Determination of glucosinolates in domestic and wild mustard by high-performance liquid chromatography with confirmation by electrospray mass spectrometry and photodiode-array detection

Glucosinolates and Omega-3 Fatty Acids from Mustard Seeds: Phytochemistry and Pharmacology

Seeds from mustard (genera Brassica spp. and Sinapsis spp.), are known as a rich source of glucosinolates and omega-3 fatty acids. These compounds are widely known for their health benefits that include reducing inflammation and lowering the risk of cardiovascular diseases and cancer. This review presented a synthesis of published literature from Google Scholar, PubMed, Scopus, Sci Finder, and Web of Science regarding the different glucosinolates and omega-3 fatty acids isolated from mustard seeds. We presented an overview of extraction, isolation, purification, and structure elucidation of glucosinolates from the seeds of mustard plants. Moreover, we presented a compilation of in vitro, in vivo, and clinical studies showing the potential health benefits of glucosinolates and omega-3 fatty acids. Previous studies showed that glucosinolates have antimicrobial, antipain, and anticancer properties while omega-3 fatty acids are useful for their pharmacologic effects against sleep disorders, anxiety, cerebrovascular disease, neurodegenerative disease, hypercholesterolemia, and diabetes. Further studies are needed to investigate other naturally occurring glucosinolates and omega-3 fatty acids, improve and standardize the extraction and isolation methods from mustard seeds, and obtain more clinical evidence on the pharmacological applications of glucosinolates and omega-3 fatty acids from mustard seeds.

Current Knowledge and Challenges on the Development of a Dietary Glucosinolate Database in the United States

Glucosinolates (GSLs) are a group of cancer chemopreventive sulfur-containing compounds found primarily in Brassica vegetables. The goals of this study were to summarize the current knowledge and discuss the challenges of developing a dietary GSL database for US foods. A systematic literature search was conducted for the period 1980-2020. Thirty articles were found to meet all inclusion and exclusion criteria; 27 GSLs were reported in 16 different vegetables. GSLs identified and quantified ranged from 3 for winter cress to 16 for cabbage. In general, the experimental designs of these 30 studies did not fully consider the factors related to the data quality. Enormous variations of GSLs are observed between different vegetables and in the same vegetables. In conclusion, the studies on GSLs in commonly consumed vegetables are still limited, and some data may be outdated. Currently available data are not sufficient to develop a valid GSL database in the United States.

Relationship between mustard pungency and allyl-isothiocyanate content: A comparison of sensory and chemical evaluations

The correlation of sensory and chemically evaluated pungency of mustard products was investigated via a time-intensity (TI) study and quantification of allyl isothiocyanate (AITC) contents using high-performance liquid chromatography (HPLC). Sweet, medium hot, hot, and extra hot commercial mustard products from different brands were examined. Notably, we found significant differences (p < 0.05) between the maximum perceived pungency intensity of various mustard products. The maximum perceived intensity (I_max ), the duration of the decreasing phase (DUR_Dec ), and the area under the curve (AUC) values increased proportionally to the increase in the sample AITC content and were also higher in products classified as hot than in sweet mustards. The AITC concentration varied greatly between products from different brands and also between different sensory evaluated pungency levels. Furthermore, sensory evaluations and analytical results were correlated using regression analysis. The best correlation (correlation coefficient 0.891) was observed between the AITC concentration and AUC, when compared to that between the AITC concentration and DUR_Dec (correlation coefficient 0.856) or the I_max value (correlation coefficient 0.803). The calculated regression model indicates that a higher AITC content induces an intensified trigeminal pungency sensation and that the sensory and chemical evaluations of mustard products were positively correlated. Therefore, by using this regression model, the sensory rating of mustard products may be predicted by chemical analysis of the AITC contents. PRACTICAL APPLICATION: This research paper provides a method to quantify the pungency inducing irritant allyl isothiocyanate in commercial mustard products and demonstrates a correlation between sensory and chemical data. Therefore, the amounts of sensory tests in product quality assurance can be reduced and replaced or at least supported by chemical quantification of pungent substances (especially AITC) in mustard products.

Glucosinolates of Lepidium graminifolium L. (Brassicaceae) from Croatia

The glucosinolate (GSL) profiles (inflorescence, stem, root, and fruit) of the wild-growing plant Lepidium graminifolium L. (Brassicaceae) from Croatia was established by LC-MS analysis. During this investigation, we confirmed the presence of benzyl- (1), 3-methoxybenzyl- (2), 4-hydroxybenzyl- (4), 4-methoxyindol-3-ylmethyl- (7) GSLs and reported for the first time in the plant the presence of (2 R)-hydroxybut-3-enyl- (11), (2S)-hydroxybut-3-enyl- (12), but-3-enyl- (13), and 2-phenylethyl- (14) GSLs. Finally, 3-hydroxybenzyl GSL (3) was isolated for the first time from L. graminifolium inflorescence and characterised by spectroscopic data interpretation.

