Isolation and characterization of two cyanogenic beta-glucosidases from flax seeds

Rutinosidase and other diglycosidases: Rising stars in biotechnology

Diglycosidases are a special class of glycosidases (EC 3.2.1) that catalyze the separation of intact disaccharide moieties from the aglycone part. The main diglycosidase representatives comprise rutinosidases that cleave rutinose (α-l-Rha-(1-6)-β-d-Glc) from rutin or other rutinosides, and (iso)primeverosidases processing (iso)primeverosides (d-Xyl-(1-6)-β-d-Glc), but other activities are known. Notably, some diglycosidases may be ranked as monoglucosidases with enlarged substrate specificity. Diglycosidases are found in various microorganisms and plants. Diglycosidases are used in the food industry for aroma enhancement and flavor modification. Besides their hydrolytic activity, they also possess pronounced synthetic (transglycosylating) capabilities. Recently, they have been demonstrated to glycosylate various substrates in a high yield, including peculiar species like inorganic azide or carboxylic acids, which is a unique feature in biocatalysis. Rhamnose-containing compounds such as rutinose are currently receiving increased attention due to their proven activity in anti-cancer and dermatological experimental studies. This review demonstrates the vast and yet underrated biotechnological potential of diglycosidases from various sources (plant, microbial), and reveals perspectives on the use of these catalysts as well as of their products in biotechnology.

Use of qNMR for speciation of flaxseeds (Linum usitatissimum) and quantification of cyanogenic glycosides

This report describes a routine method taking less than 20 min to quantify cyanogenic glycosides such as linustatin and neolinustatin from flaxseeds (Linum usitatissimum L.) using ¹H nuclear magnetic resonance. After manual dehulling, a higher linustatin content was shown in the almond fraction, while neolinustatin and total cyanogenic glycoside contents were significantly higher in hulls. Linustatin and neolinustatin were quantified in seven cultivars grown in two locations in three different years. Linustatin, neolinustatin, and total cyanogenic glycosides ranged between 91 and 267 mg/100 g, 78-272 mg/100 g, and 198-513 mg/100 g dry weight flaxseeds, respectively. NMR revealed differences of up to 70% between samples with standard deviation variations lower than 6%. This study shows that NMR is a very suitable tool to perform flaxseed varietal selection for the cyanogenic glycoside content. Graphical abstract qNMR can be used to perform flaxseed varietal selection for the cyanogenic glycoside content.

Glycosylation Is a Major Regulator of Phenylpropanoid Availability and Biological Activity in Plants

The phenylpropanoid pathway in plants is responsible for the biosynthesis of a huge amount of secondary metabolites derived from phenylalanine and tyrosine. Both flavonoids and lignins are synthesized at the end of this very diverse metabolic pathway, as well as many intermediate molecules whose precise biological functions remain largely unknown. The diversity of these molecules can be further increased under the action of UDP-glycosyltransferases (UGTs) leading to the production of glycosylated hydroxycinnamates and related aldehydes, alcohols and esters. Glycosylation can change phenylpropanoid solubility, stability and toxic potential, as well as influencing compartmentalization and biological activity. (De)-glycosylation therefore represents an extremely important regulation point in phenylpropanoid homeostasis. In this article we review recent knowledge on the enzymes involved in regulating phenylpropanoid glycosylation status and availability in different subcellular compartments. We also examine the potential link between monolignol glycosylation and lignification by exploring co-expression of lignin biosynthesis genes and phenolic (de)glycosylation genes. Of the different biological roles linked with their particular chemical properties, phenylpropanoids are often correlated with the plant's stress management strategies that are also regulated by glycosylation. UGTs can for instance influence the resistance of plants during infection by microorganisms and be involved in the mechanisms related to environmental changes. The impact of flavonoid glycosylation on the color of flowers, leaves, seeds and fruits will also be discussed. Altogether this paper underlies the fact that glycosylation and deglycosylation are powerful mechanisms allowing plants to regulate phenylpropanoid localisation, availability and biological activity.

