Glucosinolates

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.

Figure 7.20 The glucosinolinates.

Like the allyl sulphur compounds in Allium spp., the aglycones of glucosinolates lower the activity of microsomal cytochrome P450 by:

  • direct enzyme inhibition;
  • down-regulation of enzyme synthesis — it is not known whether this is at the level of transcription or translation.

They also increase the clearance of potential carcinogens and metabolites by:

  • induction of glutathione ^-transferases;
  • induction of quinone reductase.

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 11.15.3.3). Two mechanisms are involved:

  • Thiocyanate (SCN-) competes with iodide for tissue uptake; it is goitrogenic when iodine intakes are low.
  • Oxazolidine-2-thiones (e.g. progoitrin; see Figure 7.20) inhibit thyroxine synthesis by inhibition of iodination of monoiodotyrosine to di-iodotyrosine. They are goitrogenic regardless of iodine nutritional status.

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 11.15.3.3) 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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