1. GENERAL INFORMATION

Glucosinolates are a class of about 100 naturally occurring thioglucosides that are characteristic of the Cruciferae and related families in the order Capparales. At the present time the diets of people in many parts of the world include considerable amounts of Cruciferous crops and plants. These range from the consumption of processed radish and wasabi in the Far East to that of cabbage and traditional root vegetables in Europe and North America. Other crops, such as rapeseed, kale, swede and turnip may also contribute indirectly to the human food chain since they are extensively used as animal feed stuffs. The presence of some glucosinolates in agricultural crop plants, such as oilseed rape (Brassica napus) and Brassica vegetables, is undesirable because of the toxicological effects of their breakdown products. Such breakdown products include nitriles, isothiocyanates, thiocyanates, epithionitriles and vinyl oxazolidinethiones. Some glucosinolates, especially those in broccoli, have anticarcinogenic properties and are being studied for their potential therapeutic use. Glucosinolate breakdown products are responsible for the biting taste of important condiments such as horseradish and mustard, and they contribute to the characteristic flavours of many vegetables, such as cabbage, broccoli and cauliflower.
    The biological role of glucosinolates and their degradation products is not completely understood. The enzymes catalysing the hydrolysis of glucosinolates are known as myrosinases. The complexity of the myrosinase-glucosinolate system indicates an important role in the life cycle of plants. The function of this system may be diverse. The glucosinolates may be a sink for nutrients like nitrogen and sulphur, while the products of hydrolysis may have important roles in the plant defence system against insect, fungi and microorganism infections.
    Plant breeding strategies have concentrated on reducing the glucosinolate content of rape seeds. Seeds with very low glucosinolate content have been processed, but there has been a significant cost in terms of crop protection and nutrition. Modulation of the biosynthesis of specific glucosinolates is a major goal in Brassica breeding.

2. GLUCOSINOLATES

The glucosinolates are a  class of secondary metabolites found in fifteen botanical families of dicotyledonous plants. These families are the Akaniaceae, Bataceae, Brassicaceae, Bretschneideraceae, Capparaceae, Caricaceae, Euphorbiaceae, Gyrostemonaceae, Limnanthaceae, Moringaceae, Pentadiplantdraceae, Resedaceae, Salvodoraceae, Tropaeolaceae and Tovariaceae. At the present time over 100 glucosinolates have been reported. Glucosinolates are found in all parts of the plant and up to fifteen different glucosinolates have been found in the same plant. Generally, levels in the seed are high (up to ten per cent of the dry weight), whereas the levels in the leaf, stem and root are approximately ten times lower. Concentrations differ according to tissue type, physiological age, plant health and nutrition.
Studies have shown that myrosinases are localised in vacuoles of specialised plant cells, called myrosin cells. Thus the two components of the system are separated until autolysis or tissue damage brings them into contact. The precise localization of glucosinolates is not known, but they have been reported to be stored in vacuoles.

3. STRUCTURE AND NOMENCLATURE

The skeleton of glucosinolates consists of a thioglucosidic link to the carbon of a sulphonated oxime. The R group (side chain) and the sulphate group have anti stereochemical configuration. The R group is derived from amino acids and is highly variable. It can be aliphatic (e.g. alkyl, alkenyl, hydroxyalkenyl, w-methylthioalkyl), aromatic (e.g. benzyl, substituted benzyl) or heterocyclic (e.g. indolyl). The sulphate group imparts strongly acidic properties and thus the glucosinolates occur in  nature as anions counterbalanced by a cation. The cation is usually potassium, being one of the most abundant cations in plant tissues. The sulphate group and the thioglucose moiety impart nonvolatile and hydrophilic properties to all glucosinolates, the R group is variable in properties from lipophilic to marked hydrophilic. The natural forms of glucosinolates exhibit laevo rotation in solution. Glucosinolates have a large number of homologues and ß-hydroxylated analogues. As an example w-methylthioalkyl side chains range from MeS(CH₂)₃ to MeS(CH₂)₈. The general structure of glucosinolates is shown in figure 1.

Figure 1. The general structure of glucosinolates.

When glucosinolates were first discovered they were named after the plants in which they were found. With the discovery of more glucosinolates a semi-systematic system for their naming arose, based on the structure of the side chain. Table 1 shows trivial names for some glucosinolates and indicates their side chain. The name of the side chain  followed by the word "glucosinolate" gives the semi-systematic name. The suffix "ate" indicates the anionic nature of glucosinolates.

Table 1. Trivial names and their side chain for some glucosinolates.

