Cellobiose transport by mixed ruminal bacteria from a Cow

The transport of cellobiose in mixed ruminal bacteria harvested from a holstein cow fed an Italian ryegrass hay was determined in the presence of nojirimycin-1-sulfate, which almost inhibited cellobiase activity. The kinetic parameters of cellobiose uptake were 14 microM for the Km and 10 nmol/min/mg of protein for the Vmax. Extracellular and cell-associated cellobiases were detected in the rumen, with both showing higher Vmax values and lower affinities than those determined for cellobiose transport. The proportion of cellobiose that was directly transported before it was extracellularly degraded into glucose increased as the cellobiose concentration decreased, reaching more than 20% at the actually observed levels of cellobiose in the rumen, which were less than 0.02 mM. The inhibitor experiment showed that cellobiose was incorporated into the cells mainly by the phosphoenolpyruvate phosphotransferase system and partially by an ATP-dependent and proton-motive-force-independent active transport system. This finding was also supported by determinations of phosphoenolpyruvate phosphotransferase-dependent NADH oxidation with cellobiose and the effects of artificial potentials on cellobiose transport. Cellobiose uptake was sensitive to a decrease in pH (especially below 6.0), and it was weakly but significantly inhibited in the presence of glucose.

Structural and dynamic aspects of beta-glycosidase from mesophilic and thermophilic bacteria by multitryptophanyl emission decay studies

The tryptophanyl emission decay of beta-glycosidase from the extremophilic archaeon Sulfolobus solfataricus (Sbetagly) has been investigated by frequency domain fluorometry. The data were analyzed in terms of sum of discrete lifetimes as well as in terms of quasi- continuous lifetime distributions of different shape. At neutral pH the emission decay is characterized by two components: a long-lived component, centered at 7.4 ns, and a short one at 2.7 ns, irrespective of the decay scheme used for the interpretation of the experimental results. The effects of an irreversible inhibitor, that is, cyclophellitol, and that of a powerful denaturant such as guanidinium hydrochloride on the dynamics of Sbetagly has been investigated by observing the changes induced in the two components of the tryptophanyl emission decay. The addition of cyclophellitol to native Sbetagly reduces the contribution of the short-lived component but does not affect the long-lived one. Increasing concentrations of guanidinium hydrochloride differently affect the contributions of the two emission components. Higher concentrations were required to unfold the molecular regions containing the long-lived indolic fluorophores. These results indicate that the long-lived contribution arises from tryptophanyl residues deeply clustered in the interior of the protein matrix, whereas the short-lived one includes residues located in less rigid and more solvent accessible regions, some of which might be located in functionally important parts of protein. The knowledge of the crystallographic structure of Sbetagly allowed us to evaluate some average parameters for each tryptophanyl microenvironment in the Sbetagly such as hydrophobicity, structural flexibility, and ability of side chains to act as fluorescence quenchers. These results permitted to divide the tryptophanyl fluorescence of Sbetagly in the contribution of two emitting groups: one consisting of eight closely clustered tryptophans, that is, Trp 33, 36, 60, 84, 151 174, 425, and 433, responsible for the long-lived emission component and the other one, composed of nine tryptophans nearer to the subunit surface, that is, Trp 12, 156, 192, 287, 288, 316, 361, 376, 455, associable to the short-lived emission component. Finally, the examination of the tryptophanyl emission decay of the mesophilic beta-galactosidase from Escherichia coli (Cbetagal) and the Arrhenius analysis of its dependence on temperature indicated that the tryptophanyl environments of the mesophilic enzyme are rather homogeneous in consequence of a larger protein dynamics.

Homoisofagomines: chemical-enzymatic synthesis and evaluation as alpha- and beta-glucosidase inhibitors

Methyl- and hydroxymethyl derivatives of the highly potent glycosidase inhibitor isofagomine are accessible via aldolase-catalyzed C-C bond formation and competitively inhibit beta-glucosidase at low micromolar concentrations.

