Phosphoadenosine 5'-phosphosulphate Research Articles

Synthesis and structure-activity relationships of cerebroside analogues as substrates of cerebroside sulphotransferase and discovery of a competitive inhibitor

Metachromatic leukodystrophy (MLD) is a rare genetic disease characterised by a dysfunction of the enzyme arylsulphatase A leading to the lysosomal accumulation of cerebroside sulphate (sulphatide) causing subsequent demyelination in patients. The enzyme galactosylceramide (cerebroside) sulphotransferase (CST) catalyses the transfer of a sulphate group from 3′-phosphoadenosine-5'-phosphosulphate (PAPS) to cerebrosides producing sulphatides. Substrate reduction therapy for arylsulphatase A by inhibition of CST was proposed as a promising therapeutic approach. To identify competitive CST inhibitors, we synthesised and investigated analogues of the substrate galactosylceramide with variations at the anomeric position, the acyl substituent and the carbohydrate moiety, and investigated their structure–activity relationships. While most of the compounds behaved as substrates, α-galactosylceramide 16 was identified as the first competitive CST inhibitor. Compound 16 can serve as a new lead structure for the development of drugs for the treatment of this devastating disease, MLD, for which small molecule therapeutics are currently not available.

Structural and biochemical studies of sulphotransferase 18 from Arabidopsis thaliana explain its substrate specificity and reaction mechanism

Sulphotransferases are a diverse group of enzymes catalysing the transfer of a sulfuryl group from 3′-phosphoadenosine 5′-phosphosulphate (PAPS) to a broad range of secondary metabolites. They exist in all kingdoms of life. In Arabidopsis thaliana (L.) Heynh. twenty-two sulphotransferase (SOT) isoforms were identified. Three of those are involved in glucosinolate (Gl) biosynthesis, glycosylated sulphur-containing aldoximes containing chemically different side chains, whose break-down products are involved in stress response against herbivores, pathogens, and abiotic stress. To explain the differences in substrate specificity of desulpho (ds)-Gl SOTs and to understand the reaction mechanism of plant SOTs, we determined the first high-resolution crystal structure of the plant ds-Gl SOT AtSOT18 in complex with 3′-phosphoadenosine 5′-phosphate (PAP) alone and together with the Gl sinigrin. These new structural insights into the determination of substrate specificity were complemented by mutagenesis studies. The structure of AtSOT18 invigorates the similarity between plant and mammalian sulphotransferases, which illustrates the evolutionary conservation of this multifunctional enzyme family. We identified the essential residues for substrate binding and catalysis and demonstrated that the catalytic mechanism is conserved between human and plant enzymes. Our study indicates that the loop-gating mechanism is likely to be a source of the substrate specificity in plants.

Genome-wide analysis and environmental response profiling of SOT family genes in rice (Oryza sativa)

Sulphotransferase (SOT) catalyses the transfer of a sulphonate group from 3′-phosphoadenosine 5′-phosphosulphate (PAPS) to an appropriate hydroxyl group of various substrates with the parallel formation of 3′-phosphoadenosine 5′-phosphate (PAP). Although several SOTs have been identified and characterized in mammalian, their role in plant is still unclear. In this study, we report genome-wide comprehensive expression analysis of 35 putative SOT genes in rice. The 35 OsSOTs were tandemly arranged into six clusters. The phylogenetic analysis showed that there were 7 subfamilies of OsSOTs and 11 putatively conserved motifs. Six OsSOTs might be pseudogenes, 25 have the two motifs which were involved in PAPS binding regions I and IV. Microarray data indicated that all the OsSOTs were expressed almost at the same level but with different patterns: most OsSOTs were expressed exclusively in stigma and ovary and induced by IAA and BAP, several genes were induced by tZ and DMSO and 11 OsSOTs were response to abiotic stress. Further analysis showed that these 11 genes contained cis-regulatory elements responding to abiotic stresses.

