Derivatization of isothiocyanates and their reactive adducts for chromatographic analysis

The multi-factorial modes of action of urease in the pathogenesis of incontinence associated dermatitis

Background

Incontinence Associated Dermatitis (IAD) is a type of skin inflammation caused by chronic exposure to urine and/or faeces. Current treatment strategies involve creating a barrier between the skin and urine/faeces rather than targeting specific irritants. Urease expressing pathogens catalyse the conversion of urea, present in urine, into ammonia. The accumulation of ammonia causes an elevation in skin pH which is believed to activate faecal enzymes which damage skin, and opportunistic pathogens, which lead to secondary infections.

Objectives

To develop a better, multi-factorial model of IAD pathogenesis, including the effect of urease-expressing bacteria on skin, mechanism of damage of urease and urease-triggered activity of faecal enzymes and secondary pathogens. To study the effect of urease inhibition on preventing IAD skin damage.

Methods

Five separate studies were made using ex vivo porcine skin and in vivo human skin models. Measurements of the change in skin barrier function were made using skin impedance, trans-epidermal water loss (TEWL), stratum corneum moisture and pH. Skin was exposed to artificial urine, inoculated with various microbes, enzymes and chemicals to examine the influence of: 1) urease-positive Proteus mirabilis 2) ammonia, 3) combination of P. mirabilis and a faecal enzyme, trypsin, 4) combination of P. mirabilis and opportunistic pathogens, Candida albicans and Staphylococcus aureus, 5) inhibition of urease using acetohydroxamic acid (AHA) on barrier function.

Results

The urease-mediated production of ammonia had two principal effects: it elevated skin pH and caused inflammation, leading to significant breakdown in skin (stratum corneum) barrier function. Urease was found to further increase the activity of faecal enzymes and opportunistic pathogens, due to elevated skin pH. The urease inhibitor, AHA, was shown to have significantly reduced damage to skin barrier function, measured as its electrical resistance.

Conclusions

Targeted therapeutic strategies should be developed to prevent the manifestation of IAD, rather than creating a generic barrier between skin and urine/faeces. Urease has been identified as a crucial component in the manifestation of IAD, due to its role in the production of ammonia. Urease inhibition provides a promising therapeutic target to halt the progression of IAD.

Polyphenolics, glucosinolates and isothiocyanates profiling of aerial parts of Nasturtium officinale (Watercress)

Watercress (Nasturtium officinale) is a rich source of secondary metabolites with disease-preventing and/or health-promoting properties. Herein, we have utilized extraction procedures to isolate fractions of polyphenols, glucosinolates and isothiocyanates to determine their identification, and quantification. In doing so, we have utilized reproducible analytical methodologies based on liquid chromatography with tandem mass spectrometry by either positive or negative ion mode. Due to the instability and volatility of isothiocyanates, we followed an ammonia derivatization protocol which converts them into respective ionizable thiourea derivatives. The analytes' content distribution map was created on watercress flowers, leaves and stems. We have demonstrated that watercress contains significantly higher levels of gluconasturtiin, phenethyl isothiocyanate, quercetin-3-O-rutinoside and isorhamnetin, among others, with their content decreasing from flowers (82.11 ± 0.63, 273.89 ± 0.88, 1459.30 ± 12.95 and 289.40 ± 1.37 ng/g of dry extract respectively) to leaves (32.25 ± 0.74, 125.02 ± 0.52, 1197.86 ± 4.24 and 196.47 ± 3.65 ng/g of det extract respectively) to stems (9.20 ± 0.11, 64.7 ± 0.9, 41.02 ± 0.18, 65.67 ± 0.84 ng/g of dry extract respectivbely). Pearson's correlation analysis has shown that the content of isothiocyanates doesn't depend only on the bioconversion of individual glucosinolates but also on other glucosinolates of the same group. Overall, we have provided comprehensive analytical data of the major watercress metabolites thereby providing an opportunity to exploit different parts of watercress for potential therapeutic applications.

