Recycling of methylthioadenosine is essential for normal vascular development and reproduction in Arabidopsis

5'-Methylthioadenosine (MTA) is the common by-product of polyamine (PA), nicotianamine (NA), and ethylene biosynthesis in Arabidopsis (Arabidopsis thaliana). The methylthiol moiety of MTA is salvaged by 5'-methylthioadenosine nucleosidase (MTN) in a reaction producing methylthioribose (MTR) and adenine. The MTN double mutant, mtn1-1mtn2-1, retains approximately 14% of the MTN enzyme activity present in the wild type and displays a pleiotropic phenotype that includes altered vasculature and impaired fertility. These abnormal traits were associated with increased MTA levels, altered PA profiles, and reduced NA content. Exogenous feeding of PAs partially recovered fertility, whereas NA supplementation improved fertility and also reversed interveinal chlorosis. The analysis of PA synthase crystal structures containing bound MTA suggests that the corresponding enzyme activities are sensitive to available MTA. Mutant plants that expressed either MTN or human methylthioadenosine phosphorylase (which metabolizes MTA without producing MTR) appeared wild type, proving that the abnormal traits of the mutant are due to MTA accumulation rather than reduced MTR. Based on our results, we propose that the key targets affected by increased MTA content are thermospermine synthase activity and spermidine-dependent posttranslational modification of eukaryotic initiation factor 5A.

Probing the substrate specificity of Golgi alpha-mannosidase II by use of synthetic oligosaccharides and a catalytic nucleophile mutant

Inhibition of Golgi alpha-mannosidase II (GMII), which acts late in the N-glycan processing pathway, provides a route to blocking cancer-induced changes in cell surface oligosaccharide structures. To probe the substrate requirements of GMII, oligosaccharides were synthesized that contained an alpha(1,3)- or alpha(1,6)-linked 1-thiomannoside. Surprisingly, these oligosaccharides were not observed in X-ray crystal structures of native Drosophila GMII (dGMII). However, a mutant enzyme in which the catalytic nucleophilic aspartate was changed to alanine (D204A) allowed visualization of soaked oligosaccharides and led to the identification of the binding site for the alpha(1,3)-linked mannoside of the natural substrate. These studies also indicate that the conformational change of the bound mannoside to a high-energy B 2,5 conformation is facilitated by steric hindrance from, and the formation of strong hydrogen bonds to, Asp204. The observation that 1-thio-linked mannosides are not well tolerated by the catalytic site of dGMII led to the synthesis of a pentasaccharide containing the alpha(1,6)-linked Man of the natural substrate and the beta(1,2)-linked GlcNAc moiety proposed to be accommodated by the extended binding site of the enzyme. A cocrystal structure of this compound with the D204A enzyme revealed the molecular interactions with the beta(1,2)-linked GlcNAc. The structure is consistent with the approximately 80-fold preference of dGMII for the cleavage of substrates containing a nonreducing beta(1,2)-linked GlcNAc. By contrast, the lysosomal mannosidase lacks an equivalent GlcNAc binding site and kinetic analysis indicates oligomannoside substrates without non-reducing-terminal GlcNAc modifications are preferred, suggesting that selective inhibitors for GMII could exploit the additional binding specificity of the GlcNAc binding site.

Enzymatic characterization of 5-methylthioribose 1-phosphate isomerase from Bacillus subtilis

The product of the mtnA gene of Bacillus subtilis catalyzes the isomerization of 5-methylthioribose 1-phosphate (MTR-1-P) to 5-methylthioribulose 1-phosphate (MTRu-1-P). The catalysis of MtnA is a novel isomerization of an aldose phosphate harboring a phosphate group on the hemiacetal group. This enzyme is distributed widely among bacteria through higher eukaryotes. The isomerase reaction analyzed using the recombinant B. subtilis enzyme showed a Michaelis constant for MTR-1-P of 138 microM, and showed that the maximum velocity of the reaction was 20.4 micromol min(-1) (mg protein)(-1). The optimum reaction temperature and reaction pH were 35 degrees C and 8.1. The activation energy of the reaction was calculated to be 68.7 kJ mol(-1). The enzyme, with a molecular mass of 76 kDa, was composed of two subunits. The equilibrium constant in the reversible isomerase reaction [MTRu-1-P]/[MTR-1-P] was 6. We discuss the possible reaction mechanism.

In vivo hydrolysis of S-adenosyl-L-methionine in Escherichia coli increases export of 5-methylthioribose

Escherichia coli can not synthesize methionine from 5-methylthioribose (MTR) but instead exports this sulfur-containing, energy-rich molecule into the surrounding medium. Transforming E. coli with plasmids that direct expression of the cloned coliphage T3 S-adenosyl-L-methionine (SAM) hydrolase (SAMase) induces the met regulon by cleaving the SAM co-repressor to form 5'-methylthioadenosine, which is then cleaved to produce MTR. To test the effect of in vivo SAMase activity on MTR production and its fate, cultures were incubated in the presence of [35S]methionine and [methyl-3H]methionine. Cells with SAMase activity produced significantly enhanced levels (up to 40-fold in some trials) of extracellular MTR -- the only radiolabeled compound released in significant amounts -- when compared with controls. SAM synthetase (metK) mutants transformed with SAMase expression vectors did not show this increase, verifying the path through SAM as the sole route to MTR production. SAMase expression had little or no effect on intracellular MTR pools, levels of radiolabeled macromolecules, or the transfer of methyl groups to compounds that could be precipitated by trichloroacetic acid. Thus, MTR appears to be a dead-end metabolite in E. coli, begging questions about how this has evolved, the mechanism of MTR export for the cell, and whether the release of MTR is important for some other activity.