Human PAPS Synthase Isoforms Are Dynamically Regulated Enzymes with Access to Nucleus and Cytoplasm

In higher eukaryotes, PAPS synthases are the only enzymes producing the essential sulphate-donor 3′-phospho-adenosine-5′-phosphosulphate (PAPS). Recently, PAPS synthases have been associated with several genetic diseases and retroviral infection. To improve our understanding of their pathobiological functions, we analysed the intracellular localisation of the two human PAPS synthases, PAPSS1 and PAPSS2. For both enzymes, we observed pronounced heterogeneity in their subcellular localisation. PAPSS1 was predominantly nuclear, whereas PAPSS2 localised mainly within the cytoplasm. Treatment with the nuclear export inhibitor leptomycin B had little effect on their localisation. However, a mutagenesis screen revealed an Arg-Arg motif at the kinase interface exhibiting export activity. Notably, both isoforms contain a conserved N-terminal basic Lys-Lys-Xaa-Lys motif indispensable for their nuclear localisation. This nuclear localisation signal was more efficient in PAPSS1 than in PAPSS2. The activities of the identified localisation signals were confirmed by microinjection studies. Collectively, we describe unusual localisation signals of both PAPS synthase isoforms, mobile enzymes capable of executing their function in the cytoplasm as well as in the nucleus.

A heterodimer of human 3′-phospho-adenosine-5′-phosphosulphate (PAPS) synthases is a new sulphate activating complex

3′-Phospho-adenosine-5′-phosphosulphate (PAPS) synthases are fundamental to mammalian sulphate metabolism. These enzymes have recently been linked to a rising number of human diseases. Despite many studies, it is not yet understood how the mammalian PAPS synthases 1 and 2 interact with each other. We provide first evidence for heterodimerisation of these two enzymes by pull-down assays and Förster resonance energy transfer (FRET) measurements. Kinetics of dimer dissociation/association indicates that these heterodimers form as soon as PAPSS1 and -S2 encounter each other in solution. Affinity of the homo- and heterodimers were found to be in the low nanomolar range using anisotropy measurements employing proteins labelled with the fluorescent dye IAEDANS that – in spite of its low quantum yield – is well suited for anisotropy due to its large Stokes shift. Within its kinase domain, the PAPS synthase heterodimer displays similar substrate inhibition by adenosine-5′-phosphosulphate (APS) as the homodimers. Due to divergent catalytic efficacies of PAPSS1 and -S2, the heterodimer might be a way of regulating PAPS synthase function within mammalian cells.

The multi-protein family of Arabidopsis sulphotransferases and their relatives in other plant species.

All members of the sulphotransferase (SOT, EC 2.8.2.-) protein family use 3'-phosphoadenosine 5'-phosphosulphate (PAPS) as the sulphuryl donor and transfer the sulphonate group to an appropriate hydroxyl group of several classes of substrates. These enzymes have highly conserved domains and can be found in eubacteria and eukaryotes. In mammals, sulphate conjugation catalysed by SOTs constitutes an important reaction in the transformation of xenobiotics, and in the modulation of the biological activity of steroid hormones and neurotransmitters. In plants, sulphate-conjugation reactions seem to play an important role in plant growth, development, and adaptation to stress. To date only a few plant SOTs have been characterized in detail. The flavonol 3- and 4'-SOTs from Flaveria species (Asteraceae), which catalyse the sulphonation of flavonol aglycones and flavonol 3-sulphates, respectively, were the first plant SOTs for which cDNA clones were isolated. The plasma membrane associated gallic acid SOT of Mimosa pudica L. pulvini cells may be intrinsic to signalling events that modify the seismonastic response. In Brassica napus L. a SOT catalyses the O-sulphonation of brassinosteroids and thereby abolishes specifically the biological activity of 24-epibrassinolide. The fully sequenced genome of Arabidopsis thaliana Heynh. contains in total 18 genes that are likely to encode SOT proteins based on sequence similarities of the translated products with an average identity of 51.1%. So far only one SOT from A. thaliana (At5g07000) was functionally characterized: the protein was shown to catalyse the sulphonation of 12-hydroxyjasmonate and thereby inactivate excess jasmonic acid in plants. The substrates and, therefore, the physiological roles of SOTs are very diverse. By using the numerous informative databases and methods available for the model plant A. thaliana, the elucidation of the functional role of the SOT protein family will be accelerated.

