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

Progresses and Prospects on Glucosinolate Detection in Cruciferous Plants

This review provides a comprehensive summary of the latest international research on detection methods for glucosinolates in cruciferous plants. This article examines various analytical techniques, including high-performance liquid chromatography (HPLC), liquid chromatography-mass spectrometry (LC-MS), enzyme-linked immunosorbent assay (ELISA), and capillary electrophoresis (CE), while highlighting their respective advantages and limitations. Additionally, this review delves into recent advancements in sample preparation, extraction, and quantification methods, offering valuable insights into the accurate and efficient determination of glucosinolate content across diverse plant materials. Furthermore, it underscores the critical importance of the standardization and validation of these methodologies to ensure reliable glucosinolate analyses in both scientific research and industrial applications.

Kinetic Study and Modeling of Wild-Type and Recombinant Broccoli Myrosinase Produced in E. coli and S. cerevisiae as a Function of Substrate Concentration, Temperature, and pH

The myrosinase enzyme hydrolyzes glucosinolates, among which is glucoraphanin, the precursor of the anticancer isothiocyanate sulforaphane (SFN). The main source of glucoraphanin is Brassicaceae; however, its natural concentration is relatively low, limiting the availability of SFN. An option to obtain SFN is its exogenous production, through enzymatic processes and under controlled conditions, allowing complete conversion of glucoraphanin to SFN. We characterized the kinetics of wild-type (BMYR) and recombinant broccoli myrosinases produced in E. coli (EMYR) and S. cerevisiae (SMYR) in terms of the reaction conditions. Kinetics was adjusted using empirical and mechanistic models that describe reaction rate as a function of substrate concentration, temperature, and pH, resulting in R2 values higher than 90%. EMYR kinetics differed significantly from those of BMYR and SMYR probably due to the absence of glycosylations in the enzyme produced in E. coli. BMYR and SMYR were subjected to substrate inhibition but followed different kinetic mechanisms attributed to different glycosylation patterns. EMYR (inactivation Ea = 76.1 kJ/mol) was more thermolabile than BMYR and SMYR. BMYR showed the highest thermostability (inactivation Ea = 52.8 kJ/mol). BMYR and EMYR showed similar behavior regarding pH, with similar pK1 (3.4 and 3.1, respectively) and pK2 (5.4 and 5.0, respectively), but differed considerably from SMYR.

Synthesis and Evaluation of Functionalized Aryl and Biaryl Isothiocyanates Against Human MCF-7 Cells

Organic isothiocyanates (ITCs) are a class of anticancer agents which naturally result from the enzymatic degradation of glucosinolates produced by Brassica vegetables. Previous studies have demonstrated that the structure of an ITC impacts its potency and mode(s) of anticancer properties, opening the way to preparation and evaluation of synthetic, non-natural ITC analogues. This study describes the preparation of a library of 79 non-natural ITC analogues intended to probe further structure-activity relationships for aryl ITCs and second-generation, functionalized biaryl ITC variants. ITC candidates were subjected to bifurcated evaluation of antiproliferative and antioxidant response element (ARE)-induction capacity against human MCF-7 cells. The results of this study led to the identification of (1) several key structure-activity relationships and (2) lead ITCs demonstrating potent antiproliferative properties.

Kinetics of glucosinolate hydrolysis by myrosinase in Brassicaceae tissues: A high-performance liquid chromatography approach

Glucosinolates are a group of secondary metabolites occurring in all the vegetables belonging to the Brassicaceae family. Upon tissue damage, glucosinolates are hydrolyzed by myrosinase to a series of degradation products, including isothiocyanates, which are important for their health-promoting effects in humans. The glucosinolate-myrosinase system has been characterized in several Brassica species, of which white mustard (Sinapis alba) has been studied the most. In this study, a new HPLC-UV assay to evaluate the activities and kinetics of myrosinases in aqueous extracts, which closely represent the physiological conditions of plant tissues, was developed. This method was tested on myrosinases extracted from broccoli and cauliflower inflorescences, employing sinigrin and glucoraphanin as substrates. The results showed a strong inhibition of both enzymes at high substrate concentrations. The main issues related to kinetic analysis on the glucosinolate-myrosinase system were also elucidated.

Simultaneous Analysis of Glucosinolates and Isothiocyanates by Reversed-Phase Ultra-High-Performance Liquid Chromatography-Electron Spray Ionization-Tandem Mass Spectrometry

A new method to simultaneously analyze various glucosinolates (GSLs) and isothiocyanates (ITCs) by reversed-phase ultra-high-performance liquid chromatography-electron spray ionization-tandem mass spectrometry has been developed and validated for 14 GSLs and 15 ITCs. It involved derivatization of ITCs with N-acetyl-l-cysteine (NAC). The limits of detection were 0.4-1.6 μM for GSLs and 0.9-2.6 μM for NAC-ITCs. The analysis of Sinapis alba, Brassica napus, and Brassica juncea extracts spiked with 14 GSLs and 15 ITCs indicated that the method generally had good intraday (≤10% RSD) and interday precisions (≤16% RSD). Recovery of the method was unaffected by the extracts and within 71-110% for GSLs and 66-122% for NAC-ITCs. The method was able to monitor the enzymatic hydrolysis of standard GSLs to ITCs in mixtures. Furthermore, GSLs and ITCs were simultaneously determined in Brassicaceae plant extracts before and after myrosinase treatment. This method can be applied to further investigate the enzymatic conversion of GSLs to ITCs in complex mixtures.

