Glucosinolates are natural components of many pungent plants such as mustard, cabbage, and horseradish. The pungency of those plants is due to mustard oils produced from glucosinolates when the plant material is chewed, cut, or otherwise damaged. These natural chemicals most likely contribute to plant defence against pests and diseases, and impart a characteristic bitter flavor property to cruciferous vegetables. [1]
Glucosinolates occur as secondary metabolites of almost all plants of the order Brassicales. This includes the economically important family Brassicaceae as well as Capparaceae and Caricaceae. Outside of the Brassicales, the genera Drypetes [2] and Putranjiva in the family Putranjivaceae, are the only other known occurrence of glucosinolates. Glucosinolates occur in various edible plants such as cabbage (white cabbage, Chinese cabbage, broccoli), Brussels sprouts, watercress, horseradish, capers, and radishes where the breakdown products often contribute a significant part of the distinctive taste. The glucosinolates are also found in seeds of these plants. [1] [3]
Glucosinolates constitute a natural class of organic compounds that contain sulfur and nitrogen and are derived from glucose and an amino acid. They are water-soluble anions and belong to the glucosides. Every glucosinolate contains a central carbon atom, which is bound to the sulfur atom of the thioglucose group, and via a nitrogen atom to a sulfate group (making a sulfated aldoxime). In addition, the central carbon is bound to a side group; different glucosinolates have different side groups, and it is variation in the side group that is responsible for the variation in the biological activities of these plant compounds. The essence of glucosinolate chemistry is their ability to convert into an isothiocyanate (a "mustard oil") upon hydrolysis of the thioglucoside bond by the enzyme myrosinase. [4]
The semisystematic naming of glucosinolates consists of the chemical name of the group "R" in the diagram followed by "glucosinolate", with or without a space. For example, allylglucosinolate and allyl glucosinolate refer to the same compound: both versions are found in the literature. [5] Isothiocyanates are conventionally written as two words. [4]
The following are some glucosinolates and their isothiocyanate products: [4]
Sinigrin was first of the class to be isolated — in 1839 as its potassium salt. [6] Its chemical structure had been established by 1930, showing that it is a glucose derivative with β-D-glucopyranose configuration. It was unclear at that time whether the C=N bond was in the Z (or syn) form, with sulfur and oxygen substituents on the same side of the double bond, or the alternative E form in which they are on opposite sides. The matter was settled by X-ray crystallography in 1963. [7] [8] It is now known that all natural glucosinolates are of Z form, although both forms can be made in the laboratory. [5] The "ate" ending in the naming of these compounds implies that they are anions at physiological pH and an early name for this allylglucosinolate was potassium myronate. [6] Care must be taken when discussing these compounds since some older publications do not make it clear whether they refer to the anion alone, its corresponding acid or the potassium salt. [5]
About 132 different glucosinolates are known to occur naturally in plants. They are biosynthesized from amino acids: so-called aliphatic glucosinolates derived from mainly methionine, but also alanine, leucine, isoleucine, or valine. (Most glucosinolates are actually derived from chain-elongated homologues of these amino acids, e.g. glucoraphanin is derived from dihomomethionine, which is methionine chain-elongated twice.) Aromatic glucosinolates include indolic glucosinolates, such as glucobrassicin, derived from tryptophan and others from phenylalanine, its chain-elongated homologue homophenylalanine, and sinalbin derived from tyrosine. [4]
Full details of the sequence of reactions that converts individual amino acids into the corresponding glucosinolate have been studied in the cress Arabidopsis thaliana . [9] [5]
A sequence of seven enzyme-catalysed steps is used. The sulfur atom is incorporated from glutathione (GSH) and the sugar component is added to the resulting thiol derivative by a glycosyltransferase before the final sulfonation step. [10]
The plants contain the enzyme myrosinase, which, in the presence of water, cleaves off the glucose group from a glucosinolate. [11] The remaining molecule then quickly converts to an isothiocyanate, a nitrile, or a thiocyanate; these are the active substances that serve as defense for the plant. Glucosinolates are also called mustard oil glycosides. The standard product of the reaction is the isothiocyanate (mustard oil); the other two products mainly occur in the presence of specialised plant proteins that alter the outcome of the reaction. [12]
In the chemical reaction illustrated above, the red curved arrows in the left side of figure are simplified compared to reality, as the role of the enzyme myrosinase is not shown. However, the mechanism shown is fundamentally in accordance with the enzyme-catalyzed reaction.