Cytotoxic and xenoestrogenic effects via biotransformation of trans-anethole on isolated rat hepatocytes and cultured MCF-7 human breast cancer cells

The metabolism and action of trans-anethole (anethole) and the estrogen-like activity of the compound and its metabolites were studied in freshly isolated rat hepatocytes and cultured MCF-7 human breast cancer cells, respectively. The incubation of hepatocytes with anethole (0.25–2.0 mM) caused a concentration- and time-dependent cell death accompanied by losses of cellular ATP and adenine nucleotide pools. Anethole at a weakly toxic level (0.5 mM) was metabolized to 4-methoxycinnamic acid (4MCA), 4-hydroxy-1-propenylbenzene (4OHPB), and the monosulfate conjugate of 4OHPB; the levels of 4OHPB sulfate and 4MCA reached approximately 20 and 200 μM within 2 hr, respectively, whereas that of free unconjugated 4OHPB was less than approximately 0.5 μM. At a moderately toxic concentration (1.0 mM), unconjugated 4OHPB reached approximately 10 μM, followed by abrupt loss of 3′-phosphoadenosine 5′-phosphosulphate (PAPS). Based on cell viability and adenine nucleotide levels, 4OHPB was more toxic than anethole and 4MCA. The addition of 2,6-dichloro-4-nitrophenol (50 μM), an inhibitor of sulfotransferase, enhanced the anethole-induced cytotoxicity associated with losses of ATP, PAPS, and 4OHPB sulfate, and symmetrically increased the unconjugated 4OHPB concentration. 4OHPB as well as diethylstilbestrol (DES) and bisphenol A (BPA), which are known xenoestrogenic compounds, competitively displaced 17β-estradiol bound to the estrogen receptor α in a concentration-dependent manner; ic 50 values of these compounds were approximately 1×10 −5, 1×10 −8 and 5×10 −5 M, respectively. 4OHPB also caused a concentration (10 −8 to 10 −6 M)-dependent proliferation of MCF-7 cells, whereas neither anethole nor 4MCA (10 −9 to 10 −5 M) affected cell proliferation. However, at higher concentrations (>10 −4 M), 4OHPB rather than anethole and 4MCA was cytotoxic. These results suggest that the biotransformation of anethole induces a cytotoxic effect at higher concentrations in rat hepatocytes and an estrogenic effect at lower concentrations in MCF-7 cells based on the concentrations of the hydroxylated intermediate, 4OHPB.

Characterization and expression of human bifunctional 3'-phosphoadenosine 5'-phosphosulphate synthase isoforms.

Sulphonation, a fundamental process essential for normal growth and development, requires the sulphonate donor molecule 3'-phosphoadenosine 5'-phosphosulphate (PAPS), which is produced from ATP and inorganic sulphate by the bifunctional enzyme PAPS synthase. In humans, two genes encode isoenzymes that are 77% identical at the amino acid level, and alternative splicing creates two subtypes of PAPS synthase 2. The question as to whether distinctions in amino acid composition are reflected in differences in activity has been examined. The specific activity of the PAPS synthase 2 subtypes is 10- to 15-fold higher than that for PAPS synthase 1. The greater catalytic efficiency of the PAPS synthase 2 subtypes is demonstrated further by the 3- to 6-fold higher k(cat)/K(m) ratios for ATP and inorganic sulphate as compared with the ratios for PAPS synthase 1. In humans, PAPS synthase 1 is expressed ubiquitously, and is the dominant isoform in most tissues, whereas expression of the PAPS synthase 2 subtypes is variable and tissue-specific. It is noteworthy that, similar to other human tissues, PAPS synthase 1 also appears to be the dominant isoform expressed in cartilage. The latter finding initially created a conundrum, since there is a specific human dwarfing disorder that is known to be caused by a mutation in the PAPS synthase 2 gene. This apparent enigma would seem to be resolved by examination of cartilage from guinea-pigs as an animal model. Similar to humans, cartilage from mature animals predominantly expresses PAPS synthase 1. In contrast, expression of PAPS synthase 1 is relatively low in the cartilage of immature guinea-pigs, including the growth plate of long bones, whereas PAPS synthase 2 is the highly expressed isoenzyme.