Antimicrobial and Cytotoxic Activities of Lepidium latifolium L. Hydrodistillate, Extract and Its Major Sulfur Volatile Allyl Isothiocyanate

The cultivated Lepidium latifolium L. was investigated to decipher its glucosinolate profile, antimicrobial, and cytotoxic activities. HPLC/ESI-MS analyses of the intact glucosinolates and GC/MS analysis of their hydrolysis products showed the presence of sinigrin (1), glucocochlearin (2), glucotropaeolin (3), and 4-methoxyglucobrassicin (4). Hydrodistillate, extract, and allyl isothiocyanate, the main volatile resulting from sinigrin degradation, showed antimicrobial activity against all eleven tested pathogenic and food spoilage bacteria and fungi, with highest effect observed against Candida albicans with MIC₅₀ 8 and 16 μg/mL. Hydrodistillate and extract showed the best cytotoxic activity on bladder cancer UM-UC-3 cell line during an incubation time of 24 h (IC₅₀ 192.9 and 133.8 μg/mL, respectively), while the best effect on glioblastoma LN229 cell line was observed after 48 h (IC₅₀ 110.8 and 30.9 μg/mL, respectively). Pure allyl isothiocyanate displayed a similar trend in cytotoxic effect on both cell lines (IC₅₀ 23.3 and 36.5 μg/mL after 24 h and 48 h, respectively).

Comparison of the Relative Potential for Epigenetic and Genetic Variation To Contribute to Trait Stability

The theoretical ability of epigenetic variation to influence the heritable variation of complex traits is gaining traction in the study of adaptation. This theory posits that epigenetic marks can control adaptive phenotypes but the relative potential of epigenetic variation in comparison to genetic variation in these traits is not presently understood. To compare the potential of epigenetic and genetic variation in adaptive traits, we analyzed the influence of DNA methylation variation on the accumulation of chemical defense compounds glucosinolates from the order Brassicales. Several decades of work on glucosinolates has generated extensive knowledge about their synthesis, regulation, genetic variation and contribution to fitness establishing this pathway as a model pathway for complex adaptive traits. Using high-throughput phenotyping with a randomized block design of ddm1 derived Arabidopsis thaliana epigenetic Recombinant Inbred Lines, we measured the correlation between DNA methylation variation and mean glucosinolate variation and within line stochastic variation. Using this information, we identified epigenetic Quantitative Trait Loci that contained specific Differentially Methylated Regions associated with glucosinolate traits. This showed that variation in DNA methylation correlates both with levels and variance of glucosinolates and flowering time with trait-specific loci. By conducting a meta-analysis comparing the results to different genetically variable populations, we conclude that the influence of DNA methylation variation on these adaptive traits is much lower than the corresponding impact of standing genetic variation. As such, selective pressure on these traits should mainly affect standing genetic variation to lead to adaptation.

Chromatographic column evaluation for the untargeted profiling of glucosinolates in cauliflower by means of ultra-high performance liquid chromatography coupled to high resolution mass spectrometry

The untargeted profiling is a promising approach for the characterization of secondary metabolites in biological matrices. Thanks to the recent rapid development of high-resolution mass spectrometry (HRMS) instrumentations, the number of applications by untargeted approaches for biological samples profiling has widely increased in the recent years. Despite the high potentialities of HRMS, however, a major issue in natural products analysis often arises in the upstream process of compounds separation. A separation technique is necessary to avoid phenomena such as signal suppression, and it is especially needed in the presence of isomeric metabolites, which are otherwise indistinguishable. Glucosinolates (GLSs), a group of secondary metabolites widely distributed among plants, resulted to be associated to the prevention of some serious diseases, such as cancer. This led to the development of several methods for the analysis of GLSs in vegetables tissues. The issue of GLSs chromatographic separation has been widely studied in the past because of the difficulty in the analysis of this highly polar and variable class of compounds. Several alternatives to reversed phase (RP) chromatography, sometimes not compatible with the coupling of liquid chromatography with mass spectrometry, have been tested for the analysis of intact GLSs. However, the availability of new stationary phases, in the last years, could allow the re-evaluation of RP chromatography for the analysis of intact GLSs. In this work, a thorough evaluation of four RP chromatographic columns for the analysis of GLSs in cauliflower (Brassica oleracea L. var. botrytis) extracts by an ultra-high performance liquid chromatographic system coupled via electrospray source to a hybrid quadrupole-Orbitrap mass spectrometer is presented. The columns tested were the following: one column Luna Omega polar C₁₈, one column Kinetex Biphenyl, one column Kinetex core-shell XB-C₁₈, two columns Kinetex core-shell XB-C₁₈. After a previous optimization of the extraction method, cauliflower extracts were analyzed testing four different mobile phases onto the four columns for a total of sixteen different chromatographic conditions. The chromatographic systems were evaluated based on the number of detected and tentatively identified GLSs. Luna Polar stationary phase resulted to be the most suitable for the analysis of GLSs compared to Kinetex XB and Kinetex Biphenyl columns stationary phase. However, two in series Kinetex XB columns increased the number of tentatively identified GLSs compared to one Kinetex XB, showing the importance of column length in the analysis of complex mixtures. The data obtained with the best chromatographic system were deeply analyzed by MS/MS investigation for the final identification. Fiflty-one GLSs were tentatively identified, 24 of which have never been identified in cauliflower. Finally the linearity of the analytes response over the analyzed range of concentration was checked, suggesting that the developed method is suitable for both qualitative and quantitative analysis of GLSs in phytochemical mixtures.

