Purification and characterization of N-hydroxy-2-acetylaminofluorene sulfotransferase from rat liver

Identification and characterization of KpsS, a novel polysaccharide sulphotransferase in Mesorhizobium loti

Plants enter into symbiotic relationships with bacteria that allow survival in nutrient-limiting environments. The bacterium Mesorhizobium loti enters into a symbiosis with the legume host, Lotus japonicus, which results in the formation of novel plant structures called root nodules. The bacteria colonize the nodules, and are internalized into the cytoplasm of the plant cells, where they reduce molecular dinitrogen for the plant. Symbiosis between M. loti and L. japonicus requires bacterial synthesis of secreted and cell-surface polysaccharides. We previously reported the identification of an unusual sulphate-modified form of capsular polysaccharide (KPS) in M. loti. To better understand the physiological function of sulphated KPS, we isolated the sulphotransferase responsible for KPS sulphation from M. loti extracts, determined its amino acid sequence and identified the corresponding M. loti open reading frame, mll7563 (which we have named kpsS). We demonstrated that partially purified KpsS functions as a fucosyl sulphotransferase in vitro. Furthermore, mutants deficient for this gene exhibit a lack of KPS sulphation and a decreased rate of nodule formation on L. japonicus. Interestingly, the kpsS gene product shares no significant amino acid similarity with previously identified sulphotransferases, but exhibited sequence identity to open reading frames of unknown function in diverse bacteria that interact with eukaryotes.

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.

The same enzymes catalyze sulfation of minoxidil, minoxidil analogs and catecholamines

Rat liver sulfation of minoxidil and minoxidil analogs is described and the enzymes responsible are compared with those that sulfate catecholamines. Our study of minoxidil sulfation showed male-dominant sex dimorphism of the enzyme activity due to two enzymes that coelute with dopamine sulfotransferases. The most abundant isoform, in our routine assay, is minoxidil sulfotransferase 2 (P2). Sulfation ability by this enzyme parallels minoxidil analog antihypertensive ability. Minoxidil, analog and dopamine sulfotransferases were not separable by several different chromatographic methods, supporting their identity. The minoxidil sulfotransferase activity dropped in hypophysectomized males, due mostly to diminished levels of minoxidil sulfotransferase 2/dopamine sulfotransferase II, which appears to be aryl sulfotransferase IV. Its relationship to the minoxidil sulfotransferase reported by the Falany group is not clear. Here, we describe the exploration of rat liver sulfation of antihypertensive minoxidil (2,4-diamino-6-piperidinopyrimidine-3-oxide), and several minoxidil analogs. Minoxidil sulfation was first reported in rat and human liver cytosol by Johnson's group at Upjohn [1,2]. Our interest in the sulfation process arose because direct action of minoxidil sulfate had been implicated both in vivo [3,4] and in vitro [5] in blood pressure control processes.

Molecular mechanisms for the regulation of aryl sulfotransferase IV expression during 2-acetylaminofluorene-induced hepatocarcinogenesis in rat

The dietary administration of 2-acetylaminofluorene to male rats to induce hepatocarcinogenesis causes a reversible as well as persistent down-modulation of N-hydroxy-2AAF sulfotransferase activity. Studies are presented which indicate that several molecular mechanisms may be involved in the down-regulation of sulfotransferase activity and expression. These include carcinogen-mediated inactivation of sulfotransferase mRNA or protein, interference with hormonal regulation of sulfotransferase expression, and, mutation of the sulfotransferase gene.

Biochemistry of cytosolic sulfotransferases involved in bioactivation

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

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.

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.

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

Sulphation of the carcinogen 5-hydroxymethyl-chrysene (5-HCR) to the active metabolite 5-HCR sulphate occurred at significant rates in all of hepatic cytosols prepared from the male and female experimental animals, rats, mice, guinea-pigs and hamsters. The 5-HCR-sulphating activity was also found in kidney cytosols of all the experimental animals used, while their activities were much less than those of hepatic cytosols. In the male mice, the enzyme activity of testis was higher than any other examined tissue. Small intestine and adrenal of male and female guinea-pigs had relatively high enzyme activities. Small enzyme activities were also found in a variety of extrahepatic tissues of some of these animals. Marked species and sex differences (female much greater than male in the rat and mouse) were observed in the hepatic enzyme activity. In the female rat liver which showed the highest 5-HCR-sulphating activity among the examined tissues of all the animals, a typical hydroxysteroid sulphotransferase inhibitor, dehydroepiandrosterone (DHA) sulphate (1 mM), potently and competitively inhibited the sulphation of 5-HCR as well as that of DHA, a typical substrate for hydroxysteroid sulphotransferases. On the contrary, the phenol sulphotransferase inhibitors, pentachlorophenol and 2,6-dichloro-4-nitrophenol, had only a little effect on these enzyme activities even at a concentration of 50 microM that showed a potent inhibition of the phenol sulphotransferase activity. These results suggest that 5-HCR be sulphated in the female rat liver by hydroxysteroid sulphotransferases, but not by phenol sulphotransferases.