The Xanthomonas oryzae pv.◊oryzae raxP and raxQ genes encode an ATP sulphurylase and adenosine‐5′‐phosphosulphate kinase that are required for AvrXa21 avirulence activity

Xanthomonas oryzae pv. oryzae (Xoo) Philippine race 6 (PR6) is unable to cause bacterial blight disease on rice lines containing the rice resistance gene Xa21 but is virulent on non-Xa21 rice lines, indicating that PR6 carries avirulence (avrXa21) determinants required for recognition by XA21. Here we show that two Xoo genes, raxP and raxQ, are required for AvrXa21 activity. raxP and raxQ, which reside in a genomic cluster of sulphur assimilation genes, encode an ATP sulphurylase and APS (adenosine-5'-phosphosulphate) kinase. These enzymes function together to produce activated forms of sulphate, APS and PAPS (3'-phosphoadenosine-5'-phosphosulphate). Xoo PR6 strains carrying disruptions in either gene, PR6DeltaraxP or PR6DeltaraxQ, are unable to produce APS and PAPS and are virulent on Xa21-containing rice lines. RaxP and RaxQ are similar to the bacterial symbiont Sinorhizobium meliloti host specificity proteins, NodP and NodQ and the Escherichia coli cysteine synthesis proteins CysD, CysN and CysC. The APS and PAPS produced by RaxP and RaxQ are used for both cysteine synthesis and sulphation of other molecules. Mutation in Xoo xcysI, a homologue of Escherichia coli cysI that is required for cysteine synthesis, blocked APS- or PAPS-dependent cysteine synthesis but did not affect AvrXa21 activity, suggesting that AvrXa21 activity is related to sulphation rather than cysteine synthesis. Taken together, these results demonstrate that APS and PAPS production plays a critical role in determining avirulence of a phytopathogen and reveal a commonality between symbiotic and phytopathogenic bacteria.

Sulphonation of N-hydroxy-2-acetylaminofluorene by human dehydroepiandrosterone sulphotransferase

1. The aim was to determine which human recombinant sulphotransferase (ST) isoform(s) were responsible for the sulphonation and, thus, potential further bioactivation of the classical hepatic procarcinogen N-hydroxy-2-acetylaminofluorene (N-OH-2AAF). 2. N-OH-2AAF was incubated together with the cosubstrate 3'-phosphoadenosine-5'- phosphosulphate (PAPS) and either human liver cytosol or recombinant P-form phenolsulphotransferase (P-PST), M-form PST, dehydroepiandrosterone-ST (DHEA-ST) or oestrogen ST (EST). Formation of 3'-phosphoadenosine-5'-phosphate (PAP) from PAPS, measured by HPLC, was used as the assay for determination of sulphoconjugation rates. 3. The liver cytosol produced a 100% increase in PAP formation in the presence of 200 muM N-OH-2AAF as compared with baseline levels (p < 0.01), corresponding to a rate of 19 pmol/min/mg protein. Recombinant P-PST, however, was without effect. This is in contrast to previous suggestions using crude enzyme preparations. Like P-PST, recombinant M-PST and EST did not sulphonate N-OH-2AAF. On the other hand, recombinant DHEA-ST produced a 161% increase in PAP formation in the presence of 200 muM N-OH-2AAF as compared with baseline values (p < 0.001). 4. Kinetic studies of N-OH-2AAF sulphonation by DHEA-ST and human liver cytosol gave similar apparent Kms. Interestingly, the Vmax for N-OH-2AAF sulphonation by DHEA-ST was very similar to that of DHEA, the natural substrate for DHEA-ST. 5. This is the first paper to demonstrate the involvement of the human DHEA-ST in the sulphonation of an N-hydroxylated aromatic amide carcinogen.

Characterization of a spleen sulphotransferase responsible for the 6-O-sulphation of the galactose residue in sialyl-N-acetyl-lactosamine sequences.