Simultaneous analysis of derivatized allyl isothiocyanate together with its phase II metabolites by ultra-high-performance liquid chromatography/tandem mass spectrometry

RATIONALE

Allyl isothiocyanate (AITC) in food products such as wasabi is commonly analyzed by gas chromatography/mass spectrometry (GC/MS), while liquid chromatography/mass spectrometry (LC/MS) is more suitable for the polar metabolites. The development of an effective method for the simultaneous determination of AITC and its phase II metabolites is needed to support drug metabolism and pharmacokinetics (DMPK) studies.

METHODS

Derivatization of AITC by reaction with N-(tert-butoxycarbonyl)-L-cysteine methyl ester (^t BocCysME) was used for ultra-high-performance liquid chromatography/electrospray ionization mass spectrometry (UHPLC/ESI-MS) analysis, allowing the simultaneous determination with its phase II metabolites. At room temperature, reaction conditions have been optimized to maximize the reaction of AITC with ^t BocCysME quantitively. Reaction selectivity was examined by the presence of AITC metabolites. Quantification of AITC was performed by multiple reaction monitoring (MRM) with an external calibration method and applied on the serum of C57BL/6 J mouse for a DMPK study.

RESULTS

The limit of quantification (LOQ) was determined to be 0.842 nM from the derivatization method and by UHPLC/MS/MS analysis. The method accuracy in the mouse serum samples was 75 ± 2% with a relative standard deviation (RSD) of 3.0% of method variation. The UHPLC/MS/MS analysis gave RSDs of 0.2% and 1.8% for intra-day and inter-day variations. With the newly developed method, AITC could be detected in mouse serum upon oral administration for the first time.

CONCLUSIONS

An analytical method involving a rapid derivatization pre-treatment and quantitative UHPLC/ESI-QqQ-MS/MS analysis has been developed for a biological sample assay of AITC, enabling selective and sensitive detection of the AITC derivative and AITC metabolites simultaneously. The method developed could be applied to the DMPK study of AITC as well.

Assessment of Methodological Pipelines for the Determination of Isothiocyanates Derived from Natural Sources

Isothiocyanates are biologically active secondary metabolites liberated via enzymatic hydrolysis of their sulfur enriched precursors, glucosinolates, upon tissue plant disruption. The importance of this class of compounds lies in their capacity to induce anti-cancer, anti-microbial, anti-inflammatory, neuroprotective, and other bioactive properties. As such, their isolation from natural sources is of utmost importance. In this review article, an extensive examination of the various parameters (hydrolysis, extraction, and quantification) affecting the isolation of isothiocyanates from naturally-derived sources is presented. Overall, the effective isolation/extraction and quantification of isothiocyanate is strongly associated with their chemical and physicochemical properties, such as polarity-solubility as well as thermal and acidic stability. Furthermore, the successful activation of myrosinase appears to be a major factor affecting the conversion of glucosinolates into active isothiocyanates.

Plant constituents and thyroid: A revision of the main phytochemicals that interfere with thyroid function

In the past few decades, there has been a lot of interest in plant constituents for their antioxidant, anti-inflammatory, anti-microbial and anti-proliferative properties. However, concerns have been raised on their potential toxic effects particularly when consumed at high dose. The anti-thyroid effects of some plant constituents have been known for some time. Indeed, epidemiological observations have shown the causal association between staple food based on brassicaceae or soybeans and the development of goiter and/or hypothyroidism. Herein, we review the main plant constituents that interfere with normal thyroid function such as cyanogenic glucosides, polyphenols, phenolic acids, and alkaloids. In detail, we summarize the in vitro and in vivo studies present in the literature, focusing on the compounds that are more abundant in foods or that are available as dietary supplements. We highlight the mechanism of action of these compounds on thyroid cells by giving a particular emphasis to the experimental studies that can be significant for human health. Furthermore, we reveal that the anti-thyroid effects of these plant constituents are clinically evident only when they are consumed in very large amounts or when their ingestion is associated with other conditions that impair thyroid function.

