Mineral That Can Be Bound by Goitrogens and Glucosinolates in Feeds

Glucosinolates

Karyn L. Bischoff , in Nutraceuticals, 2016

Major Glucosinolates

Some well-characterized glucosinolates include progoitrin, sinigrin, thiocyanate precursors glucobrassican and neoglucobrassican, and glucoraphanin. These are predominantly metabolized to reactive isothiocyanates or mustard oils, which impart the mustard-like or garlic-like odors associated with mustard and horseradish (Burrows and Tyrl, 2013 ). Other metabolites include thiocyanates, nitriles, sulfates, and goitrins.

Progoitrin

Progoitrin is hydrolyzed to goitrin, or l-5-vinyl-2-thioöxazolidine, which is named for its antithyroid effects. Goitrin blocks tyrosine iodination and inhibits T4 formation (Burrows and Tyrl, 2013). Long-term ingestion is associated with goiter formation. Progoitrin is found in rapeseed, kale, and a variety of vegetables and seeds.

Sinagrin

Sinagrin is a prop-3-enyl glucosinolate. It is hydrolyzed to allyl isothiocyanate, which may inhibit microRNA expression and activate transcription factor Nrf2 (Wagner et al., 2013). Sinagrin is found in Brussels sprouts, mustard seeds, and other vegetables and seeds.

Glucobrassicin

Glucobrassicin is a 3-indolylmethyl glucosinolate that is hydrolyzed to an unstable isothiocyanate. One of the products of glucobrassicin metabolism is indol-3-carbinol, which inhibits nuclear factor (NF)-κβ (Wagner et al., 2013). Glucobrassicin is found in almost all of the plants in the Brassica family.

Glucoraphanin

Glucoraphanin, or 4-methylsulfinylbutyl glucosinolate, is hydrolyzed to sulforaphane, an isothiocyanate. Sulforaphane has tumor prevention properties: it blocks the cell cycle and promotes apoptosis (Halkier and Gershenzon, 2006). Glucoraphanin is a major component of broccoli and other vegetables.

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GOITROGENS AND ANTITHYROID COMPOUNDS

J. Vanderpas , in Encyclopedia of Food Sciences and Nutrition (Second Edition), 2003

Goitrin and Aliphatic Disulfides

On enzyme hydrolysis, a particular thioglucoside gives rise to progoitrin, which is rapidly converted to form goitrin. Progoitrin is present in swedes and turnips. Administration of goitrin to rats for 20 days induces an enlargement of the thyroid, decreases iodine uptake by the gland, and decreases T ₄ synthesis. Other sources of goitrin-like compounds have been detected in various species of herbs and shrubs of the Barbarea and Residea families.

The role of goitrin has been advocated in the development of endemic goiter in schoolchildren of Tasmania; this goiter could not be prevented by iodine supplementation. The endemia was attributed to a goitrogenic factor ingested by cattle from thousand-headed kale and transmitted by milk. However, only 0.05% of goitrin appears in the milk, and is rapidly denatured unless the milk is heated immediately. Goitrin has also been involved as an environmental goitrogen in Finland.

The major components of the volatile compounds from onion and garlic are small aliphatic disulfides which depress uptake of radioactive iodide by the thyroid of rats on a low-iodine diet. Disulfides are also present in high concentrations (0.3–0.5   g   l⁻¹) in aqueous effluents from coal-conversion processes; it is possible that these water contaminants intervene in the etiology of endemic goiter in the Cauca districts of Colombia.

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Goitrogens, Environmental

Eduardo Gaitan , in Encyclopedia of Endocrine Diseases, 2004

Thio-Oxazolidone (Goitrin)

The thionamide or thiourea-like goitrogens interfere in the thyroid gland with the organification of iodide and formation of the active thyroid hormones, and their action usually cannot be antagonized by iodine. Naturally occurring goitrin is representative of this category ( Fig. 1). Long-term administration of goitrin to rats results in increased thyroid weight and decreased radioactive iodide uptake and hormone synthesis by the thyroid gland. Actually, goitrin possesses 133% of the potency of propylthiouracil in humans. Goitrin is unique in that it is not degraded like thioglycosides. Additive antithyroidal effects of thiocyanate, isothiocyanate, and goitrin also occur with combinations of these naturally occurring goitrogens.