Trivial name	Side chain
Gluconasturtiin	2-Phenethyl
Glucotropaeolin	Benzyl
Progoitrin\epiprogoitrin	2-hydroxy-3-butenyl
Sinigrin	2-propenyl
(Gluco)sinalbin	p-Hydroxybenzyl

4. HYDROLYSIS OF GLUCOSINOLATES

When crushed plant tissue or seeds containing glucosinolates are added to water, myrosinases catalyse the hydrolytic cleavage of the thioglucosidic bond, giving D-glucose and a thiohydroximate-O-sulphonate (aglycone). The latter compound rearranges nonenzymatically with release of sulphate to give one of several possible products. The predominant product is dependent on the structure of the glucosinolate side chain and the presence of protein co-factors that modify the action of the enzyme. The most frequent fate of the unstable aglycone is to undergo rearrangement spontaneously via a proton independent Lossen rearrangement with a concerted loss of sulphate to yield an isothiocyanate, or a competing  proton dependent desulphuration yielding a nitrile and elemental sulphur. Some glucosinolates also give rise to the formation of thiocyanates. The normal products of glucosinolate hydrolysis are shown in figure 2.

Figure 2. The normal products of glucosinolate hydrolysis.

A mixture of  products is normally formed. At low pH the formation of the nitrile is favoured, whereas neutral or high pH favours the formation of the isothiocyanate by means of the Lossen rearrangement. The Lossen rearrangement is characterised by the migration of the nitrogen atom and subsequent loss of the sulphate group. Glucosinolates with a beta-hydroxylated side chain yield isothiocyanates which undergo spontaneous cyclization to the corresponding oxazolidone-2-thione. An example of this is the formation of goitrin from the glucosinolate progoitrin.
    The addition of ferrous ions to reaction mixtures promotes the formation of the nitrile hydrolysis product. At low pH, a proton may block the Lossen rearrangement of the aglycone, thus promoting formation of the nitrile. It is thought that the ferrous ion may serve a similar function. Fe²⁺ may act by complexing ascorbic acid, a co-factor of some myrosinase isoenzymes, thus rendering it unavailable to the isoenzyme.
    Epithiospecifier protein, ESP, is a small protein of molecular weight 30 to 40 kDa, which co-occurs with myrosinase. ESP does not have thioglucosidase activity, but interacts with myrosinase to promote the transfer of sulphur from the S-glucose moiety of terminally unsaturated glucosinolates to the alkenyl moiety, resulting in the formation of epithionitriles. The presence of ferrous ions are essential for ESP function. Enzyme characteristics (substrate affinity, temperature and pH optima) may alter relative proportions of products by causing some glucosinolates to be hydrolyzed at different rates.

5. BIOSYNTHESIS OF GLUCOSINOLATES

Studies have shown that glucosinolates are derived from amino acids. The biosynthetic studies have involved feeding experiments with labelled compounds, isolation of intermediates and isolation of some of the enzymes involved in the pathway. Aliphatic, indole and aromatic side chains are derived from methionine, tryptophan and phenylalanine respectively, from both protein and non-protein sources. The initial steps in the formation of most glucosinolates are N-hydroxylation followed by oxidative decarboxylation to yield an aldoxime. These steps are common to the biosynthesis of other groups of natural products. The biosynthetic pathways then diverge at the aldoxime to produce different compounds. The majority of glucosinolates possess aglycone structures which are not related to protein amino acids. It is generally accepted, however, that these glucosinolates are also derived from protein amino acids. These protein amino acids undergo a chain elongation process in which their 2-oxo-acids condense with acetate. The entire homologous series of glucosinolates with side chains ranging from R= MeS(CH₂)₃ to R=MeS(CH₂)₈ is considered to be derived from repeated chain extensions starting with methionine. Each time the sequence is traversed a new higher amino acid homologue is formed. A glyoxylate aminotransferase is believed to be the first enzyme of the chain elongation process. This enzyme catalyses the formation of the 2-oxo-acid from its corresponding amino acid.
    All the intermediates between the amino acid and the glucosinolate are nitrogenous and the amino acid carbon-nitrogen bond is preserved. The amino acid, whether it has undergone chain elongation or not, is specifically hydroxylated to the N-hydroxyamino acid in the presence of oxygen and NADPH. The N-hydroxyamino acid is decarboxylated to give the aldoxime, followed by a reduction step to a nitro compound which tautomerises to the aci form. The thiohydroximate is then formed by introduction of sulphur, feeding experiments have shown that cysteine is involved as the sulphur donor. The thiohydroximate is transglycosylated to the desulphoglucosinolate. An enzyme catalysing the transfer of glucose from UDP- glucose to the thiohydroximate has been isolated. The glucosinolate is obtained by sulphonation and it is known 3`-phosphoadenosine-5`-phosphosulphate (PAPS) is involved as the sulphate  donor. A summary of the biosynthesis of glucosinolates is shown in figure 3.

Figure 3. A summary of the biosynthesis of glucosinolates.

Important modifications of the side chain, such as hydroxylation and oxidative generation of the alkene from the methylthio group, occur after transglycosylation. It is clear that a number of important details, including possible enzymes involved in the early stages of the glucosinolate biosynthesis, needs to be further researched.