Identification of Asp174 and Asp175 as the key catalytic residues of human O-GlcNAcase by functional analysis of site-directed mutants

O-GlcNAcase is a family 84 beta-N-acetylglucosaminidase catalyzing the hydrolytic cleavage of beta-O-linked 2-acetamido-2-deoxy-d-glycopyranose (O-GlcNAc) from serine and threonine residues of posttranslationally modified proteins. O-GlcNAcases use a double-displacement mechanism involving formation and breakdown of a transient bicyclic oxazoline intermediate. The key catalytic residues of any family 84 enzyme facilitating this reaction, however, are unknown. Two mutants of human O-GlcNAcase, D174A and D175A, were generated since these residues are highly conserved among family 84 glycoside hydrolases. Structure-reactivity studies of the D174A mutant enzyme reveals severely impaired catalytic activity across a broad range of substrates alongside a pH-activity profile consistent with deletion of a key catalytic residue. The D175A mutant enzyme shows a significant decrease in catalytic efficiency with substrates bearing poor leaving groups (up to 3000-fold), while for substates bearing good leading groups the difference is much smaller (7-fold). This mutant enzyme also cleaves thioglycosides with essentially the same catalytic efficiency as the wild-type enzyme. As well, addition of azide as an exogenous nucleophile increases the activity of this enzyme toward a substrate bearing an excellent leaving group. Together, these results allow unambiguous assignment of Asp(174) as the residue that polarizes the 2-acetamido group for attack on the anomeric center and Asp(175) as the residue that functions as the general acid/base catalyst. Therefore, the family 84 glycoside hydrolases use a DD catalytic pair to effect catalysis.

Efficient synthesis and photodynamic activity of porphyrin-saccharide conjugates: targeting and incapacitating cancer cells

Since the role of saccharides in cell recognition, metabolism, and cell labeling is well-established, the conjugation of saccharides to drugs is an active area of research. Thus, one goal in the use of saccharide-drug conjugates is to impart a greater specificity toward a given cell type or other targets. Although widely used to treat some cancers and age related macular degeneration, the drugs used in photodynamic therapy (PDT) display poor chemical selectivity toward the intended targets, and uptake by cells most likely arises from passive, diffusional processes. Instead, the specific irradiation of the target tissues, and the formation of the toxic species in situ, are the primary factors that modulate the selectivity in the present mode of PDT. We report herein a two-step method to make nonhydrolyzable saccharide-porphyrin conjugates in high yields using a tetra(pentafluorophenyl)porphyrin and the thio derivative of the sugar. As a demonstration of their properties, the selective uptake (and/or binding) of these compounds to several cancer cell types was examined, followed by an investigation of their photodynamic properties. As expected, different malignant cell types take up one type of saccharide-porphyrin conjugate preferentially over others; for example, human breast cancer cells (MDA-MB-231) absorb a tetraglucose-porphyrin conjugate over the corresponding galactose derivative. Doseametric studies reveal that these saccharide-porphyrin conjugates exhibit varying PDT responses depending on drug concentration and irradiation energy. (1) Using 20 microM conjugate and greater irradiation energy induces cell death by necrosis. (2) When 10-20 microM conjugate and less irradiation energy are used, both necrosis and apoptosis are observed. (3) Using 10 microM and the least irradiation energy, a significant reduction in cell migration is observed, which indicates a reduction in aggressiveness of the cancer cells.

Characterisation of cellobiose dehydrogenases from the white-rot fungi Trametes pubescens and Trametes villosa

Cellobiose dehydrogenase (CDH) is an extracellular haemoflavoenzyme that is produced by a number of wood-degrading and phytopathogenic fungi and it has a proposed role in the early events of lignocellulose degradation and wood colonisation. In the presence of a suitable electron acceptor, e.g. 2,6-dichloro-indophenol, cytochrome c, or metal ions, CDH oxidises cellobiose to cellobionolactone. When screening 11 different Trametes spp. for the formation of CDH activity, all the strains investigated were found to secrete significant amounts of CDH when cultivated on a cellulose-containing medium. Amongst others, Trametes pubescens and Trametes villosa were identified as excellent, not-yet-described, producer strains of this enzyme activity that has various potential applications in biotechnology. CDH from both strains was purified to apparent homogeneity and subsequently characterised. Both monomeric enzymes have a molecular mass of approximately 90 kDa (gel filtration) and a pI value of 4.2-4.4. The best substrates are cellobiose and cellooligosaccharides; additionally, lactose, thiocellobiose, and xylobiose are efficiently oxidised. Glucose and maltose are poor substrates. The preferred substrate is cellobiose with a Km value of 0.21 mM and a kcat value of 22 s(-1) for CDH from T. pubescens; the corresponding values for the T. villosa enzyme are 0.21 mM and 24 s(-1), respectively. Both enzymes showed very high activity with one-electron acceptors such as ferricenium, ferricyanide, or the azino-bis-(3-ethyl-benzthiazolin-6-sulfonic acid) cation radical.

A functional link between RuBisCO-like protein of Bacillus and photosynthetic RuBisCO

The genomes of several nonphotosynthetic bacteria, such as Bacillus subtilis, and some Archaea include genes for proteins with sequence homology to the large subunit of ribulose bisphosphate carboxylase/oxygenase (RuBisCO). We found that such a RuBisCO-like protein (RLP) from B. subtilis catalyzed the 2,3-diketo-5-methylthiopentyl-1-phosphate enolase reaction in the methionine salvage pathway. A growth-defective mutant, in which the gene for this RLP had been disrupted, was rescued by the gene for RuBisCOfrom the photosynthetic bacterium Rhodospirillum rubrum. Thus, the photosynthetic RuBisCO from R. rubrum retains the ability to function in the methionine salvage pathway in B. subtilis.

5-Methylthiopentose: a new substituent on lipoarabinomannan in Mycobacterium tuberculosis

We have identified and characterised in several strains of Mycobacterium tuberculosis a new 5-methylthiopentose substituent on lipoarabinomannan (LAM). The 5-methylthiopentose was initially observed in heteronuclear (1)H-(13)C-NMR spectra of intact, (13)C-enriched LAM. Oligosaccharides carrying this substituent were released from (13)C-enriched LAM and from unlabelled LAM using an endo-arabinanase from Cellulomonas gellida. The presence of the methylthio group in these oligosaccharides was established using NMR, high-resolution Fourier-transform ion cyclotron resonance mass spectrometry and tandem mass spectrometry using a Q-TOF mass spectrometer. The 5-methylthiopentose is linked to a terminal mannose in the cap structures of these oligosaccharides as evidenced by tandem mass spectrometry and by NMR. We suggest interference with the signal transduction mechanisms of infected macrophages as a possible function for this newly discovered LAM substituent.

