Affinity labeling of aryl sulfotransferase IV. Identification of a peptide sequence at the binding site for 3‘-phosphoadenosine-5‘-phosphosulfate

The genetic authentication of Panax ginseng and Panax quinquefolius based on using single nucleotide polymorphism (SNP) conducted in a nucleic acid test chip

Panax ginseng and Panax quinquefolius, which are commonly called Chinese ginseng and American ginseng respectively, have different medicinal properties and market values; however, these samples can be difficult to differentiate from one another based on physical appearances of the samples especially when they are in powdery or granular forms. A molecular technique is thus needed to overcome this difficulty; this technique is based on the nucleic acid test (NAT) conducted on the microfluidic chip surface. Three single nucleotide polymorphism (SNP) sites (i.e. N1, N2, N3) on the Panax genome that differ between P. ginseng (G) and P. quinquefolius (Q) have been selected to design probes for the NAT. Primers were designed to amplify the antisense strands by asymmetric PCR. We have developed three different NAT methodologies involving surface immobilization and subsequent (stop flow or dynamic) hybridization of probes (i.e. N1G, N1Q, N2G, N2Q, N3Q) to the antisense strands. These NAT methods consist of two steps, namely immobilization and hybridization, and each method is distinguished by what is immobilized on the microfluidic chip surface in the first step (i.e. probe, target or capture strand). These three NATs developed are called probe-target method 1, target-probe method 2 and three-strand complex method 3. Out of the three methods, it was found that the capture strand-target-probe method 3 provided the best differentiation of the ginseng species, in which a 3' NH₂ capture strand is first immobilized and the antisense PCR strand is then bound, while N2G and N3Q probes are used for detection of P. ginseng (G) and P. quinquefolius (Q) respectively.

Reversible covalent inhibition of a phenol sulfotransferase by coenzyme A

Phenol sulfotransferases (SULTs), which normally bind 3'-phosphoadenosine-5'-phosphosulfate as the donor substrate, are inhibited by CoA and its thioesters. Here, we report that inhibition of bovine SULT1A1 by CoA is time-dependent at neutral pH under non-reducing conditions. The rates of inactivation by CoA indicate an initial reversible SULT:CoA complex with a dissociation constant of 5.7 microM and an inactivation rate constant of 0.07 min(-1). Titrations with CoA and prolonged incubations reveal that inactivation of the dimeric enzyme is stoichiometric, consistent with the observation of complete conversion of the protein to a slightly decreased electrophoretic mobility. Both activity and normal electrophoretic migration are restored by 2-mercaptoethanol. Mutagenesis demonstrated that Cys168 is the site of CoA adduction, and a consistent model was constructed that reveals a new SULT molecular dynamic. Cysteine reaction kinetics with Ellman's reagent revealed a PAPS-induced structural change consistent with the model that accounts for binding of CoA.

The role of lysine residues 297 and 306 in nucleoside triphosphate regulation of E. coli CTP synthase: inactivation by 2',3'-dialdehyde ATP and mutational analyses

Cytidine 5'-triphosphate synthase (CTPS) catalyzes the ATP-dependent formation of CTP from UTP using either NH3 or L-glutamine as the source of nitrogen. To identify the location of the ATP-binding site within the primary structure of E. coli CTPS, we used the affinity label 2',3'-dialdehyde adenosine 5'-triphosphate (oATP). oATP irreversibly inactivated CTPS in a first-order, time-dependent manner while ATP protected the enzyme from inactivation. In the presence of 10 mM UTP, the values of k(inact) and K(I) were 0.054 +/- 0.001 min(-1) and 3.36 +/- 0.02 mM, respectively. CTPS was labeled using (2,8-3H)oATP and subsequently subjected to trypsin-catalyzed proteolysis. The tryptic peptides were separated using reversed-phase HPLC, and two peptides were identified using N-terminal sequencing (S(492)GDDQLVEIIEVPNH(506) and Y(298)IELPDAY(K(306)) in a 5:1 ratio). The latter suggested that Lys 306 had been modified by oATP. Replacement of Lys 306 by alanine reduced the rate of oATP-dependent inactivation (k(inact) = 0.0058 +/- 0.0005 min(-1), K(I) = 3.7 +/- 1.3 mM) and reduced the apparent affinity of CTPS for both ATP and UTP by approximately 2-fold. The efficiency of K306A-catalyzed glutamine-dependent CTP formation was also reduced 2-fold while near wild-type activity was observed when NH3 was the substrate. These findings suggest that Lys 306 is not essential for ATP binding, but does play a role in bringing about the conformational changes that mediate interactions between the ATP and UTP sites, and between the ATP-binding site and the glutamine amide transfer domain. Replacement of the nearby, fully conserved Lys 297 by alanine did not affect NH3-dependent CTP formation, relative to wild-type CTPS, but reduced k(cat) for the glutaminase activity 78-fold. Our findings suggest that the conformational change associated with binding ATP may be transmitted through the L10-alpha11 structural unit (residues 297-312) and thereby mediate effects on the glutaminase activity of CTPS.

Histidine residues in human phenol sulfotransferases

Sulfotransferases are phase II drug-metabolizing enzymes that catalyze the sulfation of hydroxyl-containing compounds, leading to detoxification of xenobiotic toxicants. The universal sulfuryl donor is adenosine 3'-phosphate-5'-phosphosulfate. Human simple phenol sulfotransferase (P-PST) is one of the major human sulfotransferases that catalyze the sulfation of most phenols. Human monoamine phenol sulfotransferase (M-PST) has high affinity for monoamines and also catalyzes the sulfation of simple phenols at high substrate concentrations. In this report, the amino acid modification method was used for studies of His residues in the active site of P-PST and M-PST. The His specific modification reagent diethylpyrocarbonate was used for the modification of His residues in P-PST and M-PST. Diethylpyrocarbonate inactivation kinetic data suggest that there is one His residue in the active site that is critical for catalytic activity of both P-PST and M-PST. The modification has no effect on phenol or monoamine substrate binding for M-PST, but it does have an effect on adenosine 3'-phosphate-5'-phosphosulfate binding with M-PST. The experimental results agree with amino acid sequence alignment, mutation, and the crystal structures of P-PST and M-PST and suggest that His108 is the only critical His residue in both P-PST and M-PST. The differing roles His108 plays in P-PST and M-PST may explain the substrate specificity of the two isoforms.

