Glucosinolates, isothiocyanates and indoles

Although isothiocyanate and indole compounds derived from cruciferous vegetables appear to be structurally unrelated, they are both derived from glucosinolates. These sulfur-containing glycosides are found in the order Capparales, which includes the large family Brassicaceae. The glucosinolate molecule comprises a common glucone moiety and a variable aglucone side-chain, derived from one of seven amino acids. Isothiocyanates are formed by the degradation of glucosinolates that have either aliphatic or aromatic side-chains (Figures 4 & 5), derived (mainly) from methionine and phenylalanine, respectively, whereas indoles are derived from glucosinolates with indolyl side-chains derived from tryptophan (Figures 6 & 7) (Rosa et al., 1997). Cruciferous vegetables also contain other phytochemicals that have been associated with potential health benefits, including phenolics, vitamins and other sulfur-containing compounds. Isothiocyanates are familiar to many of us as they are largely responsible for the characteristic hot, pungent flavours of salad vegetables such as radish, cress, mustard leaves and watercress, and contribute to the flavour of cooked cruciferous vegetables. Indoles do not contribute to the flavour of crucifers. Many of the biological and chemico-physical properties of these compounds (e.g. volatility and lipophilicity) are determined by the chemical structure of the isothiocyanate side-chain (Figure 5; Appendix 1), which, in turn, is determined by the structure of the parent glucosinolate molecule. The bioactivity of these compounds to humans is thus influenced by a combination of the chemical structure of the isothiocyanates and their concentration. Major dietary sources of specific isothiocyanates and indoles