Analysis and Quantification of Glucosinolates

Recent advances in liquid chromatography and mass spectrometry have made it possible to increase the throughput of phytochemical analyses at high sensitivity. These improvements have made it more feasible to monitor metabolic processes at the metabolite level. Glucosinolates, the primary defense compounds in the model plant Arabidopsis thaliana, in particular, have gained increasing attention as model compounds for quantitative genetics and metabolic regulation. Depending on the plant species, tissue, glucosinolate content, complexity of the glucosinolate profile, and, most importantly, the overall purpose of the experiment, different choices need to be made regarding the methods of extraction and analysis. In this chapter, we describe different approaches for the analysis of glucosinolates and highlight advantages, disadvantages, and technical pitfalls. © 2016 by John Wiley & Sons, Inc.

The Glucosinolate Biosynthetic Gene AOP2 Mediates Feed-back Regulation of Jasmonic Acid Signaling in Arabidopsis

Survival in changing and challenging environments requires an organism to efficiently obtain and use its resources. Due to their sessile nature, it is particularly critical for plants to dynamically optimize their metabolism. In plant primary metabolism, metabolic fine-tuning involves feed-back mechanisms whereby the output of a pathway controls its input to generate a precise and robust response to environmental changes. By contrast, few studies have addressed the potential for feed-back regulation of secondary metabolism. In Arabidopsis, accumulation of the defense compounds glucosinolates has previously been linked to genetic variation in the glucosinolate biosynthetic gene AOP2. AOP2 expression can increase the transcript levels of two known regulators (MYB28 and MYB29) of the pathway, suggesting that AOP2 plays a role in positive feed-back regulation controlling glucosinolate biosynthesis. We generated mutants affecting AOP2, MYB28/29, or both. Transcriptome analysis of these mutants identified a so far unrecognized link between AOP2 and jasmonic acid (JA) signaling independent of MYB28 and MYB29. Thus, AOP2 is part of a regulatory feed-back loop linking glucosinolate biosynthesis and JA signaling and thereby allows the glucosinolate pathway to influence JA sensitivity. The discovery of this regulatory feed-back loop provides insight into how plants optimize the use of resources for defensive metabolites.

Long-chain Glucosinolates from Arabis turrita: Enzymatic and Non-enzymatic Degradations

C8-C10 methylsulfinylalkyl glucosinolates (GLs), and C8-C10 methylsulfonylalkyl GLs were identified in the seed of Arabis turrita L. by HPLC-MS/ESI analysis of intact GLs. Enzymatic (with myrosinase) and non-enzymatic (thermal at 100°C, and chemical at different pH) hydrolyses were performed and the volatile isolates were analyzed by GC-MS. Only the enzymatic and chemical (pH 10) degradations produced volatiles which are originating from GL degradation. GC-MS analysis showed the presence of long-chain olefinic isothiocyanates (ITCs) along with other the long-chain thiofunctionalized GL breakdown products.

Characterization of mustard seeds and paste by DART ionization with time-of-flight mass spectrometry

Direct analysis in real time (DART) is a novel technique with great potential for rapid screening analysis. The DART ionization method coupled with high-resolution time-of-flight mass spectrometry (TOF-MS) has been used for characterization of mustard seeds and table mustard. The possibility to use DART to analyse glucosinolates was confirmed on determination of sinalbin (4-hydroxybenzyl glucosinolate). The DART-TOF-MS method was optimized and validated. A set of samples of mustard seeds and mustard products was analyzed. High-performance liquid chromatography and DART-TOF-MS were used to determine glucosinolates in mustard seeds and compared. The correlation equation between these methods was DART = 0.797*HPLC + 6.987, R(2)  = 0.972. The DART technique seems to be a suitable method for evaluation of the quality of mustard seeds and mustard products.

Integration of biosynthesis and long-distance transport establish organ-specific glucosinolate profiles in vegetative Arabidopsis

Although it is essential for plant survival to synthesize and transport defense compounds, little is known about the coordination of these processes. Here, we investigate the above- and belowground source-sink relationship of the defense compounds glucosinolates in vegetative Arabidopsis thaliana. In vivo feeding experiments demonstrate that the glucosinolate transporters1 and 2 (GTR1 and GTR2), which are essential for accumulation of glucosinolates in seeds, are likely to also be involved in bidirectional distribution of glucosinolates between the roots and rosettes, indicating phloem and xylem as their transport pathways. Grafting of wild-type, biosynthetic, and transport mutants show that both the rosette and roots are able to synthesize aliphatic and indole glucosinolates. While rosettes constitute the major source and storage site for short-chained aliphatic glucosinolates, long-chained aliphatic glucosinolates are synthesized both in roots and rosettes with roots as the major storage site. Our grafting experiments thus indicate that in vegetative Arabidopsis, GTR1 and GTR2 are involved in bidirectional long-distance transport of aliphatic but not indole glucosinolates. Our data further suggest that the distinct rosette and root glucosinolate profiles in Arabidopsis are shaped by long-distance transport and spatially separated biosynthesis, suggesting that integration of these processes is critical for plant fitness in complex natural environments.

