Glucosinolates, Isothiocyanates and Indoles

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 SH R-C R—C" Myrosinase R—NCS N N Isothiocyanate os03 os03 Glucosinolate Unstable intermediate Figure 4 General scheme of the hydrolyss of glucosinolates to isothiocyarates 13 IARC Handbooks of Cancer Prevention Volume 9: Cruciferous vegetables, isothiocyanates and ind&es 3-butenyl- and 3-methylsulfinyipropyl- ITC (iberin') are derived from consumption of certain cultivars of B. 0/er- R NCS acea. Propenyl-ITC can also be obtained from leafy mustard vegetables. 3-Butenyl- and 4-pentenyl-TC R Chemical name: food are obtained from leafy B. rapa crops CH3 —S —CH 2 CH2 CH2 3 Methylthiopropyl: cabbages and particularly Chinese cabbage. Phenyl-ethyl-ITC is obtained from watercress and to a lesser extent from 2 —CH2 -- CH3 —S OH2 CH2 CH 4-Methyithiobutyl: rockets root crops such as turnips and rutabaga. 6-Methylsulfinyihexyl- ITC is derived from the Japanese vegetable OH3 S -0H2 —CH2 CH2 3-Mothylsulfinylpropyl ('iberin): broccoli, il some Brussels sprouts and cabbages wasabi (Wasabia japonica). Benzyl- o ITC is relatively rare in the diet, as it is obtained from Lepidium (cress) 0113 —S—CH2 CH2 CH2 OH2 - 4-Methy sulfinylbutyl ('su foraphane'): species. In addition to cruciferous veg- broccoli 0 etables, isothiocyanates may also be derived from mustard condiments, CH3 S —CH2 -- 6-Methylsu finylbexyl: wassbi which are eaten mainly in western o countries. Hydroxy-benzyl-ITC is found in 'English' mustard and is obtained OH3 -S—CH3]7 7 Metbylsulfiriylheptyl: watercress il from the seeds of white mustard, L) Sinapis alba. Allyl- (2-propenyl-) and 3- butenyl-ITCs are found in French and CH, S tCH2s - 8-Methylsulfinylocty watercress American mustards and are derived 0 from the seeds of brown (or Indian) 2-Properly ('ally '): mustards, cabbages, mustard, B. juncea. Allyl-ITC is also Ci —CH CH2 - some Brussels sprouts derived from the grated roots of horse- radish (Armoracia rusticana), which is OH2 OH -CH2 —CH2 3 Buteryl: Brussels sprouts, Chinese used as a condiment in western cabbages, pak-chol, turnip greens Europe and North America. Cruciferous vegetables may also be CH2 —CH CH2 0H2 —CH2-- 4-Penterryl: Chinese cabbages pak cho: processed by pickling or fermenting, as in sauerkraut in western Europe and in SH—CH2 OH2 CH2 -0H2 ---- 4-Mercaptobutyl: rockets some forms of kimchi in the Korean peninsula. Consumption of these prod- ucts is an additional source of isothiocyanates in the diet. C H2 Berizyl: Lepid:um cress Six different indolyl glucosinolates have been identified, but only four have been found in cruciferous crops (Figure 7), and, of these, only two, 2-Phenethyl. watercress, radishes, turnips indolylmethyl ('glucobrassicin') and 1- ICH2 CH2 methoxy-3-indolylmethyl ('neoglucobrassicin'), are found frequently. The types of indole glucosinolates vary from species to species (Table 6), and there is wide variation in the concen- isothiocyanates and side-chain structures (R) found in commonly Figure 5 Structures of trations of specific indole glucosino- eaten cruciferous vegetables lates. In broccoli, the concentration of 14 Glucosinolates, isothiocyanates and indoles Table 6. Main glucosinolate side-chains of isothiocyanates occurring in cruciferous vegetables Cruciferous vegetable Glucosinolate side-chain Content (!LmolIlOO g fresh weight) Broccoli (B, oleracea var. ita/ica)a 3-Meihylsulfinylpropy 0-330 4-Methylsulfinylbutyl 29-190 Indole-3-methyl 42-100 1 -Methoxyindole-3-methyl 2-18 Cabbage (B. oleracea var. capitata)L 2-Propeny! 