The myrosinase-glucosinolate interaction mechanism studied using some synthetic competitive inhibitors

Glucosinolate structural diversity, identification, chemical synthesis and metabolism in plants

The glucosinolates (GSLs) is a well-defined group of plant metabolites characterized by having an S-β-d-glucopyrano unit anomerically connected to an O-sulfated (Z)-thiohydroximate function. After enzymatic hydrolysis, the sulfated aglucone can undergo rearrangement to an isothiocyanate, or form a nitrile or other products. The number of GSLs known from plants, satisfactorily characterized by modern spectroscopic methods (NMR and MS) by mid-2018, is 88. In addition, a group of partially characterized structures with highly variable evidence counts for approximately a further 49. This means that the total number of characterized GSLs from plants is somewhere between 88 and 137. The diversity of GSLs in plants is critically reviewed here, resulting in significant discrepancies with previous reviews. In general, the well-characterized GSLs show resemblance to C-skeletons of the amino acids Ala, Val, Leu, Trp, Ile, Phe/Tyr and Met, or to homologs of Ile, Phe/Tyr or Met. Insufficiently characterized, still hypothetic GSLs include straight-chain alkyl GSLs and chain-elongated GSLs derived from Leu. Additional reports (since 2011) of insufficiently characterized GSLs are reviewed. Usually the crucial missing information is correctly interpreted NMR, which is the most effective tool for GSL identification. Hence, modern use of NMR for GSL identification is also reviewed and exemplified. Apart from isolation, GSLs may be obtained by organic synthesis, allowing isotopically labeled GSLs and any kind of side chain. Enzymatic turnover of GSLs in plants depends on a considerable number of enzymes and other protein factors and furthermore depends on GSL structure. Identification of GSLs must be presented transparently and live up to standard requirements in natural product chemistry. Unfortunately, many recent reports fail in these respects, including reports based on chromatography hyphenated to MS. In particular, the possibility of isomers and isobaric structures is frequently ignored. Recent reports are re-evaluated and interpreted as evidence of the existence of "isoGSLs", i.e. non-GSL isomers of GSLs in plants. For GSL analysis, also with MS-detection, we stress the importance of using authentic standards.

Myrosinase: insights on structural, catalytic, regulatory, and environmental interactions

Glucosinolate-myrosinase is a substrate-enzyme defense mechanism present in Brassica crops. This binary system provides the plant with an efficient system against herbivores and pathogens. For humans, it is well known for its anti-carcinogenic, anti-inflammatory, immunomodulatory, anti-bacterial, cardio-protective, and central nervous system protective activities. Glucosinolate and myrosinase are spatially present in different cells that upon tissue disruption come together and result in the formation of a variety of hydrolysis products with diverse physicochemical and biological properties. The myrosinase-catalyzed reaction starts with cleavage of the thioglucosidic linkage resulting in release of a D-glucose and an unstable thiohydroximate-O-sulfate. The outcome of this thiohydroximate-O-sulfate has been shown to depend on the structure of the glucosinolate side chain, the presence of supplementary proteins known as specifier proteins and/or on the physiochemical condition. Myrosinase was first reported in mustard seed during 1939 as a protein responsible for release of essential oil. Until this date, myrosinases have been characterized from more than 20 species of Brassica, cabbage aphid, and many bacteria residing in the human intestine. All the plant myrosinases are reported to be activated by ascorbic acid while aphid and bacterial myrosinases are found to be either neutral or inhibited. Myrosinase catalyzes hydrolysis of the S-glycosyl bond, O-β glycosyl bond, and O-glycosyl bond. This review summarizes information on myrosinase, an essential component of this binary system, including its structural and molecular properties, mechanism of action, and its regulation and will be beneficial for the research going on the understanding and betterment of the glucosinolate-myrosinase system from an ecological and nutraceutical perspective.

