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The structural basis for catalysis and specificity of the Pseudomonas cellulosa alpha-glucuronidase, GlcA67A.

Alpha-glucuronidases, components of an ensemble of enzymes central to the recycling of photosynthetic biomass, remove the alpha-1,2 linked 4-O-methyl glucuronic acid from xylans. The structure of the alpha-glucuronidase, GlcA67A, from Pseudomonas cellulosa reveals three domains, the central of which is a (beta/alpha)(8) barrel housing the catalytic apparatus. Complexes of the enzyme with the individual reaction products, either xylobiose or glucuronic acid, and the ternary complex of both glucuronic acid and xylotriose reveal a "blind" pocket which selects for short decorated xylooligosaccharides substituted with the uronic acid at their nonreducing end, consistent with kinetic data. The catalytic center reveals a constellation of carboxylates; Glu292 is poised to provide protonic assistance to leaving group departure with Glu393 and Asp365 both appropriately positioned to provide base-catalyzed assistance for inverting nucleophilic attack by water.

Binding Sites↗

[Comparison of Echinops species on the basis of the monosaccharide composition of polysaccharides].

The goal of our investigation was to compare the monosaccharide composition of polysaccharides of different Echinops species. The determination of monosaccharid composition was made after Taylor reduction, acidic hydrolysis, reduction with sodium borohydride and acetylation in pyridine/aceticanhydride as alditol-acetates with gas chromatographic analysis. 4-O-methyl-glucuronic acid was found in all studied species. The best similarity was found in the case of the monosaccharide composition of E. ruthenicus and E. exaltatus where rhamnose and arabinose are the main monosaccharides and 4-O-methyl-glucuronic acid is the uronic acid component. The main monosaccharides are glucose in E. orientalis and glucose and mannose in E. commutatus. Glucuronic acid is the uronic acid component in both species.

Asteraceae↗

1H NMR spectroscopic studies on the interactions between human plasma antithrombin III and defined low molecular weight heparin fragments.

The effects of length and composition upon the antithrombin-binding properties of heparin have been investigated for two series of structurally related heparin oligosaccharides. Each series consists of a tetrasaccharide, hexasaccharide, and octasaccharide heparin fragment composed of alternating hexuronic acid (either iduronate 2-sulfate or glucuronate) and glucosamine 6,N-disulfate residues. These two series represent dominant structural motifs in intact heparin and differ from each other by the presence of a glucuronic acid in one series in place of an iduronate 2-sulfate residue penultimate to the reducing end of the fragment. Perturbations to the 1H resonances in the NMR spectrum of antithrombin upon binding of the two series of heparin fragments are compared to those generated by intact heparin binding, as well as to the effects of binding of a synthetic high-affinity pentasaccharide. All of the heparin fragments examined appear to bind to antithrombin at the same site. Three of the heparin fragments (hexasaccharide-2, octasaccharide-2, and octasaccharide-1) produce almost identical perturbations in the antithrombin 1H NMR spectrum compared to binding of intact heparin, including perturbations of resonances from tryptophan 49. This indicates that neither the glucuronic acid nor the trisulfated glucosamine residue (structural elements known to be part of the high-affinity heparin motif) are necessary for the majority of the conformational changes induced upon heparin fragment binding to antithrombin. However, the low anticoagulant activity of these fragments indicates that the changes in protein conformation upon fragment binding, as manifested by these 1H resonance perturbations, are not sufficient for catalytic activation of the inhibitor.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

Yeast-derived recombinant DG42 protein of Xenopus can synthesize hyaluronan in vitro.

We demonstrate in this report that the Xenopus DG42 gene product made in the yeast Saccharomyces cerevisiae can synthesize authentic high molecular weight hyaluronan (hyaluronic acid; HA) in vitro. Saccharomyces are eukaryotes that do not naturally produce HA or any other molecules known to contain glucuronic acid. Therefore bakers' yeast is a good model system to determine the enzymatic activity of the DG42 protein, which is the topic of recent debate. Membrane extracts prepared from cells expressing DG42 encoded on a plasmid incorporated [14C]glucuronic acid and N-[3H]acetylglucosamine from exogenously supplied UDP-sugar nucleotides into a high molecular mass (10(6) to 10(7) Da) polymer in the presence of magnesium ions. Both sugar precursors were simultaneously required for elongation. Control extracts prepared from cells with the vector plasmid alone or the DG42 cDNA in the antisense orientation did not display this activity. The double-labeled polysaccharide product synthesized in vitro was deemed to be HA by enzymatic analyses; specific HA lyase could degrade the polymer, but it was unaffected by protease or chitinase treatments. The fragments generated by HA lyase were identical to fragments derived from authentic vertebrate HA as compared by high performance liquid chromatography. We conclude that DG42 is a membrane-associated HA synthase capable of transferring both glucuronic acid and N-acetylglucosamine groups.

