Since there is a large number of a different type of glucosinolates and different possible pathways for hydrolysis, a broad range of hydrolysis products can be found in various food sources. All these compounds show distinctive, concentration-dependent biological activities varying from acute toxicity to anticarcinogenic properties. The consumption of vegetables and fruits has always been seen as health promoting. The protective effect of Brassica veg-etables against cancer has been suggested to be due in part to the relatively high content of glucosinolates. The presence of glucosinolates distinguishes them from other vegetables. Be-sides their beneficial health effects, some glucosinolates and breakdown products also show toxicological effects. The toxicity of glucosinolates, mainly determined by the aglycones, has been described in many studies.
3.6.1 Anticarcinogenicity
The most extensively studied mechanism for inhibition of carcinogenesis by glucosinolate-derived hydrolysis products are the modulation of the antioxidant potential, enhancement of detoxification mechanisms and the induction of apoptosis in undifferentiated cells (Mithen et al., 2000).
Carcinogenesis is a multi-stage process in which at least three distinct phases can be recognized: initiation, promotion and progression. At each stage of the carcinogenic process there can be intervention. Wattenberg (1985) proposed a system of classification of dietary anticarcinogens based on the stage of carcinogenesis at which they act. Anticarcinogens
Table 3.2 Protection against chemical carcinogenesis in rat and mouse organs by a variety of isothiocyanates and glucosinolates.
Protective isothiocyanates α-Naphthyl-NCS, β-naphthyl-NCS
Phenyl-[CH2]n-NCS, where n= 0, 1, 2, 3, 4, 5, 6, 8, 10 PhCH(Ph)CH2-NCS, PhCH2CH(Ph)-NCS
CH3[CH]n-NCS, where n= 5, 11 CH3[CH2]3CH(CH3)-NCS
Sulforaphane, CH3S(O)[CH2]4-NCS 2-Acetylnorbornyl-NCS (three isomers) Protective glucosinolates
Indolylmethyl glucosinolate (glucobrassicin) Benzyl glucosinolate (glucotropaeolin) 4-Hydroxybenzyl glucosinolate (glucosinalbin) Carcinogens employed
3-Methyl-4-dimethylaminoazobenzene 4-Dimethylaminoazobenzene
N-2-Fluorenylacetamide, acetylaminofluorene 7,12-Dimethylbenz[a]anthracene (DMBA) Benzo[a]pyrene
Methylazoxymethanol acetate N-Nitrosodiethylamine
4-(Methylnitroamino)-1-(3-pyridyl)-1-butanone (NNK) N-Nitrosobenzylmethylamine (NBMA)
N-Butyl-N-(4-hydroxybutyl)nitrosamine Tumour target organs
Rat: liver, lung, mammary gland, bladder, small intestine/colon, oesophagus Mouse: lung, forestomach
Source: Talalay and Zhang (1996).
can then be divided into three major classes. The first consists of compounds that prevent the formation of carcinogens from precursor substances. The second class is called blocking agents. These have been found to be effective when given immediately before or during treatment with chemical carcinogens. The third class, called suppressing agents, is thought to act by preventing the progression of initiated cells to fully transformed tumour cells.
Isothiocyanates that arise in plants as a result of enzymatic cleavage of glucosinolates by the endogenous enzyme myrosinase are attracting increasing attention as chemical and dietary protectors against cancer. Their anticarcinogenic activities have been demonstrated in rodents (mice and rats) with a wide variety of chemical carcinogens (Table 3.2). The anticarcinogenic effects of ITCs can be explained by two different mechanisms. The first, a blocking effect, involves induction of Phase II enzymes, including quinone reductase in the small intestinal mucosa and liver (Zhang et al., 1992b; Talalay and Zhang, 1996). These enzymes are involved in the detoxification in the body of foreign compounds (xenobiotics). Increased activity will therefore block exposure of target tissues to DNA damage (Fimognari and Hrelia, 2007). The second mechanism, a suppressing effect, involves suppression of tumour development via deletion of damaged cells from colonic mucosal crypts through the induction of programmed cell death (apoptosis). Smith et al. (1996) showed that dietary supplementation with the glucosinolate sinigrin, or its breakdown product allyl isothiocyanate, can protect against chemically-induced colorectal carcinogenesis by stimulation of apoptosis.