Dirigent Protein-Mediated Lignan and Cyanogenic Glucoside Formation in Flax Seed: Integrated Omics and MALDI Mass Spectrometry Imaging

An integrated omics approach using genomics, transcriptomics, metabolomics (MALDI mass spectrometry imaging, MSI), and bioinformatics was employed to study spatiotemporal formation and deposition of health-protecting polymeric lignans and plant defense cyanogenic glucosides. Intact flax (Linum usitatissimum) capsules and seed tissues at different development stages were analyzed. Transcriptome analyses indicated distinct expression patterns of dirigent protein (DP) gene family members encoding (-)- and (+)-pinoresinol-forming DPs and their associated downstream metabolic processes, respectively, with the former expressed at early seed coat development stages. Genes encoding (+)-pinoresinol-forming DPs were, in contrast, expressed at later development stages. Recombinant DP expression and DP assays also unequivocally established their distinct stereoselective biochemical functions. Using MALDI MSI and ion mobility separation analyses, the pinoresinol downstream derivatives, secoisolariciresinol diglucoside (SDG) and SDG hydroxymethylglutaryl ester, were localized and detectable only in early seed coat development stages. SDG derivatives were then converted into higher molecular weight phenolics during seed coat maturation. By contrast, the plant defense cyanogenic glucosides, the monoglucosides linamarin/lotaustralin, were detected throughout the flax capsule, whereas diglucosides linustatin/neolinustatin only accumulated in endosperm and embryo tissues. A putative biosynthetic pathway to the cyanogens is proposed on the basis of transcriptome coexpression data. Localization of all metabolites was at ca. 20 μm resolution, with the web based tool OpenMSI enabling not only resolution enhancement but also an interactive system for real-time searching for any ion in the tissue under analysis.

A recycling pathway for cyanogenic glycosides evidenced by the comparative metabolic profiling in three cyanogenic plant species

Cyanogenic glycosides are phytoanticipins involved in plant defence against herbivores by virtue of their ability to release toxic hydrogen cyanide (HCN) upon tissue disruption. In addition, endogenous turnover of cyanogenic glycosides without the liberation of HCN may offer plants an important source of reduced nitrogen at specific developmental stages. To investigate the presence of putative turnover products of cyanogenic glycosides, comparative metabolic profiling using LC-MS/MS and high resolution MS (HR-MS) complemented by ion-mobility MS was carried out in three cyanogenic plant species: cassava, almond and sorghum. In total, the endogenous formation of 36 different chemical structures related to the cyanogenic glucosides linamarin, lotaustralin, prunasin, amygdalin and dhurrin was discovered, including di- and tri-glycosides derived from these compounds. The relative abundance of the compounds was assessed in different tissues and developmental stages. Based on results common to the three phylogenetically unrelated species, a potential recycling endogenous turnover pathway for cyanogenic glycosides is described in which reduced nitrogen and carbon are recovered for primary metabolism without the liberation of free HCN. Glycosides of amides, carboxylic acids and 'anitriles' derived from cyanogenic glycosides appear as common intermediates in this pathway and may also have individual functions in the plant. The recycling of cyanogenic glycosides and the biological significance of the presence of the turnover products in cyanogenic plants open entirely new insights into the multiplicity of biological roles cyanogenic glycosides may play in plants.