Amino Acid Sequence↗

[Study on the phase II metabolites of phenoprolamine hydrochloride in rat bile by LC/DAD/MSD].

AIM: To study the phase II metabolites of phenoprolamine hydrochloride (DDPH) in rat bile. METHODS: DDPH was administered by i.p. to bile duct-cannulated rats. Bile samples were collected before drug administration and up to 12 h after drug administration. After being purified and enriched with C-18 SPE columns the rat bile samples were analyzed by LC/DAD/MSD to identify the peaks of phase II metabolites. The fractions of phase II metabolites were prepared by HPLC and treated with beta-glucuronidase, and then were purified and enriched with C-18 SPE columns and analyzed by LC/DAD/MSD. The corresponding reference standards of DDPH phase I metabolites were analyzed by LC/DAD/MSD under identical conditions. RESULTS: The peaks M7, M8 and M9 in the chromatograms of rat bile samples were the phase II metabolites of DDPH and the enzymatic hydrolysates of M7, M8 and M9 were 1-(2, 6-dimethyl-4-hydroxyphenoxy)-2-(3, 4-methoxyphenylethylamino)-propane (M3), 1-(2, 6-dimethyl-3-hydroxyphenoxy)-2-(3, 4-methoxyphenylethylamino)-propane (M2) and 1-(2,6-dimethylphenoxy)-2-(3-methoxy-4-hydroxyphenylethyl-amino)-propane (M1) respectively. CONCLUSION: beta-1-O-[3,5-dimethyl-4-[-2-methyl-2-(3,4-dimethoxy-phenylethylamino)- ethoxy]-phenyl]-glucuronic acid (M7, glucuronide of M3), beta-1-O-[2, 4-dimethyl-3-[2-methyl-2-(3, 4-dimethoxy-phenylethylamino)-ethoxy]-phenyl]-glucuronic acid (M8, glucuronide of M2) and beta-1-O-[2-methoxy-4-[1-methyl-2-(2, 6-dimethylphenoxy)-ethylamino-ethyl]-phenyl]-glucuronic acid (M9, glucuronide of M1) were the phase II metabolites of DDPH in rat bile.

Animals↗

Disposition in male volunteers of a subanaesthetic intravenous dose of an oil in water emulsion of 14C-propofol.

1. An intravenous dose of 14C-propofol (0.47 mg/kg) administered to six male volunteers was rapidly eliminated with 88% recovered in the urine in 5 days and less than 2% in faeces. 2. The dose was cleared by metabolism with less than 0.3% excreted unchanged. The major metabolites were the glucuronic acid conjugate of propofol and the glucuronic acid and sulphate conjugates of its hydroxylated derivative, 2,6-diisopropyl-1,4-quinol. Propofol glucuronide accounted for about 53% of the urinary radioactivity and was the major metabolite in plasma from 30 min post dose. 3. The blood concentration of propofol declined in a biphasic manner from a maximum mean value of 0.44 microgram/ml, 2 min after injection. The half-lives of the first and second exponential phases, mean values 5 min and 97 min respectively, varied widely among subjects. A proportion of the dose was cleared slowly, probably due to slow release from less well perfused tissues. Propofol accounted for 94% of the total blood radioactivity at 2 min but only about 6% from 3 to 8 h post dose. 4. Propofol has a volume of distribution equivalent to about 3 to 4 times body weight, and a mean total body clearance of 2.2 1/min.

Adult↗

Pentose pathway in human liver.