Extracting biological information from DNA arrays: an unexpected link between arginine and methionine metabolism in Bacillus subtilis

BACKGROUND

In global gene expression profiling experiments, variation in the expression of genes of interest can often be hidden by general noise. To determine how biologically significant variation can be distinguished under such conditions we have analyzed the differences in gene expression when Bacillus subtilis is grown either on methionine or on methylthioribose as sulfur source.

RESULTS

An unexpected link between arginine metabolism and sulfur metabolism was discovered, enabling us to identify a high-affinity arginine transport system encoded by the yqiXYZ genes. In addition, we tentatively identified a methionine/methionine sulfoxide transport system which is encoded by the operon ytmIJKLMhisP and is presumably used in the degradation of methionine sulfoxide to methane sulfonate for sulfur recycling. Experimental parameters resulting in systematic biases in gene expression were also uncovered. In particular, we found that the late competence operons comE, comF and comG were associated with subtle variations in growth conditions.

CONCLUSIONS

Using variance analysis it is possible to distinguish between systematic biases and relevant gene-expression variation in transcriptome experiments. Co-variation of metabolic gene expression pathways was thus uncovered linking nitrogen and sulfur metabolism in B. subtilis.

Solid-phase synthesis of new S-glycoamino acid building blocks

Efficient synthesis of unprotected S-glycoamino acid building blocks in the solid phase by coupling a sugar 1-thiolate with iodine activated fluoren-9-ylmethoxycarbonyl (Fmoc) protected amino acids.

A new affinity ligand for the isolation of a single 'feruloyl esterase' (FAE-III) from Aspergillus niger

8-Aminooctyl 5'-S-coniferyl-5'-deoxy-thio-alpha-L-arabinofuranoside has been synthesised and shown to be a selective affinity ligand for the feruloyl esterase III of Aspergillus niger. The hydrolyses of methyl 5-O-coumaroyl, feruloyl, or sinapoyl alpha-L-arabinofuranosides by this enzyme proceed at comparable rates.

Characterization of the UDP-glucuronosyltransferases involved in the glucuronidation of an antithrombotic thioxyloside in rat and humans

To investigate the glucuronidation on the hydroxyl group of carbohydrate-containing drugs, the in vitro formation of glucuronides on the thioxyloside ring of the antithrombotic drug, LF 4.0212, was followed in rat and human liver microsomes and with recombinant UDP-glucuronosyltransferases (UGT). The reaction revealed a marked regioselectivity in rat and humans. Human liver microsomes glucuronidated the compound mainly on the 2-hydroxyl position of the thioxyloside ring, whereas rat was able to form glucuronide on either the 2-, 3-, or 4- hydroxyl group of the molecule, although to a lower extent. LF 4.0212 was a much better substrate of human UGT than the rat enzyme (Vmax/Km 30.0 and 0.06 microl/min/mg, respectively). Phenobarbital, 3-methylcholanthrene, and clofibrate enhanced the glucuronidation of LF 4.0212 on positions 2, 3, and 4 of the thioxyloside ring, thus indicating that several UGT isoforms were involved in this process. The biosynthesis of the 2-O-glucuronide isomer was catalyzed by the human UGT1A9 and 2B4, but not by UGT1A6 and 2B11. By contrast, the rat liver recombinant UGT1A6 and 2B1 failed to form the 2-O-glucuronide isomers. From all the recombinant UGTs tested, none catalyzed the formation of the 3-O-glucuronide isomer. Interestingly, glucuronidation on the 4-position was found in all the metabolic competent V79 cell lines considered, including the nontransfected V79 cells, suggesting the presence of an endogenous UGT in fibroblasts able to actively glucuronidate the drug. This activity, which was nonsensitive to the inhibitory effect of 7,7,7-triphenylheptanoic acid, a potent UGT inhibitor, could reflect the existence of a different enzyme.

NMR studies of the conformation of thiocellobiose bound to a beta-glucosidase from Streptomyces sp

The conformation of 4-thiocellobiose bound to beta-glucosidase from Streptomyces sp. has been studied by 1H-NMR transferred nuclear Overhauser effect spectroscopy (TR-NOE). Thiocellobiose behaves as an inhibitor of this glucosidase when cellobiose is used as substrate. NOE measurements and molecular mechanics calculations have also been performed to estimate the probability distribution of conformers of thiocellobiose when free in solution. Experimental data show that, in contrast with the natural O-analogue, thiocellobiose presents three conformational families in the free state, namely syn, anti-psi and anti-phi, whilst only one of them (syn) is recognized by the enzyme.

Chemoenzymatic synthesis of 6 omega-S-alpha-D-glucopyranosyl-6 omega-thiomaltooligosaccharides: their binding to Aspergillus niger glucoamylase G1 and its starch-binding domain

A coupling reaction of cyclodextrin glucosyltransferase (CGTase) with glucose and 6-deoxy-6-iodo-cyclomaltoheptaose (1), in the presence of glucoamylase, followed by acetylation, led to a convenient synthesis of acetylated 6III-deoxy-6III-iodo-maltotriose (2) and 6IV-deoxy-6IV-iodomaltotraose (3). Nucleophilic displacement of the iodine atom of these protected maltotriose and maltotetraose analogs by the activated form of 2,3,4,6-tetra-O-acetyl-1-S- acetyl-1-thio-alpha-D-glucose (4) afforded peracetylated 6III-S-alpha-D-glucopyranosyl-6III-thiomaltotriose (5) and 6IV-S-alpha-D-glucopyranosyl-6IV-thiomaltotetraose (6) in high yield. The interaction of OH-free tetra- and penta-saccharides (7 and 8) with both glucoamylase G1 from Aspergillus niger as well as its isolated starch-binding domain fragment were studied by UV difference spectroscopy. It was found that the starch-binding domain has higher affinity for 7 and 8 than for maltotetraose and maltopentaose.

