Glucosinolate

Glucosinolate structure; side group R varies

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 (including the family Brassicaceae, Capparidaceae and Caricaceae), but also in the genus Drypetes (family Euphorbiaceae)[2].

Sharp tasting plant molecules

Plants use substances derived from glucosinolates as natural pesticides, and as defense against herbivores; these substances are also responsible for the bitter or sharp taste of many common foods such as mustard, radish, horseradish, maca, cress, cabbage, Brussels sprouts, kohlrabi, kale, cauliflower, broccoli, turnip, swede (rutabaga), and rapeseed.

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 ketoxime) 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 120 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, or valine. (Most glucosinolates are actually derived from chain-elongated homologues of these amino acids, e.g., the cancer-preventing glucoraphanin derived from dihomomethionine, which is methionine chain-elongated twice). Aromatic glucosinolates include Indolic glucosinolates such as glucobrassicin derived from tryptophan and other ones 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. Therefore, 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 (Burow et al., 2007). 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

Bitter, toxic - or healthy?

Because the use of glucosinolate-containing crops as primary food source for animals was shown to have negative effects, food crops that contain very low amounts of glucosinolates (e.g., canola) have been developed. The glucosinolate sinigrin, among others, was shown to be responsible for the bitterness of cooked cauliflower [3] as well as in Brussels sprouts [4]. On the other hand, plants producing large amounts of glucosinolates are also desirable, because substances derived from these can serve as natural pesticides and are under investigation in the prevention of cancer (with sulforaphane in broccoli being the best known example).

Effects on humans and other mammals

Glucosinolates are well known for their toxic effects (mainly as goitrogens) in both humans and animals at high doses. In contrast, 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, viz. 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.^[1]

Relations to insect specialists

A characteristic, specialised insect fauna are 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 in these years. The whites and orange tips all possess the so-called nitrile specifier protein, which diverts glucosinolate hydrolysis toward nitriles rather than reactive isothiocyanates (Wittstock et al., 2004). In contrast, the Diamondback Moth (Plutella xylostella) possess a completely different protein, glucosinolate sulphatase, which desulphates glucosinolates, thereby making them unfit for degradation to toxic products by myrosinase^[2].

Other kinds of insects (specialised sawflies and aphids) sequester glucosinolates (Müller et al., 2001). 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 (Bridges et al., 2001). 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 (Wheat et al., 2007).

See also

• Isothiocyanate
• Gluconasturtiin
• Glucobrassicin
• Goitrin
• Sinigrin
• Sinalbin

References

• Bones AM, Rossiter JT: The myrosinase-glucosinolate system - an innate defense system in plants, Physiologia plantarum 97 (1): pages 194-208, May 1996
• Abel S: Glucosinolates and Chemoprevention of Cancer
• Reintanz B et al.: Molecules, Morphology, and Dahlgren's Expanded Order Capparales
• Srinibas Das, Amrish Kumar Tyagi and Harjit Kaur: Cancer modulation by glucosinolates: A review
• Wittstock et al., 2004. Successful herbivore attack due to metabolic diversion of a plant chemical defence. Proceedings of the National Academy of Sciences of the USA 101, 4859-4864
• Bridges et al., 2001. Spatial organization of the glucosinolate-myrosinase system in brassica specialist aphids is similar to that of the host plant. Proc. R. Soc. Lond. B 269, 187-191.
• Burow et al., 2007. Glucosinolate hydrolysis on Lepidium sativum - identification of the thiocyanate-forming protein. Plant Molecular Biology 63, 49-61.
• Ratzka et al., 2002. Disarming the mustard oil bomb. Proceedings of the National Academy of Sciences of the USA 99, 11223-11228.
• Müller et al., 2001.Sequestration of host plant glucosinolates in the defernsive hemolymph of the sawfly Athalia rosae. Journal of Chemical Ecology 27, 2505-2516.
• Wheat et al., 2007. The genetic basis of a plant-insect coevolutionary key innovation. Proceedings of the National Academy of Sciences of the USA 104, 20427-20431.

External links

• Glucosinolate metabolism pathways from MetaCyc