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Hazards and Diseases

C. Crews , D. Clarke , in Encyclopedia of Food Safety, 2014

Toxicity

When the tissues of brassica plants are macerated or cooked, the glycosides are hydrolyzed to release volatile isothiocyanates or nitriles and thiocyanates. The major toxicity is associated with oxazolidine-2-thiones, such as goitrin (5-vinyloxazolidine-2-thione), which impair thyroid function by inhibiting the formation of thyroxine by binding iodine and suppressing thyroxine secretion from the thyroid.

The oil of the brassica rapeseed is often assumed to have cardiovascular toxicity on account of its high level of erucic acid. Despite evidence for this being rather poor, special varieties of low erucic acid rapeseed are now cultivated for edible oil production.

Nitriles released from glucosinolates depress growth and cause damage to the liver and kidneys in severe cases.

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Environmental Disrupters of Thyroid Hormone Action

Françoise Brucker-Davis , in Encyclopedia of Hormones, 2003

II Natural environmental thyroid disrupters

The primary natural thyroid disrupters, excluding dietary iodine deficiency, are goitrogens found in food or water supplies as a result of bacterial contamination or mineral compound decomposition (Table 1). Goitrogenic substances are also found in many vegetables: thiocyanates and isothiocyanates in Cruciferae; goitrin in turnips; cyanogenic glucosides in cassava or sweet potatoes; disulfides in onion and garlic; and flavonoids in millet, sorghum, and beans. In addition, degradation of humic substances, i.e., soil decomposition of plant and animal tissues, leads to the production of resorcinol, a phenol derivative with potent antithyroid effects. Distinguishing between natural mineral compounds (such as coals or shales) and synthetic contamination is sometimes fuzzy, because mineral compound decomposition and industrial manufacturing processes may produce similar chemical products.

Table 1. Chemicals with Thyroid-Disrupting Properties

Family	Name a	Natural or synthetic b
Sulfurated organics	Thiocyanate, isothiocyanate, goitrin, disulfides	N
Flavonoids	Glycosides, aglycones	N
Phenol derivatives	Resorcinol, DNP, pyrogallol	N, S
Pyridines and hydroxypyridines	—	N, S
Phthalate esters and metabolites	—	N, S
Polyhalogenated hydrocarbons	PCB, PBB, dioxin	S
Polycyclic aromatic hydrocarbons	Benzopyrene, methylcolanthrene, dimethylbenzanthracene	N, S
Pesticides
Chlorinated	DDT and others	S
Others	Amides, benzonitriles, carbamates, organophosphates, pyrethroid, pyridinoxy, thiocarbamates, thiourea, triazine, triazole	S
Heavy metals, inorganics	Hg, Pb, I, Cd, perchlorate	N

a

Abbreviations: DNP, dinitrophenol; DDT, dichlorodiphenyldichloroethane; PCB, polychlorinated biphenyl; PBB, polybrominated biphenyl; Hg, mercury; Pb, lead; I, iodine; Li, lithium; Cd, cadmium.

b

N, Naturally occurring even though it may be used for industrial or other anthropological purpose; S, synthetic.

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Enzymatic activities behind degradation of glucosinolates

Karol Parchem , ... Agnieszka Bartoszek , in Glucosinolates: Properties, Recovery, and Applications, 2020