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.

Probing the Effect of Bulky Lesion-Induced Replication Fork Conformational Heterogeneity Using 4-Aminobiphenyl-Modified DNA

Bulky organic carcinogens are activated in vivo and subsequently react with nucleobases of cellular DNA to produce adducts. Some of these DNA adducts exist in multiple conformations that are slowly interconverted to one another. Different conformations have been implicated in different mutagenic and repair outcomes. However, studies on the conformation-specific inhibition of replication, which is more relevant to cell survival, are scarce, presumably due to the structural dynamics of DNA lesions at the replication fork. It is difficult to capture the exact nature of replication inhibition by existing end-point assays, which usually detect either the ensemble of consequences of all the conformers or the culmination of all cellular behaviors, such as mutagenicity or survival rate. We previously reported very unusual sequence-dependent conformational heterogeneities involving FABP-modified DNA under different sequence contexts (TG₁*G₂T [67%B:33%S] and TG₁G₂*T [100%B], G*, N-(2′-deoxyguanosin-8-yl)-4′-fluoro-4-aminobiphenyl) (Cai et al. Nucleic Acids Research, 46, 6356–6370 (2018)). In the present study, we attempted to correlate the in vitro inhibition of polymerase activity to different conformations from a single FABP-modified DNA lesion. We utilized a combination of surface plasmon resonance (SPR) and HPLC-based steady-state kinetics to reveal the differences in terms of binding affinity and inhibition with polymerase between these two conformers (67%B:33%S and 100%B).

A Simple Method for On-Gel Detection of Myrosinase Activity

Myrosinase is an enzyme present in many functional foods and spices, particularly in Cruciferous vegetables. It hydrolyses glucosinolates which thereafter rearrange into bioactive volatile constituents (isothiocyanates, nitriles). We aimed to develop a simple reversible method for on-gel detection of myrosinase. Reagent composition and application parameters for native PAGE and SDS-PAGE gels were optimized. The proposed method was successfully applied to detect myrosinase (or sulfatase) on-gel: the detection solution contains methyl red which gives intensive red bands where the HSO₄^- is enzymatically released from the glucosinolates. Subsequently, myrosinase was successfully distinguished from sulfatase by incubating gel bands in a derivatization solution and examination by LC-ESI-MS: myrosinase produced allyl isothiocyanate (detected in conjugate form) while desulfo-sinigrin was released by sulfatase, as expected. After separation of 80 µg protein of crude extracts of Cruciferous vegetables, intensive color develops within 10 min. On-gel detection was found to be linear between 0.031⁻0.25 U (pure Sinapis alba myrosinase, R² = 0.997). The method was successfully applied to detection of myrosinase isoenzymes from horseradish, Cruciferous vegetables and endophytic fungi of horseradish as well. The method was shown to be very simple, rapid and efficient. It enables detection and partial characterization of glucosinolate decomposing enzymes without protein purification.

Hydrolysis before Stir-Frying Increases the Isothiocyanate Content of Broccoli

Broccoli is found to be a good source of glucosinolates, which can be hydrolyzed by endogenous myrosinase to obtain chemopreventive isothiocyanates (ITCs); among them, sulforaphane (SF) is the most important agent. Studies have shown that cooking greatly affects the levels of SF and total ITCs in broccoli. However, the stability of these compounds during cooking has been infrequently examined. In this study, we proved that the half-lives of SF and total ITCs during stir-frying were 7.7 and 5.9 min, respectively, while the myrosinase activity decreased by 80% after stir-frying for 3 min; SF and total ITCs were more stable than myrosinase. Thus, the contents of SF and total ITCs decreased during stir-frying largely because myrosinase was destroyed. Subsequently, it was confirmed that compared to direct stir-frying, hydrolysis of glucosinolates in broccoli for 90 min followed by stir-frying increased the SF and total ITC concentration by 2.8 and 2.6 times, respectively. This method provides large quantities of beneficial ITCs even after cooking.