Arginine residues in the active site of human phenol sulfotransferase (SULT1A1)

Cytosolic sulfotransferases (STs) catalyze the sulfation of hydroxyl containing compounds. Human phenol sulfotransferase (SULT1A1) is the major human ST that catalyzes the sulfation of simple phenols. Because of its broad substrate specificity and lack of endogenous substrates, the biological function of SULT1A1 is believed to be an important detoxification enzyme. In this report, amino acid modification, computer structure modeling, and site-directed mutagenesis were used for studies of Arg residues in the active site of SULT1A1. The Arg-specific modification reagent, 2,3-butanedione, inactivated SULT1A1 in an efficient, time- and concentration-dependent manner, suggesting Arg residues play an important role in the catalytic activity of SULT1A1. According to the computer model, Arg78, Arg130, and Arg257 may be important for SULT1A1 catalytic activity. Site-directed mutagenesis results demonstrated that the positive charge on Arg78 is not critical for SULT1A1 because R78A is still active. In contrast, a negative charge at this position, R78E, completely inactivated SULT1A1. Arg78 is in close proximity to the site of sulfuryl group transfer. Arg257 is located very close to the 3'-phosphate in adenosine 3'-phosphate 5'-phosphosulfate (PAPS). Site-directed mutagenesis demonstrated that Arg257 is critical for SULT1A1: both R257A and R257E are inactive. Although Arg130 is also located very close to the 3'-phosphate of PAPS, R130A and R130E are still active, suggesting that Arg130 is not a critical residue for the catalytic activity of SULT1A1. Computer modeling suggests that the ionic interaction between the positive charge on Arg257, and the negative charge on 3'-phosphate is the primary force stabilizing the specific binding of PAPS.

Enzymatic aspects of the phenol (aryl) sulfotransferases

The sulfotransferases that are active in the metabolism of xenobiotics represent a large family of enzymes that catalyze the transfer of the sulfuryl group from 3'-phosphoadenosine 5'-phosphosulfate to phenols, to primary and secondary alcohols, to several additional oxygen-containing functional groups, and to amines. Restriction of this review to the catalytic processes of phenol or aryl sulfotransferases does not really narrow the field, because these enzymes have overlapping specificity, not only for specific compounds, but also for multiple functional groups. The presentation aims to provide an overview of the wealth of phenol sulfotransferases that are available for study but concentrates on the enzymology of rat and human enzymes, particularly on the predominant phenol sulfotransferase from rat liver. The kinetics and catalytic mechanism of the rat enzyme is extensively reviewed and is compared with observations from other sulfotransferases.

Carboxyl residues in the active site of human phenol sulfotransferase (SULT1A1)

The carboxyl-specific amino acid modification reagent, Woodward's reagent K (WK), was utilized to characterize carboxyl residues (Asp and Glu) in the active site of human phenol sulfotransferase (SULT1A1). SULT1A1 was purified using the pMAL-c2 expression system in E. coli. WK inactivated SULT1A1 activity in a time- and concentration-dependent manner. The inactivation followed first-order kinetics relative to both SULT1A1 and WK. Both phenolic substrates and adenosine 3'-phosphate 5'-phosphosulfate (PAPS) protected against the inactivation, which suggests the carboxyl residue modification causing the inactivation took place within the active site of the enzyme. With partially inactivated SULT1A1, both V(max) and K(m) changed for PAPS, while for phenolic substrates, V(max) decreased and K(m) did not change significantly. A computer model of the three-dimensional structure of SULT1A1 was constructed based on the mouse estrogen sulfotransferase (mSULT1E1) X-ray crystal structure. According to the model, Glu83, Asp134, Glu246, and Asp263 are the residues likely responsible for the inactivation of SULT1A1 by WK. According to these results, five SULT1A1 mutants, E83A, D134A, E246A, D263A, and E151A, were generated (E151A as control mutant). Specific activity determination of the mutants demonstrated that E83A and D134A lost almost 100% of the catalytic activity. E246A and D263A also decreased SULT1A1 activity, while E151A did not change SULT1A1 catalytic activity significantly. This work demonstrates that carboxyl residues are present in the active site and are important for SULT1A1 catalytic activity. Glu83 and E134 are essential amino acids for SULT1A1 catalytic activity.

Redox control of aryl sulfotransferase specificity

Aryl sulfotransferase IV from rat liver has the very broad substrate range that is characteristic of the enzymes of detoxication. With the conventional assay substrates, 4-nitrophenol and PAPS, sulfation was considered optimal at pH 5.5 whereas the enzyme in the physiological pH range was curiously ineffective. These properties would seem to preclude a physiological function for this cytosolic enzyme. Partial oxidation of the enzyme, however, results not only in a substantial increase in the rate of sulfation of 4-nitrophenol at physiological pH but also in a shift of the pH optimum to this range and radically altered overall substrate specificity. The mechanism for this dependence on redox environment involves oxidation at Cys66, a process previously shown to occur by formation of a mixed disulfide with glutathione or by the formation of an internal disulfide with Cys232. Oxidation at Cys66 acts only as a molecular redox switch and is not directly part of the catalytic mechanism. Underlying the activation process is a change in the nature of the ternary complex formed between enzyme, phenol, and the reaction product, adenosine 3',5'-bisphosphate. The reduced enzyme gives rise to an inhibitory, dead-end ternary complex, the stability of which is dictated by the ionization of the specific phenol substrate. Ternary complex formation impedes the binding of PAPS that is necessary to initiate a further round of the reaction and is manifest as profound, substrate-dependent inhibition. In contrast, the ternary complex formed when the enzyme is in the partially oxidized state allows binding of PAPS and the unhindered completion of the reaction cycle.

