A modified Ecteola cellulose assay for M and P phenol sulfotransferase

Human gastrointestinal sulfotransferases: identification and distribution

Sulfotransferases (STs) catalyze the sulfation of many structurally diverse molecules. Enzymatic assays and Western blots have been used to identify and characterize STs in the human gastrointestinal tract. Sulfation activities for 2-naphthol, dopamine, estradiol, and dehydroepiandrosterone (DHEA) from 23 donors were measured in cytosol prepared from stomach, duodenum, segments of small intestine, and colon and were compared to levels in human liver cytosol. Stomach and colon had low 2-naphthol and dopamine sulfation activities and almost no estradiol and DHEA sulfation activity. For all four substrates, small intestine has higher activities than both stomach and colon. Human small intestine 2-naphthol sulfation specific activity is approximately half that of human liver. Human small intestine dopamine sulfation activity is three times as high as that of human liver. While estrogen sulfation activity is about the same for both human intestine and human liver, human liver DHEA sulfation activity is about five times as high as that of human small intestine. The distribution of ST activities along the length of the small intestine was very different among different donors. Some donors had higher activity in the proximal segments of the small intestine, whereas other donors had higher activity in the distal segments of the small intestine. Our results also demonstrated high variation of small intestine sulfation activities compared with human liver activities among different donors. The Western blot results agreed with the enzymatic assay results. These results suggest that xenobiotics may regulate human small intestinal STs.

Enzymic sulphation of dopa and tyrosine isomers by HepG2 human hepatoma cells: stereoselectivity and stimulation by Mn2+

HepG2 human hepatoma cells, labelled with [35S]sulphate in media containing L-3,4-dihydroxyphenylalanine (L-dopa), (D-dopa), DL-m-tyrosine or D-p-tyrosine, were found to produce the [35S]sulphated forms of these compounds. Addition to the labelling media of m-hydroxybenzylhydrazine, an aromatic amino acid decarboxylase inhibitor, greatly enhanced the production of L-dopa O-[35S]sulphate and DL-m-tyrosine O-[35S]sulphate, with a concomitant decrease in the formation of dopamine O-[35S]sulphate and m-tyramine O-[35S]sulphate. With 3'-phosphoadenosine 5'-phospho[35S]sulphate as the sulphate donor., HepG2-cell cytosol was shown to contain enzymic activity catalysing the sulphation of L-dopa, D-dopa, L-m-tyrosine, D-m-tyrosine, L-p-tyrosine and D-p-tyrosine. The pH optimum of the enzyme, designated dopa/tyrosine sulphotransferase, was determined to be 8.75 with D-m-tyrosine as the substrate. The enzyme exhibited stereoselectivity for the D-form of dopa or tyrosine isomers. Addition of 10mM MnCl2 to the reaction mixture resulted in a remarkable stimulation of dopa/tyrosine sulphotransferase activity, being as high as 267.8 times with D-p-tyrosine as the substrate. Quantitative assays revealed L-dopa, D-dopa and D-m-tyrosine to be better substrates than L-p-tyrosine. When the HepG2-cell cytosol was subjected to DEAE Bio-Gel and hydroxyapatite column chromatography, dopa/tyrosine sulphotransferase was co-eluted with the thermolabile 'M-form' phenol sulphotransferase. Furthermore dopa/tyrosine sulphotransferase displayed properties similar to that of the M-form phenol sulphotransferase with respect to thermostability and sensitivity to 2,6-dichloro-4-nitrophenol. Whether the M-form phenol sulphotransferase is truly (solely) responsible for the dopa/tyrosine sulphotransferase activity present in HepG2 cells remains to be clarified.

De novo sulfation of L-tyrosine in HepG2 human hepatoma cells and its possible functional implication

