Cyanogenic glycosides

Cyanogenic glycosides are glycosides from which cyanide is formed by the activity of hydrolytic enzymes. They are widely spread in higher plants. More than 1000 plant species have been reported to be cyanophoric, mostly in edible plants (see Table 2.1).

Cyanide doses that are lethal to humans can easily be reached or even exceeded after the intake of a variety of cyanogenic foodstuffs. Lethal intakes by humans range from 0.5 to 3.5 mg per kg body weight. The quantities of cyanide produced by Asiatic varieties of lima beans range from 200 to 300 mg per 100 g (see Table 2.2). American varieties of lima beans produce less than 20 mg HCN per 100 g. Selected breeding of low-cyanide varieties has been started.

Fresh cassava cortex produces cyanide in quantities ranging from 1.0 to more than 60.0 mg per 100 g, depending on several conditions, including variety, source, time of harvest and field conditions. Damaged roots can contain even more cyanide, i.e., 245 g per 100 g.

Table 2.1 Cyanogenic glycosides in edible plants

Glycosides

Aglycone

Sugar

Food found

Amygdalin D-mandelonitrile

Dhurrin L-p-hydroxymandelonitrile

Linamarin a-hydroxyisobutyronitrile

Lotoaustralin a-hydroxy-a-methylbutyronitrile

Prunasin D-mandelonitrile

Sambunigrin L-mandelonitrile

Vicianin D-mandelonitrile

Gentiobiose Almonds, apple, apricot, cherry, peach, pear, plum, quince

D-glucose Sorghums, kaffir corns D-glucose Lima beans, flax seed, cassava or manioc D-glucose Same as linamarin: cassava D-glucose Same as amygdalin D-glucose Legumes, elderberry Vicianose Common vetch, and other vicias

Table 2.2 Hydrogen cyanide contents of some foodstuffs Food HCN (mg/100 g)

Lima beans

210-310

Almonds

250

Sorghum sp.

250

Cassava

110

Peas

2.3

Beans

2.0

Chick peas

0.8

Cyanogenic glycosides consist of a saccharide moiety and an aglycone, a P-hydroxynitrile. The saccharide group can be a monosaccharide, e.g., glucose, or a disac-charide, e.g., gentiobiose and vicianose. Glycoside linkages can be hydrolyzed by glycosi-dases. The nitrile can undergo further degradation by a lyase to hydrogen cyanide and an aldehyde, a ketone or in some cases an acid.

CH2OH

CH2OH

Linamarin

Linamarin

Glucose

H3C OH

(hydroxynitrile lyase)

Acetone

Figure 2.2 Degradation of the cyanogenic glucoside linamarin.

Glycosidases and hydroxynitrile lyase are present in plant cells. They become available when plant tissue is damaged. This inevitably occurs when food is prepared for consumption. As mentioned above, the damaged parts of cassava roots contain high concentrations of cyanide. Nevertheless, cassava, being rich in starch, remains an important food source in Africa, parts of Asia and Latin America, because preparation methods have been developed by which the cyanogenic glycosides are removed or hydrolyzed, and P-glucosi-dase is destroyed. The cassava is grated, soaked in water, and fermented for several days. The soaked plant tissue is then dried and pounded to flour. Such processes greatly reduce the cyanogen content of food to safe levels. For example, "gari," a fermented cassava preparation, contains an average of 1.0 mg HCN per 100 g. Consumption of cassava may lead to goiter, as the cyanide formed can be metabolized to thiocyanate by the enzyme rhodanase. High consumption of dry, unfermented cassava, containing high levels of cyanogen, accounts for the widespread incidence of goiter in parts of Africa.

Sorghum can be consumed safely, as it is free from or very poor in cyanogen. On germination the sorghum seedling may reach a concentration of 0.3 to 0.5% HCN (dry weight). The young green leaves, however, are rich in cyanogen. This is why cattle are not allowed to graze on young sorghum plants. If sorghum is packed in a silo, cellular degradation and fermentation may lead to the release and elimination of cyanide.

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