Isothiocyanates: An Overview of Their Antimicrobial Activity against Human Infections

The use of plant-derived products as antimicrobial agents has been investigated in depth. Isothiocyanates (ITCs) are bioactive products resulting from enzymatic hydrolysis of glucosinolates (GLs), the most abundant secondary metabolites in the botanical order Brassicales. Although the antimicrobial activity of ITCs against foodborne and plant pathogens has been well documented, little is known about their antimicrobial properties against human pathogens. This review collects studies that focus on this topic. Particular focus will be put on ITCs’ antimicrobial properties and their mechanism of action against human pathogens for which the current therapeutic solutions are deficient and therefore of prime importance for public health. Our purpose was the evaluation of the potential use of ITCs to replace or support the common antibiotics. Even though ITCs appear to be effective against the most important human pathogens, including bacteria with resistant phenotypes, the majority of the studies did not show comparable results and thus it is very difficult to compare the antimicrobial activity of the different ITCs. For this reason, a standard method should be used and further studies are needed.

HPLC-based kinetics assay facilitates analysis of systems with multiple reaction products and thermal enzyme denaturation

Glucosinolates are plant secondary metabolites abundant in Brassica vegetables that are substrates for the enzyme myrosinase, a thioglucoside hydrolase. Enzyme-mediated hydrolysis of glucosinolates forms several organic products, including isothiocyanates (ITCs) that have been explored for their beneficial effects in humans. Myrosinase has been shown to be tolerant of non-natural glucosinolates, such as 2,2-diphenylethyl glucosinolate, and can facilitate their conversion to non-natural ITCs, some of which are leads for drug development. An HPLC-based method capable of analyzing this transformation for non-natural systems has been described. This current study describes (1) the Michaelis-Menten characterization of 2,2-diphenyethyl glucosinolate and (2) a parallel evaluation of this analogue and the natural analogue glucotropaeolin to evaluate effects of pH and temperature on rates of hydrolysis and product(s) formed. Methods described in this study provide the ability to simultaneously and independently analyze the kinetics of multiple reaction components. An unintended outcome of this work was the development of a modified Lambert W(x) which includes a parameter to account for the thermal denaturation of enzyme. The results of this study demonstrate that the action of Sinapis alba myrosinase on natural and non-natural glucosinolates is consistent under the explored range of experimental conditions and in relation to previous accounts.

UGT74B1 from Arabidopsis thaliana as a versatile biocatalyst for the synthesis of desulfoglycosinolates

A glucosyltransferase is able to catalyze the formation of the thioglycosidic bond and lead to desulfoglycosinolates.

Improving red radish anthocyanin yield and off flavor removal by acidified aqueous organic based medium

In view of the high content of the highly stable anthocyanin in red radish roots, the plant is considered as a potent source of natural anthocyanins.

High-performance liquid chromatography-based method to evaluate kinetics of glucosinolate hydrolysis by Sinapis alba myrosinase

Isothiocyanates (ITCs) are one of several hydrolysis products of glucosinolates, plant secondary metabolites that are substrates for the thioglucohydrolase myrosinase. Recent pursuits toward the development of synthetic non-natural ITCs have consequently led to an exploration of generating these compounds from non-natural glucosinolate precursors. Evaluation of the myrosinase-dependent conversion of select non-natural glucosinolates to non-natural ITCs cannot be accomplished using established ultraviolet-visible (UV-Vis) spectroscopic methods. To overcome this limitation, an alternative high-performance liquid chromatography (HPLC)-based analytical approach was developed where initial reaction velocities were generated from nonlinear reaction progress curves. Validation of this HPLC method was accomplished through parallel evaluation of three glucosinolates with UV-Vis methodology. The results of this study demonstrate that kinetic data are consistent between both analytical methods and that the tested glucosinolates respond similarly to both Michaelis-Menten and specific activity analyses. Consequently, this work resulted in the complete kinetic characterization of three glucosinolates with Sinapis alba myrosinase, with results that were consistent with previous reports.