Synthesis of fluorescent and radioactive analogues of two lactosylceramides and glucosylceramide containing beta-thioglycosidic bonds that are resistant to enzymatic degradation

Condensation of 2-S-(2,3,4,6-tetra-O-acetyl-beta-D-galactopyranosyl)-2- thiopseudourea hydrobromide with 2,3,6-tri-O-benzoyl-4-O-trifluoromethylsulfonyl-beta-D-galactopyra nosyl- (1-->1)-(2S,3R,4E)-3-O-benzoyl-2-dichloroacetamido-4-octa decen-1,3-diol afforded S-(2,3,4,6-tetra-O-acetyl-beta-D-galactopyranosyl)-(1-->4)-2,3,6-tri-O- benzoyl-4-thio-beta-D-glucopyranosyl-(1-->1)-(2S,3R,4E)-3-O-benzoy l-2- dichloroacetamido-4-octadecen-1,3-diol in good yield. Removal of the protecting groups, followed by selective N-acylation of the sphingosine amino group with either a fluorescent or a radioactive fatty acid, gave labeled lactosylceramide analogues in good yield. Since these products contained a beta-thioglycosidic bond between the two sugar moieties, they were totally resistant to the action of acid lysosomal glycosidases. Likewise, condensation of 2-S-(2,3,4,6-tetra-O-acetyl-beta-D-glucopyranosyl)-2- thiopseudourea hydrobromide and 2,3,4,6-tetra-O-acetyl-beta-D-galactopyranosyl-(1-->4)-2,3,6- tri-O-acetyl-1-S-acetyl-1-thio-beta-D-glucopyranose with (2R,3R,4E)-3-O-benzoyl-2-dichloroacetamido-1-iodo-4-octad ecen-3-ol in methanolic sodium acetate afforded the corresponding beta-thioglycosides 14 and 16, respectively, in good yield. These beta-thioglycosides were converted into glucosylceramide and lactosylceramide analogues following removal of the protecting groups and by subsequent selective N-acylation using either a fluorescent or adioactive fatty acid N-succinimidyl ester. Whereas the glucosylthioceramides thus obtained proved to be completely undegradable by lysosomal glucocerebrosidase, the lactosylceramides containing the beta-thioglycosidic bond between the lactose and the ceramide residues could be degraded by lysosomal GM1-beta-galactosidase to give the corresponding glucosylthioceramides. These compound did not yield to any further enzymatic degradation.

The hydrophobic mannoside Man alpha 1-6Man alpha 1-S-(CH2)7-CH3 acts as an acceptor for the UDP-Gal:glycosylphosphatidylinositol anchor alpha 1,3-galactosyltransferase of Trypanosoma brucei

The variant surface glycoproteins (VSGs) of Trypanosoma brucei are attached to the plasma membrane via a glycosylphosphatidylinositol (GPI) membrane anchor. This anchor contains the core sequence ethanolamine-PO4-6Man alpha 1-2Man alpha 1-6Man alpha 1-4GlcN alpha 1-6myo-inositol, which is conserved in all GPI anchors, and a unique alpha Gal side chain attached to the 3-position of the alpha Man residue adjacent to the alpha GlcN residue. Here we report that trypanosome membranes can catalyse the transfer of Gal from UDP-Gal to the hydrophobic thioglycoside Man alpha 1-6Man alpha 1-S-(CH2)7-CH3. Characterization of the galactosylated products by electrospray mass spectrometry, exoglycosidase digestion and periodate-oxidation studies revealed that the major product was Man alpha 1-6(Gal alpha 1-3)Man alpha 1-S-(CH2)7-CH3. The similarity of this product to part of the mature VSG GPI anchor suggests that the thioglycoside is able to act as an acceptor for the trypanosome-specific UDP-Gal-GPI anchor alpha 1,3-galactosyltransferase.

Efficient photoaffinity labeling of human beta-hexosaminidase A. Synthesis and application of 3-azi-1-[(2-acetamido-2-deoxy-1-beta- D-glucopyranosyl)thio]- and -galactopyranosyl)thio]butane

Two photolabile thioglycosides (8 and 9) were synthesized by Koenigs-Knorr type glycosylation. These compounds, being enzyme-resistant analogues of N-acetylhexosaminides, were shown to be good competitive inhibitors of lysosomal beta-hexosaminidase (2-acetamido-2-deoxy- beta-D-hexoside acetamidodeoxyhexohydrolase, EC 3.2.1.52) action. For photoaffinity labeling 3H-labeled 8a was prepared by enzymatic oxidation with galactose oxidase followed by reduction with sodium [3H]borohydride. Compound 8a, when photolyzed in the presence of hexosaminidase, specifically labeled both subunits of the enzyme.

4-Thiocellooligosaccharides. Their synthesis and use as ligands for the separation of cellobiohydrolases of Trichoderma reesei by affinity chromatography

4-Aminophenyl 1,4-dithio-beta-cellobioside (6) was obtained by treatment of methyl 2,3,6-tri-O-benzoyl-4-O-triflyl-alpha-D-galactopyranoside with the sodium salt of 1-thio-beta-D-glucopyranose, followed by acetolysis and glycosylation of the corresponding bromide with 4-aminobenzenethiol and subsequent deacylation. A similar synthesis starting with the 1-thiolate of 1,4-dithio-beta-cellobiose led to the trisaccharide 4-aminophenyl 1,4,4'-trithiocellotrioside (16). The 4-acetamidophenyl di- and tri-thiocellooligosaccharides were found to be excellent competitive inhibitors of the hydrolysis of 4-methylumbelliferyl beta-lactoside with respective Ki values of 25 and 6.5 mM. The two 4-aminophenyl oligosaccharides 6 and 16 were coupled to CH-Sepharose 4B, and the affinity gels were used for the purification of cellobiohydrolases from a crude commercial cellulolytic extract of T. reesei. Cellobiohydrolases I or II were selectively desorbed from gels bearing ligands 6 and 16.

Spacer-modified disaccharide and pseudo-trisaccharide methyl glycosides that mimic maltotriose, as competitive inhibitors for pancreatic alpha-amylase: a demonstration of the "clustering effect"

The synthesis is reported of methyl 4,4'-dithio-alpha-maltotrioside (12) and the spacer-modified disaccharide glycosides methyl 4-S-(4-alpha-D-glucopyranosylthio-2-hydroxybutyl)-4-thio-alpha -D-glucopyranoside (20) and methyl 4-S-[(1,5/4,6)- and (4,6/1,5)-4-alpha-D-glucopyranosylthio-5,6-dihydroxy-2- cyclohexen-1-yl]-4-thio-alpha-D-glucopyranoside (29a/b), which are analogues of methyl alpha-maltotrioside. The Ki values for alpha-amylase for these compounds were determined as were those of methyl alpha-maltotrioside and maltose.