Mutational analysis of the substrate binding/catalytic domains of human M form and P form phenol sulfotransferases

Human monoamine (M) form and simple phenol (P) form phenol sulfotransferases (PSTs) are greater than 93% identical in their primary sequences and yet display distinct substrate specificities and other enzymatic properties. Through the generation and characterization of a series of chimeric PSTs, we have previously demonstrated two highly variable regions within their sequences to be responsible for determining their substrate phenotypes (Sakakibara, Y., Takami, Y., Nakayama, T., Suiko, M., and Liu, M.-C. (1998) J. Biol. Chem. 273, 6242-6247). By employing the site-directed mutagenesis technique, the present study aims to identify and quantitatively evaluate the specific amino acid residues critical to the substrate binding and catalysis in these two enzymes. Twelve mutated M-PSTs and seven mutated P-PSTs were generated, expressed, and purified. Enzymatic characterization showed that, of the twelve mutated M-PSTs, mutations at residues Asp-86, Glu-89, and Glu-146 resulted in a dramatic decrease in V(max)/K(m) with dopamine as substrate, being greater than 450 times for the D86A/E89I/E146A mutated M-PST. With p-nitrophenol as substrate, the V(max)/K(m) determined for the D86A/E89I/E146A-mutated M-PST increased more than 25 times and approached that determined for the wild-type P-PST. These results indicated that the concerted action of the three mutated residues (D86A, E89I, and E146A) is sufficient for the conversion of the substrate phenotype of M-PST to that of P-PST. Among the mutated P-PSTs, the I89E- and A146E-mutated P-PSTs displayed considerable deviations in V(max)/K(m) with dopamine or p-nitrophenol as substrate. No corresponding changes, however, were detected with the opposite compound as substrate. These results indicated that, in contrast to M-PST, mutations at Ala-86, Ile-89, and Ala-146 to the corresponding residues in M-PST are not sufficient for rendering the change of P-PST substrate phenotype to that of M-PST. For both M-PSTs and P-PSTs, mutations at Lys-48 or His-108 led to the loss of sulfotransferase activities, indicating their importance in the catalytic mechanism.

Photoaffinity labeling probe for the substrate binding site of human phenol sulfotransferase (SULT1A1): 7-azido-4-methylcoumarin

A novel fluorescent photoactive probe 7-azido-4-methylcoumarin (AzMC) has been characterized for use in photoaffinity labeling of the substrate binding site of human phenol sulfotransferase (SULT1A1 or P-PST-1). For the photoaffinity labeling experiments, SULT1A1 cDNA was expressed in Escherichia coli as a fusion protein to maltose binding protein (MBP) and purified to apparent homogeneity over an amylose column. The maltose moiety was removed by Factor Xa cleavage. Both MBSULT1A1 and SULT1A1 were efficiently photolabeled with AzMC. This labeling was concentration dependent. In the absence of light, AzMC competitively inhibited the sulfation of 4MU catalyzed by SULT1A1 (Ki = 0.47 +/- 0.05 mM). Moreover, enzyme activity toward 2-naphthol was inactivated in a time- and concentration-dependent manner. SULT1A1 inactivation by AzMC was protected by substrate but was not protected by cosubstrate. These results indicate that photoaffinity labeling with AzMC is highly suitable for the identification of the substrate binding site of SULT1A1. Further studies are aimed at identifying which amino acids modified by AzMC are localized in the binding site.

Molecular cloning and characterization of a human uronyl 2-sulfotransferase that sulfates iduronyl and glucuronyl residues in dermatan/chondroitin sulfate

A partial-length human cDNA with a predicted amino acid sequence homologous to a previously described heparan sulfate iduronyl 2-sulfotransferase (Kobayashi, M., Habuchi, H., Yoneda, M., Habuchi, O., and Kimata, K. (1997) J. Biol. Chem. 272, 13980-13985) was obtained by searching the expressed sequence-tagged data bank. Northern blot analysis was performed using this homologous cDNA as a probe, which demonstrated ubiquitous expression of messages of 5.1 and 2.0 kilobases in a number of human tissues and in several human cancer cell lines. Since the human lymphoma Raji cell line had the highest level of expression, it was used to isolate a full-length cDNA clone. The full-length cDNA was found to contain an open reading frame that predicted a type II transmembrane protein composed of 406 amino acid residues. The cDNA in a baculovirus expression vector was expressed in Sf9 insect cells, and cell extracts were then incubated together with 3'-phosphoadenosine 5'-phospho[35S]sulfate and potential glycosaminoglycan acceptors. This demonstrated substantial sulfotransferase activity with dermatan sulfate, a small degree of activity with chondroitin sulfate, but no sulfotransferase activity with desulfated N-resulfated heparin. Analysis of [35S]sulfate-labeled disaccharide products of chondroitin ABC, chondroitin AC, and chondroitin B lyase treatment demonstrated that the enzyme only transferred sulfate to the 2-position of uronyl residues, which were preponderantly iduronyl residues in dermatan sulfate, but some lesser transfer to glucuronyl residues of chondroitin sulfate.