In contrast, the reaction illustrated by red curved arrows at the right side of the figure, depicting the rearrangement of atoms resulting in the isothiocyanate, is expected to be non-enzymatic. This type of rearrangement can be named a Lossen rearrangement, or a Lossen-like rearrangement, since this name was first used for the analogous reaction leading to an organic isocyanate (R-N=C=O).
To prevent damage to the plant itself, the myrosinase and glucosinolates are stored in separate compartments of the cell or in different cells in the tissue, and come together only or mainly under conditions of physical injury (see Myrosinase).
The use of glucosinolate-containing crops as primary food source for animals can have negative effects if the concentration of glucosinolate is higher than what is acceptable for the animal in question, because some glucosinolates have been shown to have toxic effects (mainly as goitrogens and anti-thyroid agents) in livestock at high doses. [13] However, tolerance level to glucosinolates varies even within the same genus (e.g. Acomys cahirinus and Acomys russatus). [14]
The glucosinolate sinigrin, among others, was shown to be responsible for the bitterness of cooked cauliflower and Brussels sprouts. [1] [15] Glucosinolates may alter animal eating behavior. [16]
The isothiocyanates formed from glucosinolates are under laboratory research to assess the expression and activation of enzymes that metabolize xenobiotics, such as carcinogens. [17] Observational studies have been conducted to determine if consumption of cruciferous vegetables affects cancer risk in humans, but there is insufficient clinical evidence to indicate that consuming isothiocyanates in cruciferous vegetables is beneficial, according to a 2017 review. [17]
Glucosinolates and their products have a negative effect on many insects, resulting from a combination of deterrence and toxicity. In an attempt to apply this principle in an agronomic context, some glucosinolate-derived products can serve as antifeedants, i.e., natural pesticides. [18]
In contrast, the diamondback moth, a pest of cruciferous plants, may recognize the presence of glucosinolates, allowing it to identify the proper host plant. [19] Indeed, a characteristic, specialised insect fauna is found on glucosinolate-containing plants, including butterflies, such as large white, small white, and orange tip, but also certain aphids, moths, such as the southern armyworm, sawflies, and flea beetles.[ citation needed ] For instance, the large white butterfly deposits its eggs on these glucosinolate-containing plants, and the larvae survive even with high levels of glucosinolates and eat plant material containing glucosinolates. [20] The whites and orange tips all possess the so-called nitrile specifier protein, which diverts glucosinolate hydrolysis toward nitriles rather than reactive isothiocyanates. [21] In contrast, the diamondback moth possesses a completely different protein, glucosinolate sulfatase, which desulfates glucosinolates, thereby making them unfit for degradation to toxic products by myrosinase. [22]
Other kinds of insects (specialised sawflies and aphids) sequester glucosinolates. [23] In specialised aphids, but not in sawflies, a distinct animal myrosinase is found in muscle tissue, leading to degradation of sequestered glucosinolates upon aphid tissue destruction. [24] This diverse panel of biochemical solutions to the same plant chemical plays a key role in the evolution of plant-insect relationships. [25]
Plants produce glucosinolates in response to the degree of herbivory being suffered. Their production in relation to atmospheric CO2 concentrations is complex: increased CO2 can give increased, decreased or unchanged production and there may be genetic variation within the Brassicales. [26] [27]
Brassicaceae or Cruciferae is a medium-sized and economically important family of flowering plants commonly known as the mustards, the crucifers, or the cabbage family. Most are herbaceous plants, while some are shrubs. The leaves are simple, lack stipules, and appear alternately on stems or in rosettes. The inflorescences are terminal and lack bracts. The flowers have four free sepals, four free alternating petals, two shorter free stamens and four longer free stamens. The fruit has seeds in rows, divided by a thin wall.