Phosphorylated thiosugars: synthesis, properties, and reactivity in enzymatic reactions

A number of phosphorylated thiosugars have been prepared and tested as substrates for metabolic reactions. 6-Thioglucose-6-P is readily synthesized by reaction of 6-tosylglucose with trisodium thiophosphate at pH 10 in aqueous solution; the product has only sulfur between carbon and phosphorus. When ethyl glycerate is tosylated and treated similarly with thiophosphate, a 5:1 mixture of 3-thioglycerate-3-P and the 2-isomer is formed. 6-Thioglucose-6-P is converted by glycolytic enzymes to triose phosphates, 3-thioglycerol-3-P and 3-thioglycerate-3-P, and is oxidized by enzymes of the hexose monophosphate shunt to 5-thioribulose-5-P, which can be converted via phosphoribulokinase and ribulose-bis-P carboxylase into 3-P-glycerate and 3-thioglycerate-3-P. For most of the non-phosphoryl-transferring enzymes there are only moderate effects on Vmax and Km. Phosphoglucoisomerase, however, is very sensitive to the sulfur for oxygen change, with Vmax decreasing 60-fold and Km increasing 15-fold. Surprisingly, phosphoribulokinase has a V/K value for 5-thioribulose-5-P that is over 3 orders of magnitude less than for ribulose-5-P. 6-Thio-glucose-6-P was found to be a substrate for several enzymes that transfer the phosphoryl group. It is as good a substrate for alkaline phosphatase as glucose-6-P, and with phosphoglucomutase it is converted to 6-thioglucose-1-P with a rate that is 11% of the rate of reaction of glucose-1-P, with a Keq value of 45.6. The free energy of hydrolysis of the phosphorylated thiol is thus -7.2 kcal/mol at pH 7.(ABSTRACT TRUNCATED AT 250 WORDS)

Conversion of 5-S-methyl-5-thio-D-ribose to methionine in Klebsiella pneumoniae. Stable isotope incorporation studies of the terminal enzymatic reactions in the pathway

Extracts of Klebsiella pneumoniae convert 5-S-methyl-5-thio-D-ribose (methylthioribose) to methionine and formate. To probe the terminal steps of this biotransformation, [1-13C]methylthioribose has been synthesized and its metabolism examined. When supplemented with Mg2+, ATP, L-glutamine, and dioxygen, cell-free extracts of K. pneumoniae converted 50% of the [1-13C]methylthioribose to [13C]formate. The formation of [13C]formate was established by 13C and 1H NMR spectroscopy studies of the purified formate, and by 13C and 1H NMR spectroscopy and mass spectrometry studies of its p-phenylphenacyl derivative. By contrast, no incorporation of label from [1-13C]methylthioribose into the biosynthesized methionine was detected by either mass spectrometry or 13C and 1H NMR spectroscopy. The most reasonable interpretation of these results is that C-1 of methylthioribose is converted directly to formate concomitant with the conversion of carbon atoms 2-5 to methionine. The penultimate step in the conversion of methylthioribose to methionine and formate is an oxidative carbon-carbon bond cleavage reaction in which an equivalent of dioxygen is consumed. To investigate the fate of the dioxygen utilized in this reaction, the metabolism of [1-13C]methylthioribose in the presence of 18O2 was also examined. Mass spectrometry revealed the biosynthesis of substantial amounts of both [18O1]methionine and [13C, 18O1]formate under these conditions. These results suggest that the oxidative transformation in the conversion of methylthioribose to methionine and formate may be catalyzed by a novel intramolecular dioxygenase. A mechanism for this dioxygenase is proposed.

Fast measurement of galactoside transport by lactose permease

Lactose permease of Escherichia coli was reconstituted into vesicles of dimyristoylphosphatidylcholine, and the rate of galactoside counterflow was measured in the millisecond time range. The turnover number and the half-saturation constant for transport agree with the values known for cells. This result demonstrates that lactose permease is the sole protein necessary for galactoside transport. Furthermore, lactose permease seems not to require a high level of negatively charged lipids or a certain degree of unsaturation of the lipid hydrocarbon chains. However, the lipids must be in the fluid state, because the transport rate drastically decreases below the lipid ordered fluid phase transition.

Intermediates in the conversion of 5'-S-methylthioadenosine to methionine in Klebsiella pneumoniae

Extracts of Klebsiella pneumoniae oxidatively convert 1-phospho-5-S-methylthioribose (1-PMTR) to alpha-keto-gamma-methylthiobutyrate, a precursor of methionine, and to S-methylthiopropionate and formate. One equivalent of formate is produced per equivalent of alpha-keto-gamma-methylthiobutyrate and two equivalents of formate per equivalent of methylthiopropionate. Two compounds were identified as intermediates in this reaction sequence: 1-phospho-5-S-methylthioribulose (1-PMT-ribulose) and 1-phospho-2,3-diketo-5-S-methylpentane. The enzyme, 1-PMTR isomerase, which converts 1-PMTR to 1-PMT-ribulose was highly purified. In addition, a protein fraction was isolated which converts 1-PMT-ribulose to the phosphodiketone. A second protein fraction was isolated that converts the phosphodiketone to an intermediate which has not been isolated so far. This intermediate is oxidatively converted to alpha-keto-gamma-methylthiobutyrate and S-methylthiopropionate by a third protein fraction. Methylthiopropionate is not derived from free alpha-keto-gamma-methylthiobutyrate.