Multiple isoforms of heparan sulfate D-glucosaminyl 3-O-sulfotransferase. Isolation, characterization, and expression of human cdnas and identification of distinct genomic loci

3-O-Sulfated glucosaminyl residues are rare constituents of heparan sulfate and are essential for the activity of anticoagulant heparan sulfate. Cellular production of the critical active structure is controlled by the rate-limiting enzyme, heparan sulfate D-glucosaminyl 3-O-sulfotransferase-1 (3-OST-1) (EC 2.8.2.23). We have probed the expressed sequence tag data base with the carboxyl-terminal sulfotransferase domain of 3-OST-1 to reveal three novel, incomplete human cDNAs. These were utilized in library screens to isolate full-length cDNAs. Clones corresponding to predominant transcripts were obtained for the 367-, 406-, and 390-amino acid enzymes 3-OST-2, 3-OST-3A, and 3-OST-3B, respectively. These type II integral membrane proteins are comprised of a divergent amino-terminal region and a very homologous carboxyl-terminal sulfotransferase domain of approximately 260 residues. Also recovered were partial length clones for 3-OST-4. Expression of the full-length enzymes confirms the 3-O-sulfation of specific glucosaminyl residues within heparan sulfate (Liu, J., Shworak, N. W., Sinaÿ, P., Schwartz, J. J. Zhang, L., Fritze, L. M. S., and Rosenberg, R. D. (1999) J. Biol. Chem. 274, 5185-5192). Southern analyses suggest the human 3OST1, 3OST2, and 3OST4 genes, and the corresponding mouse isologs, are single copy. However, 3OST3A and 3OST3B genes are each duplicated in humans and show at least one copy each in mice. Intriguingly, the entire sulfotransferase domain sequence of the 3-OST-3B cDNA (774 base pairs) was 99.2% identical to the same region of 3-OST-3A. Together, these data argue that the structure of this functionally important region is actively maintained by gene conversion between 3OST3A and 3OST3B loci. Interspecific mouse back-cross analysis identified the loci for mouse 3Ost genes and syntenic assignments of corresponding human isologs were confirmed by the identification of mapped sequence-tagged site markers. Northern blot analyses indicate brain exclusive and brain predominant expression of 3-OST-4 and 3-OST-2 transcripts, respectively; whereas, 3-OST-3A and 3-OST-3B isoforms show widespread expression of multiple transcripts. The reiteration and conservation of the 3-OST sulfotransferase domain suggest that this structure is a self-contained functional unit. Moreover, the extensive number of 3OST genes with diverse expression patterns of multiple transcripts suggests that the novel 3-OST enzymes, like 3-OST-1, regulate important biologic properties of heparan sulfate proteoglycans.

Molecular characterization and expression of heparan-sulfate 6-sulfotransferase. Complete cDNA cloning in human and partial cloning in Chinese hamster ovary cells

Heparan-sulfate 6-sulfotransferase (HS6ST) catalyzes the transfer of sulfate from 3'-phosphoadenosine 5'-phosphosulfate to position 6 of the N-sulfoglucosamine residue of heparan sulfate. The enzyme was purified to apparent homogeneity from the serum-free culture medium of Chinese hamster ovary (CHO) cells (Habuchi, H., Habuchi, O., and Kimata, K. (1995) J. Biol Chem. 270, 4172-4179). From the amino acid sequence data of the purified enzyme, degenerate oligonucleotides were designed and used as primers for the reverse transcriptase-polymerase chain reaction using poly(A)+ RNA from CHO cells as a template. The amplified cDNA fragment was then used as a probe to screen a cDNA library of CHO cells. The cDNA clone thus obtained encoded a partial peptide sequence composed of 236 amino acid residues that included the sequences of six peptides obtained after endoproteinase digestion of the purified enzyme. This cDNA clone was applied to the screening of a human fetal brain cDNA library by cross-hybridization. The isolated cDNA clones contained a whole open reading frame that predicts a type II transmembrane protein composed of 401 amino acid residues. No significant amino acid sequence identity to any other proteins, including heparan-sulfate 2-sulfotransferases, was observed. When the cDNA for the entire coding sequence of the protein was inserted into a eukaryotic expression vector and transfected into COS-7 cells, the HS6ST activity increased 7-fold over the control. The FLAG fusion protein purified by anti-FLAG affinity chromatography showed the HS6ST activity alone. Northern blot analysis revealed the occurrence of a single transcript of 3.9 kilobases in both human fetal brain and CHO cells. The results, together with the ones from our recent cDNA analysis of heparan-sulfate 2-sulfotransferase (Kobayashi, M., Habuchi, H., Yoneda, M., Habuchi, O., and Kimata, K. (1997) J. Biol. Chem. 272, 13980-13985), suggest that at least two different gene products are responsible for 6- and 2-O-sulfations of heparan sulfate.

Localization and functional analysis of the substrate specificity/catalytic domains of human M-form and P-form phenol sulfotransferases

Human monoamine (M)-form and simple phenol (P)-form phenol sulfotransferases (PSTs), which are greater than 93% identical in their primary sequences, were used as models for investigating the structural determinants responsible for their distinct substrate specificity and other enzymatic properties. A series of chimeric PSTs were constructed by reciprocal exchanges of DNA segments between cDNAs encoding M-form and P-form PSTs. Functional characterization of the recombinant wild-type M-form, P-form, and chimeric PSTs expressed in Escherichia coli and purified to homogeneity revealed that internal domain-spanning amino acid residues 84-148 contain the structural determinants for the substrate specificity of either M-form or P-form PST. Data on the kinetic constants (Km, Vmax, and Vmax/Km) further showed the differential roles of the two highly variable regions (Region I spanning amino acid residues 84-89 and Region II spanning amino acid residues 143-148) in substrate binding, catalysis, and sensitivity to the inhibition by 2,6-dichloro-4-nitrophenol. In contrast to the differential sulfotransferase activities of M-form and P-form PSTs toward dopamine and p-nitrophenol, the Dopa/tyrosine sulfotransferase activities were found to be restricted to M-form, but not P-form, PST. Furthermore, the variable Region II of M-form PST appeared to play a predominant role in determining the Dopa/tyrosine sulfotransferase activities of chimeric PSTs. Kinetic studies indicated the role of manganese ions in dramatically enhancing the binding of D-p-tyrosine to wild-type M-form PST. Taken together, these results pinpoint unequivocally the sequence encompassing amino acid residues 84-148 to be the substrate specificity/catalytic domain of both M-form and P-form PSTs and indicate the importance of the variable Regions I and II in determining their distinct enzymatic properties.

