Purification, and comparison, of two forms of dextransucrase from Streptococcus sanguis

Recent Progress of Basic Studies of Natural Products and Their Dental Application

The present article reviews the research progress of three major polyphenols (tannins, flavonoids and lignin carbohydrate complexes), chromone (backbone structure of flavonoids) and herbal extracts. Chemical modified chromone derivatives showed highly specific toxicity against human oral squamous cell carcinoma cell lines, with much lower toxicity against human oral keratinocytes, as compared with various anticancer drugs. QSAR analysis suggests the possible correlation between their tumor-specificity and three-dimensional molecular shape. Condensed tannins in the tea extracts inactivated the glucosyltransferase enzymes, involved in the biofilm formation. Lignin-carbohydrate complexes (prepared by alkaline extraction and acid-precipitation) and crude alkaline extract of the leaves of Sasa species (SE, available as an over-the-counter drug) showed much higher anti-HIV activity, than tannins, flavonoids and Japanese traditional medicine (Kampo). Long-term treatment with SE and several Kampo medicines showed an anti-inflammatory and anti-oxidant effects in small size of clinical trials. Although the anti-periodontitis activity of synthetic angiotensin II blockers has been suggested in many papers, natural angiotensin II blockers has not yet been tested for their possible anti-periodontitis activity. There should be still many unknown substances that are useful for treating the oral diseases in the natural kingdom.

Characterization of the Different Dextransucrase Activities Excreted in Glucose, Fructose, or Sucrose Medium by Leuconostoc mesenteroides NRRL B-1299

When grown in glucose or fructose medium in the absence of sucrose, Leuconostoc mesenteroides NRRL B-1299 produces two distinct extracellular dextransucrases named glucose glucosyltransferase (GGT) and fructose glucosyltransferase (FGT). The production level of GGT and FGT is 10 to 20 times lower than that of the extracellular dextransucrase sucrose glucosyltransferase (SGT) produced on sucrose medium (traditional culture conditions). GGT and FGT were concentrated by ultrafiltration before sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis. Their molecular masses were 183 and 186 kDa, respectively, differing from the 195 kDa of SGT. The structural analysis of the dextran produced from sucrose and of the oligosaccharides synthesized by acceptor reaction in the presence of maltose showed that GGT and FGT are two different enzymes not previously described for this strain. The polymer synthesized by GGT contains 30% alpha(1-->2) linkages, while FGT catalyzes the synthesis of a linear dextran only composed of alpha(1-->6) linkages.

Identification of a single and non-essential cysteine residue in dextransucrase of Leuconostoc mesenteroides NRRL B-512F

Amino acid analysis of purified dextransucrase (sucrose: 1,6-alpha-D-glucan 6-alpha-D-glucosyltransferase EC 2.4.1.5) from Leuconostoc mesenteroides NRRL B-512F was carried out. The enzyme is virtually devoid of cysteine residue there being only one cysteine residue in the whole enzyme molecule comprising over 1500 amino acid residues. The enzyme is rich in acidic amino acid residues. The number of amino acid residues was calculated based on the molecular weight of 188,000 (Goyal and Katiyar 1994). Amino sugars were not found, implying that the enzyme is not a glycoprotein. It has been shown earlier that the cysteine residue in dextransucrase is not essential for enzyme activity (Goyal and Katiyar 1998). The presence of only one cysteine residue per enzyme molecule illustrates that its tertiary structure is solely dependent on other types of non-covalent interactions such as hydrogen bonding, ionic and nonpolar hydrophobic interactions.

Streptococcus mutans glucan-binding protein-A affects Streptococcus gordonii biofilm architecture

The glucan-binding protein-A (GbpA) of Streptococcus mutans has been shown to contribute to the architecture of glucan-dependent biofilms formed by this species and influence virulence in a rat model. As S. mutans synthesizes multiple glucosyltransferases and nonglucosyltransferase glucan-binding proteins (GBPs), it is possible that there is functional redundancy that overshadows the full extent of GbpA contributions to S. mutans biology. Glucan-associated properties such as adhesion, aggregation, and biofilm formation were examined independently of other S. mutans GBPs by cloning the gbpA gene into a heterologous host, Streptococcus gordonii, and derivatives with altered or diminished glucosyltransferase activity. The presence of GbpA did not alter dextran-dependent aggregation nor the initial sucrose-dependent adhesion of S. gordonii. However, expression of GbpA altered the biofilm formed by wild-type S. gordonii as well as the biofilm formed by strain CH107 that produced primarily alpha-1,6-linked glucan. Expression of gbpA did not alter the biofilm formed by strain DS512, which produced significantly lower quantities of parental glucan. These data are consistent with a role for GbpA in facilitating the development of biofilms that harbor taller microcolonies via binding to alpha-1,6-linkages within glucan. The magnitude of the GbpA effect appears to be dependent on the quantity and linkage of available glucan.