Rapid profiling of intact glucosinolates in Arabidopsis leaves by UHPLC-QTOFMS using a charged surface hybrid column

INTRODUCTION

The analysis of glucosinolates (GS) is traditionally performed by reverse-phase liquid chromatography coupled to ultraviolet detection after a time-consuming desulphation step, which is required for increased retention. Simpler and more efficient alternative methods that can shorten both sample preparation and analysis are much needed.

OBJECTIVE

To evaluate the feasibility of using ultrahigh-pressure liquid chromatography coupled to quadrupole time-of-flight mass spectrometry (UHPLC-QTOFMS) for the rapid profiling of intact GS.

METHODOLOGY

A simple and short extraction of GS from Arabidopsis thaliana leaves was developed. Four sub-2 µm reverse-phase columns were tested for the rapid separation of these polar compounds using formic acid as the chromatographic additive. High-resolution QTOFMS was used to detect and identify GS.

RESULTS

A novel charged surface hybrid (CSH) column was found to provide excellent retention and separation of GS within a total running time of 11 min. Twenty-one GS could be identified based on their accurate mass as well as isotopic and fragmentation patterns. The method was applied to determine the changes in GS content that occur after herbivory in Arabidopsis. In addition, we evaluated its applicability to the profiling of other Brassicaceae species.

CONCLUSION

The method developed can profile the full range of GS, including the most polar ones, in a shorter time than previous methods, and is highly compatible with mass spectrometric detection.

Composition of Dithyrea wislizenii fruit extract and free-radical scavenging activity of its constituents

Glucolesquerellin (2), glucohesperin (3), quercetin 3-O-sophoroside (4), and quercetin 3-O-sophoroside-7-O-glucoside (5), isolated from the fruit of Dithyrea wislizenii , were quantified by HPLC. The fruit extract and flavonoids were not found to be toxic by using a brine shrimp lethality assay. The fruit extract and the flavonoids and glucosinolates were submitted to a free-radical scavenging activity assay with the diphenylpicrylhydrazyl radical (DPPH•). The concentration of quercetin (6) (a positive control for the flavonoids) able to scavenge 50% of DPPH• (SC50) was 32 ± 2 µmol/L (or 4 ± 1 µg/mL), which was about 27 times more potent than the crude extract. Compounds 4 and 5 had a SC50, the concentration of the compound required to scavenge 50% of the DPPH•, of 78 ± 1 µmol/L and 113 ± 10 µmol/L, respectively. The positive control for the glucosinolates, glucoraphasatin, (1) had a SC50 of 1768 ± 60 µmol/L. The glucosinolates 2 and 3 had a SC50 of 7819 ± 1968 and 970 ± 63 µmol/L, respectively.

Inhibition of Listeria monocytogenes on bologna sausages by an antimicrobial film containing mustard extract or sinigrin

The ability of Listeria (L.) monocytogenes to convert glucosinolates into antimicrobial isothiocyanates was investigated. Mustard glucosinolates in pure (sinigrin) or extract forms (sinigrin, oriental; sinalbin, yellow mustard) were used in broth media and in a polyvinyl polyethylene glycol graft copolymer (PPG) packaging film with bologna to examine their value as antimicrobial precursors for the control of L. monocytogenes viability and extension of bologna shelf-life at 4 °C. During broth tests with deodorized (myrosinase-inactivated) mustard extracts (10 d at 20 °C) or with purified sinigrin (21 d at 20 °C) L. monocytogenes was only inhibited when exogenous myrosinase was added. None the less, the organism was able to hydrolyze almost half the pure sinigrin by 21 d in tests without added enzyme. Reductions in sinigrin levels were measured by reversed-phase liquid chromatography, and in the absence of L. monocytogenes or added myrosinase the glucosinolate was stable. When pure sinigrin, oriental or yellow mustard extracts were incorporated in PPG films containing 3, 5 and 6% (w/w) of the corresponding glucosinolate and used to package bologna inoculated with 4 log CFU/g L. monocytogenes, the pathogen became undetectable in bologna packed with the oriental mustard extract at 52 d storage and remained undetectable at 70 d. The yellow mustard extract was less inhibitory and the pure sinigrin was not antimicrobial. L. monocytogenes numbers reached >7 log CFU/g in the film and untreated controls at 17 d storage. At 35 d storage, samples packed with control film contained sufficient numbers of lactic acid bacteria (LAB) (>7 log CFU/g) to be considered spoiled, whereas treatments containing mustard or sinigrin remained <7 log CFU/g LAB for ≤ 70 d. L. monocytogenes played a key role in exerting control over its own viability in bologna by hydrolysis of the glucosinolate in the oriental mustard film, but other antimicrobials in treatments may have contributed.

Stability and antimicrobial activity of allyl isothiocyanate during long-term storage in an oil-in-water emulsion

This study investigated the stability and antimicrobial activity of allyl isothiocyanate (AITC) in medium chain triglyceride (MCT) or soybean oil (SBO) dispersed in an oil-in-water (o/w) system during long-term storage. Oil type, content, and oxidative stability affect the stability and antimicrobial activity of AITC during storage. High oil content is favorable for AITC stability in the emulsion. Notably, AITC with MCT is more stable than AITC with SBO with the same oil content. Consequently, AITC with MCT is more effective than AITC with SBO in inhibiting G(-) bacteria (E. coli O157:H7, Salmonella enterica, and Vibrio parahaemolyticus) and G(+) bacteria (Staphylococcus aureus and Listeria monocytogenes).