Structural and functional characterisation of human sulfotransferases

The human aryl sulfotransferases HAST4 and HAST4v vary by only two amino acids but exhibit markedly different affinity towards the sulfonate acceptor p-nitrophenol and the sulfonate donor 3'-phosphoadenosine-5'-phosphosulfate (PAPS). To determine the importance of each of these amino acid differences, chimeric constructs were made of HAST4 and HAST4v. By attaching the last 120 amino acids of HAST4v to HAST4 (changing Thr235 to Asn235) we have been able to produce a protein that has a Km for PAPS similar to HAST4v. The reverse construct, HAST4v/4 produces a protein with a Km for PAPS similar to HAST4. These data suggests that the COOH-terminal of sulfotransferases is involved in co-factor binding.

A review of the effects of manipulation of the cysteine residues of rat aryl sulfotransferase IV

Aryl sulfotransferase IV from rat liver has the broad substrate range that is characteristic of the enzymes of detoxication. With the standard assay substrates, 4-nitrophenol and 3'-phosphoadenosine 5'-phosphosulfate (PAPS), sulfation is optimum at pH 5.4 whereas the reaction is minimal in the physiological pH range. These properties preclude a physiological function for this cytosolic enzyme. Partial oxidation of the enzyme, however, results not only in an increase in the rate of sulfation but also in a shift of the pH optimum to the physiological pH range. The mechanism for this dependence on the redox environment involves oxidation at Cys66, the cysteine residue that is conserved throughout the phenol sulfotransferase family. As documented by mass spectroscopic methods, oxidation by GSSG leads to the formation of an internal disulfide between Cys66 and Cys232; for mutants at Cys232, the oxidation product is a mixed disulfide of Cys66 and glutathione. Both of these disulfide species activate the enzyme and allow it to function at a pH optimum in the physiological range. The activated enzyme differs from the reduced form by a more circumscribed substrate spectrum. All five mutants, in which each of the cysteines of the sulfotransferase subunit have been changed to serine, are catalytically active. Only Cys66 is required for the redox response.

Studies on an affinity label for the sulfuryl acceptor binding site in an aryl sulfotransferase

Active site-directed affinity labeling was utilized to elucidate peptide sequences at the binding site for sulfuryl acceptors in rat hepatic aryl sulfotransferase (AST) IV (also known as tyrosine-ester sulfotransferase, EC 2.8.2.9). The affinity labeling reagent, N-bromoacetyl-4-hydroxyphenylamine, was designed on the basis of substrate specificity studies with para-substituted phenols, utilization of a bromoacetamido group for reactivity with active site amino acid residues and its similarity to acetaminophen, a known substrate for aryl (phenol) sulfotransferases. AST IV utilized N-bromoacetyl-4-hydroxyphenylamine as a substrate with kinetic constants that compared favorably to those obtained with acetaminophen. Incubation of AST IV with N-bromoacetyl-4-hydroxyphenylamine at pH 7.0 in the absence of PAPS and other substrates resulted in an irreversible inactivation of the enzyme that was both time- and concentration-dependent. [14C]-N-bromoacetyl-4-hydroxyphenylamine was synthesized and used to analyze the regions of protein sequence that were involved in the binding of the affinity label. AST IV was incubated with [14C]-N-bromoacetyl-4-hydroxyphenylamine, hydrolyzed with endoproteinase Lys-C and the labeled peptides were purified by HPLC. Control incubations of AST IV with the affinity label in the presence of 4-propylphenol and PAP were utilized to ascertain the specificity of the interaction. Sequence analysis of the labeled peptides, carried out by automated Edman degradation, revealed labeling sites on cysteine (Cys-232, Cys-283 and Cys-289) and lysine (Lys-286) residues near the C-terminus of the protein. The locations of these labeling sites were further evaluated both by sequence-alignment with other sulfotransferases and by theoretical calculations on predicted secondary structure.

Molecular cloning and expression of mouse and human cDNAs encoding heparan sulfate D-glucosaminyl 3-O-sulfotransferase

The cellular rate of anticoagulant heparan sulfate proteoglycan (HSPGact) generation is determined by the level of a kinetically limiting microsomal activity, HSact conversion activity, which is predominantly composed of the long sought heparan sulfate D-glucosaminyl 3-O-sulfotransferase (3-OST) (Shworak, N. W., Fritze, L. M. S., Liu, J., Butler, L. D., and Rosenberg, R. D. (1996) J. Biol. Chem. 271, 27063-27071; Liu, J., Shworak, N. W., Fritze, L. M. S., Edelberg, J. M., and Rosenberg, R. D. (1996) J. Biol. Chem. 271, 27072-27082). Mouse 3-OST cDNAs were isolated by proteolyzing the purified enzyme with Lys-C, sequencing the resultant peptides as well as the existing amino terminus, employing degenerate polymerase chain reaction primers corresponding to the sequences of the peptides as well as the amino terminus to amplify a fragment from LTA cDNA, and utilizing the resultant probe to obtain full-length enzyme cDNAs from a lambda Zap Express LTA cDNA library. Human 3-OST cDNAs were isolated by searching the expressed sequence tag data bank with the mouse sequence, identifying a partial-length human cDNA and utilizing the clone as a probe to isolate a full-length enzyme cDNA from a lambda TriplEx human brain cDNA library. The expression of wild-type mouse 3-OST as well as protein A-tagged mouse enzyme by transient transfection of COS-7 cells and the expression of both wild-type mouse and human 3-OST by in vitro transcription/translation demonstrate that the two cDNAs directly encode both HSact conversion and 3-OST activities. The mouse 3-OST cDNAs exhibit three different size classes because of a 5'-untranslated region of variable length, which results from the insertion of 0-1629 base pairs (bp) between residues 216 and 217; however, all cDNAs contain the same open reading frame of 933 bp. The length of the 3'-untranslated region ranges from 301 to 430 bp. The nucleic acid sequence of mouse and human 3-OST cDNAs are approximately 85% similar, encoding novel 311- and 307-amino acid proteins of 35,876 and 35,750 daltons, respectively, that are 93% similar. The encoded enzymes are predicted to be intraluminal Golgi residents, presumably interacting via their C-terminal regions with an integral membrane protein. Both 3-OST species exhibit five potential N-glycosylation sites, which account for the apparent discrepancy between the molecular masses of the encoded enzyme (approximately 34 kDa) and the previously purified enzyme (approximately 46 kDa). The two 3-OST species also exhibit approximately 50% similarity with all previously identified forms of the heparan biosynthetic enzyme N-deacetylase/N-sulfotransferase, which suggests that heparan biosynthetic enzymes share a common sulfotransferase domain.

