The Brassica family of vegetables, including broccoli, kale, water cress, rape seed, Brussels sprouts, cabbage, turnip, mustard and radishes, are the only plant source of the sulphur containing glucosinolate compounds. Chemically, glucosinolates are β-thioglucoside N-hydroxysulfates with a variable side chain and a sulphur linked β-glucopyranose moiety. Although there are over 100 known glucosinolates with different variable groups, only around 15 are common in plants eaten by humans. Hydrolysis of the thioglucosidic bond in glucosinolates results in the formation of isothiocyanates, compounds that give the recognisable bitter taste of Brassica vegetables. Glucosinolate are of interest because their metabolites may protect humans from various types of cancer. However, polymorphism is metabolic enzymes and the reduction of glucosinolates in plant breeding programmes to produce less bitter tasting plants, suggest that the association between Brassica consumption and cancer is not simple.
Epidemiological studies suggest that Brassica vegetable consumption is beneficial in preventing some types of cancer, but questions remain as to the identity of the protective chemical within the plants. The bioavailability of the glucosinolate compounds themselves is very low and this calls into question their anti-cancer effects. Animals and cell culture work suggests that rather than the glucosinolates themselves, it is the isothiocyanate metabolites that are protective of cancer. As with flavonoids, the absorption, metabolism and excretion of isothiocyanates appears to be complex, and is not fully understood. Mechanistically, the protection that isothiocyanates show in models of cancer is similar to that shown by flavonoids. Possible mechanisms that have been researched include down-regulation of phase I detoxification enzymes such as CYP3A, induction of phase II detoxification enzymes such as glutathione S-transferase, cellular antioxidant defence, apoptosis, cell cycle arrest and reductions in cellular proliferation.
Glucosinolates themselves have been shown to be absorbed in very small quantities in animal and human studies, but the more lipophilic isothiocyanates appear to be well absorbed and are present in humans plasma in much higher amounts. Glucosinolates can be converted to isothiocyanates by hydrolysis of the thioglucosidic bond, but an enzyme containing thioglucosidase activity in not believed to be present in human cells. Two mechanisms exist that are thought to account for the formation of isothiocyanates which are subsequently absorbed by either the small intestine or colon via passive diffusion. Myrosinase is an enzyme in plants of the Brassica family that possesses thioglucosidase activity. Breakdown of the cells, as may occur through cooking or chewing, brings the myrosinase and glucosinolates together to cause hydrolysis of the thioglucosidic bond and the formation of isothiocyanates. In addition, colonic microflora are believed to possess thioglucosidase activity.
Because myrosinase is a protein, cooking Brassica vegetables may denature the enzyme and prevent the formation of isothiocyanates. However, cooking destroys the cell structure and therefore is beneficial at increasing enzyme and substrate interactions. Isothiocyanate production could increase with light cooking, such as steaming, which has an effect to softens the cellular constituents, but does not denature the enzyme and inhibit myrosinase activity. If myrosinase activity is lost, then glucosinolates will remain intact throughout the small intestine, until they come into contact with colonic microorganisms possessing thioglucosidase activity. Following this route would slow the absorption of any subsequent isothiocyanates formed. Research shows that uncooked Brassica vegetables show an absorption peak in plasma at around 2 hours but cooking extends this to around 6 hours. Urinary excretion of isothiocyanate also appears to be higher following consumption of cooked, rather than uncooked Brassica vegetables.