Proteolytic modification of Leuconostoc mesenteroides B-512F dextransucrase

Multiple active lower molecular weight forms from Leuconostoc mesenteroides B512F dextransucrase have been reported. It has been suggested that they arise from proteolytic processing of a 170 kDa precursor. In this work, the simultaneous production of proteases and dextransucrase was studied in order to elucidate the dextransucrase proteolytic processing. The effect of the nitrogen source on protease and dextransucrase production was studied. Protease activity reaches a maximum early in the logarithmic phase of dextransucrase synthesis using the basal culture medium but the nitrogen source plays an important effect on growth: the highest protease concentration was obtained when ammonium sulfate, casaminoacids or tryptone were used. Two active forms of 155 and 129 kDa were systematically obtained from dextransucrase precursor by proteolysis. The amino termini of these forms were sequenced and the cleavage site deduced. Both forms of the enzyme obtained had the same cleavage site in the amino terminal region (F209-Y210). From dextransucrase analysis, various putative cleavage sites with the same sequence were found in the variable region and in the glucan binding domain. Although no structural differences were found in dextrans synthesized with both the precursor and the proteolyzed 155 kDa form under the same reaction conditions, their rheological behaviour was modified, with dextran of a lower viscosity yielded by the smaller form.

Invasion and killing of human endothelial cells by viridans group streptococci

Colonization of the cardiovascular endothelium by viridans group streptococci can result in infective endocarditis and possibly atherosclerosis; however, the mechanisms of pathogenesis are poorly understood. We investigated the ability of selected oral streptococci to infect monolayers of human umbilical vein endothelial cells (HUVEC) in 50% human plasma and to produce cytotoxicity. Planktonic Streptococcus gordonii CH1 killed HUVEC over a 5-h period by peroxidogenesis (alpha-hemolysin) and by acidogenesis but not by production of protein exotoxins. HUVEC were protected fully by addition of supplemental buffers and bovine liver catalase to the culture medium. Streptococci were also found to invade HUVEC by an endocytic mechanism that was dependent on polymerization of actin microfilaments and on a functional cytoskeleton, as indicated by inhibition with cytochalasin D and nocodazole. Electron microscopy revealed streptococci attached to HUVEC surfaces via numerous fibrillar structures and bacteria in membrane-encased cytoplasmic vacuoles. Following invasion by S. gordonii CH1, HUVEC monolayers showed 63% cell lysis over 4 h, releasing 64% of the total intracellular bacteria into the culture medium; however, the bacteria did not multiply during this time. The ability to invade HUVEC was exhibited by selected strains of S. gordonii, S. sanguis, S. mutans, S. mitis, and S. oralis but only weakly by S. salivarius. Comparison of isogenic pairs of S. gordonii revealed a requirement for several surface proteins for maximum host cell invasion: glucosyltransferase, the sialic acid-binding protein Hsa, and the hydrophobicity/coaggregation proteins CshA and CshB. Deletion of genes for the antigen I/II adhesins, SspA and SspB, did not affect invasion. We hypothesize that peroxidogenesis and invasion of the cardiovascular endothelium by viridans group streptococci are integral events in the pathogenesis of infective endocarditis and atherosclerosis.

Deletions in the carboxyl-terminal region of Streptococcus gordonii glucosyltransferase affect cell-associated enzyme activity and sucrose-associated accumulation of growing cells