Effects of food formulation and thermal processing on flavones in celery and chamomile

Flavones isolated from celery varied in their stability and susceptibility to deglycosylation during thermal processing at pH 3, 5, or 7. Apigenin 7-O-apiosylglucoside was converted to apigenin 7-O-glucoside when heated at pH 3 and 100°C. Apigenin 7-O-glucoside showed little conversion or degradation at any pH after 5h at 100°C. Apigenin, luteolin, and chrysoeriol were most stable at pH 3 but progressively degraded at pH 5 or 7. Chamomile and celery were used to test the effects of glycosidase-rich foods and thermal processing on the stability of flavone glycosides. Apigenin 7-O-glucoside in chamomile extract was readily converted to apigenin aglycone after combination with almond, flax seed, or chickpea flour. Apigenin 7-O-apiosylglucoside in celery leaves was resistant to conversion by β-glucosidase-rich ingredients, but was converted to apigenin 7-O-glucoside at pH 2.7 when processed at 100°C for 90min and could then be further deglycosylated when mixed with almond or flax seed. Thus, combinations of acid hydrolysis and glycosidase enzymes in almond and flax seed were most effective for developing a flavone-rich, high aglycone food ingredient from celery.

Functional analyses of the digestive β-glucosidase of Formosan subterranean termites (Coptotermes formosanus)

The research was to elucidate the function of the β-glucosidase of Formosan subterranean termites in vitro and in vivo. The gene transcript was detected predominantly in the salivary gland tissue, relative to the midgut and the hindgut of the foraging worker caste, indicating salivary glands were the major expression sites of the β-glucosidase. Using recombinant β-glucosidase produced in Escherichia coli, the enzyme showed higher affinity and activity toward cellobiose and cellotriose than other substrates tested. In assessing impacts of specific inhibitors, we found that the β-glucosidase could be irreversibly inactivated by conduritol B epoxide (CBE) but not gluconolactone. Termite feeding assays showed that the CBE treatment reduced the glucose supply in the midgut and resulted in the body weight loss while no effect was observed for the gluconolactone treatment. These findings highlighted that the β-glucosidase is one of the critical cellulases responsible for cellulose degradation and glucose production; inactivation of these digestive enzymes by specific inhibitors may starve the termite.

The beta-glucosidases responsible for bioactivation of hydroxynitrile glucosides in Lotus japonicus

Lotus japonicus accumulates the hydroxynitrile glucosides lotaustralin, linamarin, and rhodiocyanosides A and D. Upon tissue disruption, the hydroxynitrile glucosides are bioactivated by hydrolysis by specific beta-glucosidases. A mixture of two hydroxynitrile glucoside-cleaving beta-glucosidases was isolated from L. japonicus leaves and identified by protein sequencing as LjBGD2 and LjBGD4. The isolated hydroxynitrile glucoside-cleaving beta-glucosidases preferentially hydrolyzed rhodiocyanoside A and lotaustralin, whereas linamarin was only slowly hydrolyzed, in agreement with measurements of their rate of degradation upon tissue disruption in L. japonicus leaves. Comparative homology modeling predicted that LjBGD2 and LjBGD4 had nearly identical overall topologies and substrate-binding pockets. Heterologous expression of LjBGD2 and LjBGD4 in Arabidopsis (Arabidopsis thaliana) enabled analysis of their individual substrate specificity profiles and confirmed that both LjBGD2 and LjBGD4 preferentially hydrolyze the hydroxynitrile glucosides present in L. japonicus. Phylogenetic analyses revealed a third L. japonicus putative hydroxynitrile glucoside-cleaving beta-glucosidase, LjBGD7. Reverse transcription-polymerase chain reaction analysis showed that LjBGD2 and LjBGD4 are expressed in aerial parts of young L. japonicus plants, while LjBGD7 is expressed exclusively in roots. The differential expression pattern of LjBGD2, LjBGD4, and LjBGD7 corresponds to the previously observed expression profile for CYP79D3 and CYP79D4, encoding the two cytochromes P450 that catalyze the first committed step in the biosyntheis of hydroxynitrile glucosides in L. japonicus, with CYP79D3 expression in aerial tissues and CYP79D4 expression in roots.

beta-Glucosidases as detonators of plant chemical defense

Some plant secondary metabolites are classified as phytoanticipins. When plant tissue in which they are present is disrupted, the phytoanticipins are bio-activated by the action of beta-glucosidases. These binary systems--two sets of components that when separated are relatively inert--provide plants with an immediate chemical defense against protruding herbivores and pathogens. This review provides an update on our knowledge of the beta-glucosidases involved in activation of the four major classes of phytoanticipins: cyanogenic glucosides, benzoxazinoid glucosides, avenacosides and glucosinolates. New aspects of the role of specific proteins that either control oligomerization of the beta-glucosidases or modulate their product specificity are discussed in an evolutionary perspective.