Catalytic mechanism and specificity for hydrolysis and transglycosylation reactions of cytosolic beta-glucosidase from guinea pig liver

Cytosolic beta-glucosidase (CBG) from mammalian liver is known for its broad substrate specificity and has been implicated in the transformation of xenobiotic glycosides. CBG also catalyzes a variety of transglycosylation reactions, which have been been shown with other glycosylhydrolases to function in synthetic and genetic regulatory pathways. We investigated the catalytic mechanism, substrate specificity, and transglycosylation acceptor specificity of guinea pig liver CBG by several methods. These studies indicate that CBG employs a two-step catalytic mechanism with the formation of a covalent enzyme-sugar intermediate and that CBG will transfer sugar residues to primary hydroxyls and equatorial but not axial C-4 hydroxyls of aldopyranosyl sugars. Kinetic studies revealed that correction for transglycosylation reactions is necessary to derive correct kinetic parameters for CBG. Further analyses revealed that for aldopyranosyl substrates, the activation energy barrier is affected most by the presence of a C-6 carbon and by the configuration of the C-2 hydroxyl, whereas the binding energy is affected modestly by the configuration and substituents at C-2, C-4, and C-5. These data indicate that the transglycosylation activity of CBG derives from the formation of a covalently linked enzyme-sugar intermediate and that the specificity of CBG for transglycosylation reactions is different from its specificity for hydrolysis reactions.

The family 1 beta-glucosidases from Pyrococcus furiosus and Agrobacterium faecalis share a common catalytic mechanism

Comparisons of catalytic mechanisms have not previously been performed for homologous enzymes from hyperthermophilic and mesophilic sources. Here, the beta-glucosidase from the hyperthermophilic archaeon Pyrococcus furiosus was recombinantly produced in Escherichia coli and shown to have biophyscial and biochemical properties identical to those of the wild-type enzyme. Moreover, the recombinant enzyme was subjected to a detailed kinetic investigation at 95 degreesC to compare its catalytic mechanism to that determined at 37 degreesC for the beta-glucosidase (abg) from the mesophilic bacterium, Agrobacterium faecalis [Kempton, J., and Withers, S. G. (1992) Biochemistry 31, 9961]. These enzymes have amino acid sequences that are 33% identical and have been classified as family 1 glycosyl hydrolases on the basis of amino acid sequence similarities. Both enzymes have similar broad specificities for both sugar and aglycone moieties and exhibit nearly identical pH dependences for their kinetic parameters with several different substrates. Bronsted plots were constructed for bgl at several temperatures using a series of aryl glucoside substrates. These plots were concave downward at all temperatures, indicating that bgl utilized a two-step mechanism similar to that of abg and that the rate-limiting step in this mechanism did not change with temperature for any given aryl glucoside. The Bronsted coefficient for bgl at 95 degreesC (beta1g = -0.7) was identical to that for abg at 37 degreesC and implies that these enzymes utilize nearly identical transition states, at least in regard to charge accumulation on the departing glycosidic oxygen. In addition, a high correlation coefficient (rho = 0.97) for the linear free energy relationship between these two enzymes and similar inhibition constants for these two enzymes with several ground state and transition state analogue inhibitors further indicate that these enzymes stabilize similar transition states. The mechanistic similarities between these two enzymes are noteworthy in light of the large difference in their temperature optima. This suggests that, in the presumed evolution that occurred between the hyperthermophilic archaeal enzyme and the mesophilic bacterial enzyme, structural modifications must have been selected which maintained the integrity of the active site structure and, therefore, the specificity of transition state interactions, while adapting the overall protein structure to permit function at the appropriate temperature.

Deglycosylation of flavonoid and isoflavonoid glycosides by human small intestine and liver beta-glucosidase activity

Flavonoid and isoflavonoid glycosides are common dietary phenolics which may be absorbed from the small intestine of humans. The ability of cell-free extracts from human small intestine and liver to deglycosylate various (iso)flavonoid glycosides was investigated. Quercetin 4'-glucoside, naringenin 7-glucoside, apigenin 7-glucoside, genistein 7-glucoside and daidzein 7-glucoside were rapidly deglycosylated by both tissue extracts, whereas quercetin 3,4'-diglucoside, quercetin 3-glucoside, kaempferol 3-glucoside, quercetin 3-rhamnoglucoside and naringenin 7-rhamnoglucoside remained unchanged. The Km for hydrolysis of quercetin 4'-glucoside and genistein 7-glucoside was approximately 32+/-12 and approximately 14+/-3 microM in both tissues respectively. The enzymatic activity of the cell-free extracts exhibits similar properties to the cytosolic broad-specificity -glucosidase previously described in mammals.