Collision-induced dissociation mass spectra of glucosinolate anions

Collision-induced dissociation (CID) mass spectra of differently substituted glucosinolates were investigated under negative-ion mode. Data obtained from several glucosinolates and their isotopologues ((34)S and (2)H) revealed that many peaks observed are independent of the nature of the substituent group. For example, all investigated glucosinolate anions fragment to produce a product ion observed at m/z 195 for the thioglucose anion, which further dissociates via an ion/neutral complex to give two peaks at m/z 75 and 119. The other product ions observed at m/z 80, 96 and 97 are characteristic for the sulfate moiety. The peaks at m/z 259 and 275 have been attributed previously to glucose 1-sulfate anion and 1-thioglucose 2-sulfate anion, respectively. However, based on our tandem mass spectrometric experiments, we propose that the peak at m/z 275 represents the glucose 1-thiosulfate anion. In addition to the common peaks, the spectrum of phenyl glucosinolate (beta-D-Glucopyranose, 1-thio-, 1-[N-(sulfooxy)benzenecarboximidate] shows a substituent-group-specific peak at m/z 152 for C(6)H(5)-C(=NOH)S(-), the CID spectrum of which was indistinguishable from that of the anion of synthetic benzothiohydroxamic acid. Similarly, the m/z 201 peak in the spectrum of phenyl glucosinolate was attributed to C(6)H(5)-C(=S)OSO(2)(-).

Phytochemical constituents of Cardamine diphylla

From the methanol extract of the Cardamine diphylla rhizome, methylethyl- (1), 2-methylbutyl- (2), 3-methylpentyl- (4), 3-indolylmethyl- (5), 1-methoxy-3-indolylmethyl- (6), 4-methoxy-3-indolylmethyl- (7) glucosinolates, and desulfo-2-methylbutylglucosinolate (3) were isolated. The structure elucidation of the compounds was performed by spectroscopic methods. The toxicity on brine shrimp larvae of the methanol extract of the C. diphylla rhizome was evaluated. In addition, the free-radical-scavenging activity of the crude extract was carried out by the 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay.

Updated glucosinolate profile of Dithyrea wislizenii

Fruit extracts of Dithyrea wislizenii were analyzed for desulfoglucosinolates and intact glucosinolates using HPLC-APCI-MS and HPLC-ESI-MS, respectively. 2-Propenylglucosinolate (sinigrin) was shown to be present in the extracts. 6-Methylsulfanylhexyl- (glucolesquerellin 9), 6-methylsulfinylhexyl- (glucohesperin 10), 7-methylsulfanylheptyl- (11), and 5-methylsulfanylpentylglucosinolate (glucoberteroin 12) were isolated from the extracts and characterized by NMR and MS data. 7-Methoxyglucobrassicin was not detected in D. wislizenii extracts.

The genetic basis of constitutive and herbivore-induced ESP-independent nitrile formation in Arabidopsis

Glucosinolates are a group of thioglucosides that are components of an activated chemical defense found in the Brassicales. Plant tissue damage results in hydrolysis of glucosinolates by endogenous thioglucosidases known as myrosinases. Spontaneous rearrangement of the aglucone yields reactive isothiocyanates that are toxic to many organisms. In the presence of specifier proteins, alternative products, namely epithionitriles, simple nitriles, and thiocyanates with different biological activities, are formed at the expense of isothiocyanates. Recently, simple nitriles were recognized to serve distinct functions in plant-insect interactions. Here, we show that simple nitrile formation in Arabidopsis (Arabidopsis thaliana) ecotype Columbia-0 rosette leaves increases in response to herbivory and that this increase is independent of the known epithiospecifier protein (ESP). We combined phylogenetic analysis, a screen of Arabidopsis mutants, recombinant protein characterization, and expression quantitative trait locus mapping to identify a gene encoding a nitrile-specifier protein (NSP) responsible for constitutive and herbivore-induced simple nitrile formation in Columbia-0 rosette leaves. AtNSP1 is one of five Arabidopsis ESP homologues that promote simple nitrile, but not epithionitrile or thiocyanate, formation. Four of these homologues possess one or two lectin-like jacalin domains, which share a common ancestry with the jacalin domains of the putative Arabidopsis myrosinase-binding proteins MBP1 and MBP2. A sixth ESP homologue lacked specifier activity and likely represents the ancestor of the gene family with a different biochemical function. By illuminating the genetic and biochemical bases of simple nitrile formation, our study provides new insights into the evolution of metabolic diversity in a complex plant defense system.