Evaluation of biological activities of Barbarea integrifolia and isolation of a new glucosinolate derivated compound

The aim of the present study is to determine the potent biological activities and carry out isolation studies on Barbarea integrifolia. The antioxidant capacity of the species was evaluated by total phenolic content, FRAP, CUPRAC, and DPPH radical scavenging activity. Anticancer activity studies were performed by MTT assay in MDA-MB-231, MCF-7, Hep3B, PC-3, A549, HCT116, L-929 cell lines. It was observed that the remaining aqueous fraction has higher total phenolic content while higher activity in the CUPRAC and FRAP assays was displayed for the methanolic extract and chloroform fraction. The extracts showed anticancer activity as compared with vincristine. It was observed that chloroform fraction has the highest anticancer activity on MCF-7 cell line, while ethyl acetate fraction has the highest anticancer activity on Hep-3B and A549 cell lines. Methanolic extract has the highest anticancer activity on HCT116 and MDA-MB-23 cell lines. The isolation studies have been performed using several chromatographic methods. The chemical structures of compounds have been identified by means of ¹H NMR, ¹³C NMR, 2D-NMR, and MS. Five major compounds, one steroid (β-Sitosterol), one phenolic acid (Rosmarinic acid), one flavonol heteroside (kaempferol 7-O-α-l-rhamnoside-3-O-β-d-(2-O-β- d -glucosyl)-β-d-glucoside), and two glucosinolates (Gluconasturtiin, Gluconasturtiin choline salt) have been isolated.

Biomarkers of Broccoli Consumption: Implications for Glutathione Metabolism and Liver Health

Diet and lifestyle choices contribute to obesity and liver disease. Broccoli, a brassica vegetable, may mitigate negative effects of both diet and lifestyle. Currently, there are no clinically relevant, established molecular biomarkers that reflect variability in human absorption of brassica bioactives, which may be the cause of variability/inconsistencies in health benefits in the human population. Here, we focused on the plasma metabolite profile and composition of the gut microbiome in rats, a relatively homogenous population in terms of gut microbiota, genetics, sex and diet, to determine if changes in the plasma metabolite profiles caused by dietary broccoli relate to molecular changes in liver. Our aim was to identify plasma indicators that reflect how liver health is impacted by dietary broccoli. Rats were fed a 10% broccoli diet for 14 days. We examined the plasma metabolite composition by metabolomics analysis using GC-MS and gut microbiota using 16S sequencing after 0, 1, 2, 4, 7, 14 days of broccoli feeding. We identified 25 plasma metabolites that changed with broccoli consumption, including metabolites associated with hepatic glutathione synthesis, and with de novo fatty acid synthesis. Glutamine, stearic acid, and S-methyl-L-cysteine (SMC) relative abundance changes correlated with changes in gut bacteria previously implicated in metabolic disease and with validated increases in expression of hepatic NAD(P)H dehydrogenase [quinone] 1 (NQO1) and nuclear factor (erythroid-derived 2)-like 2 (Nrf2), associated with elevated hepatic glutathione synthesis. Circulating biomarkers following broccoli consumption reflect gut-liver axis health.

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.

The Role of the Glucosinolate-Myrosinase System in Mediating Greater Resistance of Barbarea verna than B. vulgaris to Mamestra brassicae Larvae

We investigated the influences of two structurally similar glucosinolates, phenethylglucosinolate (gluconasturtiin, NAS) and its (S)-2-hydroxyl derivative glucobarbarin (BAR), as well as their hydrolysis products on larvae of the generalist Mamestra brassicae (Lepidoptera: Noctuidae). Previous results suggested a higher defensive activity of BAR than NAS based on resistance toward M. brassicae larvae of natural plant genotypes of Barbarea vulgaris R. Br. (Brassicaceae) dominated by BAR. In the present study, the hypothesis of a higher defensive activity of BAR than NAS was tested by comparing two Barbarea species similarly dominated either by BAR or by NAS and by testing effects of isolated BAR and NAS on larval survival and feeding preferences. Larvae reared on leaf disks of B. verna (Mill.) Asch. had a lower survival than those reared on B. vulgaris P- and G-chemotypes. Leaves of B. verna were dominated by NAS, whereas B. vulgaris chemotypes were dominated by BAR or its epimer. In addition, B. verna leaves showed a threefold higher activity of the glucosinolate-activating myrosinase enzymes. The main product of NAS from breakdown by endogenous enzymes including myrosinases ("autolysis") in B. verna leaves was phenethyl isothiocyanate, while the main products of BAR in autolyzed B. vulgaris leaves were a cyclized isothiocyanate product, namely an oxazolidine-2-thione, and a downstream metabolite, an oxazolidin-2-one. The glucosinolates BAR and NAS were isolated and offered to larvae on disks of cabbage. Both glucosinolates exerted similar negative effects on larval survival but effects of NAS tended to be more detrimental. Low concentrations of BAR, but not of NAS, stimulated larval feeding, whereas high BAR concentrations acted deterrent. NAS only tended to be deterrent at the highest concentration, but the difference was not significant. Recoveries of NAS and BAR on cabbage leaf disks were similar, and when hydrolyzed by mechanical leaf damage, the same isothiocyanate-type products as in Barbarea plants were formed with further conversion of BAR to cyclic products, (R)-5-phenyloxazolidine-2-thione [(R)-barbarin] and (R)-5-phenyloxazolidin-2-one [(R)-resedine]. We conclude that a previously proposed generally higher defensive activity of BAR than NAS to M. brassicae larvae could not be confirmed. Indeed, the higher resistance of NAS-containing B. verna plants may be due to a combined effect of rather high concentrations of NAS and a relatively high myrosinase activity or other plant traits not investigated yet.