Expression of cytosolic beta-glucosidase in guinea pig liver cells

The cytosolic beta-glucosidase of mammalian liver has been implicated in the metabolic transformation of plant glycosides, such as vicine and amygdalin, which are associated with the development of toxic syndromes. We investigated which cell types express cytosolic beta-glucosidase in guinea pig liver, and characterized the contribution of this enzyme to the hydrolysis of aromatic glucosides in cultured cells and in tissue slices. Cytosolic beta-glucosidase was expressed in hepatocytes and not in Kupffer or endothelial cells as determined by enzyme-specific activity and Western blots of liver cell extracts. Intracellular beta-glucosidase activity was visualized using the fluorescent beta-glucosidase substrate, resorufin beta-D-glucoside, and shown to be caused by the cytosolic beta-glucosidase using the inhibitors, conduritol beta-epoxide and dinitrophenol-2-deoxy-2-fluoro-beta-D-glucopyranoside (DNP2FGlc). Staining of fresh liver slices with resorufin beta-glucoside revealed that cytosolic beta-glucosidase is expressed in all hepatocytes, with no significant portal-central gradient. These data indicate that cytosolic beta-glucosidase is a hepatocyte-specific enzyme, and support the hypothesis that cytosolic beta-glucosidase in the liver functions to hydrolyze small glucosides absorbed by the intestine. Furthermore, toxic injury to cultured hepatocytes by CCl4 resulted in release of cytosolic beta-glucosidase in parallel with the hepatocyte marker enzymes alanine transaminase and lactate dehydrogenase. This suggests that acute increases in serum levels of cytosolic beta-glucosidase in animal models of liver injury may reflect direct injury of hepatocytes.

Studies on the constituents of Broussonetia species. III. Two new pyrrolidine alkaloids, broussonetines G and H, as inhibitors of glycosidase, from Broussonetia kazinoki Sieb

Two new pyrrolidine alkaloids, broussonetines G and H, were isolated from the branches of Broussonetia kazinoki SIEB. (Moraceae). Broussonetines G and H were formulated as 2 beta-hydroxymethyl-3 alpha, 4 beta-dihydroxy-5 alpha-(1-hydroxy- 6:10;10:13-diepoxytridecyl)-pyrrolidine (1) and 2 beta-hydroxymethyl-3 alpha, 4 beta-dihydroxy-5 alpha-(1-hydroxy- 5:9;9:13-diepoxytridecyl)-pyrrolidine (2), respectively, by spectroscopic methods. 1 and 2 inhibited beta-glucosidase, beta-galactosidase and beta-mannosidase.

Salicortin: a repeat-attack new-mechanism-based Agrobacterium faecalis beta-glucosidase inhibitor

Salicortin, a natural product abundant in most members of the Salicaceae family, is a mechanism-based inactivator of Agrobacterium faecalis beta-glucosidase. Inactivation is delayed in the presence of competitive inhibitors, thereby demonstrating the requirement for an enzyme-bound salicortin before inactivation. Product studies suggest that inactivation proceeds via a quinone methide intermediate formed by the fragmentation of the aglycone of salicortin while it is bound to the enzyme. Tryptic digest and HPLC/MS studies confirm the role of quinone methide attack and also show that the enzyme undergoes multiple modifications. In addition, when the inactivation was run in the presence of a mutant inactive form of the enzyme, HPLC/MS analyses clearly showed no modification of the mutant enzyme, demonstrating that the quinone methide does not exist in free solution and suggesting that inactivation is active-site directed.

Folding of insulin receptor monomers is facilitated by the molecular chaperones calnexin and calreticulin and impaired by rapid dimerization

Many complex membrane proteins undergo subunit folding and assembly in the ER before transport to the cell surface. Receptors for insulin and insulin-like growth factor I, both integral membrane proteins and members of the family of receptor tyrosine kinases (RTKs), are unusual in that they require homodimerization before export from the ER. To better understand chaperone mechanisms in endogenous membrane protein assembly in living cells, we have examined the folding, assembly, and transport of the human insulin receptor (HIR), a dimeric RTK. Using pulse-chase labeling and nonreducing SDS-PAGE analysis, we have explored the molecular basis of several sequential maturation steps during receptor biosynthesis. Under normal growth conditions, newly synthesized receptor monomers undergo disulfide bond formation while associated with the homologous chaperones calnexin (Cnx) and calreticulin (Crt). An inhibitor of glucose trimming, castanospermine (CST), abolished binding to Cnx/Crt but also unexpectedly accelerated receptor homodimerization resulting in misfolded oligomeric proreceptors whose processing was delayed and cell surface expression was also decreased by approximately 30%. Prematurely-dimerized receptors were retained in the ER and more avidly associated with the heat shock protein of 70 kD homologue binding protein. In CST-treated cells, receptor misfolding followed disordered oligomerization. Together, these studies demonstrate a chaperone function for Cnx/Crt in HIR folding in vivo and also provide evidence that folding efficiency and homodimerization are counterbalanced.

