The 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, but are also enjoyed in small amounts by humans and are believed to contribute to the health promoting properties of cruciferous vegetables.
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 can be leached into the water during cooking. Glucosinolates belong to the glucosides. Every glucosinolate contains a central carbon atom, which is bound via a sulfur atom to the thioglucose group (making a sulfated aldoxime) and via a nitrogen atom to a sulfate group. 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. Some glucosinolates:
- Sinigrin is the precursor to allyl isothiocyanate
- Glucotropaeolin is the precursor to benzyl isothiocyanate
- Gluconasturtiin is the precursor to phenethyl isothiocyanate
- Glucoraphanin is the precursor to sulforaphane
Plants with glucosinolates
Glucosinolates occur as secondary metabolites of almost all plants of the order Brassicales (e.g. families Brassicaceae = Cruciferae, Capparidaceae, and Caricaceae), but also in the genus Drypetes (family Euphorbiaceae). For example, glucosinolates occur in cabbages (white cabbage, Chinese cabbage, broccoli), watercress, horseradish, capers and radishes. They are typically in parts consumed, with the pungent taste of these vegetables due to breakdown products (isothiocyanates or mustard oils) of glucosinolates. The glucosinolates are also found in the seeds of these plants.
Natural diversity from a few amino acids
About 132 different glucosinolates are known to occur naturally in plants. They are synthesized from certain 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.
The plants contain the enzyme myrosinase, which, in the presence of water, cleaves off the glucose group from a glucosinolate. 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.
To prevent damage to the plant itself, the myrosinase and glucosinolates are stored in separate compartments of the cell and come together only or mainly under conditions of physical injury.
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Humans and other mammals
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. Some glucosinolates have been shown to have toxic effects (mainly as goitrogens) in both humans and animals at high doses. However, tolerance level to glucosinolates varies even within the same genus (e.g. Acomys cahirinus and Acomys russatus).
Taste and eating behavior
The glucosinolate sinigrin, among others, was shown to be responsible for the bitterness of cooked cauliflower and Brussels sprouts. Glucosinolate have been shown to alter animal eating behavior.
Substances derived from plants producing large amounts of glucosinolates can serve as natural pesticides.
A characteristic, specialised insect fauna is found on glucosinolate-containing plants, including familiar butterflies such as Large White, Small White, and Orange Tip, but also certain aphids, moths, saw flies, flea beetles, etc. For instance, the Large White butterfly oviposits its eggs on these glucosinolate-containing plants because they help the larvae survive. The biochemical basis of these specialisations are being unraveled. The whites and orange tips all possess the so-called nitrile specifier protein, which diverts glucosinolate hydrolysis toward nitriles rather than reactive isothiocyanates. In contrast, the diamondback moth (Plutella xylostella) possesses a completely different protein, glucosinolate sulfatase, which desulfates glucosinolates, thereby making them unfit for degradation to toxic products by myrosinase.
Other kinds of insects (specialised sawflies and aphids) sequester glucosinolates. 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. This diverse panel of biochemical solutions to the same plant chemical plays a key role in current attempts to understand the evolution of plant-insect relationships.
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