Micellar electrokinetic capillary chromatography—Synchronous monitoring of substrate and products in the myrosinase catalysed hydrolysis of glucosinolates

A micellar electrokinetic capillary chromatography (MECC) method has been developed for monitoring the myrosinase catalysed hydrolysis of 2-hydroxy substituted glucosinolates and the simultaneous formation of the corresponding degradation products (oxazolidine-2-thiones (OZTs) and nitriles). Glucosibarin ((2R)-2-hydroxy-2-phenylethylglucosinolate) was chosen as the model glucosinolate owing to the difficulties in determining hydrolysis rates of this type of substrates in traditional UV-assays. The method was afterwards validated with glucobarbarin ((2S)-2-hydroxy-2-phenylethylglucosinolate) and progoitrin ((2R)-2-hydroxybut-3-enylglucosinolate). Aromatic glucosinolates without a 2-hydroxy group in their side chains, such as glucotropaeolin (benzylglucosinolate) and gluconasturtiin (phenethylglucosinolate) were also tested. Formation of the glucosinolate hydrolysis products was monitored simultaneously at 206 nm and 230 nm. This allowed estimation of the extinction coefficient of the OZT derived from glucosibarin, which was found to be 18,000M−1 cm−1 and 12,000M−1 cm−1 at 206 nm and 230 nm, respectively. The developed method has limit of detection of 0.04mM and 0.06mM and limit of quantification of 0.2mM and 0.3mM for the glucosibarin derived OZT and nitrile, respectively. Linearity of the glucosinolate concentration was examined at six concentration levels from 2.5mMto 100mMand at 206 nm a straight line (R2 = 0.9996) was obtained. The number of theoretical plates (N) at the optimal system conditions was 245,000 for the intact glucosibarin, 264,000 for the OZT and 252,000 for the nitrile

Effect of ascorbic acid and glutathione on the production of nitriles by myrosinase

Biofumigation is based on the use of glucosinolate-containing plants for the control of soil-borne pest and diseases. Upon tissue damage, glucosinolates are hydrolyzed by endogenous enzymes (myrosinase) and a range of biologically active compounds are formed. Isothiocyanates (ITCs) are the quantitatively dominating products formed at neutral pH. Most of these compounds are volatile and only sparingly soluble in aqueous systems, and depending on the R-group structure and the presence of nucleophiles, further transformation of ITCs occurs. At lower pH and in the presence of certain molecules able to deliver two redox equivalents, the proportion of nitriles increases at the expense of ITC. The effect of ascorbic acid and glutathione on the production of nitriles at pH 5 was investigated by micellar electrokinetic capillary chromatography (MECC). The presence of 0.25 µmol ascorbic acid increased the production of nitriles although at higher concentrations the proportion of nitriles decreased. Increasing amounts of GSH favored the production of nitriles (40% of the total degradation products were nitriles in the presence of 2 µmol GSH). The oxidation of GSH gives the redox equivalents needed for the liberation of the sulfur from the unstable intermediate of the glucosinolate hydrolysis leading to the formation of the nitrile

Glucosinolate hydrolysis products for weed control

Glucosinolates are allelochemicals present in all Brassica plants. Upon hydrolysis by endogenous enzymes they produce a series of biologically active compounds, such as isothiocyanates and their deriva-tives among others. These compounds have marked fungicidal, nematocidal and herbicidal effects and therefore their use as biodegradable natural products for crop protection has attracted much attention in the last years. A number of these compounds, either individually or in combination, were tested against Sinapis alba and Lollium perenne in Petri dishes bio-assays. C50 values as low as 0.7 and 0.2 mM were obtained. This may open the possibility for using glucosinolate hydrolysis products as herbicides

Glucosinolate types and concentrations in seedlings of different Brassica species used for food