A cell wall-bound beta-glucosidase from germinated rice: purification and properties

A large portion of beta-glucosidase (EC 3.2.1.21) in germinating rice seeds, which appears to be ionically bound to cell walls, can be solubilized with 1 M NaCl. Its activity increased more than eight-fold within five days of germination. It was purified to electrophoretic homogeneity from the extracts of germinated rice seeds by fractionation with (NH4)2SO4 followed by CM-Sepharose, Polybuffer exchanger 118, Concanavalin A-Sepharose and Bio-Gel P-100. The Mr of the purified enzyme, estimated by SDS-PAGE, was 56,000 and the isoelectric point was > 10.0. Its N-terminal amino acid sequence (44 residues) exhibited high homology to those of beta-glucosidases from other plants, such as barley and white clover. Its activity was optimal at pH 4.5 and 50 degrees, and it was strongly inhibited by glucono-1,5-lactone. The enzyme showed hydrolytic as well as transglycosylation activity towards (1-->3)-beta- and (1-->4)-beta-linked oligosaccharides with degree of polymerization of 2-4. The results suggest that the beta-glucosidase is probably involved not only in hydrolysis but also in modification of oligosaccharides in cell walls of germinating rice seeds.

New imidazoles as probes of the active site topology and potent inhibitors of beta-glucosidase

Series of 4-arylimidazoles, omega-N-acylhistamines and 4-(omega-phenylalkyl)imidazoles were synthesized in order to probe the active site topology of sweet almond beta-glucosidase. These imidazole derivatives were shown to be very powerful competitive inhibitors. Among the 20 tested compounds, omega-N-benzoylhistamine and 4-(3'-phenylpropyl)imidazole are the most potent inhibitors of the enzyme, with pH-independent Ki values of 0.06 and 0.07 microM, respectively. The inhibition of 4-(omega-phenylalkyl)imidazoles exhibited an interesting trend as to Ki values: 4-phenylimidazole (6.6 microM)>4-benzylimidazole (1.4 microM)>4-(2'-phenylethyl)imidazole (0.82 microM)>4-(3'-phenylpropyl)imidazole (0.07 microM)<4-(4'-phenylbutyl)imidazole (0.13 microM)<4-(5'-phenylpentyl)imidazole (0.3 microM). This revealed that the imidazole and aryl binding sites (which result from favorable interactions within the corresponding glycone and aglycone binding subsites) are separated by the optimal distance equivalent to the length of a -CH2-CH2-CH2- group. Substitutions of the phenyl moieties of 4-phenylimidazole and 4-benzoylhistamine result in weaker inhibition. These classes of imidazoles are particularly powerful inhibitors of sweet almond beta-glucosidase.

Purification and some properties of beta-glucosidase from Aspergillus niger IBT-90

beta-glucosidase (EC 3.2.1.21) was isolated from the culture filtrate of Aspergillus niger IBT-90. The crude extracellular enzyme preparation was fractionated by six step purification procedure, (NH4)2SO4 precipitation, gel filtration on Bio-Gel P-10 and P-100, an ion-exchange chromatography on DEAE Bio-Gel A, yielding beta-glucosidase with an isoelectric point at pH 4.05. The enzyme was found to be a dimer with an apparent molecular weight of approximately 200 kDa as determined by size exclusion chromatography. It is composed of two apparently identical subunits of about 100 kDa (determined by sodium dodecyl sulphate polyacrylamide gel electrophoresis). A. niger IBT-90 beta-glucosidase contains 33% carbohydrates. It is most active towards cellobiose at pH 4.8 and 65 degrees C. The enzyme sequentially splits off glucose units from non reducing ends of collodextrins. Kinetic studies on cellobiose and salicin hydrolysis, in concentration from 0.1 to 5.0 mM, resulted in non-linear Lineweaver-Burk and Hanes plots, whereas p-nitrophenyl-beta-D-glucopiranoside (pNPG) did not induce this type of effect. No metal ion is required for the enzyme catalytic activity. Hg2+ and N-bromosuccinimide (NBS) are its strong inhibitors. Glucono-delta-lactone and glucose are competitive inhibitors of the enzyme and glucono-delta-lactone is more potent of the two.