Localization and catabolism of cyanogenic glycosides

The catabolism of cyanogenic glycosides is initiated by cleavage of the carbohydrate moiety by one or more beta-glycosidases, which yields the corresponding alpha-hydroxynitrile. Until recently, the mode by which cyanogenic disaccharides are hydrolysed was largely unclear. Investigation of highly purified beta-glycosidases from plants containing cyanogenic disaccharides has now indicated that these compounds may be degraded via two distinct pathways, depending on the plant species. beta-Glycosidases from Davallia trichomanoides and Vicia angustifolia hydrolysed (R)-vicianin and (R)-amygdalin at the aglycone-disaccharide bond producing mandelonitrile and the corresponding disaccharide. Alternatively, hydrolysis of cyanogenic disaccharides in Prunus serotina, almonds, and Linum usitatissimum involves stepwise removal of the sugar residues. The nature of these enzymes and of other beta-glycosidases responsible for hydrolysis of simple cyanogenic monosaccharides is discussed. Hydroxynitriles may decompose either spontaneously or enzymically in the presence of a hydroxynitrile lyase to produce hydrogen cyanide and an aldehyde or ketone. The major kinetic and molecular properties of hydroxynitrile lyases purified from species accumulating aromatic and aliphatic cyanogens are reviewed. Cyanogenesis occurs rapidly only after cyanogenic plant tissues are macerated, allowing glycosides access to their catabolic enzymes. The possible nature of the compartmentation which prevents cyanogenesis under normal physiological conditions is discussed in relation to our knowledge of the tissue and subcellular localizations of cyanogens and catabolic enzymes.

Cyanogenesis and the role of cyanogenic compounds in insects

The cyanogenic system comprising cyanogenic glycosides, hydroxynitriles (cyanohydrins), beta-glucosidases and nitrile lyases is widespread in the plant kingdom but also occurs in several arthropods. A few insects were found to contain mandelonitrile and, in one case, a small amount of prunasin was detected. Cardiospermin and gynocardin occur in one insect, and the cyanoglucosides linamarin and lotaustralin are found in several species of the lepidopterans. Biosynthesis of these cyanoglucosides has been studied in two of these species and their sequestration has been investigated in one species. For Zygaena trifolii the presence of the entire cyanide-handling system indicates an important function of these compounds. So far, their function as defensive compounds seems likely on the basis of their ability to generate HCN and their localization, and appears to be indicated by some feeding experiments with potential predators.

The molecular biology of cyanogenesis

The cyanogenic polymorphism in Trifolium repens L. (white clover) has been used as the basis of a study of the genetic control of cyanogenesis. The Ac locus controls the presence of two cyanoglucosides in white clover. Biochemical characterization of cyanoglucoside biosynthesis in plants containing different Ac alleles has shown that this is a complex locus which affects more than one step in the pathway. A study of the in vivo synthesis and processing of the cyanogenic beta-glucosidase (linamarase) of white clover led to the isolation of cDNA clones for this enzyme. The cloning strategy and structure of the cDNA clones is described. Together with biochemical and genetic data, these clones have been used to characterize the Li locus which controls linamarase activity in white clover. It has been shown that 'null' alleles of the Li locus result in very reduced levels of transcription of homologous mRNA sequences. The use of these cDNA clones to investigate the genomic organization of cyanogenesis genes in both white clover and other cyanogenic species is described, and their use in structural analysis of the cyanogenic beta-glucosidase is discussed.