Horseradish is a perennial plant of the family Brassicaceae. It is a root vegetable, cultivated and used worldwide as a spice and as a condiment. The species is probably native to southeastern Europe and western Asia.
In organic chemistry, isothiocyanate is the functional group −N=C=S, formed by substituting the oxygen in the isocyanate group with a sulfur. Many natural isothiocyanates from plants are produced by enzymatic conversion of metabolites called glucosinolates. These natural isothiocyanates, such as allyl isothiocyanate, are also known as mustard oils. An artificial isothiocyanate, phenyl isothiocyanate, is used for amino acid sequencing in the Edman degradation.
Mustard oil can mean either the pressed oil used for cooking, or a pungent essential oil also known as volatile oil of mustard. The essential oil results from grinding mustard seed, mixing the grounds with water, and isolating the resulting volatile oil by distillation. It can also be produced by dry distillation of the seed. Pressed mustard oil is used as cooking oil in some cultures, but sale is restricted in some countries due to high levels of erucic acid. Varieties of mustard seed also exist that are low in erucic acid.
Allyl isothiocyanate (AITC) is an organosulfur compound (formula CH2CHCH2NCS). The colorless oil is responsible for the pungent taste of mustard, radish, horseradish, and wasabi. This pungency and the lachrymatory effect of AITC are mediated through the TRPA1 and TRPV1 ion channels. It is slightly soluble in water, but more soluble in most organic solvents.
Sulforaphane is a compound within the isothiocyanate group of organosulfur compounds. It is produced when the enzyme myrosinase transforms glucoraphanin, a glucosinolate, into sulforaphane upon damage to the plant, which allows the two compounds to mix and react.
Glucoraphanin is a glucosinolate found in broccoli, mustard and other cruciferous vegetables.
Sinigrin or allyl glucosinolate is a glucosinolate that belongs to the family of glucosides found in some plants of the family Brassicaceae such as Brussels sprouts, broccoli, and the seeds of black mustard. Whenever sinigrin-containing plant tissue is crushed or otherwise damaged, the enzyme myrosinase degrades sinigrin to a mustard oil, which is responsible for the pungent taste of mustard and horseradish. Seeds of white mustard, Sinapis alba, give a less pungent mustard because this species contains a different glucosinolate, sinalbin.
Cruciferous vegetables are vegetables of the family Brassicaceae with many genera, species, and cultivars being raised for food production such as cauliflower, cabbage, kale, garden cress, bok choy, broccoli, Brussels sprouts, mustard plant and similar green leaf vegetables. The family takes its alternative name from the shape of their flowers, whose four petals resemble a cross.
Barbarea vulgaris, also called wintercress, or alternatively winter rocket, rocketcress, yellow rocketcress, yellow rocket, wound rocket, herb barbara, creases, or creasy greens, is a biennial herb of the genus Barbarea, belonging to the family Brassicaceae.
Glucobrassicin is a type of glucosinolate that can be found in almost all cruciferous plants, such as cabbages, broccoli, mustards, and woad. As for other glucosinolates, degradation by the enzyme myrosinase is expected to produce an isothiocyanate, indol-3-ylmethylisothiocyanate. However, this specific isothiocyanate is expected to be highly unstable, and has indeed never been detected. The observed hydrolysis products when isolated glucobrassicin is degraded by myrosinase are indole-3-carbinol and thiocyanate ion, which are envisioned to result from a rapid reaction of the unstable isothiocyanate with water. However, a large number of other reaction products are known, and indole-3-carbinol is not the dominant degradation product when glucosinolate degradation takes place in crushed plant tissue or in intact plants.