[1-14C]Ribose and [2-14C]glucose were given to normal subjects along with glucose loads (1 g per kg of body weight) after administration of diflunisal and acetaminophen, drugs that are excreted in urine as glucuronides. Distributions of 14C were determined in the carbons of the excreted glucuronides and in the glucose from blood samples drawn from hepatic veins before and after glucagon administration. Eighty percent or more of the 14C from [1-14C]ribose incorporated into the glucuronic acid moiety of the glucuronides was in carbons 1 and 3, with less than 8% in carbon 2. In glucuronic acid from glucuronide excreted when [2-14C]glucose was given, 3.5-8.1% of the 14C was in carbon 1, 2.5-4.3% in carbon 3, and more than 70% in carbon 2. These distributions are in accord with the glucuronides sampling the glucose unit of the glucose 6-phosphate pool that is a component of the pentose pathway and is intermediate in glycogen formation. It is concluded that the glucuronic acid conjugates of the drugs can serve as a noninvasive means of sampling hepatic glucose 6-phosphate. In human liver, as in animal liver, the classical pentose pathway functions, not the L-type pathway, and only a small percentage of the glucose is metabolized via the pathway.

Acetaminophen↗

Identification of novel N-glucuronides in rat bile after administration of 450191-S, a 1H-1,2,4-triazolyl benzophenone derivative.

5-[2-Aminoacetamido)methyl]-1-[p-chloro-2-(o-chlorobenzoyl)phenyl] -N,N-dimethyl-1H-s-triazole-3-carboxamide hydrochloride dihydrate (450191-S) is a ring-opened derivative of 1,4-benzodiazepine, which is activated by desglycylation and subsequent cyclization. After 450191-S administration, rat bile contained three novel conjugates which released active metabolites possessing the 1,4-benzodiazepine structure through beta-glucuronidase hydrolysis. Since the released metabolites have no functional groups to conjugate with glucuronic acid, we speculated that the aglycone might be the ring-opened form of 1,4-benzodiazepine which spontaneously cyclizes after the release of glucuronic acid. This possibility was tested by chemically reducing the ketone group of the ring-opened 1,4-benzodiazepine glucuronate conjugates, which would prevent the spontaneous ring closure reaction after the release of the glucuronic acid moiety. The NaBH4 reduction of the ketone of the benzophenone moiety of the conjugates and subsequent treatment with beta-glucuronidase allowed identification of the reduced aglycones with authentic samples using gas chromatography-mass spectrometry.

Animals↗

Substrate specificity of the heparan sulfate hexuronic acid 2-O-sulfotransferase.

The interaction of heparan sulfate with different ligand proteins depends on the precise location of O-sulfate groups in the polysaccharide chain. We have previously shown that overexpression in human kidney 293 cells of a mouse mastocytoma 2-O-sulfotransferase (2-OST), previously thought to catalyze the transfer of sulfate from 3'-phosphoadenosine 5'-phosphosulfate to C2 of L-iduronyl residues, preferentially increases the level of 2-O-sulfation of D-glucuronyl units [Rong, J., Habuchi, H., Kimata, K., Lindahl, U., and Kusche-Gullberg, M. (2000) Biochem. J. 346, 463-468]. In the study presented here, we further investigated the substrate specificity of the mouse mastocytoma 2-OST. Different polysaccharide acceptor substrates were incubated with cell extracts from 2-OST-transfected 293 cells together with the sulfate donor 3'-phosphoadenosine 5'-phospho[(35)S]sulfate. Incubations with O-desulfated heparin, predominantly composed of [(4)alphaIdoA(1)-(4)alphaGlcNSO(3)(1)-](n)(), resulted in 2-O-sulfation of iduronic acid. When, on the other hand, an N-sulfated capsular polysaccharide from Escherichia coli K5, with the structure [(4)betaGlcA(1)-(4)alphaGlcNSO(3)(1)-](n)(), was used as an acceptor, sulfate was transferred almost exclusively to C2 of glucuronic acid. Substrates containing both iduronic and glucuronic acid residues in about equal proportions strongly favored sulfation of iduronic acid. In agreement with these results, the 2-OST was found to have a approximately 5-fold higher affinity for iduronic acid-containing substrate disaccharide units (K(m) approximately 3.7 microM) than for glucuronic acid-containing substrate disaccharide units (K(m) approximately 19.3 microM).

Animals↗

Effects of model traumatic injury on hepatic drug metabolism in the rat. IV. Glucuronidation.