Isoferuloyl derivatives of five seed glucosinolates in the crucifer genus Barbarea

Five acylated glucosinolates (GSLs) were isolated as desulfated derivatives after enzymatic desulfation of anionic metabolites from seeds of two chemotypes of Barbareavulgaris, and their structures were elucidated by a combination of spectroscopic methods and HPLC analysis of products of enzymatic de-acylation. The acyl group was in all cases found to be a trans isoferuloyl group at the 6'-position of the thioglucose moiety. The GSL moieties of the native metabolites were found to be one Trp derived; indol-3-ylmethylGSL, as well as four homoPhe derived; phenethylGSL, (S)-2-hydroxy-2-phenylethylGSL, (R)-2-hydroxy-2-phenylethylGSL, and (R)-2-hydroxy-2-(4-hydroxyphenyl)ethylGSL. GSL analysis of B. vulgaris seed extracts by the commonly employed 'desulfoGSL' method (based on binding to anion exchange columns, enzymatic desulfation, elution and HPLC) was optimized for 6'-isoferuloyl derivatives of GSLs. From peak areas before and after de-acylation of the isolated desulfoGSL, the response factor of the 6'-isoferuloyl derivative of (S)-2-hydroxy-2-phenylethylGSL was estimated to be 0.37 (relative to 1.00 for sinigrin), allowing us to estimate the level in B. vulgaris to 3μmol/g dry wt. in mature seeds and less than 0.1μmol/g dry wt. in seedlings and floral parts of the insect resistant G-type of B. vulgaris var. arcuata. HPLC analysis of intact GSLs in crude extracts and after group separation did not reveal additional derivatives, but confirmed the existence of the deduced intact GSLs. A taxonomic screen showed that most (14/17) B. vulgaris accessions (with the exception of three accessions of var. vulgaris) contained relatively high levels of 6'-isoferuloyl GSLs. The profiles of 6'-isoferuloylated GSLs matched the profiles of non-acylated GSLs in the same seed accessions, suggesting a low side chain specificity of the isoferuloylation mechanism. A minor peak tentatively identified as a dimethoxycinnamoyl derivative of (S)-2-hydroxy-2-phenylethylGSL was detected by HPLC-MS of one accession, suggesting that GSLs with other acyl groups may occur at low levels. A single analyzed B. plantaginae accession contained relatively high levels of 6'-isoferuloylated phenethylGSL and (S)-2-hydroxy-2-phenylethylGSL. Five other tested Barbarea species (B. australis, B. bracteosa, B. intermedia, B. stricta, B. verna) also contained isoferuloylated GSLs, albeit at lower levels than in B. vulgaris and B. plantaginae, suggesting that seed GSL acylation is a general character of the Barbarea genus and possibly also of related genera including Arabidopsis.

Revised determination of free and complexed myrosinase activities in plant extracts

The enzyme myrosinase (thioglucoside glucohydrolase, EC 3.2.1.147, formerly EC 3.2.3.1) catalyzes the hydrolysis of glucosinolates after tissue damage in plants of the order Brassicales. The various myrosinase isoforms occur either as free soluble dimers or as insoluble complexes. We propose a reliable method for determination of both soluble and insoluble myrosinase activity concentrations in partially purified plant extracts. The procedure requires the removal of endogenous glucosinolates through ion-exchange columns previous to enzyme measurements. Myrosinase activity was assayed in continuous mode by photometric quantification of the released glucose using glucose-oxidase with peroxidase and colorimetric indicators. The measurement of the colored product at 492nm has a favorable signal to noise ratio both in clear extract solutions (free dimers) and in turbid pellet suspensions (insoluble complexes). No interferences by ascorbic acid were found in continuous analyses. With the recommended sample preparation methods and assay conditions potential activities in damaged plant tissues can be characterized which are involved in plant defense mechanisms.

The first naturally occurring aromatic isothiocyanates, rapalexins A and B, are cruciferous phytoalexins

The discovery of the first naturally occurring aromatic isothiocyanates, indole-3-isothiocyanates, their first synthesis, antimicrobial activity and proposed biogenetic origin in canola plants are reported.

The glucosinolate-myrosinase system. New insights into enzyme-substrate interactions by use of simplified inhibitors

Myrosinase, a thioglucoside glucohydrolase, is the only enzyme able to hydrolyse glucosinolates, a unique family of molecules bearing an anomeric O-sulfated thiohydroximate function. Non-hydrolysable myrosinase inhibitors have been devised and studied for their biological interaction. Diverse modifications of the O-sulfate moiety did not result in a significant inhibitory effect, whereas replacing the D-glucopyrano residue by its carba-analogue allowed inhibition to take place. X-Ray experiments carried out after soaking allowed for the first time inclusion of a non-hydrolysable inhibitor inside the enzymatic pocket. Structural tuning of the aglycon part in its pocket is being used as a guide for the development of simplified and more potent inhibitors.