Glucosinolate diversity within a phylogenetic framework of the tribe Cardamineae (Brassicaceae) unraveled with HPLC-MS/MS and NMR-based analytical distinction of 70 desulfoglucosinolates

As a basis for future investigations of evolutionary trajectories and biosynthetic mechanisms underlying variations in glucosinolate structures, we screened members of the crucifer tribe Cardamineae by HPLC-MS/MS, isolated and identified glucosinolates by NMR, searched the literature for previous data for the tribe, and collected HPLC-MS/MS data for nearly all glucosinolates known from the tribe as well as some related structures (70 in total). This is a considerable proportion of the approximately 142 currently documented natural glucosinolates. Calibration with authentic references allowed distinction (or elucidation) of isomers in many cases, such as distinction of β-hydroxyls, methylthios, methylsulfinyls and methylsulfonyls. A mechanism for fragmentation of secondary β-hydroxyls in MS was elucidated, and two novel glucosinolates were discovered: 2-hydroxy-3-methylpentylglucosinolate in roots of Cardamine pratensis and 2-hydroxy-8-(methylsulfinyl)octylglucosinolate in seeds of Rorippa amphibia. A large number of glucosinolates (ca. 54 with high structural certainty and a further 28 or more suggested from tandem MS), representing a wide structural variation, is documented from the tribe. This included glucosinolates apparently derived from Met, Phe, Trp, Val/Leu, Ile and higher homologues. Normal side chain elongation and side chain decoration by oxidation or methylation was observed, as well as rare abnormal side chain decoration (hydroxylation of aliphatics at the δ rather than β-position). Some species had diverse profiles, e.g. R. amphibia and C. pratensis (19 and 16 individual glucosinolates, respectively), comparable to total diversity in literature reports of Armoracia rusticana (17?), Barbarea vulgaris (20-24), and Rorippa indica (>20?). The ancestor or the tribe would appear to have used Trp, Met, and homoPhe as glucosinolate precursor amino acids, and to exhibit oxidation of thio to sulfinyl, formation of alkenyls, β-hydroxylation of aliphatic chains and hydroxylation and methylation of indole glucosinolates. Two hotspots of apparent biochemical innovation and loss were identified: C. pratensis and the genus Barbarea. Diversity in other species mainly included structures also known from other crucifers. In addition to a role of gene duplication, two contrasting genetic/biochemical mechanisms for evolution of such combined diversity and redundancy are discussed: (i) involvement of widespread genes with expression varying during evolution, and (ii) mutational changes in substrate specificities of CYP79F and GS-OH enzymes.

Simultaneous determination of individual isothiocyanates in plant samples by HPLC-DAD-MS following SPE and derivatization with N-acetyl-l-cysteine

The procedure for the isothiocyanates (ITCs) determination that involves derivatization with N-acetyl-l-cysteine (NAC) and separation by HPLC was developed. Prior to derivatization, plant ITCs were isolated and purified using solid-phase extraction (SPE). The optimum conditions of derivatization are: 500μL of isopropanolic eluate obtained by SPE combined with 500μL of derivatizing reagent (0.2M NAC and 0.2M NaHCO3 in water) and reaction time of 1h at 50°C. The formed dithiocarbamates are directly analyzed by HPLC coupled with diode array detector and mass spectrometer if required. The method was validated for nine common natural ITCs. Calibration curves were linear (R(2)⩾0.991) within a wide range of concentrations and limits of detection were below 4.9nmol/mL. The recoveries were in the range of 83.3-103.7%, with relative standard deviations <5.4%. The developed method has been successfully applied to determine ITCs in broccoli, white cabbage, garden cress, radish, horseradish and papaya.