Rapid screening method for intact glucosinolates in Chinese medicinal herbs by using liquid chromatography coupled with electrospray ionization ion trap mass spectrometry in negative ion mode

An optimized method using liquid chromatography coupled with electrospray ionization ion trap mass spectrometry (LC/ESI-ITMS) in negative ion mode has been developed for screening different structural classes of intact glucosinolates in six Chinese medicinal herbs. The glucosinolates were extracted with hot methanol/water (70:30 v/v) and separation of the individual glucosinolates was achieved using a reversed-phase C18 column with an aqueous ammonium acetate/methanol gradient. Identification of the intact glucosinolates was based on the detection of compounds with a constant neutral loss of 242 Da corresponding to the combined loss of anhydroglucose (162 Da) and sulfur trioxide (80 Da) in collision-induced dissociation. The structures of the identified glucosinolates were confirmed with the use of group-specific product ions at m/z 195, 241, 259, 275 in their corresponding MS/MS product ion spectra. Differentiation of intact glucosinolates was achieved through their respective retention times and molecular masses as well as the characteristic product ions. The limits of detection were at the low nanogram level per injection, based on constant neutral loss scans. Significant variation in the compositions of intact glucosinolates was identified in the cruciferous herbs. This method was applied in the differentiation and quality control of two pairs of easily confused herbs.

Identification of desulphoglucosinolates in Brassicaceae by LC/MS/MS: comparison of ESI and atmospheric pressure chemical ionisation-MS

In order to develop a sensitive method for the detection of desulphoglucosinolates by HPLC-MS, the two most common interfaces for HPLC-MS, atmospheric pressure chemical ionisation (APCI) and ESI, were compared. While working with the APCI-interface the evaporation temperature and corona amperage were optimised. In doing so 300 degrees C and 6 muA proved to be most suitable for aliphatic and indole desulphoglucosinolates. The use of formic acid instead of water in the eluent in HPLC-ESI-MS measurements increased the sensitivity for the indole desulphoglucosinolates in the presence of 1 mM formic acid, while the sensitivity for the aliphatic desulphoglucosinolate desulphoglucoraphanin was substantially increased by the presence of 5 mM formic acid. Using an Agilent ion trap, two optimisation procedures for the MS parameters, smart and expert mode, were available. In smart mode the software optimises several parameters automatically, which is much more time efficient than expert mode, in which the optimisation is done manually. It turned out that ESI-MS is most sensitive in smart mode, while for APCI-MS a higher sensitivity could be gained using the expert mode. Comparing both interfaces, APCI-MS was more sensitive than ESI-MS. However, no additional information, in terms of structure determination, was obtained by APCI-MS.

Extraction and analysis of intact glucosinolates--a validated pressurized liquid extraction/liquid chromatography-mass spectrometry protocol for Isatis tinctoria, and qualitative analysis of other cruciferous plants

Glucosinolates have attracted significant interest due to the chemopreventive properties of some of their transformation products. Numerous protocols for the extraction and analysis of glucosinolates have been published, but limited effort has been devoted to optimize and validate crucial extraction parameters and sample preparation steps. We carried out a systematic optimization and validation of a quantitative assay for the direct analysis of intact glucosinolates in Isatis tinctoria leaves (woad, Brassicaceae). Various parameters such as solvent composition, particle size, temperature, and number of required extraction steps were optimized using pressurized liquid extraction (PLE). We observed thermal degradation of glucosinolates at temperatures above 50 degrees C, and loss of >60% within 10min at 100 degrees C, but no enzymatic degradation in the leaf samples at ambient temperature. Excellent peak shape and resolution was obtained by reversed-phase chromatography on a Phenomenex Aqua column using 10mM ammonium formate as ion-pair reagent. Detection was carried out by electrospray ionisation mass spectrometry in the negative ion mode. Analysis of cruciferous vegetables and spices such as broccoli (Brassica oleracea L. var. italica), garden cress (Lepidium sativum L.) and black mustard (Sinapis nigra L.) demonstrated the general applicability of the method.

Glucosinolate hydrolysis in Lepidium sativum--identification of the thiocyanate-forming protein

Glucosinolates are a class of thioglycosides found predominantly in plants of the order Brassicales whose function in anti-herbivore defense has been attributed to the products formed by myrosinase-catalyzed hydrolysis upon plant tissue damage. The most common type of hydrolysis products, the isothiocyanates, are toxic to a wide range of organisms. Depending on the glucosinolate side-chain structure and the presence of certain protein factors, other types of hydrolysis products, such as simple nitriles, epithionitriles and organic thiocyanates, can be formed whose biological functions are not well understood. Of the proteins controlling glucosinolate hydrolysis, only epithiospecifier proteins (ESPs) that promote the formation of simple nitriles and epithionitriles have been identified on a molecular level. We investigated glucosinolate hydrolysis in Lepidium sativum and identified a thiocyanate-forming protein (TFP) that shares 63-68% amino acid sequence identity with known ESPs and up to 55% identity with myrosinase-binding proteins from Arabidopsis thaliana, but differs from ESPs in its biochemistry. TFP does not only catalyze thiocyanate and simple nitrile formation from benzylglucosinolate but also the formation of simple nitriles and epithionitriles from aliphatic glucosinolates. Analyses of glucosinolate hydrolysis products in L. sativum autolysates and TFP transcript accumulation revealed an organ-specific regulation of thiocyanate formation. The identification of TFP defines a new family of proteins that control glucosinolate hydrolysis and challenges the previously proposed reaction mechanism of epithionitrile formation. As a protein that promotes the formation of a wide variety of hydrolysis products, its identification provides an important tool for further elucidating the mechanisms of glucosinolate hydrolysis as well as the ecological role and the evolutionary origin of the glucosinolate-myrosinase system.