A previously validated small mammal trauma model, hind-limb ischemia secondary to infrarenal aortic ligation in the rat, was utilized to investigate the effects of traumatic injury on hepatic glucuronidation activity. As was previously observed with hepatic oxidative drug metabolism, model trauma resulted in a significant decrease in the in vivo glucuronidation of chloramphenicol, with a 23% drop in clearance of this drug. The effect on in vivo pharmacokinetics appeared to result from a complex interaction between trauma's differential influences on conjugating enzyme(s), deconjugating enzyme(s), and hepatic UDP-glucuronic acid levels, as well as the relative physiological importance of these variables. Hepatic UDP-glucuronyltransferase activities towards both p-nitrophenol and chloramphenicol were elevated (44-54%) after model injury when measured in native hepatic microsomes. However, microsomes which had been "activated" by treatment with Triton X-100 showed no significant difference between control and traumatized animals. Serum beta-glucuronidase activities were elevated by 58%, while hepatic beta-glucuronidase rose by about 16%. Nevertheless, in vivo deconjugation showed no significant change. Model trauma also resulted in a 46% decrease in hepatic UDP-glucuronic acid content. Thus, the observed post-traumatic depression of in vivo chloramphenicol glucuronidation could be due either to a diminished availability of a necessary cofactor (UDP-glucuronic acid) or to an alteration in enzyme kinetics or function in vivo.

Animals↗

Synthesis, isolation and identification of glucuronides and mercapturic acids of a novel antiparasitic agent, licochalcone A.

1. Four glucuronic acid conjugates of licochalcone A (Lica), and their metabolites, have been synthesized using rabbit and pig liver microsomes and purified by preparative hplc. 2. The glucuronides were identified as E-Lica 4'-O-beta-glucuronide, E and Z-Lica 4-O-beta-glucuronide and a mono-glucuronide conjugate of a beta-hydroxylated Lica metabolite. The metabolites were identified by hplc-nmr (one and two-dimensional nmr) as well as hplc-ms. 3. At pH 8.5 Lica reacted with N-acetyl-L-cysteine giving the two epimeric conjugates, which were then isolated by preparative hplc and identified by one and two-dimensional nmr spectroscopic methods. 4. Only two glucuronic acid conjugates (E- and Z-Lica 4-O-beta-glucuronide) were found in the urine of rat after i.p. administration of a single dose of Lica.

Acetylcysteine↗

Milligram-scale preparation and purification of oligosaccharides of defined length possessing the structure of chondroitin from defructosylated capsular polysaccharide K4.

Escherichia coli K4 bacterium synthesizes a nonsulfated capsule polysaccharide (K4) composed of a repeating disaccharide subunit of D-glucuronic acid (beta1-->3) and N-acetyl-D-galactosamine (beta1-->4) to which beta-fructofuranose units are linked to C-3 of D-glucuronic acid residues. The K4 polyanion is easily defructosylated under acid conditions with no fragmentation of the polymer to produce a polysaccharide having a repeated disaccharide unit of chondroitin consisting of D-glucuronic acid (beta1-->3) and N-acetyl-D-galactosamine (beta1-->4) (K4d). K4 and K4d were depolymerized by partial digestion with testicular hyaluronidase and separated into uniform-size oligosaccharides from 4-mers to 16-mers by preparative anion-exchange chromatography after removal of the hyaluronidase. The purity and size of each oligosaccharide was confirmed by using anion-exchange HPLC, HPSEC analysis, and FACE. Mg-scale K4d oligosaccharides were obtained from 50 mg K4d starting material. Under the conditions used to degrade the K4 polysaccharide by testicular hyaluronidase, fructose is slowly liberated forming the defructosylated K4. As a consequence, a mixture of uniform- size K4 and K4d oligosaccharide species, from approximately 4- to 20-mers, are generated and size-separated by anion-exchange chromatography. These pure, uniform-size, and large ranges of K4d oligosaccharides having the structure of a chondroitin, -->4)-GlcUA-beta(1-->3)GalNAc-beta(1-->, will be available for investigating important biological functions of this polymer.

Bacterial Capsules↗

Human liver glucuronate 2-sulphatase. Purification, characterization and catalytic properties.