Inducer expulsion in Streptococcus pyogenes: properties and mechanism of the efflux reaction

Expulsion of preaccumulated methyl-beta-D-thiogalactoside-phosphate (TMG-P) from Streptococcus pyogenes is a two-step process comprising intracellular dephosphorylation of TMG-P followed by rapid efflux of the intracellularly formed free galactoside (J. Reizer, M.J. Novotny, C. Panos, and M.H. Saier, Jr., J. Bacteriol. 156:354-361, 1983). The present study identifies the mechanism and the order and characterizes the temperature dependency of the efflux step. Unidirectional efflux of the intracellularly formed [14C]TMG was only slightly affected when measured in the presence of unlabeled TMG (25 to 400 mM) in the extracellular medium. In contrast, pronounced inhibition of net efflux was observed in the presence of relatively low concentrations (1 to 16 mM) of extracellular [14C]TMG. Since net efflux was nearly arrested when the external concentration of [14C]TMG approached the intracellular concentration of this sugar, we propose that a facilitated diffusion mechanism is responsible for efflux and equilibration of TMG between the intracellular and extracellular milieus. The exit reaction was markedly dependent upon temperature, exhibited a high energy of activation (23 kcal [ca. 96 kJ] per mol), and followed first-order kinetics, indicating that the permease mediating this efflux was not saturated under the conditions of expulsion employed.

Chemical and biological properties of indole glucosinolates (glucobrassicins): a review

Glucosinolates are a group of secondary products commonly, but not exclusively, found in plants of the family Cruciferae. They give rise, upon enzymic hydrolysis, to a range of volatile, pungent and physiologically active compounds. Recently, particular attention has been focused upon those that are trytophan-derived--the indole glucosinolates (glucobrassicins). When chemically or enzymically hydrolysed these compounds give rise to a range of involatile indole compounds which have been implicated in the anti-carcinogenic and mixed-function-oxidase stimulatory activities of brassica vegetables. This review details the chemical and physiological properties of indole glucosinolates and their products and suggests possible areas for future research.

Regulation of beta-galactoside transport and accumulation in heterofermentative lactic acid bacteria

Galactose-grown cells of the heterofermentative lactic acid bacteria Lactobacillus brevis and Lactobacillus buchneri transported methyl-beta-D-thiogalactopyranoside (TMG) by an active transport mechanism and accumulated intracellular free TMG when provided with an exogenous source of energy, such as arginine. The intracellular concentration of TMG resultant under these conditions was approximately 20-fold higher than that in the medium. In contrast, the provision of energy by metabolism of glucose, gluconate, or glucosamine promoted a rapid but transient uptake of TMG followed by efflux that established a low cellular concentration of the galactoside, i.e., only two- to fourfold higher than that in the medium. Furthermore, the addition of glucose to cells preloaded with TMG in the presence of arginine elicited a rapid efflux of the intracellular galactoside. The extent of cellular TMG displacement and the duration of the transient effect of glucose on TMG transport were related to the initial concentration of glucose in the medium. Exhaustion of glucose from the medium restored uptake and accumulation of TMG, providing arginine was available for ATP generation. The nonmetabolizable sugar 2-deoxyglucose elicited efflux of TMG from preloaded cells of L. buchneri but not from those of L. brevis. Phosphorylation of this glucose analog was catalyzed by cell extracts of L. buchneri but not by those of L. brevis. Iodoacetate, at a concentration that inhibits growth and ATP production from glucose, did not prevent efflux of cellular TMG elicited by glucose. The results suggested that a phosphorylated metabolite(s) at or above the level of glyceraldehyde-3-phosphate was required to evoke displacement of intracellular TMG from the cells. Counterflow experiments suggested that glucose converted the active uptake of TMG in L. brevis to a facilitated diffusion mechanism that allowed equilibrium of TMG between the extra- and intracellular milieux. The means by which glucose metabolites elicited this vectorial regulation is not known, but similarities to the inducer expulsion that has been described for homofermentative Streptococcus and Lactobacillus species suggested the involvement of HPr, a protein that functions as a phosphocarrier protein in the phosphotransferase system, as well as a presumptive regulator of sugar transport. Indeed, complementation assays wit extracts of Staphylococcus aureus ptsH mutant revealed the presence of HPr in L. brevis, although this lactobacillus lacked a functional phaosphoenolpyruvate-dependent phosphortransferase system for glucose, 2-deoxyglucose, or TMG.

Valorization of rapeseed meal. 2. Nutritive value of high or low-glucosinolate varieties and effect of dehulling

Five groups of 12 growing rats each were fed a control diet or one of 4 experimental diets composed of either high glucosinolate (Jet Neuf) or low glucosinolate (Tandem) rapeseed meal. After a 3-day adaptation period, the rats were fed ad libitum for 15 days. Both meals were given dehulled or non-dehulled. The Tandem rapeseed meal provided all the protein and Jet Neuf half the protein of the corresponding diets. Dehulling the seeds significantly increased the energy and protein digestibility of the rapeseed meals (+ 16.5 and + 6.3 or 10.4 points, respectively), their digestible energy and digestible protein contents (+ 28 and + 30%) and metabolic utilization of digestible protein in the indirect nitrogen balance trial. Conversely, in the feeding trial, the feed efficiency of the dehulled meal diets was lower than that of the non-dehulled meal diets. However, dehulling the Tandem rape seeds did not seem to reduce lysine availability. Finally, feeding dehulled Tandem rapeseed meal as the sole protein source did not significantly increase the liver weight of the rats.

Substrate recognition by a sucrose transporting protein

Protoplasts derived from developing soybean cotyledons were used to study substrate recognition by a sucrose transporting protein in plant membranes. When used as alternate substrate inhibitors of [14C] sucrose influx, five different fructosyl-substituted sucrose derivatives, phenyl-alpha-D-glucopyranoside, and phenyl-alpha-D-thioglucopyranoside proved to bind effectively to the sucrose carrier-active site. These results are interpreted to indicate that a large portion of substrate recognition by this carrier may arise from the interaction of a relatively hydrophobic portion of the sucrose molecule and a hydrophobic region of the carrier protein binding site. Binding of phenyl-alpha-D-thioglucopyranosides in which various substitutions were made for the glucosyl hydroxyls shows that the glucosyl hydroxyls at positions 3, 4, and 6 are involved in substrate recognition by the carrier protein.