Naturally occurring glucosinolates in plant extracts of rocket salad (Eruca sativa L.) identified by liquid chromatography coupled with negative ion electrospray ionization and quadrupole ion-trap mass spectrometry

A method for the comprehensive profiling of intact glucosinolates (GLSs), major and minor, occurring in leaves and seeds of rocket salad (Eruca sativa L.) is presented using optimized reversed-phase liquid chromatography (RP-LC) with electrospray ionization (ESI) ion trap mass spectrometry (ITMS). ESI-ITMS in the negative mode was confirmed to be very suitable to analyze these compounds in crude extracts. After extraction from the plant material with methanol/water (70:30 v/v) at 70 degrees C, the analytes of interest were separated on a C18 column using an eluent acidified with formic acid (0.1%) and modified with acetonitrile. All the GLSs found in leaves of rocket salad gave good signals corresponding to the deprotonated precursor ion, [M-H]-. Although the mass spectra also exhibited an analytically important non-covalent adduct ion at [2M-H]-, the structures of glucosinolates were confirmed by extensive sequential MS analysis, thereby substantially improving the identification of unknown compounds. The results obtained not only revealed in leaves of E. sativa at least twelve species of GLSs including seven aliphatic compounds (glucoraphanin with [M-H]- at m/z ratio of 436, glucoerucin at m/z 420, 4-mercaptobutyl-GLS at m/z 406, progoitrin/epiprogoitrin at m/z 388, sinigrin at m/z 358, 4-methylpentyl- and n-hexyl-GLS at m/z 402) and three indole glucosinolates (i.e., three N-heterocyclic compounds: 4-hydroxyglucobrassicin and 5-hydroxyglucobrassicin at m/z 463, and 4-methoxy-glucobrassicin at m/z 477), but also two structurally related compounds containing one intermolecular disulfide linkage (4-(beta-D-glucopyranosyldisulfanyl)butyl-GLS at m/z 600 and a dimeric 4-mercaptobutyl-GLS at m/z 811). This latter symmetric disulfide was previously considered as an artefact formed during extraction of GLSs from vegetative tissues. Glucosinolates were detected in the leaves with a wide range of contents (10-200 micromol/g) and a great variation in the composition. Only three GLSs were identified in seeds of rocket salad, namely glucoraphanin, glucoerucin and 4-methoxyglucobrassicin. As expected, the most abundant GLS in seeds is glucoerucin. The feasibility of the strategy was also demonstrated using a rapeseed extract of certified reference material (BCR367R). The results indicated the usefulness of this method for a rapid, sensitive and comprehensive profiling of the GLS family naturally occurring in extracts of crude plant matter.

Comparative biochemical characterization of nitrile-forming proteins from plants and insects that alter myrosinase-catalysed hydrolysis of glucosinolates

The defensive function of the glucosinolate-myrosinase system in plants of the order Capparales results from the formation of isothiocyanates when glucosinolates are hydrolysed by myrosinases upon tissue damage. In some glucosinolate-containing plant species, as well as in the insect herbivore Pieris rapae, protein factors alter the outcome of myrosinase-catalysed glucosinolate hydrolysis, leading to the formation of products other than isothiocyanates. To date, two such proteins have been identified at the molecular level, the epithiospecifier protein (ESP) from Arabidopsis thaliana and the nitrile-specifier protein (NSP) from P. rapae. These proteins share no sequence similarity although they both promote the formation of nitriles. To understand the biochemical bases of nitrile formation, we compared some of the properties of these proteins using purified preparations. We show that both proteins appear to be true enzymes rather than allosteric cofactors of myrosinases, based on their substrate and product specificities and the fact that the proportion of glucosinolates hydrolysed to nitriles does not remain constant when myrosinase activity varies. No stable association between ESP and myrosinase could be demonstrated during affinity chromatography, nevertheless some proximity of ESP to myrosinase is required for epithionitrile formation to occur, as evidenced by the lack of ESP activity when it was spatially separated from myrosinase in a dialysis chamber. The significant difference in substrate- and product specificities between A. thaliana ESP and P. rapae NSP is consonant with their different ecological functions. Furthermore, ESP and NSP differ remarkably in their requirements for metal ion cofactors. We found no indications of the involvement of a free radical mechanism in epithionitrile formation by ESP as suggested in earlier reports.

