Cruciferous vegetables such as broccoli, cauliflower, brussel sprouts, kale and water cress (brassicaceae family) are a good source of glucosinolates. Glucosinolates are phytochemicals that can be converted into isothiocyanates by the plant based enzyme myrosinase, or by bacterial myrosinase in our small intestine. Isothiocyanates are of interest to nutritional scientists because evidence in mounting that they are protective of cancer. For example, research published in the Journal of Nutrition in 20041, suggest that consumption of cruciferous vegetables is inversely associated with the development of breast cancer in premenopausal women. Of the cruciferous vegetables, broccoli showed the strongest protective effect. Isothiocyanates are thought to be protective of cancer because they are potent inhibitors of phase I detoxification, but strong inducers of phase II detoxification. Sulforophane is an isothiocyanate that has been identified as having potent anti cancer effects for this reason.
Sulphorophane has been shown to be able to inhibit the phase I detoxification pathway. During phase I detoxification, chemicals are metabolised by enzymes of cytochrome P450 system (e.g. Cyp3A4, Cyp1A1, Cyp1A2) present mainly in the liver. These enzymes metabolise various drugs, chemicals and endogenous compounds by oxidising or reducing them, often to more toxic chemicals. Some of the phase I products are carcinogens, and can induce cancer if left to accumulate. However, this is prevented from happening by phase II detoxification. Here the metabolites from the phase I pathway are further metabolised to a less toxic, more water soluble compound that can be excreted. This is achieved by conjugation reactions that can involve glutathione conjugation, methylation, amino acid conjugation, sulfation, acetylation or glucuronidation. Sulphorophane can induce phase II detoxification
Because cruciferous vegetables contain sulphorophane and other isothiocyanates, they are able to inhibit phase I detoxification and induce phase II detoxification. This is important because it prevents the accumulation of carcinogenic phase I intermediates in the body. Broccoli has been the most researched of the brassicaceae family, but water cress has the highest known glucosinolate content of any vegetable. Ideally a mixture of the of the brassicaceae family vegetables should be consumed on a regular basis. The enzyme myrosinase is present in the cells of the plants and is needed to convert the glucosinolates to isothiocyanates. Cooking deactivates this enzyme, but also breaks open cells walls allowing greater bioavailability of their content. Light cooking is the best way to achieve high amounts of isothiocyanates because it destroys the cells walls, but does not fully inactivate myrosinase.
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