4-160 3 Mathylsulfinylpropyl 5-280 Indolylmethyl 9-200 Brussels sprouts (B. oleracea var. gemmifera)G 2-Propenyl 4-390 3-Methylsulfinylpropyl 0-150 3-Buterryl 0-220 4-Methylsulfinylbutyl 0-23 2-1--Iydroxy-3-bulenyl 1-300 lndcle-3-methyl 45-470 1 -Methoxyindole-3-methyl 2-34 Cauliflower (B. oleracea var. botrytis) 2-Propenyl 1-160 3-MethylsuJfinylpropyl 0-330 4-Methylsulfiny butyl 2-190 lndole-3-methyl 14-160 1 -Methoxyindole-3-methyl 1-32 Kale (B. oleracea var. acephafa) 2-Propenyl 62-200 3-Methylsulfinylpropyl 0-50 3-But eryl 6-38 2-Hydroxy-3-butenyl 17 130 I ndole-3-methyl 67-160 Rape (B. rapa, including Chinese cabbage 3-Butenyl 38-290 and turnip tops)d 4-Fertenyl 20-150 All the crops can also contain low concentrations of other glucosinolates (Carlson et al., 1987a,b; Hill et al., 1987; Rosa et aL, 1997; Kushad et al., 1999). The concentrations must be interpreted with care, as different methods of analyses were used in the different studies, and the concentrations are affected by a wide range of environmental factors. In addition, generalizations cannot be made about the glucosinolate content of cabbages and Brussels sprouts as there are large genetic differences between cultivars. The values quoted ndicate the greatest range of values in the reviews referred to above. a All broccoli cultivars produce predominantly 4-netbylsulfinylbutyl glucosinolate, the precursor of sulforaphane. b Some cabbage varieties produce the elongated glucosinolates, 3-butenyl and 4-methylsulfirylbutyl; some also produce their precur- sors, 3-meihylthiopropy and 4-nethylthiobutyl. Surpr singly, the glucosinolate content of Brussels sprouts vanes: some cultivars produce high concentrations of 2-propenyl, while others produce high concentrations of 3-butenyl and 2-bydroxy-3-buteny . Recently developed cu tivars either have low concentrations of these glucosinolates or moderate concentrations of 3-methy sulfinyipropyl or 4-methyl sultinylbutyl. Cultivars tend to have both 3-butenyl and 4 pentenyl (and hydroxylated forms) or just 3-butenyl 15 IARC Handbooks of Cancer Prevention Volume 9 Cruciferous vegetables, isothiocyanates and indoles /S —Glucose OH2 N N2 0s03- R Irdolyl glucosinolate R = H or 00H3 Myrosinase C R H20 pH 3-4 Ascorbic acid j Y H1OHO f CH2-CN CH2 0H (F 0H 0H CR2 +s R R +SCN N lndolyl-3-carbinOl Ascorbigen lrdolyI3-acetonitriIe Figure 6 Degradation pathways of indole gluco&nolates neoglucobrassicin was found in some indole glucosinolate is glucobrassicin, side-chain and partially determines studies to be even higher than that of and 4- met hoxy-3-Lndolylrnethyl glu- the overall amount; glucobrassicin (yang et at., 2001; cosinolate is present at half the con- abiotic and biotic environmental Vallejo et al., 2003a). In Brussels centration (Ciska et al., 2000). factors, which can influence the sprouts, neoglucobrassicin occurs at a Four factors interact to determine overall amount of isothiocyanates much lower concentration than gluco- the exposure of the gastrointestinal produced by the plant; brassicin (Kushad et al., 1999; Ciska tract to isothiocyanates and indoles: post-harvest storage, processing et al., 2000), while 4-hydroxy-3- the biochemical genetics of the and cooking; and indolylmethyl glucosinolate and gluco- biosynthesis of glucosinolates and the thioglucosidase activity of the brassicin are found at similar concen- isothiocyanates within the crop intestinal microbial flora. trations in cauliflower (Kushad et al., plant, which

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