Aliphatic glucosinolates are sulphonated thioglucosides which are found in all hitherto investigated plants of the order Capparales, which includes the genera Brassica and Arabidopsis. Although these secondary metabolites share the general structure (see previous chapter and Fig. 1), differences in their ‘R’ substituents enable classification into several distinct structural classes. The aliphatic glucosinolates found in Brassica typically include methylthioalkyl, methylsulphinylalkyl, alkenyl and hydroxyalkenyl homologues of propyl, butyl and pentyl glucosinolates. A.thaliana contains a similar variety of glucosinolates, with the addition of hydroxyalkyl glucosinolates and trace levels of longer side-chain homologues (Hogge et al, 1988). Following disruption of cell integrity, aliphatic glucosinolates are hydrolysed by endogenous β-thioglucoside glucohydrolases (‘myrosinases’; EC 3.2.3.1) to give a complex mixture of products, of which D-glucose, sulphate and isothiocyanates are major components (Cole, 1976; Benn, 1977; Fenwick, Heaney & Mullin, 1983). In addition to being responsible for the characteristic flavour of cruciferous condiments, hydrolytic products of glucosinolates such as isothiocyanates, thiocyanates and nitriles are known to impart antinutritional properties to protein-rich oilseed ‘meals’ commonly used as animal-feed (Bell, Benjamin & Giovanetti, 1972; Fenwick, 1984). In particular, isothiocyanate derivatives of hydroxyalkenyl glucosinolates undergo spontaneous cyclization to produce substituted oxazolidine-2-thiones (Macleod & Rossiter, 1987) which have potent goitrogenic properties (Astwood, Greer & Ettlinger, 1949; Langer, 1966). The occurrence of these antinutritional compounds in seed-meals of major oilseed Brassica crops has led to efforts to reduce the level of aliphatic glucosinolates in the seeds of oilseed rape.