Preparation of isomaltose oligosaccharides labelled with 14C in the non-reducing terminal unit, and their use in studies of dextranase activity

Will isomalto-oligosaccharides, a well-established functional food in Asia, break through the European and American market? The status of knowledge on these prebiotics

This critical review article presents the current state of knowledge on isomalto-oligosaccharides, some well known functional oligosaccharides in Asia, to evaluate their potential as emergent prebiotics in the American and European functional food market. It includes first a unique inventory of the different families of compounds which have been considered as IMOs and their specific structure. A description has been given of the different production methods including the involved enzymes and their specific activities, the substrates, and the types of IMOs produced. Considering the structural complexity of IMO products, specific characterization methods are described, as well as purification methods which enable the body to get rid of digestible oligosaccharides. Finally, an extensive review of their techno-functional and nutritional properties enables placing IMOs inside the growing prebiotic market. This review is of particular interest considering that IMO commercialization in America and Europe is a topical subject due to the recent submission by Bioneutra Inc. (Canada) of a novel food file to the UK Food Standards Agency, as well as several patents for IMO production.

Preparation and reactivity of a novel disaccharide, glucosyl 1,5-anhydro-D-fructose (1,5-anhydro-3-O-alpha-glucopyranosyl-D-fructose)

A novel disaccharide, glucosyl 1,5-anhydro-D-fructose (1,5-anhydro-3-O-alpha-glucopyranosyl-D-fructose, GAF) was enzymatically prepared from 1,5-anhydro-D-fructose (1,5-AF) and cyclomaltoheptaose (beta-cyclodextrin). Cyclodextrin glucanotransferase transferred various sizes of maltooligosaccharide to 1,5-AF. Glucoamylase digested the maltooligosyl chain of the products to a glucosyl residue giving a final product, GAF. An NMR analysis of GAF elucidated that the glucose residue was linked to C-3 of the 1,5-AF residue with an ether linkage. Reactivity on the aminocarbonyl reaction of GAF with bovine serum albumin was lower than that of 1,5-AF, but was higher than that of glucose.

Formation of alpha-(1-->6), alpha-(1-->3), and alpha-(1-->2) glycosidic linkages by dextransucrase from Streptococcus sanguis in acceptor-dependent reactions

Dextransucrase from Streptococcus sanguis 10558 was found to synthesize alpha-(1-->6), alpha-(1-->3), and alpha-(1-->2) linkages during an acceptor-dependent glucosyl transfer reaction. Normally, new glucosyl residues are added at C-6 of monosaccharide acceptors. However, sugars blocked at C-6 also can serve as good acceptors. The disaccharide and trisaccharide products formed when methyl 6-bromo-6-deoxy-alpha-D-glucopyranoside was used as acceptor were isolated and characterized. Both were found to contain only alpha-(1-->3) glycosidic bonds. This supports the hypothesis that when C-6 is blocked the acceptor binds to the enzyme in a flipped orientation, resulting in an approximate exchange in space of the C-3 and C-6, thereby putting C-3 adjacent to the active site. The second alpha-(1-->3) links in the trisaccharide are formed by a single-chain mechanism without release of the intermediate disaccharide. With maltose as acceptor, new glucosyl residues are added at C-6'. However, if that position is blocked with a bromine atom, the resulting compound, 6'-bromo-6'-deoxy-maltose, can still serve as an acceptor. The product in this case was isolated and characterized. The new glycosidic link was found to be alpha-(1-->2).

Dynamic reactivities of dextransucrase

Dextransucrase, from Streptococcus sanguis ATCC 10558, was immobilized on hydroxylapatite and was "charged" in short pulses with labeled sucrose, as previously described [V. K. Parnaik, G. A. Luzio, D. A. Grahame, S. L. Ditson, and R. M. Mayer (1983) Carbohydr. Res. 121, 257-268]. The "charged" enzyme has been shown to contain both bound glucose and gluco-oligosaccharides. The reactivity of this form of the enzyme has been studied, and shown to have unexpected behavior. Earlier pulse-chase experiments [J. F. Robyt, B. K. Kimble, and T. F. Walseth (1979) Arch. Biochem. Biophys. 165, 634-640; S. L. Ditson and R. M. Mayer (1984) Carbohydr. Res. 126, 170-175], carried out with high concentrations of unlabeled sucrose in the chase, resulted in a rapid decrease in isotope at the reducing termini of enzyme-bound oligosaccharides. However, in the present work, in which the pulsed enzyme was chased with low concentrations of unlabeled sucrose, we observed an increase in the radioactive reducing termini. The possibility that this was due to the enzymatic hydrolysis of dextran has been ruled out. Data presented demonstrate that the enzyme catalyzes the depolymerization of the bound oligosaccharides. Individual glucosyl residues of the oligosaccharides are transferred to acceptors, such as added maltose to form a trisaccharide, or water to form glucose. Similarly, the glucosyl residues can be transferred to added fructose to form sucrose. The studies also provide evidence that the oligosaccharides are slowly released from the enzyme. The ability of the enzyme to catalyze the reverse of the glucosyl transfer reaction involving acceptors was also examined. It was observed that glucose residues transferred by dextransucrase to an acceptor can also be removed to produce sucrose when fructose is added.