HepG2 human hepatoma cells, labeled with [35S]sulfate in the presence of 10-30 micrograms/ml of cycloheximide, released up to 64% of the amount of free tyrosine-O-[35S]sulfate produced and released by cells labeled in the absence of cycloheximide. A time-course study revealed that, in cells incubated in medium containing [3H]tyrosine, free [3H]tyrosine-O-sulfate was produced within 5 min of incubation, whereas no [3H]tyrosine-sulfated proteins were detected until 20 min after the incubation had begun. Using 3'-phosphoadenosine, 5'-phospho[35S]sulfate as the sulfate donor, HepG2 cell homogenate was shown to contain enzymic activity catalyzing the sulfation of L-tyrosine with the formation of tyrosine-O-[35S]sulfate. Upon subcellular fractionation, the majority of the enzyme activity was found in the cytosolic fraction. The enzyme, designated tyrosine sulfotransferase, displayed the optimum activity at pH 8.0 in the presence of 10 mM Mn2+. Under optimum conditions, the apparent Km of the enzyme for L-tyrosine, at 4.5-microM concentration of 3'-phosphoadenosine, 5'-phosphosulfate, was determined to be 1.95 mM, while that for 3'-phosphoadenosine, 5'-phosphosulfate, at 1 mM L-tyrosine concentration, was 8.3 microM. The Vmax determined under these conditions was 1.05 pmol.min-1.mg protein-1. A tyrosine-dependence study showed that, for cells labeled with [35S]sulfate, the production and release of free tyrosine-O-[35S]sulfate appeared to proceed actively and increase proportionally to the L-tyrosine concentration when it was raised above a threshold level in the culture medium. These results may imply a possible involvement of sulfation in removing excess intracellular L-tyrosine.

Common food additives are potent inhibitors of human liver 17 alpha-ethinyloestradiol and dopamine sulphotransferases

Interactions between dietary xenobiotics, drugs and biologically active endogenous compounds are a potential source of idiosyncratic adverse pathology. We have examined the inhibition of the sulphation of a number of xenobiotics and endobiotics in human liver cytosol by 15 food additives and constituents. Sulphation of dehydroepiandrosterone was resistant to inhibition by all compounds tested; however, dopamine sulphotransferase (ST) activity was inhibited strongly by (+/-)-catechin, (+)-catechin, octyl gallate, tartrazine and vanillin. Sulphation of the xenobiotic steroid 17 alpha-ethinyloestradiol (EE2) was inhibited by vanillin, erythrosin B and octyl gallate. Of these compounds, only vanillin was found to be sulphated to a significant extent by both human liver and platelets, and vanillin was determined to be a substrate for the monoamine-sulphating isoenzyme of phenolsulphotransferase. Vanillin was found to inhibit 50% of liver EE2 ST activity (IC50) at a concentration of approximately 1.3 microM and the mode of inhibition was non-competitive. The implications of these results for the adverse side effects associated with food additives and oral contraceptives are discussed.

Purification and characterization of human liver phenol-sulfating phenol sulfotransferase

The phenol-sulfating form of phenol sulfotransferase (P-PST) was purified and characterized from human liver cytosol using DEAE-cellulose, Sephacryl S-200, and 3',5'-diphosphoadenosine-agarose affinity chromatography. During the purification procedure, P-PST was resolved from the monoamine-sulfating form of phenol sulfotransferase (M-PST) and dehydroepiandrosterone sulfotransferase, which are also present in human liver cytosol. P-PST activity was purified 560-fold as compared to liver cytosol and the purified enzyme possessed a specific activity of 340 nmol phenol sulfated per minute per milligram protein. Enzymatically active P-PST has an apparent molecular size of 68,000 Da as determined by Sephacryl S-200 chromatography and a subunit molecular weight of 32,000 Da as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, suggesting that P-PST exists in vivo as a homodimer. Antibodies raised to human platelet M-PST cross-reacted strongly with pure P-PST suggesting the two PSTs are structurally closely related. Two types of P-PST activity have been identified in different human livers by their thermostability and elution during anion-exchange chromatography. Each of the livers examined possessed only one type of P-PST activity. Both types of P-PST were shown to possess the same subunit molecular weight and immunoreactivity, whereas the differences in thermostability of the two P-PST activities appeared to be related to the method of preparation of liver cytosol. Both types of P-PST activity were inhibited to similar extents by incubation with 50 microM N-ethylmaleimide or 5 mM phenylglyoxal. These results suggest that the two types of P-PST in different human livers are very similar and probably represent different allelic forms of the enzyme.

O2-dependent methionine auxotrophy in Cu,Zn superoxide dismutase-deficient mutants of Saccharomyces cerevisiae