Analysis of the developmental regulation of the cyanogenic compounds in seedlings of two lines of Linum usitatissimum L

The developmental profiles and tissue distribution of the four cyanogenic compounds in seedlings of two developmentally contrasting inbred lines of flax (Linum usitatissimum L.) were examined using HPLC. During germination, the isoleucine-derived compound, neolinustatin, was hydrolysed faster in the more vigorous of the two lines. Furthermore, in this line, the neolinustatin content was higher in seeds and the accumulation of the other isoleucine-derived compound, lotaustralin, was also higher in the cotyledons of seedlings. In contrast, with one exception, the hydrolysis and accumulation of the valine-derived compounds, linustatin and linamarin, was the same in both lines. Differences in the levels of the compounds during germination, and in the hypocotyls, are interpreted as evidence for the involvement of transient levels of hydrogen cyanide in the autocatalytic regulation of ethylene production.Key words: HPLC, germination, hypocotyl, neolinustatin, lotaustralin, ethylene.

Flaxseed increased alpha-linolenic and eicosapentaenoic acid and decreased arachidonic acid in serum and tissues of rat dams and offspring

The effects of dietary flaxseed (FS), and defatted flaxseed meal (FLM) on serum and tissue fatty acid profiles were investigated. Pregnant Sprague-Dawley rats were fed AIN-93 based diets balanced in calories, fat, nitrogen, and fiber. Diets contained 0, 20%, 40% FS or 13% or 26% FLM by weight. The control, FS and FLM diets differed in linoleic acid to alpha-linolenic acid (ALA) fatty acid ratio. These diets were fed continuously during gestation, suckling period and 8 weeks post-weaning (F(1)). FS fatty acids were bioavailable and metabolized by pregnant and F(1) rats. ALA and eicosapentaenoic acid increased; linoleic and arachidonic acid decreased; and docosahexaeonic acid was unchanged in serum, 'gastric milk' and liver of FS and FLM-fed pregnant and F(1) rats. FS more than FLM, changed fatty acids profiles, but FLM and 40% FS significantly reduced serum cholesterol. Dietary 40% FS may have increased oxidative stress as evidenced by a reduction in liver vitamin E.

Process-induced compositional changes of flaxseed

Flaxseed has been used as an edible grain in different parts of the world since ancient times. However, use of flaxseed oil has been limited due to its high content of polyunsaturated fatty acids. Nonetheless, alpha-linolenic acid, dietary fiber and lignans of flaxseed have regained attention. New varieties of flaxseed containing low levels of alpha-linolenic acid are available for edible oil extraction. Use of whole flaxseed in foods provides a means to utilise all of its nutrients and require minimum processing steps. However, the presence of cyanogenic glucosides and diglucosides in the seeds is a concern as they may release cyanide upon hydrolysis. In addition, the polyunsaturated fatty acids may undergo thermal or autooxidation when exposed to air or high temperatures that are used in food preparation. Studies todate on oxidation products of intact flaxseed lipids have not shown any harmful effects when flaxseed is included, up to 28%, in the baked products. Furthermore, cyanide levels produced as a result of autolysis are below the harmful limits to humans. However, the meals left after oil extraction require detoxification but, by solvent extraction, to reduce the harmful effects of cyanide when used in animal rations. Flaxseed meal is a good source of proteins; these could be isolated by complexation with sodium hexametaphosphate without changing their nutritional value or composition. In addition, the effect of germination on proteins, lipids, cyanogenic glycosides, and other minor constituents of flaxseed is discussed.