An enzyme which catalyses the transfer of sulphate from 3'-phosphoadenosine 5'-phosphosulphate (PAPS) to C-6 of galactose in the NeuAcalpha2-3Galbeta1-4GlcNAc (3'SLN) sequence has been found in rat spleen microsomes and its specificity indicates that it is well suited to participate in the assembly of 3'-sialyl-6'-sulpho-LacNAc [NeuAcalpha2-3Gal(6-SO4)beta1-4GlcNAc] and 3'-sialyl-6'-sulpho-LewisX [NeuAcalpha2-3Gal(6-SO4)beta1-4(Fucalpha1-3)GlcNAc] saccharide groups which have been implicated as selectin ligands. This sulphotransferase has a strict requirement for oligosaccharide acceptors which are capped by an alpha2-3-linked sialic acid residue, although GlcNAc in 3'SLN can be substituted by Glc, and Galbeta1-4GlcNAc can be replaced by Galbeta1-3GlcNAc without loss of activity. The finding that 3'-sialyl LewisX was inert as an acceptor suggested that fucosylation, in contrast with sialylation, follows the addition of the sulphate group. Since fetuin glycopeptides containing the NeuAcalpha2-3Galbeta1-4GlcNAc sequence had a similar affinity for the enzyme as the unattached 3'SLN, it would appear that the acceptor determinants reside primarily in the peripheral trisaccharide constellation. The position of the sulphate on C-6 of galactose was elucidated by Smith periodate oxidation, hydrazine/nitrous acid/NaBH4 treatment and elder (Sambucus nigra) bark lectin chromatography of the desialylated [35S]sulphate-labelled products of the enzyme. Assays carried out with 3'SLN as acceptor indicated that the sulphotransferase had a pH optimum between 6.5 and 7.0 and a dependence on a bivalent cation best met by Mn2+ (12-25 mM); Triton X-100 (0.02 to 0.35%) brought about maximal stimulation. Tentative Km values determined for this enzyme were 4.7 microM for PAPS, and 0.72 mM and 1.16 mM for 3'SLN and fetuin glycopeptides respectively. A survey of several rat organs indicated that the PAPS:3'SLN-6-O-sulphotransferase is selectively distributed with maximal activity occurring in spleen which was substantially greater than thymus or lymph nodes. In contrast, other enzymes (i.e. PAPS:Gal-3-O-and GlcNAc-6-O-sulphotransferases) involved in the sulphation of sialyl-lactosamine and lactosamine sequences, which in the sulphated form are believed to also be selectin ligands, were more evenly distributed in lymphoid tissues. Relatively high activities for all three enzymes were found in brain.

Ion-pair reversed-phase high-performance liquid chromatography of adenine nucleotides and nucleoside using triethylamine as a counterion

An isocratic HPLC method for the simple and selective determination of adenine nucleoside and nucleotides has been developed. The separation is achieved at room temperature by reversed-phase chromatography (Shiseido, Capcell Pak C18). A mixture of 0.1 M triethylamine (TEA) phosphate buffer and methanol (95:5, v/v) is used as a standard eluent. Influence of pH and concentrations of organic modifiers and TEA ion on capacity factors of adenine compounds has been investigated. It has been also found that the TEA ion in the eluent is adsorbed onto the reversed-phase surface. The results clearly demonstrate that ion-pair formation with TEA ion occurs probably both in the mobile phase and on the stationary phase and governs the retention of adenine and nucleotides in the present system. The HPLC system is applied to the analysis of adenine nucleotides formed as intermediates in the synthesis of 3′-phosphoadenosine 5′-phosphosulphate (PAPS) and to the assays of ATPases and 5′-nucleotidase activities in rat liver plasma membrane. This method is a new type of ion-pair reversed-phase HPLC system and is suitable for the separation of highly polar organic anions, especially for adenine nucleotides.

Characterization of a rat liver Golgi sulphotransferase responsible for the 6-O-sulphation of N-acetylglucosamine residues in beta-linkage to mannose: role in assembly of sialyl-galactosyl-N-acetylglucosamine 6-sulphate sequence of N-linked oligosaccharides.