Effects of combined pressure and temperature on enzymes related to quality of fruits and vegetables: from kinetic information to process engineering aspects

Throughout the last decade, high pressure technology has been shown to offer great potential to the food processing and preservation industry in delivering safe and high quality products. Implementation of this new technology will be largely facilitated when a scientific basis to assess quantitatively the impact of high pressure processes on food safety and quality becomes available. Besides, quantitative data on the effects of pressure and temperature on safety and quality aspects of foods are indispensable for design and evaluation of optimal high pressure processes, i.e., processes resulting in maximal quality retention within the constraints of the required reduction of microbial load and enzyme activity. Indeed it has to be stressed that new technologies should deliver, apart from the promised quality improvement, an equivalent or preferably enhanced level of safety. The present paper will give an overview from a quantitative point of view of the combined effects of pressure and temperature on enzymes related to quality of fruits and vegetables. Complete kinetic characterization of the inactivation of the individual enzymes will be discussed, as well as the use of integrated kinetic information in process engineering.

Isolation and biochemical characterization of a basic myrosinase from ripe Crambe abyssinica seeds, highly specific for epi-progoitrin

On the basis of previous studies on the mechanism-based inhibition, activation, and active site structure of myrosinase(s) isolated from Sinapis alba and other cruciferous seeds, crambe myrosinase shows uncommon properties and behavior. For this reason homogeneous crambe myrosinase was isolated and investigated to establish the most important physicochemical features, including kinetic properties determined with the epimers progoitrin (R) and epi-progoitrin (S) as substrates, with and without ascorbate as an activator. The results of this study demonstrate that crambe myrosinase is highly specific for epi-progoitrin due to a better stabilization of the enzyme-substrate complex. This stabilization is caused by additional hydrogen bonding that only epi-progoitrin can set up between its hydroxyl group and a suitable residue in the hydrophobic pocket where the "docking" of the glucosinolates side chain takes place.

Comparison of the bioactivity of two glucoraphanin hydrolysis products found in broccoli, sulforaphane and sulforaphane nitrile

Epidemiological and laboratory studies suggest that dietary broccoli may prevent or delay a variety of cancers. Broccoli and other crucifers contain a relatively unique family of secondary metabolites called glucosinolates. Glucoraphanin, the major glucosinolate in broccoli, is hydrolyzed by an endogenous plant myrosinase to form either the potent anticarcinogen sulforaphane (SF) or sulforaphane nitrile (SF nitrile). The bioactivities of SF and SF nitrile were compared in rats and in mouse hepatoma cells. Male, 4-week-old, Fischer 344 rats were administered SF or SF nitrile (200, 500, or 1000 micromol/kg) by gavage daily for 5 days. Hepatic, colonic mucosal, and pancreatic quinone reductase and glutathione S-transferase activities were induced by high doses of SF, but not by SF nitrile. When Hepa 1c1c7 cells were exposed to increasing levels of each compound for 24 h, quinone reductase showed a 3-fold maximal induction over control at 2.5 microM SF and a 3.5-fold maximal induction over control at 2000 microM SF nitrile, the highest concentration tested. These results demonstrate that SF nitrile is substantially less potent than SF as an inducing agent of phase II detoxification enzymes. Therefore, glucoraphanin hydrolysis directed toward the production of SF rather than SF nitrile could increase the potential chemoprotective effects of broccoli.

The biosynthesis of benzoic acid glucosinolate esters in Arabidopsis thaliana

The siliques and seeds of Arabidopsis thaliana accumulate a series of glucosinolates containing an alkyl side chain of varying length with a terminal benzoate ester function. The biosynthesis of these unusual nitrogen- and sulfur-containing natural products was investigated by feeding isotopically-labeled precursors to detached flowering stems. Glucosinolates were extracted, purified and analyzed by tandem mass spectrometry. Phenylalanine and benzoic acid were incorporated into the benzoate ester function, and methionine and acetate were incorporated into the aliphatic portion of the side chain in a position-specific manner. The labeling patterns observed were consistent with the chain extension of methionine by a three-step elongation cycle which begins with the condensation of acetyl-CoA with a 2-oxo acid derived from methionine and ends with an oxidative decarboxylation forming a new 2-oxo acid with an additional methylene group. Incorporation of desulfo-4-methylthiobutyl glucosinolate into 4-benzoyloxybutyl olucosinolate suggested chain-extended methionine derivatives are first converted to their corresponding methylthioalkyl glucosinolates with further side chain modification occurring later. Transformation of the methylthiol function to a hydroxyl group is followed by esterification with benzoic acid. The siliques appear to possess the complete machinery for carrying out all of the reactions in the biosyntheis of these complex glucosinolates.

