Glucosinolates (Figure 7.20) occur in brassicas (cabbage, cauliflower, broccoli, sprouts) and some other Cruciferae. The enzyme myrosinase in vacuoles in the plant cell is released when cells are damaged; it catalyses cleavage of glucosinolates to yield a variety of isothiocyanates, thiocyanates and nitriles plus the aglycone. Intestinal bacteria have a similar enzyme, so glucosinolates from cooked vegetables yield similar products.
Like the allyl sulphur compounds in Allium spp., the aglycones of glucosinolates lower the activity of microsomal cytochrome P450 by:
There is a potential hazard associated with excessive consumption of brassicas — a number of the glucosinolates have a goitrogenic action, reducing synthesis of the thyroid hormones (section 22.214.171.124). Two mechanisms are involved:
Goitre is a well-known problem in cattle fed on brassicas, but there is no evidence of reduced thyroid hormone status in people consuming, for example 150 g sprouts per day for several weeks. It is, however, noteworthy that iodine-deficiency goitre was a problem in The Netherlands (a country that cannot be considered to be upland, over limestone soil or inland, the usual criteria for iodine deficiency; section 126.96.36.199) until the introduction of iodide enrichment of flour at the beginning of the twentieth century. The traditional Dutch diet included a considerable amount of sauerkraut (fermented cabbage) — to such an extent that during the sixteenth and seventeenth centuries, when seafarers from most countries suffered from scurvy (vitamin C deficiency; section 11.14.3) during long voyages of exploration, the Dutch mariners did not.
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