Cytosolic pyridoxine-beta-D-glucoside hydrolase from porcine jejunal mucosa. Purification, properties, and comparison with broad specificity beta-glucosidase

During studies of the nutritional utilization of pyridoxine 5'-beta-D-glucoside, a major form of vitamin B6 in plants, we detected two cytosolic beta-glucosidases in jejunal mucosa. As expected, one was broad specificity beta-glucosidase that hydrolyzed aryl beta-D-glycosides but not pyridoxine beta-D-glucoside. We also found a previously unknown enzyme, designated pyridoxine-beta-D-glucoside hydrolase, that efficiently hydrolyzed pyridoxine beta-D-glucoside. These were separated and purified as follows: broad specificity beta-glucosidase 1460-fold and pyridoxine-beta-D-glucoside hydrolase 36,500-fold. Purified pyridoxine-beta-D-glucoside hydrolase did not hydrolyze any of the aryl glycosides tested but did hydrolyze cellobiose and lactose. Pyridoxine-beta-D-glucoside hydrolase exhibited a pH optimum of 5.5 and apparent molecular mass of 130 kDa by SDS-polyacrylamide gel electrophoresis and 160 kDa by nondenaturing gel filtration, in contrast to 60 kDa for native and denatured broad specificity beta-glucosidase. Glucono-delta-lactone was a strong inhibitor of both enzymes. Ionic and nonionic detergents were inhibitory for each enzyme. Conduritol B epoxide, a potent inhibitor of lysosomal acid beta-glucosidase, inhibited pyridoxine-beta-D-glucoside hydrolase but not broad specificity beta-glucosidase, but both were inhibited by the mechanism-based inhibitor 2-deoxy-2-fluoro-beta-D-glucosyl fluoride. Our findings indicate major differences between these two cytosolic beta-glucosidases. Studies addressing the role of vitamin B6 nutrition in regulating the activity and its consequences regarding pyridoxine glucoside bioavailability are in progress.

The effects of calystegines isolated from edible fruits and vegetables on mammalian liver glycosidases

The polyhydroxylated nortropane alkaloids called calystegines occur in many plants of the Convolvulaceae, Solanaceae, and Moraceae families. Certain of these alkaloids exhibit potent inhibitory activities against glycosidases and the recently demonstrated occurrence of calystegines in the leaves, skins, and sprouts of potatoes (Solanum tuberosum), and in the leaves of the eggplant (S. melongena), has raised concerns regarding the safety of these vegetables in the human diet. We have surveyed the occurrence of calystegines in edible fruits and vegetables of the families Convolvulaceae, Solanaceae, and Moraceae by GC-MS. Calystegines A3, B1, B2, and C1 were detected in all the edible fruits and vegetables tested; sweet and chili peppers, potatoes, eggplants, tomatoes, Physalis fruits, sweet potatoes, and mulberries. Calystegines B1 and C1 were potent competitive inhibitors of the bovine, human, and rat beta-glucosidase activities, with Ki values of 150, 10, and 1.9 microM, respectively for B1 and 15, 1.5, and 1 microM, respectively, for C1. Calystegine B2 was a strong competitive inhibitor of the alpha-galactosidase activity in all the livers. Human beta-xylosidase was inhibited by all four nortropanes, with calystegine C1 having a Ki of 0.13 microM. Calystegines A3 and B2 selectively inhibited the rat liver beta-glucosidase activity. The potent inhibition of mammalian beta-glucosidase and alpha-galactosidase activities in vitro raises the possibility of toxicity in humans consuming large amounts of plants that contain these compounds.

