Localization and catabolism of cyanogenic glycosides.
The catabolism of cyanogenic glycosides is initiated by cleavage of the carbohydrate moiety by one or more beta-glycosidases, which yields the corresponding alpha-hydroxynitrile. Until recently, the mode by which cyanogenic disaccharides are hydrolysed was largely unclear. Investigation of highly purified beta-glycosidases from plants containing cyanogenic disaccharides has now indicated that these compounds may be degraded via two distinct pathways, depending on the plant species. beta-Glycosidases from Davallia trichomanoides and Vicia angustifolia hydrolysed (R)-vicianin and (R)-amygdalin at the aglycone-disaccharide bond producing mandelonitrile and the corresponding disaccharide. Alternatively, hydrolysis of cyanogenic disaccharides in Prunus serotina, almonds, and Linum usitatissimum involves stepwise removal of the sugar residues. The nature of these enzymes and of other beta-glycosidases responsible for hydrolysis of simple cyanogenic monosaccharides is discussed. Hydroxynitriles may decompose either spontaneously or enzymically in the presence of a hydroxynitrile lyase to produce hydrogen cyanide and an aldehyde or ketone. The major kinetic and molecular properties of hydroxynitrile lyases purified from species accumulating aromatic and aliphatic cyanogens are reviewed. Cyanogenesis occurs rapidly only after cyanogenic plant tissues are macerated, allowing glycosides access to their catabolic enzymes. The possible nature of the compartmentation which prevents cyanogenesis under normal physiological conditions is discussed in relation to our knowledge of the tissue and subcellular localizations of cyanogens and catabolic enzymes.