Synthesis and spectral characterization of 2,2-diphenylethyl glucosinolate and HPLC-based reaction progress curve data for the enzymatic hydrolysis of glucosinolates by Sinapis alba myrosinase

The data presented in this article are related to the research article, "HPLC-based enzyme kinetics assay for glucosinolate hydrolysis facilitate analysis of systems with both multiple reaction products and thermal enzyme denaturation" (C.K. Klingaman, M.J. Wagner, J.R. Brown, J.B. Klecker, E.H. Pauley, C.J. Noldner, J.R. Mays,) [1]. This data article describes (1) the synthesis and spectral characterization data of a non-natural glucosinolate analogue, 2,2-diphenylethyl glucosinolate, (2) HPLC standardization data for glucosinolate, isothiocyanate, nitrile, and amine analytes, (3) reaction progress curve data for enzymatic hydrolysis reactions with variable substrate concentration, enzyme concentration, buffer pH, and temperature, and (4) normalized initial velocities of hydrolysis/formation for analytes. These data provide a comprehensive description of the enzyme-catalyzed hydrolysis of 2,2-diphenylethyl glucosinolate (5) and glucotropaeolin (6) under widely varied conditions.

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.

Myrosinase Compatible Simultaneous Determination of Glucosinolates and Allyl Isothiocyanate by Capillary Electrophoresis Micellar Electrokinetic Chromatography (CE-MEKC)

INTRODUCTION

The functional food Cruciferous vegetables contain glucosinolates which are decomposed by the myrosinase enzyme upon tissue damage. The isothiocyanates are the most frequent decomposition products. Because of their various bioactivities, these compounds and the myrosinase is of high interest to many scientific fields.

OBJECTIVE

Development of a capillary electrophoresis method capable of myrosinase-compatible, simultaneous quantification of glucosinolates and isothiocyanates.

METHODS

Capillary electrochromatography parameters were optimised, followed by optimisation of a myrosinase-compatible derivatisation procedure for isothiocyanates. Vegetable extracts (Brussels sprouts, horseradish, radish and watercress) were tested for myrosinase activity, glucosinolate content and isothiocyanate conversion rate. Allyl isothiocyanate was quantified in some food products.

RESULTS

The method allows quantification of sinigrin, gluonasturtiin and allyl isothiocyanate after myrosinase compatible derivatisation in-vial by mercaptoacetic acid. The chromatograhpic separation takes 2.5 min (short-end injection) or 15 min (long-end injection). For the tested vegetables, measured myrosinase activity was between 0.960-27.694 and 0.461-26.322 µmol/min/mg protein, glucosinolate content was between 0-2291.8 and 0-248.5 µg/g fresh weight for sinigrin and gluconastrutiin, respectively. The possible specificity of plants to different glucosinolates was also shown. Allyl isothiocyanate release rate was different in different vegetables (73.13 - 102.13%). The method could also be used for quantification of allyl isothiocyanate from food products.

CONCLUSIONS

The presented capillary electrophoresis method requires a minimal amount of sample and contains only a few sample preparation steps, and can be used in several applications (glucosinolate determination, myrosinase activity measurement, isothiocyanate release estimation). Copyright © 2016 John Wiley & Sons, Ltd.

De novo Transcriptome Analysis of Sinapis alba in Revealing the Glucosinolate and Phytochelatin Pathways

Sinapis alba is an important condiment crop and can also be used as a phytoremediation plant. Though it has important economic and agronomic values, sequence data, and the genetic tools are still rare in this plant. In the present study, a de novo transcriptome based on the transcriptions of leaves, stems, and roots was assembled for S. alba for the first time. The transcriptome contains 47,972 unigenes with a mean length of 1185 nt and an N50 of 1672 nt. Among these unigenes, 46,535 (97%) unigenes were annotated by at least one of the following databases: NCBI non-redundant (Nr), Swiss-Prot, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway, Gene Ontology (GO), and Clusters of Orthologous Groups of proteins (COGs). The tissue expression pattern profiles revealed that 3489, 1361, and 8482 unigenes were predominantly expressed in the leaves, stems, and roots of S. alba, respectively. Genes predominantly expressed in the leaf were enriched in photosynthesis- and carbon fixation-related pathways. Genes predominantly expressed in the stem were enriched in not only pathways related to sugar, ether lipid, and amino acid metabolisms but also plant hormone signal transduction and circadian rhythm pathways, while the root-dominant genes were enriched in pathways related to lignin and cellulose syntheses, involved in plant-pathogen interactions, and potentially responsible for heavy metal chelating, and detoxification. Based on this transcriptome, 14,727 simple sequence repeats (SSRs) were identified, and 12,830 pairs of primers were developed for 2522 SSR-containing unigenes. Additionally, the glucosinolate (GSL) and phytochelatin metabolic pathways, which give the characteristic flavor and the heavy metal tolerance of this plant, were intensively analyzed. The genes of aliphatic GSLs pathway were predominantly expressed in roots. The absence of aliphatic GSLs in leaf tissues was due to the shutdown of BCAT4, MAM1, and CYP79F1 expressions. Glutathione was extensively converted into phytochelatin in roots, but it was actively converted to the oxidized form in leaves, indicating the different mechanisms in the two tissues. This transcriptome will not only benefit basic research and molecular breeding of S. alba but also be useful for the molecular-assisted transfer of beneficial traits to other crops.