[ON DATA ON SWEET ALMOND EMULSIN]

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.

beta-Glucosidase activity in fetal bovine serum renders the plant glucoside, hypoxoside, cytotoxic toward B16-F10-BL-6 mouse melanoma cells

By using p-nitrophenyl-beta-D-glucopyranoside as substrate, beta-glucosidase activity was observed in fetal bovine serum (FBS). This activity could be inhibited by heat inactivation of the serum. Gel chromatography of FBS indicated the presence of beta-glucosidase activity with an apparent molecular mass of 29 kDa. In McCoy's 5A medium supplemented with non-heat inactivated FBS, the diglucoside hypoxoside ([E]-1,5-bis[4'beta-D-glucopyranosyloxy-3'-hydroxyphenyl]pent-4-en - 1-yne) showed cytotoxicity toward B16-F10-BL-6 mouse melanoma cells. In incubations where the media were supplemented with FBS previously heat inactivated at 56 degrees C for 1 h or more, no cytotoxicity was observed in the presence of hypoxoside. The aglucone of hypoxoside, rooperol ([E]-1,5-bis[3',4'-dihydroxyphenyl]pent-4-en-1-yne), showed cytotoxicity regardless of whether the serum was heat inactivated or not. The kinetics of the heat inactivation of the beta-glucosidase activity in FBS coincided with the loss of apparent cytotoxicity of hypoxoside. High performance liquid chromatography analysis showed that rooperol could be generated by incubation of hypoxoside in non-heat inactivated FBS, but that this ability was lost in serum that was heat inactivated for 1 h or longer. Newborn bovine serum did not contain any beta-glucosidase activity whereas it was found in three different commercial sources of FBS. This observation is of practical importance because conventional heat inactivation of FBS at 56 degrees C for 30 min was not sufficient to inactivate the beta-glucosidase activity completely.

Comparison of kinetic and molecular properties of two forms of amygdalin hydrolase from black cherry (Prunus serotina Ehrh.) seeds

Two forms of the beta-glucosidase amygdalin hydrolase (AH I and II), which catalyze the hydrolysis of (R)-amygdalin to (R)-prunasin and D-glucose, have been purified over 200-fold from mature black cherry (Prunus serotina Ehrh.) seeds. These proteins showed very similar molecular and kinetic properties but could be resolved by chromatofocusing and isoelectric focusing. AH I and II were monomeric (Mr 60,000) and had isoelectric points of 6.6 and 6.5, respectively. Their glycoprotein character was indicated by positive periodic acid-Schiff staining and by their binding to concanavalin A-Sepharose 4B with subsequent elution by alpha-Me-D-glucoside. Of the natural glycosidic substrates tested, both enzymes showed a pronounced preference for the endogenous cyanogenic disaccharide (R)-amygdalin. They also hydrolyzed at the same active site the synthetic substrates p-nitrophenyl-beta-D-glucoside and 4-methylumbelliferyl-beta-D-glucoside but were inactive towards (R)-prunasin, p-nitrophenyl-alpha-D-glucoside, and 4-methylumbelliferyl-alpha-D-glucoside. Maximum hydrolytic activity was shown in citrate-phosphate buffer in the pH range 4.5-5.0. AH I and II were inhibited competitively by the reaction product (R)-prunasin and noncompetitively (mixed type) by delta-gluconolactone and castanospermine.

Inhibition of beta-glucosidases from almonds by cationic and neutral beta-glucosyl derivatives