Metabolism, excretion and avoidance of cyanogenic glucosides in insects with different feeding specialisations

Cyanogenic glucosides (CNglcs) are widespread plant defence compounds releasing toxic hydrogen cyanide when hydrolysed by specific β-glucosidases after plant tissue damage. In contrast to specialist herbivores that have mechanisms to avoid toxicity from CNglcs, it is generally assumed that non-adapted herbivores are negatively affected by CNglcs. Recent evidence, however, implies that the defence potential of CNglcs towards herbivores may not be as effective as previously anticipated. Here, performance, metabolism and excretion products of insects not adapted to CNglcs were analysed, including species with different degrees of dietary specialisation (generalists, specialists) and different feeding modes (leaf-snipping lepidopterans, piercing-sucking aphids). Insects were reared either on cyanogenic or acyanogenic plants or on an artificial cyanogenic diet. Lepidopteran generalists (Spodoptera littoralis, Spodoptera exigua, Mamestra brassicae) were compared to lepidopteran glucosinolate-specialists (Pieris rapae, Pieris brassicae, Plutella xylostella), and a generalist aphid (Myzus persicae) was compared to an aphid glucosinolate-specialist (Lipaphis erysimi). All insects were tolerant to cyanogenic plants; in lepidopterans tolerance was mainly due to excretion of intact CNglcs. The two Pieris species furthermore metabolized aromatic CNglcs to amino acid conjugates (Cys, Gly, Ser) and derivatives of these, which is similar to the metabolism of benzylglucosinolates in these species. Aphid species avoided uptake of CNglcs during feeding. Our results imply that non-adapted insects tolerate plant CNglcs either by keeping them intact for excretion, metabolizing them, or avoiding uptake.

Taste detection of the non-volatile isothiocyanate moringin results in deterrence to glucosinolate-adapted insect larvae

Isothiocyanates (ITCs), released from Brassicales plants after hydrolysis of glucosinolates, are known for their negative effects on herbivores but mechanisms have been elusive. The ITCs are initially present in dissolved form at the site of herbivore feeding, but volatile ITCs may subsequently enter the gas phase and all ITCs may react with matrix components. Deterrence to herbivores resulting from topically applied volatile ITCs in artificial feeding assays may hence lead to ambiguous conclusions. In the present study, the non-volatile ITC moringin (4-(α-L-rhamnopyranosyloxy)benzyl ITC) and its glucosinolate precursor glucomoringin were examined for effects on behaviour and taste physiology of specialist insect herbivores of Brassicales. In feeding bioassays, glucomoringin was not deterrent to larvae of Pieris napi (Lepidoptera: Pieridae) and Athalia rosae (Hymenoptera: Tenthredinidae), which are adapted to glucosinolates. Glucomoringin stimulated feeding of larvae of the related Pieris brassicae (Lepidoptera: Pieridae) and also elicited electrophysiological activity from a glucosinolate-sensitive gustatory neuron in the lateral maxillary taste sensilla. In contrast, the ITC moringin was deterrent to P. napi and P. brassicae at high levels and to A. rosae at both high and low levels when topically applied to cabbage leaf discs (either 12, 120 or 1200 nmol moringin per leaf disc of 1cm diameter). Survival of A. rosae was also significantly reduced when larvae were kept on leaves treated with moringin for several days. Furthermore, moringin elicited electrophysiological activity in a deterrent-sensitive neuron in the medial maxillary taste sensillum of P. brassicae, providing a sensory mechanism for the deterrence and the first known ITC taste response of an insect. In simulated feeding assays, recovery of moringin was high, in accordance with its non-volatile nature. Our results demonstrate taste-mediated deterrence of a non-volatile, natural ITC to glucosinolate-adapted insects.