Myrosinase is a family of enzymes involved in plant defense against herbivores, specifically the mustard oil bomb. The three-dimensional structure has been elucidated and is available in the PDB.
Sinalbin is a glucosinolate found in the seeds of white mustard, Sinapis alba, and in many wild plant species. In contrast to mustard from black mustard seeds which contain sinigrin, mustard from white mustard seeds has only a weakly pungent taste.
Gluconasturtiin or phenethyl glucosinolate is one of the most widely distributed glucosinolates in the cruciferous vegetables, mainly in the roots, and is probably one of the plant compounds responsible for the natural pest-inhibiting properties of growing crucifers, such as cabbage, mustard or rape, in rotation with other crops. This effect of gluconasturtiin is due to its degradation by the plant enzyme myrosinase into phenethyl isothiocyanate, which is toxic to many organisms.
Brevicoryne brassicae, commonly known as the cabbage aphid or cabbage aphis, is a destructive aphid native to Europe that is now found in many other areas of the world. The aphids feed on many varieties of produce, including cabbage, broccoli (especially), Brussels sprouts, cauliflower and many other members of the genus Brassica, but do not feed on plants outside of the family Brassicaceae. The insects entirely avoid plants other than those of Brassicaceae; even though thousands may be eating broccoli near strawberries, the strawberries will be left untouched.
Allyl cyanide is an organic compound with the formula CH2CHCH2CN. Like other small alkyl nitriles, allyl cyanide is colorless and soluble in organic solvents. Allyl cyanide occurs naturally as an antifeedant and is used as a cross-linking agent in some polymers.
Antifeedants are organic compounds produced by plants to repel herbivores through distaste or toxicity. These chemical compounds are typically classified as secondary metabolites in that they are not essential for the metabolism of the plant, but instead confer longevity. Antifeedants exhibit a wide range of activities and chemical structures as biopesticides. Examples include rosin, which inhibits attack on trees, and many alkaloids, which are highly toxic to specific insect species.
The mustard oil bomb, formerly known as the glucosinolate–myrosinase complex, is a chemical herbivory defense system found in members of the Brassicaceae. The mustard oil bomb requires the activation of a common plant secondary metabolite, glucosinolate, by an enzyme, myrosinase. The defense complex is typical among plant defenses to herbivory in that the two molecules are stored in different compartments in the leaves of plants until the leaf is torn by an herbivore. The glucosinolate has a β-glucose and a sulfated oxime. The myrosinase removes the β-glucose to form mustard oils that are toxic to herbivores. The defense system was named a "bomb" by Matile, because it like a real bomb is waiting to detonate upon disturbance of the plant tissue.
Glucotropaeolin or benzyl glucosinolate is a glucosinolate found in cruciferous vegetables, particularly garden cress. Upon enzymatic activity, it is transformed into benzyl isothiocyanate, which contributes to the characteristic flavor of these brassicas.
Scaptomyza flava is an herbivorous leaf mining fly species in the family Drosophilidae. In Latin, flava means golden or yellow. The fly is amber to dark brown in color and approximately 2.5 mm in length. In Europe and New Zealand the larvae are pests of plants in the order Brassicales, including arugula, brassicas, broccoli, Brussels sprouts, bok choy, cabbage, canola, cauliflower, horseradish, kale, kohlrabi, napa cabbage, nasturtium, radish, rapini, rutabaga, turnip, wasabi and watercress. In New Zealand, its range has expanded to include host species that are intercropped with salad brassicas, including gypsophila, otherwise known as baby's breath, which is in the pink family (Caryophyllaceae) and the pea in the Fabaceae. More typically, S. flava is oligophagous within the Brassicales. Scaptomyza are unusual within the Drospophilidae because the group includes species that are truly herbivorous. Other herbivorous drosophilids include D. suzukii, which attacks fruit very early during ripening and species within the genus Lordiphosa, from Africa and Asia, which also include leaf miners. Most drosophilids feed on microbes associated with decaying vegetation and sap fluxes.
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