The single glucosyltransferase (GTF) of Streptococcus gordonii Challis CH1 makes alpha 1,3- and alpha 1,6-linked glucans from sucrose. The GTF carboxyl-terminal region has six direct repeats thought to be involved in glucan binding. Strains with defined mutations in this region have been described recently (M. M. Vickerman, M. C. Sulavik, P. E. Minick, and D. B. Clewell, Infect. Immun. 64:5117-5128, 1996). Strain CH107 GTF has three internal direct repeats deleted; the 59 carboxyl-terminal amino acids are identical to those of the parental strain. This deletion resulted in decreased enzyme activity but did not affect the amount of cell-associated GTF protein. The GTFs of strains CH2RPE and CH4RPE have six and eight direct repeats, respectively, but are both missing the 14 carboxyl-terminal amino acids. Strain CH2RPE had significantly decreased levels of cell-associated GTF; this decrease was not obviated by the increased number of direct repeats in strain CH4RPE. Thus, the carboxyl-terminal amino acids appeared to influence the amount of cell-associated GTF more than the direct repeats. The qualitative and quantitative differences in the GTFs did not affect the abilities of these strains to accumulate on hydroxyapatite beads in the absence of sucrose. However, when sucrose was added as a substrate for GTF, the mutant strains were unable to accumulate on these surfaces to the same extent as the parent. These differences in sucrose-associated accumulation may be due to changes in the nature of the glucans produced by the different enzymes and/or cohesive interactions between these glucans and the GTF on the surfaces of the growing streptococci.

Molecular analysis of representative Streptococcus gordonii Spp phase variants reveals no differences in the glucosyltransferase structural gene, gtfG

Streptococcus gordonii glucosyltransferase polymerizes sucrose to form glucans, which confer a hard, sucrose-promoted phenotype (Spp+) to colonies on sucrose agar plates. The glucosyltransferase structural gene, gtfG, is positively regulated by the upstream determinant, rgg. Strain Challis undergoes a spontaneous, reversible phase variation between high (Spp+) and low (Spp-) levels of glucosyltransferase activity. Representative strains were examined to gain insights into the basis of glucosyltransferase phase variation. Western blots indicated that the level of glucosyltransferase activity was related to the amount of extracellular glucosyltransferase protein produced by Spp- and Spp+ strains. The nucleotide sequence of rgg and gtfG of the Spp- strain CH97 was found to be identical to that of the Spp+ parent, indicating that DNA differences in these regions are not the basis for glucosyltransferase phase variation. Indeed, 13C-NMR spectroscopy suggested that glucans synthesized by strain CH97 glucosyltransferase were similar to those synthesized by glucosyltransferase of the Spp+ parental strain, indicating a quantitative rather than qualitative change. However, one Spp- strain, CH1C1, had a point mutation in rgg; replacement of the parent rgg with the CH1C1 allele resulted in decreased levels of glucosyltransferase protein and activity. The results indicate that glucosyltransferase phase variation can occur in more than one way, and suggest that glucosyltransferase regulation may involve distally located regulatory gene(s) that affect rgg and/or gtfG expression.

Nucleotide sequence analysis of the Streptococcus gordonii glucosyltransferase gene, gtfG

Streptococcus gordonii has an extracellular glucosyltransferase (GTF) that polymerizes the glucose moiety of sucrose to form both water-soluble and water-insoluble glucans. Whereas multiple gtf genes have been identified in strains of mutans streptococci and Streptococcus salivarius, a single gene, designated gtfG, encodes the GTF of S. gordonii Challis. gtfG is also unique among the characterized gtfs in that it has a described regulatory determinant, rgg. Furthermore, the GTF activity in S. gordonii undergoes reversible phase variation between high and low levels. In order to gain insight into this novel GTF system, the nucleotide sequence of gtfG was determined and found to consist of a 4,734 base pair open reading frame encoding a protein with a deduced molecular weight of ca. 174,000. gtfG was similar to other sequenced gtfs with a conserved signal sequence followed by a ca. 600-bp region distinctive for gtfG, a conserved region encoding a putative catalytic active site and a series of six direct repeats in the carboxyl terminal region implicated in glucan binding. Although comparison of gtfG to other gtfs did not show a basis for the primer-independence of the encoded enzyme or the nature of the glucan products, the gtfG sequence data provide an important basis for further studies of these enzymes.