The crystal structure of a cyanogenic beta-glucosidase from white clover, a family 1 glycosyl hydrolase

BACKGROUND

beta-glucosidases occur in a variety of organisms and catalyze the hydrolysis of aryl and alkyl-beta-D-glucosides as well as glucosides with only a carbohydrate moiety (such as cellobiose). The cyanogenic beta-glucosidase from white clover (subsequently referred to as CBG) is responsible for the cleavage of cyanoglucosides. Both CBG and the cyanoglucosides occur within the plant cell wall where they are found in separate compartments and only come into contact when the leaf tissue experiences mechanical damage. This results in the eventual production of hydrogen cyanide which acts as a deterrent to grazing animals. beta-glucosidases have been assigned to particular glycosyl hydrolase families on the basis of sequence similarity; this classification has placed CBG in family 1 (there are a total of over 40 families) for which a three-dimensional structure has so far not been determined. This is the first report of the three-dimensional structure of a glycosyl hydrolase from family 1.

RESULTS

The crystal structure of CBG has been determined using multiple isomorphous replacement. The final model has been refined at 2.15 A resolution to an R factor of 18.9%. The overall fold of the molecule is a (beta/alpha)8 [or (alpha/beta)8] barrel (in common with a number of glycosyl hydrolases) with all residues located in a single domain.

CONCLUSIONS

Sequence comparisons between beta-glucosidases of the same family show that residues Glu183 and Glu397 are highly conserved. Both residues are positioned at the end of a pocket located at the C terminus of the barrel and have been assigned the respective roles of proton donor and nucleophile on the basis of inhibitor-binding and mutagenesis experiments. These roles are consistent with the environments of the two residues. The pocket itself is typical of a sugar-binding site as it contains a number of charged, aromatic and polar groups. In support of this role, we present crystallographic data on a possible product complex between CBG and glucose, resulting from co-crystallization of the native enzyme with its natural substrate, linamarin.

An examination of the beta-glucosidase (linamarase) banding pattern in flax seedlings using Ferguson plots and sodium dodecyl sulphate-polyacrylamide gel electrophoresis

The reported polyacrylamide gel electrophoresis banding pattern for the main beta-glucosidase (linamarase) component from flax seed consists of five bands, made up of 62.5 and 65 kDa subunits; this component has an estimated molecular weight of 570-670 kDa. The present study used Ferguson plots to estimate the molecular weight of each electrophoretic band, plus two additional bands which were detected. From low to high relative mobility, the seven bands formed a linear series with estimated molecular weights from 1200 to 245 kDa. Each was 160 kDa smaller, and less charged, than the preceding band. This 160 kDa difference between bands did not appear to be consistent with the reported subunit size. Each band produced a corresponding band on sodium dodecyl sulfate (SDS)-gels. The decreases in molecular weight between the bands on nondenaturing gels and their corresponding bands on SDS-gels were multiples of the 62-65 kDa value. However, the estimated molecular weights of the SDS bands themselves and of the differences between the SDS bands were, again, not consistent with the proposed subunit size. The results suggest that active forms of this enzyme may contain a second minor component (possibly a 30-35 kDa component) in addition to the 62.5 and 65 kDa subunits.

A molecular and biochemical analysis of the structure of the cyanogenic beta-glucosidase (linamarase) from cassava (Manihot esculenta Cranz)

The cyanogenic beta-glucosidase (linamarase) of cassava is responsible for the first step in the sequential break-down of two related cyanoglucosides. Hydrolysis of these cyanoglucosides occurs following tissue damage and leads to the production of hydrocyanic acid. This mechanism is widely regarded as a defense mechanism against predation. A linamarase cDNA clone (pCAS5) was isolated from a cotyledon cDNA library using a white clover beta-glucosidase heterologous probe. The nucleotide and derived amino acid sequence is reported and five putative N-asparagine glycosylation sites are identified. Concanavalin A affinity chromatography and endoglycosidase H digestion demonstrate that linamarase from cassava is glycosylated, having high-mannose-type N-asparagine-linked oligosaccharides. Consistent with this structure and the extracellular location of the active enzyme is the identification of an N-terminal signal peptide on the deduced amino acid sequence of pCAS5.

