Evidence for an ordered reaction mechanism for bile salt: 3'phosphoadenosine-5'-phosphosulfate: sulfotransferase from rhesus monkey liver that catalyzes the sulfation of the hepatotoxin glycolithocholate

Paradigms of sulfotransferase catalysis: the mechanism of SULT2A1

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

Structural rearrangement of SULT2A1: effects on dehydroepiandrosterone and raloxifene sulfation

BACKGROUND: Human cytosoloic sulfotransferase (SULT) 2A1 is a major hepatic isoform and sulfates hydroxyl groups in structurally diverse sterols and xenobiotics. SULT2A1 crystal structures resolved in the presence and absence of 3',5'-diphosphoadenosine (PAP) or dehydropeiandrosterone (DHEA) suggest a significant rearrangement of the peptide that forms the surface of the active site in the presence of PAP. MATERIALS AND METHODS: Molecular modeling was used to examine the effects of the rearrangement in SULT2A1 associated with 3'-phosphoadenosine 5'-phosphosulfate (PAPS) binding on the binding of DHEA and raloxifene. The kinetics of DHEA and raloxifene sulfation was analyzed to investigate the effects of the rearrangement on SULT2A1 activity. RESULTS: Molecular models indicate that DHEA is able to bind to SULT2A1 in both conformations (open, without PAP; closed, with PAP) in a catalytic configuration, whereas raloxifene bound in a catalytic conformation only in the open structure. Raloxifene did not bind in the smaller, closed substrate binding pocket. Kinetic analysis of DHEA sulfation was consistent with a random Bi-Bi reaction mechanism, whereas raloxifene sulfation was more indicative of an ordered reaction mechanism with raloxifene binding first. Initial burst kinetics with DHEA yielded similar results after preincubation of SULT2A1 with DHEA or PAPS. Preincubation of SULT2A1 with raloxifene showed a burst of raloxifene sulfate formation with the addition of PAPS. In contrast, little raloxifene sulfate was formed if SULT2A1 was preincubated with PAPS and the reaction initiated with raloxifene. CONCLUSIONS: The structural rearrangements in SULT2A1 caused by PAPS binding can alter the sulfation mechanism and kinetics of different substrates.

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

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

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

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

Crystal structure-based studies of cytosolic sulfotransferase

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

Effects of 3'-phosphoadenosine 5'-phosphate on the activity and folding of phenol sulfotransferase

Known spectroscopic and kinetic data are used to formulate pathways of the physiological and transfer reactions and the substrate inhibition of phenol sulfotransferase. Kinetic mechanisms indicate that release of PAP from enzyme complex is required for the physiological reaction but not for the transfer reaction. The pathways explain rate difference between the physiological and transfer reactions since the release of PAP is the rate-limiting step of the former reaction. Two enzyme species of phenol sulfotransferase which distinguish the physiological and transfer reaction were found to involve the binding of PAP. Differences between two forms of phenol sulfotransferase, alpha and beta, indicate that they assemble through different folding process. It is demonstrated that only alpha enzyme renatures in the presence of PAP and beta enzyme renatures only in the absence of PAP in vitro. In the over-expressed system, formation of alpha and beta phenol sulfotransferase is also dependent on the availability of PAP in Escherichia coli. It is concluded that folding of phenol sulfotransferase is assisted by PAP to form alpha enzyme. In the absence of PAP, beta form of phenol sulfotransferase is produced.

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

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

2-Fluoro-beta-alanine, a previously unrecognized substrate for bile acid coenzyme A:amino acid:N-acyltransferase from human liver

Our laboratory has demonstrated recently that conjugates of 2-fluoro-beta-alanine (FBAL) and bile acids are the major biliary metabolites of 5-fluorouracil (FUra) in cancer patients. Bile acids are normally conjugated with glycine or taurine, and therefore the identification of the FBAL-bile acid conjugates suggested that FBAL may also be a substrate for the bile acid conjugating enzyme, bile acid CoA:amino acid:N-acyltransferase. Enzyme activity detected using glycine and taurine as substrates was purified 8-fold from human liver cytosol using a DEAE-cellulose column. This preparation when tested for its activity towards beta-alanine and FBAL using cholyl CoA as the bile acid substrate only catalyzed the formation of FBAL-cholate. beta-Alanine was not a substrate. Confirmation of FBAL-cholate as the enzymatic product was demonstrated by (1) coelution of the product of this reaction on HPLC with authentic FBAL-cholate, (2) specific hydrolysis of this product by cholylglycine hydrolase, and (3) molecular weight of the product (497) being identical to that of the authentic FBAL-cholate. Kinetic experiments demonstrated that the enzyme had an affinity for FBAL (Km 1.45 mM) comparable to taurine (Km 1.32 mM), but greater than glycine (Km 6.45 mM). Formation of FBAL-cholate was inhibited competitively by taurine (Ki 1.27 mM) and glycine (Ki 4.47 mM), suggesting that a single enzyme is responsible for conjugation of glycine, taurine and FBAL with bile acids. These data indicate that the formation of the FBAL-bile acid conjugates in patients receiving FUra results from high affinity of the bile acid conjugating enzyme for FBAL.