Specific alpha-galactosidase inhibitors, N-methylcalystegines--structure/activity relationships of calystegines from Lycium chinense

An examination of the roots of Lycium chinense (Solanaceae) has resulted in the discovery of 14 calystegines, a cycloheptane bearing an amino group and three hydroxyl groups, and two polyhydroxylated piperidine alkaloids. Calystegines A7 and B5, in addition to the previously known calystegines A3, A5, A6, B1, B2, B3, B4, C1, C2 and N1, were isolated and determined as 1alpha,2beta,4alpha-trihydroxy-nortropane and 1alpha,2alpha,4alpha,7alpha-tetrahydroxy-nort ropane, respectively. L. chinense also had two polyhydroxytropanes bearing a methyl group on the nitrogen atom, unlike the previously reported nortropane alkaloids. They were established as N-methylcalystegines B2 and C1, and their N-methyl groups were found to be axially oriented from NOE experiments. 1Beta-amino-3beta,4beta,5alpha-trihydroxycyclohepta ne was also present in L. chinense and may be a biosynthetic precursor of the calystegines that occur in this plant. Two polyhydroxypiperidine alkaloids, fagomine and 6-deoxyfagomine, were isolated. Calystegine B2 is a potent competitive inhibitor of almond beta-glucosidase (Ki = 1.9 microM) and coffee bean alpha-galactosidase (Ki = 0.86 microM), while N-methylcalystegine B2 was a more potent competitive inhibitor of the latter enzyme (Ki = 0.47 microM) than the parent compound but showed a marked lack of inhibitory activities towards most other glycosidases. Since this compound is a very specific inhibitor of alpha-galactosidase and inhibits rat liver lysosomal alpha-galactosidase with a Ki of 1.8 microM, it may provide a useful experimental model for the lysosomal storage disorder, Fabry's disease. The addition of a hydroxyl group at C6exo, as in calystegines B1 and C1, enhances the inhibitory potential towards beta-glucosidase and beta-galactosidase but markedly lowers or abolishes inhibition towards alpha-galactosidase. Hence, the N-methylation of calystegine C1 did not enhance its inhibition of alpha-galactosidase. The chemical N-methylation of calystegines A3 and B4 markedly enhanced inhibition of coffee bean alpha-galactosidase, with Ki values of 5.2 microM and 36 microM, respectively, but almost eliminated their inhibitory potential towards beta-glucosidase and trehalase, respectively. Thus, methylation of the nitrogen atom significantly altered the specificity of the inhibitors.

Inhibition of glycohydrolase enzymes by aqueous extracts of Chinese medicinal herbs in a microplate format

A microplate assay, for use with a variety of glycohydrolase enzymes, was developed to aid the screening of Chinese medicinal herb extracts for the presence of potential anti-viral and anti-lymphoma compounds. The microplate assay method described offers greater convenience, speed and reproducibility over existing methods. The enzymes tested were alpha-glucosidase, beta-glucosidase and beta-glucuronidase. The assay can be easily adapted for use with other glycohydrolase enzymes. Of the 12 herb extracts examined four did not inhibit any of the enzymes (< 50% inhibition), one inhibited alpha-glucosidase only (> 50% inhibition), six inhibited beta-glucuronidase only, and one inhibited both alpha-glucosidase and beta-glucuronidase. None of the extracts were capable of inhibiting beta-glucosidase to any significant extent.

Purification and characterization of a glucose-tolerant beta-glucosidase from Aspergillus niger CCRC 31494

An extracellular glucose-tolerant beta-glucosidase was purified to homogeneity by alcohol fractionation and preparative isoelectric focusing from Aspergillus niger CCRC 31494. The enzyme was a dimeric protein with a subunit of 49,000, and had its optimum activity at pH 5.0 and 55 degrees C. The enzyme was completely inhibited by 5 mM Ag+. Thiol groups and serine residues were not essential for its activity. Low concentrations of alcohols (10%) except for methanol could activate the enzyme. It was very specific for para-nitrophenyl-beta-D-glucoside (pNPG) and cellobiose. However, the enzyme also had some beta-xylosidase activity, but showed no activity towards alpha-linked glycosidic substrates. The Vmax of 124.4 U/mg and 21.6 U/mg were found for pNPG (Km = 21.7 mM) and para-nitrophenyl-beta-D-xyloside (pNPX) (Km = 14.2 mM), respectively. The enzyme was tolerant to glucose inhibition with a Ki of 543 mM, while fructose, galactose, mannose, and xylose were not inhibitory.