Sulfation of mono- and diaryl oximes by aryl sulfotransferase isozymes

Aryl sulfotransferases (3'-phosphoadenylsulfate:phenol sulfotransferase, EC 2.8.2.1) catalyze the sulfonation of a wide variety of hydroxyl-containing substrates, including numerous xenobiotics. The chemical diversity of aryl sulfotransferase substrates is in part attributable to the presence of multiple isozymes, each of which has broad substrate specificity. Of the aryl sulfotransferase isozymes in rat liver cytosol, two (designated isozymes I and II) have previously been shown to sulfonate phenolic compounds exclusively and, moreover, have very similar substrate specificity patterns. The recently reported unusually efficient, rapid isozyme I-catalyzed sulfonation of 9-fluorenone oxime (Mangold, J.B., Mangold, B.L.K. and Spina, A. (1986) Biochim. Biophys. Acta 874, 37-43) was therefore unexpected and suggested that aryl oximes may represent a useful class of model compounds to probe isozymic differences in substrate steric and electronic requirements. In the present study, several mono- and diaryl oximes have been prepared and tested as potential substrates for partially purified aryl sulfotransferases I and II from rat liver cytosol. The results indicate that steric factors, specifically planarity and hydroxyl group position, appear to be important requirements for enzyme-catalyzed sulfonation. In addition, although isozymes I and II had comparable activity with diaryl oximes, some striking differences in the ability of these two isozymes to sulfonate both substituted and unsubstituted monoaryl oximes were observed. This dissimilarity is consistent with distinct differences in the active sites of these isozymes.

Hepatic dopamine sulfotransferases in untreated rats and in rats subjected to endocrine or hypertension-related treatments

Here we describe the dopamine sulfotransferase activity of rat liver cytosol. With cytosol, 3'-phosphoadenosine-5'-phosphosulfate and dopamine Km values were 17.2 +/- 4.1 and 22.4 +/- 3.5 microM. Females possessed 23 to 37% of dopamine sulfotransferase levels, per gm liver, in males. DEAE-Sephadex A-50 chromatography resolved dopamine sulfotransferase activity to dopamine sulfotransferase I and dopamine sulfotransferase II. Dopamine sulfotransferase II comprised 79 +/- 10 or 61 +/- 18% of dopamine sulfotransferase in males or females in routine assays. 4-Methoxytyramine gave 609 or 179% of mean dopamine sulfotransferase activity with dopamine sulfotransferase I or II. Dopamine and 3-methoxytyramine were comparable substrates. Epinephrine was less effective. Mn++, Cd++, Zn++, Na+ and K+ inhibited dopamine sulfotransferase II. Mg++ activated it. Dopamine sulfotransferase II from males was purified 184 +/- 64-fold. Its Km values for 3'-phosphoadenosine-5'-phosphosulfate and dopamine were 12.7 +/- 1.5 and 47.5 +/- 6.7 microM, respectively. Its dopamine sulfotransferase mechanism was sequential. The molecular weight of dopamine sulfotransferase II was 49,100 +/- 4,000 by Sephadex G-100 chromatography. Dopamine sulfotransferase II preferred phenol to catecholamines. Dopamine and 3,4-dihydroxybenzylamine were its best catecholamine substrates. Adrenalectomy or castration of males led to 35 or 45% mean decreases of dopamine sulfotransferase levels, indicating adrenal and gonadal participation in control of dopamine sulfotransferase production. Testosterone had no effect in either sex, whereas estradiol led to 40% mean decreases of dopamine sulfotransferase levels in males. This suggested a role for ovaries in dopamine sulfotransferase production, supported by 55 to 102% increased dopamine sulfotransferase levels after ovariectomy. Okamoto-hypertensive males or males given hypertensogenic doses of cortisol exhibited 37 or 48% mean increases of dopamine sulfotransferase levels per gm liver. Antihypertensive spironolactone or hydralazine led to 30% mean decreases of dopamine sulfotransferase levels. Altered dopamine sulfotransferase levels after all experimental manipulations were due mostly to changed dopamine sulfotransferase II content. Dopamine sulfotransferase II is compared to other reported enzymes that sulfate catecholamines.