Human glucuronate 2-sulphatase (GAS), which is involved in the degradation of the glycosaminoglycans heparan sulphate and chondroitin 6-sulphate, was purified almost 2,000,000-fold to homogeneity in 8% yield from liver with a four-step six-column procedure, which consists of a concanavalin A-Sepharose/Blue A-agarose coupled step, a DEAE-Sephacel/octyl-Sepharose coupled step, CM-Sepharose chromatography and gel-permeation chromatography. Although more than 90% of GAS activity had a pI of greater than 7.5, other forms with pI values of 5.8, 5.3, 4.7 and less than 4.0 were also present. The pI greater than 7.5 form of GAS had a native molecular mass of 63 kDa. SDS/polyacrylamide-gel-electrophoretic analysis resulted in two polypeptide subunits of molecular mass 47 and 19.5 kDa. GAS was active towards disaccharide substrates derived from heparin [O-(beta-glucuronic acid 2-sulphate)-(1----4)-O-(2,5)-anhydro[1-3H]mannitol 6-sulphate (GSMS)] and chondroitin 6-sulphate [O-(beta-glucuronic acid 2-sulphate-(1----3)-O-(2,5)-anhydro[1-3H]talitol 6-sulphate (GSTS)]. GAS activity towards GSMS and GSTS was at pH optima of 3.2 and 3.0 respectively with apparent Km values of 0.3 and 0.6 microM respectively and corresponding Vmax values of 12.8 and 13.7 mumol/min per mg of protein respectively. Sulphate and phosphate ions are potent inhibitors of enzyme activity. Cu2+ ions stimulated, whereas EDTA inhibited enzyme activity. It was concluded that GAS is required together with a series of other exoenzyme activities in the lysosomal degradation of glycosaminoglycans containing glucuronic acid 2-sulphate residues.

Catalysis↗

Biosynthesis of heparin/heparan sulphate: mechanism of epimerization of glucuronyl C-5.

In the biosynthesis of heparin and heparan sulphate, D-glucuronic acid residues are converted into L-iduronic acid (IdoA) units by C-5 epimerization, at the polymer level. The reaction catalysed by the epimerase occurs by reversible abstraction and readdition of a proton at C-5 of target hexuronic acid residues, through a carbanion intermediate, with or without an inversion of configuration at C-5 [Prihar, Campbell, Feingold, Jacobsson, Jensen, Lindahl and Rodén (1980) Biochemistry 19, 495-500]. Incubation of chemically N-sulphated capsular polysaccharide from Escherichia coli K5 ([4GlcAbeta1-4GlcNSO(3)alpha1-](n)), or of O-desulphated heparin (predominantly [4IdoAalpha1-4GlcNSO(3)alpha1-](n)) with purified C-5 epimerase from bovine liver, resulted in the interconversion of glucuronic acid and IdoA residues, which reached equilibrium (30-40% IdoA/total hexuronic acid) after approx. 1 h of incubation. Similar incubations performed in the presence of (3)H(2)O resulted in progressive labelling at C-5 of the target hexuronic acid units of either substrate polysaccharide. Contrary to chemical D-gluco/L-ido equilibrium, established within 1 h of incubation, the accumulation of (3)H label continued for at least 6 h. This isotope effect suggests that the second stage of the reaction, i.e. the re-addition of a proton to the carbanion intermediate, is the rate-limiting step of the overall process. Analysis of the 5-(3)H-labelled polysaccharide products showed that the (3)H was approximately equally distributed between glucuronic acid and IdoA units, irrespective of incubation time (from 15 min to 72 h) and of the relative proportions of the two epimers in the substrate. This finding points to a catalytic mechanism in which the abstraction and re-addition of C-5 protons are effected by two polyprotic bases, presumably lysine residues. Previous experiments relating to the biosynthesis of dermatan sulphate were similarly interpreted in terms of a two-base epimerization mechanism but differed from the present findings by implicating one monoprotic and one polyprotic base function [Hannesson, Hagner-McWhirter, Tiedemann, Lindahl and Malmström (1996) Biochem. J. 313, 589-596].

Animals↗

Species differences in the metabolism of suprofen in laboratory animals and man.