The most characterized isothiocyanate is sulforaphane because of its ability to simulta-neously modulate multiple cellular targets involved in cancer development (Fimognari and Hrelia, 2007).
The evidence for anticarcinogenic effects of Brassica vegetables in humans is strongly supported by evidence obtained from epidemiological and human intervention studies as well as with experimental animals. Verhoeven et al. (1996) reviewed 7 cohort studies and 87 case–
control studies and showed an inverse correlation between the consumption of individual Brassica vegetables and the risk of lung, stomach and second primary cancers. Broccoli consumption in particular shows a uniform protective effect, with no contrary evidence in any study. Consumption of Brassica vegetables, which might be expected to yield high levels of indoles and ITCs, was particularly strongly associated with a lower risk of colon cancer.
Brennan et al. (2005) showed that the protective effects of a diet rich in cruciferous vegetables towards the occurrence of lung cancer is strongly dependent on the human genetic background (GSTM1 and GSTT1 background). Reduction of risks up to 72% was reported in this study.
High intake of glucoraphanin can be obtained by the consumption of broccoli sprouts. The effect of this intake is studied in clinical trials now (Shapiro et al., 2006).
3.6.2 Toxicity
Initially, glucosinolates were studied because of their potentially deleterious effects. A lot of attention was given to removal of glucosinolates from dietary sources and animal feed by breeding (‘double zero’ rapeseed) and processing of plant material.
In the field of animal production it is well-known that ingestion of substantial amounts of glucosinolates may result in a variety of toxic and antinutritional effects. The fodder and seed meals of genus Brassica such as crambe, kale, mustard, rape, cabbage and turnips are the main source of glucosinolates in animal diets. Major deleterious effects of glucosinolates ingestion in animals are reduced palatability, decreased growth and production. Nitriles are known to affect liver and kidney functions. The ITCs interfere with iodine availability, whereas 5-vinyl-2-oxazolidinethione (VOT) is responsible for the morphological and physiological changes of the thyroid. Other adverse effects of glucosinolate metabolites are goitrogenicity and mutagenicity. Deleterious effects of glucosinolates are greater in non-ruminant animals compared to ruminants. Also, in general, young animals appear to be more sensitive to glucosinolates than adults or older animals. Animal species are affected in different degrees of severity; pigs are more severely affected by dietary glucosinolates than rabbits, poultry and fish (Tripathi and Mishra, 2007).
The oil meal of Brassica origin is a good source of protein for animal feeding but its glu-cosinolate content limits its efficient utilization. Presently, very low-gluglu-cosinolate rapeseed varieties are available that contain less than 25 μmol g−1of total glucosinolates. Furthermore, various processing techniques were applied to remove glucosinolates in order to minimize their deleterious effects on animals. Most of these methodologies, like microwave irradiation, micronization and extrusion, included hydrolysis or decomposition of glucosinolate before feeding (Tripathi and Mishra, 2007). Also, a reduction in glucosinolate content could be obtained by autoclaving rapeseed meal for 1.5 hours (Mansour et al., 1993), treatment of meal with Cu2+ and the use of ammonia in conjugation with other processing (Keith and Bell, 1982). During seed processing most glucosinolate breakdown products are formed.
The degree of degradation depends on seed properties and on processing conditions such as moisture level, pressure and temperature (Mawson et al., 1993).
In principle, glucosinolate breakdown products can be capable of inducing goitrogenic effects in humans, but there is little or no epidemiological evidence that this is an important cause of human disease. Experimentally, consumption of 150 g of Brussels sprouts in the diets of adult volunteers had no effect on their levels of thyroid hormones (McMillan et al., 1986).
The current evidence suggests that normal consumption of glucosinolate containing veg-etables does not damage human health. In contrast, beneficial effects are prevailing. On the other hand, there is a need for further studies (e.g. human intervention studies) because the growing interest in the anticarcinogenic properties of glucosinolates and the possibility that this may lead to increased human exposure to these compounds raises important questions about the balance of adverse and beneficial effects of Brassica vegetables or derived products with enhanced levels.