Glucosinolates

Thyroid Factor

The Natural Thyroid Diet

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A variety of plants contain a third group of antiminerals, the so-called glucosinolates, a class of thioglucosides, whose general structure is shown below.

Glucosinolates Found Cabbage

H OH

Glucosinolate

H OH

Glucosinolate

Many glucosinolates are goitrogenic. Three types of goiter are distinguished: cabbage goiter (struma), brassica seed goiter, and legume goiter.

Cabbage goiter can be induced by the excessive consumption of cabbage. It seems that cabbage goitrogens inhibit iodine uptake by directly affecting the thyroid gland. Cabbage goiter can be treated by iodine supplementation.

Brassica seed goiter can result from the consumption of the seeds of Brassica plants, such as rutabaga (swede), turnip, cabbage, rape, and mustard, which contain substances that prevent thyroxine synthesis. This type of goiter can only be treated by administration of the thyroid hormone.

Legume goiter is induced by goitrogens in legumes such as soybeans and peanuts. It differs from cabbage goiter in that the thyroid gland is not involved directly. Inhibition of the intestinal absorption of iodine or the reabsorption of thyroxine has been shown in this case. Legume goiter can be treated by iodine therapy.

Fifty types of glucosinolates have been identified. Table 3.3 gives a list of foodstuffs which have been shown to induce goiter, at least in experimental animals.

Rutabaga, turnips, cabbage, peaches, strawberries, spinach, and carrots can cause a significant reduction in the iodine uptake by the human thyroid gland, with rutabaga being the most active.

Table 3.3 Goitrogenic foodstuffs

Food or feedstuff Glucosinolatea Specific chemical (R) Group

Table 3.3 Goitrogenic foodstuffs

Food or feedstuff Glucosinolatea Specific chemical (R) Group

Broccoli (buds)

Glucobrassicinb

3-Indolylmethyl

Gluconapinc

3-Butenyl

Neoglucobrassicinc

3-(N-methoxyindolyl)methyl

Progoitrinc

2-Hydroxy-3-butenyl

Sinigrinb

Allyl

Brussel sprouts (head)

As in broccoli

Cabbage (head)

As in broccoli

Cauliflower (buds)

As in broccoli

Charlock (seed)

Sinalbinc

p-Hydroxybenzyl

Crambe (seed)

Gluconapinc

(See broccoli above)

Gluconasturtlinc

2-Phenylethyl

Garden cress (leaves)

Glucotropasolinc

Benzyl

Horseradish (roots)

Gluconasturtlinc

(See crambe above)

Sinigrinb

Kale (leaves)

As in broccoli

Kohlrabi (head)

As in broccoli

Mustard, black (seed)

Sinigrinb

(See broccoli above)

Mustard, white (seed)

Sinalbinb

(See charlock above)

Radish (root)

4-Methylthio-3-butenyl

Glucobrassicinc

(See broccoli above)

Rutabaga (root)

Glucobrassicinc

(See broccoli above)

Neoglucobrassicinc

(See broccoli above)

Progoitrinb

(See broccoli above)

Turnips (root)

Gluconasturtlinb

(See crambe above)

Progoitrinc

(See broccoli above)

2-Hydroxy-4-pentenyl

a Trivial or common name of glucosinolate. The chemical name is formed by the designation of the R group prefixed to the term glucosinolate, e.g., glucobrassicin is 3-indolylmethyl-glucosinolate. b Major glucosinolate. c Minor glucosinolate component.

a Trivial or common name of glucosinolate. The chemical name is formed by the designation of the R group prefixed to the term glucosinolate, e.g., glucobrassicin is 3-indolylmethyl-glucosinolate. b Major glucosinolate. c Minor glucosinolate component.

Hydrolysis, which takes place in damaged plant tissue before or after ingestion, yields the actual goitrogens: thiocyanates, isothiocyanates, cyclic sulfur substances, and nitriles. The thiocyanates and isothiocyanates formed from the glucosinolates are probably the substances responsible for cabbage goiter. As discussed in Chapter 2, goitrogens can also be formed from cyanogens, as their major biotransformation products are thiocyanates. An illustrative example of activation of a glucosinolate by hydrolysis is the potent antithyroid progoitrin occurring in the seeds of Brassica plants and the roots of rutabaga. This substance undergoes hydrolysis as shown in Figure 3.1.

The cyclization product of isothiocyanate III, goitrin (IV), is a powerful goitrogen. The R-enantiomer of goitrin found in crambe seeds is also a strong antithyroid agent. These substances interfere with the iodination of thyroxine precursors so that iodine therapy is not successful. The nitriles I and II obtained from progoitrin are highly toxic, but it is uncertain whether they are goitrogens. Mustard and rapeseed varieties have been bred with low thioglucoside concentration.

S — glucose ch2=ch-ch-ch2-< thioglucosidas% I, II, III, IV

Progoitrin OH 3

S OH

Figure 3.1 Hydrolysis of progoitrin.

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