Glucosinolates are water-soluble anions and belong to the glucosides. Every glucosinolate contains a central carbon atom which is bond 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 bond 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.
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 valin. (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. 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.
Because the use of glucosinolate-containing crops as primary food source for animals was shown to have negative effects, food crops have been developed that contain very low amounts of glucosinolates (e.g. canola). The glucosinolate sinigrin amongst others was shown to be responsible for the bitterness of cooked cauliflower as well as in Brussels sprouts 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).
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.
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 unravelled in these years. The whites and orange tips all possess the so-called nitrile specifier protein, which diverts glucosinolate hydrolysis towards 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 (Ratzka et al., 2002). Other kinds of insects (specialised sawflies and aphids) sequester glucosinolates, presumably after having in some way inhibited the plant myrosinase (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).