ABSTRACT
Cyanogenic glycosides (CGs) are nitrogen-containing secondary metabolites whose biosynthetic origin is similar to that of glucosinolates. They are intermediately polar, water-soluble compounds that often accumulate in the vacuoles of plant cells (1-13). These compounds are usually O-β-glycosides of cyanohydrins (α-hydroxynitriles). They are optically active because of chirality of the hydroxylated C-atom 2, which occurs in either the S or the R configuration. Cyanogenic glycosides are formed in the cytoplasm but stored in the central vacuole. Storage of cyanogenic glycosides is tissue specific and takes place in epidermal vacuoles in sorghum. The enzymes that degrade cyanogenic glycosides, namely, β-glucosidase and hydroxynitrile lyase, are present in the adjacent mesophyll cells, which is safely away from the cyanogenic glycosides. More than 75 different cyanogenic glycosides occur among plants. A closely related nitrile glycosides and cyanolipids also occur in plants, but their distribution is restricted. Cyanolipids are found only in Sapindaceae and Hippocastanaceae (14). Several reviews on cyanogenic compounds have been published (1-13). All plants produce cyanide; however, in most cases cyanide is present in extremely small quantities. A level of 10 mg HCN per kg is the minimum for a plant to be considered cyanogenic. Cyanogenic glycosides are stored in plant cell vacuoles and enzyme in the cytosol. When the plant tissues are damaged due to herbivory, trampling, intense heat, or frost, cyanide release occurs. Since nonruminants have more acidic stomach, they are not sensitive to cyanogenic glycosides. Tiger beetles, millipedes, and centipedes are able to use cyanogenic glycosides as a defense against predators. These insects sequester the cyanide in their cells. When an insect is attacked by a predator, cyanide is released as a defense.