Disproportionation reactions catalyzed by Leuconostoc and Streptococcus glucansucrases

Glucansucrases from Leuconostoc mesenteroides NRRL B-512F and Streptococcus mutans 6715 were found to utilize a number of D-gluco-oligosaccharides as D-glucosyl donors and as acceptors. These donors included isomaltotriose and its homologs, panose, maltotriose, and dextran. In each case, D-glucosyl groups were transferred from the donor to an acceptor sugar. When the donor sugar also acted as an acceptor, disproportionation reactions occurred. Isomaltotriose, for example, gave rise to isomaltose and isomaltotetraose initially, and to a series of isomalto-oligosaccharides eventually. In addition to forming alpha-D-(1----6) linkages in the reactions, dextransucrase from S. mutans 6715 was capable of forming alpha-D-(1----3)-linked products.

A D-glucosylated form of dextransucrase: preparation and characteristics

Dextransucrase was treated with [14C]sucrose, and the product applied to gel-permeation columns. In the absence of the detergents SDS and Triton X-100, poor recovery of enzyme was observed; however, that enzyme which was recovered was labeled. In the presence of detergents, recovery was increased, but the material appeared to be a large aggregate (mol. wt. greater than 5 X 10(6) ). In addition, the ratio of D-glucose to enzyme suggested that a polymer had been formed. Disc-gel electrophoresis in the presence of a mixture of SDS and Triton X-100 showed similar results, and indicated that the aggregate was disrupted upon treatment with dextranase. Native enzyme that had been immobilized on hydroxylapatite could also be labeled with [14C]sucrose, and the labeling followed saturation kinetics. The labeled protein could be released from the gel with 8M urea, but was aggregated. Radioactive sugars, free from protein, could be released by heating the labeled enzyme. The sugars released consisted of a mixture of D-glucose with oligosaccharides having an average chain-length of 17 D-glucosyl residues. The significance of these observations is discussed.

A D-glucosylated form of dextransucrase: demonstration of partial reactions

A D-glucosylated form of dextransucrase, whose preparation and characteristics have just been reported in Carbohydr. Res., was employed in a series of studies designed to explore the question of whether the bound sugars participate in the reactions catalyzed by the enzyme. When exposed to maltose, a good acceptor-substrate, monomeric D-glucosyl groups were rapidly transferred to the disaccharide, affording a trisaccharide. In the absence of an acceptor, monomeric D-glucose was released from the enzyme by hydrolysis. In a reaction with D-fructose, the charged enzyme catalyzed the formation of sucrose. Finally, in the presence of unlabeled sucrose, monomeric D-glucosyl groups were chased into enzyme-associated oligomers. Evidence is also presented which indicates that the various pathways for the bound D-glucosyl groups are competitive. The significance of these observations is discussed.

An enzyme from Streptococcus mutans forms branches on dextran in the absence of sucrose

An enzyme in glucosyltransferase preparations from Streptococcus mutans catalyzed the transfer of [14C]glucopyranoside from purified isomaltosaccharides, each containing [14C]glucopyranoside at its non-reducing terminus, to acceptor dextran, in the absence of sucrose. Half of the radioactivity present in the resulting [14C]dextrans was resistant to hydrolysis by amylo-1,6-glucosidase. Treatment of the [14C]dextrans with endodextranase resulted in extensive hydrolysis and produced [14C]-labeled limit oligosaccharides containing branch sites. Acetolysis of the [14C]-labeled limit oligosaccharides yielded [14C]nigerose, thus indicating the formation of branch sites on dextran in the absence of sucrose. The enzyme catalyzing this reaction has not been identified but appears to be independent of the major extracellular glucosyltransferases of S. mutans.