Toxicological, nutritional and microbiological evaluation of tempe fermentation with Rhizopus oligosporus of bitter and sweet apricot seeds

Bitter and sweet apricot seeds are by-products of the apricot processing industry. Bitter seeds, in particular, contain toxic levels of the cyanogenic substance amygdalin. Tempe was made from both kinds of seeds. The bitter seeds contain antimicrobial substances which must be removed by leaching and boiling prior to tempe fermentation. Apricot seed tempe had an agreeable taste. It contained approx. 21% (w/w) crude protein, 52% (w/w) crude fat, 1.5% (w/w) crude fibre and 25.5% (w/w) carbohydrates based on dry matter. The extent of biological acidification during soaking prior to fungal inoculation was inadequate to prevent growth of Bacillus cereus, and requires further optimisation. Bitter seeds were detoxified by the tempe process (approx. 70% of total cyanide was removed). However, additional improvement of the detoxification process is required to obtain a completely safe product.

Kinetic investigation of the substrate specificity of the cyanogenic beta-D-glucosidase (linamarase) of white clover

Partially purified linamarase from Trifolium repens (genotype Lili acac) plants was kinetically characterized. Kinetic evidence was found to support the assumption that this cyanogenic beta-D-glucosidase has a broad substrate spectrum. p-Nitrophenyl-beta-D-xylopyranoside and p-nitrophenyl-alpha-L-arabinopyranoside substrates bound almost as tightly to the active center of the enzyme as the glucono(1----5)lactone transition-state analog inhibitor. Substrate specificity investigation also indicated that positions C-4 and C-6 on the pyranoside ring play an essential role in both substrate orientation and splitting. Recently very similar kinetic characteristics were reported on some mammalian cytosolic beta-D-glucosidases and a possible physiological interpretation of this coincidence is discussed. Inhibition studies with glucono(1----5)lactone revealed that the carbohydrate moiety of each substrate attached to the same binding site in the active center. Inhibition experiments with 1-thio substrate analogs demonstrated that the aglycon and the angular arrangement around the glycosidic linkage were the major determinants in the observed substrate specificity.

Characterization of cyanogenic beta-glucosidase (linamarase) from cassava (Manihot esculenta Crantz)

Linamarase (EC 3.2.1.21) was purified from cassava petiole, stem, and root cortex by ammonium sulfate precipitation, column chromatography on Sepharose 6B, and chromatofocusing. The last step resolved the enzyme from each source into three forms with pI values of 4.3, 3.3, and 2.9. Each form was found to be oligomeric, consisting of one kind of subunit, Mr 63,000. The major isozyme with a pI of 4.3 from petiole showed a Km for linamarin of 0.6 mM and possessed both beta-glucosidase and beta-fucosidase activities. The former was sensitive to inhibition by delta-gluconolactone, isopropyl-beta-D-thioglucoside, and HgCl2, whereas the latter was inhibited by Tris ion.

Purification and characterization of acetone cyanohydrin lyase from Linum usitatissimum

The hydroxynitrile lyase (EC 4.1.2.--) which catalyzes the dissociation of the cyanohydrins of acetone and 2-butanone has been isolated and purified from young seedlings of flax (Linum usitatissimum L.). The purification procedure involved precipitation with (NH4)2SO4, chromatofocusing, and chromatography on DEAE-cellulose, hydroxylapatite, Sephacryl 200, and Matrex Red A gel columns with a final recovery of 21%. Purification of 136-fold yielded an apparently homogeneous preparation that, in contrast to the lyases isolated from Prunus species, is not a flavoprotein. The subunit molecular weight of 42,000 was estimated by gel electrophoresis in the presence of sodium dodecyl sulfate. The native molecular weight of the enzyme was estimated by gel filtration (HPLC) to be 82,000. The enzyme has a narrow pH optimum around 5.5 and is highly stable at 4 degrees C.