The role of the proton motive force and electron flow in solute transport in Escherichia coli

Transport of lactose and methyl beta-D-thiogalactopyranoside, a melibiose analogue, was studied in intact cells of Escherichia coli. A proton motive force could drive the translocation of these solutes via these two transport systems, but the initial rates and steady-state levels of solute accumulation increased upon initiation of electron transfer. When the absolute value of the proton motive force was decreased by ionophores the steady-state levels of lactose accumulation did not decrease as expected if thermodynamic equilibrium with the proton motive force had existed. Accumulation of lactose was also observed in the absence of any measurable proton motive force as long as electron transfer took place. Since both proton/lactose and sodium/methyl beta-D-thiogalactopyranoside symport showed the same characteristics, an explanation based on local proton diffusion pathways is unlikely.

The metabolism of 5'-methylthioadenosine and 5-methylthioribose 1-phosphate in Saccharomyces cerevisiae

Cordycepin sensitive mutants of Saccharomyces cerevisiae, which are permeable to 5'-deoxy-5'-methylthioadenosine (MTA), were used to study the fate of the methylthioribose carbons of this purine nucleoside. Evidence is presented for the recycling of the methylthio group and part of the ribose portion of MTA in a biosynthetic pathway which leads to the synthesis of methionine. The main pathway involves the phosphorylytic cleavage of MTA by MTA phosphorylase yielding 5-methylthioribose 1-phosphate and adenine as products. Loss of the phosphate group of 5-methylthioribose 1-phosphate, concurrent with the rearrangement of the ribose carbons, leads to the synthesis of 2-keto-4-methylthiobutyric acid. In the final step of the sequence, 2-keto-4-methylthiobutyric acid is converted to methionine via transamination. Several compounds not directly associated with the biosynthesis of methionine were also isolated. These compounds, which may arise through the degradation of intermediates in the pathway, were: 5'-methylthioinosine, a deaminated catabolite of MTA; 5-methylthioribose, a result of the phosphorylysis of 5-methylthioribose 1-phosphate, and 3-methylthiopropionaldehyde, 3-methylthiopropionic acid and 2-hydroxy-4-methylthiobutyric acid, all arising from the catabolism of 2-keto-4-methylthiobutyric acid.

The metabolism of some xenobiotics in germ-free and conventional rats

The different participation of the gastrointestinal microflora in the biotransformation of gamma-hexachlorcyclohexane (gamma-HCH), azobenzene and the isolated glucosinolate progoitrin was studied. Marked dehydrochlorination of gamma-HCH to gamma-pentachlorcyclohexene (PCCH) also occurs without participation of the microflora. Azobenzene was reduced in germ-free (g) and conventional (c) rats to hydrazobenzene but was split to aniline only in c-rats. Progoitrin was split with and without the influence of the microflora, but the splitting was more complete in rats with intact gastrointestinal flora.

Characteristics of n-octyl beta-D-thioglucopyranoside, a new non-ionic detergent useful for membrane biochemistry

n-Octyl beta-D-thioglucopyranoside, a new non-ionic detergent, was synthesized. Properties, and applicability to membrane proteins, of this detergent were investigated. The detergent was easily removed by dialysis. The solubilizing power of this detergent for Escherichia coli membrane proteins was similar to that of n-octyl beta-D-glucopyranoside, which has been widely used in membrane biochemistry. No inactivation of proteins was observed after the solubilization. n-Octyl beta-D-thioglucopyranoside was superior to n-octyl beta-D-glucopyranoside in that it was much more stable and could be synthesized at much lower cost.

Characterization of a defect in the pathway for converting 5'-deoxy-5'-methylthioadenosine to methionine in a subline of a cultured heterogeneous human colon carcinoma

5'-Deoxy-5'-methylthioadenosine (methylthioadenosine) is cleaved to adenine and 5-methylthioribose-1-phosphate (methylthioribose-1-P). Methylthioribose-1-P is converted to 2-keto-4-methylthiobutyrate ( ketomethylthiobutyrate ) which is transaminated to methionine. We report that one subline of a heterogeneous human colon carcinoma, DLD-1 Clone D, only forms methylthioribose-1-P from methylthioadenosine or 5'-deoxy-5'-methylthioinosine (methylthioinosine), a deaminated derivative of methylthioadenosine, whereas Clone A converts methylthioadenosine and methylthioinosine to methionine, as shown by growth studies in culture of Clone A and Clone D cells and radioactive studies utilizing [methyl-14C]methylthioadenosine or [methyl-14C]methylthioinosine in the presence of extracts of these cells lines. To characterize this defect, we utilized three protein fractions isolated from rat liver which together convert methylthioribose-1-P to ketomethylthiobutyrate . Addition of only Fraction A to Clone D sonicates restores its ability to convert methylthioadenosine to methionine. This fraction is responsible for converting methylthioribose-1-P to 5- methylthioribulose -1-phosphate; radioactive studies confirm this observation. Thus, Clone D is deficient in an enzyme contained in Fraction A; this represents a qualitative biochemical difference between the two clones derived from a single human tumor.

Mechanism of inducer expulsion in Streptococcus pyogenes: a two-step process activated by ATP

The mechanism of methyl-beta-D-thiogalactoside-phosphate (TMG-P) expulsion from Streptococcus pyogenes was studied. The expulsion elicited by glucose was not due to exchange vectorial transphosphorylation between the expelled TMG and the incoming glucose since more beta-galactoside was displaced than glucose taken up, and the stoichiometry between TMG and glucose transport was inconstant. Instead, two distinct and sequential reactions, intracellular dephosphorylation of TMG-P followed by efflux of free TMG, mediated the expulsion. This was shown by temporary accumulation of free TMG effected by competitive inhibition of its efflux and by the aid of arsenate, which arrested dephosphorylation of TMG-P but did not affect efflux of free TMG formed intracellularly before arsenate addition. The competitive inhibition of TMG efflux by its structural analogs suggests that a transport protein facilitates the expulsion. Iodoacetate or fluoride prevented TMG-P dephosphorylation and its expulsion. However, provision of ATP via the arginine deiminase pathway restored these activities in the presence of the glycolytic inhibitors and stimulated expulsion in their absence. Other amino acids tested did not promote this restoration, and canavanine or norvaline severely inhibited it. Arginine without glucose neither elicited the dephosphorylation nor evoked the expulsion of TMG-P. Ionophores or ATPase inhibitors did not prevent the expulsion as elicited by glucose or its restoration by arginine. The results suggest that activation of the dephosphorylation-expulsion mechanism occurs independently of a functional glycolytic pathway, requires ATP provision, and is possibly due to protein phosphorylation controlled by a yet unknown metabolite. The in vivo phosphorylation of a protein (approximate molecular weight - 10,000) under the conditions of expulsion was demonstrated.