The beta-glucosidase (beta-D-glucoside glucohydrolase, EC 3.2.1.21) isoenzymes B from sweet and bitter almonds showed considerable differences in their kinetic and inhibition parameters, but both were inhibited much more strongly by basic beta-glucosyl derivatives than by their neutral analogs. The additional interaction energy apparently due to the basic character ranged from 18 kJ/mol (4.3 kcal/mol) for beta-glucosylamine compared to beta-glucose to 28 kJ/mol (6.9 kcal/mol) for N-benzyl-beta-glucosylamine compared to N-beta-glucosyl-p-toluidine. N-beta-Glucosylpyridinium ion and N-beta-glucosylimidazol which both cannot be protonated at the glucosylated nitrogen are very weak inhibitors. beta-2-Amino-2-deoxyglucose is bound with half the affinity of beta-glucosylamine. The structural requirement for strong inhibition is thus the protonation of the inhibitor at the glucosylated nitrogen. The additional binding energy is assumed to be due to the electrostatic interaction of the inhibitor cation with a carboxylate group in an environment of low polarity. The failure of the pyridinium ion to show this interaction is attributed to the presence of a positively charged group at the active site which acts as proton donor. The pKa values of beta-glucosylamine and its derivatives have been determined and found to be 3.5 units lower than those of the corresponding parent amines. An exception is beta-glucosylimidazol (pKa 5.4) which is protonated on the non-glycosylated nitrogen.

Studies on almond emulsin beta-D-glucosidase. I. Isolation and characterization of a bifunctional isozyme

A beta-D-glucosidase (beta-D-glucoside glucohydrolase, EC 3.2.1.21) isozyme has been isolated from almond emulsin. The isolated enzyme is a glycoprotein and migrates as a single band on Sephadex G-200 filtration, CM 52 ion exchange chromatography, polyacrylamide gel electrophoresis, sodium dodecyl sulfate polyacrylamide gel electrophoresis and isoelectric focussing. The glucosidase and galactosidase activities traverse together during Sephadex G-200 gel filtration. Polyacrylamide gels stained specifically for the 2 enzymes reveal that the two activities comigrate. The molecular weight of the isozyme has been found to be 135 180 +/- 770, and that of its protomers to be 65 150 +/- 650.

Studies on almond emulsin beta-D-glucosidase. II. Kinetic evidence for independent glucosidase and galactosidase sites

A purified beta-D-glucosidase (beta-D-glucoside glucohydrolase, EC 3.2.1.21) isozyme isolated from almond emulsin was found to catalyze hydrolysis of beta-D-glucopyranosides and beta-D-galactopyranosides but not the corresponding alpha-D-derivatives. Hydrolysis of the corresponding beta-D-thioglycopyranosides at rates 10(3)--10(4) times lower than those for the hydrolysis of the beta-D-glycopyranosides was also noted. The enzyme does not exhibit any transferolytic activity using D-glucose or D-galactose as acceptors. D-glucose, p-nitrothiophenyl-beta-D-glucopyranoside, 5-deoxy-5-thio-D-glucose and D-glucono-delta-lactone are shown to exert mainly competitive inhibition on beta-D-galactopyranoside hydrolysis. D-galactose, p-nitrothiophenyl-beta-D-galactopyranside and methylthio-beta-D-galactopyranoside are shown to inhibit the glucopyranoside hydrolysis mainly non-competitively and to exert competitive inhibition of galactopyranoside hydrolysis. The inhibition caused by the antibiotic Nojirimycin (5-amino-5-deoxy-D-glucose) is shown to be more complex. Analysis of the kinetic data indicates that the catalytic site of the enzyme responsible for the beta-D-glucosidase activity is kinetically distinct from the beta-D-galactosidase site.

Specfic irreversible inhibition of sweet-almond beta-glucosidase by some beta-glycopyranosylepoxyalkanes and beta-d-glucopyranosyl isothiocyanate

beta-D-Glucopyranosyl-(1S and 1R)-epoxyethanes (I and II), 1-(beta-D-glucopyranosyl)-(2R and 2S)-2,3-epoxypropanes (III and IV), beta-D-glucopyranosyl isothiocyanate (V) and beta-D-galactopyranosylepoxyethane (VI) are active-site-directed irreversible inhibitors of sweet-almond beta-glucosidase B (beta-D-Glucoside glucohydrolase, EC 3.2.1.21). Formation of the covalent bond is preceded by the binding of these inhibitors in the active site of the enzyme. This is testitified by the competitive character of inhibition of beta-glucosidase component B by compounds I-VI at the early period and by the protection of the enzyme from inactivation by its competitive inhibitors D-glucose and 1,5-D-gluconolactone. Epoxides I-IV are bound covalently with componet B at a molar ratio 1 : 1 as shown with the aid of 14C-labelled inhibitors. The release of the label from modified enzyme (E-I covalent) by treatment with hydroxylamine suggests the formation of an ester bond between inhibitors I-IV and the carboxyl group of the enzyme active site. The pH dependence curve of the inactivation rate of beta-glucosidase B is of a bell-shaped form for V and of a sigmoid character for I-IV and points to the involvement of the active site groups with pKa 5.6-5.9 and 4.2-4.4.