Isolation and characterization of multiple forms of prunasin hydrolase from black cherry (Prunus serotina Ehrh.) seeds

Three forms of prunasin hydrolase (PH I, PH IIa, and PH IIb), which catalyze the hydrolysis of (R)-prunasin to mandelonitrile and D-glucose, have been purified from homogenates of mature black cherry (Prunus serotina Ehrh.) seeds. Hydroxyapatite chromatography completely resolved PH I from PH IIa and PH IIb. PH IIa and IIb, which coeluted on hydroxyapatite, were resolved by gel filtration. PH IIa was a dimer with a native molecular weight of 140,000. Both PH I and PH IIb were monomeric with molecular weights of 68,000. The isozymes appeared to be glycoproteins based on their binding to concanavalin A-Sepharose 4B with subsequent elution by alpha-methyl-D-glucoside. When presented several potential glycosidic substrates, these enzymes exhibited a narrow specificity towards (R)-prunasin. Km values for (R)-prunasin for PH I, PH IIa, and PH IIb were 1.73, 2.3, and 1.35 mM, respectively. PH I and PH IIb possessed fivefold greater Vmax/Km values than PH IIa. Ortho- and para-nitrophenyl-beta-D-glucosides were hydrolyzed at the same active site. All forms had a pH optimum of 5.0 in citrate-phosphate buffer. PH I and PH IIb were competitively inhibited by castanospermine with Ki values of 0.19 and 0.09 mM, respectively. PH activity was not stimulated by any metal ion tested and was unaffected by diethyldithiocarbamate, o-phenanthroline, 2,2'-dipyridyl, and EDTA.

Characterization of beta-glucosidases with high specificity for the cyanogenic glucoside dhurrin in Sorghum bicolor (L.) moench seedlings

Two beta-glucosidases exhibiting high specificity for the cyanogenic glucoside dhurrin have been purified to near homogeneity from seedlings of Sorghum bicolor. Dhurrinase 1 was isolated from shoots of seedlings grown in the dark. Dhurrinase 2 was isolated from the green shoots of young seedlings grown in the light. The two enzymes were similar in following characteristics: their optimum activity is around pH 6.2; the enzymes are stable above pH 7; they are effectively inhibited by the beta-glycosidase inhibitors nojirimycin delta-gluconolactone and 1-amino-beta-D-glucoside. On the other hand, they clearly differed in other properties, e.g., molecular weights, isoelectric points, and substrate specificity. Moreover, dithiothreitol has no effect on dhurrinase 1, but is necessary for the activity of dhurrinase 2. Preliminary investigations indicate that the two enzymes are located in different parts of the sorghum seedlings: dhurrinase 1 is found in the coleoptiles and hypocotyls; dhurrinase 2 occurs in the leaves. Dhurrin (p-hydroxy-(S)-mandelonitrile-beta-D-glucoside) and its structural analog without the hydroxyl group, sambunigrin, were the only substrates hydrolyzed at high rate, the Km values with both enzymes being 0.15 and 0.3 mM, respectively. All other cyanogenic glucosides tested, as well as synthetic substrates such as 4-nitrophenyl-beta-D-glucoside, were in general poor substrates, especially for dhurrinase 1, the enzyme isolated from coleoptile and hypocotyl tissue. Dhurrinase 1 appears to exist within the seedlings as a tetramer (Mr - 2-2.4 X 10(5)) which dissociates without loss of activity into a dimeric form (Mr = 1-1.1 X 10(5)) upon extraction and purification. There is only one monomeric subunit with Mr = 5.7 X 10(4). Isolectric focusing and chromatofocusing of purified dhurrinase 1 showed the presence of at least three isomeric forms, but their relationship to each other is not known at the present time. Dhurrinase 2 appears to be a tetrameric protein with Mr = 2.5-3 X 10(5); it also has only one monomeric subunit of Mr = 6.1 X 10(4). In contrast to many other beta-glucosidases, the dhurrinases are not glycoproteins.

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.