Involvement of lactose enzyme II of the phosphotransferase system in rapid expulsion of free galactosides from Streptococcus pyogenes

Streptococcus pyogenes accumulated thiomethyl-beta-galactoside as the 6-phosphate ester due to the action of the phosphoenolpyruvate:lactose phosphotransferase system. Subsequent addition of glucose resulted in rapid efflux of the free galactoside after intracellular dephosphorylation (inducer expulsion). Efflux was shown to occur in the apparent absence of the galactose permease, but was inhibited by substrate analogs of the lactose enzyme II and could not be demonstrated in a mutant of S. lactis ML3 which lacked this enzyme. The results suggest that the enzymes II of the phosphotransferase system can catalyze the rapid efflux of free sugar under appropriate physiological conditions.

Methionine synthesis from 5'-S-Methylthioadenosine. Resolution of enzyme activities and identification of 1-phospho-5-S methylthioribulose

5'-S-Methylthioadenosine is converted to methionine in mammalian systems, microorganisms and plants. 5'-S-Methylthioadenosine is first converted to 1-phospho-5-S-methylthioribofuranoside (1-PMTR) which is then converted to 2-keto-4-S-methylthiobutyrate, the precursor of methionine. We have now investigated the conversion of 1-PMTR to the keto acid. This conversion requires at least three protein fractions designated A, B, and C. Fraction A catalyzes an isomerization of 1-PMTR to form 1-phospho-5-S-methylthioribulose. The identification of this compound is based in part on the products obtained after NaIO4 oxidation, i.e. S-methylthioacetaldehyde, formate, and phosphoglycolic acid. When fractions A and B are added to 1-PMTR, two additional compounds, designated II and III, were detected. No O2 was consumed in the formation of compounds II and III. These compounds are, therefore, at the oxidation state of 5-S-methylthioribose. Compound II is phosphorylated as evidenced by its electrophoretic behavior before and after alkaline phosphatase treatment. Addition of fraction C to compounds II and III leads to O2 consumption and to the conversion of these compounds to 2-keto-4-S-methylthiobutyrate. Thus, compounds II and III are precursors of the keto acid. These compounds have not been fully characterized.

Kinetic analysis of H+/methyl beta-D-thiogalactoside symport in Saccharomyces fragilis

A theoretical description of initial uptake kinetics of H+/sugar symport is given, with emphasis on the differences between carrier and non-carrier systems. Transport of methyl beta-D-thiogalactoside in Saccharomyces fragilis is shown to proceed via the inducible lactose transporter. Uptake of this sugar stimulates electrogenic H+ influx. Together with the correlation between methyl beta-D-thiogalactoside accumulation and the proton-motive force this shows that transport proceeds via H+ symport. Kinetic analysis of initial influx revealed that transport proceeds via a single transport system, sensitive to changes in membrane potential. The pH dependence of the kinetic parameters showed that Kapp is almost pH insensitive, whereas Vapp decreases strongly at increasing extracellular pH. It is shown that transport proceeds, most likely, via a non-carrier system, with random binding of H+ and sugar, in a system where binding of the first ligand does not influence binding of the second.

Induction of cellulose in Schizophyllum commune: thiocellobiose as a new inducer

Several mono-, di, tetra-, and polysaccharides were screened for their ability to induced cellulase production by the tetrapolar hymenomycete Schizophyllum commune. Out of 21 carbohydrates screened, 4 (thiocellobiose, carboxymethylcellulose, cellobiose, and xylan) induced all three enzymes tested (carboxymethylcellulase, beta-glucosidase, and xylanase). The inducing effect increased with rising concentrations of the inducers up to a certain value, beyond which there was either a leveling off or a decrease of the enzymatic activities. The most powerful inducer, thiocellobiose, showed the highest activity at 0.5 mM. Cellobiose, carboxymethylcellulose, and xylan showed their highest activities at 1 mM and 1%, respectively. Surprisingly, sophorose did not enhance enzyme production. The enzymatic activities were monitored over a period of 24 h. Thiocelloboise elicited a response immediately after incubation, but with all other inducers there was a latency period before their effect could be measured. High-performance liquid chromatography showed no hydrolysis of thiocellobiose when incubated in the presence of S. commune extracellular enzymes.

Regulation of methyl-beta-d-thiogalactopyranoside-6-phosphate accumulation in Streptococcus lactis by exclusion and expulsion mechanisms

Starved cells of Streptococcus lactis ML3 (grown previously on galactose, lactose, or maltose) accumulated methyl-beta-D-thiogalactopyranoside (TMG) by the lactose:phosphotransferase system. More than 98% of accumulated sugar was present as a phosphorylated derivative, TMG-6-phosphate (TMG-6P). When a phosphotransferase system sugar (glucose, mannose, 2-deoxyglucose, or lactose) was added to the medium simultaneously with TMG, the beta-galactoside was excluded from the cells. Galactose enhanced the accumulation of TMG-6P. Glucose, mannose, lactose, or maltose plus arginine, was added to a suspension of TMG-6P-loaded cells of S. lactis ML3, elicited rapid expulsion of intracellular solute. The material recovered in the medium was exclusively free TMG. Expulsion of galactoside required both entry and metabolism of an appropriate sugar, and intracellular dephosphorylation of TMG-6P preceded efflux of TMG. The rate of dephosphorylation of TMG-6P by permeabilized cells was increased two-to threefold by adenosine 5'-triphosphate but was strongly inhibited by fluoride. S. lactis ML3 (DGr) was derived from S. lactis ML3 by positive selection for resistance to 2-deoxy-D-glucose and was defective in the enzyme IIMan component of the glucose:phosphotransferase system. Neither glucose nor mannose excluded TMG from cells of S. lactic ML3 (DGr), and these two sugars failed to elicit TMG expulsion from preloaded cells of the mutant strain. Accumulation of TMG-6P by S. lactis ML3 can be regulation by two independent mechanisms whose activities promote exclusion or expulsion of galactoside from the cell.