Mutational analysis of domain II of flavonol 3-sulfotransferase

The flavonol 3- and 4'-sulfotransferases (ST) from Flaveria chloraefolia catalyze the transfer of the sulfonate group from 3'-phosphoadenosine 5'-phosphosulfate (PAdoPS) to position 3 of flavonol aglycones and position 4' of flavonol 3-sulfates. We identified previously a protein segment, designated domain II, that contains all the determinants responsible for the specificity of these enzymes. Within domain II, at least five amino acids specific to the 4'-ST that could bind the sulfate group of quercetin 3-sulfate were identified. In this study, these amino acid residues were introduced at equivalent positions in the flavonol 3-ST sequence by site-directed mutagenesis of the cloned cDNA. No reversal of the substrate specificity was observed after the individual mutations. However, mutation of Leu95 to Tyr had different effects on the kinetic constants depending on the substitution pattern of the flavonoid B ring, suggesting that the tyrosine side chain may be in direct contact with this part of the molecule. The function of conserved amino acids present in domain II was also investigated. Unconservative mutations at Lys134, Tyr137 and Tyr150 resulted in protein instability in solution, suggesting that these residues might be important for the structural stability of the enzyme. Replacement of Arg140 with Lys or Ser had no effect on protein stability, but resulted in a strong reduction in specific activity. The results of photoaffinity-labeling experiments with PAdoP[35S]S suggest that this residue is required to bind the cosubstrate. In addition, the reduced affinity of [Ser140]ST for 3'-phosphoadenosine 5'-phosphate (PAdoP)-agarose indicates that Arg140 is also involved in binding the coproduct. Replacement of His118 with Glu or Ala resulted in a strong reduction in catalytic activity. However, [Lys118]ST retained a significant amount of catalytic activity. The results of photoaffinity-labeling experiments with PAdoP[35S]S and affinity chromatography on PAdoP-agarose suggest that His118 might be involved in catalysis in the flavonol 3-ST.

Molecular cloning and expression of Chinese hamster ovary cell heparan-sulfate 2-sulfotransferase

Heparan-sulfate 2-sulfotransferase (HS2ST), which catalyzes the transfer of sulfate from 3'-phosphoadenosine 5'-phosphosulfate to L-iduronic acid at position 2 in heparan sulfate, was purified from cultured Chinese hamster ovary (CHO) cells to apparent homogeneity (Kobayashi, M., Habuchi, H., Habuchi, O., Saito, M., and Kimata, K. (1996) J. Biol. Chem. 271, 7645-7653). The internal amino acid sequences were obtained from the peptides after digestion of the purified protein with a combination of endoproteinases. Mixed oligonucleotides based on the peptide sequences were used as primers to obtain a probe fragment by reverse transcriptase-polymerase chain reaction using CHO cell poly(A)+ RNA as template. The clone obtained from a CHO cDNA library by screening with the probe is 2.2 kilobases in size and contains an open reading frame of 1068 bases encoding a new protein composed of 356 amino acid residues. The protein predicts a type II transmembrane topology similar to other Golgi membrane proteins. Messages of 5.0 and 3.0 kilobases were observed in Northern analysis. Evidence that the cDNA clone corresponds to the purified HS2ST protein is as follows. (a) The predicted amino acid sequence contains all five peptides obtained after endoproteinase digestion of the purified protein; (b) the characteristics of the predicted protein fit those of the purified protein in terms of molecular mass, membrane localization, and N-glycosylation; and (c) when the cDNA containing the entire coding sequence of the enzyme in a eukaryotic expression vector was transfected into COS-7 cells, the HS2ST activity increased 2.6-fold over controls, and the FLAG-HS2ST fusion protein purified by affinity chromatography showed the HS2ST activity alone.

Construction and expression of chimeric rat liver hydroxysteroid sulfotransferase isozymes

The St-20 and ST-40 cDNAs encode rat liver hydroxysteroid sulfotransferases (HS-ST) that are 90% identical in amino acid sequence but exhibit different substrate preferences for dehydroepiandrosterone (DHEA), androsterone (AD), and cortisol (CS). ST-40 is active for all three substrates, whereas ST-20 is mainly active for cortisol. To determine the domain responsible for the substrate preferences of the HS-STs, 20 chimeric HS-STs were constructed by reciprocal exchanges of DNA fragments derived from the cDNAs and were expressed in Escherichia coli. Some chimeric enzymes were enzymatically active for all three substrates, and some displayed reduced or lost CS-ST activity, with retention of DHEA- and AD-ST activities. Others lost all HS-ST activity. Analysis revealed that a central region (region III spanning amino acids 102-164 with five amino acid differences between ST-20 and ST-40) is essential for HS-ST activity, whereas regions II (amino acids 65-101) and IV (amino acids 165-219) are unimportant with regard to substrate preference. It was also shown that the parental combination of regions I (amino acids 1-64) and V (amino acids 220-284) is essential for CS-ST activity. Photoaffinity labeling with [35S]3'-phosphoadenosine 5'-phosphosulfate (PAPS) revealed that some inactive chimeras lost affinity for PAPS. These results suggested that an ordered structure formed by regions I, III, and V is required for HS-ST activity, especially for substrate preference and PAPS binding.

