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Glucosinolates

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Glucosinolates

The glucosinolates are a class of organic compounds that contain sulfur and nitrogen and are derived from glucose and an amino acid. They 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).[1]

Chemistry

Glucosinolates are water-soluble anions and 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.

Biochemistry

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.

Enzymatic activation

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.[2]

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.

Biological effects

Humans and other mammals

Toxicity

The use of glucosinolate-containing crops as primary food source for animals has negative effects.Glucosinolates are well known for their 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).[3]

Taste and eating behavior

The glucosinolate sinigrin, among others, was shown to be responsible for the bitterness of cooked cauliflower and Brussels sprouts.[4] Glucosinolate have been shown to alter animal eating behavior. [5]

Cancer inhibitor

Plants producing large amounts of glucosinolates are under investigation for mitigating cancer, with sulforaphane from broccoli being the best known example. Consumers of higher levels of Brassica vegetables, particularly broccoli, Brussels sprouts and cabbage, may benefit from a lower risk of cancer at a variety of organ sites, although this information is neither scientifically conclusive nor approved by any national regulatory authority as of 2011. At subtoxic doses, their hydrolytic and metabolic products act as chemoprotective agents against chemically-induced carcinogens by blocking the initiation of tumors in a variety of rodent tissues, such as the liver, colon, mammary gland, pancreas, etc. They exhibit their effect by inducing Phase I and Phase II enzymes, inhibiting the enzyme activation, modifying the steroid hormone metabolism and protecting against oxidative damages.[6] In particular, the chemopreventive effects of the glucosinolates present in cruciferous vegetables are related to their activity as Histone deacetylase inhibitors.[7]

Insects

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. 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.[8] 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.[9]

Other kinds of insects (specialised sawflies and aphids) sequester glucosinolates.[10] 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.[11] 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.[12]

See also

References

External links

  • Glucosinolate metabolism pathways from MetaCyc
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