Introduction

It is well known that diet plays a fundamental role in the etiology of human cancer. Lifestyle choices such as the high consumption of alcohol, dietary fats, and smoking can all contribute to cancer risk. Extensive reviews of epidemiological evidence by Block et al. (1), among others, indicated that dietary intake of fruits and vegetables had a significant impact on several forms of cancers, the mechanisms of which have been described by Steinmetz and Potter (2-4). The general consensus is that increased consumption of fruit and vegetables was associated with a reduced risk of human cancers. Highlighted in these reviews was the strong correlation between a reduction in stomach, esophageal, lung, and colon cancer risk with high consumption of cruciferous vegetables, particularly members of the genus Brassica (1,5-7). The Brassi-caceae comprise a large number of vegetables such as cabbage, cauliflower, brussels sprouts, broccoli, and watercress. One characteristic of cruciferous vegetables is their ability to synthesize phytochemicals known as the gluco-sinolates (GSLs) (Fig. 1). Upon tissue damage GSLs are hydrolyzed by endogeneous plant myrosinase or alternatively by intestinal bacteria in the gut and converted to bioactive isothiocyanates (ITCs, Fig. 1). ITCs have a wide range of biological functions including antibacterial, antifungal, and anticarcinogenic properties. Furthermore, they are involved in plant herbivore interactions; we recognize them as the characteristic spicy flavors

Figure 1 Glucosinolate-mediated hydrolysis by myrosinase. (a) Formation of isothiocyanates, nitriles, and thiocyanates from a glucosinolate precursor. (b) Hydrolysis of 4-hydroxy-3-butenyl and 3-butenyl glucosinolates to their goitrogenic products, (—) 5-vinyloxazolidine-2-thione and 1-cyano-2-hydroxy-3-butene. (Adapted from Refs. 28,33.)

Figure 1 Glucosinolate-mediated hydrolysis by myrosinase. (a) Formation of isothiocyanates, nitriles, and thiocyanates from a glucosinolate precursor. (b) Hydrolysis of 4-hydroxy-3-butenyl and 3-butenyl glucosinolates to their goitrogenic products, (—) 5-vinyloxazolidine-2-thione and 1-cyano-2-hydroxy-3-butene. (Adapted from Refs. 28,33.)

associated with many of these vegetables, thus giving them their alternative name of mustard oils.

This chapter will give a brief overview of cruciferous vegetables and their potential in chemoprevention, highlighting the role of GSLs in various metabolic pathways and the formation of ITCs. The importance of these compounds on Phase I enzyme inhibition and Phase II enzyme induction, as well as apoptosis, will also be discussed. The biological significance of hydrolytic products derived from indole GSLs will not be addressed as they have been extensively reviewed (8-11). Likewise, other phytochemical constituents, such as flavonoids, isoflavones, and carotenoids, that may also contribute to the chemoprotective effects of cruciferous vegetables will not be addressed as they are beyond the scope of this chapter (12-15). Some of these compounds are covered in other chapters of this book.

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