Analysis of glucosinolates, isothiocyanates, and amine degradation products in vegetable extracts and blood plasma by LC–MS/MS

Dietary glucosinolates are under intensive investigation as precursors of cancer-preventive isothiocyanates. Quantitation of the dose and bioavailability of glucosinolates and isothiocyanates requires a comprehensive analysis of the major dietary glucosinolates, isothiocyanates, and related metabolites. We report a liquid chromatography with tandem mass spectrometric detection (LC-MS/MS) analytical method for the comprehensive analysis of the seven major dietary glucosinolates, related isothiocyanates, and putative amine degradation products. The parent glucosinolates were sinigrin, gluconapin, progoitrin, glucoiberin, glucoraphanin, glucoalyssin, and gluconasturtiin. The LC-MS/MS analysis method for these compounds was developed and validated; a standard addition analysis protocol was used generally to avoid the requirement for stable isotopic standards. Where stable isotopic standards were available, internal standardization with these gave estimates in agreement with those obtained by the standard addition analysis protocol. For glucosinolates, negative ion electrospray LC-MS/MS analysis was performed. Isothiocyanates and amines were prederivatized to the corresponding thiourea and N-acetamides, respectively, and were quantified by positive ion electrospray LC-MS/MS. The limits of detection were 0.5-2 pmol; the recoveries for glucosinolates, isothiocyanates, and amines were 85-90%, 50-85%, and 60-70%, respectively; and the intra- and interbatch coefficients of variation were 1-4% and 3-10%, respectively. These methods provide facile access to comprehensive analytical data on the major dietary glucosinolates and related metabolites to quantify inputs and metabolic formation of these compounds in cancer prevention and related studies.

Analysis of the glucosinolate pattern ofArabidopsis thalianaseeds by capillary zone electrophoresis coupled to electrospray ionization-mass spectrometry

An easy and rapid method for the analysis of intact, non-desulfated glucosinolates by capillary zone electrophoresis (CZE) coupled to electrospray ionization-time of flight-mass spectrometry (ESI-TOF-MS) is described. Surprisingly, an electrolyte and a sheath liquid based on formic acid provided the best results. In this strongly acidic system, the glucosinolates were separated and detected as anions, resulting in an excellent selectivity. Thus, crude plant extracts could be analyzed without any interference of matrix constituents. The sensitivity together with mass accuracy and true isotopic pattern of the TOF-MS allowed identification of a broad series of glucosinolates in Arabidopsis thaliana seeds.

Successful herbivore attack due to metabolic diversion of a plant chemical defense

Plants protect themselves against herbivory with a diverse array of repellent or toxic secondary metabolites. However, many herbivorous insects have developed counteradaptations that enable them to feed on chemically defended plants without apparent negative effects. Here, we present evidence that larvae of the specialist insect, Pieris rapae (cabbage white butterfly, Lepidoptera: Pieridae), are biochemically adapted to the glucosinolate-myrosinase system, the major chemical defense of their host plants. The defensive function of the glucosinolate-myrosinase system results from the toxic isothiocyanates that are released when glucosinolates are hydrolyzed by myrosinases on tissue disruption. We show that the hydrolysis reaction is redirected toward the formation of nitriles instead of isothiocyanates if plant material is ingested by P. rapae larvae, and that the nitriles are excreted with the feces. The ability to form nitriles is due to a larval gut protein, designated nitrile-specifier protein, that by itself has no hydrolytic activity on glucosinolates and that is unrelated to any functionally characterized protein. Nitrile-specifier protein appears to be the key biochemical counteradaptation that allows P. rapae to feed with impunity on plants containing glucosinolates and myrosinases. This finding sheds light on the ecology and evolution of plant-insect interactions and suggests novel highly selective pest management strategies.

Probing crucial metabolic pathways in fungal pathogens of crucifers: biotransformation of indole-3-acetaldoxime, 4-hydroxyphenylacetaldoxime, and their metabolites

Indole-3-acetaldoxime is an intermediate of crucial importance in the biosynthesis of diverse plant secondary metabolites of Cruciferae. The metabolism of indole-3-acetaldoxime to indole-3-acetic acid via indole-3-acetonitrile by fungi that cause important plant diseases in crucifers, Leptosphaeria maculans (asexual stage Phoma lingam) causative agent of blackleg disease, Rhizoctonia solani causative agent of root rot disease, and Sclerotinia sclerotiorum causative agent of stem rot disease, is described. As well, the antifungal activity of indole-3-acetaldoxime and metabolites and the synthesis and biotransformation of 4-hydroxyphenylacetaldoxime by the same plant pathogens and by an insect fungal pathogen, Beauveria bassiana, are reported.

Intact glucosinolate analysis in plant extracts by programmed cone voltage electrospray LC/MS: performance and comparison with LC/MS/MS methods

We present a comprehensive, sensitive, and highly specific negative ion electrospray LC/MS method for identifying all structural classes of glucosinolates in crude plant extracts. The technique is based on the observation of simultaneous maxima in the abundances of the m/z 96 and 97 ions, generated by programmed cone voltage fragmentation, in the mass chromatogram. The abundance ratios lie in the range 1:2-1:4 ([m/z 96]/[m/z 97]). Examination of the corresponding full-scan mass spectra allows individual glucosinolates of all structural classes to be identified rapidly and with confidence. The use of linearly programmed cone voltage fragmentation enhances characteristic fragment ions without compromising the abundance of the analytically important [M - H]- ion and its associated (and analytically useful) sulfur isotope peaks. Detection limits are in the low nanogram range for full-scan, programmed cone voltage spectra. Comparison of the technique with LC/MS/MS methods (product ion, precursor ion, and constant neutral loss scans) has shown that the sensitivity and selectivity of the programmed cone voltage method is superior. Data obtained on a variety of plant extracts confirmed that the methodology was robust and reliable.