Rat liver Golgi membranes were found to contain an enzyme that can transfer sulphate from 3'-phosphoadenosine 5'-phosphosulphate (PAPS) to C-6 of the terminal GlcNAc in beta-linkage to mannose and has properties indicating that it is involved in the synthesis of the NeuAc alpha 2-3(6)Gal beta 1-4GlcNAc(6-SO4) sequences observed in the N-linked carbohydrate units of various glycoproteins. Assays performed with [35S]PAPS (Km 0.67 microM) and GlcNAc beta 1-6Man alpha 1-O-Me (GnMaMe) acceptor (Km 0.71 mM) indicated that the sulphotransferase had a pH optimum of approx. 7.0 and is markedly stimulated by Mn2+ ions (maximum approx. 15 mM) and Triton X-100 (0.05-0.1%). Hydrazine/nitrous acid/NaBH4 treatment of the 35S-labelled product yielded radiolabelled 2,5-anhydromannitol(6-SO4). The sulphated GnMaMc product of the GlcNAc-6-O-sulphotransferase could be galactosylated by a rat liver Golgi enzyme that was shown to have the same properties as the UDP-Gal:GlcNAc beta-1,4-galactosyltransferase from bovine milk. Competition studies performed with GlcNAc and GlcNAc-6-SO4 furthermore indicated that the same liver enzyme acted on both acceptors to produce Gal beta 1-4GlcNAc and Gal beta 1-4GlcNAc(6-SO4) with Km values of 1.04 and 1.68 mM respectively. Because the sulphated N-acetyl-lactosaminc could in turn serve as an acceptor for rat liver sialyltransferase, it seems that this enzyme, together with the Golgi galactosyltransferase and the GlcNAc-6-O-sulphotransferase, could act in concert in assembling the NeuAc alpha 2-3(6)Gal beta 1-4GlcNAc(6-SO4) branches of complex N-linked oligosaccharides.

Effect of lipids on glycoprotein sulphotransferase activity in rat submandibular salivary glands

Although glycoprotein sulphation has been implicated in the processing of salivary mucin, little is known about the regulation of the enzyme responsible for this event. Using desulphated glycoprotein as sulphate acceptor, the glycoprotein sulphotransferase (GPST) from Golgi membranes of submandibular salivary gland was used to study the effect of various lipids on its activity. The GPST activity in the Golgi membrane was 0.7 pmol/mg protein per min and the activity was extractable by Triton S-100. The K m of the solubilized GPST for glycoprotein and 3'-phosphoadenosine 5′-phosphosulphate (PAPS) were 1 l and 0.2 μM, respectively. Among the various lipids tested, phosphatidylinositol and sphingosine stimulated the GPST activity, while other lipids such as sphingomyelin, phosphatidylcholine and phosphatidylserine did not produce a significant effect. At 12 mot% (when expressed as mot% of sphingosine to total phospholipids plus Triton X-100) of sphingosine concentration, the enzyme activity was increased nearly 1.7-fold. The stimulatory effect of sphingosine was accompanied by a significant decrease in K mfor glycoprotein from I I to 2 μM but the increase in V max was small. In contrast, the sphingosine effect did not change the K m for PAPS but increased the V max nearly two fold. Of the two sphingosine analogues tested, threosphinganine and erythrosphinganine had a lesser stimulatory effect than sphingosine. Stearylamine was partially active, whereas the amino acids (glutamate, aspartate, glutamine, asparagine and serine) were not. These observations and our earlier finding of tyrosylprotein sulphotransferase inhibition by sphingosine demonstrate diverse sphingosine effects on the post-translational sulphation involved in the processing of salivary proteins and suggest an important role for sphingosine in the regulation of salivary protein sulphation.