Covalent binding of 4-nitrobenzyl mercaptan S-sulfate to the sulfhydryl groups of hepatic cytosolic proteins and bovine serum albumin with mixed disulfide bond formation

4-Nitrobenzyl [35S]mercaptan S-sulfonic acid ([35S]NBM S-sulfate), a new type of reactive metabolite of the thiol [35S]NBM in rat liver cytosol fortified with 3'-phosphoadenosine 5'-phosphosulfate, bound rapidly and covalently at pH 7.4 and 37 degrees C to the sulfhydryl groups of rat liver cytosolic proteins with formation of disulfide bonds. From the radioactive proteins was isolated and identified the sole amino acid adduct, S-([35S]NBM)cysteine, after their acid hydrolysis under the anaerobic conditions. Bovine serum albumin (BSA), a model protein with a single SH group, also reacted readily with radioactive NBM S-sulfate to form a disulfide bond in stoichiometric manner. S-([35S]NBM)-cysteine was also isolated and identified as the sole amino acid adduct from the well-washed, radioactive BSA after the same anaerobic acid hydrolysis. A normal hepatic level of GSH not only retarded the BSA-NBM adduct formation completely, but also detached the radioactivity from BSA by the reduction of the disulfide bond with formation of [35S]NBM and its disulfide. Of twenty-one amino acids examined at pH 7.4 and 37 degrees C, only cysteine reacted with NBM S-sulfate and afforded S-(NBM)cysteine with concomitant formations of S-sulfocysteine, cystine, NBM, and its disulfide.

Sex differences in sulfation and glucuronidation of phenol, 4-nitrophenol and N-hydroxy-2-acetylaminofluorene in the rat in vivo

Sulfation and glucuronidation of phenol, 4-nitrophenol (4NP) and N-hydroxy-2-acetylaminofluorene (N-OH-AAF) were studied in adult (60 days) male and female rats. Within 3 hours almost 50% of a dose of phenol was excreted in urine as phenyl sulfate; male rats excreted slightly more phenyl sulfate than females. This probably was due to a slower excretion of phenyl sulfate by the females. No sex difference in glucuronidation of phenol was found. Over a period of 24 hours male and female rats excreted almost 35% of a dose of 4NP as 4NP-sulfate in urine and almost 40% as 4NP-glucuronide. No differences in the excretion of 4NP-conjugates were found between sexes. However, almost twice as much of a dose of N-OH-AAF was excreted after 4 hours as the N-O-glucuronide in bile and urine in female than in males. On the other hand, females excreted less of the AAF-glutathione conjugates that are derived from the reaction of AAF-N-sulfate with glutathione in vivo [Meerman et al., Chem.-Biol. Interactions, 39, 149, 1982] in bile, than males. This indicates that sulfation of N-OH-AAF is less active in females than in males. Most likely, sulfation of the phenols is catalyzed by a different sulfotransferase than that of N-OH-AAF.

Covalent binding of a mercaptan S-sulfate to hepatic cytosolic proteins and its inhibition by glutathione

4-Nitrobenzyl mercaptan (NBM) S-sulfate, a new type of the sulfate conjugate enzymatically formed from NBM in the presence of 3'-phosphoadenosine 5'-phosphosulfate in rat liver cytosol, bound covalently to rat liver cytosolic proteins at pH 7.4. The protein binding of NBM S-sulfate was strongly retarded by GSH. GSH not only played a role as a scavenger for NBM S-sulfate with formation of NBM and GSSG via S-(4-nitrobenzyl)thioglutathione, but also cleaved the covalent bonds, possibly disulfides formed from NBM S-sulfate and sulfhydryl groups of the cytosolic proteins. Thus, evidence was provided that NBM S-sulfate be a new type of the reactive metabolite.

The S-sulfate formation from 4-nitrobenzyl mercaptan in rat liver cytosol

4-Nitrobenzyl mercaptan (NBM) was enzymatically transformed at pH 6.0 into its S-sulfate in rat liver cytosol fortified with 3'-phosphoadenosine 5'-phosphosulfate. At pH 7.4, the S-sulfate was not detected from the incubation mixture. 4-Nitrobenzyl alcohol was also transformed under the same incubation conditions into the corresponding O-sulfate at a higher rate at pH 6.0 than at pH 7.4. Under the incubation conditions used, NBM S-sulfate reacted with the substrate NBM at a significant rate to afford 4-nitrobenzyl disulfide. The disulfide formation from NBM and the S-sulfate occurred more readily at pH 7.4 than at pH 6.0, so that biologically formed NBM S-sulfate was strongly suggested not to remain unchanged in the incubation mixture at pH 7.4.