3.4.5 Oxazolidine-2-thiones

Oxazolidine-2-thione arises as a result of spontaneous arrangement of unstable ITCs containing hydroxyl group in position 2 (β-hydroxy) (Carlson et al., 1987). In the case of this reaction (Fig. 3.1 ), neither enzymatic proteins nor additional metal ion cofactors were identified as necessary. The best known compound from this group is the thiocarbamate 5-ethenyl-1,3-oxazolidine-2-thione, known under common name goitrin. Goitrin was demonstrated to impair the iodine homeostasis in animal organisms by the deregulation of the function of the enzyme thyroid peroxidase, which results in the decreased production of thyroid hormones such as thyroxine. The GL precursor of this compound is progoitrin, that is present among others in cabbage, Brussels sprouts, cauliflower, and broccoli ( Vig et al., 2009). Its concentration is so low, that typical intake of these vegetables does not lead to the mentioned antinutritional, so-called goitrogenic effects in healthy subjects. However, the presence of progoitrin and the product of its degradation—goitrin creates a problem for oil industry as this prevents utilization of side products, e.g., expeller produce during oil pressing from rape seeds, as a component of animal feeds. In animals consuming feeds containing rape addition, the enlargement of the liver and kidneys was diagnosed; also the decrease in appetite and generally slower growth was observed. Moreover, goitrin can pass to the milk of animals consuming feeds with rape addition (Bachmann et al., 1985), which in turn may be dangerous for children drinking such milk. To ensure appropriate quality of oils as well as satisfactory feeding value of rapeseed side products, Common Catalog of Varieties of Agricultural Plant Species contains only such varieties in which GL content does not exceeds 15 and 25   μmol/g of defatted dry mass. An acceptable level in the case of rape supplemented feeds for animals is 15–25   μmol/g of defatted dry mass. It should be pointed out that during oil production some processes decrease the goitrin content. For example, the additional treatments with high temperature and vapor are aimed at removing GLs and possible products of their degradation. Also during oil pressing, such stages as heating the plant raw material up to 100°C (seed conditioning) cause the inactivation of myrosinase and thereby blockage of GL hydrolysis, including progoitrin to goitrin conversion. This process, though to lesser extent, may still be promoted by intestinal microbes.

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PLANT ANTINUTRITIONAL FACTORS | Detoxification

I.E. Liener , in Encyclopedia of Food Sciences and Nutrition (Second Edition), 2003

Goitrogens

Goiter-causing agents in the form of glycosides (also referred to as 'glucosinolates') are found in members of the cabbage family, which includes not only cabbage but also broccoli, Brussels sprouts, cauliflower, turnips, rapeseed, and mustard seed. These compounds are biologically inactive as long as they remain bound to glucose ((1)Figure 2 ), but an enzyme, myrosinase, present in the same plant, serves to release the active goitrogenic principle, goitrin ( (2) Figure 2). The glucosinolates found in the cabbage family appear to pose little risk to human health since the enzyme responsible for the release of goitrin is inactivated by household cooking. The glucosinolates may also be removed to a great extent by leaching out into the cooking water. The common usage of rapeseed meal in the feeding of livestock may prove to be toxic unless it has been treated with moist heat. Alternative methods of detoxification of rapeseed may involve prior extraction of the glucosinolates with water or acetone or by decomposition with iron salts or soda ash. These procedures, however, do not preclude the possibility that some goitrin produced enzymatically prior to processing may still remain in the meal. A more effective means of detoxification is one in which an aqueous slurry of the meal is deliberately allowed to undergo autolysis, which serves to liberate virtually all of the goitrin; the latter is then removed by extraction with water or acetone. Lactic acid fermentation or treatment with a specific fungus (Geotrichum candidum) has also been reported to be an effective means for the biological destruction of the glucosinolates in rapeseed meal. The immobilization of myrosinase on a solid matrix offers a promising approach for the hydrolysis of the glucosinolates, provided the goitrogenic end products are subsequently removed by extraction, as described above. (See PLANT ANTINUTRITIONAL FACTORS | Characteristics.)

Figure 2. Structure of one of the goitrogenic compounds present in the cabbage family. Progoitrin (1) is biologically inactive, but, upon hydrolysis by the enzyme myrosinase, the active principle goitrin (2) is released.