Sulphate conjugation of minoxidil in rat skin

Minoxidil sulphotransferase (MST) activity was determined in the cytosolic fraction of rat skin and liver. MST of rat skin is similar to the P (phenol)-form of phenolsulphotransferase (PST) of human tissues with respect to thermostability and inhibition by 2,6-dichloro-4-nitrophenol (DCNP). p-Nitrophenol, a prototype substrate of human P-PST form, inhibits MST at micromolar concentration while millimolar concentrations of dopamine and tyramine, substrates of human M-(monoamine)-PST, are required to elicit a similar degree of inhibition. The enzymatic transfer of 35S from sodium 35sulphate to minoxidil was also demonstrated suggesting that the rat skin is potentially capable of synthesizing 3'-phosphoadenosine-5'-phosphosulphate (PAPS) from inorganic sulphate and utilizing it for the biosynthesis of minoxidil sulphate, its active metabolite. Thus, it is conceivable that the pharmacological action of minoxidil as a promotor of hair growth could be carried out by the cutaneous tissues without the contribution of hepatic or other extrahepatic organs.

Biosynthesis of PAPS in vitro by human liver : Measurement by two independent assay procedures

The biosynthesis of 3'-phosphoadenosine-5'-phosphosulphate (PAPS) by extracts of human liver from inorganic sulphate and APS was assayed by transferring the “active sulphate” to two different acceptors, namely N-acetyldopamine (NADA) and 4-methylumbelliferone (4-MU). NADA-sulphate was quantified by an HPLC-ECD method while the decrease in 4-MU was monitored continuously by a fluorimetric procedure. The optimum pH was 8.0 for both the PAPS generation and APS-kinase reaction. The apparent K m value for APS determined by the fluorimetric and HPLC-ECD procedures was 17.6 and 16.8 μM, respectively. PAPS-generation measured in 13 samples of human liver showed excellent correlation between the two assay procedures, with correlation coefficients ( r) of 0.96 and 0.95 for PAPS generation from inorganic sulphate and APS, respectively. The fluorimetric assay was preferred as it is simple, equally sensitive and more reproducible. There was also a good correlation between the APS-kinase reaction and the two-step PAPS-generation from inorganic sulphate, with r = 0.97 and 0.91, as determined by the fluorimetric and HPLC-ECD procedures. The rate of PAPS generation from inorganic sulphate was only marginally less than that from APS in each of the 13 human liver samples, suggesting that the coupling of ATP-sulphurylase to APS-kinase facilitates “sulphate activation” and releases it from the constraints imposed by the unfavourable ATP sulphurylase reaction.

Purification and properties of 3′-phosphoadenosine-5′-phosphosulphate: Desulphoglucosinolate sulphotransferase from Brassica juncea cell cultures

The enzyme 3′-phosphoadenosine-5′-phosphosulfate:desulphoglucosinolate sulphotransferase was partially purified from Brassica juncea cell cultures by fractional precipitation with ammonium sulphate, anion-exchange chromatography and gel filtration. The resulting enzyme preparation had a specific activity of ⋔ ∼ 2 nmol/hr mg protein, representing a 230-fold purification and an overall yield of 58%, compared with crude cell extract. The enzyme had no absolute requirements for cations, and was inhibited by various SH-group reagents. The enzyme was stable up to 40° for at least 1 hr. Maximum activity was exhibited between pH 8.5 and 9.0 at 30°. The enzyme had an apparent pI and M r of 4.84 and 44 000, respectively. The K r values for desulphobenzylglucosinolate (DSBG) and 3′-phosphoadenosine-5′-phosphosulphate (PAPS) were 2.3 and 0.78 μM, respectively. The enzyme did not sulphate DSBG in the presence of adenosine-5′-phosphosulphate. Product inhibition studies established that 3′-phosphoadenosine-5′-phosphate was competitive with PAPS ( K i 0.4 μM).