Preparative HPLC method for the purification of sulforaphane and sulforaphane nitrile from Brassica oleracea

An extraction and preparative HPLC method has been devised to simultaneously purify sulforaphane and sulforaphane nitrile from the seed of Brassica oleracea var. italica cv. Brigadier. The seed was defatted with hexane, dried, and hydrolyzed in deionized water (1:9) for 8 h. The hydrolyzed seed meal was salted and extracted with methylene chloride. The dried residue was redissolved in a 5% acetonitrile solution and washed with excess hexane to remove nonpolar contaminants. The aqueous phase was filtered through a 0.22-microm cellulose filter and separated by HPLC using a Waters Prep Nova-Pak HR C-18 reverse-phase column. Refractive index was used to detect sulforaphane nitrile, and absorbance at 254 nm was used to detect sulforaphane. Peak identification was confirmed using gas chromatography and electron-impact mass spectrometry. Each kilogram of extracted seed yielded approximately 4.8 g of sulforaphane and 3.8 g of sulforaphane nitrile. Standard curves were developed using the purified compounds to allow quantification of sulforaphane and sulforaphane nitrile in broccoli tissue using a rapid GC method. The methodology was used to compare sulforaphane and sulforaphane nitrile content of autolyzed samples of several broccoli varieties.

Soluble and total myrosinase activity in defatted Crambe abyssinica meal

Crambe defatted meal contains 4-6% w/w of glucosinolates, with epiprogoitrin accounting for >90% of the total. This feature limits the use of the meal as feed due to the antinutritional properties of myrosinase-glucosinolate breakdown products. In this context, myrosinase activity assumes particular importance. In this study the total and soluble myrosinase activities have been evaluated directly on defatted meals of eight Crambe abyssinica varieties. The pH-stat method, which is the most suitable for assays in heterogeneous solid-water systems, was used. The total myrosinase activity in C. abyssinica varieties, determined using epiprogoitrin as substrate, ranged from 288 to 653 units g(-1). These activity values were up to 26 times higher than those obtained using other substrates, namely, sinigrin, glucosinalbin, glucotropaeolin, progoitrin, and glucoraphenin. Crambe myrosinase is unusual in that, unlike other Brassicaceae containing a typical main glucosinolate, it does not show the same specificity toward its natural substrates.

High resolution X-ray crystallography shows that ascorbate is a cofactor for myrosinase and substitutes for the function of the catalytic base

Myrosinase, an S-glycosidase, hydrolyzes plant anionic 1-thio-beta-d-glucosides (glucosinolates) considered part of the plant defense system. Although O-glycosidases are ubiquitous, myrosinase is the only known S-glycosidase. Its active site is very similar to that of retaining O-glycosidases, but one of the catalytic residues in O-glycosidases, a carboxylate residue functioning as the general base, is replaced by a glutamine residue. Myrosinase is strongly activated by ascorbic acid. Several binary and ternary complexes of myrosinase with different transition state analogues and ascorbic acid have been analyzed at high resolution by x-ray crystallography along with a 2-deoxy-2-fluoro-glucosyl enzyme intermediate. One of the inhibitors, d-gluconhydroximo-1,5-lactam, binds simultaneously with a sulfate ion to form a mimic of the enzyme-substrate complex. Ascorbate binds to a site distinct from the glucose binding site but overlapping with the aglycon binding site, suggesting that activation occurs at the second step of catalysis, i.e. hydrolysis of the glycosyl enzyme. A water molecule is placed perfectly for activation by ascorbate and for nucleophilic attack on the covalently trapped 2-fluoro-glucosyl-moiety. Activation of the hydrolysis of the glucosyl enzyme intermediate is further evidenced by the observation that ascorbate enhances the rate of reactivation of the 2-fluoro-glycosyl enzyme, leading to the conclusion that ascorbic acid substitutes for the catalytic base in myrosinase.

