Cyanogenesis in Higher Plants and Animals

Abstract

Abstract Cyanogenesis describes the ability of living organisms to liberate hydrogen cyanide from stored cyanogenic glycosides, cyanogenic lipids or cyanohydrins on tissue damage by hydrolysis and/or decomposition. It has been described for over 2650 species of higher plants and some animals. Together with plant β‐glucosidases and hydroxynitrile lyases cyanogenic glycosides are the constituent part of a preformed defence system. Today it's generally accepted that cyanogenic glycosides also may serve as storage form for reduced nitrogen and sugar. Chemically, cyanogenic glycosides are glycosides of α‐hydroxynitriles (cyanohydrins). The structures are biogenetically related to only a few precursor amino acids. Biogenetically closely related are some noncyanogenic glucosides of β‐ and γ‐hydroxynitriles ( cyanoglucosides ). Their biological function is not fully understood until today. Many food plants are cyanogenic and great efforts are made to optimise detoxification. Humans are able to detoxify considerable amounts of HCN. Although acute intoxication is rare, the daily consumption of subacute amounts of cyanogenic glycosides leads to chronic diseases, caused by increased plasma levels of the human detoxification products. The presence of cyanogenic glycosides in the animal kingdom appears to be restricted to Arthropoda. Some insects, often aposematically coloured, either de novo synthesise cyanogenic glycosides and/or sequester them from their host plants. Key Concepts: Cyanogenic glycosides are secondary metabolites known from more than 2650 different plant species. Cyanogenic glycosides are glycosides of α‐hydroxynitriles, derived from five proteinogenic amino acids (Phe, Tyr, Val, Ile and Leu) and from the nonproteinogenic amino acid cyclopentenyl glycine. Acalyphin is apparently derived from nicotinic acid. Cyanogenic plants are able to liberate hydrogen cyanide from their cyanogenic glycosides upon disruption of plant tissue. Cyanogenic glycosides and their corresponding degrading enzymes (β‐glucosidases; hydroxynitrile lyases) are part of a preformed defence system. Thus, they can be regarded as phytoanticipins. Additional roles and functions of cyanogenic glycosides include storage of reduced nitrogen and sugar, transportation of nitrogen and the turnover of nitrogen into primary metabolism. The presence of cyanogenic glycosides in animals appears to be restricted to Arthropoda. Some insects are strongly associated with their cyanogenic host plants. They sequester the cyanogenic glycosides from these pants as well as carry out de novo biosynthesis of these compounds.