Inhibition by acarbose, nojirimycin and 1-deoxynojirimycin of glucosyltransferase produced by oral streptococci

Acarbose is known to inhibit glucoamylase, maltase and sucrase. Our aim was to test whether it would also inhibit glucosyltransferase (GTF), to determine the type of inhibition and to compare the inhibitor potency of acarbose with that of nojirimycin and deoxynojirimycin, two other glucosidase inhibitors. Enzyme inhibition was measured either by chemical assay or by incorporation of radioactivity into product. Acarbose effectively inhibited the synthesis of polysaccharide by GTF from strains of Streptococcus mutans and Streptococcus sanguis, but not by fructosyltransferase from Streptococcus salivarius. Acarbose and 1-deoxynojirimycin were more potent inhibitors of GTF than maltose, nojirimycin or various amino sugars. The mechanism of action of these compounds is consistent with competitive inhibition.

The mechanism of acceptor reactions of Leuconostoc mesenteroides B-512F dextransucrase

Reactions of dextransucrase and sucrose in the presence of sugars (acceptors) of low molecular weight have been observed to give a dextran of low molecular weight and a series of oligosaccharides. The acceptor reaction of dextransucrase was examined in the absence and presence of sucrose by using D-[14C]glucose, D-[14C]fructose, and 14C-reducing-end labeled maltose as acceptors. A purified dextransucrase was preincubated with sucrose, and the resulting D-fructose and unreacted sucrose were removed from the enzyme by chromatography of columns of Bio-Gel P-6. The enzyme, which migrated at the void volume was collected and referred to as "charged enzyme". The charged enzyme was incubated with 14C-acceptor in the absence of sucrose. Each of the three acceptors gave two fractions of labeled products, a high molecular weight product, identified as dextran, and a product of low molecular weight that was an oligosaccharide. It was found that all three of the acceptors were incorporated into the products at the reducing end. Similar results were obtained when the reactions were performed in the presence of sucrose, but higher yields of labeled products were obtained and a series of homologous oligosaccharides was produced when D-glucose or maltose was the acceptor. We propose that the acceptor reaction proceeds by nucleophilic displacement of glucosyl and dextranosyl groups from a covalent enzyme-complex by a specific, acceptor hydroxyl group, and that this reaction effects a glycosidic linkage between the D-glucosyl and dextranosyl groups and the acceptor. We conclude that the acceptor reactions serve to terminate polymerization of dextran by displacing the growing dextran chain from the active site of the enzyme; the acceptors, thus, do not initiate dextran polymerization by acting as primers.

Metabolism of the polysaccharides of human dental plaque. Part II. Purification and properties of Cladosporium resinae (1 leads to 3)-alpha-D-glucanase, and the enzymic hydrolysis of glucans synthesised by extracellular D-glucosyltransferases of oral streptococci

Cladosporium resinae (1 leads to 3)-alpha-D-glucanase has been characterized as an endoglucanase capable of completely hydrolysing insoluble (1 leads to 3)-alpha-D-glucans isolated from fungal cell-walls. D-Glucose was the major product, but a small amount of nigerose was also produced. The enzyme was specific for the hydrolysis of (1 leads to 3) bonds that occur in sequence, and nigerotetraose was the smallest substrate that was rapidly attacked. Isolated (1 leads to 3)-alpha-D-glucosidic linkages that occur in mycodextran, isolichein, dextrans, and oligosaccharides derived from dextran were not hydrolysed. Insoluble glucan synthesised from sucrose by culture filtrates of Streptococcus spp. were all hydrolysed to various limits; the range was 11-61%. A soluble glucan, synthesised by an extracellular D-glucosyltransferase of S. mutans OMZ176, was not a substrate, whereas insoluble glucans synthesised by a different D-glucosyltransferase, isolated from S. mutans strains OMZ176 and K1-R, were extensively hydrolysed (84 and 92%, respectively). It is suggested that dextranase-CB, a bacterial endo(1 leads to 6)-alpha-D-glucanase that does not release D-glucose from any substrate, could be used together with C. resinae (1 leads to 3)-alpha-D-glucanase to determine the relative proportions of (1 leads to 6)-linked to (1 leads to 3)-linked sequences of D-glucose residues in the insoluble glucans produce by oral streptococci. The simultaneous action of the two D-glucanoses was highly effective in solubilizing the glucans.