Changes in the carboxyl-terminal repeat region affect extracellular activity and glucan products of Streptococcus gordonii glucosyltransferase

Glucans produced by the glucosyltransferase (GTF) of Streptococcus gordonii confer a hard, cohesive phenotype (Spp+) on colonies grown on sucrose agar plates. S. gordonii strains with specific mutations in the region of gtfG that encodes the GTF carboxyl terminus were characterized. In the parental strain Challis CH1, this region included a series of six direct repeats thought to function in glucan binding. The spontaneous mutant strain CH107 had a 585-bp deletion resulting in the loss of three internal direct repeats. Insertional mutagenesis was used to construct strain CH2RPE, which had the parental repeat region but was missing 14 carboxyl-terminal amino acids. The similarly constructed strain CH4RPE had an in-frame addition of 390 nucleotides encoding two additional direct repeats. Although strains CH1, CH2RPE, and CH4RPE all had similar levels of extracellular GTF activity, strain CH107 had less than 15% of the parental activity; however, Western blots (immunoblots) indicated that the amounts of extracellular GTF protein in all four strains were similar. 13C NMR analyses indicated that partially purified GTFs from the Spp+ strains CH1, CH2RPE, and CH4RPE all produced glucans with similar ratios of alpha1,6 and alpha1,3 glucosidic linkages, whereas the Spp- strain CH107 GTF produced primarily alpha1,6-linked glucans. Transformation of strain CH107 with pAMS57, which carries the gtfG positive regulatory determinant, rgg, increased the amount of GTF activity and GTF antibody-reactive protein ca. fivefold but did not confer a hard colony phenotype on sucrose agar plates, suggesting that the type of glucan product affects the sucrose-promoted colony phenotype.

Purification and properties of extracellular glucosyltransferase from Streptococcus bovis

Eight Streptococcus bovis strains were classified into 3 types on the basis of isoelectric point (pI) and molecular mass (M(r)) of extracellular glucosyltransferase. Strains ATCC 9809, 35034 and 43143 produced glucosyltransferase of pI 3.7 and M(r) 165 kDa; strains ATCC 15351, 27960 and 33317 produced glucosyltransferase of pI 4.1 and M(r) 140 kDa; strains ATCC 43085 and 43144 did not produce any glucosyltransferase. The glucosyltransferase form S. bovis 9809 was purified by Bio-Gel hydroxyapatite chromatography and DEAE-Toyopearl chromatography. The S. bovis 9809 glucosyltransferase was immunologically identical with the other 5 S. bovis glucosyltransferases and not related to mutants streptococcal glucosyltransferases. The specific activity, the optimum pH and the Km value for sucrose were 17.9 U/mg protein, 6.0 and 5.0 mM, respectively. The first 11 N-terminal amino acid residues of the glucosyltransferases were DETSAVTLTRE, and the region was hydrophilic. The glucosyltransferases from S. bovis 9809 and 3317 synthesized from sucrose 1, 6-alpha-D-glucan with 9 and 2 mol%, 1, 3, 6-alpha-branched glucose, respectively.

Glucosyltransferase mediates adhesion of Streptococcus gordonii to human endothelial cells in vitro

Human umbilical vein endothelial cells (HUVEC) were used as an experimental host model to investigate the mechanism(s) of streptococcal adhesion in infective endocarditis. Adhesion activity of Streptococcus gordonii was maximal during the logarithmic phase of growth and was greatly reduced or eliminated by pretreatment of bacteria with heat, formaldehyde, or trypsin. At saturating numbers of streptococci, an average of 81 bacteria were bound per HUVEC. Streptococcal adhesion was inhibited by low-molecular-weight dextran and heparin but not by sucrose, fibronectin, or laminin. Adhesion was also prevented by pretreatment of HUVEC with proteins dissociated from the surface of S. gordonii with 10 mM EDTA or isolated from spent culture medium. Western blot (immunoblot) assays detected a single adhesion protein of 153 kDa (AP153) on HUVEC after incubation with unfractionated extracts of streptococci. The adhesin exhibited glucosyltransferase (GTF) activity when incubated with sucrose and Triton X-100 after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The AP153 was purified by affinity chromatography on dextran beads and show to have binding activity for HUVEC, GTF activity, an amino acid composition similar to that reported for GTF of S. gordonii, and the ability to inhibit S. gordonii adhesion. Incubation of the streptococci with antibodies to the adhesin inhibited bacterial attachment to HUVEC monolayers. These results indicate that surface-localized GTF mediates adhesion of S. gordonii to HUVEC in vitro and may serve as a mechanism for colonization of the endocardium in infective endocarditis.

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).