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GLUCOSINOLATES

B. Holst , G.R. Fenwick , in Encyclopedia of Food Sciences and Nutrition (Second Edition), 2003

Structure of the Side Chain

At a pH of 6–7, most glucosinolates yield stable isothiocyanates (11, 12). However, those possessing a β-hydroxylated side chain form unstable hydroxyisothiocyanates that spontaneously cyclize to oxazolidine-2-thiones (13 ), the best-known example being 5-vinyloxazolidine-2-thione (goitrin). Indole glucosinolates also form unstable isothiocyanates; these undergo lysis, initially forming the corresponding alcohol, e.g., indole-3-carbinol ( 14) and subsequently condensing to the dimer (15), trimer, or tetramer. In the presence of ascorbic acid, ascorbigen (16) is the major product (Figure 3).

Figure 3. Glucosinolate breakdown products – Isothiocyanates (2.1, Figure 2). ^a Refers to Figure 1.

Depending on the plant species, autolysis of fresh material (pH ∼ 6) can yield nitriles (17–21). Several authors have discussed a 'nitrile-forming factor' but this has neither been isolated nor characterized. Under acidic conditions, nitriles are the major degradation products (17, 18, 20, 21: Figure 4). In the presence of active epithiospecifier protein (ESP) and Fe²⁺ ions, glucosinolates with terminally unsaturated side chains transfer sulfur from the S-glucose moiety to the alkenyl moiety to form an epithionitrile (19). ESP is a small protein, which co-occurs and interacts with myrosinase but is not present in all glucosinolate-containing species. In the absence of ESP the addition of ferrous ions to reaction mixtures promotes nitrile formation (18).

Figure 4. Glucosinolate breakdown products – nitriles (2.2, Figure 2). ^a Refers to Figure 1.

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Assessment of Iodine Intake and Iodine Status in Vegans

Helen J. Lightowler , in Comprehensive Handbook of Iodine, 2009

High Intake of Dietary Goitrogens

Another confoundingfactor in the assessment of iodine intake may be environmental goitrogens, substances that interfere with thyroid hormone production or utilization (Gaitan, 1990). Vegetables of the genus Brassica have been considered to possess goitrogenic properties, due to a simultaneous effect of thiocyanate, isothiocyanate and thiooxazoliclones. Moreover, a potent antithyroid compound, "goitrin," is found in some Brassica seeds, and seaweeds of the genus Laminaria (such as kombu) have a high content of the potent antithyroid compound, phloroglucinol and other polyhydroxyphenols. Thus, sustained seaweed ingestion may play an additional role to that of iodine excess in the development of goiter.

Environmental goitrogens may normally be ineffective when in low concentration, and most are not of major clinical importance unless there is coexisting iodine deficiency. However, a vegan diet is likely to include more foods possessing goitrogenic properties, such as brassicas and seaweed, compared with an omnivorous diet, and previous research has shown that vegans are at increased risk of iodine deficiency. Thus, excess intake of these substances may affect iodine metabolism in vegans.

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Glycosides

M. Bartnik , P.C. Facey , in Pharmacognosy, 2017

8.11.5 Adverse Effects of Thioglycosides

The tyroid gland, pancreas, kidney, and liver are the main targets of the toxicity due to the hydrolysis products of THGs. Thyroid-toxicity in animal experiments shows that certain isothiocyanates interfere with the synthesis of thyroid hormones, whereas thiocyanates compete with iodine and inhibit iodine uptake by the thyroid gland (goitrin, which inhibits iodine incorporation and the formation of thyroxin) [261]. Oxazoline-thiones interfere with thyroxine synthesis. Goitrogenic as well as antigoitrogenic glucosinolates are present sometimes in the same plant material [11,24]. Damage to liver, kidney, and pancreas by some decomposition products has been also observed [261]. The unstable isothiocyanates formed from indole glucosinolates decompose to carbinols. Under the acid conditions of the stomach, the indole-3-carbinol may spontaneously condense to form compounds (e.g., 2,3,7,8-tetrachlorodibenzo-p-dioxin) which in structure, toxicity, and carcinogenicity closely resemble dioxin [251]. Caution should be exercised if recommending long-term intake of THGs containing plants at doses well in excess of optimum dietary levels because of their possible mutagenicity, tumor promotion, and carcinogenicity [262]. However, at normal dietary levels, they seem to be nontoxic and even beneficial (as discussed before).

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