The metabolism of the oral anti-inflammatory agent suprofen (S), 2-4-(2-thienylcarbonyl)phenyl)propionic acid, has been studied in mice, rats, guinea pigs, dogs, monkeys, and human volunteers. The major metabolites of S in the serum, urine, and feces of these species were determined by GC/MS and HPLC techniques. The metabolic pathways of S in these species involved reduction of the ketone group to an alcohol (S-OH), hydroxylation of the thiophene ring (T-OH), elimination of the thiophene ring to a dicarboxylic acid (S-COOH), and conjugation with glucuronic acid or taurine. In 72-hr urine and feces of these species after po dosing of 1.6 to 2 mg/kg of S, S and these metabolites accounted for 46 to 92% of the dose and were mainly excreted in the urine. S was present as a major product (excreted mainly in conjugated form) in all species. S-OH was a major component in guinea pig and dog but a minor one in other species. T-OH was identified as a major metabolite in monkey, rat, mouse, and man, but a minor one in guinea pig, and it was absent in the dog. S-COOH was present as the minor metabolite in mouse and rat, and present at trace levels in dog, monkey, and man. Conjugation of the propionic acid functionality with taurine was observed only in the dog; in the other species, conjugation with glucuronic acid was extensive. Absorption parameters of S in the rat and monkey were similar to those in man; however, other species were very different from man.

Animals↗

Intracellular inhibition of UDP-glucose dehydrogenase during ethanol oxidation.

The enzymatic basis for inhibition of drug glucuronidation during ethanol oxidation was investigated in isolated rat hepatocytes. The intracellular rate of glucuronidation was varied independently by controlling the steady-state O2 concentration and the concentrations of UDP-glucose and UDP-glucuronic acid were measured in the absence and presence of 20 mM ethanol. Ethanol caused substantial inhibition of the glucuronidation rate which corresponded to a significant decrease in UDP-glucuronic acid concentration but not in UDP-glucose concentration. A plot of glucuronidation rate as a function of cellular UDP-glucuronic acid concentration yielded a single curve for incubations with or without ethanol; a similar plot of glucuronidation against UDP-glucose concentration gave separate curves for the two incubation conditions. These results clearly define the UDP-glucose dehydrogenase reaction as the site of inhibition during ethanol oxidation.

Acetaminophen↗

A Sinorhizobium meliloti lipopolysaccharide mutant altered in cell surface sulfation.

The Rhizobium-legume symbiosis involves the formation of a novel plant organ, the nodule, in which intracellular bacteria reduce molecular dinitrogen in exchange for plant photosynthates. Nodule development requires a bacterial signal referred to as Nod factor, which in Sinorhizobium meliloti is a beta-(1,4)-linked tetramer of N-acetylglucosamine containing N-acyl and O-acetyl modifications at the nonreducing end and a critical 6-O-sulfate at the reducing end. This sulfate modification requires the action of three gene products: nodH, which catalyzes the sulfonyl transfer, and nodPQ, which produce the activated form of sulfate, 3'-phosphoadenosine-5'-phosphosulfate. It was previously reported that S. meliloti cell surface polysaccharides are also covalently modified by sulfate in a reaction dependent on NodPQ. We have further characterized this unique form of bacterial carbohydrate modification. Our studies have determined that one of the nodPQ mutant strains used in the initial study of sulfation of cell surface harbored a second unlinked mutation. We cloned the gene affected by this mutation (referred to as lps-212) and found it to be an allele of lpsL, a gene previously predicted to encode a UDP-glucuronic acid epimerase. We demonstrated that lpsL encoded a UDP-glucuronic acid epimerase activity that was reduced in the lps-212 mutant. The lps-212 mutation resulted in an altered lipopolysaccharide structure that was reduced in sulfate modification in vitro and in vivo. Finally, we determined that the lps-212 mutation resulted in a reduced ability to elicit the formation of plant nodules and by altered infection thread structures that aborted prematurely.

Bacterial Proteins↗

Composition of an extracellular poly-saccharide produced by Sphaerotilus natans.

The capsular polysaccharide of Sphaerotilus natans has been isolated, purified, and analyzed. Chromatographic and chemical analyses performed on acid hydrolyzates of the purified material have shown that the major components are fucose, galactose, glucose, and glucuronic acid in approximately equimolar amounts. Glucose and glucuronic acid are believed to occur as an aldobiuronic acid unit.

Chromatography↗