Sulfation of minoxidil by liver sulfotransferase

The 100,000 g supernatant fraction of rat liver homogenate contains a sulfotransferase activity which catalyzes the sulfation of minoxidil. Synthetic minoxidil N-O sulfate and the enzyme synthesized product had identical chromatographic characteristics on high pressure liquid chromatography. Minoxidil sulfate, which yields minoxidil when treated with sulfatase, was slowly hydrolyzed in water. Several N-oxides of other heterocycles, including several other pyrimidines, triazines and imidazoles, were also substrates for this sulfotransferase.

Enzymatic sulfation of steroids. XV. Studies differentiating between rat liver androgen, estrogen, bile acid, glucocorticoid and phenol sulfotransferases

Earlier reports left the number of enzymes that catalyzed phenol, androgen, estrogen, bile acid and glucocorticoid sulfation obscure. Here, we have utilized chromatographic, immunochemical and endocrinologic methods to compare and differentiate these enzymes in rat liver. Sulfotransferases I, II, and III--which sulfate glucocorticoids--were used in this comparison. We found that phenols were sulfated by phenol sulfotransferases 1 and 2, which were unrelated to the other enzymes studied here. Large amounts of phenol sulfotransferase 1 were found in both sexes. Large amounts of phenol sulfotransferase 2 were restricted to males. By contrast, the small amount of androgen sulfation found in both sexes appeared to be mediated by sulfotransferase II, which preferred 3 beta-hydroxysteroids, but also sulfated estrogens and glucocorticoids to lesser extents. The sulfation of estrogens presented a more complex picture. Most estradiol sulfotransferase activity in both sexes was due to an enzyme that sulfated estrone poorly, and did not sulfate the other steroids tested. This specific estradiol sulfotransferase was unrelated to the other sulfotransferases described here. Smaller amounts of estrogen sulfotransferase activity that sulfated estradiol and estrone equally well were present at concentrations dependent on the sex of test animals. This enzyme activity appeared to be due to sulfotransferases I, II and III. Most bile acid sulfotransferase activity eluted from DEAE-Sephadex A-50 columns with sulfotransferases I and II. However, data with males suggested that these enzymes were not responsible. Thus, phenols, androgens, estrogens and glucocorticoids were sulfated by six enzymes of differing substrate specificity: phenol sulfotransferases 1 and 2, specific estradiol sulfotransferase, and sulfotransferases I, II, and III. Unique bile acid sulfotransferases also appeared probable.

Biotransformation of xenobiotics in Drosophila melanogaster and its relevance for mutagenicity testing

Biotransformation of lipophilic xenobiotics may lead to formation of reactive intermediates which can give rise to irreversible toxic events such as carcinogenesis, mutagenesis, teratogenesis, and tissue necrosis. In recent years considerable attention has been paid to the problem of testing for these effects. Short-term mutagenicity tests have been shown to have value for predicting the occurrence of delayed toxic effects in mammals following administration of indirectly acting harmful xenobiotics. In any test system the capacity to bioactivate the compound under test is a necessary prerequisite, and in most short-term test assays this is provided for by adding a metabolic activation system generally consisting of the 9,000 g supernatant fraction of a rat liver homogenate supplied with cofactors. The fruitfly Drosophila melanogaster constitutes an organism well-suited for mutagenicity testing, and it was shown that a number of precarcinogens evoke mutagenic effects in this species. Thus Drosophila is apparently able to metabolize xenobiotics to reactive intermediates, which in turn induce mutagenicity. However, knowledge about the presence and characteristics of the xenobiotic-metabolizing enzymes involved is lacking. Since knowledge of these enzymes contributes to the evaluation and interpretation of observed mutagenic events, this paper described studies concerning some important xenobiotic-metabolizing enzymes of Drosophila. Files were homogenized and subcellular fractions were investigated with respect to enzymatic activities. It was possible to demonstrate cytochrome P-450 and some related mixed-function oxidase activities. Cytochrome b5, epoxide hydrolase, and glutathione S-transferase are also present, while preliminary experiments suggest the presence of UDP-glucosyltransferase and phosphotransferase activities. The enzymes which have been found are discussed with respect to their similarities with rat liver enzymes and their relevance for mutagenicity testing with Drosophila melanogaster.