Synthesis and biological evaluation of potent glycosidase inhibitors: N-phenyl cyclic isourea derivatives of 5-amino- and 5-amino-C-(hydroxymethyl)-1,2,3,4-cyclopentanetetraols

Twenty-four stereoisomers of 5-amino- and 5-amino-C-(hydroxymethyl)-1,2,3,4-cyclopentanetetraols and twenty-six of the corresponding N-phenyl cyclic isourea derivatives were assayed for inhibitory activity against six glycosidases. Among them, as has been expected for structure mimics of putative transition state glucopyranosyl cation for glycoside hydrolysis, 1L-(1,2,4,5/3)-5-amino-1-C-(hydroxymethyl)-1,2,3,4-cyclopentanetetrao l L-4 and its N-phenyl cyclic isourea derivative S-19 were shown to have strong inhibitory activity, IC50 4 x 10(-7) and 7.6 x 10(-9) M, respectively, against baker's yeast alpha-glucosidase. It has been analogously explained that compounds R,S-22 and R,S-26 possessed high inhibitory potency against Escherichia coli and bovine liver beta-galactosidases, respectively.

Purification and characterization of a microsomal bile acid beta-glucosidase from human liver

A human liver microsomal beta-glucosidase has been purified to apparent homogeneity in sodium dodecyl sulfate-polyacrylamide gel electrophoresis where a single protein band of Mr 100,000 was obtained under reducing conditions. The enzyme was enriched about 73, 000-fold over starting microsomal membranes by polyethylene glycol fractionation, anion exchange chromatographies on DEAE-Trisacryl, and Mono Q followed by affinity chromatography on N-(9-carboxynonyl)-1-deoxynojirimycin-AH-Sepharose 4B. The purified enzyme had a pH optimum between 5.0 and 6.4, was activated by divalent metal ions, and required phospholipids for exhibition of activity. The enzyme catalyzed the hydrolysis of 3beta-D-glucosido-lithocholic and 3beta-D-glucosido-chenodeoxycholic acids with high affinity (Km, 1.7 and 6.2 microM, respectively) and of the beta-D-glucoside (Km, 210 microM) and the beta-D-galactoside of 4-methylumbelliferone. The ratio of relative reaction rates for these substrates was about 6:3:11:1. No activity was detectable toward 6beta-D-glucosido-hyodeoxycholic acid, glucocerebroside, and the following glycosides of 4-methylumbelliferone: alpha-D-glucoside, alpha-L-arabinoside, beta-D-fucoside or beta-D-xyloside. Immunoinhibition and immunoprecipitation studies using antibodies prepared against lysosomal glucocerebrosidase showed no cross-reactivity with microsomal beta-glucosidase suggesting that these two enzymes are antigenically unrelated.

Fagomine isomers and glycosides from Xanthocercis zambesiaca

50% aqueous MeOH extracts from the leaves and roots of Xanthocercis zambesiaca (Leguminosae) were subjected to various ion-exchange column chromatographic steps to give fagomine (1), 3-epi-fagomine (2), 3,4-di-epi-fagomine (3), 3-O-beta-D-glucopyranosylfagomine (4), and 4-O-beta-D-glucopyranosylfagomine (5). Their structures were determined by spectroscopic analyses, particularly by extensive 1D and 2D NMR studies. Compounds 3 and 4 are new natural products. Compound 1 is a good inhibitor of isomaltase and certain alpha- and beta-galactosidases. Whereas 2 is a more potent inhibitor of isomaltase and beta-galactosidases than 1, it does not inhibit alpha-galactosidase. Compounds 3-5 exhibited no significant inhibition against the glycosidases used.

Purification and characterization of a beta-glucosidase from Aspergillus niger

The high-molar mass form of beta-glucosidase from Aspergillus niger strain NIAB280 was purified to homogeneity with a 46-fold increase in purification by a combination of ammonium sulfate precipitation, hydrophobic interaction, ion-exchange and gel-filtration chromatography. The native and subunit molar mass was 330 and 110 kDa, respectively. The pH and temperature optima were 4.6-5.3 and 70 degrees C, respectively. The K(m) and kcat for 4-nitrophenyl beta-D-glucopyranoside at 40 degrees C and pH 5 were 1.11 mmol/L and 4000/min, respectively. The enzyme was activated by low and inhibited by high concentrations of NaCl. Ammonium sulfate inhibited the enzyme. Thermolysin periodically inhibited and activated the enzyme during the course of reaction and after 150 min of proteinase treatment only 10% activity was lost with concomitant degradation of the enzyme into ten low-molar-mass active bands. When subjected to 0-9 mol/L transverse urea-gradient-PAGE for 105 min at 12 degrees C, the nonpurified beta-glucosidase showed two major bands which denatured at 4 and 8 mol/L urea, respectively, with half-lives of 73 min.