Brassicaceous food crops contain in their tissues different quantities of the glucoside allelochemicals known as glucosinolates (Bellostas et al., 2004; Sørensen, 1990). These compounds are alkyl-N-hydroximine sulphate esters with a β-D-thioglucopyranoside group attached to the hydroximine carbon in Z-configuration relative to the sulphate group (Ettlinger and Kjær, 1968; Kjær, 1960). Glucosinolates are biosynthetically derived from amino acids (Hill et al., 2003) and they occur in all plants of the order Capparales and in some other plants (Bjerg and Sørensen, 1987; Kjær, 1960; Rodman, 1978). These compounds co-occur with myrosinase isoenzymes (Thioglucosidase; EC 3.2.1.147), which catalyze the hydrolysis of the β-D-thioglucopyranoside bond releasing an aglucone that forms a variety of biologically active products with structures defined by the type of glucosinolate and the reaction conditions (Bjergegaard et al., 1994; Buskov et al., 2000a; Buskov et al., 2000b; Buskov et al., 2000c; Palmieri et al., 1998). These breakdown products are chemically very reactive and they have for a long time been related to the pungent odour and flavour typical for Brassicaceous plants. These compounds show a various range of biological activities that goes from antinutritional (Bjerg et al., 1989; Hansen et al., 1997), to fungicidal, nematicidal and bactericidal (Brown and Morra, 1997; Buskov et al., 2002; Kirkegaard and Sarwar, 1998). In the last years, interest in their anticarcinogenic properties has increased and research has mainly focused on the effect of the isothiocyanates present in sprouts of certain Brassica food crops, especially broccoli (Zhang et al., 1992; Zhang, 2004). These isothiocyanates have been related to the increase in the activity of the Phase 2 enzymes, which is related to detoxification of xenobiotica and protection against cancer (Bonnesen et al., 1999). They have also been related to an increased antioxidative metabolism by induction of the scavenging of oxygen radicals, which may contribute to a decreased risk of coronary diseases (Wu et al., 2004). Given the biological effects of Brassica crops used for food, it was considered of interest to investigate the glucosinolate profile during early development of the Brassica plant in order to be able to determine the stages at which the desired biologically active compounds are present. It would also allow determining the presence of other potentially active compounds as well as to allow better understanding the metabolic changes occurring during germination and early growth. Five B. oleracea used for food (white cabbage, red cabbage, broccoli, cauliflower and savoy cabbage) and two B. napus (a low and a high-glucosinolate rapeseeds) were used in the present experiments. The content of glucosinolates in seeds, seedlings and the individual parts of grown plants was followed from germination to one-month growth. Samples were taken at one, two, three, four, seven, 14, 21 and 28 days and plants were separated into cotyledons, leaves, epicotyle and roots. Glucosinolates were isolated and their concentration determined by HPLC following standard procedures developed at our laboratory

In vitro screening of the effect of three glucosinolate derived nitriles on soil-borne fungi

Glucosinolates are allelochemicals present in all plants of the order Capparales that are hydrolysed by endogenous enzymes (myrosinases) forming a variety of compounds with biological activity. ‘Biofumigation’ is the term used to describe the effect of these compounds on soil-borne pathogens and it has normally been attributed to isothiocyanates. At acidic pH and in the presence of redox co-factors such as glutathione, glucosinolate hydrolysis yields also nitriles, which are more hydrophilic and stable than isothiocyanates. Three nitriles (allyl-, benzyl- and phenethyl cyanide) were tested against soil borne fungi of economic importance: Aphanomyces euteiches var. pisi, Gaeumannomyces graminis var. tritici and Verticillium dahliae. The nitriles were initially tested at 1 mM and four additional concentrations were further tested in order to determine LD50. At 1 mM, allyl cyanide showed in all cases less than 10% inhibition and it did not inhibit fungi growth at higher concentrations. LD50 of benzyl cyanide was 2.5 mM for Verticillium and Aphanomyces, whereas it was as low as 0.5 mM for Gaeumannomyces. LD50 of phenyl ethyl cyanide was 2.5 mM for Verticillium, 1.4 mM Gaeumannomyces and 1.25 mM Aphanomyces. Although nitriles are generally less toxic than ITCs, their role in biofumigation should not be disregarded