Biosynthesis of heparin. Relationship between the polymerization and sulphation processes

Incubation of a mouse mastocytoma microsomal fraction with UDP-[3H]GlcA and UDP-GlcNAc yielded proteoglycans containing non-sulphated polysaccharide chains. Similar incubations performed in the presence of sulphate donor 3'-phosphoadenosine 5'-phosphosulphate (PAPS) produced both sulphated and non-sulphated proteoglycans, which were separated by chromatography on DEAE-cellulose Analysis by gel chromatography of single polysaccharide chains, released from the proteoglycans by alkali treatment, showed that the non-sulphated chains produced during incubation for 5 min or 25 min, either in the absence or in the presence of PAPS, were of fairly small molecular size, with an average peak Mr of approx. 10 x 10(3)-15 x 10(3). In contrast, the sulphated chains exceeded Mr 100 x 10(3) Pulse-chase experiments suggested that sulphated chains were capable of further elongation. These results indicate that sulphation promotes, by so far unknown mechanisms, further chain elongation. Sulphated proteoglycan (retarded on DEAE-cellulose chromatography) isolated after similar incubation of the microsomal fraction for 1 min only was found to contain a mixture of sulphated and virtually non-sulphated polysaccharide chains. However, when [35S]PAPS was included in the incubations, some 35S was found to be associated, essentially as N-sulphate groups, also with the latter type of chains, preferentially the high-Mr fraction. These results are interpreted in terms of a biosynthetic model by which the heparin proteoglycan is generated through transient interactions of macromolecular intermediates with distinctly separate complexes of membranebound enzymes.

Phosphorylation of an 85-kd membrane protein by a novel mechanism.

Protein phosphorylation has been recognized as a major mechanism for the regulation of cellular functions. The classical phosphate donor in protein phosphorylation reactions is ATP. Here we show that 3'-phosphoadenosine-5'-phosphosulphate (PAPS), a ubiquitous nucleotide so far known to have a central role in sulphate transfer, serves as phosphate donor for protein phosphorylation. In a very specific, rapid and probably autocatalytic reaction, the 3'-phosphate group of PAPS was found to be transferred to a serine residue of an 85-kd membrane protein (p85). ATP did not serve as phosphate donor in this reaction. Radioactive phosphate incorporated into p85 in a membrane fraction was rapidly lost by dephosphorylation after removal of PAPS or by exchange with unlabelled phosphate after addition of nonradioactive PAPS. PAPS-dependent phosphorylation of the 85-kd protein and other proteins was observed in all rat and bovine tissues examined, as well as in various mammalian cell lines. Our results indicate the existence of a novel widespread form of protein phosphorylation.

Sulphotransferase-mediated covalent binding of the carcinogen 7,12-dihydroxymethylbenz[a]anthracene to calf thymus DNA and its inhibition by glutathione transferase.

The carcinogen, 7,12-dihydroxymethylbenz[a]anthracene (DHBA), bound covalently through its 7-methylene carbon to calf thymus DNA via DHBA 7-sulphate, a regiospecifically formed, reactive metabolite, when incubated with rat liver cytosol in the presence of 3'-phosphoadenosine 5'-phosphosulphate (PAPS). The hydroxysteroid sulphotransferase inhibitor, dehydroepiandrosterone sulphate, strongly retarded the covalent binding of DHBA to DNA as well as the DHBA 7-sulphate formation from DHBA in the biological system, while pentachlorophenol and dichloronitrophenol showed little effect on these reactions. DHBA 7-sulphate was a good substrate for rat liver cytosolic glutathione (GSH) transferases, so that the PAPS-dependent covalent binding of DHBA to DNA could be markedly retarded in the presence of GSH with concomitant formation of a significant amount of the stable conjugate, S-(12-hydroxymethylbenz[a]anthracen-7-yl)methylglutathione. From DNA, incubated with DHBA in the presence of the hepatic cytosol and PAPS as well as with DHBA 7-sulphate alone, two purine base adducts were isolated after hydrolysis. The purine base adducts accounted for 70% of the total covalent binding of the carcinogen or its 7-sulphate to the nucleic acid and were identified with synthetic specimens as N6-(12-hydroxymethylbenz[a]anthracen-7-yl)-methyladenine and N2-12-hydroxymethylbenz[a]anthracen-7-yl)-methylguanine. The ratio of the adenine to guanine adducts was 1:2.5 with DNA, incubated with DHBA in the presence of hepatic cytosol and PAPS as well as with various concentrations of DHBA 7-sulphate.