Control of activity through oxidative modification at the conserved residue Cys66 of aryl sulfotransferase IV

Oxidation at Cys66 of rat liver aryl suflotransferase IV alters the enzyme's catalytic activity, pH optima and substrate specificity. Although this is a cytosolic detoxification enzyme, the pH optimum for the standard assay substrate 4-nitrophenol is at pH 5.5; upon oxidation, the optimum changes to the physiological pH range. The principal effect of the change in pH optimum is activation, which is manifest by an increase in K'cat without any major influence on substrate binding. In contrast, with tyrosine methyl ester as a substrate, the enzyme's optimum activity occurs at pH 8.0; upon oxidation, it ceases to be a substrate at any pH. The presence of Cys66 was essential for activation to occur, thereby providing a putative reason underlying the conserved nature of this cysteine throughout the phenol sulfotransferase family. Mapping of disulfides by mass spectrometry showed the critical event to be the oxidation of Cys66 to form a disulfide with either Cys232 or glutathione, either one is effective. These results point to a mechanism for regulating the activity of a key enzyme in xenobiotic detoxication during cellular oxidative stress.

Molecular cloning and expression of cDNA encoding human 3'-phosphoadenylylsulfate:galactosylceramide 3'-sulfotransferase

We have isolated a cDNA clone encoding human 3'-phosphoadenylylsulfate:galactosylceramide 3'-sulfotransferase (EC 2.8.2.11). Degenerate oligonucleotides, based on amino acid sequence data for the purified enzyme, were used as primers to amplify fragments of the gene from human renal cancer cell cDNA by the polymerase chain reaction method. The amplified cDNA fragment was then used as probe to screen a human renal cancer cell cDNA library. The isolated cDNA clone contained an open reading frame encoding 423 amino acids including all of the peptides that were sequenced. The deduced amino acid sequence predicts a type II transmembrane topology and contains two potential N-glycosylation sites. There is no significant homology between this sequence and either the sulfotransferases cloned to date or other known proteins. Northern blot analysis demonstrated that a 1.9-kilobase mRNA was unique to renal cancer cells. When the cDNA was inserted into the expression vector pSVK3 and transfected into COS-1 cells, galactosylceramide sulfotransferase activity in the transfected cells increased from 8- to 16-fold over that of controls, and the enzyme product, sulfatide, was expressed on the transformed cells.

New applications of bacterial systems to problems in toxicology

Bacterial systems have long been of use in toxicology. In addition to providing general models of enzymes and paradigms for biochemistry and molecular biology, they have been adapted to practical genotoxicity assays. More recently, bacteria also have been used in the production of mammalian enzymes of relevance to toxicology. Escherichia coli has been used to express cytochrome P450, NADPH-cytochrome P450 reductase, flavin-containing monooxygenase, glutathione S-transferase, quinone reductase, sulfotransferase, N-acetyltransferase, UDP-glucuronosyl transferase, and epoxide hydrolase enzymes from humans and experimental animals. The expressed enzymes have been utilized in a variety of settings, including coupling with bacterial genotoxicity assays. Another approach has involved expression of mammalian enzymes directly in bacteria for use in genotoxicity systems. Particularly with Salmonella typhimurium. Applications include both the reversion mutagenesis assay and a system using a chimera with an SOS-response indicator and a reporter.

Purification, cloning, and bacterial expression of retinol dehydratase from Spodoptera frugiperda

Anhydroretinol and 14-hydroxy-4,14-retro-retinol, retro-retinoids endogenous to both mammals and insects, act as agonist and antagonist, respectively, in controlling proliferation in lymphoblasts and other retinol-dependent cells. We describe here the identification, purification, cloning, and bacterial expression of the enzyme retinol dehydratase, which converts retinol to anhydroretinol in Spodoptera frugiperda. Retinol dehydratase has nanomolar affinity for its substrate and is, therefore, the first enzyme characterized able to utilize free retinol at physiological intracellular concentrations. The enzyme shows sequence homology to the sulfotransferases and requires 3'-phosphoadenosine 5'-phosphosulfate for activity.

Photoaffinity labeling of human recombinant sulfotransferases with 2-azidoadenosine 3',5'-[5'-32P]bisphosphate

Photoaffinity labeling with 2-azidoadenosine 3', 5'-[5'-32P]bisphosphate was used to identify and characterize adenosine 3',5'-bisphosphate-binding proteins in human liver cytosol and recombinant sulfotransferase proteins. The sulfotransferases investigated in these studies were the human phenol sulfotransferases, HAST1, -3, and -4, dehydroepiandrosterone sulfotransferase, and estrogen sulfotransferase. The cDNAs for these enzymes have been previously cloned and expressed in COS-7 cells or Escherichia coli. Photoaffinity labeling of all proteins was highly dependent on UV irradiation, was protected by co-incubation with unlabeled adenosine 3',5'-bisphosphate and phosphoadenosine phosphosulfate, and reached saturation at concentrations above 10 microM. To verify that the 31 35-kDa photolabeled proteins were indeed sulfotransferases, specific antibodies known to recognize human sulfotransferases were used for Western blot analyses of photolabeled proteins. It was shown unequivocally that the proteins in the 31-35-kDa region recognized by the antibodies also photoincorporated 2-azidoadenosine 3',5'-[5'-32P]bisphosphate. This is the first application of photoaffinity labeling with 2-azidoadenosine 3',5'-[5'-32P]bisphosphate for the characterization of recombinant human sulfotransferases. Photoaffinity labeling will be also useful in the purification and functional identification of other adenosine 3',5'-bisphosphate-binding proteins and to determine amino acid sequences at or near their active sites.

