Glucosinolates

Glucosinolates are glucosides of sulfur-containing amino acid derivatives found mainly in brassicas; the enzyme myrosinase in the plant and similar intestinal bacterial enzymes catalyze cleavage of the glycoside to form a variety of isothiocyanates, thiocyanates, and the aglycone. Figure 14.11 shows the general structure of glucosinolates.

The glucosinolates have two actions that may protect against the development of cancer:

1. They reduce the activation of many procarcinogens to the active proximate carcinogen by both inhibiting and reducing the synthesis of mi-crosomal cytochrome P450 enzymes.

Figure 14.11. Glucosinolates.

2. They increase the metabolic clearance of potential carcinogens and their metabolites, by the induction of glutathione S-transferases and quinone reductases.

Myrosinase action on the glucosinolate progoitrin yields goitrin (see Figure 14.11), which inhibits the synthesis of thyroid hormones by inhibition of the iodination of monoiodotyrosine to diiodotyrosine and thus has a goitrogenic action. In addition, the thiocyanate released by myrosinase competes with iodide for uptake into the thyroid and may be goitrogenic when iodine status is marginal. There is no evidence that normal consumption of vegetables has any effect on thyroid hormone status, although goiter is a problem when cattle are fed on large amounts of brassicas.

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