Polyphenol-protein interactions

The association of some natural and synthetic polyphenols with beta-glucosidase was examined and some observations on the chemical nature of the complex were made.

Purification of beta-glucosidase from Saccharomyces lactis strain Y-123

Saccharomyces lactis strain Y-123, a constitutive high producer of beta-glucosidase (B(h)), was grown in an enriched medium. beta-Glucosidase, extracted most easily by cell autolysis, was purified by successive ammonium sulfate precipitation, ethyl alcohol precipitation, gel filtration, calcium phosphate gel adsorption-elution, and hydroxyapatite column chromatography. The specific activity of the enzyme increased 200-fold during the purification. The electrophoretic and catalytic properties of the enzyme did not change during the procedure. Polyacrylamide gel disc electrophoresis of the partially purified enzyme revealed one major and several minor protein-staining bands. beta-Glucosidase activity in the polyacrylamide gel columns was measured directly by assaying sections of columns frozen and sliced immediately after electrophoresis. Most of the activity coincided with the major protein-staining band. Prolonged assay produced minor activity coinciding with the less intense protein bands. Properties of the enzyme suggest that it is a single, unconjugated, intracellular, high molecular weight protein. The purification procedure is applicable to strains of S. lactis which possess alleles of the B locus for beta-glucosidase synthesis.

Inducible system for the utilization of beta-glucosides in Escherichia coli. II. Description of mutant types and genetic analysis

Two types of mutants obtained by treating beta-gl(+) cells with nitrosoguanidine are described. One type, beta-gl(+)c, is constitutive for the biosynthesis of the aryl beta-glucoside splitting enzyme(s) and for the beta-glucoside permease; the other (beta-gl(+)sal(-)) has lost the capacity to ferment salicin, but has retained the capacity to ferment arbutin and other aryl beta-glucosides. By two successive mutational steps, beta-gl(+)sal(-)c double mutants can be obtained. Determinations of the enzymatic splitting of salicin and p-nitrophenyl beta-glucoside by beta-gl(+)sal(-) cells and extracts showed that these mutants have lost the capacity to split salicin but do split p-nitrophenyl beta-glucoside; they possess the beta-glucoside permease, and in them salicin is a gratuitous inducer for enzyme and permease biosynthesis. Studies on a beta-gl(+) strain, which splits salicin as well as p-nitrophenyl beta-glucoside, have shown that the splitting of salicin is more temperature-sensitive than that of p-nitrophenyl beta-glucoside and other beta-glucosides. Other properties of the two activities are similar. Interrupted mating experiments and cotransduction with P1kc phage showed that the genetic determinants of the beta-glucoside system map between the pyrE and ile loci. Three distinct mutational sites were found and are presumed to have the following functions: beta-glA, a structural gene for an aryl beta-glucoside splitting enzyme; beta-glB, either the structural gene for the beta-glucoside-permease or a regulatory gene; and beta-glC, a regulatory gene (or site). Escherichia coli wild-type strains are of the genotype A(+) B(-) C(+). The beta-gl(+) mutation determining the ability to ferment beta-glucosides is considered to be a permease or regulatory mutation, and the resulting genotype is A(+) B(+) C(+). The beta-gl(+)sal(-) phenotype results from a mutation in the beta-glA gene (genotype A' B(+) C(+)), and the constitutive phenotype results from a mutation in the beta-glC gene, the genotypes A(+) B(+)C(a) and A' B(+)C(a) corresponding to the phenotypes beta-gl(+)c and beta-gl(+)sal(-)c.