Streptoccus mutans dextransucrase: purification, properties, and requirement for primer dextran

We attempted to purify dextransucrase from S mutans strain 6715 to investigate its properties and determine if multiple species of the enzyme existed. It was concluded that the properties of this enzyme such as the pH (5.5), temperature (37 C) optimum, and Km for sucrose (3 mM) are very similar to those reported for S sanguis, S bovis, S mutans strain OMZ-176 isozymes, S mutans strain GS-5, and the single dextransucrase purified from S mutans strain HS-6. The IEF enzyme preparation consisted of two enzyme species, possibly differing in their ability to synthesize different dextran linkages. The minor enzyme activity demonstrated a strict primer dependency. Similarly, primer dependency has been reported for dextransucrases from S mutans, S sanguis, and L mesenteroides. S mutans strain 6715 dextransucrase also showed both the insertion and stepwise mechanisms for dextran synthesis. Sucrose was the sole glucose donor, whereas dextran was a specific, highly efficient glucose acceptor. The complex primer kinetics are not fully understood at this time and require further investigation. Without linkage analysis of the products of our enzymes, we can only postulate that each enzyme has a different function in the synthesis of interresidue and interchain alpha1-3 and alpha1-6 bonds. Insoluble dextran synthesis may involve a special enzyme mechanism characteristic of S mutans. This synthesis would require both enzymes, possibly in some aggregated form, with one enzyme synthesizing endogenous primer dextran. This endogeneous primer or some cell wall polysaccharide could stimulate both enzymes to rapidly synthesize heterogeneously linked insoluble dextran.

Streptococcus mutans dextransucrase: mode of interaction with high-molecular-weight dextran and role in cellular aggregation

The interaction between Streptococcus mutans dextransucrase (EC 2.4.1.5) and high-molecular-weight dextran was studied in both the presence and absence of substrate sucrose. Equivalent weight-percent solutions of primer dextrans that differed 200-fold in molecular weight were found to be equally efficient in priming new dextran synthesis. Sodium borohydride reduction of dextran had no effect on its priming ability. These results suggest that dextran synthesis proceeds by addition of glucosyl residues to nonreducing termini of primer dextrans and that several enzyme molecules simultaneously bind to single high-molecular-weight dextran molecules. Kinetic data suggested that dextransucrase contains only one dextran binding site per enzyme molecule. The nature of the commonly observed highly aggregated state of dextransucrase was also studied. Two types of enzyme aggregates were distinguished: (i) oligomeric enzyme aggregates that formed in the absence of dextran and were dissociated by 1 M KCl; and (ii) dextran-induced enzyme aggregates that were stable to 3 M salt. Oligomeric enzyme aggregates were obtained from supernatants of fructose-grown cultures, whereas dextran-induced enzyme aggregates appeared to be present in glucose-grown cultures. The molecular weight of the smallest species of dextran-free detransucrase observed in solutions of 1 M KCl was estimated to be 40,000 by gel column chromatography. Addition of dextran to primer-dependent dextransucrase resulted in formation of complexes that were stable in CsCl density gradients and exhibited a buoyant density of 1.382 g/cm3 as compared with a buoyant density of 1.302 g/cm3 exhibited by dextransucrase. The enzyme-dextran complexes observed in CsCl density gradients contained about 25% dextran. This corresponded to 150 enzyme molecules (molecular weight, 40,000) per dextran molecule (molecular weight, 2 X 10(6)). The implication of these results to the mechanism of sucrose- and dextran-induced aggregation of S. mutans is discussed.

The action pattern of Penicillium lilacinum dextranase

The product distributions resulting from the action of Penicillium lilacinum dextranase on end-labelled oligosaccharides of the isomaltose series have been determined. The initial rates of formation of labelled products were measured for isomaltotriose up to isomalto-octaose, and the molar proportions and radioactivity of the final products from isomaltotriose up to isomaltohexaose were determined. D-Glucose was released only from isomaltotriose and isomaltotetraose, by hydrolysis of the first linkage from the reducing end (linkage 1); the terminal bonds of higher members of the series were not attacked. All oligosaccharides except isomaltotriose were hydrolyzed at more than one linkage. The main points of attack on isomaltotetraose up to isomalto-octaose were at linkage 2, and at the third linkage from the non-reducing end; these two positions coincide for isomaltopentaose. The degradation of isomaltotriose up to isomalto-octaose was entirely hydrolytic. The enzyme also catalyzed an extremely slow, concentration-dependent degradation of isomaltose, and this may have occurred via a condensation to isomaltotetraose, followed by hydrolysis of linkage 1 to give D-glucose and isomaltotriose.