Chapter 2 Glucosinolates, isothiocyanates and indoles
Although isothiocyanate and ing phenolics, vitamins and other sul- Major dietary sources of indole compounds derived from crucif- fur-containing compounds. specific isothiocyanates erous vegetables appear to be struc- Isothiocyanates are familiar to turally unrelated, they are both derived many of us as they are largely respon- and indoles from glucosinolates. These sulfur-con- sible for the characteristic hot, pun- The concentrations and forms of glu- taining glycosides are found in the gent flavours of salad vegetables such cosinolates in commonly eaten crucif- order Capparales, which includes the as radish, cress, mustard leaves and erous vegetables are summarized in large family Brassicaceae. The glu- watercress, and contribute to the Table 6. Several surveys of variations cosinolate molecule comprises a com- flavour of cooked cruciferous vegeta- in glucosinolate content between mon glucone moiety and a variable bles. Indoles do not contribute to the Brass/ca cultivars have been reported, aglucone side-chain, derived from one flavour of crucifers. Many of the bio- for example for B. rapa (Carlson et al., of seven amino acids. Isothiocyanates logical and chemico-physical proper- 1987a; Hill et al., 1987) and B. oler- are formed by the degradation of glu- ties of these compounds (e.g. volatility acea (Carlson et al., 1987b; Kushad et cosinolates that have either aliphatic and lipophilicity) are determined by al., 1999), and these are also summa- or aromatic side-chains (Figures 4 & the chemical structure of the isothio- rized in Table 6. While over 90 different 5), derived (mainly) from methionine cyanate side-chain (Figure 5; Appen- isothiocyanate (ITC) glucosinolates and phenylalanine, respectively, dix 1), which, in turn, is determined by have been described, only about six whereas indoles are derived from glu- the structure of the parent glucosino- occur frequently in the diet (Figure 5; cosinolates with indolyl side-chains late molecule. The bioactivity of these Table 6). Sulforaphane (4-methyl- derived from tryptophan (Figures 6 & compounds to humans is thus influ- sulfinylbutyl-ITC) is obtained predomi- 7) (Rosa et al., 1997). Cruciferous enced by a combination of the chemi- nantly from broccoli but may also be vegetables also contain other phyto- cal structure of the isothiocyanates obtained from rocket (Eruca sat/va) chemicals that have been associated and their concentration. and some cultivars of cabbage and with potential health benefits, includ- Brussels sprouts. 2-Propenyl (allyl')-,
S—Glucose