N1-alkyl-D-gluconamidines: are they 'perfect' mimics of the first transition state of glucosidase action?

The inhibition of four beta-glucosidases of plant, fungal, and mammalian origin by N1-butyl- and N1-dodecyl-D-gluconamidine was determined. Comparison with the inhibition by the corresponding N-alkyl-D-glucosylamines revealed that the strongly basic amidines (pKa 10.8) were at the most 10-times more inhibitory than the weakly basic glucosylamines (pKa 6.5). The small enhancement of inhibitory potency, resulting from transforming the tetrahedral C-1 geometry of the glucosylamines to the planar sp2-geometry of the amidines, was ascribed to the inability of the fully protonated amidines to function as hydrogen bond acceptors with the catalytic acid of the enzyme. Additional evidence for the importance of a hydrogen bond for strong inhibition came from the comparison of K1-values of the weakly basic 5-amino-5-deoxyhexopyranoses and 1,5-iminohexitols with those of the corresponding glyconamidrazones (pKa 8.4), which also have a planar C-1 geometry but are largely protonated under the assay conditions and which had similar or up to 10(4)-times larger K1-values than the former. Transition state resemblance was judged from the ratio KS(alkyl beta-glucoside)/K1(alkyl gluconamidine) relative to the rate acceleration factor kcat/kuncat (Wolfenden, Acc, Chem. Res., 5 (1972) 10-16). Compared to ratios of kcat/kuncat from > or = 10(11) to > or = 10(13), the ratios for KS/K1 were only from 10(3) to 2 x 10(4) except for beta-glucosidase A3 from Asp. wentii which had KS/K1 2.8 x 10(6). This enzyme differs from the others by being strongly inhibited by cationic glycon and substrate analogues rather than by basic ones. The pH-dependence of 1/K1 and the 'slow' approach to the inhibition is discussed with respect to transition state resemblance.

Production, purification, and characterization of a highly glucose-tolerant novel beta-glucosidase from Candida peltata

Candida peltata (NRRL Y-6888) produced beta-glucosidase when grown in liquid culture on various substrates (glucose, xylose, L-arabinose, cellobiose, sucrose, and maltose). An extracellular beta-glucosidase was purified 1,800-fold to homogeneity from the culture supernatant of the yeast grown on glucose by salting out with ammonium sulfate, ion-exchange chromatography with DEAE Bio-Gel A agarose, Bio-Gel A-0.5m gel filtration, and cellobiose-Sepharose affinity chromatography. The enzyme was a monomeric protein with an apparent molecular weight of 43,000 as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and gel filtration. It was optimally active at pH 5.0 and 50 degrees C and had a specific activity of 108 mumol.min-1.mg of protein-1 against p-nitrophenyl-beta-D-glucoside (pNP beta G). The purified beta-glucosidase readily hydrolyzed pNP beta G, cellobiose, cellotriose, cellotetraose, cellopentaose, and cellohexaose, with Km values of 2.3, 66, 39, 35, 21, and 18 mM, respectively. The enzyme was highly tolerant to glucose inhibition, with a Ki of 1.4 M (252 mg/ml). Substrate inhibition was not observed with 40 mM pNP beta G or 15% cellobiose. The enzyme did not require divalent cations for activity, and its activity was not affected by p-chloromercuribenzoate (0.2 mM), EDTA (10 mM), or dithiothreitol (10 mM). Ethanol at an optimal concentration (0.75%, vol/vol) stimulated the initial enzyme activity by only 11%. Cellobiose (10%, wt/vol) was almost completely hydrolyzed to glucose by the purified beta-glucosidase (1.5 U/ml) in both the absence and presence of glucose (6%). Glucose production was enhanced by 8.3% when microcrystalline cellulose (2%, wt/vol) was treated for 24 h with a commercial cellulase preparation (cellulase, 5 U/ml; beta-glucosidase, 0.45 U/ml) that was supplemented with purified beta-glucosidase (0.4 U/ml).