol ST. Analysis of the tissue distribution of this protein demonstrated that its expression was restricted to the liver, as was the enzyme activity. The observed sex difference in paracetamol ST (males greater than females) was determined by immunoblot analysis to be the result of reduced enzyme protein levels in females. In human liver cytosol, the antibody recognized two polypeptides, probably corresponding to M- and P-phenol STs, suggesting significant sequence similarity between rat and human phenol sulphotransferases.