Glucosyltransferase production by Streptococcus sanguis Challis and comparison with other oral streptococci

Glucosyltransferase (GTF) activity in batch cultures of Streptococcus sanguis strain Challis in defined medium was maximum at peak growth and declined rapidly on further incubation into stationary phase. Activity was present in spent culture medium and was associated with 2 polypeptides of approximate Mrs 170 kDa (90% of activity) and 155 kDa (about 10% of activity). Other S. sanguis strains produced similar antigenically-related polypeptides but with varying activities, and antibodies to the S. sanguis enzyme did not react with enzymes produced by other species of oral streptococci. Increasing the [Na+] reduced GTF production by S. sanguis strains Challis and NCTC 7865, and by Streptococcus sobrinus, but not by Streptococcus salivarius. Addition of Tween 80 to cultures caused a rapid inhibition of GTF synthesis by strain Challis, but not by strain NCTC 7865. Benzyl alcohol had a slower and less inhibitory effect on GTF production by strain Challis. Tween 80 generally inhibited to varying degrees GTF production by other oral streptococci through in S. sobrinus it was stimulated. The results suggest controls operating on GTF synthesis and secretion by different strains of oral streptococci appear to be diverse.

Spontaneous switching of the sucrose-promoted colony phenotype in Streptococcus sanguis

Streptococcus sanguis on media containing 3% sucrose gives rise to characteristic hard cohesive colonies (designated Spp+). Populations of Spp+ bacteria (strain Challis) on sucrose media switch to a soft noncohesive phenotype (designated Spp-) at a frequency of 10(-4) to 10(-3). Spp- bacteria switch back to Spp+ bacteria at a similar frequency. Successive rounds of Spp variation were observed. The Spp phenotypic switch was associated with changes in extracellular glucosyltransferase activity.

Purification and characterization of invertase from a novel industrial yeast, Schwanniomyces occidentalis

The use of yeast as an expression system for heterologous proteins offers several potential advantages with respect to industrial scale-up and genetics over other expression systems, but suffers from several drawbacks. For example, the secreted proteins of S. cerevisiae, found in the periplasm, are hyperglycosylated and the organism has a limited range of usable substrates. Other yeasts have similar disadvantages in addition to producing a variety of proteases. We have investigated the use of Schwanniomyces occidentalis as a host for developing a gene expression system in which these and several disadvantages are minimized. The present paper describes the isolation and characterization of an invertase from cell free supernatants of the yeast Schwanniomyces occidentalis grown on lactose. The enzyme is a beta-D-fructofuranoside-fructohydrolyase, composed of two identical subunits of 76,000 to 78,000 da. with a native molecular mass of 125,000 +/- 25,000 da. of which approximately 17% can be attributed to N-linked carbohydrate. The enzyme has a Vmax of 0.49 +/- 0.025 units, a Km of 21 +/- 1.5 mM, and temperature and pH optima of 55 degrees C and 3.9-4.5, respectively. The amino acid sequences of the amino terminal region and an internal tryptic peptide support an 81% identity with the invertase from Saccharomyces cerevisiae. The enzyme is induced by low glucose and is catabolite repressed.

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.

Inhibition of dextransucrase by Zn2+, Ni2+, Co2+, and Tris(hydroxymethyl)aminomethane (Tris)

Initial rate kinetics of polysaccharide formation indicate that Zn2+, Ni2+, and Co2+ inhibit dextransucrase [sucrose: 1,6-alpha-D-glucan 6-alpha-D-glucosyltransferase, EC 2.4.1.5] by binding to two types of metal ion sites. One type consists of a single site and has a low apparent affinity for Ca2+. At the remaining site(s), Ca2+ has a much higher apparent affinity than Zn2+, Ni2+, or Co2+, and prevents inhibition by these metal ions. These findings are consistent with a two-site model previously proposed from studies with Ca2+ and EDTA. Initial rate kinetics also show that Tris is competitive with sucrose, but that, unlike Zn2+, Tris does not bind with significant affinity to a second site. This argues that there is a site which is both the sucrose binding site and a general cation site.

Interaction of deoxyhalosucrose derivatives with dextransucrase

Members of a series of deoxyhalosucrose analogs substituted at one, two, or three primary carbon atoms with bromine or chlorine were prepared. Dextransucrase isolated from Streptococcus sanguis was separately treated with 6-bromo-6-deoxysucrose, 6,6'-dibromo-6,6'-dideoxysucrose, 6,1',6'-tribromotrideoxysucrose, and 6,6'-dichlorodideoxysucrose, in order to determine if they were inactivators. Variation in time of exposure, and in the concentration of the sucrose analogs, did not yield significant irreversible inactivation. In supplementary studies, it was found that the compounds serve as weak, reversible inhibitors.