Identification of amino acid residues critical for catalysis and cosubstrate binding in the flavonol 3-sulfotransferase

The comparison of the deduced amino acid sequences of plant and animal sulfotransferases (ST) has allowed the identification of four well conserved regions, and previous experimental evidence suggested that regions I and IV might be involved in the binding of the cosubstrate, 3'-phosphoadenosine 5'-phosphosulfate (PAPS). Moreover, region IV is homologous to the glycine-rich phosphate binding loop (P-loop) motif known to be involved in nucleotide phosphate binding in several protein families. In this study, the function of amino acid residues within these two regions was investigated by site-directed mutagenesis of the plant flavonol 3-ST. In region I, our results identify Lys59 as critical for catalysis, since replacement of this residue with alanine resulted in a 300-fold decrease in specific activity, while a 15-fold reduction was observed after the conservative replacement with arginine. Photoaffinity labeling of K59R and K59A with [35S]PAPS revealed that Lys59 is not required for cosubstrate binding. However, the K59A mutant had a reduced affinity for 3'-phosphoadenosine 5'-phosphate (PAP)-agarose, suggesting that Lys59 may participate in the stabilization of an intermediate during the reaction. In region IV, all substitutions of Arg276 resulted in a marked decrease in specific activity. Conservative and unconservative replacements of Arg276 resulted in weak photoaffinity labeling with [35S]PAPS and the R276A/T73A and R276E enzymes displayed reduced affinities for PAP-agarose, suggesting that the Arg276 side chain is required to bind the cosubstrate. The analysis of the kinetic constants of mutant enzymes at residues Lys277, Gly281, and Lys284 allowed to confirm that region IV is involved in cosubstrate binding.

Inactivation of Escherichia coli phosphoribosylpyrophosphate synthetase by the 2',3'-dialdehyde derivative of ATP. Identification of active site lysines

The enzyme 5-phosphoribosyl-alpha-1-pyrophosphate (PRPP) synthetase from Escherichia coli was irreversibly inactivated on exposure to the affinity analog 2',3'-dialdehyde ATP (oATP). The reaction displayed complex saturation kinetics with respect to oATP with an apparent KD of approximately 0.8 mM. Reaction with radioactive oATP demonstrated that complete inactivation of the enzyme corresponded to reaction at two or more sites with limiting stoichiometries of approximately 0.7 and 1.3 mol of oATP incorporated/mol of PRPP synthetase subunit. oATP served as a substrate in the presence of ribose-5-phosphate, and the enzyme could be protected against inactivation by ADP or ATP. Isolation of radioactive peptides from the enzyme modified with radioactive oATP, followed by automated Edman sequencing allowed identification of Lys181, Lys193, and Lys230 as probable sites of reaction with the analog. Cysteine 229 may also be labeled by oATP. Of these four residues, Lys193 is completely conserved within the family of PRPP synthetases, and Lys181 is found at a position in the sequence where the cognate amino acid (Asp181) in human isozyme I PRPP synthetase has been previously implicated in the regulation of enzymatic activity. These results imply a functional role for at least two of the identified amino acid residues.

Molecular cloning and expression of chick chondrocyte chondroitin 6-sulfotransferase

Chondroitin 6-sulfotransferase (C6ST) catalyzes the transfer of sulfate from 3'-phosphoadenosine 5'-phosphosulfate to position 6 of the N-acetylgalactosamine residue of chondroitin. The enzyme has been purified previously to apparent homogeneity from the serum-free culture medium of chick chondrocytes. The purified enzyme also catalyzed the sulfation of keratan sulfate. We have now cloned the cDNA of the enzyme. This cDNA contains a single open reading frame that predicts a protein composed of 458 amino acid residues. The protein predicts a Type II transmembrane topology similar to other glycosyltransferases and heparin/heparan sulfate N-sulfotransferase/N-deacetylases. Evidence that the predicted protein corresponds to the previously purified C6ST was the following: (a) the predicted sequence of the protein contains all of the known amino acid sequence, (b) when the cDNA was introduced in a eukaryotic expression vector and transfected in COS-7 cells, both the C6ST activity and the keratan sulfate sulfotransferase activity were overexpressed, (c) a polyclonal antibody raised against a fusion peptide, which was expressed from a cDNA containing the sequence coding for 150 amino acid residues of the predicted protein, cross-reacted to the purified C6ST, and (d) the predicted protein contained six potential sites for N-glycosylation, which corresponds to the observation that the purified C6ST is an N-linked glycoprotein. The amino-terminal amino acid sequence of the purified protein was found in the transmembrane domain, suggesting that the purified protein might be released from the chondrocytes after proteolytic cleavage in the transmembrane domain.

Chimeric flavonol sulfotransferases define a domain responsible for substrate and position specificities

The pFST3 and pFST4' cDNAs encode flavonol sulfotransferases (ST) that are 69% identical in amino acid sequence yet exhibit strict substrate and position specificities. To determine the domain responsible for the properties of the flavonol STs, several chimeric flavonol STs were constructed by the reciprocal exchange of DNA fragments derived from the plasmids pFST3 and pFST4' and by the expression of the corresponding chimeric proteins in Escherichia coli. The chimeric enzymes were enzymatically active even though their activities were reduced compared to the parent enzymes. The specificity of the resulting hybrid proteins indicates that an interval of the flavonol STs spanning amino acids 92-194 of the flavonol 3-ST sequence contains the determinant of the substrate and position preferences. From the comparison of the amino acid sequences between plant and animal STs, this interval can be subdivided into a highly conserved region corresponding to positions 134-152 of the flavonol 3-ST, flanked by two regions of high divergence from 98 to 110 and 153 to 170. In view of the similarities in length and hydropathic profiles as well as the presence of four conserved regions between plant and animal STs, the results of these experiments suggest that this interval is involved in the recognition of substrates and/or catalysis in all STs.