Glucosinolates (Goitrogenic Glycosides)
Glucosinolates are thioethers. They generally consist of a sugar entity, b-D-thioglucose, with an ester bond to an organic aglycone that is an alkyl group yielding isothiocyanate, nitrile, thiocyanate or a similar compound upon hydrolysis. These compounds often contribute a bitter, "hot" taste to condiments (mustard, horseradish) and may exhibit goitrogenic or antithyroid activity.
Glycosinolates are hydrolyzed by either the enzyme glucosinolase or thioglucosidase into glucose, HSO4- , and one of the following aglycone derivatives: isothiocynates, thiocyanates, nitriles, or related compounds such as oxazolidine-2-thiones. The enzymes for hydrolysis are produced by plants and by rumen organisms. They react with the glucosinolates when plant tissue is crushed, for example by mastication, or when the plant is consumed into the rumen of a ruminant animal.
- Isothiocyanates - are irritating to mucous membranes and not readily consumed in sufficient quantities to be toxic. However, if they are consumed as glycosinolates and then hydrolyzed to isothiocyanates in the gut, they can have powerful antithyroid effects and interfere with the synthesis of necessary thyroid hormones.
- Oxazolidine-2- thiones - are closely related to isothiocyanates. One way they are produced is by the conversion of the glucosinolate progoitrin in rapeseed meal to goitrin which in turn is hydolyzed to these compounds. Oxazolidine-2-thiones depress growth and increase the incidence of goiters. They inhibit thyroid function by blocking the incorporation of iodine into thyroxine precursors and by suppressing thyroxine secretion from the thyroid.
- Nitriles - depress growth, cause liver and kidney lesions, and in severe cases --> liver necrosis, bile duct hyperplasia, and megalocytosis of tubular epithelium in the kidney.
- Thiocyanates - inhibit iodine uptake by the thyroid --> leading to reduced iodination of tyrosine --> resulting in decreased production of the important thyroid hormone thyroxine.
Some common plants that contain glucosinolates include:
- Amoracia lapathifolia, Horseradish
- Brassica campestris, Turnips, yellow-hulled rape
- Brassica chinensis, Pak-choi
- Brassica napus, Rutabaga, brown-hulled rape
- Brassica nigra, Black mustard
- Brassica oleracea, Cabbage, brusselssprouts, brocolli, cauliflower, kale, kohlrabi
- Crambe abyssinica, Crambe
- Limnanthes alba, Meadowfoam
- Nasturtium officinalis, Watercress
- Raphanus sativus, Radish
- Thiaspi arvense, Stinkweed
Glucosinolates are found in several oil meals that have been used traditionally in the northern states, Canada, and Europe as protein supplements for livestock. Some examples include crambe, mustard, and meadowfoam meal, and most importantly, rapeseed meal. Rapeseed meal contains several glucosinolates and produces not only oxazolidine-2-thiones but also isothiocyanates, nitriles and thiocyanates.
Poultry and swine and other nonruminants can tolerate 5-10% rapeseed meal in their diets. Symptoms of poisoning in poultry may include depressed growth , goiters, perosis, poor egg production, off-flavored eggs, enlarged thyroid in chick embryos, and liver damage. Symptoms of poisoning in swine include growth depression, goiters, and enlarged livers.
Although rumen enzymes break down glucosinolate aglucones to their toxic derivatives, other rumen enzymes are able to metabolize these toxicants into less toxic compounds. Hence, ruminants can generally tolerate diets of 10% rapeseed meal. More than this canl cause antithyroid symptoms.
Glucosinolates and their derivatives can be transferred through milk and placenta to the young of female animals.
Addition of extra iodine to diets can help to counteract the antithyroid effects of thiocyanates but not of oxazolidine-2-thiones.
Canadian plant breeders have developed low-glucosinolate cultivars of rapeweed. These cultivars are referred to as canola rather than rapeweed. Meal from them can be used in high enough quantities to provide all the protein supplementation demanded in livestock diets that require high levels of protein.
Glucosinolates are responsilble for the unique taste of many of the condiments that make our foods more interesting to taste.
Research indicates that glucosinolates and their derivatives may have potential in fighting human cancers. Inclusion of Brassica vegetables appear to help protect against rectal and colon cancer. These vegetables aid in the detoxification of carcinogens such as aflatoxins and polybromobiphenyl. They enhance the activity of several hepatic enzymes used in detoxification processes. Benzyl isothiocyanate and thiocyanate have been shown in the lab to inhibit tumor development in animals exposed to carcinogens. Indole-3- carbinol, a product of glucosinolates, is a compound with promise in anticancer research.