Mutant strains of the yeast Saccharomyces cerevisiae which lack functional Cu,Zn superoxide dismutase (SOD-1) do not grow aerobically unless supplemented with methionine. The molecular basis of this O2-dependent auxotrophy in one of the mutants, Dscd1-1C, has been investigated. Sulfate supported anaerobic but not aerobic mutant growth. On the other hand, cysteine and homocysteine supported aerobic growth while serine, O-acetylserine, and homoserine did not, indicating that the interconversion of cysteine and methionine (and homocysteine) was not impaired. Thiosulfate (S2O3(2-] and sulfide (S2-) also supported aerobic growth; the activities of thiosulfate reductase and sulfhydrylase in the aerobic mutant strain were at wild-type levels. Although the levels of SO4(2-) and adenosine-5'-sulfate (the first intermediate in the SO4(2-) assimilation pathway) were elevated in the aerobically incubated mutant strain, this condition could be attributed to a decrease in protein synthesis caused by the de facto sulfur starvation and not to a block in the pathway. Therefore, the activation of SO4(2-) (to form 3'-phosphoadenosine-5'-phosphosulfate) appeared to be O2 tolerant. Sulfite reductase activity and substrate concentrations [( NADPH] and [SO3(2-)]) were not significantly different in aerobically grown mutant cultures and anaerobic cultures, indicating that SOD-1- mutant strains could reductively assimilate sulfur oxides. However, the mutant strain exhibited an O2-dependent sensitivity to SO3(2-) concentrations of less than 50 microM not exhibited by any SOD-1+ strain or by SOD-1- strains supplemented with a cytosolic O2(-)-scavenging activity. This result suggests that the aerobic reductive assimilation of SO4(2-) at the level of SO3(2-) may generate a cytotoxic compound(s) which persists in SOD-(1-) yeast strains.

Purification and characterization of human liver dehydroepiandrosterone sulphotransferase

A form of sulphotransferase capable of sulphating dehydroepiandrosterone and other steroids was purified from cytosol prepared from human liver. Dehydroepiandrosterone sulphotransferase was purified 621-fold when compared with the activity in cytosol using DEAE-Sepharose CL-6B and adenosine 3',5'-bisphosphate-agarose affinity chromatography. During affinity chromatography, dehydroepiandrosterone sulphation activity could be resolved from p-nitrophenol sulphation activity catalysed by phenol sulphotransferase by using a gradient of adenosine 3'-phosphate 5'-phosphosulphate. The purified enzyme was most active towards dehydroepiandrosterone but was capable of conjugating a number of other steroids, including pregnenolone, androsterone and beta-oestradiol. No activity towards p-nitrophenol or dopamine, substrates for the phenol sulphotransferase, was observed with the pure enzyme. A single band with a subunit molecular mass of 35 kDa was observed by Coomassie Blue staining following SDS/polyacrylamide-gel electrophoresis of the purified enzyme. A molecular mass of 68-70 kDa was calculated for the active form of the enzyme by chromatography on Sephacryl S-200, suggesting that the active form of the enzyme is a dimer.

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.

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

The identification of three forms of phenol sulfotransferase (PST) in human brain and the subsequent purification and kinetic characterization of a phenol-sulfating form of the enzyme are described. Two forms of PST which were capable of conjugating phenol and a third form which sulfated dopamine were resolved from one another using DEAE-cellulose chromatography. One of the phenol-sulfating forms (P1-PST) was subsequently purified on Affi-Gel blue and Sephacryl S-200, giving a final purification of almost 390-fold, with an overall yield of approximately 5%. The purified enzyme was sensitive to NaCl and showed an optimum for phenol conjugation at pH 8.5. Kinetic analysis demonstrated that sulfation by P1-PST proceeds via a sequential ordered, bi-substrate reaction mechanism, where 3'-phosphoadenosine-5'-phosphosulfate (PAPS) is the leading substrate. The true Km and Kia values for PAPS were both 0.35 microM, while the true Km value for phenol was 2.8 microM.

Effect of phosphatase inhibition of in vitro dopamine sulfation and 3'-phosphoadenosine-5'-phosphosulfate catabolism in human brain

The effects of inhibition of phosphatase activity in 100,000 g supernatant solution from human frontal cortex on dopamine (DA) conjugation were examined using the phosphatase substrate p-nitrophenyl phosphate (pNPO4). The increases in DA sulfation seen in the presence of pNPO4 suggested that inhibition of phosphatase activity in high speed supernatant solutions of brain may substantially alter the pattern of 3'-phosphoadenosine-5'-phosphosulfate (PAPS) metabolism and subsequently the rate of DA sulfation. Accordingly, the effects of the pyrophosphate analog phosphonoacetic acid (PAA) on the extent of DA sulfation and PAPS metabolism were examined in 100,000 g supernatant solution from human frontal cortex. At concentrations up to 10 mM, PAA markedly reduced PAPS hydrolysis to inorganic sulfate and 3'-phosphoadenosine-5'-phosphate (PAP) and significantly extended the linear time period for the sulfation of DA. These findings suggest that the phosphatase enzymes that degrade PAPS to produce the end product inhibitor, PAP, and possibly other break-down products of PAP, play an important role in determining the observed levels of phenol sulfotransferase activity in tissue from human brain in vitro.