The crystal structures of Sinapis alba myrosinase and a covalent glycosyl-enzyme intermediate provide insights into the substrate recognition and active-site machinery of an S-glycosidase

BACKGROUND

Myrosinase is the enzyme responsible for the hydrolysis of a variety of plant anionic 1-thio-beta-D-glucosides called glucosinolates. Myrosinase and glucosinolates, which are stored in different tissues of the plant, are mixed during mastication generating toxic by-products that are believed to play a role in the plant defence system. Whilst O-glycosidases are extremely widespread in nature, myrosinase is the only known S-glycosidase. This intriguing enzyme, which shows sequence similarities with O-glycosidases, offers the opportunity to analyze the similarities and differences between enzymes hydrolyzing S- and O-glycosidic bonds.

RESULTS

The structures of native myrosinase from white mustard seed (Sinapis alba) and of a stable glycosyl-enzyme intermediate have been solved at 1.6 A resolution. The protein folds into a (beta/alpha)8-barrel structure, very similar to that of the cyanogenic beta-glucosidase from white clover. The enzyme forms a dimer stabilized by a Zn2+ ion and is heavily glycosylated. At one glycosylation site the complete structure of a plant-specific heptasaccharide is observed. The myrosinase structure reveals a hydrophobic pocket, ideally situated for the binding of the hydrophobic sidechain of glucosinolates, and two arginine residues positioned for interaction with the sulphate group of the substrate. With the exception of the replacement of the general acid/base glutamate by a glutamine residue, the catalytic machinery of myrosinase is identical to that of the cyanogenic beta-glucosidase. The structure of the glycosyl-enzyme intermediate shows that the sugar ring is bound via an alpha-glycosidic linkage to Glu409, the catalytic nucleophile of myrosinase.

CONCLUSIONS

The structure of myrosinase shows features which illustrate the adaptation of the plant enzyme to the dehydrated environment of the seed. The catalytic mechanism of myrosinase is explained by the excellent leaving group properties of the substrate aglycons, which do not require the assistance of an enzymatic acid catalyst. The replacement of the general acid/base glutamate of O-glycosidases by a glutamine residue in myrosinase suggests that for hydrolysis of the glycosyl-enzyme, the role of this residue is to ensure a precise positioning of a water molecule rather than to provide general base assistance.

Mechanism-based inhibition and stereochemistry of glucosinolate hydrolysis by myrosinase

Myrosinase is a particular glucosidase which hydrolyzes a variety of plant 1-thio-beta-D-glucosides known as the glucosinolates. This enzyme, which is the only glycosidase able to hydrolyze these naturally occurring thioglucosides, has been found previously to display strong sequence similarities with family 1 O-glycosidases. Myrosinase therefore offers the opportunity to compare the mechanism of enzymatic cleavage of S- vs O-glycosidic bonds. The stereochemistry of hydrolysis of sinigrin by Sinapis alba myrosinase was followed by 1H NMR and the enzyme was found to operate with a mechanism retaining the anomeric configuration at the cleavage point exactly like the related O-glycosidases found in family 1. Myrosinase was readily inactivated by 2-deoxy-2-fluoroglucotropaeolin with inactivation kinetic parameters of Ki = 0.9 mM and ki = 0.083 min-1. Reactivation kinetic parameters were determined in buffer only, with k(react) = 0.015 h-1 and t1/2 = 46 h, and also in the presence of acceptors of transglycosylation. No significant changes were observed in the presence of methyl beta-D-glucoside, but with azide anion the half-life of reactivation was found to be reduced to t1/2 = 20 h. These results suggest that myrosinase inhibition by 2-deoxy-2-fluoroglucotropaeolin occurs via the accumulation of a long-life glucosyl-enzyme intermediate and that the catalytic machinery of the enzyme is composed of only one catalytic residue, a nucleophilic glutamate, while the acid catalyst residue found in the corresponding O-glycosidases is missing.